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

32 32 32

32 32 32

32 32 32

32 32 32

32 32 32

34 34 34

34 34 34

34 34 34

34 34 34

34 34 34

34 34 34

34 34 34

34 34 34

34 34 34

34 34 34

34 34 34

34 34 34

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.