Image Acquisition Errors

What is an important step in analysis of the image data?

Recognizing exposure filed borders

If a PSP is unable to recognized an exposure field border, the image analysis data will also include all the information _________ that border

Outside

Inappropriate collimation margins and/or _________ may also result in both histogram and rescaling errors

Beam alignment

Is this single exposure field ideal, permitted, or inappropriate?

Ideal

Is this single exposure field ideal, permitted, or inappropriate?

Permitted

Is this single exposure field ideal, permitted, or inappropriate?

Inappropriate

Is this beam alignment correct or inappropriate?

Correct

Is this beam alignment correct or inappropriate?

Correct

Is this beam alignment correct or inappropriate?

Inappropriate

For appropriate multiple field distribution, the field must be:

• center to plate segment

• collimation must be “clean” with margins between fields and between edge of field and plate’s edge

What do acceptable multiple fields look like?

• symmetrical field distribution

• clean collimation between fields

• no overlap

What is this image showing?

• clearly demonstrates that exposure field borders, collimation borders, were recognized

• everything outside the black area is clear, which shows that the borders were recognized

What is this rescaling error occurred by?

The image incorporate the extra exposure in the form of scatter and off-focus radiation

Prior to image creation, the exposure values extracted from the PSP are subject to a process known as?

Quantization

Quantization is a unique ______ assigned to each sample or pixel

Value

What can quantization also be referred to as?

Sampling frequency

Both sampling (time between samples) and quantization are the processes of?

Analog-to- digital conversion (ADC)

Histograms graphically represent a collection of exposure values extracted from the _______

Receptor

An image is created after a histogram of the extracted image data is analyzed using one of two formats:

• priori histogram analysis

• neural histogram analysis

Priori histogram analysis

• three types

• involves comparing the exposure data set to a single standardized exposure data set fir a matching examination

Where does the standardized data set come from for priori histogram analysis?

Derived during experiments that used like subjects under ideal conditions

Neural histogram analysis

• data extracted from the PSP are compared to two or more pre-defined histograms

• an image is created after extracted image data is matched to one of the pre-defined histograms

What are some examples of histogram analysis errors?

• incorrect anatomic menu selection (by user)

• exposure field not detected

• unexpected material in data set

• large exposure error (plate saturation)

• inappropriate rescaling (light or dark image)

The predetermined values used to create the image are known as?

Values of interest

The process of matching the captured image values to a standardized set of values is called?

Automatic rescaling

What does automatic rescaling permit for?

The consistent output of image data and image display appearance even when errors in exposure technique are present

What is this image showing?

Excessive exposure without rescaling

What is this image showing?

Excessive exposure with rescaling

Low intensity radiation response: background radiation:

• cassette-based system is constantly refreshed

• cassette-less system plate is storage phosphor

  • stores background exposure

PSP responds to an exposure as low as _____ uR

60

Background radiation is between ____-_____ uR/day

40-80

Imaging plates unused for more than ____ hours should be erased

48

Scatter radiation is much _______ intense than background radiation

More

When is scatter radiation produced?

During Compton interaction

Compton reaction

• a primary photon interacts with an outer shell electron and changes direction

• not part of useful beam

Ways at reducing scatter include:

1. beam limiting

2. optimal exposure

3. grid use

Beam limiting

As beam is restricted, fewer primary photons are emitted from the tube and collimator, thus creating fewer scattered photons

What are some examples of beam limiting?

Aperture diaphragms, cones/cylinders, and collimators

Overexposure produces _____ scatter radiation

More

What is one of the principal factors affecting the amount of scatter radiation produced?

kVp

As kVp _____ , the % of primary photons that will undergo scattering also increases

Increases

With digital radiography, _____ should be manipulated in order to compensate for grid use rather than mAs

kVp

Changes in ____ will not greatly affect radiographic contrast as it did with film/screen combinations

kVp

With digital imaging, a ___________ that occurs when the grid frequency is approximately equal to the Nyquist frequency

Grid error

Moire effect

• the grid line is running in the sam direction as the movement of the laser beam that is scanning the imaging plate

• occurs with digital image receptor systems when the grid lines are captured and scanned parallel to the scan lines in the imaging plate readers

To avoid the Moire effect, use a high-frequency grid of ____-____ lines per inch when using a stationary grid when a stationary grid is needed

103-178

The maximum spatial resolution in digital imaging is equal to the ________, which is ½ x the pixel pitch (mm)

Nyquist frequency

When does aliasing occur?

• when the nyquist criterion is violated

  • when the spatial frequency exceeds the nyquist frequency

  • the incoming data would be sampled less than twice per cycle

  • aliased = misrepresentation of signal frequency