Fluoroscopy

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
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 PerformanceConversion 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 PerformanceBrightness 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 PerformanceBrightness 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
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
Magnification
LARGE NON-MAG FoV e.g., 12 INCH
SMALL, MAG FoV e.g., 6 INCH
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
TELEVISION IMAGE
HORIZONTAL DIRECTION
RASTER LINE
VERTICALDIRECTION
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 1.8-2.2 LP/mm 2.7-3.2 LP/mm
• 6 INCH 2.5-2.8 LP/mm 3.7-4.5 LP/mm
• 4.5 INCH 3.2-3.7 LP/mm 5.0-6.0 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
41
FLAT PANEL-LIGHT SENSOR
Very High Fill Factor
Fill Factor
FROM GE
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 10242 or 20482 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 objects 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 QualityContrast 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 QualityContrast 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 QualityTemporal 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 QualityTemporal 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 (510)
  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.
• http//www.rsna.org/education/archive/aapm/toc.htm
  lfluoroscopy