Accurate attenuation correction is essential for quantitative positron emission
tomography. Typically, this correction is based on a coincidence transmission
measurement using an external source of positron emitter, which is positioned close to
the detectors. This technique suffers from poor statistical quality and high dead time
losses, especially with a high transmission source strength.
We have proposed and tested the use of single photon transmission measurement
with a rotating rod source, to measure the attenuation correction factors (ACFs). The
singles projections are resampled into the coincidence geometry using the detector
positions and the r,)d source location. A nonparalyzable dead time correction algorithm
was developed for the block detectors used in the McMaster PET scanner.
Transaxial resolution is approximately 6 mm, which is comparable to emission
scanning performance. Axial resolution is about 25 mm, with only crude source
collimation. ACFs are underestimated by approximately 10% due to increased crossplane
scatter, compared to coincidence transmission scanning. Effective source
collimation is necessary to obtain suitable axial resolution and improved accuracy. The
response of the correction factors to object density is linear to within 15%, when
comparing singles transmission measurement to current coincidence transmission
measurement.
The major advantage of using singles transmission measurement IS a
dramatically increased count rate. A factor of seven increase in count rate over
coincidence scanning is possible with a 2 mCi transmission rod source. There are no
randoms counted in singles transmission scans, which makes the measured count rate
nearly linearly proportional with source activity. Singles detector dead time is
approximately 6% in the detectors opposite a 2 mCi rod source.
Present hardware and software precludes the application of this technique in a
clinical environment. We anticipate that real time acquisition of detector singles can
reduce the transmission scanning time to under 2 minutes, and produce attenuation
coefficient images with under 2% noise. This is a significant improvement compared
to the current coincidence transmission technique. / Thesis / Master of Science (MS)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/24192 |
| Date | 09 1900 |
| Creators | Dekemp, Robert A. |
| Contributors | Nahmias, C, Physics |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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