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Section III: Chapter 6 1
RADT 3463 - COMPUTERIZED IMAGING RADT 3463 - COMPUTERIZED IMAGING
RADT 3463 Computerized Imaging 1
Section III: Chapter 6 3
SECTION III - CHAPTER 6
DIGITAL FLUOROSCOPY
RADT 3463 COMPUTERIZED IMAGING
3 RADT 3463 Computerized Imaging
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ACKNOWLEDGEMENTS This presentation is a professional collaboration of
development time prepared by:
Rex Christensen
Terri Jurkiewicz
and
Diane Kawamura
References and images are gathered from many sources including
those copyrighted by Elsevier / Mosby Publishing as they appear in:
Bushong, S. C. (2008). Radiologic science for
technologists: Physics, biology, and protection, 9th ed.
Chapter 27, St. Louis, MO: Elsevier Mosby.
Seeram, E. Digital Radiography: An Introduction, Delmar
Cengage Learning.
Section III: Chapter 6 RADT 3463 Computerized Imaging 4
DEFINITION - FLUOROSCOPY
• An imaging modality that produces dynamic or moving images, displayed in real time.
• Study of anatomical structures and the motion of organs and contrast media in organs and blood vessels.
• Identifies the function of organs and blood vessels.
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CONVENTIONAL FLUOROSCOPY PRINCIPLES
• X-ray source -> Image intensifier -> Video camera -> Television monitor (analog signal)
• 30 frames per second (fps)
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X-RAY TUBE AND GENERATOR
• Continuous – 30 fps @ 33 ms
• Pulsed – lower patient dose (3-10 ms/image), less blurring
• High frequency generators
• Low mA (1-3 mA) and high kV (65-120 kV)
• Switch from fluoroscopic mode to radiographic mode (spot films, radiographic cassettes)
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IMAGE INTENSIFICATION
• The brightening of the fluoroscopic image using an image intensifier.
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IMAGE INTENSIFIER TUBE
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IMAGE INTENSIFICATION
• Parts of the image intensifier
(enclosed in a vacuum tube)
include:
– Input screen (x-ray to light) • Phospher – Cesium Iodide CsI
– Photocathode (photoelectrons) • Phospher - Antimony Cesium SbCs
– Electrostatic lens (focus) • 20 – 30 kV
– Output screen (light - Increase) • Phospher – Zinc Cadmium Sulfide ZnCdS
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IMAGE INTENSIFICATION
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BRIGHTNESS GAIN (BG)
• Increase in brightness from the input phospher to the output phospher (5,000 – 30,000)
• BG = Minification Gain (MG) x Flux Gain (FG)
MG = Diameter of the input screen2
Diameter of the output screen2
FG= Number of light photons at the output screen
Number of light photons at the input screen
BUT………….
Section III: Chapter 6 RADT 3463 Computerized Imaging 13
CONVERSION FACTOR (CF)
• The Brightness Gain (BG) method has been replaced by the Conversion Factor (CF)
• This is the light gain at the output phospher
• CF = Luminance of the output screen
Exposure rate at the input screen
Luminance is measured in candela/square meter (Cd/m2)
Exposure rate is measured in milliroentgens/second (mR/sec)
Conversion Factor (CF) ranges between 50-300. The higher being more efficient.
Section III: Chapter 6 RADT 3463 Computerized Imaging 14
FLUX GAIN • 1000 light photons at the
photocathode • from 1 x-ray photon • photocathode decreased
the # of ë’s so that they could fit through the anode
• Output phosphor = • 3000 light photons (3 X
more than at the input phosphor!)
• This increase is called the flux gain
Section III: Chapter 6 RADT 3463 Computerized Imaging 15
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Magnification
• Magnification enhances the image to help improve diagnostic interpretation.
