Accuracy and calibration of integrated radiation output indicators in

Accuracy and calibration of integrated radiation output indicators in
Accuracy and calibration of integrated radiation output indicators
in diagnostic radiology: A report of the AAPM Imaging Physics
Committee Task Group 190
Pei-Jan P. Lina)
Virginia Commonwealth University Medical Center, Richmond, Virginia 23298
Beth A. Schueler
Mayo Clinic, Rochester, Minnesota 55905
Stephen Balter
Columbia University Medical Center, New York, New York 10032
Keith J. Strauss
Children’s Hospital Medical Center, Cincinnati, Ohio 45229
Kevin A. Wunderle
Cleveland Clinic Foundation, Cleveland, Ohio 44195
M. Terry LaFrance
Baystate Health Systems, Inc., Springfield, Massachusetts 01199
Don-Soo Kim
Children’s Hospital Boston, Boston, Massachusetts 02115
Richard H. Behrman
Boston University Medical Center, Boston, Massachusetts 02118
S. Jeff Shepard
University of Texas MD Anderson Cancer Center, Houston, Texas 77096
Ishtiaq H. Bercha
Children’s Hospital Colorado, Aurora, Colorado 80045
(Received 29 April 2015; revised 4 September 2015; accepted for publication 19 October 2015;
published 6 November 2015)
Due to the proliferation of disciplines employing fluoroscopy as their primary imaging tool and the
prolonged extensive use of fluoroscopy in interventional and cardiovascular angiography procedures,
“dose-area-product” (DAP) meters were installed to monitor and record the radiation dose delivered
to patients. In some cases, the radiation dose or the output value is calculated, rather than measured,
using the pertinent radiological parameters and geometrical information. The AAPM Task Group
190 (TG-190) was established to evaluate the accuracy of the DAP meter in 2008. Since then, the
term “DAP-meter” has been revised to air kerma-area product (KAP) meter. The charge of TG 190
(Accuracy and Calibration of Integrated Radiation Output Indicators in Diagnostic Radiology) has
also been realigned to investigate the “Accuracy and Calibration of Integrated Radiation Output
Indicators” which is reflected in the title of the task group, to include situations where the KAP
may be acquired with or without the presence of a physical “meter.” To accomplish this goal,
validation test protocols were developed to compare the displayed radiation output value to an
external measurement. These test protocols were applied to a number of clinical systems to collect
information on the accuracy of dose display values in the field. C 2015 American Association of
Physicists in Medicine. [http://dx.doi.org/10.1118/1.4934831]
Key words: fluoroscopy, dose-area-product, kerma-area-product, calibration of KAP meters, patient
exposure
TABLE OF CONTENTS
1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2.C Digital imaging and communications in
medicine (DICOM) radiation dose
structured report (RDSR) . . . . . . . . . . . . . . . . .
2 BACKGROUND INFORMATION . . . . . . . . . . . . . . .
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2.D Integrated radiation output indicators . . . . . . .
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2.A KAP, the units, and the geometry . . . . . . . . . .
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2.B Regulatory requirement . . . . . . . . . . . . . . . . . . .
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3 INTEGRATED RADIATION OUTPUT SYSTEM
VALIDATION METHOD . . . . . . . . . . . . . . . . . . . . . . .
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3.A Protocol overview . . . . . . . . . . . . . . . . . . . . . . . .
3.A.1 An external radiation dosimeter . . . . .
3.A.2 Attenuator . . . . . . . . . . . . . . . . . . . . . . . .
3.A.3 Field-size measurement plate . . . . . . .
3.A.4 Radiation detector/FSMP stand . . . . .
3.B Interventional fluoroscopy system protocol
(vertical geometry) . . . . . . . . . . . . . . . . . . . . . . .
3.B.1 Determination of focal spot location.
3.B.2 Determination of isocenter location .
3.B.3 Measurement setup for Ka,r and
radiation field size . . . . . . . . . . . . . . . . .
3.B.4 Measurement process . . . . . . . . . . . . . .
3.B.5 Calculation process . . . . . . . . . . . . . . . .
3.C Interventional fluoroscopy system protocol
(horizontal geometry) . . . . . . . . . . . . . . . . . . . .
3.C.1 Determination of focal spot location.
3.C.2 Measurement setup . . . . . . . . . . . . . . . .
3.C.3 Measurement process . . . . . . . . . . . . . .
3.C.4 Calculation process . . . . . . . . . . . . . . . .
3.D Undertable fluoroscopy system protocol . . . .
3.D.1 Measurement setup . . . . . . . . . . . . . . . .
3.D.2 Measurement process . . . . . . . . . . . . . .
3.D.3 Calculation process . . . . . . . . . . . . . . . .
3.E Overtable fluoroscopy system protocol . . . . .
3.E.1 Measurement setup . . . . . . . . . . . . . . . .
3.E.2 Measurement process . . . . . . . . . . . . . .
3.E.3 Calculation process . . . . . . . . . . . . . . . .
3.F Multipurpose fluoroscopy system protocol . .
3.G Mobile C-arm system protocol . . . . . . . . . . . .
3.G.1 Measurement setup . . . . . . . . . . . . . . . .
3.G.2 Measurement process . . . . . . . . . . . . . .
3.G.3 Calculation process . . . . . . . . . . . . . . . .
3.H Mini-C-arm protocol . . . . . . . . . . . . . . . . . . . . .
3.H.1 Measurement setup . . . . . . . . . . . . . . . .
3.H.2 Measurement process . . . . . . . . . . . . . .
3.H.3 Calculation process . . . . . . . . . . . . . . . .
3.I Radiographic systems protocol . . . . . . . . . . . .
3.I.1 Measurement setup . . . . . . . . . . . . . . . .
3.I.2 Measurement process . . . . . . . . . . . . . .
3.I.3 Calculation process . . . . . . . . . . . . . . . .
DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.A Sources of uncertainty . . . . . . . . . . . . . . . . . . . .
4.A.1 External dosimeter reading
uncertainty . . . . . . . . . . . . . . . . . . . . . . .
4.A.2 External detector location
uncertainty . . . . . . . . . . . . . . . . . . . . . . .
4.A.3 Displayed dose value accuracy. . . . ..
4.A.4 X-ray field-size measurement
uncertainty . . . . . . . . . . . . . . . . . . . . . . .
4.B KAP meter performance variation with
beam quality . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SUMMARY AND CONCLUSIONS . . . . . . . . . . . . .
APPENDIX A: THE FIELD-SIZE
MEASUREMENT PLATE AND STAND . . . . . . . . .
APPENDIX B: THE HORIZONTAL
GEOMETRICAL ARRANGEMENT . . . . . . . . . . . .
NOMENCLATURE . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. INTRODUCTION
Over the past two decades, several publications have addressed
concerns on the dramatic increase of radiation dose patients
receive from fluoroscopic examinations has increased dramatically.1–3 This has been due in part to the proliferation of medical disciplines that are new to the application of fluoroscopy,
in employing this imaging tool in their patient care.1,2 On the
other hand, the development of complex surgical procedures
resulted in prolonged extensive use of fluoroscopy in interventional and cardiovascular angiography procedures which
contributed substantial increase in radiation exposures to patients.3–5 To better understand and control this increase in
dose, manufacturers began providing real-time displays of
the radiation dose that a patient was receiving. These realtime displays work by either performing calculations using the
pertinent generator parameters and geometrical information or
by measuring the x-ray beam directly with what is known as a
dose-area-product (DAP) meter.
