1. Fluoroscopy
Robert Metzger, Ph.D.

2. Real-Time Imaging
Fluoroscopy is an imaging procedure that allows real-time x-ray viewing of the patient with high temporal resolution
Use TV technology, which provides 30 frames per second imaging
Allows acquisition of a real-time digital sequence of images (digital video), that can be played back as a movie loop
Cine cameras offer up to 120 frame per second acquisition rates using 35-mm cine film. Digital cine also available

3. Fluoroscope Imaging Chain
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 232.

4. The Image Intensifier There are 4 principal components of an II:
(a) a vacuum bottle to keep the air out
(b) an input layer that converts the x-ray signal to electrons
(c) electronic lenses that focus the electrons, and
(d) an output phosphor that converts the accelerated electrons into visible light
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

5. The Image Intensifier X-RAYS CCD TV CAMERA MIRROR ADC LENS
OUTPUT PHOSPHOR
APERTURE
FOCUSING ELECTRODES
DISPLAY
ELECTRONS
PHOTO-CATHODE LAYER
INPUT PHOSPHOR ...CsI
X-RAYS

6. The Input Screen
The input screen of the II consists of 4 different layers:
(a) vacuum window, a 1 mm aluminum window that is part of the vacuum bottle
keeps the air out of the II, and its curvature is designed to withstand the force of the air pressing against it
a vacuum is necessary in all devices in which electrons are accelerated across open space
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

7. The Input Screen
The input screen of the II consists of 4 different layers:
(b) support layer, which is strong enough to support the input phosphor and photocathode layers, but thin enough to allow most x-rays to pass through it
0.5 mm of aluminum, is the first component in the electronic lens system, and its curvature is designed for accurate electronic focusing
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

8. The Input Screen
The input screen of the II consists of 4 different layers:
(c) input phosphor, whose function is to absorb the x-rays and convert their energy into visible light
cesium iodide (CsI) is used
long, needle-like crystals which function as light pipes, channeling the visible light toward the photochathode with minimal lateral spreading
400 mm tall, 5 mm in diameter
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

9. The Input Screen
The input screen of the II consists of 4 different layers:
(d) photocathode is a thin layer of antimony and alkali metals that emits electrons when struck by visible light
10 to 20% conversion efficiency
23 to 35 cm diameter input image (FOV)
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

10. Input Phosphor Energy Conversion
Aluminum Support
Photocathode
CsI Needles
Figure courtesy from Jonathan Tucker, Brooke Army Medical Center, SA, TX

11. Input Phosphor Energy Conversion
60 keV X-Ray
Aluminum Support
Photocathode
Figure courtesy from Jonathan Tucker, Brooke Army Medical Center, SA, TX
CsI Needles

12. Input Phosphor Energy Conversion
Aluminum Support
3,000 light photons
 = ~ 420 nm
Photocathode
Figure courtesy from Jonathan Tucker, Brooke Army Medical Center, SA, TX
CsI Needles

13. Input Phosphor Energy Conversion
Aluminum Support
Photocathode
~ 400 electrons To
Anode
CsI Needles
Figure courtesy from Jonathan Tucker, Brooke Army Medical Center, SA, TX

14. Electron Optics Electrons are accelerated by an electric field
Energy of each electron is substantially increased and this gives rise to electron gain
Focusing is achieved using an electronic lens, which requires the input screen to be a curved surface, and this results in unavoidable pincushion distortion of the image
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

15. Electron Optics
The G1, G2, G3 electrodes along with the input screen and the anode near the output phosphor comprise the five-component electronic lens system of the II
The electrons under the influence of the 25K to 35K V electric field, are accelerated and arrive at the anode with high velocity and considerable kinetic energy
After penetrating the very thin anode, the energetic electrons strike the output phosphor
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

16. The Output Phosphor
The output phosphor is made of zinc cadmium sulfide
Anode is very thin coating of aluminum on the vacuum side of the output phosphor, which is electrically conductive to carry away the electrons once they deposit their energy in the phosphor
Each electron causes the emission of approximately 1000 light photons from the output phosphor
2.5 cm diameter output phosphor
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

