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FLUOROSCOPY & INTERVENTIONAL IMAGING BUSHBERG – CHAPTER 9 RSNA & AAPM PHYSICS CURRICULUM: MODULE 13 Renée (Dickinson) Butler, MS, DABR Medical Physicist.

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Presentation on theme: "FLUOROSCOPY & INTERVENTIONAL IMAGING BUSHBERG – CHAPTER 9 RSNA & AAPM PHYSICS CURRICULUM: MODULE 13 Renée (Dickinson) Butler, MS, DABR Medical Physicist."— Presentation transcript:

1 FLUOROSCOPY & INTERVENTIONAL IMAGING BUSHBERG – CHAPTER 9 RSNA & AAPM PHYSICS CURRICULUM: MODULE 13 Renée (Dickinson) Butler, MS, DABR Medical Physicist University of Washington Medical Center Department of Radiology Diagnostic Physics Section rdickins@uw.edu

2 General Fluoroscopy: Modes of Operation & System Components © UW and Renée Butler, MS, DABR

3 Fluoroscopy vs Projection Radiography PARAMETERG.I. FLUOROSCOPYRADIOGRAPHY kVp’s60 - 12050 - 130 mA values 0 - 5 (continuous) 200 - 800 0 - 100 (pulsed) X-ray Tube Focal Spot Size0.3 - 0.6 mm1.0 - 1.2 mm Exposure Duration0.5 - 15 minutes0.01 - 0.3 seconds Image Receptor Input Radiation Dose per image0.01 - 0.15 mGy / image4 - 10 mGy/image Patient skin Dose Rates10 - 60 mGy / min0.2 - 10 mGy / image Source-to- Skin Distance (SSD)30 - 50 cm60 - 145 cm Source-to-Image Receptor Distance (SID)80 - 120 cm100 or 182 cm Typical Spatial Resolution 1 - 2.0 LP/mm (Image Int.) 3 - 10 LP/mm 2.5 - 3.0 LP/mm(Flat Panel) Image Quantum MottleHighLow Staff Exposure to Scattered RadiationYesNo c.f. AAPM/RSNA Web Module: Fluoroscopy systems. Section IV. Table1. © UW and Renée Butler, MS, DABR

4 “Low(er) Exposure Rates” – general fluoroscopy Continuous – typically display is 30 fps or greater Maximum dose rate = 10 R per min [87.3 mGy/min] (10 CFR Part 20) Pulsed fluoroscopy Maximum dose rate = 10 R per min [87.3 mGy/min] (10 CFR Part 20) “High(er) Exposure Rates” – leads to rapid dose accumulation High dose rate – Specially activiated fluoroscopy (“enable” buttons or separate pedal) Maximum dose rate = 20 R/min [175 mGy/min] with audible signal (10 CFR Part 20) Not found on modern fluoroscopy suite systems, common on c-arms Road Mapping & Digital Subtraction Angiography (DSA): real-time subtraction of pre- and post-contrast injection images to improve the perception of low-contrast vessels 3D rotational angiography or CT acquisitions Fluoroscopy Modes of Operation © UW and Renée Butler, MS, DABR

5 Typical entrance exposure rates to the patient: For thin patients/body parts: dose rate is roughly 1-2 R per min [8.7 to 17 mGy per min] for thin body parts Skin injury threshold in normal mode can be reached in approximately 118 to 230 min For average patient sizes: dose rate is roughly 3-5 R per min [26 to 44 mGy/min Skin injury threshold in normal mode can be reached in approximately 45 to 77 min For heavy patient sizes: dose rate is roughly 8-10 R per min [70 to 87.3 mGy/min Skin injury threshold in normal mode can be reached in approximately 23 to 29 min Fluoroscopy Modes of Operation © UW and Renée Butler, MS, DABR

6 If temporal resolution is not needed (e.g. guiding a catheter from the femoral artery to aortic arch), use pulsed fluoro to reduce patient (and personnel!) dose Also reduces motion 30 fps 15 fps display c.f. AAPM/RSNA Physics Tutorials for Residents: MS Van Lysel. Fluoroscopy: Optical Coupling and the Video System. Radiographics. Nov 2000; 20: 1769-1786. Fluoroscopy Modes of Operation © UW and Renée Butler, MS, DABR