The AAPM Task Group 190 (TG-190) was established in
2008 initially to assess the accuracy of the DAP meter. Since
then, the term “DAP-meter” has been revised to air kerma-area
product (KAP) meter. Furthermore, the charge of TG 190 has
been realigned to investigate the “Accuracy and Calibration of
Integrated Radiation Output Indicators” which is reflected in
the title of the task group (Accuracy and Calibration of Integrated Radiation Output Indicators in Diagnostic Radiology),
to include situations where the KAP may be acquired with or
without a physical meter. To accomplish that goal, validation
test protocols for different equipment types were developed to
compare the displayed radiation output value to an external
measurement. These test protocols were applied to a number
of clinical systems to collect information on the accuracy of
radiation output in the field.
2. BACKGROUND INFORMATION
2.A. KAP, the units, and the geometry
The KAP is the air kerma integrated over the area of the
exposure field in the plane perpendicular to the beam axis.
The value of KAP is independent of the distance from the xray source since the air kerma decreases proportionally to the
square of the distance from the source while the x-ray beam
area increases proportionally to the square of that distance.
When displayed on radiographic and fluoroscopic systems,
the units of KAP used will vary with manufacturer, equipment
type, and software revision, including (Gy-cm2), (cGy-cm2),
(mGy-m2), and (µGy-m2). The mGy displayed by fluoroscopic
systems is the accumulated air kerma at a demarcated reference point (RP) Ka,r . This quantity is defined under lowscatter conditions with all removable attenuators (e.g., tabletop
and pad) removed from the beam-path between the x-ray
source and the measurement point.
The RP is defined to approximate the “location” of the
patient’s entrance skin surface. This location will differ for
various fluoroscopic equipment configurations. The RP may
also be specified by the manufacturer at an alternative
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location which represents the location of the patient’s entrance
skin surface. Listed in Table I is a summary of RP locations
defined by the U.S. Food and Drug Administration (FDA,
21CFR1020.32).6
It should be noted that actual patient skin dose will likely
be different from the displayed Ka,r value. Ka,r represents
an approximate sum of air kerma delivered to all areas of
the patient’s skin exposed to fluoroscopic radiation during the
procedure. In most procedures, the x-ray beam is moved to
different entrance locations, exposing different skin areas, so
that the air kerma to any one anatomical area will be lower
than the accumulated total. As pointed out by Jones and Pasciak, skin dose estimation must also account for backscatter,
tabletop and pad attenuation, soft tissue f -factor, and the actual
source to skin distance.7
result, both Ka,r and KAP are displayed on most fluoroscopic
systems. These requirements and/or regulations also specify
the accuracy requirements for Ka,r and KAP values. The
displayed Ka,r value shall not deviate from the actual value
by more than ±35% above 100 mGy (Refs. 6 and 9) and KAP
shall not deviate from the actual value by more than ±35%
above 2.5 Gy-cm2.7,9
2.B. Regulatory requirement
Both International Electrotechnical Commission (IEC
2000)8 and (IEC 2010)9 and the FDA,6 for equipment manufactured after June 2006, require fluoroscopy equipment
to display the cumulative Ka,r and the Ka,r rate during the
procedure at the operator’s working position. IEC also requires that an indication of the KAP value be provided. As a
T I. Nominal reference point (RP) location specifications for different
fluoroscope types.a
Fluoroscope type
Vertical orientation
fixed C-arm system
Horizontal orientation
fixed C-arm, L-arm, or
lateral system
Undertable x-ray tube
Overtable x-ray tube
Mobile C-arm
Mini-C-arm
Radiographic unit
Reference point (RP) location
15 cm from the isocenter toward the
x-ray source along the centerline of x-ray
beam
15 cm from the centerline of the x-ray
table and in the direction of the x-ray
source with the end of the beam-limiting
device or spacer positioned as closely as
possible to the point of measurement
1 cm above the tabletop or cradle
30 cm above the tabletop with the end of
the beam-limiting device or spacer
positioned as closely as possible to the
point of measurement
30 cm from the entrance surface of the
image receptor toward the x-ray source
along the centerline of x-ray beam
Typically located 3–6 cm from the image
receptor toward the x-ray source along
the centerline of x-ray beam
30 cm above the tabletop toward the
x-ray source along the centerline of x-ray
beam, with the end of the beam-limiting
device or spacer positioned as closely as
possible to the point of measurement
Note: The cumulative air kerma represents the value for conditions of free-in-air
irradiation at one of the RP locations specified according to the type of fluoroscope. Alternative locations of the RP may be specified by the manufacturer. The
user is advised to refer to Instructions for Use or Operator’s Manual to verify the
RP location for the system.
a
21CFR1020.32, Code of Federal Regulations, Title 21, Volume 8, Performance
Standards for Ionizing Radiation Emitting Products: Fluoroscopic Equipment,
April 1, 2013.
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2.C. Digital imaging and communications in medicine
(DICOM) radiation dose structured report (RDSR)
More recently, some fluoroscopic systems have begun to
provide a report file that contains a summary of procedure dose
information for later analysis and recording. The RDSR, as
specified by DICOM 2011 standards,10 contains detailed dose
and geometry data for each irradiation event (individual fluoroscopy or image acquisition sequence) and an accumulated
dose summary for the entire procedure. In addition, the RDSR
includes fields for the recording of information related to the
calibration of dose values and calibration factors to account for
the deviation of the displayed dose from the external measurement. Table II is a summary of the dose calibration data fields
specified.
It should be noted that the calibration factor is not utilized by the equipment manufacturer to modify the displayed
dose values. Instead, the calibration information is to be
utilized by the customer, typically a medical physicist. The
calibration factor, accounting for tabletop attenuation, backscatter, and geometry considerations, for example, may be
applied to the dose values that are displayed or recorded in the
RDSR to allow for greater accuracy in individual patient dose
estimation.
Additional specification of RDSR content has been provided in a publicly available specification from IEC (IEC
2007).11 This prestandard defines four levels of conformance
with specified RDSR elements required for each level. For all
levels above level 0 limited conformance (level 1 limited dose
monitoring, level 2 general dose monitoring, level 3 advanced
dose monitoring), the dose calibration elements included in
Table II must be provided. Furthermore, level 2 conformance
includes the source to reference point distance and the collimated field area per irradiation event.
T II. RDSR dose calibration definitions.
Attribute
Calibration
Calibration date
Calibration
factor
Calibration
uncertainty
Calibration
responsible party
Definition
Procedure used to calibrate
measurements or measurement devices
Last calibration date for the integrated
dose meter or dose calculation
Factor by which a measured or calculated
value is multiplied to obtain the estimated
real-world value
Uncertainty of the “actual” value. Value
ranges from 0% to 100%
Individual or organization responsible for
calibration
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Section 2.D of this report includes detailed methods for
measurement of the calibration factor for various equipment
types. To provide a method to enter this information into the
RDSR for individual interventional fluoroscopy systems, a
user quality control mode has been specified in a National
Electrical Manufacturers Association standard (NEMA XR
27-2012).12 Note that currently the RDSR provides for entry
of a single calibration factor. A single calibration factor may
not be sufficient to account for the complexity encountered
in all types of fluoroscopy equipment and clinical geometry.
However, the attenuation due to the tabletop for a given system
may be applied. Therefore, it is possible that the dose display
correction factor may be different for Ka,r and KAP and/or
differ for variations in beam quality and dose rate. Additional
examination of this issue is included in Sec. 4.
