Update on Fluoroscopy Physics AAPM MO-A-210A-1 Stephen Balter, Ph.D.

Transcription

1 Update on Fluoroscopy Physics Stephen Balter, PhD Columbia University Draft for MO-A-210A AAPM Educational objectives Understand dosimetric concepts relating to interventional fluoroscopy Characterize the dosimetric features and performance of a modern fluoroscope Be able to set up a clinical dose recording and reporting policy that will meet clinical and JC requirements. SB0907 UFP AAPM - 1 SB0907 UFP AAPM - 2 Outline Notation This presentation uses the notation given in ICRU-74 (2006) The notation has been extended to encompass additional quantities specifically related to fluoroscopy Organizations that have accepted or are in the process of accepting this notation include the IAEA, IEC, NCRP and NEMA. SB0907 UFP AAPM - 3 SB0907 UFP AAPM - 4 Air KERMA (K a ) Measurements are made under lowscatter conditions Metric usually used by calibration laboratories to report instrument factors. Unless the detector is air, there may be additional corrections needed to account for instrument construction and X-ray spectrum. From the lab to the clinic K a,i is the air kerma at the location of the patient s skin in the absence of the patient. K a,e is the air kerma at the location of the patient s skin with the patient present. SB0907 UFP AAPM - 5 SB0907 UFP AAPM - 6

2 Backscatter factors K a,i K a,e Reference Point Air Kerma (K a,r ) Reference point originally defined in IEC (2000) Included in 2005 FDA fluoro regulations. Intent is to approximate the location of the patient s skin Reported as K a without scatter IEC, FDA ± 35 % SB0907 UFP AAPM - 7 SB0907 UFP AAPM - 8 Reference Point Air Kerma (K a,r ) Reference Point FDA Point Total air kerma accumulated at the reference point from the beginning of the procedure. (called cumulative dose in the regs) Displayed to operators at the working position and in the control room. Defined under low scatter conditions. Table top and mattress attenuation? Not standardized. Propose measurement without attenuation unless the attenuators are always in the beam. SID = Any FDA Dose Compliance Point 15 cm Image Receptor Isocenter IEC & FDA K a,r Reference Point Focal Spot SB0907 UFP AAPM - 9 SB0907 UFP AAPM - 10 Reference point skin Conversion of K a,r to D skin D max,skin is seldom numerically equal to K a,r For teaching purposes, assume that the beam does not move during the procedure. K a,i = K a,r * (SRD/SSD) 2 where SRD is the source to reference point distance K a,e = K a,i * Backscatter f(kvp, filter, field size) D skin = K a,e * (µ skin / µ air ) SB0907 UFP AAPM - 11 SB0907 UFP AAPM - 12

3 Kerma Area Product (P KA ) P KA is independent of distance! Total KAP accumulated from the beginning of the procedure. Displayed to operators at the working position and in the control room. Can be used to estimate K a,r (need to know field size at reference point) Defined under low scatter conditions. Table top and mattress attenuation? Where should DAP be measured? Area = a * d 2 Output = k *d -2 P KA = a * d 2 * k*d -2 = a * k SB0907 UFP AAPM - 13 SB0907 UFP AAPM - 14 Outline Geometry (generic system) Min SSD = 38 cm (<) SID 80 cm 125 cm SAD = cm (focus to isocenter) Table top height - 30 to + 20 cm (relative to isocenter) 110º LAO 110º RAO 45º CRA 45º CAU FS cm 2 (at image receptor) SB0907 UFP AAPM - 15 SB0907 UFP AAPM - 16 Radiation technique range Table-Top Outputs FP K a,i range <1 4,000 mgy/min kvp Fixed filtration 2 6 mm Al Added Filter mm Cu Brand A : Added Cu fluoro filter independent of kvp Brands B,C: Added Cu fluoro filter tends to decrease as kvp increases All Brands: Less or no Cu for acquisition modes New systems do not provide physicist level control of basic X-ray factors mgy/min Chamber from FP Minimum SID 25NF 20NF 15NF 25CI 20CI 15CI 38 mm Al +0.5 mm +1.0 mm +1.5 mm +2.0 mm +3 mm Cu +4 mm Cu 6 mm Cu +10 mm Cu Cu Cu Cu Cu Attenuator 0902D SB0907 UFP AAPM - 17 SB0907 UFP AAPM - 18

