R. Harrop

Positron emission tomography in the early diagnosis of Huntington's disease

We studied 10 patients with early Huntingtons disease and 7 normal age-matched controls with positron emission tomography (PET) using fluorodeoxyglucose. Subjects had little or no caudate nucleus atrophy and had not received any medications. The results demonstrated that hypometabolism of glucose preceded tissue loss. Furthermore, patients with minimal neurologic or psychiatric symptoms and no obvious CT changes may be differentiated from normal persons with high accuracy by PET. PET is helpful in the early diagnosis of Huntingtons disease irrespective of the mode of presentation. PET may also be useful for preclinical detection and may supplement information from DNA studies.

Comparison of PET, MRI, and CT with pathology in a proven case of Alzheimer's disease

Positron emission tomography (PET) with fluorodeoxyglucose (FDG), magnetic resonance imaging (MRI), and CT were carried out in a patient with Alzheimers disease 16 months before he died. At autopsy, the gross appearance of the brain correlated with MRI and CT, which showed some regional atrophy. These were much less revealing than PET, which correlated with microscopic findings of neuronal loss and proliferation of glia. In areas of moderately impaired local cerebral metabolic rate of glucose, as revealed by reduced FDG uptake, there was some gliosis, primarily around the numerous senile plaques. In areas of severe metabolic impairment, there was a profound loss of neurons, extensive gliosis, and a diminished appearance of plaques. PET-FDG is a better measure of the severity of Alzheimers disease than MRI or CT, because it reflects the degree of neuronal pathology.

Positron emission tomography and histopathology in Pick's disease

PET using 18F-2-fluoro-2-deoxyglucose was carried out on a case of Picks disease established by necropsy. A sharply decreased cortical metabolic rate for glucose was obtained in specific gyri, especially in the frontal lobes, where there was extensive gliosis and neuronal loss. More moderate decreases were found in areas with numerous Pick bodies and inflated neurons but less gliosis. The PET pattern was sufficiently distinctive to suggest that it might be possible to distinguish Picks from Alzheimers disease premortem.

Analytical calculation of scatter distributions in SPECT projections

In this paper the authors present a method for analytically calculating the distribution of photons detected in single photon emission computed tomography (SPECT) projections. The technique is applicable to sources in homogeneous and nonhomogeneous media. The photon distribution (primary, first-, and second-order Compton scatter) is computed using a precalculated camera-dependent look-up table in conjunction with an attenuation map of the scattering object and a map of the activity distribution. The speed and accuracy of this technique is compared to that of Monte Carlo simulations. The cases considered are a point source in a homogeneous and also in a nonhomogeneous scattering medium, an extended source in a nonhomogeneous medium, and a homogeneous cylinder filled uniformly with activity. The method is quantitatively accurate and faithfully reproduces the spatial distribution of the unscattered and scattered photons. For comparable statistical precision in the peak of the calculated distribution, their approach can result in a gain in calculation time over Monte Carlo simulators. For point sources, the computation times are improved by a factor of 20-150. However, this gain depends on the source configuration, and calculation times become comparable for an 800 voxel source and are five times slower for a 55000 voxel source. The method also offers an increase in the speed of computation of higher order Compton scatter events over a similar analytical technique.

18Fluorodeoxyglucose positron emission tomography studies in presumed Alzheimer cases, including 13 serial scans.

Positron emission tomographic (PET) data on local cerebral metabolic rates for glucose (LCMR) are reported for 32 regions of interest (ROI)s in cross-sectional studies on 57 patients with clinically diagnosed Alzheimers disease (AD) and 20 neurologically normal controls, and in serial studies on 13 of the AD cases, including a familial, young-onset case where the diagnosis has been confirmed at autopsy. Extensive psychological testing was done on all the AD cases. Almost all cortical regions showed a significant decline in LCMR with age in the control subjects. There were the expected cortical metabolic deficits in AD and the serial studies showed a general increase in such deficits over time in 12 of the 13 cases. The regions showing the greatest declines with time in serial studies are the same as those showing the most severe deficiencies in cross-sectional studies. The young-onset case did not show a greater rate of metabolic decline than many of the older cases studied. Results on individual psychological tests tended to correlate with metabolic rates in multiple, rather than single, cortical regions, suggesting intact neuronal networks are required for good performance. The correlations with cortical metabolic activity found were of a sign indicating that the higher the metabolic rates and the better the left:right asymmetry index, the better was the performance.