For convenience, a summary of acronyms and abbreviations employed in this report is provided in Nomenclature.
2.D. Integrated radiation output indicators
Several different methods are currently used by manufacturers of radiographic and fluoroscopic equipment to provide
a measurement or estimate of Ka,r and KAP. One common
method is direct measurement of KAP with a KAP-meter.
The KAP-meter is a thin, parallel-plate transmission ionization
chamber that is fixed in the x-ray tube-collimator assembly,
typically at the end of the collimator and at all times intercepts
the entire x-ray field.
In order to estimate Ka,r , the KAP is divided by the irradiated field area at the RP location. The exposure area is determined by system indicators of the collimator blade position.
Using this method, KAP accuracy will depend on the accuracy
of the KAP-meter itself and Ka,r accuracy will depend on both
the KAP-meter accuracy and the field-size measurement.
Instead of direct KAP measurement, some models determine Ka,r computationally based on x-ray tube output for
given technique factors and added filtration. KAP can then be
estimated by multiplying Ka,r by the x-ray beam area. For this
method, Ka,r accuracy will depend on the accuracy of x-ray
output values that are used and KAP accuracy will depend on
both the Ka,r computation and the field-size measurement.
3. INTEGRATED RADIATION OUTPUT SYSTEM
VALIDATION METHOD
3.A. Protocol overview
The following sections describe integrated radiation output
indicator validation protocols for various types of fluoroscopic
and radiographic equipment configurations.
3.B.
3.C.
3.D.
3.E.
3.F.
Interventional fluoroscopy, vertical x-ray beam.
Interventional fluoroscopy, horizontal x-ray beam.
Undertable x-ray fluoroscopy.
Overtable x-ray fluoroscopy.
Multipurpose fluoroscopy (C-arm with integrated table).
3.G. Mobile C-arms.
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3.H. Minimobile C-arms.
3.I. Radiographic systems.
For each equipment configuration, irradiation data are
collected from both an external dosimeter and the system’s
integrated radiation output display. The measurements from
the external dosimeter are compared to the system’s internal
display. This process yields a correction factor C where C(x)
= measured external value/system’s displayed value, where x
is either Ka,r or KAP.
A system with C < 1.0 will display a value that is greater
than the external value, and a system with C > 1.0 will display
a value that is smaller than the external value.
The following materials are required to perform validation
measurements.
3.A.1. An external radiation dosimeter
The dosimeter should be able to make accurate measurements over the appropriate air kerma and beam quality range.
It should have a traceable current calibration based on relevant
beam qualities. The measured external air kerma value should
include necessary adjustments for the calibration factor of the
dosimeter and appropriate temperature-pressure corrections.
A note on the requirements of this section: Typically, the
dosimeter is calibrated with standard radiation quality RQR 5
[70 kilovolt peak (kVp), homogeneity coefficient 0.71, nominal first HVL 2.58 mm Al] and/or RQR 8 (100 kVp, homogeneity coefficient 0.68, nominal first HVL 3.97 mm Al) as
specified in Table 4 of IEC 61267 ed. 2.0. The energy response
over the 50–150 kV should be better than ±2.5% of the RQR
calibration points.
It is possible to use an external KAP-meter to perform
validation measurements. This device will allow for direct
validation of displayed KAP values and calculated Ka,r values
with measurement of the field area. Also available are dual
chamber KAP-meters which incorporate a small detector to
allow for simultaneous KAP and Ka,r measurement. For either
type of meter, a current calibration is needed for accurate
validation measurements.
It is important to note that when performing validation
measurements, external KAP-meters and radiation dosimeters
without incorporated lead backing should be placed so that
the sensitive area of the dosimeter is not in a region where
the measurement may be impacted by backscatter from the
image receptor. A spacing of at least 10 cm away from the
image receptor is recommended, for an irradiated field size of
50–100 cm2.
The dosimeter will be operated in “integrate” mode to
allow for accumulation of sufficient air kerma (a minimum
of 50 mGy is suggested) for improved radiation output accuracy. In integrate mode, multiple acquisitions or fluoroscopy
irradiation events can be combined to achieve the desired air
kerma total. Use of an “automatic reset” mode is allowable;
in this mode, the integrated dose is reset at the start of each
exposure. Use of a dose rate mode for fluoroscopy measurements is not recommended due to variations in the radiation
output delivered over time. This is particularly important in
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interventional systems in acquisition (cine) modes, which can
have substantial variations by design.
3.A.2. Attenuator
An attenuator consisting of copper sheets of approximately
1 mm totaling at least 8 mm is required. Copper is selected
as the attenuator to reduce weight and excessive x-ray scatter
production. The copper sheets should be large enough (at least
30 × 30 cm) to cover the collimated exposure field and placed
as close as possible to the image receptor. The copper sheets do
not need to be high purity. Commonly available copper sheets,
type C101 with 99.99% purity, are acceptable. The purpose of
the copper is only to drive the fluoroscope within the target
kVp range. Individual copper sheets approximately 1 mm
(or, alternatively 1/32 in.) thick are recommended to allow
for adjustment of the total attenuator thickness as needed.
This thickness of attenuator will typically drive a fluoroscopic
system under automatic dose rate control to moderately high
kVp and Ka,r rates. Use of high kVp minimizes deviations in
KAP-meter accuracy that are common at low kVp and use of
high Ka,r rates allows for more rapid accumulated air kerma
measurements.13
F. 2. Enlarged view of FSMP.
vides a 49 cm2 area for KAP measurement. (To be exact, a
7.071 cm2 would provide a 50 cm2 area.) The 10, 15, and 20 cm
square boxes and the radio-opaque ruled demarcations work
as landmarks aiding setup and measurements of the radiation
field size. Small lead numbers may be embedded to assist in
identifying the field size.
One enlarged section of the radio-opaque ruler on the FSMP
is shown in Fig. 2. Note that the linewidth of radio-opaque ruler
and the square boxes is 1 mm.
3.A.3. Field-size measurement plate
The plate should contain radio-opaque ruled demarcations
in orthogonal directions to allow for measurement of the exposure field area. A sample field-size measurement plate (FSMP)
is depicted in Fig. 1.
On the FSMP, the cutout is 7 × 7 cm in size which serves
two purposes. It is a blank space to accommodate a 30 cm3 flat
pancake-type ionization chamber typically 5 cm in diameter
or smaller. The physical size of the square (7 × 7 cm) pro-
3.A.4. Radiation detector/FSMP stand
If using the interventional fluoroscopy system protocol
(vertical geometry) method described below, a stand to hold
the radiation dosimeter and field-size measurement plate is
useful. The stand may be as simple as using the FSMP itself, as
described later in Sec. 3.C.3, securely affixed and extended out
from the tabletop for the field-size measurement. The FSMP
may also be employed as a supporting device to hold the
radiation detector. Alternatively, a more elaborate stand can
be fabricated to hold the FSMP and the detector in a more
convenient configuration. One such example of FSMP stand
is shown in Appendix A.
3.B. Interventional fluoroscopy system protocol
(vertical geometry)
F. 1. Field-size measurement plate (FSMP).
Medical Physics, Vol. 42, No. 12, December 2015
This section of the report is specific to isocentric fluoroscopes with a fixed focal spot to isocenter distance [source
to axis distance (SAD)]. The default RP location is along the
central ray of the x-ray beam at a distance of 15 cm from the
isocenter toward the x-ray tube. Note that any particular system
might have a different RP location. This and other important
geometric dimensions are available in the Instructions for Use
or Operator’s Manual that the manufacturer is required to
supply with each system.