4 Observed values Dose rates are down; BUT!!! SB0907 UFP AAPM - 19 SB0907 UFP AAPM - 20 Conventional Wisdom FDA limits normal-mode table-top outputs to 87 mgy/min (10 R/min) Using more magnification increases patient skin-dose-rate Using more magnification increases limiting spatial resolution Source to Image Receptor Tracking FDA regulations limit normal mode fluoroscopy to less than 87 mgy/min at from the image receptor. Source to image receptor distance (SID) is not mentioned in the regulations. Most interventional limit output to less than 87 mgy/min at any SID What happens at the table top when SID is varied? SB0907 UFP AAPM - 21 SB0907 UFP AAPM - 22 Minimum SID Table-Top K a,i Maximum SID Table-Top K a,i Minimum SID Maximum SID Flu 75 mgy/m Cin 1700 mgy/m Flu 75 mgy/m Flu 230 mgy/m Cin 1700 mgy/m SB0907 UFP AAPM - 23 SB0907 UFP AAPM - 24

5 Low Table/Max SID Table-Top K a,i Table Top Maximum Air Kerma Rate Maximum SID 2.75 Table Top (chamber) fixed 55 cm from FS Flu 75 mgy/m Flu 270 mgy/m Cin 2000 mgy/m Relative Table Top Output Fluoroscopy Table Top Rate Inverse Square Cine Table Top Rate SB0907 UFP AAPM - 25 Minimum SSD Source to Image Receptor Distance (cm) 0907 Alpha SB0907 UFP AAPM - 26 Effect of zoom on EERII Field of view considerations An image intensifier zooms a variable size input field onto a constant output field. Unmodified: Dose per frame 1/ FOV 2 Programmed: Dose per frame 1/ FOV A first generation flat-panel electronically zooms a variable number of pixels onto a full size image display. Dose per frame constant Dose per frame 1/ FOV Dose per frame 1/ FOV 2 SB0907 UFP AAPM - 27 SB0907 UFP AAPM - 28 Measured Phantom EER vs. f(fov) Image intensifier zoom Program kvp Beam filter Dose Rate Constraint Normalized Phantom Input (fluoroscopy) 6 Base 38 Al 0.5 Cu Cu 2.0 Cu 4.0 Cu 8.0 Cu 1/r 4 1/r cm 42 cm 32 cm 22 cm 16 cm 11cm Field of View - Flat Panel 0805 BETA SB0907 UFP AAPM - 29 SB0907 UFP AAPM - 30

6 Pixel size image intensifier zoom DR Indirect Detector X-ray Light Structured X-ray phosphor (CsI) S G D + Photodiode Storage capacitor Charge X-rays to light to electrons to electronic signal: a -Si TFT/ CsI phosphor SB0907 UFP AAPM - 31 SB0907 UFP AAPM - 32 Pixel size: flat panel zoom (1 st gen) Smaller FOV = Fewer Pixels SB0907 UFP AAPM - 33 SB0907 UFP AAPM - 34 Pixel size: zoom comparison Rebinning SB0907 UFP AAPM - 35 SB0907 UFP AAPM - 36

7 Display Display Processing Zoomed FP images are digitally transformed from the detected pixel matrix to the full display matrix. All images are highly processed to enhance perception of detail. Dose per frame is programmed by the system to standardize the perception of noise. These measures are designed to maximize perception High Contrast Objects (small interventional devices < 1 mm) Low Contrast Objects (relatively large tissue blush > 5 mm) Different acquisition and display optimization is needed for different clinical procedures. Outline SB0907 UFP AAPM - 37 SB0907 UFP AAPM - 38 What instrument? Reference Point Locations If you are super-fussy (and want to try to stay below the ICRU-74 tolerance of 7%) the functionally closer to a free air chamber the better. For most work, energy compensation of solid-state detectors is adequate Absolute accuracy should improve when appropriate calibration beams are defined and implemented. Consider the influence of any instrument on the fluoroscope s ADRC. Caution: You, your service engineers, and your regulators may be using different types of instrumentation. SID = Any FDA Dose Compliance Point 15 cm Image Receptor Isocenter IEC & FDA K a,r Reference Point Focal Spot SB0907 UFP AAPM - 39 SB0907 UFP AAPM - 40 Compliance Setup Compliance minimum SID 50% Equipment placed on table top Attenuators 15 cm above ion chamber Minimum SID Adjust table height for between center of chamber to entrance surface of image receptor Collimate to just inside smallest FOV Test common modes at all FOVs. SB0907 UFP AAPM - 41 SB0907 UFP AAPM - 42

