PM tube and associated electronics

2.2.8.1. Example: Uniformity — defective PM tube (old generation camera)

No uniformity correction

With uniformity correction map

Analogue camera with no linearity or energy corrections, ^99mTc, 20% energy window, 3 million counts each image. Images of flood field uniformity and spatial resolution using a four quadrant bar pattern.

The uniformity correction in this camera was obtained by a count skim calibration.

T: No uniformity correction applied.

B: With a uniformity correction applied.

Results: The flood field image obtained without the uniformity correction map shows a diffuse cold area in the upper part of the FOV (T). This was due to a malfunctioning PM tube. The spatial resolution image shows poor linearity (curved images of bars) and overall poor spatial resolution, especially over the area of the defective PM tube. Uniformity correction corrected for the non-uniformity but not for the poor spatial resolution and linearity. Service is required.

Comments: Often a uniformity correction map is applied in order to improve residual non-uniformity.

Such a correction map will disguise a problem due to a defective PM tube. Therefore the camera uniformity should be checked both with and without such a correction map applied.

QC of the spatial resolution image is a necessary supplement to the uniformity flood image.

Deteriorated spatial resolution will indicate any problem that has not already been detected with the uniformity image.

2.2.8.2. Example: Uniformity — defective PM tube

Daily QC image of flood field uniformity, ^99mTc, 15% energy window, 4 million counts.

Results: The image shows a large, circular cold area that was due to a defective PM tube. Note the inner halo of lower counts and the outer halo of higher counts at the edge of the defect. Service is required.

2.2.8.3. Example: Uniformity — defective PM tube (new digital generation camera)

99mTc, 15% energy window, 10 million counts, modern camera.

Results: Large, circular cold area due to non-functioning PM tube. Service is required.

Comments: The image registration in new generation cameras is such that a non-functioning PM tube will give an unmistakable circumscribed cold spot in the location of the PM tube. Note that the tube itself, the preamplifier or subsequent circuits could be defective. In this example, the fault is in the middle of the FOV and the scintillation camera cannot be used. Service is required before clinical work can resume.

2.2.8.4. Example: Uniformity — defective PM tube at edge of FOV

99mTc, 15% symmetric energy window, 256 × 256 matrix, 10 million counts.

Results: A discrete, semicircular cold indentation is seen at the top of the FOV. This is due to a defective PM tube. Service personnel replaced the PM tube.

Comments:Although service must take place to repair the defect, this camera could still be used for planar and SPECT imaging, provided that the organ of interest lies well below the defective PM tube, but it could not be used for whole body imaging.

2.2.8.5. Example: Clinical bone scan — defective PM tube

Bone scan of patient using ^99mTc phosphonate, 15% energy window.

TL, TM and BL: Camera A — different single spot views of the skeleton: posterior, right anterior and right anterior oblique, respectively.

BM: Camera A — flood field image taken after the spot views.

TR and BR: Camera B — posterior spot views of the same patient.

Results: The spot views from camera A show an area of apparent decreased activity in the lower skeletal area that is especially evident in the TL and BL images. This was due to a defective PM tube, as demonstrated in the uniformity flood field image (BM).

Images of the same patient made with scintillation camera B show a normal ^99mTc phosphonate distribution in the lower spinal column (TR and BR).

Comments:This patient was imaged on two different scintillation systems because the cold area in the lower spine was not considered to indicate pathology but to be an artefact. The artefact, caused by a defective PM tube, was confirmed by a subsequent uniformity image (BM).

If a second camera had not been available, the patient position could have been shifted so that the lower skeleton could be imaged in another part of the camera FOV.

Unusually, the patient positions in the TL, TM and BL images are such that the defective area is over the same part of the spine. More usually there would be translation between successive views, so that the artefact would be more easily seen. A smaller defect, however, would be more difficult to see.

2.2.8.6. Example: Clinical scan — defective PM tube

A

B

A: Clinical ¹¹¹In somatostatin receptor study. Upper and lower abdomen in the anterior view (top two images), and upper and lower abdomen in the posterior view (bottom two images).

B: Uniformity image obtained after the clinical study.

Results:The clinical study showed two large, circular colder areas (indicated by arrows). These were verified by the uniformity image on the right, which shows two large cold areas with a hot border due to defective PM tubes.

Comments:The non-uniformities in this example were large enough to be recognized in the patient’s images. Routine uniformity QC images were only taken on a weekly basis on this camera, and the non-uniformity was not detected prior to the patient study.

Later this non-uniformity turned out to recur intermittently, with no obvious reason, and appeared even after tuning. The scintillation camera was replaced.

2.2.8.7. Example: Maladjusted PM tube amplifier

Routine intrinsic uniformity, ^99mTc, 15% energy window, 128 × 128 matrix. The image has been contrast enhanced.

Results: Within the upper right quadrant of the image, there is a distinct non-uniformity: a hexagonal pattern delineated by two borders of increased counts. This was due to a fault in the adjustment of a PM tube amplifier.

2.2.8.8. Example: Loss of coupling between PM tubes and light pipe

A 4 million count intrinsic ^99mTc image was acquired with a symmetric 20% window at

~75 000 counts/s, with the intrinsic ^99mTc uniformity map correction turned off.

Results: The cold region in the upper left quadrant was considerably larger than a single tube and has a distinct edge. Service personnel opened the detector head and recoupled eight PM tubes to the plastic light pipe.

Comments: This condition had been seen previously using off-peak images, but use of a ^99mTc intrinsic flood correction made the problem less visible.

2.2.8.9. Example: Faulty electronics associated with PM tubes

Routine intrinsic uniformity check, ^99mTc, 20% energy window.

Results:Three cold indentations are seen at the upper and lower edges of the image. These were due to faulty electronics associated with the PM tubes.

2.2.8.10. Example: Faulty PM tube preamplifier

Routine intrinsic uniformity check with ^99mTc, 15% symmetric energy window.

L: Uniformity image.

R: Four quadrant bar pattern image.

Results:A diffuse area of reduced counts is seen in the top right corner of the FOV. Note also the deformity in the bar pattern at the same location. This problem was due to a faulty preamplifier.

2.2.8.11. Example: Faulty PM tube circuitry

Dual head scintillation camera, intrinsic uniformity, ^99mTc, 15% symmetric energy window, 5 million counts each image.

L: Detector 1.

R: Detector 2.

Results: Detector 1 shows good uniformity, whereas detector 2 shows two large, circular non-uniformities, each consisting of several rings of different intensities. The PM tube circuitry was faulty and required servicing.

2.2.9. Collimator