This method makes use of the FSMP at the plane of radiation measurements for the positioning of the dosimeter as
well as the placement and thickness adjustment of the copper
sheets. In addition, the physical sizes of the copper sheets need
to be large enough to encompass the entire area of the x-ray
beam at the location of the tabletop.
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The measurement procedure includes a method to determine the exact location of the focal spot within the x-ray tube
housing and a method to verify the location of the isocenter
of the C-arm gantry. Once this information is obtained for a
system, it need not be repeated for subsequent measurements.
on bottom right of Fig. 3. When both sheets appear
the same in size, by triangulation, “A” is equal to “B”
(A = B).
(4) Without moving the vertical position of the examination table or the detector, carefully measure the distance
between the two copper sheets. Make sure that the tape
measure is vertical during this measurement; it may be
necessary to move the table horizontally to ensure this.
Then, measure the distance from the 1 × 1 cm copper
sheet test object to the x-ray tube. This will provide
the location of the focal spot. This location can be
marked permanently on the x-ray tube housing surface
for future reference.
(5) Using the determined focal spot location, measure the
distance from the focal spot to the exit point of the x-ray
tube housing assembly (SHD).
3.B.1. Determination of focal spot location
In some cases, the focal spot location may not be marked
on the x-ray tube housing or a precise determination of the
focal spot location is desired. To find the location, two test
objects will be employed; a 1 × 1 cm and a 2 × 2 cm copper
sheets (1 mm thick). The 1 × 1 cm copper sheet can be placed
on the tabletop or a stand where the FSMP is located while
the 2 × 2 cm copper sheet is attached to the front face of the
image receptor. The exact location of the focal spot can then
be determined as follows.
(1) All clinically removable attenuators except the x-ray
table (i.e., removable without tools) shall be removed
from the path between the x-ray tube assembly and the
measuring detector before acquiring data.
(2) Place the 1×1 cm copper sheet on the FSMP (which is
securely affixed on and extended from the tabletop) and
attach the 2×2 cm copper sheet on the front cover of the
image receptor housing assembly as shown in Fig. 3.
Both copper sheets should be aligned and placed in the
center of the imaging field as shown on the top right of
Fig. 3.
(3) During fluoroscopy with the x-ray beam vertical, either
change the elevation of the tabletop or the elevation
of the image receptor until the size of the two copper
sheet test objects in the fluoroscopic image is identical.
This will place the small 1 × 1 cm copper sheet exactly
midway between the focal spot and the 2×2 cm copper
sheet. The images of copper sheets appear as shown
3.B.2. Determination of isocenter location
(1) All clinically removable attenuators (i.e., removable
without tools) shall be removed from the path between
the x-ray tube assembly and the measuring detector
before acquiring data. This can be accomplished either
by retracting the examination table or using a horizontal x-ray beam.
(2) Set the system to the medium field-of-view (FOV) with
the collimator opened to its fullest extent. The medium
FOV, typically, is a 23 cm image intensifier input field
size or a 25 × 25 cm flat panel image receptor.
(3) Set the gantry to the maximum source to image receptor
distance (SID) that allows free rotation of the gantry.
(4) Tape a small lead marker to the surface of the tabletop.
Or, use the FSMP in place of the tabletop as continuation of Sec. 3.B.1.
(5) Set the x-ray beam to a vertical orientation: The small
radio-opaque marker is placed approximately at isocenter by moving the tabletop horizontally.
(6) Set the x-ray beam to a horizontal orientation: Center
the radio-opaque marker by adjusting tabletop height.
(7) Repeat steps (5) and (6) until the marker does not move
across the field as the gantry is rotated. The marker is
now at the isocenter.
(8) Orient the x-ray beam vertically with the tube over the
table.
(9) Without moving the vertical elevation of the x-ray table, carefully measure the vertical distance from the
focal spot marking on the x-ray tube housing to the lead
marker taped to the tabletop. Note that moving the table
horizontally may be included during this measurement
to ensure that the tape measure is vertically oriented.
Record this distance as the focal spot to isocenter distance, SAD.
3.B.3. Measurement setup for Ka,r and radiation
field size
F. 3. Geometry of utilizing two copper sheets for localization of focal spot,
under vertical geometry.
Medical Physics, Vol. 42, No. 12, December 2015
(1) All clinically removable attenuators (i.e., removable
without tools) shall be removed from the path between
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(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
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the x-ray tube assembly and the measuring detector
before acquiring data.
The examination tabletop should be moved to its
maximum height. To the extent practicable, there
should be no objects, including the x-ray table, in the
x-ray beam that can scatter x-rays from within 10 cm
of the sensitive volume of the measuring detector.
Set the x-ray beam to a vertical orientation.
Set the gantry to the maximum SID.
Set the system to a medium FOV with open collimation.
Affix the FSMP on the tabletop as shown in Fig. 4. The
field-size measurement pattern is extended out from
the tabletop and into air. Place the external dosimeter
in the cutout of the FSMP and centered within FSMP.
Using a tape measure, carefully measure and record
the distance from the focal spot to the center of the
external dosimeter (source to detector distance, SDD).
Set the system to a FOV of approximately 17 cm
(diagonal) at the image receptor and maximum SID.
Note: A legacy of circular image intensifier specification. A 17 cm diagonal flat panel image receptor is
equivalent to 6–7 in. round FOV of an image intensifier.
Set the collimators using the 10×10 cm2 marker on the
FSMP. All edges of the irradiated field should be seen
on the monitor almost superimposed on the square
box and well within the full FOV border. (If the penumbra makes the location of the edge uncertain, place
the corresponding edge of the 10 × 10 cm2 marker on
the FSMP to the middle of the penumbra.)
Place an attenuator (approximately a total of 8 mm
Cu) on the tabletop so it completely intercepts the xray beam.
Select the medium dose rate modes for fluoroscopy. If
the system only has two dose rate mode choices, then
select the higher.
(12) Do a test irradiation. Adjust the attenuator thickness
if this is necessary to bring the displayed tube voltage
into the 90–100 kVp range and record the attenuator
thickness required.
F. 4. Measurement setup for K a, r and radiation field size.
Medical Physics, Vol. 42, No. 12, December 2015
Note on the selection of tube voltage in the range of
90–100 kVp.
(i) It is preferred to have one measurement point for
verification of accuracy and calibration of integrated
output indicators.
(ii) The tube potential for various fluoroscopic examinations, typically, falls in the 90–100 kVp range.
(iii) The TG 190 members compared a half dozen radiation
detectors typically employed in the field and found
better agreement amongst different types of detectors
at 90–100 kVp (<5%) than at 70–90 kVp (∼15%).
3.B.4. Measurement process
The following procedure should be done both with the fluoroscopic and the acquisition mode, using the amount of filtration appropriate for each to bring the displayed tube voltage
into the 90–100 kVp range.
(1) Collect data using the external dosimeter in the integrate mode.
(2) Precision is increased if each measurement is appropriately replicated (three is suggested). The coefficient of variation in the value of C calculated from the
repeated measurements (see Sec. 3.B.5) should be less
than 0.01.