17. The Output Phosphor
The reduction in image diameter leads to amplification (analogy: magnifying glass and sunlight)
Minification gain of an II is simply the ratio of the area of the input phosphor to that of the output phosphor, e.g., 9’’ input phosphor, 1’ output phosphor, area is square of the diameter ratio, minification gain is 81
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

18. The Output Phosphor
The output phosphor is coated right onto the output window
Some fraction of the light emitted by the output phosphor is reflected at the glass window
Light bouncing around the output window is called veiling glare, and can reduce image contrast
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

19. Image Intensifier Performance Conversion Factor
Light out of image intensifier (cd/m2)
Conversion Factor =
Exposure rate into image intensifier (mR/sec)
Defined as a measure of the gain of an image intensifier
ratio of light output to exposure rate input
100 to 200 for new image intensifier
Degrades over time, ultimately can lead to II replacement

20. Image Intensifier Performance Brightness Gain
BG = minification gain x electronic gain (flux gain)
Minification gain = increase in image brightness that results from reduction in image size from the input phosphor to output phosphor size
(di/do)2, di is input diameter which varies, do is output diameter typically 2.5 cm
For 30 cm (12”) II, minification gain = 144

21. Image Intensifier Performance Brightness Gain
BG = minification gain x electronic gain (flux gain)
Electronic gain or flux gain is typically 50
The brightness gain therefore ranges from about 2,500 – 7,000
As the effective diameter of the input phosphor decreases (magnification increases), the brightness gain decreases

22. Field of View/Magnification
FOV specifies the size of the input phosphor of the image intensifier
Different sizes: 23 cm (9”), 30 cm (12”), 35 cm (14”), 40 cm (16”) FOV
Magnification is accomplished electronically using electronic focusing that projects part of the input layer onto the output phosphor
Since brightness gain decreases in mag. mode, the x-ray exposure rate is boosted. (12/9)2 = 1.8, (12/7)2 = 2.9
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 237.

23. ALL OF INPUT SURFACE USED TO GATHER X-RAYS
NON-MAGNIFY MODE OF I.I.
OUTPUT IMAGE
ALL OF INPUT SURFACE USED TO GATHER X-RAYS

24. MAGNIFIED MODE OF IMAGE INTENSIFIER
OUTPUT IMAGE
LESS IMAGED ANATOMY IS EXPANDED OVER THE SAME OUTPUT SURFACE AND LOOKS MAGNIFIED
ONLY A PORTION OF INPUT SURFACE USED TO GATHER X-RAYS

25. LARGE NON-MAG FoV e.g., 12 INCH
SMALL, MAG FoV e.g., 6 INCH
Magnification

26. Pincushion Distortion

27. Optical Coupling
Parallel rays of light enter the optical chamber, are focused by lenses, and strike the video camera where an electronic image in produced
A partially silvered mirror is used to shunt the light emitted by the image intensifier to an accessory port
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 239.

28. Video Camera
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 240.

29. Video Camera
Analog video systems typically have 30 frames/sec operation, but they work in an interlaced fashion to reduce flicker, the perception of the image flashing on and off
The human eye-brain system can detect temporal fluctuations slower than about 47 images/sec, and therefore at 30 frames/sec flicker would be perceptible
With interlaced systems, each frame is composed of two fields and each field is refreshed at a rate of 60 times per second, which is fast enough to avoid perception of flicker

30. Lag
Lag means that each new TV image actually contains residual image information from the last several frames
Lag is good and bad
Lag acts to smooth the quantum noise in the image, but can also cause motion blurring

31. Lag
Effect of camera lag. Angiogram of a rapidly moving coronary artery shows a trailing "ghost" due to excessive camera lag (the direction of travel is from right to left).

32. Video Resolution
Spatial resolution of a video in the vertical direction (top to bottom) of the TV image is governed by the number of scan lines
By convention, 525 lines are used in N. America for TV
490 lines usable
In the early days of TV, a man named Kell determined that about 70% of theoretical video resolution is appreciated visually, and this psychophysical effect is now called the Kell factor
490 x 0.7 = 343 lines or 172 line pairs useful for resolution
For 9” field, resolution = 172 lp/229 mm = 0.75 lp/mm
17 cm or 7” field, resolution is 1.0 lp/mm
12 cm or 5” field, resolution is 1.4 lp/mm