7 Continuous vs. pulsed Continuous – 30 fps w/ 33 msec per frame @ 2 mA (0.066 mAs) Pulsed – 30 fps w/ 10 msec per frame @ 6.6 mA (0.066 mAs) ** Same exposure to patient, but less motion artifact ** Pulsed – 15 fps w 33 msec per frame @ 2 mA (0.033 mAs) 1 second exposure Continuous Fluoro 30 fps, 33 msec per frame Pulsed Fluoro 15 fps, 33 msec per frame “Pulsed” Fluoro 30 fps, 10 msec per frame 2 mA 6.6 mA 2 mA Fluoroscopy Modes of Operation © UW and Renée Butler, MS, DABR

8 8.76 mGy per R 14.7 R per min = 128.8 mGy per min 34.6 R per min = 303.1 mGy per min 63.1 R per min = 552.8 mGy per min Fluoroscopy Modes of Operation © UW and Renée Butler, MS, DABR

9 Digital Subtraction Angiography (DSA): real-time subtraction of pre- and post-contrast injection images to improve the perception of low-contrast vessels Removal of background anatomy and tissue Increased image noise Clinically used for diagnostic and therapeutic applications of vessel visualization throughout the entire body c.f. AAPM/RSNA Physics Tutorial for Residents: Digital Fluoroscopy. Radiographics. Vol 21. March 2001. DSA cerebral arteriogram. a. Unsubtracted digital fluoro. b-d. Subtracted DSA images obtained at 3 progressive time points during contrast injection. Fluoroscopy Imaging Techniques © UW and Renée Butler, MS, DABR

10 Road mapping A DSA sequence is performed and the frame w/ maximum vessel opacification is identified The road map mask is subtracted from subsequent live fluoro images to produce real-time subtracted fluoro images e.g.: a wire is “steered” by using the road map for cues on maneuvering through vasculature 3D rotational angiography Philips Xper CT c.f. AAPM/RSNA Physics Tutorial for Residents: Digital Fluoroscopy. Radiographics. Vol 21. March 2001. Fluoroscopy Imaging Techniques © UW and Renée Butler, MS, DABR

11 What type of configuration is preferred for a urology suite? Why? Kidneys & bladder are closer to the image receptor, therefore, reducing focal spot blur. What is the negative of this room set-up? Because the tube is above the patient, the scatter radiation is projected back into the procedure room; whereas for GI fluoroscopy rooms, the entrance point is below the table and lead shield curtains attenuate the scatter radiation. © UW and Renée Butler, MS, DABR Web Modules Clinical Applications

12 Under table tube Gastrointestinal room Remote rooms C-arm (mobile) Operating rooms or floor clinics Orthopedic joint replacement, endoscopy, colonoscopy Positioning flexibility Angiography, IR rooms Over table tube Urology c.f. AAPM/RSNA Web Module: Fluoroscopy systems. Section III.A-E. © UW and Renée Butler, MS, DABR Web Modules Clinical Applications

13 Fluoroscopy System Components Image Intensifier (II) vs Flat-Panel Detectors (FPD) Flat-panel detector c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 232. © UW and Renée Butler, MS, DABR

14 Image intensifier system components: II – vacuum bottle (housing); input screen (x-ray to e - ); electronic lenses; output phosphor (e - s to visible light) Lenses and aperature Optical coupling w/ accessory port Viewing electronic output image – video or more commonly charged-coupled device (CCD) detectors c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 232, 239. Fluoroscopy System Components Image Intensifier (II) vs Flat-Panel Detectors (FPD) © UW and Renée Butler, MS, DABR

15 Review Questions For the image intensifier shown, match the following: (answers may be used more than once) A. Light photons. B. X-ray photons. C. Microwaves. D. Electrons. E. Infrared photons. I represents II represents III represents IV represents V represents B. A. D. A. © UW and Renée Butler, MS, DABR

