chapter 7: digital applications and image enhancements
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Chapter 7: Digital Applications and Image Enhancements
In the early days of digital imaging, computer memory was insufficient forcapturing dynamic images digitally,but one use, DSA, became popular in the 1980’s, and is still used today.
Types of Subtraction
Serial Angiography
Zero Film Mask
+
ContrastFilm
Plain Film Subtraction
=
What kind of film?
Subtraction Film
What kind of film?Subtraction Print
Film
Mask Mode DSA
Temporal Modes of DSA
A. Mask Mode
B. Time Interval Difference
Reregistrationor
Pixel Shifting
Misregistration
Time Interval Difference (TID) DSA
Finished Products
Etc.
Utilizes Remasking
1. Temporal Subtraction
Modes of DSA
A. Mask Mode
B. TID
1. Temporal Subtraction
Modes of DSA
A. Mask Mode
B. Time Interval Difference
2. Energy Subtraction
PhotoelectricAbsorbtion
PhotoelectricAbsorbtion
K
L
I53K Shell Binding Energy of 33 keV
Pro
bab
ilit
y of
Ab
sorb
tion
keV
33 keV
MuscleBone
Iodine
32 keV34 keV
32 32 32
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Energy Subtraction in ActionGlobules or
Pixels
Spacesbetween
the Globulesor pixels
(where imagedata is lost)
Colored globules/pixels represent contrast in vessel.Numbers represent digital values (shades of gray)
Energy Subtraction In Reality
70 to 90 kVp
60 to 130 kVp
Via pulsed beams or flywheels
Bottom Line: Because the kVp must behigher than optimal to penetrate the part,energy subtraction is not a magic bullet.
1. Temporal Subtraction
Modes of DSA
A. Mask Mode
B. Time Interval Difference
2. Energy Subtraction
3. Hybrid Subtraction
What is the difference?
1011
101110114 bits vs. 8 bits
Contrast Resolution as a Function of the Processor
Windowing
The human eye candistinguish 32 shadesof gray.
25
It’s like contrast and brightness, but it’s not.
32
Windowing
25
But a digitized image may contain thousands of shadesof gray, known as the dynamic range
210
102432
Bone = + 1000
CT Numbers (Hounsfield Units)
Based on Water = 0
Air = - 1000 CT numbers (HU), express attenuation values relative to water
The range of displayed pixel values
Window Width
For example: A window width of 5
The range of displayed pixel values
Window Width
For example: A window width of 5
A narrow window is the digitalequivalent of a short scale ofcontrast on a radiograph
The range of displayed pixel values
Window Width
For example: A window width of 500
A wide window is the digitalequivalent of a long scale ofcontrast on a radiograph
The middle of the range of any given window width
Window Center (or level)
At a center of 2
For example: A windowwidth of 5
0
7
-7
12345
*012
Window Center (or level)
At a center of - 2
A window width of 5
0
7
-7
*
*
This CT section through the abdomenwas windowed with a width of 300 shadesof gray
The median number(the level or center)is 2 HU below water,-2
A soft tissue window set at a width of 110, at a levelof 43
The same CT section of the head set at a width of 2010,at a level of 800
Note the fracture through the frontal sinus
Width 2290Level 907
Bone windows demonstratingtrauma to the left orbit, maxilla, and sinuses.
Two windows of the same section through the thorax
Width 1269Level 202
Width 1269Level 2
DSA images usenarrow windows toenhance the contrastdifference betweenthe vessel and thesurrounding tissue
Double Windows
A separate window width and center of the lungs is cut and pasted into a soft tissue windowed image of the thorax.
Heart
Diaphragm
Lung tissue
Bronchi
Loss of image information at interface
Annotation
Gray Scale Inversion
SPR CT Scout
CT of Abdomen
Thumbnails, or tokenimages are especiallyuseful for simultaneousviewing of numerousexaminations on a patient,on a PACS workstation
Region of Interest (ROI)
The ROI cursor may be shaped and sized to select an area of animage
Using the ROI cursor to sample CT numbers
This ROI has beensized to measure adensity in the right kidney.
The mean density of 15.9 indicates a fluidfilled cyst.
Sampling the Voxel in Cross Sectional Images
Cross sectional images have depth, which is selected priorto a scan. When a two dimensional section is viewed, thedensity of each pixel actually represents all the tissues inthe volume of the section. This density represents the volume element, or the voxel.
Three contiguous CTsections with parts ofa frontal lobe mass ineach one
The CT numbers from these samples would represent an average of the mass and healthy tissue.
The CT number from this sample would represent the true density of the mass.
Histograms
In this example histogramswere superimposed over sixsections of a neck for the purpose of identifying areasin the field that need bolusmaterial to equalize theexposure.
Without a bolus, the carotidarteries would have beenover-exposed on this DSA.
Histograms
Linear measurement through a lesion, plotted as a histogram
Linear measurements
6.3 cm hematoma from aruptured aortic aneurysm
Diameter of commoniliac arteries
Bone densitometry performed on CT softwareusing the SPR mode of CT
Bone densitometry
Repeated applicationsof a high passfilter demonstratesthe effect of suppressing lowspatial frequencies
No filter High
Ultra High Ridiculously High
No filter
Low pass or smoothingfilter is similar to aconvolving filter
Edge sharpening filterhas an algorithm similarto a high pass filter
Selected by ROI
Electronic Zoom
Electronic Zoomand bone window of IAC
Images zoomed toexcess show pixelation,or tiling.
This image looks muchlike early CT scansthat used a small matrixsize
A small matrix size, andzoom, produce the sameeffect.
Magnification is often available in an ROI window that is moved around the screen, providing zoom on the fly.
Mag off
Mag on
Zoomed to thepoint where lossof spatial resolutionis visible, though pixelation is notclearly evident.
An image acquired with a small field of viewis preferable to a zoomed image
Large FOV Small FOV
Colorization
Nuclear Medicinestudy demonstrating myocardial ischemia
Color enhancementdemonstrates the profusion of blood inthe heart muscle
Multiple Plane Reconstructions
3 sections of the headreconstructed fromaxial sections done infine increments
Pelvis reconstructed from 1 cm increments
3D or Volumetric Imaging
Test image demonstrates theuse of shading to create the appearanceof depth.
3D or Volumetric Imaging
After loading volumetricdata, an image may be rotated on the screen, and displayed from any perspective
3D or Volumetric Imaging
3D images help surgeonsvisualize the extent ofinjury.
This trauma victim hasnumerous facial fractures. The extent ofinjury is easier to appreciate in 3D
3D or Volumetric Imaging
3D or Volumetric Imaging
CT scan of Joseph Merrick’s (the elephant man) skull.
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