• Improves spatial resolution
• Controlled by the input
screen diameter
Section III: Chapter 6 RADT 3463 Computerized Imaging 16
Multi-field Units
• Allows selection of different input phosphor sizes
• Types of multi-field units:
– Dual focus - 9/6 inches
– Tri focus - 12/9/6 inches
• Smaller input magnifies output by moving focal point away from output
• Greater voltage to electrostatic lenses
– Increases acceleration of electrons
– Shifts focal point away from anode
• Requires more x-rays to maintain brightness
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Intensifier Format and Modes
Note focal point
moves farther from
output in mag
mode
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Magnification and Patient Dose
• Magnification is used to enlarge small structures or to penetrate through larger parts
• Patient dose is INCREASED in the magnification mode
• Dose is dependent on the size of the Input Phosphor (IP)
• FORMULA:
Section III: Chapter 6 RADT 3463 Computerized Imaging 19
MAGNIFICATION MODE FORMULA
IP OLD SIZE
IP NEW SIZE = % mag
Section III: Chapter 6 RADT 3463 Computerized Imaging 20
PT dose in MAG MODE
IP OLD SIZE 2
IP NEW SIZE 2 = ↑(x) pt dose
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Image Quality Characteristics
• Spatial Resolution
• Contrast Ratio
• Noise
• Artifacts
Section III: Chapter 6 RADT 3463 Computerized Imaging 22
Spatial Resolution • The ability to resolve fine details of the object
being viewed (patient)
• Input screen is convex – better resolution in the center
• Resolution gets better as the Input diameter gets smaller
• Measured in line pairs per mm (lp/mm) - how close lines can be to each other and still be visibly resolved. The more line pairs the better the resolution (spatial frequency).
Section III: Chapter 6 RADT 3463 Computerized Imaging 23
Spatial Frequency
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• One line pair = the line and an interspace the
same width as the line
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Spatial Frequency
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An imaging system with high spatial
frequency has better spatial resolution
APPROXIMATE SPATIAL RESOLUTION - MEDICAL IMAGING SYSTEMS
Gamma camera 0.1 lp/mm
Magnetic resonance imaging 1.5 lp/mm
Computed tomography 1.5 lp/mm
Diagnostic sonography 2 lp/mm
Fluoroscopy 3 lp/mm
Digital radiography 4 lp/mm
Computed radiography 6 lp/mm
Radiography 8 lp/mm
Mammography 15 lp/mm
Line pair gauges
GOOD RESOLUTION POOR RESOLUTION
Section III: Chapter 6 RADT 3463 Computerized Imaging 26
Spatial Resolution
• A 1024 x 1024 image matrix
is sometimes described as a
1000-line system
• Spatial resolution (how
much information is stored
within the space given) is
determined by both the
image matrix and by the
size of the image intensifier.
Section III: Chapter 6 RADT 3463 Computerized Imaging 27
Pixel Size = Image intensifier Size / Matrix
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Noise
• Low mA produces high amount of noise
• If you increase the mA to minimize the noise you increase patient dose.
Section III: Chapter 6 RADT 3463 Computerized Imaging 28
How Noise Effects Contrast
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Artifacts • Image lag – continuous emission of light from the screen after
the radiation beam has been turned off.
• Vignetting - reduction of an image's brightness or saturation at the periphery compared to the image center.
• Veiling glare – light is scattered in the intensifier tube
• Distortion artifacts:
– Pincushion
– S distortion
– Barrel Distortion
Section III: Chapter 6 RADT 3463 Computerized Imaging 30
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Artifacts – Vignetting
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FALL-OFF OF BRIGHTNESS
AT PERIPHERY (EDGES)
OF THE IMAGE
Artifacts – Veiling glare
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Scatter in the form of
x-rays, light &
electrons can
reduce contrast of
an image intensifier
tube.