16 In vacuum – electrons (e - s) are influence by environment Input screen – converts x-rays to e - s 1 mm aluminum window creates vacuum Support layer Input phosphor – Cesium Iodine (CsI) crystal; Converts x-rays to light Photocathode – Layer of antimony and alkali metal; emits electrons when struck by light; 10-20% conversion efficiency Electronic lenses – 25 kV – 35 kV electric field between input and output (electronic gain) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233. © UW and Renée Butler, MS, DABR Fluoroscopy System Components Image Intensifier (II)

17 Output phosphor – converts e - s to visible light Zinc cadmium sulfide (ZnCdS) Anode – thin coating of aluminum on the vacuum side of output phosphor Each e - interacts in the phosphor creating ~1000 light photons Some fraction of the output light emitted by ZnCdS phosphor is reflected at the glass window; known as veiling glare, which reduces image contrast c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233. © UW and Renée Butler, MS, DABR Fluoroscopy System Components Image Intensifier (II)

18 Solid-state devices Thin, carbon fiber – protects CsI phosphor and photodiode array INDIRECT digital detector Input phosphor – CsI converts x-rays to light photons X-rays interact in CsI and create ionizations; some deposited energy is emitted as light Photodiode array An array of detector elements – each element is 200 microns (0.2 mm) Absorbs light and converts energy into free electron charge that is stored in each cell of the array Charge stored is proportional to the incident light, which is proportional to the # incident (absorbed) x-ray c.f. Granfors & Albagli. Scintillator-based flat-panel x-ray imaging detectors. Journal of the Society for Information Display. June 2009. © UW and Renée Butler, MS, DABR Fluoroscopy System Components Flat-Panel Detectors (FPD)

19 Patient & Personnel Safety in Fluoroscopy © UW and Renée Butler, MS, DABR

20 Identify the technique factors and appropriate system features to use to optimize image quality while minimizing patient dose. 1.FPD (or II) close to patient 2.Grid in 3.Collimate! 4.Increase SSD (source-to-skin distance) to decrease ESD (entrance skin dose) *Note: geometry limitations with shorter personnel AAPM/RSNA Resident Physics Curriculum: Module 13: Fluoroscopy & Interventional Imaging Clinical Application © UW and Renée Butler, MS, DABR

21 Describe the geometric factors that affect operator dose during an IR procedure. 1.Lead apron 2.Thyroid shield 3.Protective eyewear 4.Radiation badge 5.Adjust dose settings when possible (pulse) to reduce scatter 6.Distance, when possible 7.Shielding, when possible AAPM/RSNA Resident Physics Curriculum: Module 13: Fluoroscopy & Interventional Imaging Clinical Application © UW and Renée Butler, MS, DABR

22 Describe the geometric factors that affect operator dose during an IR procedure – scatter geometry for the frontal and lateral tubes in a NIR suite. c.f. ZH Anastasian et.al. Radiation Exposure of the Anesthesiologist in the Neurointerventional Suite. Anesthesiology 2011; 114: 512-20. General set-up for angio/IR suites: Frontal tube: positioned w/ tube below patient Scatter to personnel minimized by lead drapes Lateral tube: positioned so radiologist is on the same side as the FPD or II Scatter is projected from the skin back toward the x-ray tube Higher scatter for personnel on “tube side” of lateral tube. Use moveable shields! AAPM/RSNA Resident Physics Curriculum: Module 13: Fluoroscopy & Interventional Imaging Clinical Application © UW and Renée Butler, MS, DABR

23 General set-up for angio/IR suites: Frontal tube: positioned w/ tube below patient Scatter to personnel minimized by lead drapes Lateral tube: positioned so radiologist is on the same side as the FPD or II Scatter is projected from the skin back toward the x-ray tube Higher scatter for personnl on “tube side” of lateral tube. Use moveable shields! AAPM/RSNA Resident Physics Curriculum: Module 13: Fluoroscopy & Interventional Imaging Clinical Application c.f. ZH Anastasian et.al. Radiation Exposure of the Anesthesiologist in the Neurointerventional Suite. Anesthesiology 2011; 114: 512-20. © UW and Renée Butler, MS, DABR Describe the geometric factors that affect operator dose during an IR procedure – scatter geometry for the frontal and lateral tubes in a NIR suite.

