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The contribution of the scattered events originating from Radioactivity Outside the tomograph Field of View (ROFOV) is treated differently by different scatter correction algorithms. Using measured data we have evaluated the amount of scatter originating from ROFOV, its axial and transaxial distribution in two energy windows for direct and a subset of oblique planes in 3D PET. Nonuniform distributions were observed with the total contribution found to be non negligible when the amount of ROFOV was comparable to the radioactivity in the Field of View (FOV).

Since many different configurations of brain SPECT systems are available for clinical imaging, it is informative to investigate whether there are significant differences in their performance potentials. However, the difference in configurations and performance characteristics make direct comparison between systems difficult. To clarify the issue, we have developed a method to analyze the relative merits of different basic system geometries and configurations. This method is based on the calculated system geometric efficiency (SGE) which is obtained when the system spatial resolution is normalized to a reference level under a specified imaging condition. During the normalization process, the design parameters and performance of the collimators are iteratively adjusted to match the system resolution of an appropriately-defined reference specification. After the collimator is normalized, the geometric efficiency (GE) of the collimator is calculated by summing the point source geometric efficiency (PSGE) over a brain source model while imposing narrow-beam attenuation effect. The SGE is then derived for each system configuration based on the calculated GE for each collimator. This analysis has been applied to many existing geometries and detector-collimator configurations of current brain SPECT systems.

We have shown in previous works that distinct non-stationary analytical scatter kernels can be extracted from line source measurements and used to independently subtract object scatter and subtract or restore detector scatter in high resolution PET. In this work, the dependence of the scatter components on energy threshold and source position was investigated. Line source measurements were acquired in multispectral mode using the Sherbrooke PET simulator. Scatter parameters were extracted from data summed in energy windows with a lower threshold varying from 129 keV to 516 keV in steps of 42 keV, and a fixed upper threshold of 644 keV. Decreasing the lower threshold from 344 keV to 129 keV increases the trues by only 25%, but increases object scatter by 136% and almost triples detector scatter. A gain in efficiency by a factor of 2 or more would result from recovering the latter by restoration in the broad window. The intensity and shape of the scatter functions for both object and detector are shown to have a significant dependence on energy and position. This dependence needs to be taken into account in the design of kernels for accurate scatter correction over a broad energy range.

We investigated the shift of local energy spectra with SPECT rotation for the GE 400 AT and the Picker Prism 3000 tomographs. A Co-57 flood source was taped to the parallel-beam collimator of the GE 400 AT; a Tc-99m line source was placed at the focus of the fan-beam collimator of one head of the Picker Prism. The count-based method, which employs a narrow window (about 4 keV) on the maximum slope of the photopeak, was used with both systems. Non-linear, polynomial spectral fitting was applied to x-y-E data acquisitions with the GE camera. The fitting yielded either shifts or shifts and width changes. Results show 1) the shifts are pseudo-sinusoidal with angle and similar for different spatial locations, 2) the average of their absolute value is 0.71 keV and 0.13 keV for the GE and Picker cameras, respectively, 3) width changes for the GE camera are small and appear random, 4) the calculated shifts from the count-based method for the central part of the GE camera are correlated with those from the spectral fitting method. They are 12% smaller. The conclusion is that energy shifts with angle may be present with many rotating cameras although they may be smaller with newer cameras. It might be necessary to account for them in schemes designed for high-accuracy compensation of Compton-scattered gamma rays although they possibly could be ignored for newer cameras.

A dedicated 3D brain PET scanner has several advantages, most notably increased sensitivity, over a whole body scanner for neurological studies. However brain scanners have higher scatter fractions, random count-rates and deadtime for the same activity concentration. We have used noise effective count-rate (NECR) analysis to compare brain scanners of 53, 60, and 66 cm diameter with the GE ADVANCE whole body scanner (93 cm diameter). Monte Carlo simulations of a brain-sized phantom (16 cm diameter, 13 length) in the ADVANCE geometry were used to develop a model for NECR performance, which was reconciled to results from a decay series measurement. The model was then used to predict the performance of the brain scanner designs. The brain scanners have noise effective sensitivities (the slope of the NECR curve at zero activity) as much as 40% higher than the body scanner. However, their NECR, advantage diminishes quickly as t;he activity concentration increases. The brain scanners' NECR equals the body scanner with about 0.7-0.8 mCi in the phantom; the body scanner has superior NECR performance at higher activity levels. An imaging center concentrating on only very low activity imaging tasks would find the efficiency advantage of a smaller detector diameter valuable, while a center performing higher activity studies such as bolus water injections or 5mCi FDG injections might prefer the count rate performance of a whole body scanner.