(3) Record the initial displayed system values of Ka,r and
KAP and the external dosimeter reading before each
irradiation. If using the external dosimeter’s “autodose” mode, you need not record the external reading.
(4) Each irradiation should be long enough so that the
resolution of the digital displays does not significantly
limit the accuracy of the measurements. For example,
for a system that displays Ka,r in units of mGy in a
whole number (rounded to the nearest integer value),
the external dosimeter should show approximately
50 mGy after each irradiation.
(5) Record the final displayed system values of Ka,r and
KAP and the external dosimeter reading after each
irradiation. Record the associated generator factors
(kVp, mA, ms, beam filter) if available.
(6) Calculate C(Ka,r ) using the process described in
Eq. (3) in Sec. 3.B.5.
(7) Remove the external dosimeter from the beam without
disturbing the collimator setting.
(8) Place the FSMP at isocenter by rotating the gantry between the vertical and the horizontal imaging projections to confirm the correct placement of the FSMP at
the isocenter.
(9) Perform a fluorographic irradiation (nonsubtracted) to
image the FSMP, or a fluoroscopic irradiation with
last image hold.
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(10) Determine the field size at isocenter by observing the
image of the FSMP on the system monitor. If the system is equipped with “black shutters” or “automatic
masking” these should be turned off before making
this measurement. Record the two dimensions of the
rectangular field as AL and AW.
the x-ray tube assembly and the measuring detector
before acquiring data.
As much as practicable and possible, there should be
no objects that can scatter x-rays within 10 cm of
the sensitive volume of the measuring detector. Specific attention should be directed toward the location
of the tabletop and the supports for the measuring
detector.
Set the system to a medium FOV with open collimation. (Typically 6–9 in. or 17–22 cm FOV.)
Set the gantry to the maximum SID that allows free
rotation of the gantry.
Set the x-ray beam to a vertical orientation. The
external dosimeter is placed approximately at isocenter by moving the table horizontally. Note that the
radiation detector may be suspended in air or placed
on the FSMP. This is described in Sec. 3.B.3, Fig. 4,
and the corresponding photographs in Fig. 7 of the
Appendix.
Set the x-ray beam to a horizontal orientation. Center the detector by adjusting table height. See Appendix B for a photograph and description of the
setup.
When the detector appears to rotate in the image but
remains fixed at the center of the field of view as the
gantry rotates, it is at isocenter. Repeat steps (5) and
(6) until the detector does not move across the field as
the gantry is rotated.
Use a tape measure to determine the distance from the
focal spot to the chamber and record this distance as
the SAD.
Rotate the system for a horizontal x-ray beam.
Set the system to a FOV of approximately 17 cm (or
22 cm) at the image receptor and maximum SID.
Set the collimators using the 10×10 cm2 marker on the
FSMP. All edges of the irradiated field should be seen
on the monitor almost superimposed on the square
box and well within the full FOV border.
Attach an attenuator (∼8 mm Cu) to the face of the
image receptor. The attenuator thickness may need to
be adjusted later to yield 90–100 kVp while taking
measurements.
Select the medium dose rate modes for fluoroscopy. If
the system only has two dose rate mode choices, then
select the higher.
Do a test irradiation. Adjust the attenuator thickness
if this is necessary to bring the displayed tube voltage
into the 90–100 kVp range and record the attenuator
thickness required.
3.B.5. Calculation process
(2)
(3)
(1) Calculate the measured KAP,
KAP = [measured Kamas] ∗ AW ∗ AL,
(4)
(1)
where Kamas is the accumulated air kerma measured
with the external dosimeter at isocenter. If using integrate rather than autodose mode, this is the value
recorded after an irradiation event minus the value
recorded before the event.
(2) Calculate the air kerma Ka,r at the RP by multiplying
the measured air kerma at isocenter Ka,SAD by a geometric factor G1,
G1 = [SAD/(SAD − RPD)]2,
(6)
(2)
where RPD = isocenter to reference point distance
Ka,r = [measured Ka,SAD] ∗ G1.
(5)
(7)
(3)
For example, for SAD = 750 mm, RPD = 150 mm,
G1 = [750/(750 − 150)]2 = 1.56.
(3) Calculate the correction factor C separately for each
irradiation event for both Ka,r and KAP. Caution: The
following processes may need further corrections to
account for the dose units in which a particular fluoroscope displays KAP.
(a) C(KAP) is determined by dividing the measured
KAP value by the KAP value displayed by the
system.
(b) C(Ka,r ) is determined by dividing the measured
Ka,r value by the Ka,r value displayed by the
system.
(4) Average the individual C factors over the measurement
repetitions for Ka,r , and separately for KAP.
(8)
(9)
(10)
(11)
(12)
(13)
3.C. Interventional fluoroscopy system protocol
(horizontal geometry)
While this section is similar to Sec. 3.B, an important
difference is that the dosimetry measurements are completed
with a horizontal as opposed to a vertical x-ray beam geometry.
(14)
3.C.1. Determination of focal spot location
Follow the procedure in Sec. 3.B.1.
3.C.3. Measurement process
Follow the procedure described in Sec. 3.B.4.
3.C.2. Measurement setup
(1) All clinically removable attenuators (i.e., removable
without tools) shall be removed from the path between
Medical Physics, Vol. 42, No. 12, December 2015
3.C.4. Calculation process
Follow the procedure described in Sec. 3.B.5.
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3.D. Undertable fluoroscopy system protocol
This section of the report addresses fluoroscopic systems
with x-ray tubes mounted under the procedure table. These
units typically have variable SIDs between 75 and 120 cm. The
default RP is located 1 cm above the procedure table. (Note that
any particular make or model may have a different RP location
and this should be confirmed in the vendor documentation.)
3.D.1. Measurement setup
(1) Remove any table pads from the path between the x-ray
tube and the measuring detector before acquiring data.
To the extent practicable, there should be no objects that
can scatter x-rays within 10 cm of the sensitive volume
of the external dosimeter.
(2) Set the system to a FOV of approximately 22 cm and
raise the image receptor tower to the maximum height.
(3) Place the FSMP and the external dosimeter 1 cm above
the tabletop or at the manufacturer specified RP if
different from “1 cm above the tabletop.” If necessary,
raise the FSMP with spacers. Set the collimators using
the 10 × 10 cm2 marker on the FSMP. All edges of the
irradiated field should be seen on the monitor almost
superimposed on the square box and well within the
full FOV.
(4) Place an attenuator (approximately 5 mm Cu) above the
radiation detector in the primary beam. The attenuator
sheets may be attached to the image receptor tower or
placed on a stand to hold the sheets at least 10 cm above
the radiation detector.
(5) Select the medium dose rate mode for fluoroscopy. If
the system only has two dose modes, select the higher.
(6) Do a test irradiation. Adjust the attenuator thickness
if this is necessary to bring the tube voltage into the
90–100 kVp range.
3.D.2. Measurement process
Follow the procedure described in Sec. 3.B.4.
3.D.3. Calculation process
Follow the procedure described in Sec. 3.B.5. Note that
since air kerma is measured at the RP, the geometric factor
G1 = 1. If the external dosimeter could not be placed at the
RP, use an appropriate value of G1.
3.E. Overtable fluoroscopy system protocol
This section of the report addresses fluoroscopic systems
with x-ray tubes mounted above the procedure table. These
units typically have SIDs between 115 and 150 cm which
can be fixed or variable. The typical RP is located along the
central ray of the x-ray beam 30 cm above the procedure table.