33. Video Resolution
The horizontal resolution is determined by how fast the video electronics can respond to changes in light intensity
This is influenced by the camera, the cable, the monitor but the horizontal resolution is governed by the bandwidth of the system
The time necessary to scan each video line (525 lines at 30 frame/sec) is 63 msec
11 msec required for horizontal retrace, 52 msec available
To achieve 172 cycles in 52 msec, the bandwidth required is 172 cycles/52 x 10-6 sec = 3.3 x 106 cycles/sec = 3.3 MHz
Higher bandwidths are required for high-line video systems

34. TV LINES ARE COMPOSED OF DOTS
TELEVISION IMAGE
HORIZONTAL DIRECTION
RASTER LINE
VERTICAL DIRECTION
TV LINES ARE COMPOSED OF DOTS

35. INTERLACED SCANS

36. INTERLACED SCANS

37. TYPICAL MEASURED RESOLUTION
[ 1023 LINE T.V. ]
FoV T.V. I.I. [CINE]
9 INCH LP/mm LP/mm
6 INCH LP/mm LP/mm
4.5 INCH LP/mm LP/mm

38. Summary Fluoroscopy is a live imaging procedure
Image Intensifier main component and consists of the input phosphor, electronic lens system and output phosphor
Input phosphor – Cesium Iodide, converts x-rays to light
Photocathode – converts light into electrons
Output phosphor – Zinc cadmium sulphide, converts electrons into light
Artifacts – pincushion distortion, veiling glare, lag
Brightness gain = minification gain x electronic (flux) gain
Several magnification modes available, typically exposure rate increases with magnification
Video camera produces the electronic image which we see on the TV monitor
Use interlaced scanning to avoid flicker
Horizontal (determined by bandwidth) and vertical (determined by the number of scan lines) video resolution

39. Flat Panel Digital Fluoroscopy
Flat panel devices are thin film transistor (TFT) arrays that are rectangular in format and are used as x-ray detectors
CsI, a scintillator is used to convert the incident x-ray beam into light
TFT systems have a photodiode at each detector element which converts light energy to an electronic signal
Flat panel detectors would replace the image intensifier, video camera, and other peripheral devices
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 242.

40. DIAGRAM OF GE FLAT PANEL IMAGE DETECTORS
FROM GE

41. FLAT PANEL-LIGHT SENSOR
scan line
FET
Pitch
Very High Fill Factor
Sensitive Area
Pitch x Pitch
Fill Factor=
data line
FROM GE
Pitch

42. Peripheral Equipment Photo-spot camera
used to generate images on photographic film, 100-mm cut film or 105-mm roll film
full resolution of the II system, hardly seen nowadays
Digital photo-spot
high resolution, slow-scan TV cameras in which the TV signal is digitized and stored in computer memory
Or CCD cameras with or pixel formats
near-instantaneous viewing of the image on a video monitor
allows the fluoroscopist to put together a number of images to demonstrate the anatomy important to the diagnosis
digital images can be printed on a laser imager

43. Peripheral Equipment Spot-film devices
attaches to the front of the II, and produces conventional radiographic screen-film images
better resolution than images produced by II
Cine-radiography cameras
attaches to a port and can record a very rapid sequence of images on 35-mm film
used in cardiac studies, 30 frames/sec to 120 frames/sec or higher
uses very short radiographic pulses
digital cine are typically CCD-based cameras that produce a rapid sequence of digital images instead of film sequence

44. Fluoroscopy Modes of Operation
Continuous fluoroscopy
continuously on x-ray beam, 0.5 – 4 mA or higher
display at 30 frames/sec, 33 msec/frame acquisition time
blurring present due to patient motion, acceptable
10 R/min is the maximum legal limit
High dose rate fluoroscopy
specially activated fluoroscopy
20 R/min is the maximum legal limit
audible signal required to sound
used for obese patients

45. Fluoroscopy Modes of Operation
Pulsed fluoro:
series of short x-ray pulses, 30 pulses at ~10 msec per pulse
exposure time is shorter, reduces blurring from patient motion
Can be used where object motion is high, e.g., positioning catheters in highly pulsatile vessels
15 frames/sec, 7.5 frames/sec also available
Variable frame pulsed fluoroscopy is instrumental in reducing dose
Ex., initially guiding the catheter up from the femoral artery to the aortic arch does not require high temporal resolution and 7.5 frames/sec could potentially be used instead of 30 frames/sec
7.5 frames/sec instead of 30 frames/sec, dose savings of (7.5/30) 25%