A series of 2D and 3D count rate measurements have been carried out on the Advance PET system. The measurements include: NEMA standard protocols; simultaneous imaging of two small vials containing [O-15]water and [F-18]; a cardiac insert placed in a water filled elliptical tank; two different head sized cylindrical tanks; and a Hoffman 3D phantom. The data were processed with and without dead time and scatter corrections. Plots were made of the total count rate, randoms, trues + scatter, scatter corrected trues, and noise equivalent count rate as a function of total activity in the machine and as the [Mu]Ci/cc in the phantoms. The accuracy of deadtime corrections was also measured. In 2D mode, the NEMA standard protocol resulted in maximum 'trues' count rates of 540,000 cps in high sensitivity mode. In 3D mode the head sized cylindrical phantoms yielded a maximum trues rate of approximately 950,000 cps. Recovery of the water half-life was accurate with up to 50 mCi for the cardiac insert in 2D mode.

QPET is a positron imaging system developed at Queen's University for high-resolution, 3D imaging of small volumes; it includes a pair of planar gamma-ray detectors 25.4 cm square, which rotate about a central axis with a quasi-cylindrical geometry. We describe here the performance of this system. Basic characteristics of the detectors are evaluated: a spatial sampling of 1 mm, a quantum efficiency of 9.3% for 511 keV gamma rays with normal incidence), and a time resolution of 88 ns. Models are developed to characterize the system deadtime and the sensitivity in terms of the noise-equivalent counting rate. With an 8 cm diameter spherical source, the noise-equivalent counting rate reaches a maximum at just over 3 kcps for an activity concentration of 2 [Mu]Ci/cc; the random coincidence events and the deadtime losses both contribute significantly and the scatter contribution is small. Spatial resolution and uniformity over the field of view are evaluated by imaging short and long line sources; a spatial resolution of 2.7 mm in the transverse directions and 2.0 mm in the axial direction is achieved, with excellent uniformity throughout the field of view. The detector response is amplitude invariant across a 20 cm transverse diameter and a 9 cm axial length with the acceptance angle limited to [+ or -]25[degrees] in the axial direction. As an example of the imaging capabilities of QPET, we show 3D images of l8F uptake in the bones of a rat, showing the excellent spatial resolution. This system is best suited to limited-volume applications where high counting rates are not necessary, but where high spatial resolution and uniform detector response are priorities.

Recent decades have witnessed the introduction of remarkable new technologies in many fields, including neurology. However, not all new technology, even that which is of tremendous scientific benefit, necessarily has a role in clinical practice. How, then, to assess the value of new technology for the evaluation and treatment of patients? Traditional assessment generally asks how accurately a new technology might diagnose a particular condition. When such tests are conducted, they usually involve patients already diagnosed with confidence. Therefore, even successful results only confirm that the technology can make diagnoses that were already possible. What of the borderline cases? What about unsuspected cases? Rarely is it demonstrated that a new technology can deal with these questions. Another approach is to compare the new technology with existing technology, but this approach often fails to adequately consider the differences between approaches and the information they obtain. The author proposes that new technologies should be evaluated with a question-driven approach, and he illustrates this approach with SPECT, or single photon emission computed tomography. In this approach, the series of questions asked by a neurologist in the course of evaluating a patient are considered in light of what information would be provided by the new technology. For example, 'Where is the lesion?' would be more satisfactorily answered by computed tomography or MRI, and SPECT would add little. This is also true of the question 'What is the lesion?' If the lesion turns out to be an intracerebral hemorrhage, the additional information provided by SPECT might be interesting, but would not result in better treatment for the patient. However, if the hemorrhage is a subarachnoid hemorrhage, then SPECT might be able to show regions of blood flow restriction prior to the development of signs and symptoms. Such an application may prove to be of great value, and this is an area of SPECT application worthy of further research. The author maintains that by keeping the clinical questions in mind an appropriate evaluation of the role of new technology will become clearer. (Consumer Summary produced by Reliance Medical Information, Inc.)

