NM Quality Assurance

Radionuclide

1) Radionuclide purity:

Testing for contamination of 99mTc with 99Mo (which give unnecessary dose to the patient)

By measuring γ radiation after blocking off the gamma rays from 99mTc with 6mm lead

2) Radiochemical purity:

Testing for free pertechnetate in a labeled compound - By Chromatography

3) Chemical purity:

Testing for alumina (which would interfere with labeling) - By Spot colour test

4) Sterility testing and pyrogens testing: → available only retrospectively

5) Response of the radionuclide calibrator

Radionuclide dose calibrator

• Re-entrant ionization chamber that is used to check the activity of radionuclide vial before patient administration

• Ionization current is dependent on

– Activity of the sample

– Sensitivity of the chamber to the energy of the gamma rays assessed

– Geometry of the source within the calibrator (Thus radionuclide and syringe type must be selected on the control panel)

• Accuracy of the calibrator is checked regularly using a long lived source (Co-57)

• Used also to measure radionuclide purity

Gamma Camera

• Intrinsic measurement = without collimator
• System measurement = with collimator

1) Uniformity

Field uniformity refers to the ability to produce a uniform distribution of activity

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 & energy response of the system.

Daily figures for uniformity acquired

+2 SD from mean = remedial actions required

In modern cameras, a field uniformity better than 2% between two adjacent areas

Non-uniformity > 10% is not acceptable for clinical imaging

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 (flood field or sheet phantom).

Either liquid filled with technetium-99 or plastic resin with Cobalt-57

• Acquired with linearity, energy and sensitivity corrections applied

2) 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 (bar test pattern)

Quantitative measurements:

• Image a point or line source & 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

3) Linearity

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

4) Sensitivity (efficiency):

• Measures proportion of emitted radiation detected within the photopeak of the collimated gamma camera (Total counts per second per MBq of activity)
• Image small phantom (small version of the flood field phantom) containing known amount of radioactivity measured for a known amount of time at a distance of 10 cm from the camera face

5) Count rate capability

• Measures ability of gamma camera to record the count rate linearly as it 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 due to dead time

6) Energy resolution

The ability to distinguish between separate gamma rays of different energies.

• Importance: better energy resolution → better scatter rejection & better spatial resolution
• Measured as: the FWHM of the photopeak (maximum recorded energy of incident gamma photons)
• Energy resolution = FWHM / energy photopeak x 100%
• Typically 12% of the peak energy
• Better for high-energy gamma photons because they produce more light photons, therefore better with 99mTc than with 201Tl