Test of variations of uniformity and sensitivity with angle
4.3.8. Test of variations of uniformity and sensitivity with angle
Purpose of test To determine the variations in system sensitivity as a function of angular
position of the detector. Materials
A flood source of about 200 MBq (5 mCi) of 99m Tc, which can be safely attached to the collimator face such that the system can be rotated. A 57 Co flood source could be used if the field uniformity is not computed for each angle.
Procedure (1) Attach the flood source firmly to the camera so that it cannot shift when
the system rotates. (2) Perform a tomographic acquisition with at least about 10 6 counts per angle, for the normal matrix size used. (3) For a system collecting data by continuous rotation, record the total rotation time. Repeat the test for both fast and slow rotation, for example, total rotation times of 4 min and 30 min.
(4) Repeat for any other heads. Data analysis (1) Find the total number of counts collected at each angle. Correct for decay
of the radionuclide used if total acquisition time is significant, e.g. 60 min for 99m Tc.
(2) Calculate the mean, standard deviation and maximum deviation from the mean. (3) Perform the same calculations for a central region of interest. (4) If suitable software exists, calculate the NEMA integral uniformity for
each angular position. If such software is available, it might also be useful to compare each view on a pixel by pixel basis.
(5) For a continuous rotation system, perform the analysis for both a slow and a rapid rotation speed. (6) If the speed of rotation (or time per acquisition) can be measured independently, this should be recorded.
Observations This test is intended to be performed as a reference test.
Variations in uniformity and sensitivity as a function of angle can be caused by lack of magnetic shielding, changes in temperature and (for continuous rotation) mechanical drive problems.
Interpretation of results Some systems show considerable variation in sensitivity and uniformity as
a function of angle, probably because of the influence of the earth’s magnetic field. These variations can cause considerable differences in uniformity, etc., as
a function of angle, but are eliminated primarily by either good magnetic shielding of the photomultipliers, an appropriate energy correction, or both.
An alternative source of variation in sensitivity and uniformity with angle results from changes in temperature within the head as a function of angle. Check the deviations from the mean for the total number of counts and the central region of interest.
Limits of acceptability The variation in sensitivity should be less than ±1% of the mean value. If
it is greater than this, then the system is not performing satisfactorily. It may, however, be possible to perform an appropriate correction with software.
The results for continuous rotation should be similar for both rotation speeds. In particular, note if there is an apparent increase in counting time, corresponding to an increase in the number of collected counts for those angles when the camera head is rising, compared with the angles where the head is descending.
Conclusion Record whether or not the results confirm acceptable performance. If
not, indicate the follow-up action taken.