8 FL FN CL CN Update on Fluoroscopy Physics AAPM MO-A-210A-1 Attenuator protocol Acquisition modes 19 mm Al (children) 38 mm Al (small adult) 38 mm Al mm Cu (medium adult) 38 mm Al mm Cu (large adult) 38 Al + 2 Cu + 3 mm Pb (maximum) or 38 mm Al mm Cu (maximum) There is reluctance to test acquisition modes at full output due to concerns about damaging the fluoroscope. This is reflected in regulatory requirements Modern systems are actually clinically operated at or near full output a significant fraction of the time. Maximum outputs are often more than double the 38 Al + 2 Cu outputs SB0907 UFP AAPM - 43 SB0907 UFP AAPM - 44 Clinical kvp in one laboratory Typical compliance data Compliance Image Intensifier - 17 cm FOV FDA Geometry kvp Cine n = 1856 Fluoro n = 6594 fkvp ckvp mgy/min Al Cu + 1 Cu + 2 Cu + 4 Cu + 8 Cu + 8Cu+3Pb Attenuator Display = Cine actual/2, fc actual mgy/min cine S fluoro mm Al + X mm Cu 80 Flat Panel Detector Image Intensifier % 5% 10% 20% 25% 30% 40% 50% 60% 70% 75% 80% 90% 95% 99% Both have variable Cu beam filters Percentile SB0907 UFP AAPM - 45 SB0907 UFP AAPM - 46 Dose instrumentation Dose displays in the lab Locations Integrated Interfaced External Measurement technologies None Calculated from settings P KA chamber + calculation K a,r chamber + calculation P KA & K a,r dual channel chamber SB0907 UFP AAPM - 47 The same RPDose will display as 5678 on all three systems The same KAP will display as on GE and SIEMENS and on PHILIPS SB0907 UFP AAPM - 48

9 Verification of displayed dose Accuracy of display IEC ± 50% (RPDose &KAP) FDA ± 35% (RPDose) Stability usually better Usually validated at factory Seldom validated by installers When verified as part of QA constancy should be ± 5% Stability of K a,r instruments AVE STDEV Jul-Dec 05Jan-Jun 06Jul-Dec 06Jan-Jun 07Jul-Dec 07 A B C DA DB E F GB GA SB0907 UFP AAPM - 49 SB0907 UFP AAPM - 50 K a,i and P KA at isocenter Field size measurement SB0907 UFP AAPM - 51 SB0907 UFP AAPM - 52 Outline Skin dose map and beam position Illustration courtesy of Siemens SB0907 UFP AAPM - 53 SB0907 UFP AAPM - 54

10 What can be measured in the lab? Dose displays in the lab Fluoroscopy time Number of acquisition Runs Frames Kerma Area Product P KA Reference Point Air Kerma K a,r Peak Skin Dose Dose maps SB0907 UFP AAPM - 55 The same RPDose will display as 5678 on all three systems The same KAP will display as on GE and SIEMENS and on PHILIPS SB0907 UFP AAPM - 56 In the control room SIR Standard of Practice Record all available data for every interventional procedure Order of Precedence Skin dose map (not currently available) Reference Point Air Kerma K a,r Kerma Area Product P KA Fluoro time and number of images Uses for the data Patient pre discharge instruction Dose reconstruction when necessary Clinical QA Physics QA against guidance levels SB0907 UFP AAPM - 57 SB0907 UFP AAPM - 58 Manual log A New IHE Profile IHE Radiation Exposure Monitoring Profile Integration of systems reporting dose and systems which receive, store, or process those reports Modalities, PACS, RIS, Workstations, Registries Technician hand writes data to log Data retyped into clinical records or dose data base Time lags and overflow can be problems Process is error prone Facilitate compliance with Euratom 97/43, ACR Guidelines, etc. Directly based on DICOM Dose Reports Creation, Collection, Distribution, Processing SB0907 UFP AAPM - 59 IHE 2008 Webinar Series

11 Outline JC Radiation Sentinel Event An unexpected occurrence involving death or serious physical or psychological injury, or the risk thereof. Serious injury specifically includes loss of limb or function. The phrase or the risk thereof includes any process variation for which a recurrence would carry a significant chance of a serious adverse outcome. Such events are called sentinel because they signal the need for immediate investigation and response. Prolonged fluoroscopy with cumulative dose >1500 rads to a single field or any delivery of radiotherapy to the wrong body region or >25% above the planned radiotherapy dose Can you demonstrate to the surveyors that Sentinel Events did not occur? How to be accurate enough? (not easy) SB0907 UFP AAPM - 61 SB0907 UFP AAPM - 62 MedPhys list May 2007 A facility without a full-time physicist needs a quick, easy number that they can refer to, so we use fluoro time, as archaic and inaccurate as it might be. If you want to know if your patient received more than the threshold for skin injuries, look at the patient's skin, not the KAP meter or fluoro time! Etiology not identified for more than one year SB0907 UFP AAPM - 63 Fluoro 35 minutes Cine 12 minutes What was covered SB0907 UFP AAPM - 64