Dynamic SPECT imaging using a single camera rotation (dSPECT)

Techniques to estimate dynamic parameters from the data acquired during a single rotation of a standard (single, dual and triple head) SPECT camera are being investigated. The performance of the constrained linear least squares (C-LS) method is being assessed. This method uses a simple assumption that the activity within each pixel at a given projection angle is less than or equal to the activity seen at the prior angle. In simulations, we have used an analytical heart model with single and bi-exponential activity decrease, with and without background activity and statistical noise in the data. Studies using a single, a 90/spl deg/ dual and a 120/spl deg/ triple head camera were modeled with a total acquisition time of 20 min, performing rotation over 1800. These simulations were followed by experimental scans of our dynamic phantom used with and without attenuation and subsequently a preliminary study involving a human subject was performed. Dynamic images from simulations and phantom experiments reconstruct well with an accuracy of reconstructed half-lives for most reconstructions within a 20% error range. Some streaking artifacts which are evident in images corresponding to early reconstruction times subside upon incorporating further constraints and regularizations.

Object shape dependent scatter simulations for PET

The correctness of an analytical simulation for single-scattered gamma rays in positron volume imaging was verified by comparing it to Monte Carlo simulation of single scattering. The Monte Carlo simulation was verified using measured tomograph data. The single-scatter analytical simulation was extended to estimate multiple scattering. The multiple-scatter analytical simulation showed good agreement with the Monte Carlo simulation of total object scatter. It is shown using the analytical simulation that the position of a source along the line of response and the shape of the scattering object make a substantial difference in the scatter distribution in the projections. Such projections can be generated in a much shorter time using the analytical simulation than using the Monte Carlo simulation. A scatter correction method which uses the analytical simulation and exploits the inherent smoothness of the scatter distribution to account for three-dimensional effects in scatter distribution and object shape is also proposed. >

Design of an efficient position sensitive gamma ray detector for nuclear medicine

The shortcomings of conventional scintillation cameras are analysed theoretically with a view towards improving performance at gamma ray energies above 140 keV. A camera design is proposed which incorporates several new features to obtain good spatial resolution from thicker crystals of sodium iodide. Computer simulations show that in addition to having good efficiency and spatial resolution, the new design allows parallax error correction and (possibly) Compton scattering correction at gamma energies up to 511 keV.

PET studies of cerebral glucose metabolism in idiopathic torticollis

Regional cerebral glucose metabolism was studied in 16 patients with idiopathic torticollis, using positron emission tomography. Analysis of subcortical regions revealed no consistent focal abnormality of cerebral metabolic rate for glucose, but there was a bilateral breakdown of the normal relationships between the thalamus and basal ganglia. The findings suggest disruption of the pallidothalamic projections in this focal dystonia and may imply a disturbance of GABA.

Performance of the dynamic single photon emission computed tomography (dSPECT) method for decreasing or increasing activity changes

Radionuclide imaging is now widely used whenever functional information is required. We present a new approach to dynamic SPECT imaging (dSPECT method) that uses a single slow rotation of a conventional camera and allows us to reconstruct a series of 3D images corresponding to the radiotracer distribution in the body at various times. Using simulations of various camera configurations and acquisition protocols, we have shown that this method is able to reconstruct washout half-lives with an accuracy greater than 90% when used with triple-head SPECT cameras. Accuracy decreases when using fewer camera heads, but dual-head geometries still give an accuracy greater than 80% for short and 90% for long half-lives and about 50-75% for single-head systems. Dynamic phantom experiments have yielded similar results. Presence of attenuation and background activity does not affect the accuracy of the dSPECT reconstructions. In all situations investigated satisfactory dynamic images were produced. A preliminary normal volunteer study measuring renal function was performed. The reconstructed dynamic images may be presented as a three-dimensional movie showing movement of the tracer through the kidneys and the measurement of the regional renal function can be performed. The time-activity curves determined from this dSPECT data are very similar to those obtained from dynamic planar scans.