24 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p791. Bushberg Table 23-18. Nuclear Regulatory Commission (NRC) Regulatory Requirements: Maximum Permissible Dose Equivalent Limits a Maximum Possible Annual Dose Limit LimitsmSvrem Occupational Limits Total effective dose equivalent (ED)505 Total dose equivalent to any individual organ (except lens of eye) 50050 Dose equivalent to the lens of the eye15015 Dose equivalent to the skin or any extremity50050 Minor (< 18 years old)10% of adult limit Dose to an embryo/fetus b 5 in 9 months0.5 in 9 months Non-occupational (Public) Limits Individual members of the public1.0 per yr0.1 per yr Unrestricted area0.02 in any 1 hr c 0.002 in any 1 hr c a These limits are exclusive of natural background and any dose the individual has received for medical purposes; inclusive of internal committed dose equivalent & external effective dose equivalent (i.e., total effective dose equivalent). b Applies only to conceptus of a worker who declares her pregnancy. If the limit exceeds 4.5 mSv (450 mrem) at declaration, conceptus dose for remainder of gestation is not to exceed 0.5 mSv (50 mrem). c This means the dose to an area (irrespective of occupancy) shall not exceed 0.02 mSv (2 mrem) in any 1 hour. This is not a restriction of instantaneous dose rate to 0.02 mSv per hour (2 mrem per hour). Radiation Protection and Fluoroscopy/IR © UW and Renée Butler, MS, DABR

25 Radiation Protection and Fluoroscopy/IR Badging – OUTSIDE the lead near the neck/collar Generally, no double badging (except pregnant personnel) Monitors total exposure, eye dose, extremity Total annual limit for occupation radiation worker is 50 mSv/year Total eye lens dose limit is 150 mSv (or 20 mSv; ICRP) per year Lead aprons Lead equivalent = 0.25 mm: absorbs > 90% of scatter Lead equivalent = 0.35 - 0.50 mm: absorbs 95 - 99% of scatter (but heavier, so is it feasible to wear??) Use a lead thyroid shield at all times Protective gloves of 0.5 mm lead of greater should be worn if hands are going to be near but outside the primary beam To protect hands during fluoroscopy, it is recommended: Keep hands out of and away from the x-ray field when the beam is on unless physician control of invasive devices is required for patient care during fluoroscopy Work on the exit-beam side of the patient whenever possible Monitor hand dose © UW and Renée Butler, MS, DABR

26 Radiation Protection and Fluoroscopy/IR Leaded glasses: Recommended that all full-time radiology interventionists and anesthesiologists should wear leaded eye protection Measured lens entrance dose is ~2.5 +/- 2.0 µSv (radiologist); therefore ~30,000 procedures to reach current 150 mSv/year limit (based on current NRC Dose Limits) NEW!! The ICRP recently released a statement stating lower dose thresholds for cataracts were appropriate. The previous ICRP threshold (and current NCRP – or US – threshold) of 4 Gy (acute exposure) and 8 Gy (chronic exposure) Reduced to 0.5 Gy for acute and chronic exposures, based on recent studies of patients and occupational workers. Note, even though the USA has yet to adopt this ICRP threshold, it is anticipated change. Because of this lower cataract threshold, the ICRP slashed the occupational dose limit for the lens of the eye to 20 mSv in a year, averaged over a defined period of five years. The cumulative lens dose should not exceed 50 mSv in any single year.