Artifacts – Distortion
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Geometric problems in
shape of input screen
• Pincushion –
rectangular grid used with
a round input screen
• S distortion –
electromagnetic field is
close to the intensifier
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Fluoroscopic Television Chain
Section III: Chapter 6 RADT 3463 Computerized Imaging 34
Fluoroscopic Television Chain
Video Camera
• Television pick-up camera
• Charged Couple Device (CCD) – more common
– More compact
– No image lag
– No spatial distortion
– High dynamic range 3000:1
• Connected to the image intensifier by an image distributor
• Converts light to an electrical signal
Display Monitor
• Cathode Ray Tube (CRT)
• Liquid Crystal Display (LCD)
• Scanning: – Interlaced - odd/even
– Progressive – sequentially (important in digital fluoro)
Section III: Chapter 6 RADT 3463 Computerized Imaging 35
VIDEO CAMERA - CHARGE-COUPLED DEVICE
Charge-coupled
device is mounted
to the output
phosphor of the
image-intensifier
tube and is coupled
through fiber optics
or a lens system
Section III: Chapter 6 RADT 3463 Computerized Imaging 36
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DISPLAY MONITOR
Conventional Fluoroscopy System Digital Fluoroscopy System
Interlaced Mode Progressive Mode
Signal-to-noise ratio 200:1 Signal-to-noise ratio 1000:1
Conventional Fluoroscopy System
• Usually a 525-line system
Limitations restrict application in digital techniques
1. Interlaced mode of reading the target of the TV
camera can significantly degrade a digital
image
2. Conventional TV camera tubes are relatively
noisy (compare signal-to-noise ratios on table)
Section III: Chapter 6 RADT 3463 Computerized Imaging 37
DISPLAY MONITOR
Interlaced Mode
• 2 fields
• 525-line system/2 = 262½
lines
• 262½ lines are read
individually in 1/60 s (17 ms)
to form a 525-line video
frame in 1/30 s (33 ms)
Section III: Chapter 6 RADT 3463 Computerized Imaging 38
DISPLAY MONITOR
Progressive Mode
• The video signal is read and the
electron beam of the TV camera
tube sweeps the target
assembly continuously from top
to bottom in 33 ms
• The video image is formed
similarly on the TV monitor with
no interlace of one field with
another occurs
Section III: Chapter 6 RADT 3463 Computerized Imaging 39
Interlaced Mode
Progressive Mode
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DISPLAY MONITOR
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Compared to cathode ray
tubes (CRT), flat panel
monitors are:
1. Easier to view
2. Easier to manipulate
3. Provide better images
4. Light in weight
5. Easy to See
6. Easy to mount or
suspend in an
angiographic room
Digital Fluoroscopy with Image Intensifiers
Section III: Chapter 6 RADT 3463 Computerized Imaging 41
• Projecting a radiographic image on an image-
intensifying fluorescent screen coupled to a
digital video image processor.
DIGITAL FLUOROSCOPY
Advantages
• Low dose fluoroscopic imaging (digital average,
last frame hold)
• Pulsed fluoroscopy and variable frame rate
• Speed of image acquisition
• Postprocessing to enhance image artifacts
• Uses hundreds of mA settings compared to 5Ma
or less in conventional
• Digital Subtraction Angiography (DSA) and non
subtraction acquisition and display
• Image distribution and archiving, PACS
Section III: Chapter 6 RADT 3463 Computerized Imaging 42
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DIGITAL FLUOROSCOPY IMAGING SYSTEM If x-ray tube were energized continuously
thermal overloading would cause tube failure
patient dose would be high or exceeded quickly
Pulse-Progressive Fluoroscopy
Images obtained by pulsing the x-ray beam
Section III: Chapter 6 RADT 3463 Computerized Imaging 43
DIGITAL FLUOROSCOPY IMAGING SYSTEM
PULSE-PROGRESSIVE FLUOROSCOPY
INTERROGATION TIME
• Time required for
the x-ray tube to be
switched on and
reach selected
levels of kVp and
mA
EXTINCTION TIME
• Time required for
the x-ray tube to be
switched off
High frequency generators
must be fast enough to
have interrogation and
extinction times of less than 1 ms
Section III: Chapter 6 RADT 3463 Computerized Imaging 44
DIGITAL FLUOROSCOPY IMAGING SYSTEM
PULSE-PROGRESSIVE FLUOROSCOPY
DUTY CYCLE
• The fraction of time the x-ray tube is energized
• The illustration shows the x-ray tube is
energized for 100 ms every second which
equals a duty cycle of 10%
(100/1000 = 0.1 = 10%)
• Can result in significant
radiation dose reduction
Section III: Chapter 6 RADT 3463 Computerized Imaging 45
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ADC – Analog Digital Convertor
• Receives the output video signal (Analog) from the video camera and converts it into Binary code (0’s and 1’s) – digital language.
