Fluoroscopy Arun Ganguly PhD (Rebecca Fahrig)

1. FluoroscopyArun Ganguly PhD(Rebecca Fahrig)

2. A good quick reference resource: • J. Wang, T. J. Blackburn, “The AAPM/RSNA Physics Tutorial for Residents X-ray Image Intensifiers for Fluoroscopy”, September 2000 RadioGraphics, 20, 1471-1477.

3. Image Intensifiers

4. Image Intensifier Electrostatic lenses anode • Brightness gain = electronic gain (also called ‘flux’ gain) • X minification gain • For each e- ~1000 light photons are created • For each light photon at input phosphor, output 50 or 100 light photons generated (depending on mode chosen) electrons output fluorescent screen (fluorescent CsI)

5. II Magnification Modes

6. II resolution Note: x-axis is in lp/cm so divide by 10 to get lp/mm

7. II Artefacts • Veiling glare • Lag • Pin cushion • Vignetting • S-distortion

8. Dose/Noise Trade-off • Automatic Brightness Control (ABC): • Photocell located between image intensifier and camera sends signals to x-ray tube to change mA for constant brightness in image on monitor • Automatic Gain Control (AGC): • Amplifier gain in TV electronics maintains video signal during the time it takes ABC to change x-ray tube output. Does not affect input exposure.

9. Dose/Noise Trade-off • Automatic Exposure Control (AEC): • Using photo-timers the actual amount of radiation reaching the image receptor (input phosphor) is measured to allow consistent amount of x-rays to be incident

10. Image quality based automatic exposure control Today's exposure control tube voltage Exposure parameters are today highly correlated Dependencies are defined on fixed points, where the behaviour suddenly changes tube current exposure time prefilter patient thickness

11. Flat Panel Detector

12. II vs FPD II FPD II FPD

13. II vs FPD

14. II vs FPD • FPD can have many different zoom mode (upto 5 in some cases) • Higher zoom does not change spatial resolution for FPD. For II, resolution increases with higher zoom • If zoom involves binning pixels then there is loss of resolution • In IIs, the input dose increases as FOV-1

15. Sample Question Set

16. Raphex 2010

17. The main dose to a fluoroscopist during a procedure is • Leakage from x-ray tube • Scatter from the x-ray tube • Scatter from the table • Scatter from the patient • Scatter from the image intensifier

18. ANSWER: D : Scatter from the patient X-ray tubes are shielded to limit leakage radiation to a level below 0.87 mGy/h(100 mR/h) at 1m at maximum rated kVp. Leakage at fluroroscopic kV is typically very much less. Most of the scatter from x-ray tube components is absorbed by the shielding used for leakage. The table is designed not to attenuate the x-ray beam. This minimizes its scatter. The image intensifier is only illuminated by incident x-rays. The full intensity of the beam irradiates the patient and most scatter originates where the beam enters the patient.

19. The stage of image intensified fluoroscopy which determines the noise content of the image is the number of ---------------. • Electron produced at the photocathode • Light produced at the exit phosphor • Light absorbed in the photocathode • X-ray photons absorbed in the input phosphor

20. ANSWER: D : X-ray photons absorbed in the input phosphor The number of x-ray photons absorbed in the input phosphor. Beyond that stage the number of quanta is always greater

21. In a 15/23/30-cm image intensifier with automatic brightness control, the greatest patient input exposure rate will be for the • 15-cm mode • 23-cm mode • 30-cm mode • Depends on whether kV or mA is varied

22. ANSWER: A : 15 cm The greatest patient input exposure rate will be for smallest input size, 15 cm, since it has the smallest minification gain

23. Using a 25cm field of view II fluro system with video monitor used for GI studies will typically resolved • 1-2 • 3-4 • 5-6 • 7-8

24. ANSWER: A: 1-2 lp/mm The overall resolution is limited mainly by the video system. The intensifier itself may have 3-4lp/mm resolution

25. If a flat panel can resolve 2.4lp/mm in the 20-cm mode, it will be able to resolved ----lp/mm in 40cm mode • 0.6 • 1.2 • 2.4 • 4.8

26. ANSWER: C: 2.4 lp/mm The resolution of a flat-panel image intensifier does not generally change with input field size.

27. According to federal regulations, the largest total does that can be received by the patient. • 50mGy • 100mGy • 200mGy • Not defined

28. ANSWER: D: Not defined Federal regulations limit the dose rate but not the total dose. It is however expected that ALARA be observed by the system operators.

29. Which is true of solid-state fluoroscopic image detectors (flat panels) • A vacuum envelope is used to maintain electron focus • Approximately 40V is applied between the anode and cathode • CsI is used as an x-ray detector in both II and FPD • Increasing magnification (small FOV) always results in better spatial resulting • Increasing magnification (small FOV)

30. ANSWER: c: CsI is used as an x-ray detector in both II and FPD

31. In the diagram of the image intensifier shown below, all of the numbered arrows represent photons, except: • I. • II. • III. • IV. • V.

32. ANSWER: D : IV the photocathode converts visible light photons to electrons, which are accelerated and hit the output phosphor

33. Using a fluoroscope with automatic dose-rate control: Which one of the following configurations will result in the greatest skin dose-rate to the patient?

