Image Display

But first a review

Remember the 3 main steps

• 1. Data Acquisition• 2. Image Reconstruction• 3. Image Display

Review

Image formation reviewA ray is the pathway of a portion of the x-ray beam from one specific focal-spot position to a specific detector position.

As the ray passes through the body, it measures the total x-ray attenuation (or penetration) along it's path. 

This is the data recorded by the detector.

A view is made-up of many individual rays

Ref. Sprawls.com

Image formation review

A view is made-up of many individual rays

As the x-ray beam is scanned around the body, forming many views, the data recorded by the detectors are stored in computer memory for later image reconstruction.

Information from the view is then projected onto a matrix

Ref. Sprawls.com

Beam Attenuation• Structures in a CT image are represented

by varying shades of gray

• The creation of these shades of gray is based on basic radiation principles

• The degree to which an x-ray beam is reduced by an object is referred to as attenuation

Beam Attenuation (cont’d)• Photons that pass through objects

unimpeded are represented by a black area on the image

• When photons are completely absorbed by an object, the corresponding area on the image is white

• Areas of intermediate attenuation are represented by various shades of gray

Beam Attenuation (cont’d)• The number of photons that interact

depends on the object’s– Thickness– Density– Atomic number

A CT SCANNER IS BASICALLY A DENSITY MEASURING DEVICE !

FROM GE

DETERMINE THE X-RAY ATTENUATION OF ALL THE TISSUES IN EACH MATRIX ELEMENT. DETERMINE THE LINEAR ATTENUATION COEFFICIENT FOR EACH

MATRIX ELEMENT, X .

CT NUMBERS

x - water

water1000 x

In HOUNSFIELD UNITS (H.U.)

CT Number – Hounsfield Unit (HU)

• Each Voxel represents a numerical value – aka CT Number, which is the average of all the attenuation values within the voxel.

• This scale assigns water a HU value of 0. Air of -1000. Each number represents a shade of grey with +1000 as white and -1000 as black.

ReviewAir in the lungs is considered

a. High attenuation

b. Low attenuation

c. Intermediate attenuation

Display Monitors

• CT images are usually displayed on a black-and-white or color monitor

• Display devices can be either– A cathode-ray tube (CRT)– Some form of flat panel, such as TFT LCD

• Monitors consist of– The display device– Circuitry to generate an image from electronic signals– An enclosure

Display Monitors (cont’d)

• CRT monitors are heavier, bulkier, hotter, and less durable than the newer LCD monitors

• LCD monitors produce higher luminance and high spatial resolution

• Digital-to-analog converters change the digital signal from the computer memory back to an analog format so that the image can be displayed on the monitor

Monitors

• AWS – Acquisition workstation – typically 1 or 2 MP color monitor.

• RWS/PDM – Review workstation/Primary Diagnostic Monitor. 2, 3 or 5 MP monitor. DICOM GSDF calibration, Higher maximum luminance and higher monochrome bit depth -> better image contrast.

Storage format

• DICOM - Digital Imaging and Communications in Medicine

• Standard format for image storage. Uncompressed images.

• Lots of other info in header (technical factors, scanner info, technologist id!)

• Stored in PACS – Picture archiving and communications system (computer network and servers)

Window Settings

• Adjusting the window width and window level will change the way an image is

viewed on the monitor

Window Settings (cont’d)• Many studies require each image to be viewed at two or

more window settings

• “Ideal” window settings are somewhat subjective

• Imaging departments typically have an established setting for each type of examination

• However, because many factors have an effect, technologists must adjust settings in individual situations

Window Settings (cont’d)

• Images cannot be displayed with a different shade of gray for each HU because– There are more 2,000 Hounsfield values;

most monitors can only display 256 shades of gray

– Different Window & Level settings can bring out contrast in different areas of the image.

Gray Scale

• The gray scale is used to display CT images

• This system assigns a certain number of HU to each shade of gray– By convention, the gray scale assigns higher

HU values lighter shades of gray

Window Width

• The window width determines the number of HUs represented on a specific image– Values higher than the selected range will be

white– Values lower than the selected range will be

black

• Increasing the window width assigns more HU to each shade of gray

Window Width

• To illustrate, assume – we have just 10 shades of gray available– 300 is selected as the window width

• 300 density values will be represented on the image as a shade of gray

• All others will be either black or white

Window Width/Window Level

• If the window width is set at 300, which 300 Hounsfield values, from the more than 2,000 possible, will be displayed?

• Window width selects the quantity of HU

• Window level selects the range of HU

Window Level

• Selects the center CT value of the window width

• Window level is also called window center

• Window level determines which HUs are displayed as shades of gray

Window Level (cont’d)

• Continuing with the previous example, assume – 0 is selected as the level

• The HUs that are represented as a shade of gray on this image will range from –150 to 150

Review

• The selected window width is 400

• The selected window level is 50

• Which HUs are displayed on the image as shades of gray?a. –200 to 200b. –150 to 450c. –150 to 250d. 350 to 450

Answer

a. –150 to 250

General Rule for Setting Window Level

• The window level should be set at a point that is roughly the same value as the average attenuation number of the tissue of interest

Setting Window Width

• In general, wide window widths (500–2,000) are best for imaging tissue types that vary greatly, when the goal is to see all of the various tissues on one image, such as lung

• Wider window widths encompass greater anatomic diversity, but subtle density discrimination is lost

• Because wider window width settings decrease image contrast, they suppress the display of noise on an image

Review

• A window width of 90 is most likely used to display an image of thea. Lung

b. Femur

c. Brain

d. Abdomen

Answer

c. Brain

Region of Interest (ROI)

• An ROI is most often circular, but may be elliptic, square, or rectangular, or may be custom drawn by the operator

• Defining the size, shape, and location of the ROI is the first step in many display and measurement functions

HU Measurement and Standard Deviation

• HU measurements may be affected by volume averaging or image noise

• A cursor (+) displays a measurement of the HU of the pixel that the cursor covers

• Conversely, an ROI provides an averaged measurement of all of the pixels within the ROI– When an ROI is used, the standard deviation

is also displayed

Other Image Display Options

• Distance measurements

• Image annotation

• Reference image

• Image magnification

• Multiple image display

• Histogram

• Multiplanar and three-dimensional reformatting

WINDOW WIDTH

- 500 0 +500

WINDOW LEVEL

SIEMENS VOLUME ZOOM 4 SLICE LEVEL = 50 H.U. : WIDTH = 350 H.U.

SIEMENS VOLUME ZOOM 4 SLICE LEVEL = 166 H.U. : WIDTH = 350 H.U.

SIEMENS VOLUME ZOOM 4 SLICE LEVEL = -106 H.U. : WIDTH = 350 H.U.

SIEMENS VOLUME ZOOM 4 SLICE LEVEL = 50 H.U. : WIDTH = 2000 H.U.

GE LIGHTSPEED 16 SLICE LEVEL = 40 H.U. & WIDTH = 400 H.U.

GE LIGHTSPEED 16 SLICE LEVEL = -99 H.U. & WIDTH = 400 H.U.

GE LIGHTSPEED 16 SLICE LEVEL = 200 H.U. & WIDTH = 400 H.U.

GE LIGHTSPEED 16 SLICE LEVEL = 40 H.U. & WIDTH = 57 H.U.

GE LIGHTSPEED 16 SLICE LEVEL = 350 H.U. & WIDTH = 2000 H.U.