46. Frame Averaging
Fluoroscopy systems provide excellent temporal resolution
However, fluoroscopy images are relatively noisy, and in some applications it is beneficial to compromise temporal resolution for lower noise images
This can be achieved by averaging a series of images or frames
Real-time averaging in the computer memory for display
Can cause noticeable image lag but noise in image is reduced as well
Could also reduce dose in some circumstances
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 245.

47. Last Frame Hold Last-frame hold
when the fluoroscopist takes his or her foot off the fluoroscopy pedal, rather than seeing a blank monitor, last-frame-hold enables the last live image to be shown continuously
useful at training institutions
no unnecessary radiation used on patient

48. Road-Mapping Road Mapping
software-enhanced variant of the last-frame-hold feature
side-by-side video monitors, one shows captured image, the other live image
In angiography, subtracted image can be overlayed over live image to give the angiographer a vascular “road map” right on the fluoroscopy image
is useful for advancing catheters through tortuous vessels

49. Automatic Brightness Control
The purpose of the automatic brightness control (ABC) is to keep the brightness of the image constant at monitor
It does this by regulating the x-ray exposure rate (control kVp, mA or both)
Automatic brightness control triggers with changing patient size and field modes

50. Automatic Brightness Control
The top curve increases mA more rapidly than kV as a function of patient thickness, and preserves subject contrast at the expense of higher dose
The bottom curve increases kV more rapidly than mA with increasing patient thickness, and results in lower dose, but lower contrast as well
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 247.

51. Image Quality Spatial Resolution
A 2D image really has 3 dimensions: height, width, and gray scale
Height and width are spatial and have units such as millimeters
The classic notion of spatial resolution is the ability of an image system to distinctly depict two objects as they become smaller and closer together
The closer together they are, with the image still showing them as separate objects, the better the spatial resolution
At some point, the two objects become so close that they appear as one, and at this point, spatial resolution is lost

52. Image Quality Spatial Resolution
The spatial domain simply refers to the two spatial dimensions of an image, width (x-dimension) and length (y-dimension)
Another useful way to express the resolution of an imaging system is to make use of the spatial frequency domain
F (line pairs/mm or cycles/mm) =1/2, where  is the size of the object (mm)
Smaller objects (small ) correspond to higher spatial frequencies and larger objects (large ) correspond to lower spatial frequencies
So, objects that are
0.36 mm correspond to 1.4 lp/mm
0.19 mm corresponds to 2.7 lp/mm
1 mm correspond to 0.5 lp/mm

53. Image Quality: Spatial Resolution
Spatial frequency is just another way of thinking of object size
A device used to measure the spatial resolution is the bar pattern
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 249.

54. Image Quality Spatial Resolution
The modulation transfer function, MTF of an image system is a very complete description of the resolution properties of an imaging system
The MTF illustrates the fraction (or percentage) of an object’s contrast that is recorded by the imaging system, as a function of the size (i.e., spatial frequency) of the object
The limiting spatial resolution is the size of the smallest object that an imaging system can resolve
The limiting resolution of modern image intensifiers is between 4 and 5 cycles/mm
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 248.

55. Image Quality Contrast Resolution
The ability to detect a low-contrast object on an image is highly related to how much noise (quantum noise and otherwise) there is in the image
The ability to visualize low-contrast objects is the essence of contrast resolution. Better contrast resolution implies that more subtle objects can be routinely seen on the image
The contrast resolution of fluoroscopy is low by comparison to radiography, because the low exposure levels produce images with relatively low signal-to-noise ratio (SNR)

56. Image Quality Contrast Resolution
Contrast resolution is increased when higher exposure rates are used, but the disadvantage is more radiation dose to the patient
Fluoroscopic systems with different dose settings allow the user flexibility from patient to patient to adjust the compromise between contrast resolution and patient exposure

57. Noise and Contrast
Comparison of x-ray noise amplitudes in coronary angiograms acquired at fluoroscopic (2 µR per frame) (a) and angiographic (16 µR per frame) (b) exposure levels.

