CR Reader Functions, Image Histograms, Nyquist Theorem Notes

CR Reader Functions

  • In cassette-based and cassette-less systems, once X-ray photons are converted into electrical signals, these signals are available for processing and manipulation.
  • Computed Radiography (CR) imaging plate records a wide range of X-ray exposures.
  • If the entire range of exposure were digitized, values at extremely high and low ends of range would also be digitized, resulting in low-density resolution.
  • To avoid this, exposure data recognition processes only the optimal density exposure range.

Data Recognition

  • Data recognition program searches for anatomy recorded on the imaging plate.
    1. Finding collimation edges and scatter.
    2. Eliminating data outside the collimation.
  • Failure of the system to find the collimation edges can result in incorrect data collection.
  • Images may be too bright or too dark.

Image Histogram

  • Centering anatomy to the center of the imaging plate is equally important.
    1. Ensures that appropriate intensities are recorded.
    2. Failure to do so could result in an image that is too bright or too dark.
  • Histogram: graphical representation of exposure values collected from the IR.
    • Horizontal axis: tone values.
    • Vertical axis: number of pixels in each tone.
  • Histogram is generated to find useful signal by locating the minimum (S1S1) and maximum (S2S2) signal within the anatomic regions of interest in the image.
  • Histogram identifies all intensities within the collimated borders in the form of a graph.
    • X-axis displays exposure values, related to the amount of exposure signal.
    • Y-axis displays the number of pixels for each exposure.
  • Graphic representation appears as a series of peaks and valleys and has a pattern that varies for each body part.

Histogram Analysis

  • Histogram analysis is complex.
  • The shape of the histogram stays fairly constant for each part exposed (anatomy specific).
  • For example:
    • The shape of the histogram for a chest radiograph on a large adult patient looks different from a knee histogram generated from a pediatric knee exam.
  • It is important to choose the correct anatomic region on the menu before exposing the patient.
  • Raw data used to form the histogram are compared with a "normal" histogram of the same body part by the computer.
  • Image correction takes place at this time.

The Nyquist Theorem

  • In 1928, Harry Nyquist described a way to convert analog signals into digital signals that would more accurately transmit over telephone lines.
  • The analog signal could be captured, transmitted digitally, and created in analog form on the receiver.
  • In 1948, Claude Shannon presented a mathematical proof of the Nyquist's theory.
  • It is often referred to simply as the sampling theorem.
  • The theorem states that 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.
    • At least twice the number of pixels needed to form the image must be sampled.
    • If too few pixels are sampled, the result is a lack of resolution.
    • Oversampling does not result in additional useful information.
  • During image acquisition, energy conversions allow for signal loss. Conversion include:
    • X-rays to light to electrical signal (Indirect capture).
    • X-ray to electrical signal (direct capture).
  • The number of conversions in CR—electron to light, light to digital information, digital to analog signal—results in loss of detail.
  • Some light is lost during the light-to-digital conversion because of the spreading out of light photons.
  • Because there is a small distance between the phosphor plate surface and the photosensitive diode of the photomultiplier, some light spreads out there as well, resulting in loss of information.
  • The indirect method of image acquisition has the highest potential for loss of signal.
  • In PSP plates, the longer the electrons are stored, the more energy they lose.
  • When the laser stimulates electrons, some lower energy electrons escape the active layer.
  • If enough energy was lost, some lower energy electrons are not stimulated enough to escape, and information is lost.
  • All manufacturers suggest that imaging plates be read as soon as possible to avoid this loss.
  • Both FPD systems lose fewer signals to light spread than conventional radiography.
  • The Nyquist theorem is still applied to ensure that sufficient signal is sampled.
  • Because the sample is preprocessed by the computer immediately, signal loss is minimized but still occurs.