Mathematics and Physics of Emerging Biomedical Imaging (1996) / Chapter Skim
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5 SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY
5 SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY
Pages 89-104
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From page 89... ...
give information based on the spatial: concentration of injected radiopharmaceuticals, in contrast to the other medical imaging modalities used for clinical diagnostic purposes. Figure 5.1 shows the imaging process and the components of a typical nuclear medicine imaging system.
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From page 90... ...
The scintillation camera provides a two-dimensional projection image of the three-dimensional radioactivity distribution or radiopharmaceutical uptake within the patient. SPECT takes conventional two-climensional nuclear medicine images acquired at different views around the patient and provides an estimate of the three-dimensional radioactivity distribution using methods of image reconstruction from multiple projections.
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From page 91... ...
In particular, the non-uniform attenuation distribution in the thorax is a major problem in cardiac SPECT image reconstructions. Photons that have been scattered before reaching the radiation detector give erroneous spatial information about the origin of the radioactive source, and a significant fraction of the photons detected in SPECT have been scattered: typical ratios of scattered to unscattered photons are about 20-30X0 in brain SPECT studies and about 30-40% in cardiac and body
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From page 92... ...
one or more scintillation cameras, or (3) hybrid scintillation detectors combining the first two approaches.
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From page 93... ...
A set of pinholes on a hemispherical lead collimator allows formation of projection images from multiple views that can be reconstructed directly to form a threedimensional reconstructed image. Since no rotation motion is required to collect the complete projection data set, the system is capable of acquiring fast dynamic three-dimensional SPECT data.
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From page 94... ...
arranged on a half hemispherical surface surrounding the patient's head. A set of pinholes on a hemispherical lead collimator allows formation of projection images from multiple views that can be reconstructed directly to form a three-dimensional reconstructed image.
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From page 95... ...
During this time its performance has been thoroughly studied and optimized, and further improvement will likely be dependent on the development of solid-state detector arrays. Current scintillation cameras have about 3-mm intrinsic spatial resolution and 10% energy resolution at 140 keV, numbers that have remained essentially unchanged for over a decade.
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From page 96... ...
Assume an ideal situation in which the emission photons do not experience attenuation and scatter in the patient and the collimator-(letector has perfect spatial resolution without blurring effects in the measured (lata. A naive approach to the SPECT reconstruction is to consider the projections pit, 8)
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From page 97... ...
The SPECT reconstruction problem is different from the classical problem of image reconstruction from projections because each photon is attenuated and scattered by the material between its source and the detector. When attenuation is taken into consideration, the two-dimensional attenuated Radon transform can be written as I+= ptt, {3)
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From page 98... ...
Often, the image reconstruction algorithm is inseparable from the compensation technique, resulting in SPECT reconstruction methods that are not found in other tomographic imaging modalities. Iterative Reconstruction Algorithms A typical iterative reconstruction algorithm starts with an initial estimate of the object source ctistribution.
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From page 99... ...
Interest in the applications of iterative reconstruction algorithms was initially propelled by the need to compensate for non-uniform attenuation in the imaging of heart muscle with radiopharmaceuticals, which illuminate relative perfusion. More recently, iterative reconstruction algorithms have been applied to compensate for the spatially variant system response function (collimator-detector response)
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From page 100... ...
The total response function is then used to generate a restoration filter that approximately compensates for the geometric and scatter response. Another class of scatter compensation methods determines the exact scatter response function and incorporates it into iterative reconstruction algorithms.
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From page 101... ...
Three-Dimensional Reconstruction Methods for Special Collimator Designs As discussed in the previous section, fan-beam and conebeam collimators have been used in SPECT to improve the trade-o~ between detection efficiency and spatial resolution as compared to the conventional paraHel-hole design. Due to the special geometries of such converging-hole collimators (others include astigmatic and variable focal collimators)
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From page 102... ...
In the area of better SPECT system configurations, research is recommended for the following: · Development of new SPECT system configurations for improved de tection efficiency, spatial resolution, and ease of operation; and · Development of new collimator designs for improved trade-off between spatial resolution and detection efficiency. The most promising research directions related to image reconstruction and mathematical approaches include the following: · Development of analytical solutions for the SPECT reconstruction problem that include the effects of non-uniform attenuation, spatially varying scatter, and collimator-~letector response; · Development of fast and stable iterative reconstruction methods that incorporate non-uniform attenuation, spatially varying scatter, and collimator-detector response for accurate SPECT image reconstruction; and · Development of new three-dimensional reconstruction methods and ciata acquisition strategies for converging-beam SPECT.
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From page 103... ...
4. Jaszczak, Rat., and Tsui, B.M.W., Single photon emission computed tomography, in Principles of Nuclear Medicine, H.N.
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