NM quality assurance
There is a legal requirement for quality assurance (QA) for all equipment used for medical exposure as specified in Ionising Radiations Regulations 1999 (IRR99).
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
- 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%)
- When collimator in place
- Measured using flood source. Either liquid filled with technetium-99 or plastic resin resin with Cobalt-57
- Acquired with linearity, energy and sensitivity corrections applied
- Anger pie phantom: segments with holes of different diameters separated by a distance of 4x the hole diameter
- Quadrant bar phantom: four sets of lead bars of different spacing
- 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
- Measures spatial distortion of an image
- Parallel line equal spacing phantom (PLES) imaged
- 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
- 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%
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Centre of rotation
- Reconstruction assumes centre of rotation matches centre of detector gantry
- Measured by point source of technetium-99m in centre of field of view but offset by ~15cm from axis of rotation
- Measured using Jaszczak phantom - a cylindrical, liquid filled filled with rods and spheres