Positron emission tomography, more commonly called the PET scan, has been a little slow in entering the realm of clinical practice. This may be because unlike CT scans and MRI (magnetic resonance imaging), which provide better pictures of structural lesions, PET scanning provides new information on blood flow and biochemistry in the brain which is simply unprecedented. There are no historical references for comparison and, in many cases, the normal range of values have yet to be established. The fact that PET scans are complementary to CT and MRI, rather than an alternative, will likely become the strong point of PET scanning in the long run. PET requires the use of pharmaceuticals labelled with radioactive elements that emit positrons (free positive electrons) . Perhaps the most commonly used radiopharmaceutical is F-18 fluorodeoxyglucose. This substance is taken up into cells by the glucose transport machinery, but is not metabolized. The level of radioactivity in different regions of the brain therefore represents the level of metabolism in that region. PET is therefore a measure of dynamic activity rather than structure, and in this way resembles the electroencephalogram (EEG). However, the EEG detects only electrical activity near the surface of the brain that is strong enough to produce voltage differences on the scalp. PET, on the other hand, is not limited to the brain's surface, and monitors the entire three-dimensional volume of the brain. The PET scan resolves the brain into cubes about one-quarter milliliter in volume. (This equals a linear resolution of just under two-thirds centimeter.) PET scanning is not limited to glucose metabolism, however, and is indeed limited only by the imagination of the pharmaceutical chemists. Clinical applications include the visualization of lesions prior to the point at which structural lesions appear on MRI or CT and the estimation of boundaries for brain tumors, which rarely appear clear-cut on imaging techniques like CT. Promising applications include the use of PET scanning to distinguish between parkinsonian syndromes with very similar symptoms and to evaluate psychiatric and behavioral disorders. Since the PET scan provides information that is available in no other way, its value to the clinician as well as the researcher is certain to increase. (Consumer Summary produced by Reliance Medical Information, Inc.)

A wide variety of different underlying pathological processes may contribute to the seizures of epilepsy. In the extremely severe cases in which surgery is the last recourse, the portions of the brain removed sometimes are found to contain tumors while other no abnormalities are observed. Sometimes, even patients whose seizures are well controlled by drug therapy are ultimately found to have tumors many years after the initial onset of epilepsy. Positron emission tomography, or PET scanning, has proved to be useful in evaluating patients with uncontrolled complex partial seizures who are candidates for surgical treatment; the technique can reveal areas of depressed cellular metabolism within the brain. Nevertheless, the underlying pathological mechanisms that are responsible for the depressed metabolism often remain unknown. A study was undertaken of 26 patients undergoing surgery for intractable epilepsy to attempt to correlate the appearance of the brain prior to surgery with the pathological findings in the surgical specimens afterwards. Prior to surgery, the patients were evaluated with electroencephalography (EEG); computed tomography (CT) and magnetic resonance imaging (MRI), which are imaging techniques; and PET scanning, which can provide information about the brain's metabolism. Twenty-one of the 26 patients had depressed metabolism in the region corresponding to the seizure focus. Five patients had abnormalities of the seizure focus visible on MRI, while two others had MRI abnormalities that were located elsewhere in the brain, not at the place the seizures began. Eleven patients were found to have gliosis, a 'scarring' process indicating the brain is responding to some sort of damage. In ten of these patients, abnormalities had been detected on the PET scan. Three patients were found to have tumors; in two tumor patients, abnormalities were visible on CT. One tumor patient had a normal PET scan. The results of this study show that PET is a sensitive indicator of physiological abnormalities even when pathological analysis does not reveal any noticeable structural abnormalities. The study was not able to obtain sufficient information, however, to determine if any of the factors measured may be of value in predicting which patients are likely to benefit most from the surgical procedure. (Consumer Summary produced by Reliance Medical Information, Inc.)

Magnetic resonance imaging (MRI) and computed tomography (CT) are different techniques for producing images of internal body structures; these scans are important in diagnosing diseases and injuries. Research has shown that MRI provides more information than CT concerning injuries to the brain, but there has been little research to test the usefulness of MRI specifically for identifying brain injuries in boxers. Twenty-one boxers were evaluated for traumatic brain injuries with both MRI and CT scans; the results from the two methods were compared. The study group included 16 active professional boxers, four active amateur boxers, and one retired professional boxer. Ages ranged from 21 to 66 years. For 18 subjects, the MRI and CT results agreed; 11 boxers showed normal results by both methods and seven showed abnormalities by both methods. For the remaining three boxers, CT results were inconclusive, but MRI confirmed that they were normal. MRI detected some abnormalities that were missed by CT, but the reverse was not true, and thus MRI appears to be the diagnostic imaging technique of choice when examining boxers for traumatic brain injuries. (Consumer Summary produced by Reliance Medical Information, Inc.)