PACS- Chapter 3

Pre-processing

Takes place in the computer where the algorithms determine the image histogram

Algorithm

Step by step mathematical procedures by a computer to execute a specific task

Basic steps of exposure recognition

Exposure field recognition

Histogram creation

Grayscale rendition

Exposure data recognition

Program searches for anatomy recorded on the IR:

Finding collimation edges

Eliminating scatter outside collimation

How much of the plate should be exposed?

25%

Why would exposure recognition fail?

Too much scatter

Metallic components

Histogram

A graph plotting the pixel count for each brightness level (density) within an entire image

The values to the left of a histogram

Darker

The values to the right of a histogram are

Lighter

Wide histogram

High contrast

Low energy kvp

Narrow histogram

Low contrast

High energy kvp

Look up table

A histogram of the luminescence values derived during image acquisition

Automatic rescaling

Occurs in effort to display the pixels for the area of interest

A typical PSP plate loses approximately ____% of its stored energy with __ hours after exposure

25%, 8 hours

The longer the electrons are stored

The more energy they lose

Flat panal detector systems lose fewer signals to light spread than

Conventional radiography

Aliasing

Occurs in digital imaging when spatial frequency is greater than the nyquost frequency and when sampling occurs less than twice per cycle

Results in a moiŕe effect (the same effect can occur with grid errors)

Aliasing

Nyquist theorem

Applied to digital images to ensure that sufficient signal sampling occurs for maximum resolution

Sampling

The measurement of brightness level

Pixel pitch

The physical distance between adjacent pixels and is measured from center to center

Contrast enhancement parameters

Convert the digital input data to an image with appropriate brightness and contrast

Spatial frequency resolution is controlled by:

Focal spot

OID

Computer algorithms

Spatial frequency filtering is used to:

Sharpen

Smooth

Blur

Reduce noise

Pull elements of interest from an image

Smoothing

Aka low-pass filtering

Results from averaging of the frequency of each pixel with surrounding pixel values to remove high frequency noise

Window level

Controls how light or dark the image is (brightness)

Window width

Controls the ratio of black to white (contrast)

Veil glare

Causes oversensitization of a chemical with the eye called rhodopsin, causes temporary white light blindness

Automatic shuttering

Used to blacken out the white collimation borders

Image stitching

Allows multiple images to be joined when the anatomy is too large for one exposure

Querying

Retrieval of radiographic studies can be specific to a patient, examination date, or type etc.