Planar imaging


Types of planar imaging

Planar imaging is the acquisition of 2D nuclear images, similar to plain films in x-ray imaging.

Static

  • Used for studies in which the distribution of the radiopharmaceutical is effectively static throughout the acquisition e.g. bone scan
  • Inject → wait → image
  • The time from injection to imaging depends on the study being performed.
  • The total time of imaging can be determined by a preset time or a preset number of counts
  • A static image can provide information on:
    • Organ size, shape and position
    • Regions of increased or decreased uptake
  • Examples: DMSA renal scan, bone scan, lung perfusion scan

Dynamic

  • Used for studies in which the distribution of the radiopharmaceutical changes rapidly with time
  • Inject → image immediately → acquire series of frames over time
  • The time between frames varies depending on the study being performed
  • A dynamic study provides information on:
    • Variation of radiopharmaceutical distribution over time
  • Examples: MAG3 renal scan, gallbladder emptying scan, gastric emptying scan

Gated

  • Used to study organs with regular physiological motion
  • Example: cardiac gated blood pool imaging - acquisition is triggered by the R wave of the ECG. Images are then acquired. When the R wave occurs again the new images are overlaid onto the images from the previous cardiac cycle.

 

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Image acquisition

The operator can alter several variables during image acquisition depending on the nature of the scan being performed.

Collimator

The appropriate collimator needs to be selected for the study (more detail in Gamma camera chapter)

  1. Low, medium or high energy collimator depending up the radionuclide used
  2. General purpose, high resolution or high sensitivity collimator
    • High resolution gives better spatial resolution at the expense of a lower count rate and, therefore, longer imaging times. Usually used when spatial resolution of small structures important e.g. bone scan
    • High sensitivity gives better count rates but lower spatial resolution. Usually used in dynamic imaging when count rate is more important than anatomy e.g. renography.
  3. Parallel, diverging, converging or pinhole configuration

Number of counts

  • The aim is to increase the count density to achieve a high signal to noise ratio (SNR). Count density is increased by:
    • Increasing imaging time
    • Increasing the amount of administered radiation
    • Ensuring acceptable gamma camera sensitivity

Matrix

  • A larger matrix = more pixels in the image and, therefore, better spatial resolution. But this comes at a cost of fewer counts per pixel (lower SNR) and higher processing power required.
  • Small matrix = fewer, larger pixels but better SNR
  • Typical matrices:
    • Static imaging: 256 x 256
    • Dynamic imaging: 128 x 128 or 256 x 256
    • Gated cardiac imaging: 64 x 64

Orientation

  • The orientation of the images can be changed by altering the position of the patient or the camera
  • Oblique / lateral imaging can help differentiate structures that are overlying each other on the AP views
  • If an object larger than the field of view needs to be imaged (e.g. whole body bone scan) this can be done by:
    • Continuous: the couch travels between the camera heads at a constant speed and the computer reconstructs the image
    • Step and shoot: 4-6 static images are taken along the body and these are then stitched together during processing

Position of camera

  • The gamma camera should be as close to the patient as possible to optimise spatial resolution and signal. A smaller air gap means the radioactivity has a smaller spread as it passes through the collimator and, therefore, better spatial resolution.
  • A small air gap can be achieved with infrared autocontouring which maintains the camera at a close but safe distance from the patient as the gamma camera scans along.

List mode acquisition

  • In dynamic and gated imaging we can record the time information with the detected radioactivity. We can then split the signal acquired according to different time intervals. However, this requires a large amount of computer memory to be able to store all the data which includes data that will not be incorporated into the final image.

Image display

Lookup tables

  • The relationship between the pixel signal count and the displayed colour / brightness is determined by a lookup table.
    • Linear = linear relationship between signal count and displayed value.
    • Non-linear (i.e. logarithmic or exponential) = used for images in which the signal is concentrated in an area that is not of interest (e.g. the bladder in a bone scan) to prevent the majority of display values being used to display this small number of pixels and reduce overall contrast.

Contrast enhancement

The display of pixel brightness / colour is adjusted by adjusting the windowing. The displayed values should be set so that the pixels of interest are displayed best.


Image processing

Image filtering

Convolution is used to smooth and sharpen the image by altering the count density values in the image (i.e. not just adjusting the display of the counts).

Region of interest (ROI)

A region of interest analysis can be used to calculate the total number of counts in a specified area. The region can be drawn by the user or drawn automatically by the processing system. If counts are calculated for anterior and posterior views the mean can be taken which corrects for depth (e.g. in a DMSA)

Time activity curves

The count rate in a specific ROI on a study can be shown over time in the form of a graph. This can then be used to calculate parameters and display these as a colour / brightness scale rather than just the number of counts:

  • Time to reach the peak
  • Area under the curve
  • Washout rate of a radiopharmaceutical

Σ Summary

Types of imaging

  • Static: inject → wait → image
  • Dynamic: inject → image immediately → acquire series of frames over time
  • Gated: inject → image and collect timing data → reconstruct data into time periods

Image acquisition

  • Collimator:
    • Low, medium or high energy
    • General purpose, high resolution or high sensitivity
    • Parallel, diverging, converging or pinhole
  • Count number:
    • Increase imaging time
    • Increase administered radiation
  • Matrix size:
    • Large matrix = better spatial resolution but more noise
  • Orientation of camera / patient:
    • Oblique / lateral for superimposed structures
    • Continuous or step-and-shoot for large object
  • Position of camera:
    • Close as possible to patient - use infrared contouring system.

Image display

  • Lookup tables: linear or non-linear
  • Contrast enhancement: i.e. windowing

Image processing

  • Image filtering with convolution
  • ROI analysis
  • Time activity curves in dynamic / gated studies

Next page: SPECT imaging


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