• Each 0 or 1 is called a BIT (BInary DigiT)
Section III: Chapter 6 RADT 3463 Computerized Imaging 46
BIT Depth
• 1 bit (21) = 2 tones
• 2 bits (22) = 4 tones
• 3 bits (23) = 8 tones
• 4 bits (24) = 16 tones
• 8 bits (28) = 256 tones
• 16 bits (216) = 65,536 tones
• 24 bits (224) = 16.7 million tones
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BIT Depth and Dynamic Range (Shades of Gray)
N = 2n
N = Number of values
n = number of bits
BIT DEPTH POWER DYNAMIC
RANGE BIT DEPTH POWER
DYNAMIC
RANGE
2 21 2 10 210 1024
4 24 16 12 212 4096
6 26 64 14 214 16,384
8 28 256 16 216 65,536
9 29 512 20 220 1,048,576
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BIT Depth and Imaging Modalities
• With digital imaging systems, dynamic range is
identified by the bit capacity of each pixel
DIGITAL MEDICAL IMAGING SYSTEMS DYNAMIC RANGE
Imaging System Dynamic Range
Bit Depth Shades of Gray
Diagnostic Sonography 28 256
Nuclear Medicine 210 1024
Computed Tomography 212 4096
Magnetic Resonance Imaging 212 4096
Digital Radiography 214 16,384
Digital Mammography 216 65,536
Section III: Chapter 6 RADT 3463 Computerized Imaging 49
BIT Depth
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DIGITAL FLUOROSCOPY IMAGING SYSTEM
Operating Console: The right side module
contains:
• Computer-interactive
video controls
• A pad for cursor and
region-of interest
(ROI) manipulation • May use trackball, joystick
or a mouse instead
Section III: Chapter 6 RADT 3463 Computerized Imaging 51
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DIGITAL FLUOROSCOPY IMAGING SYSTEM
Monitors • Two or more monitors are
used
• One is used to edit • Patient data
• Examination data
• Annotate final image
• One is used for subtraction
images
Section III: Chapter 6 RADT 3463 Computerized Imaging 52
Computers in Digital Fluoroscopy
• Digital fluoroscopy employs the
use of minicomputers and
microprocessors
• Computer capacity is an
important factor in determining:
1. Image quality
2. The manner and speed of
image acquisition
3. The means of image
processing and manipulation
Section III: Chapter 6 RADT 3463 Computerized Imaging 53
DIGITAL SUBTRACTION ANGIOGRAPHY
• Usually image storage occurs in primary
memory where data acquisition and transfer can
be as rapid as 30 images per second
• A system might be capable of acquiring 30
images per second in the 512 x 512 matrix
mode
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DIGITAL SUBTRACTION ANGIOGRAPHY
Data Transfer Limitation
• If a higher spatial resolution image is required
and the 1024 x 1024 mode is requested, then
only 8 images per second can be acquired
• Limitation on data transfer is imposed by the
time required to transport enormous quantities of
data from one segment on memory to another
Section III: Chapter 6 RADT 3463 Computerized Imaging 55
Digital Fluoroscopy with Flat-Panel
Detectors (FPD)
Section III: Chapter 6 RADT 3463 Computerized Imaging 56
• FPDs are used in regular radiographic imaging.