27 Fluoroscopy – the Ten Commandments 1. As patient size increases As image quality decreases, patient dose increases, personnel dose increases 2. Exposure time Total fluoro time directly affects patient dose, but also distributing dose over the skin (can you rotate/move tube to a different position??) 3. Use appropriate dose and dose-rate settings Pulsed vs continuous Standard FOV vs mag modes © UW and Renée Butler, MS, DABR

28 Fluoroscopy – the Ten Commandments 4. X-ray tube position Raise/lower patient away from x-ray tube to decrease ESD Lateral and oblique tube positions general have higher ESDs c.f. LK Wagner & BR Archer. Minimizing Risks from Fluoroscopic X Rays: Bioeffects, Instrumentation, and Examination. 2004: 4 th Edition. © UW and Renée Butler, MS, DABR

29 c.f. LK Wagner & BR Archer. Minimizing Risks from Fluoroscopic X Rays: Bioeffects, Instrumentation, and Examination. 2004: 4 th Edition. Fluoroscopy – the Ten Commandments 5. Proximity of II or FPD to Patient – improves image quality and decreases radiation dose © UW and Renée Butler, MS, DABR SID = 110 cm, varied SOD

30 Fluoroscopy – the Ten Commandments 6. Inverse Square Law Reduce the dose through the use of distance if and when you can. Given: Exposure rate at 2 ft is 90 mR/hr © UW and Renée Butler, MS, DABR 9 ft 2 4 ft 2 1 ft 2 2 ft 4 ft 6 ft Exposure Rate at 4 ft = (90 mR/hr)(2ft/4ft) = 22.5 mR/hr 2 Exposure Rate at 6 ft = (90 mR/hr)(2ft/6ft) = 10 mR/hr 2 1 ft 2 ft 3 ft

31 Fluoroscopy – the Ten Commandments Magnification Electronic mag – generally higher doses when using mag modes (though may not always be Geometric mag – increase distance between patient and II; typically increases dose by the square of the magnification Grid – remove grid for thin patients or if the image contrast is not affected by the scatter Collimation!! Personnel Safety – use time, distance and shielding to your advantage whenever possible; always wear lead aprons, use badges to monitor individual dose © UW and Renée Butler, MS, DABR

32 Skin Injury Case Reports & Radiation Dose © UW and Renée Butler, MS, DABR

33 Skin Injuries – Case Reports This is a 49-yr old woman with 8-yr history of refractory supraventricular tachycardia. The patient was exposed to about 20 min of fluoroscopy with her elbow about 20-25 cm from the x-ray source. The circular port of the x-ray system defined the sharply demarcated border of the injury. Fig.8b show sharply demarcated erythema above right elbow at 3 weeks after RF cardiac cath ablation. Fig. 8c shows tissue necrosis 5 months after procedure. Fig. 8d shows injury with humerus visible about 6.5 months after procedure. Fig. 8e shows the surgical flap about 10 months after fluoroscopy procedure. Fig 8b Fig 8c Fig 8d Fig 8e © UW and Renée Butler, MS, DABR

34 Skin Injuries – Case Reports ref: ICRP Publication 85, Case 1 (photographs courtesy of T. Shope). (a) The patient’s back 6–8 weeks after multiple coronary angiography and angioplasty procedures. (b) The injury approximately 16–21 weeks after the procedures. A small, ulcerated area is present. (c) The injury approximately 18–21 months after the procedures. Tissue necrosis is evident. (d) Close-up photograph of the lesion shown in (c). © UW and Renée Butler, MS, DABR

35 Skin Injuries – Case Reports Radiation injury in a 60-year-old woman subsequent to successful neurointerventional procedure for the treatment of acute stroke. Estimated fluoroscopy time was more than 70 minutes; 43 imaging series were performed during course of the procedure. The head was not shaved. Note focal epilation on scalp and skin injury on neck but not on scalp. No dose estimates were available for this case. ref: Balter et al. Radiology. Feb 2010. Vol 254:2 © UW and Renée Butler, MS, DABR

36 © UW and Renée Dickinson, MS

37 FDA Advisory In the late 1980-early 1990s, the US FDA documented reports of at least 40 cases of radiation-induced burns to patients from fluoroscopically guided procedures. Fluoroscopic radiation is a carcinogen. While the risk of cancer from fluoroscopy is usually very small, it is essential that the radiation be properly controlled to minimize this risk to patients, to operators and to personnel. On September 9 th, 1994, the FDA issued an advisory for facilities that use fluoroscopy for invasive procedures. © UW and Renée Butler, MS, DABR