34. ANSWER: D. Position D. This position will result in the greatest skin dose-rate to the patient because it involves a thick patient, it is close to the x-ray tube, and it has a long tissue path due to angulation.

35. A patient is examined with an x-ray fluoroscope using auto brightness control. The source to skin distance, SSD=65 cm. The source to image intensifier distance SID=90 cm. The patient’s entrance air kerma rate (EAKr) is 20 mGy/min when the 14 cm FOV image intensifier is used & the x-ray beam is collimated to the full FOV. If the SID is increased to 120 cm without changing SSD, the EAKr: • Increased by a factor of approximately 2.6. • Increased by a factor of approximately 1.8. • Increased by a factor of approximately 1.3. • Is unaffected. • Decreases by a factor of approximately 1.3.

36. ANSWER: Increased by a factor of approximately 1.8. A constant dose rate is needed at the image intensifier at any SID. Increasing the SID requires an increased output from the X-ray tube. Using the inverse square law, the ratio is (120/90)2 = about 1.8.

37. Consider a fluoroscopy system with ABC & a dual FOV images intensifier capable of a 9” mode & a 4.5” mode. Which image intensifier mode will provide (A = 9” mode, B = 4.5” mode, C = neither) • Better spatial resolution • Larger area of patient imaged • Greater minification gain • Greater electronic (flux) gain • Greater quantum mottle

38. ANSWER: • B, 4.5” mode presents a magnified image • A, 9” mode has larger FOV • A • C, flux gain independent of mode (depends on voltage across II) • C, In 4.5” mode area being imaged is down by a factor of 4, but ABC increases exposure by factor of 4, therefore # x-ray photons/picture element remains unchanged.

39. A patient is examined with an auto brightness triple mode FOV, image intensifier x-ray fluoroscope. The patient’s entrance air kerma rate (EAKr) is 20 mGy/min when the 14 cm FOV is used & the x-ray beam is collimated to the full FOV. The patient’s EAKr: Increases more than 30% when the FOV is decreased to 10 cm (full field collimation). Increases more than 30% when the FOV is unchanged & the beam is collimated to 10 cmx10cm. Is unaffected by FOV & collimation changes. Decreases more than 30% when the FOV is unchanged & the beam is collimated to 10 cmx10 cm. Decreases more than 30% when the FOV is decreased to 10 cm (full field collimation).

40. ANSWER: Increases more than 30% when the FOV is decreased to 10 cm (full field collimation). Decreasing the FOV decrease the geometric light amplification factor of the image intensifier. More radiation is therefore needed to produce the required output light level. Also, less scatter is produced (and reaches the image intensifier) when the beam is collimated. A small increase in EAKr is needed to offset this effect.

41. Assuming constant film density with a 35 mm cine fluoro system using automatic brightness control, indicate whether each of the following will: • Increase image quality • Increase patient dose • Both • neither Change frame rate from 30 frames/s to 60 frames/s Increase mA on x-ray tube with constant kVp. Increase kVp with a constant mA. Change lens aperture from f5.6 to f2.8 (i.e. increase lens aperture)

42. ANSWER: • B, doubling frame rate, double patient exposure • C, increased mA implies increased patient exposure & reduced image noise • D, increase kVp decrease image contrast as well as patient dose • D, increased lens aperture implies more light to camera, thus ABC will decrease mA’s accordingly, therefore, image noise will increase & patient dose will go down.

43. When an image intensifier is zoomed from the 12-in. mode: • The system spatial resolution increases. • The patient’s skin dose increases. • The patient’s effective dose increases. • A, B, and C are true. • A and B are true.

44. ANSWER: E. A and B are true. A is true because a smaller input screen is focused on te same size output screen. B is true because there is less minification gain in the 5-inch mode. C is false: the dose-rate in the beam increases as the field-of-view decreases. The two effects offset each other; the same total number of x-ray photons is delivered to the patient. Therefore the effective dose is constant.

45. Rank the resolution of the following viewing modes using the same image intensifier from highest to lowest: • 35 mm cine • 525 line TV • Video cassette recorder • 100 mm film • 1025 line TV 1,2,3,4,5 5,4,1,2,3 4,1,5,2,3 1,4,3,5,2 4,5,1,2,3

46. ANSWER: C. 100 mm has almost same resolution as II output phosphor, 35 mm slightly less. All TV images are much lower in resolution with the high line TV being best, VCR always degrades TV image.

47. The reason for over-framing in cine filming is to: • Improve spatial resolution • Increase the field of view • Reduce patient dose • Change the shape of the cine image • Increase the framing rate

48. ANSWER: • Improve spatial resolution.

49. Which recording method used in conjunction with fluoroscopy has the lowest patient exposure per image? • Digital photospot • Spot film/par screen • 35 mm cine • spot film/rare earth screen • 100 mm photospot

50. ANSWER: C. The approximate exposure to the input surface of the II for a 9” field of coverage is 15 µR for a 35 mm cine, 100 µR for a 100 mm photospot, 50 to 100 µR for a digital photospot, 1000 µR for a par 4 screen conventional spot, and 250 µR for a 400 speed rare earth system. Of course, the cine image is too small for routine imaging and a single frame is never taken.