• When used in Fluoroscopy it is referred to as Dynamic FPD
IMAGE CAPTURE - FLAT PANEL IMAGE RECEPTOR
Flat Panel Image Receptors (FPIRs)
• Composed of cesium iodide (CsI) / amorphous
silicon (a-Si) pixel detectors
• Much smaller and lighter and is manipulated
more easily than an image intensifier
• Provides easier patient manipulation and
radiologist / technologist movement
• There are no radiographic cassettes
Section III: Chapter 6 RADT 3463 Computerized Imaging 57
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IMAGE CAPTURE - FLAT PANEL IMAGE RECEPTOR
• In contrast to an image-intensifier tube, a flat
panel image receptor is insensitive to external
magnetic fields
May allows advanced application in:
• Cardiology Radiology
• Neurovascular Radiology
• Interventional - Vascular Radiology
• Image-guided catheter - magnetic tip in vessels is
manipulated remotely by two large steering magnets
located on either side of the patient
Section III: Chapter 6 RADT 3463 Computerized Imaging 58
IMAGE CAPTURE - FLAT PANEL IMAGE RECEPTOR
DF equipped with
a flat panel image
receptor
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POSTPROCESSING – LAST IMAGE HOLD
Section III: Chapter 6 RADT 3463 Computerized Imaging 60
Displays the last image continuously when the x-ray
beam is turned off
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POSTPROCESSING – TEMPORAL FRAME AVERAGING
• Averages the current frame with previous frames to reduce the noise in the image.
• Reduces noise by 44%
• This is sometimes called “over-sampling” an image
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Postprocessing - Edge Enhancement
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A.Original image
B.Blurred image
C.A and B digitally
subtracted
D.C is added to the
original (A) image
to produce the
edge-enhanced
image
Postprocessing Images
• Image contrast can be enhanced
electronically using subtraction
techniques
• Subtraction techniques provide
instantaneous viewing of the subtracted
image during passage of a bolus of
contrast medium
Section III: Chapter 6 RADT 3463 Computerized Imaging 63
Digital fluoroscopy provides better
contrast resolution through
postprocessing of image subtraction.
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DIGITAL SUBTRACTION ANGIOGRAPHY
DSA Techniques
1. Temporal Subtraction (used most often)
2. Energy Subtraction
3. Hybrid Subtraction (combines temporal and
energy subtraction)
Section III: Chapter 6 RADT 3463 Computerized Imaging 64
DIGITAL SUBTRACTION ANGIOGRAPHY
COMPARISON OF TEMPORAL AND ENERGY SUBTRACTION
TEMPORAL SUBTRACTION ENERGY SUBTRACTION
A single kVp setting is used Rapid voltage switching is required
Normal x-ray beam filtration is
adequate
X-ray beam filter switching is preferred
Contrast resolution of 1 mm at 1% is
achieved
High x-ray intensity is required for
comparable contrast resolution
Simple arithmetic image subtraction is
necessary
Complex image subtraction is
necessary
Motion artifacts are a problem Motion artifacts are greatly reduced
Total subtraction of common structures
is achieved
Some residual bone may survive
subtraction
Subtraction possibilities are limited by
the number of images
Many more types of subtraction
images are possible
Section III: Chapter 6 RADT 3463 Computerized Imaging 65
Temporal Subtraction
• A number of computer-assisted techniques
where an image obtained at one time is
subtracted form an image obtained at a later
time (temporal = time)
• In the interval period, if contrast material is
introduced into the vasculature, the subtracted
image will contain only the vessels filled with
contrast material
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DIGITAL SUBTRACTION ANGIOGRAPHY
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Temporal Subtraction
Two methods are commonly used to obtain the
temporal subtracted image are:
1. The mask mode
2. The time interval difference mode (TID)
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DIGITAL SUBTRACTION ANGIOGRAPHY
Mask Mode –Temporal
Subtraction
Section III: Chapter 6 RADT 3463 Computerized Imaging 68
Mask mode results in successive
subtraction images of contrast vessels.