38 FDA Advisory Recommendations… Appropriate credentials and training for physicians performing fluoroscopy Operators be trained and understand system operation, and implications of radiation exposure for each mode of operation Physicians be educated in assessing risks and benefits on a case-by-case basis for patients Patients be counseled regarding the symptoms and risks of large radiation exposures Physicians justify and limit use of high dose rate modes of operation © UW and Renée Butler, MS, DABR

39 Washington State Law WAC 246-225-020 Operators shall be adequately instructed in safe operating procedures and shall be able to demonstrate competence A medical x-ray machine operator shall be licensed, certified or registered by the department as either: a licensed health care practitioner a certified diagnostic or therapeutic RT a registered x-ray technician Nurses or PAs need training if asked to operate x-ray equipment Physician is ultimately responsible for assuring that the x- rays are safely and properly applied and that appropriate radiation protection measures are followed © UW and Renée Butler, MS, DABR

40 Image Quality (Dose) in Fluoroscopy © UW and Renée Butler, MS, DABR

41 Review Questions A 9-in. multi-mode image intensifier (II) is switched to the 6-in. mode. As a result, the image will be ________, and the automatic brightness control system (ABC) will _________ the exposure to the II and the patient. A. magnified, decrease B. magnified, increase C. minified, increase D. magnified, not change E. minified, decrease © UW and Renée Butler, MS, DABR

42 Small FS – general fluoroscopy (minimize blurring) Large FS – digital spots only (tube loading) In general, the spatial resolution of the I.I. alone is 3.5-6.0 LP/mm Smaller structures are minified less (spread over a larger portion of the output phosphor), this enlargement of the displayed image improves the limiting resolution of the imaging system The typical spatial resolution of most current FPD image receptors is about 2.5 – 3.0 LP/mm for all FOV’s. © UW and Renée Butler, MS, DABR Web Modules Clinical Applications – Spatial Resolution

43 With regard to a flat panel, what is binning? What is the advantage? What is the disadvantage? Example: 8 x 8 matrix; 10 photons per pixel 4 x 4 matrix; 160 photons per pixel 10 160 Less Quantum Mottle, the patient radiation dose can be significantly reduced while maintaining the same image noise. However, binning does reduce the spatial resolution of the image. Binning is especially useful for a large FOV where there would be too many pixels in the image σ = sqrt(10) = 3.2 SNR = N/σ = sqrt(N) = 3.2 σ = sqrt(160) = 12.6 SNR = 12.6 © UW and Renée Butler, MS, DABR Web Modules Clinical Applications – Spatial Resolution (and noise)

44 What percent of dose is a single fluoroscopy image relative to a general radiography image? About 1% (typically 450 to 1800 images per minute of fluoroscopy) 0.01-0.15 mGy per image (fluoro) 4-10 mGy per image (radiography) © UW and Renée Butler, MS, DABR Web Modules Clinical Applications – Radiation Dose

45 Spatial resolution System detector limitations – FOV, matrix, DELs, video capabilities, binning e.g.: FPD detector element sizes GI studies @ 2.5-3 lp per mm Using mag, the resolution improves to @ 3.5-6 lp per mm Television systems limit resolution to about 1-2 lp per mm for GI systems and 2-4 lp per mm for angio systems Focal spot size and geometry – keep patient adjacent to detector!! This reduces focal spot blur Motion, temporal factors affecting image blur In general, pulsed fluoro reduces motion blur → improve resolution © UW and Renée Butler, MS, DABR Fluoroscopy Factors Affecting Image Quality

46 Contrast Scattered x-rays (grid), veiling glare (II only) kVp and filtration – if the average (effective) energy of the x-rays is increased, then contrast decreases Collimation – decreases scatter contribution Radiation dose and noise – increasing the mA, decreases the noise and therefore improved contrast Image processing – smoothing algorithms and frame averaging reduces image noise, which improves contrast; edge enhancement algorithms increase image noise, therefore contrast degrades Contrast media (iodine, barium, or air) enhances contrast of anatomical structures © UW and Renée Butler, MS, DABR Fluoroscopy Factors Affecting Image Quality