(Note that any particular make or model may have a different
RP location and this should be confirmed in the vendor
documentation.)
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3.E.1. Measurement setup
(1) Set the system to a FOV of approximately 22 cm field
of view at the image receptor. If adjustable, set the SID
to the maximum possible.
(2) Place attenuator (approximately 5 mm Cu) on the
procedure table in the primary beam.
(3) Select the medium dose rate mode for fluoroscopy. If
the system only has two dose modes, select the higher.
(4) Place the FSMP and the external dosimeter at the RP
and center in the field. Typically, the RP is 30 cm above
the tabletop. Raise the FSMP with spacers as needed.
Set the collimators using the 10×10 cm2 marker on the
FSMP. All edges of the irradiated field should be seen
on the monitor almost superimposed on the square box
and well within the full FOV border.
(5) Do a test irradiation. Adjust the attenuator thickness
if this is necessary to bring the tube voltage into the
90–100 kVp range.
3.E.2. Measurement process
Follow the procedure described in Sec. 3.B.4.
3.E.3. Calculation process
Follow the procedure described in Sec. 3.B.5. Note that
since air kerma is measured at the RP, the geometric factor
G1 = 1.
3.F. Multipurpose fluoroscopy system protocol
Multipurpose fluoroscopy systems (sometimes referred to
as “universal” or “tilt-C” systems) generally consist of a floormounted C-arm stand with an integrated patient table. The
fluoroscopic C-arm is capable of angulation about the table and
the table and C-arm can be tilted together from the stand base.
Either the interventional fluoroscopy system protocol (vertical geometry) described in Sec. 3.B or the interventional
fluoroscopy system protocol (horizontal geometry) described
in Sec. 3.C can be used to determine Ka,r and KAP correction
factors for this configuration. However, some manufacturers
have chosen to design their multipurpose fluoroscopy systems
to comply with IEC radiography standards (IEC, 2009)14 and
as a result, assume that the patient table is present in the x-ray
beam when calibrating Ka,r and KAP displayed values.
Consultation with the manufacturer is recommended to
determine if a particular model of multipurpose fluoroscopy
system includes the table in the x-ray beam for Ka,r and KAP
displays. For these systems, the vertical geometry measurement setup should be followed with the external dosimeter and
FSMP positioned above the tabletop.
3.G. Mobile C-arm system protocol
This section of the report addresses mobile C-arm fluoroscopic systems. For mobile C-arms, typically the RP is
located 30 cm from the entrance surface of the image receptor
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assembly. Note that any particular model might have a different
RP location and this should be confirmed in the vendor
documentation.
3.G.1. Measurement setup
(1) Remove any material such as tabletop and pad from the
path between the x-ray tube and the measuring detector
before acquiring data.
(2) To the extent practicable, there should be no objects that
can scatter x-rays within 10 cm of the sensitive volume
of the measuring detector.
(3) Set the system to a normal FOV for that system.
(4) Set the x-ray beam to either a vertical or horizontal
orientation. For discussion purposes, the description in
the main text will employ a vertical orientation with
the x-ray tube at the top and the image receptor at the
bottom.
(5) Place attenuator (approximately 3–5 mm Cu) on the
image receptor.
(6) Select the medium dose rate mode for fluoroscopy. If
the system only has two dose modes, select the higher.
(7) Secure and suspend the FSMP and the external dosimeter, mechanically with appropriate means like a stand,
at the RP and center in the field. As mentioned previously, typically the RP is set to 30 cm above the image
receptor.
(8) Set the collimators using the 10×10 cm2 marker on the
FSMP. All edges of the irradiated field should be seen
on the monitor almost superimposed on the square box
and well within the full FOV border.
(9) Do a test irradiation. Adjust the attenuator thickness
if this is necessary to bring the tube voltage into the
90–100 kVp range.
3.G.2. Measurement process
Follow the procedure described in Sec. 3.B.4, with the
following addition.
(1) If the radiation field is hexagonal, octagonal, or circular, determine the diameter of field (D) with the radioopaque ruler on the FSMP.
3.H. Mini-C-arm protocol
This section of the report is specific to mobile C-arm fluoroscopes with a fixed SID less than or equal to 45 cm. Typically,
the RP for these devices is located 3–6 cm from the entrance
surface of the image receptor assembly. (Note that any particular make or model may have a different RP location and this
should be confirmed in the vendor documentation.)
3.H.1. Measurement setup
(1) Remove any material such as tabletop and pad from the
path between the x-ray tube and the measuring detector
before acquiring data.
(2) To the extent practicable, there should be no objects that
can scatter x-rays within 10 cm of the sensitive volume
of the measuring detector.
(3) Set the system to a normal FOV for that system. If
collimation is adjustable, open the collimation to its
fullest extent.
(4) Set the x-ray beam to either a vertical or horizontal
orientation. For discussion purposes, the description in
the main text will employ a vertical orientation with
the x-ray tube at the top and the image receptor at the
bottom.
(5) Place attenuator (approximately 2–3 mm Cu) on the
face of the image receptor.
(6) Select the normal dose rate mode for fluoroscopy. If the
system only has two dose modes, select the higher.
(7) Secure and suspend the FSMP and the external dosimeter, mechanically with appropriate means like a stand,
at the RP and center in the field. As mentioned previously, typically the RP is 3–6 cm above the image
receptor.
(8) Set the collimators using the 7 × 7 cm2 marker on the
FSMP. All edges of the irradiated field should be seen
on the monitor almost superimposed on the 49 cm2
square box and well within the full FOV border.
(9) Do a test irradiation. Adjust the attenuator thickness
if this is necessary to bring the tube voltage to the
maximum value (typically less than 80 kVp).
3.H.2. Measurement process
Follow the procedure described in Sec. 3.B.4, with the
following addition.
3.G.3. Calculation process
Follow the procedure described in Sec. 3.B.5, with the
following change.
(1) The measured KAP is
KAP = [measured Ka,r ] ∗ AW ∗ AL
(1) If the radiation field is hexagonal, octagonal, or circular, determine the diameter of field (D) with the radioopaque ruler on the FSMP.
3.H.3. Calculation process
or
KAP = [measured Ka,r ] ∗ π ∗ (D/2)2.
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(4)
Follow the procedure described in Sec. 3.B.5, with the
following change.
(1) The measured KAP is
Note that since air kerma is measured at the RP, the geometric factor G1 = 1.
Medical Physics, Vol. 42, No. 12, December 2015
KAP = [measured Ka,r ] ∗ AW ∗ AL
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or
KAP = [measured Ka,r ] ∗ π ∗ (D/2)2.
(5)
Note that since air kerma is measured at the RP, the geometric factor G1 = 1.
3.I. Radiographic systems protocol
This section of the report is specific to radiographic systems. The RP location of these systems may be based on user
data entry or set to a default location. Review of the vendor
documentation is recommended to determine how the RP is
defined for any individual radiographic system. For measurement of the radiographic x-ray field size, any standard radiographic collimation test tool measurement plate with radioopaque fiducial markers can be used.
6825
(3) Orient the x-ray tube so the central ray of the beam is
normal (90◦) to the tabletop/flat surface and the image
receptor. Set the SID to the default setup of the radiographic room typically; SID = 100 cm.
(4) Place the FSMP 30 cm over the tabletop or the image
receptor, using spacer rods to support the FSMP. Center
the FSMP within the radiation field.
