Simultaneous positron and single photon emission tomography
Emission computed tomography involves external measurements of gamma
photons emitted from within the object under investigation in order to map the
radioactive distribution into a two-dimensional array within a slice of interest.
Both positron emission tomography (PET) and single photon emission computed
tomography (SPECT) constitute the two types of emission computed tomography.
PET and SPECT differ radically in almost every aspect of system design; radionuc1ide
employed, radiation detectors and arrangement, collimation (electronic, mechanical),
processing electronics as well as data acquisition, handling and correction.
A prototype scanning-rig incorporating two collimated BOO scintillation
detectors has been used to carry out PET experiments utilising 6SOe line sources
(positron-emitter) and a perspex phantom of 50-mm in diameter to simulate a small
animal i.e. a rat's head. Modifications for the experimental scanning-rig allowed the
collection of the singles events in the PET studies in such a way that they could be
reconstructed to provide SPECT images for the radioactive distribution under
investigation. This property allowed a simultaneous collection of PET and SPECT data
for the same object under exactly the same conditions. Two data sets are generated
from each tomographic experiment; one is for PET and the other is for SPECT. Each
data set is corrected separately for the required corrections i.e. scattering and
attenuation before reconstruction, and then two images are produced for each study.
The outcome from this work is the comparison between the two images of PET
and positron SPECT obtained. The line spread function curves taken for various
depths and the image profiles for studies in air and perspex show that PET provides
better spatial resolution than positron SPECT. This property of PET is further
confirmed by the MTF curves and the fidelity test. Using a collimation aperture of 3-
mm wide, the spatial resolution values in air were found to be 3.2 +/- 0.45 mm and 7.4
+/- 0.45 mm FWHM for PET and SPECT respectively. The images of the two line
sources with a 10-mm centre-to-centre separation are partially resolved in the SPECT
images whereas a sufficient separation between the two sources is achieved in PET.
Image combination has been applied in order to obtain a hybrid image which
contains the advantages from both PET and SPECT. A straightforward averaging and
multiplication of the two images of PET and SPECT were found useful to provide
images with enhanced quality. The multiplication process provided images with
significantly improved quality for the PE T images. When evaluating the image quality
of the line source in air, the fidelity test values are 0.71 and -1.11 for PET and SPECT
respectively. The image combination resulted in an image with fidelity values of 0.92
when the two images are multiplied and 0.12 when their averaging was obtained.