DIGITAL SUBTRACTION
ANGIOGRAPHY
Mask Mode – Temporal Subtraction
A, The preinjection mask.
B, A postinjection image.
C, Image produced when the preinjection mask is
subtracted from the postinjection image.
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DIGITAL SUBTRACTION ANGIOGRAPHY
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Time-Interval
Difference Mode –
Temporal
Subtraction
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Time-Interval Difference mode produces
subtracted images form progressive
masks and following frames
DIGITAL
SUBTRACTION
ANGIOGRAPHY
Time-Interval Difference Mode – Temporal
Subtraction
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DIGITAL SUBTRACTION ANGIOGRAPHY
Time-Interval
Difference Mode –
Temporal
Subtraction
Misregistration
Artifacts - due to
patient motion occurring
between the mask
image and a
subsequent image.
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DIGITAL SUBTRACTION
ANGIOGRAPHY
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Energy Subtraction
• Based on abrupt
change in photoelectric
absorption at the K
edge of contrast media
compared with that for
soft tissue Illustration shows the
probability of x-ray
interaction with iodine,
bone, and muscle as a
function of x-ray energy
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DIGITAL SUBTRACTION
ANGIOGRAPHY
Energy Subtraction
• When the incident x-ray energy is sufficient to
overcome the K-shell electrons binding energy
of iodine, an abrupt and large increase in
absorption occurs
• Graphically, this increase is known as the K
absorption edge
Section III: Chapter 6 RADT 3463 Computerized Imaging 74
The probability of photoelectric
absorption in all three decreases
with increasing x-ray energy.
DIGITAL SUBTRACTION
ANGIOGRAPHY
Hybrid Subtraction
• Combines temporal and energy subtraction
techniques.
• Produces highest quality digital fluoroscopy
images if patient motion can be controlled Section III: Chapter 6 RADT 3463 Computerized Imaging 75
DIGITAL SUBTRACTION ANGIOGRAPHY
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PATIENT DOSE – DIGITAL FLUOROSCOPY
• Should result in reduced patient dose
• Uses pulsed beams to fill one or more 33-ms
video frames
• mA settings are higher with digital fluoroscopy
but the fluoroscopic dose rate is lower than
continuous analog fluoroscopy
Section III: Chapter 6 RADT 3463 Computerized Imaging 76
PATIENT DOSE – DIGITAL FLUOROSCOPY
• With digital fluoroscopy, static images are
made with lower dose per frame than
those attained with a 100 mm spot film
camera
Section III: Chapter 6 RADT 3463 Computerized Imaging 77
PATIENT DOSE – DIGITAL FLUOROSCOPY
• Digital spot images are easy to acquire
making it possible to make more than the
needed exposures
• Even with more exposures, the patient
dose is lower with digital fluoroscopy
Section III: Chapter 6 RADT 3463 Computerized Imaging 78
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DIGITAL SUBTRACTION ANGIOGRAPHY
PATIENT DOSE Approximate Patient Dose in Representative Fluoroscopic Examinations
Patient Dose
Imaging Mode Conventional Digital
5 minutes
fluoroscopy
20 rad
(200 mGy)
10 rad
(100 mGy)
3 spot films-
normal mode
0.6 rad
(6 mGy)
0.2 rad
(2 mGy)
3 spot films-
mag 1 mode
1.0 rad
(10 mGy)
0.3 rad
(3 mGy)
Total dose 21.6 rad
(216 mGy)
10.5 rad
(105 mGy)
Section III: Chapter 6 RADT 3463 Computerized Imaging 79
QUESTIONS??