(5) Collimate the x-ray field so that it falls inside the image
receptor and the FSMP. The collimated beam should be
no smaller than 15 × 15 cm at the plane of the FSMP.
It is desirable to use the light field projected on the
15 × 15 cm box drawn on the FSMP. Or, a larger field
size should be employed to account for radiographic
applications.
(6) For systems with a Ka,r display, place the external
dosimeter at the RP if different from the setup shown
in Fig. 5. Record the SDD. The setup in Fig. 5 allows
for direct air kerma measurement assuming a typical
patient size of 30 cm.
3.I.1. Measurement setup
(1) As much as practicable and possible, ensure that there
are no objects that could potentially produce scatter
within 10 cm of the sensitive detector volume.
(2) For a stationary system, position the x-ray tube above
the tabletop [Fig. 5(A)]. If a radiographic table is not
present (e.g., in a dedicated chest room), either direct
the x-ray tube downward toward a flat horizontal surface or position the image receptor on supports for a
horizontal x-ray beam. For a portable or mobile x-ray
system, place the x-ray tube above a flat horizontal
surface such as a tabletop of an examination room
[Fig. 5(B)]. Notice the difference in the location of the
image receptor. A 35 × 43 cm (14 × 17 in.) cassette
size image receptor is employed for the test procedure
described here.
3.I.2. Measurement process
(1) Set the x-ray generator to 100 kVp and a minimum of
50 mAs.
(2) Precision is increased if each measurement is appropriately replicated (3× is suggested). The coefficient of
variation in the repeated measurements should be less
than 0.01.
(3) Make an exposure and record the measured Ka,r and
the displayed KAP and Ka,r , if applicable.
Note: For screen-film systems, make one additional
exposure at a lower technique to ensure the fiducial
markers of the field-size measurement device can be
read on the developed film. Do not record the Ka,r or
KAP for this exposure.
(4) Calculate C(Ka,r ) using the process described below.
(5) Determine the field size by observing the image of the
FSMP on the recorded image. Record the two dimensions of the rectangular field as AL and AW.
3.I.3. Calculation process
(1) Calculate the measured KAP,
KAP = [measured Ka,r ] ∗ AW ∗ AL.
F. 5. Measurement setup for radiographic systems. (A) is for a radiographic x-ray room with a Bucky tray or an integrated DR image receptor.
(B) is for a portable radiographic unit using a tabletop for support.
Medical Physics, Vol. 42, No. 12, December 2015
(6)
(2) Calculate the correction factor C separately for each
irradiation event for both Ka,r and KAP. Caution: The
following processes may need further corrections to
account for the dose units in which a particular fluoroscope displays KAP.
(a) C (KAP) is determined by dividing the measured
KAP value by the KAP value displayed by the
system.
(b) C(Ka,r ) is determined by dividing the measured
Ka,r value by the Ka,r value displayed by the
system.
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6826
(3) Average the individual C factors over the measurement
repetitions for Ka,r , and separately for KAP.
film, a CR, or a DR cassette to record the exposure FOV size
instead of reading the field size from the monitor. Use of the
FSMP will assist in setting up the actual radiation field at
the time of data collection. However, if a software distance
measurement function is built into the control console, the
field size can be determined with a higher accuracy. If any
magnification/minification exists, the ruler on the FSMP can
be employed to provide the scaling corrections.
4. DISCUSSION
4.A. Sources of uncertainty
There are multiple factors that contribute to inaccuracy in
the correction factor value. These factors include uncertainty
in the external detector reading, the external detector location relative to the x-ray source, the accuracy of displayed
dose values, and accuracy of the x-ray field-size measurement. Additional discussion of each source of error is provided
below.
4.A.1. External dosimeter reading uncertainty
Accurate calibration of the external dosimeter is critical to
minimize this source of variation. A mismatch between the
calibration beam spectrum and the clinical beam spectrum
causes an additional source of error for x-ray beams with heavy
filtration (AAPM Report No. 125).15 Depending on the type of
calibration, the external dosimeter reading should be accurate
to within 2% and 6%.
4.A.2. External detector location uncertainty
Inaccuracy in positioning of the external detector at the
correct distance from the x-ray source (either at isocenter or
the RP) will result in an erroneous air kerma measurement.
Utilization of the isocenter localization technique for fixed Carm systems described in Sec. 3.A will generally reduce this
error. An error in detector positioning of ±1 cm at a 65 cm
source-chamber distance will result in an air kerma error of
approximately ±3%.
4.B. KAP meter performance variation
with beam quality
It is known that KAP meters may have a significant x-ray
beam energy dependence which is affected by the materials
and design of the chamber.16 The greatest dependence on
beam filtration occurs at low kVp values (50–80 kVp). Heavier
filtered beams (0.1–0.2 mm Cu) exhibit a higher dependence
as compared to less filtered beams in the range of 10%–15%
for 70–90 kVp beams. Since clinical systems may have modes
that include heavier beam filters, it is recommended that an
initial validation of the system includes expanded measurements of the calibration for a range of operating modes. If a
single calibration factor is desired (as for entry in the RSDR),
the average of these values over the range of clinically used
operating modes is recommended. Alternatively, if additional
accuracy in the correction factor is desired, separate correction
factors for different modes may be determined.
Whether the radiological imaging equipment in question
is a radiographic unit or a fluoroscopic unit, both types of
KAP meters, physical and virtual, employed for dose measurements must be properly calibrated to the radiation beam quality
encountered in clinical practices. This is also applicable to the
external dosimeter to evaluate the accuracy of the KAP meters.
5. SUMMARY AND CONCLUSIONS
4.A.3. Displayed dose value accuracy
Error in the displayed dose value can become particularly
large if Ka,r is displayed in units of mGy in a whole number
(rounded to the nearest integer value) and an insufficient total
air kerma is not accumulated during the measurement. For
example, a 20 mGy displayed Ka,r value may have an error of
±0.5 mGy in the initial value and ±0.5 mGy in the final value,
resulting in a total error of ±1 mGy or 5%. Accumulating a
dose of at least 50 mGy will reduce this error to ±2%.
4.A.4. X-ray field-size measurement uncertainty
Uncertainty in the x-ray exposure field size will affect the
accuracy of the measured KAP value only. For a rectangular
field with the exposure field edges clearly visible, an error
of ±1 mm in reading the template rule on each side would
yield a KAP error of ±3% for a 50 cm2 area. Collimating to
a larger FOV decreases this error. When electronic imaging
shutters obstruct the exposure edges and cannot be readily
eliminated, increased accuracy can be accomplished by using
Medical Physics, Vol. 42, No. 12, December 2015
In this report, the accuracy of a KAP-meter or more precisely, the integrated radiation output indicators employed in
diagnostic radiology has been investigated and the calibration
methodology described in detail for various types of fluoroscopy system. The TG 190 Report has also been reviewed by
various x-ray equipment manufacturers so that the calibration
protocol is also acceptable to the industry. For the patient
radiation dose estimation, the measurement protocol yields the
radiation dose that can be employed for further modification to
include corrections due to the geometry, the backscatter, and
attenuation of the tabletop and the mattress.
The RP for the interventional angiography fluoroscopy systems has been specified by IEC. The location of RP for other
fluoroscopy systems was left in the hands of equipment manufacturers. TG 190 Report deals with the measurement and
verification of the KAP-meter accuracy at or associated with
the RP independent of manufacturers while permitting medical
physicists to follow a unified approach in achieving more realistic radiation dose estimation. For these reasons, this report is
likely to be of great interest to standard organizations such as
IEC, MITA, and NEMA.
