Digital Radiography and PACS Chapter 3

In both photostimulable phosphor (PSP) and flat-panel detector systems (FPD), after the x-ray photons have been converted into ___________, these signals are available for processing and manipulation.

electrical

Preproccessing

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

Postprocessing

Done by the technologist through various user functions

In image formation, using values at the extreme high and low ends of the exposure range would result in a........

low-density resolution

*To avoid this, exposure data recognition processes only the optimal density exposure range.

Data recognition program searches for anatomy recorded on the imaging plate such as

1. Finding collimator edges

2. Eliminating scatter outside collimation

Information within the collimated area is the signal used for image data. This is the source for a __________-____________ exposure data indicator.

vendor-specific

Failure of the system to find the collimation edges can result in.......

incorrect data collection

Equally important is centering anatomy to the center of the imaging plate. Ensures that appropriate recorded densities are located. Failure to do so could result in an image that is..............

too bright or too dark

Histogram

A graphical representation of exposure values collected from the imaging plate

The value of each tone is represented on the....

horizontal axis (of histogram)

The number of pixels in each tone is represented on the.....

vertical axis (of histogram)

Values to the left are _______; as tones vary towards the right, they get ___________. Middle=medium tones

darker;brighter (histogram)

A dark image will show the majority of its data on the?

left side. (histogram)

a light image will show the majority of its data on the?

right side (histogram)

Image Sampling

(order of)

1. Plate is scanned.

2. Image location and orientation are determined.

3. Size of the signal is determined.

4. Value is placed on each pixel.

5. A histogram is generated from the image data

Histogram is generated that allows system to find useful signal by locating the ___________ (S1) and ____________ (S2) signal within the anatomic regions of interest in the image.

minimum;maximum

Histogram identifies all densities on the imaging plate in the form of a graph:

X-axis is related to ____________ ____ ___________.

Y-axis displays the ___________ ___ ___________ for each exposure

amount of exposure; number of pixels

Histogram:

Graphic representation appears as a series of ___________ ____ __________ and has a pattern that varies for each body part.

peaks and valleys

Low energy (kVp) gives a _________ histogram.

wider

High energy (kVp) gives a _______ histogram.

narrow

A Histogram shows...

the distribution of pixel values for any given exposure.

For example:

Pixels have a value of 1, 2, 3, and 4 for a specific exposure.

Histogram shows the frequency of each of those values and actual number of values.

Histogram sets the minimum (S1) and maximum (S2) "useful" pixel values.

*Histogram Analysis

Analysis is complex.

Shape of the histogram stays fairly constant for each part exposed (anatomy specific).

Histogram analysis: Image correction

Raw data used to form the histogram are compared with a "normal" histogram of the same body part by the computer.

The Nyquist Theorem

when sampling a signal, the sampling frequency must be greater than twice the bandwidth of the input signal so that the reconstruction of the original image will be as close to the original signal as possible.

Many scientists have revised the Nyquist Theorem. It is often referred to simply as the _____________ _______________.

Sampling theorem

In the Nyquist theorem, at least _______ the number of pixels needed to form the image must be ____________.

twice;sampled

In the Nyquist theorem, if too few pixels are sampled, the result is a .........

lack of resolution

Does oversampling result in additional information according to the Nyquist theorem?

No

*The number of conversions in PSP (Nyquist Theorem)

1. electron to light

2. light to digital information

3. digital to analog

*results in lack of detail

Some light is lost during the light-to-digital conversion because of the........

spreading of light photons. (Nyquist theorem)

The longer the electrons are stored, the more energy they gain or lose?

lose

Aliasing occurs in digital imaging when.....

the spatial frequency is greater than the Nyquist frequency.

When sampling occurs less than twice per cycle, what happens?

Information is lost

Fluctuating signal is produced

also know as Foldover or biasing

Aliasing results in a ________ effect.

moire

Automatic Rescaling

Images are produced that have uniform brightness and contrast regardless of the amount of exposure

Problems occur with rescaling:

Too little exposure results in quantum mottle.

Too much exposure results in a loss of contrast and loss of distinct edges because of detector saturation.

Look-Up Table

All pixels are changed to a new gray value.

Image will have appropriate appearance in brightness and contrast.

(LUT) Contrast can be increased or decreased by changing the _________ of the graph.

slope

(LUT) Brightness (density) can be increased or decreased by moving the line.....

up or down the y-axis

Digital systems have greater ______________ _____________ than film/screen imaging.

dynamic range (latitude)

Spatial frequency resolution

Refers to detail or sharpness of the image

Edge Enhancement

Smooths images by averaging pixels

Amplification, also known as high-pass filtering

results in an increase of contrast and edge enhancement

Suppression of frequencies (masking)

, can result in loss of small details.

This technique is useful for enhancing large structures, such as organs and soft tissues, but can be noisy.

Smoothing (low-pass filtering)

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

Result is a reduction of noise and contrast

Low-pass filtering is useful for viewing small structures such as fine bone tissuesImage Manipulation

Image Manipulation

-Window level and window width

-Background removal or shutter

-Image orientation

-Image stitching

-Image annotation

-Magnification

Window level

Controls how light or dark the image is

Window width

Controls the ratio of black to white, or contrast

Background Removal or Shutter