NM quality assurance

Gamma camera

Uniformity

When irradiated by a uniform source the gamma camera should produce an image in which all pixels have the same count value. The uniformity depends on the spatial linearity and energy response of the system.

• Daily figures for uniformity acquired
• + 2SD from mean = remedial actions required

Intrinsic uniformity

• When collimator is removed
• Measured using point source of Cobalt-57
• Measured with point source positioned on central axis at 5x the diameter of the crystal (reduces variations in photon flux that reach detector to < 1%)

System uniformity:

• When collimator in place
• Measured using flood source. Either liquid filled with technetium-99 or plastic resin with Cobalt-57
• Acquired with linearity, energy and sensitivity corrections applied
  
  

Spatial resolution

Qualitative measurements

1. Anger pie phantom: segments with holes of different diameters separated by a distance of 4x the hole diameter
2. Quadrant bar phantom: four sets of lead bars of different spacing

Quantitative measurements

• Image a point or line source and calculate the full width at half maximum (FWHM)
  • Rayleigh criterion = FWHM is minimum separation required between two line sources to be seen as separate
• Emission phantom (e.g. Williams’ liver phantom) – thinned phantom (hot spots) or perspex discs of different thicknesses (cold spots) layered over a radioactive source to assess resolution with different depths
  
  

Linearity

• Measures spatial distortion of an image
• Parallel line equal spacing phantom (PLES) imaged
  
  

Sensitivity

• Measures proportion of emitted radiation that is detected within the photopeak of the collimated gamma camera
• Image small phantom containing known amount of radioactivity measured for a known amount of time at a distance of 10 cm from the camera face
  
  

Count rate capability

• Measures ability of gamma camera to record count rate linearly as the count rate increases
• Usually expressed as 20% count rate loss – the count rate at which the recorded value is 20% lower than the expected value
• The lower recorded value is usually due to dead time
  
  

Energy resolution

• Spread of the recorded energy of incident gamma photons
• Measured as the FWHM of the photopeak (maximum energy recorded energy of incident gamma photons)
• Energy resolution = FWHM / energy photopeak x 100%