Biomedical Engineering Reference
In-Depth Information
the principles of CT (see Section 8.4.3) are applied to provide a 3D quantitative
estimate of the distribution of gamma-emitting radionuclides. Projection data are
acquired in SPECT at a given view angle, but over multiple planes simultaneously,
with a gamma camera. The camera is then rotated around the subject in steps
of
covering the full 360° range. Images are reconstructed from these multiple-
view projections similar to CT using either a filtered backprojection technique or
an iterative reconstruction algorithm. In some situations, for example imaging the
heart with a low energy emitter like Tl-201, a 180° scan may be preferred to a 360°
rotation.
Despite certain imperfections in some early combined devices, their perform-
ance made SPECT a widespread clinical tool within a short period of time. In
SPECT, radioactivity distribution of the radionuclide inside the patient is deter-
mined instead of the attenuation coefficient distribution from different tissues as
obtained from X-rays. Further, the radiation source is within the body instead of
outside the patient. Since the source distribution is unknown, an exact solution to
the attenuation correction problem is theoretically very difficult to attain. Hence,
the technique was varied from CT. Further, the cameras have improved, providing
better mechanical stability, more efficient scatter and attenuation corrections, and
shorter reconstruction times. However, the amount of radionuclide that can be ad-
ministered is limited by the allowable dose of radiation to the patient. Furthermore,
SPECT with a single rotating gamma camera suffers from low sensitivity compared
to PET. If the spatial resolution of PET device is in the order of 5 mm, a SPECT sin-
gle rotating camera can be made with a spatial resolution in the order of 15 to 20
mm. To increase the detection efficiency while improving the spatial resolution of
the imaging system, a three-headed gamma camera has been developed. Nonethe-
less, several approaches leading to approximate corrections for both iterative and
direct reconstruction techniques are also developed.
Emitted gamma radiation interacts with the body by photoelectric absorption
and Compton scattering processes, producing a significant attenuation in the pri-
mary beam at energies used in SPECT. Since attenuation depends on the properties
of the medium interposed between the point of origin of the photons and the object
boundary, it is necessary to know the distribution of attenuation coefficients cor-
responding to the energy of the emitted radiation within the object and the source
distribution to accurately compensate for attenuation. In addition to attenuation
of photons due to photoelectric absorption, Compton scattering of the emitted
photons within the object introduces another error in the acquired data. Due to the
finite energy resolution of the detection system, a portion of the scattered photons
is indistinguishable from the primary photons and is recorded under the photo-
peak. Since the scattered photons originate mostly from regions outside the region
delineated by the collimator line spread function, counts produced by the scattered
photons cause blurring and a reduction of contrast in the image. Although the ef-
fects of scattered photons are not as prominent as attenuation losses, corrections
for scatter become necessary when higher quantitation accuracy, for example in
compartmental modeling, is sought in SPECT. Several methods, depending on the
type of image reconstruction algorithm, are proposed to correct for scattering in
SPECT.
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