Key words: fluoroscopy, dose-area-product, kerma-area-product, calibration of KAP meters, patient exposure

Transcription

1 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. Lin a) Virginia Commonwealth University Medical Center, Richmond, Virginia Beth A. Schueler Mayo Clinic, Rochester, Minnesota Stephen Balter Columbia University Medical Center, New York, New York Keith J. Strauss Children s Hospital Medical Center, Cincinnati, Ohio Kevin A. Wunderle Cleveland Clinic Foundation, Cleveland, Ohio M. Terry LaFrance Baystate Health Systems, Inc., Springfield, Massachusetts Don-Soo Kim Children s Hospital Boston, Boston, Massachusetts Richard H. Behrman Boston University Medical Center, Boston, Massachusetts S. Jeff Shepard University of Texas MD Anderson Cancer Center, Houston, Texas Ishtiaq H. Bercha Children s Hospital Colorado, Aurora, Colorado (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 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. [ Key words: fluoroscopy, dose-area-product, kerma-area-product, calibration of KAP meters, patient exposure TABLE OF CONTENTS 1 INTRODUCTION BACKGROUND INFORMATION A KAP, the units, and the geometry B Regulatory requirement C Digital imaging and communications in medicine (DICOM) radiation dose structured report (RDSR) D Integrated radiation output indicators INTEGRATED RADIATION OUTPUT SYSTEM VALIDATION METHOD Med. Phys. 42 (12), December /2015/42(12)/6815/15/$ Am. Assoc. Phys. Med. 6815

2 6816 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy A Protocol overview A.1 An external radiation dosimeter A.2 Attenuator A.3 Field-size measurement plate A.4 Radiation detector/fsmp stand B Interventional fluoroscopy system protocol (vertical geometry) B.1 Determination of focal spot location. 6 3.B.2 Determination of isocenter location. 6 3.B.3 Measurement setup for K a,r and radiation field size B.4 Measurement process B.5 Calculation process C Interventional fluoroscopy system protocol (horizontal geometry) C.1 Determination of focal spot location. 8 3.C.2 Measurement setup C.3 Measurement process C.4 Calculation process D Undertable fluoroscopy system protocol D.1 Measurement setup D.2 Measurement process D.3 Calculation process E Overtable fluoroscopy system protocol E.1 Measurement setup E.2 Measurement process E.3 Calculation process F Multipurpose fluoroscopy system protocol G Mobile C-arm system protocol G.1 Measurement setup G.2 Measurement process G.3 Calculation process H Mini-C-arm protocol H.1 Measurement setup H.2 Measurement process H.3 Calculation process I Radiographic systems protocol I.1 Measurement setup I.2 Measurement process I.3 Calculation process DISCUSSION A Sources of uncertainty A.1 External dosimeter reading uncertainty A.2 External detector location uncertainty A.3 Displayed dose value accuracy A.4 X-ray field-size measurement uncertainty B KAP meter performance variation with beam quality SUMMARY AND CONCLUSIONS A APPENDIX A: THE FIELD-SIZE MEASUREMENT PLATE AND STAND B APPENDIX B: THE HORIZONTAL GEOMETRICAL ARRANGEMENT NOMENCLATURE 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 x- ray 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-cm 2 ), (cgy-cm 2 ), (mgy-m 2 ), and (µgy-m 2 ). The mgy displayed by fluoroscopic systems is the accumulated air kerma at a demarcated reference point (RP) K a,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

3 6817 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6817 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, 21CFR ). 6 It should be noted that actual patient skin dose will likely be different from the displayed K a,r value. K a,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 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 K a,r and the K a,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 TABLE 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 21CFR , Code of Federal Regulations, Title 21, Volume 8, Performance Standards for Ionizing Radiation Emitting Products: Fluoroscopic Equipment, April 1, result, both K a,r and KAP are displayed on most fluoroscopic systems. These requirements and/or regulations also specify the accuracy requirements for K a,r and KAP values. The displayed K a,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-cm 2. 7,9 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. TABLE 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

4 6818 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6818 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 ). 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 K a,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 K a,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 K a,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 K a,r accuracy will depend on both the KAP-meter accuracy and the field-size measurement. Instead of direct KAP measurement, some models determine K a,r computationally based on x-ray tube output for given technique factors and added filtration. KAP can then be estimated by multiplying K a,r by the x-ray beam area. For this method, K a,r accuracy will depend on the accuracy of x-ray output values that are used and KAP accuracy will depend on both the K a,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. Interventional fluoroscopy, vertical x-ray beam. 3.C. Interventional fluoroscopy, horizontal x-ray beam. 3.D. Undertable x-ray fluoroscopy. 3.E. Overtable x-ray fluoroscopy. 3.F. Multipurpose fluoroscopy (C-arm with integrated table). 3.G. Mobile C-arms. 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 K a,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 ed The energy response over the 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 K a,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 K a,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 cm 2. 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

5 6819 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6819 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 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 K a,r rates. Use of high kvp minimizes deviations in KAP-meter accuracy that are common at low kvp and use of high K a,r rates allows for more rapid accumulated air kerma measurements 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 cm 3 flat pancake-type ionization chamber typically 5 cm in diameter or smaller. The physical size of the square (7 7 cm) pro- FIG. 2. Enlarged view of FSMP. vides a 49 cm 2 area for KAP measurement. (To be exact, a cm 2 would provide a 50 cm 2 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.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. FIG. 1. Field-size measurement plate (FSMP). 3.B. Interventional fluoroscopy system protocol (vertical geometry) 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.

