Point sources General comments

A point source of radioactivity used for intrinsic tests must fulfil criteria for size and shape:

(a) For distances of >5 FOV, the source must be in as small a volume as possible.

(b) If a syringe is used, the volume should be <0.05 mL in a 1 mL syringe and the syringe needle must not be radioactive (remove the original needle used for preparing the source, and preferably use a small cap to seal the syringe, or replace with a non-radioactive needle).

(c) For distances of <1 FOV (e.g. in the case of multihead systems), the source must be a small round point, i.e. a drop of radioactivity.

(d) The syringe or capsule that contains the radioactivity must be suspended or fixed in such a way as to produce an energy spectrum with minimum scatter. If placed on a surface, it should be put in a lead container in order to provide backscatter absorption and to limit the exposure to personnel.

Note:The shape and size of the point source affect the uniformity image obtained. Care must also be exercised to produce a consistent point source and detector geometry in order to compare images. An alternative to a ^99mTc point source could be a ⁵⁷Co point marker, or a ⁵⁷Co spot marker, of the appropriate amount of radioactivity. Used consistently, these can easily be employed to routinely monitor detector sensitivity and therefore to monitor changes in energy window setting or window width.

The dome of the measured count distribution is dependent on the source to detector distance. This effect is minimal if a distance of 5 FOV is used between source and detector. When a shorter distance is used, and software corrects for the observed count distribution, then the same distance should be used each time.

2.2.10.3A. Example: Intrinsic uniformity — off-centred point source

Intrinsic uniformity, ^99mTc point source, 15% energy window, 3 million counts.

Results: The image shows a gradual increase in count density from bottom to top (in the ydirection).

The source was not positioned centrally beneath the detector but was off-centred too far towards the top of the image.

Comments: Even when a point source is at a distance of 5 FOV away from the detector, it must be positioned on a perpendicular line through the centre of the detector in order to irradiate the detector uniformly. This is even more critical when the source is placed at a closer distance.

Note:The same applies if the surface of the crystal does not form a 90º angle with the source–detector axis.

2.2.10.3B. Example: Intrinsic uniformity — geometry of point source and detector — source too close to detector

Intrinsic uniformity, ^99mTc point source placed at a distance of about 3 times the maximum FOV from the detector, 15% energy window, 3 million counts.

Results: The count distribution shows an increase in intensity towards the centre of the FOV because the source was too close to the detector and not at a distance of 5 FOV.

Comments: With large FOV detectors it becomes difficult to find an appropriate solution to positioning a point source at a sufficiently large distance from the detector for intrinsic uniformity QC.

For routine QC, when monitoring changes in uniformity is the essence of the test, a point source placed slightly too close, as shown here, is acceptable, provided that the user is aware of the influence of the point source proximity in evaluating the result both visually and quantitatively.

Note:There is a slight, cold, crescent shaped artefact along the right hand part of the upper border of the image. This was due to a PM tube that had been replaced. This artefact was obscured with the collimator in place.

2.2.10.3C. Example: Multihead systems — geometry of point source and detector

Multihead SPECT system. Intrinsic uniformity using a ^99mTc point source positioned at about 30 cm centrally between the detectors in order to obtain simultaneous flood field images from each detector, 15% energy window, 30 million counts (each detector). The images are from one detector of the system.

L: Raw flood image.

R: Raw image corrected for the curvature in count response (dome effect).

Results:

L: The image shows the pronounced curvature or dome of increased counts towards the centre of the FOV.

R: The vendor provided a program to correct for the curvature, in order to produce a flat field response for further evaluation, quantification and, if a sufficiently high number of counts have been collected, use as an intrinsic part of a uniformity correction map. The image has been corrected using this software.

Comments: With multihead systems it is convenient (and may also be necessary according to the manufacturer’s instructions, as in the situation shown here) to place a point source symmetrically between the detector heads in order to acquire data simultaneously with all detectors. In this geometry, the point source is not at the required 5 FOV distance from the detectors but is much closer, thereby forming a dome-like image. Correction for the dome effect by suitable software is required in order to properly assess and monitor the non-uniformity.

The convenience of obtaining a simultaneous intrinsic uniformity by multiple detectors may depend on such a geometry and subsequent correction. The results are highly dependent on the point source being indeed a tiny point of radioactivity and not a slightly elongated source, which will distort the image and give false quantitative values and, if applied, an incorrect uniformity (or sensitivity) map.

Always adhere to the manufacturer’s instructions and recommendations.

2.2.10.3D. Example: Intrinsic uniformity — elongated point source/attenuation in source holder

Intrinsic uniformity, ^99mTc radioactivity in a syringe placed in a lead holder suspended from the ceiling above the detector, 20% energy window, 3 million counts. A lead mask was in position on the detector in order to define the collimated UFOV, and a thin sheet of Perspex (Lucite) (supported by the collimator housing) was placed over the detector to protect the crystal from accidental breakage due to impact by an object falling on the crystal.

L: Analogue flood field image.

R: Digital flood field image (128 × 128 matrix, no contrast enhancement).

Results: The flood images show a diffuse area of diminished counts over the upper right quadrant of the image. This was absent in the previous uniformity QC check, suggesting a user error. Investigation showed that the point source had not been properly prepared. The point source was a full 3 mL of

99mTc solution in a 3 mL syringe, producing an elongated volume of radioactivity that was partially obscured by the lead syringe holder. When this source was replaced by a proper point source, the large area of decreased activity disappeared and a uniform image was produced throughout the FOV, as was expected.

Comments:It is always advisable to check the set-up, as well as to compare new QC results with previous results.

Reference: NATIONAL ELECTRICAL MANUFACTURERS ASSOCIATION, Performance Measurements of Scintillation Cameras, NEMA Standards Publication No. NU1, Washington, DC (1994).

2.3. SPATIAL RESOLUTION AND LINEARITY 2.3.1. Intrinsic spatial resolution and linearity