Mathematics and Physics of Emerging Biomedical Imaging (1996) / Chapter Skim
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6 POSITRON EMISSION TOMOGRAPHY
6 POSITRON EMISSION TOMOGRAPHY
Pages 105-120
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From page 105... ...
It was recognized then that the high-energy photons produced by the annihilation of the positron-emitting isotopes could be used to describe, in three dimensions, the physiological distribution of "tagged" chemical compounds. After 2 decades of moderate technological developments by a few research centers, widespread interest and broadly based clinical research activity began in earnest following the development of sophisticated reconstruction algorithms and improvements in detector technology.
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From page 106... ...
POSITRON EMISSION TOMOGRAPHY Research with PET has added immeasurably to our current understanding of flow, oxygen utilization, and the metabolic changes that accompany disease and that change during brain stimulation and cognitive activation. CTinical uses inclu(le studies of Alzheimer's disease, Parkinson's disease, epilepsy, and coronary artery disease affecting heart muscle metabolism and flow.
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From page 107... ...
the recorded projection data: P = e-: Ads ,/ f(~syds (6.1~) where ~ is the projection data, ats)
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From page 108... ...
Additionally, connecting round photomultiplier tubes of sufficiently small diameter to rectangular scintillation crystals in order to form a solid ring lea(ls to problems in packaging. The contemporary method of packaging many scintiBators for 511 keV around the patient is to use what is caned a block detector design.
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From page 109... ...
For statistical reasons this limits the number of individual crystals that can be decoded without ambiguity within a detector block that spreads the light into the area of a number of crystals. Improving the spatial resolution of block detectors without increasing the number of expensive photomultiplier tubes thus requires the use of scintilIators with higher light output.
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From page 110... ...
POSITRON EMISSION TOMOGRAPHY 6. 2.2 Limitations of the Spatial Resolution Aside from the obvious effect of the size of the detector elements on the spatial resolution of PET systems, there are two fundamental effects limiting the resolution of the reconstructed images: 1.
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From page 111... ...
A modern commercial PET system employs a total of approximately 1S,000 detector crystals grouped into about 300 block detectors. Three or four rings of block detectors allow an axial field of view of 15 cm to be measured without repositioning the patient.
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From page 112... ...
For each incoming coincidence pair, the correct address in projection space is determined in real time and one event is added to the corresponding memory location. This circuitry allows correction of random events in real time by subtracting the delayed coincidence pairs or "randoms" from the appropriate projection space data ac3 cress.
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From page 113... ...
After transmission and emission data have been collected, the ranclom-corrected emission rate is divided by the attenuation factor and efficiency for each line of response. The resulting corrected projection data are reconstructed, usually with the filtered backprojection algorithm.
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From page 114... ...
. The most efficient use of the available photon flux can be realized by removing the interplane septa, which usually shield the detectors from the more oblique lines of response, and reprogramming the data acquisition hardware to accept all possible coincidence events (Fig.
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From page 115... ...
These line integrals can be arranged in groups of parallel lines, to form a set of two-dimensional parallel projections of the object. This three-dimensional data set has three key properties: 1.
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From page 116... ...
This algorithm uses the Towerstatistics image obtained by reconstructing only the straight projection data in order to obtain the missing line integrals for the oblique data through forward projection. Tests have demonstrated an image quality equal to the quality obtained from a two-dimensional reconstruction, and they also demonstrate the ability of the reprojection algorithm to take full advantage of the increased sensitivity allowed for by the three-dimensional acquisition, i.e., the ability to translate that increased sensitivity into an improved image signal-to-noise ratio.
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From page 117... ...
This necessitates improvements of the scatter correction routines that in two dimensions usually just rely on simple deconvolution methods. One way is to employ the admittedly limited energy resolution of PET detectors by simultaneously acquiring data with a lower energy window representing mostly scattered events and a higher energy window for the unscattered events.
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From page 118... ...
With the computing power available now for three-dimensional reconstruction, however, such calculations can be performed with acceptable computation times, if appropriate sampling techniques are used. 6.3.4 Attenuation Correction in Three Dimensions Transmission scans in three dimensions suffer from the fact that, because of the increase in count rate associated with the removal of the septa, significantly weaker transmission sources have to be employed in order to keep the detector dead timed within acceptable limits.
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From page 119... ...
It has been shown that the transmission count rates that can be achieved with such a point source arrangement can be increased by more than a factor of 10 above conventional coincidence based transmission scans in three dimensions. 6.4 Research Opportunities To summarize, the following major areas of scientific or technological development appear to bear the greatest potential of enhancing the applications and clinical usefulness of PET: Cost-effective y-ray detectors with high spatial, time, and energy resolution and the capability for measuring the angle and depth of inter actions; · Effective and fast means of acquiring low-noise transmission data; · Mathematical techniques to take advantage of improved detector technologies, such as the possible inclusion of time-of-flight information into the reconstruction process · Fast three-dimensional reconstruction algorithms, in particular for fast dynamic or whole-body studies; .
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From page 120... ...
5. Grootoonk, S., Spinks, T.~., Kenne(ly, A.M., Bloomfield, P.M., Sashin, D., and Jones, T., The practical implementation and accuracy of dual window scatter correction in a neuro-PET scanner with the septa retracted, IEEE Conf.
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