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F. 6. Drawing of the FSMP and the stand.
APPENDIX A: THE FIELD-SIZE MEASUREMENT
PLATE AND STAND
It should be pointed out that the “stand” shown here is
designed for a convenient measurement of KAP and Ka,r ,
as well as to perform other tasks related to the evaluation
of fluoroscopy systems. While the details of the FSMP
are given in the main text, the stand shown here may be
replaced by a simple cardboard box of appropriate size.
A sample FSMP with associated stand parts is depicted in
Fig. 6.
The drawing, including the actual dimensions, is intended
for illustration purposes only. Note that the “bottom plate of
FSMP” shown on the top left of the drawing is the same
as that described in the main text. All parts are made of
6827
1/2 in. (1.27 cm) thick PMMA plastic with the exception of
the embedded FSMP gratings section. The FSMP section is
embedded in an 8 × 8 in. (20 × 20 cm) square and is machined
on a 1/4 in. (0.635 cm) thick PMMA plate. The length of the
bottom plate of FSMP needs to be sufficiently long (∼18 in.,
∼46 cm) so that the device can be properly secured at the edge
of the examination table.
In addition, the 8 × 8 × 1/4 in. (20 × 20 × 0.635 cm)
FSMP can be removed from the bottom plate of FSMP and
interchanged with others designed for different tasks involved
in the testing of fluoroscopic equipment.
The “top plate of FSMP” is designed to hold the copper sheets for attenuation. The “spacer rods” shown are
fabricated to provide a distance of 30 cm from the bottom
to the top of the entire stand. Different sets of spacer
rods may be necessary for specific fluoroscopic units being
evaluated.
Depicted in Fig. 7 is a series of photographs showing the
experimental arrangement of the FSMP stand setup with a
mobile C-arm fluoroscopy unit. (Note, the FSMP stand is of
different design and size than the drawing.) Inset (A) on the
left of the photographs shows the measurement arrangement
similar to Fig. 4 in the main text. However, as described
previously, the FSMP is embedded in a 1/4 in. (0.635 cm) plate
which is interchangeable and may be removed or replaced with
other test objects if desired.
Inset (B) in Fig. 7 shows a closeup view of the FSMP with
an ionization chamber placed in the middle, aligned to the
center of the radiation field. Note that the ionization chamber
and the copper attenuation sheets shown are for illustration
purpose only. The copper sheets should be positioned close to
the image receptor in actual measurements.
Under this measurement arrangement, two corrections
may be necessary for better accuracy;1 the locations of the
FSMP and the sensing volume of the ionization chamber are
displaced by one half of the ionization chamber thickness
(0.6 cm), and2 attenuation due to the FSMP itself which is
made of PMMA plastic (1/4 in., 0.635 cm).
F. 7. Photographs of the FSMP stand.
Medical Physics, Vol. 42, No. 12, December 2015
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Lin et al.: AAPM TG 190 Report on KAP-meter accuracy
6828
F. 8. Fluoroscopy images.
Inset (C) in Fig. 7 shows that the small solid state detector
is held in an interchangeable holder (with a thin transparency
film) so that the sensing detector lies in the same plane as
the FSMP. The corrections needed due to the attenuation of
FSMP plastic plate and distance displacement are eliminated.
The small amount of scattered radiation from the 10 × 10 cm
radiation field, for example, is further minimized.
Shown in Fig. 8 are fluoroscopy images of the FSMP.
Inset (A) is a last-image-hold image corresponding to the
experimental arrangement shown in Fig. 7, inset (B).
In Fig. 8, inset (B) shows the fluoroscopy image with
the ionization chamber removed. The center circular cutout
should be large enough to accommodate the ionization
chamber employed. As indicated previously, the FSMP is
fabricated such that it can be removed and replaced with a
holder designed to accommodate the detector.
In Fig. 8, inset (C) is the fluoroscopy image of an ionization
chamber place in a holder so that the center plane of the
ionization chamber is placed at the same plane where the
FSMP is located.
APPENDIX B: THE HORIZONTAL GEOMETRICAL
ARRANGEMENT
Isocentric fluoroscopes can also be evaluated without the
stand. Figure 9 illustrates a horizontal-beam measurement
setup. The radiation detector is shown at system isocenter.
Its location was confirmed by rotating the gantry 90◦.
Note that the description of each key component is
identified with the annotation in the photograph with the
description of each item listed in Fig. 9. The SAD may be
obtained either from system documentation or measurement.
Field size at isocenter is measured after radiation data
are collected by removing the attenuator (C) and dosimeter
(D) and then, sliding the field-size plate (F) such that the
plate is perpendicular to the radiation beam (B) and at
the fluoroscope’s isocenter. As shown in Fig. 9, the plate
can be slid into position by sliding it along the tight gap
between blocks (G). Plate position can be confirmed by
rotating the gantry 90◦ and verifying that the plate is seen on
edge.
F. 9. The horizontal geometry measurement setup. (A) is the x-ray tube assembly. (B) is the central ray of horizontal x-ray beam. (C) is image receptor with
copper attenuation plate. (D) is the radiation detector (note that the sensitive volume is placed at the fluoroscope’s isocenter). (E) is the tabletop (note that the
table height was adjusted to place the horizontal scale of the field-size plate at approximately the same height as the central ray of the beam). (F) is the field-size
plate with radio-opaque scale (any appropriate plate may be used). For illustrative purposes, the plate is shown outside the x-ray beam in this picture. (G) are
aluminum blocks used as the supports to hold the field-size plate (F).
Medical Physics, Vol. 42, No. 12, December 2015
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Lin et al.: AAPM TG 190 Report on KAP-meter accuracy
NOMENCLATURE
Length and width of a rectangular exposure field
of view
C(x)
Correction factor equal to the measured external
value divided by the system’s displayed value,
where x is either Ka,r or KAP
D
Diameter of a circular or octagonal exposure field
of view
DAP
Dose-area-product
DICOM Digital imaging and communications in medicine
DID
Dosimeter to image receptor distance, detector to
image receptor distance
FSMP
Field-size measurement plate
FDA
U.S. Food and Drug Administration
f -factor Factor used to convert air kerma to dose in
tissue.
FOV
Field of view
FSMP
Field-size measurement plate
G1
Geometric factor 1 = [SAD/(SAD − RPD)]2
IEC
International Electrotechnical Commission
Ka,SAD
Air kerma at source to axis distance
Ka, SDD Air kerma at source to external dosimeter
distance
Ka,r
Air kerma at the reference point
KAP
Air kerma-area-product
KAP-meter A thin, parallel-plate transmission ionization
chamber that is fixed in the x-ray tube housing,
typically at the end of the collimator
kVp
Kilovolt peak
mA
Milliampere
mGy
Milligray
NEMA
National Electrical Manufacturers Association
RDSR
Radiation Dose Structured Report
RP
Reference point
RPD
Isocenter to reference point distance
SAD
Source to axis distance (i.e., source-isocenter
distance)
SDD
Source to external dosimeter distance
SHD
Source to the exit point of the x-ray tube housing
assembly distance
SID
Source to image receptor distance
AL, AW
Medical Physics, Vol. 42, No. 12, December 2015
6829
a)Author
to whom correspondence should be addressed. Electronic mail:
Pei-Jan.Lin@vcuhealth.org
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