6 6820 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6820 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. 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 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.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 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. FIG. 3. Geometry of utilizing two copper sheets for localization of focal spot, under vertical geometry. 3.B.3. Measurement setup for K a,r and radiation field size (1) All clinically removable attenuators (i.e., removable without tools) shall be removed from the path between

7 6821 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6821 the x-ray tube assembly and the measuring detector before acquiring data. (2) 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. (3) Set the x-ray beam to a vertical orientation. (4) Set the gantry to the maximum SID. (5) Set the system to a medium FOV with open collimation. (6) 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. (7) 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). (8) 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. (9) Set the collimators using the cm 2 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 cm 2 marker on the FSMP to the middle of the penumbra.) (10) Place an attenuator (approximately a total of 8 mm Cu) on the tabletop so it completely intercepts the x- ray beam. (11) Select the medium dose rate modes for fluoroscopy. If the system only has two dose rate mode choices, then select the higher. FIG. 4. Measurement setup for K a,r and radiation field size. (12) Do a test irradiation. Adjust the attenuator thickness if this is necessary to bring the displayed tube voltage into the kvp range and record the attenuator thickness required. Note on the selection of tube voltage in the range of 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 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 kvp (<5%) than at 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 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 (3) Record the initial displayed system values of K a,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 K a,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 K a,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(K a,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.

8 6822 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6822 (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. 3.B.5. Calculation process (1) Calculate the measured KAP, KAP = [measured K amas ] AW AL, (1) where K amas 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 K a,r at the RP by multiplying the measured air kerma at isocenter K a,sad by a geometric factor G1, G1 = [SAD/(SAD RPD)] 2, (2) where RPD = isocenter to reference point distance K a,r = [measured K a,sad ] G1. (3) For example, for SAD = 750 mm, RPD = 150 mm, G1 = [750/( )] 2 = (3) Calculate the correction factor C separately for each irradiation event for both K a,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(K a,r ) is determined by dividing the measured K a,r value by the K a,r value displayed by the system. (4) Average the individual C factors over the measurement repetitions for K a,r, and separately for KAP. 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. the x-ray tube assembly and the measuring detector before acquiring data. (2) 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. (3) Set the system to a medium FOV with open collimation. (Typically 6 9 in. or cm FOV.) (4) Set the gantry to the maximum SID that allows free rotation of the gantry. (5) 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. (6) 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. (7) 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. (8) Use a tape measure to determine the distance from the focal spot to the chamber and record this distance as the SAD. (9) Rotate the system for a horizontal x-ray beam. (10) Set the system to a FOV of approximately 17 cm (or 22 cm) at the image receptor and maximum SID. (11) Set the collimators using the cm 2 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. (12) Attach an attenuator ( 8 mm Cu) to the face of the image receptor. The attenuator thickness may need to be adjusted later to yield kvp while taking measurements. (13) Select the medium dose rate modes for fluoroscopy. If the system only has two dose rate mode choices, then select the higher. (14) Do a test irradiation. Adjust the attenuator thickness if this is necessary to bring the displayed tube voltage into the kvp range and record the attenuator thickness required. 3.C.1. Determination of focal spot location Follow the procedure in Sec. 3.B.1. 3.C.2. Measurement setup (1) All clinically removable attenuators (i.e., removable without tools) shall be removed from the path between 3.C.3. Measurement process Follow the procedure described in Sec. 3.B.4. 3.C.4. Calculation process Follow the procedure described in Sec. 3.B.5.

9 6823 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 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 cm 2 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 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.) 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 cm 2 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 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 K a,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 K a,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 K a,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

10 6824 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6824 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 cm 2 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 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.G.3. Calculation process Follow the procedure described in Sec. 3.B.5, with the following change. (1) The measured KAP is KAP = [measured K a,r ] AW AL or KAP = [measured K a,r ] π (D/2) 2. (4) Note that since air kerma is measured at the RP, the geometric factor G1 = 1. 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 cm 2 marker on the FSMP. All edges of the irradiated field should be seen on the monitor almost superimposed on the 49 cm 2 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. (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 Follow the procedure described in Sec. 3.B.5, with the following change. (1) The measured KAP is KAP = [measured K a,r ] AW AL

11 6825 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6825 or KAP = [measured K a,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. 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 cm (14 17 in.) cassette size image receptor is employed for the test procedure described here. (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 cm at the plane of the FSMP. It is desirable to use the light field projected on the cm box drawn on the FSMP. Or, a larger field size should be employed to account for radiographic applications. (6) For systems with a K a,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.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 (3) Make an exposure and record the measured K a,r and the displayed KAP and K a,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 K a,r or KAP for this exposure. (4) Calculate C(K a,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. FIG. 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. 3.I.3. Calculation process (1) Calculate the measured KAP, KAP = [measured K a,r ] AW AL. (6) (2) Calculate the correction factor C separately for each irradiation event for both K a,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(K a,r ) is determined by dividing the measured K a,r value by the K a,r value displayed by the system.

12 6826 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6826 (3) Average the individual C factors over the measurement repetitions for K a,r, and separately for KAP. 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 C- arm 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.A.3. Displayed dose value accuracy Error in the displayed dose value can become particularly large if K a,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 K a,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 cm 2 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 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.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 ( mm Cu) exhibit a higher dependence as compared to less filtered beams in the range of 10% 15% for 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 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.

13 6827 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 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 /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. ( 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., cm). F. 7. Photographs of the FSMP stand.

14 6828 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 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).