58. Noise and Contrast
16 µR per frame. Note improved resolution and contrast due to the higher exposure.

59. Digital Image Quality
Effect of Matrix Size.
512 x 512 matrix

60. Digital Image Quality
Effect of Matrix Size.
256 x 256 matrix

61. Digital Image Quality
Effect of Matrix Size.
128 x 128 matrix

62. Digital Image Quality
Effect of Matrix Size.
64 x 64 matrix

63. Digital Image Quality Gray Levels at a constant 512 x 512 matrix size.
256 Grey Levels (8 bit)

64. Digital Image Quality Gray Levels at a constant 512 x 512 matrix size.
4 Grey Levels (2 bits)

65. Digital Image Quality Gray Levels at a constant 512 x 512 matrix size.
8 Grey Levels (3 bits)

66. Image Quality Temporal Resolution
Fluoroscopy has excellent temporal resolution, that is over time
Blurring in the time domain is typically called image lag
Lag implies that a fraction of the image data from one frame carries over into the next frame
Video cameras such as the vidicon demonstrate a fair amount of lag

67. Image Quality Temporal Resolution
Lag in general is undesirable, beneficial for DSA
Frame averaging improves contrast resolution at the expense of temporal resolution
With DSA and digital cine, cameras with low-lag performance (plumbicons or CCD cameras) are used to maintain temporal resolution

68. Fluoroscopy Suites Gastrointestinal Suites
R and F room, large table that can be rotated from horizontal to vertical to put the patient in a head-down or head-up position
II above or under the table, spot film device usually there
Remote Fluoroscopy Rooms
Designed for remote operation by the radiologist
Tube above table, II under table
Reduce dose to the physician and no lead apron needed
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 250.

69. Fluoroscopy Suites Peripheral Angiography Suites
Table floats, allows patient to be moved from side to side and head to toe
C-arm or U-arm configuration
30 to 40 cm image intensifier used
Power injectors are normally ceiling- or table-mounted
Cardiology Catheterization Suite
Similar to angiography suite, 23 cm II used to permit more tilt in cranial caudal direction
Cine cameras used, biplane rooms common
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 250.

70. Fluoroscopy Suites Biplane Angiographic Systems
Two complete x-ray tube/II systems used, PA and Lateral
Simultaneous acquisition of 2 views allows a reduction of the volume of contrast media injected in patient
Portable Fluoroscopy- C Arms
C-Arm devices with an x-ray tube placed opposite from the II
18-cm (7-inch) and 23-cm (9-inch) and several other field sizes available
Operating rooms and ICUs
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 250.

71. Radiation Dose Patient Dose
The maximum exposure rate permitted in the US is governed by the Code of Federal Regulations (CFR), and is overseen by the Center for Devices and Radiological Health (CDRH), a branch of the Food and Drug Administration (FDA)
The maximum legal entrance exposure rate for normal fluoroscopy to the patient is 10 R/min
For specially activated fluoroscopy, the maximum exposure rate allowable is 20 R/min

72. Radiation Dose Patient Dose
Typical entrance exposure rates for fluoroscopic imaging are
About 1 to 2 R/min for thin (10-cm) body parts
3 to 5 R/min for the average patient
8 to 10 R/min for the heavy patient
Maximum dose at 120 kVp for most vendors
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 252.

73. Dose to Personnel
Rule of Thumb: standing 1 m from the patient, the fluoroscopist receives from scattered radiation (on the outside of apron) approximately 1/1,000 of the exposure incident upon the patient
The scatter field incident upon the radiologist while performing a fluoroscopic procedure is shown
A radiologist of average height, 178 cm (5’10”) is shown overlaid on the graph and key anatomic levels are indicated
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 253.

74. Dose to Personnel
The dose rate as a function of height above the floor in the room is shown for 6 different distances D, representing the distance between the edge of the patient and the radiologist
80 kVp beam and 20 cm patient thickness assumed for calculation
Roentgen-area product (RAP) or dose-area product (DAP) meters can be used to provide real-time estimate of the amount of radiation the patient has received
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 253.

75. Additional Reading
Additional topics on digital fluoroscopy and digital subtraction angiography can be found at the RSNA Education Portal.