47 Image Quality: II vs FPD IMAGE QUALITYII SystemsFPD Systems If FOV is reduced (mag mode)… improved spatial resolution Improves or stays the same (no geometric “minification” for FPD) Binning n/a reduces spatial resolution, improved SNR by decreasing mottle Focal spot size use small FS to decrease focal spot blur, except for digital spots Limiting resolution II resolution (3.5-6 LP/mm), but system resolution is limited by television system (1-4 LP/mm) limited by DEL (2.5-3 LP/mm) Dynamic range limited generally, no limitation Contrast increasing the kVp and/or filtration increases the average energy of the x-rays and the number of Compton scatter events, therefore image contrast is degraded Collimation limits number of scattered photons and extraneous irradiation of tissue c.f. AAPM/RSNA Web Module: Fluoroscopy systems. Section VIII: A-F. © UW and Renée Butler, MS, DABR

48 Goal is to keep the # photons constant → maintains SNR regardless of patient thickness (or attenuation) c.f. AAPM/RSNA Web Module: Fluoroscopy systems. Section X.A-7. Programming the generator: If generator responds by increase kV for thicker (more attenuating) regions, then contrast is compromised but dose is lower For procedures where contrast is critical (e.g. angiography) the generator programmed to increase mA first. Preserves contrast, but at the cost of increased dose to patient © UW and Renée Butler, MS, DABR Fluoroscopy Factors affecting Radiation Dose (and Image Quality)

49 ABC = automatic brightness control higher kVp’s on the x-ray tube more x-ray tube current (higher mA) opening the aperture to increase the brightness longer x-ray pulse width less x-ray beam filtration or some combination of these factors © UW and Renée Butler, MS, DABR Fluoroscopy Factors affecting Radiation Dose (and Image Quality) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 247.

50 ABC systems X-ray beam kVp and filtration Aperture – smaller aperture blocks more light from output phosphor, and decreases dose rate (the aperture is used to balance an acceptable amount of noise w/ an acceptable level of patient dose rate) Pulse vs continuous fluoro Conversion gain – as an II ages, the amount of light produced in the input phosphor decreases and the conversion gain decreases… this results in GREATER RADIATION DOSE because the II is less efficient Geometry Decreased SID → ↓ dose Image receptor close to patient FOV selection II system: smaller (mag) FOVs increase the dose FPD: may not increase x-ray output c.f. AAPM/RSNA Web Module: Fluoroscopy systems. Section IX.B. © UW and Renée Butler, MS, DABR Fluoroscopy Factors affecting Radiation Dose (and Image Quality)

51 Radiation Dose: II vs FPD Radiation DoseII SystemsFPD Systems If FOV is reduced (mag mode)… increases the radiation dose not necessary to increase the dose Increasing the SID… increases the radiation dose Adding filtration (e.g. Cu)… preferentially removes low energy x-rays, therefore decreasing the dose Smaller aperature (large F)… increases the radiation dose (aperature balances acceptable amount of image noise with an acceptable level of patient dose) n/a Increasing kVp… reduction in radiation dose (more x-rays penetrate the patient) Reducing the pulse rate… reduction in radiation dose Decreasing conversion gain… increases the radiation dose (in aging IIs) n/a c.f. AAPM/RSNA Web Module: Fluoroscopy systems. Section IX: A-J. © UW and Renée Butler, MS, DABR

52 Review Questions An interventional radiologist performed 5000 fluoroscopically guided procedures last year. His annual occupational exposure was reported as “background.” What is the most likely explanation? A. All procedures were performed remotely from the x-ray control room. B. His badge was worn under a 0.5mm lead apron. C. The radiologist never wore his radiation badge while working. D. The department’s control badges were stored in the interventional control room. E. There was a persistent failure at the radiation badge company. © UW and Renée Butler, MS, DABR

53 Review Questions Which one of the following scenarios will result in the highest skin dose to the patient? A. B. C. D. Short SSD plus large SID results in higher patient dose. Use of the grid also requires higher patient dose. © UW and Renée Butler, MS, DABR

54 Questions? © UW and Renée Butler, MS, DABR


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