Computed Tomography Positron Research Articles

C-11 labeled palmitic acid for the noninvasive evaluation of regional myocardial fatty acid metabolism with positron-computed tomography: II. Kinetics of C-11 palmitic acid in acutely ischemimic myocardium

C-11 labeled palmitic acid for the noninvasive evaluation of regional myocardial fatty acid metabolism with positron-computed tomography: II. Kinetics of C-11 palmitic acid in acutely ischemimic myocardium

Analytical study of the performance of a multilayer positron computed tomography scanner.

The image-forming performance of multilayer positron tomographs for extended sources is evaluated analytically. The analysis is simplified by "rotation transform," by which three-dimensional photon detection problems are solved by two-dimensional treatment. Event rates of singles, unscattered true coincidence, and a single- and double-scattered coincidence are formulated for a uniform cylinder phantom as functions of various design parameters. Angle factors for Compton scattering and other parameters used in the evaluation are presented. Scatter components in projections and their effect on the reconstructed images are also evaluated. The scatter component in the reconstructed image depends critically on the detector ring radius, phantom radius, method of attenuation correction, etc. When the director radius is relatively small (40 or approximately 45 cm in diameter), the scatter/true ratio at the image center of a 20 cm diameter phantom may be larger than the scatter/true ratio in the event rates. Comparison with experimental data obtained with a head positron tomograph, POSITOLOGICA, showed reasonable agreement both in the total coincidence rates and in the scatter components in the images for a cylindrical phantom of 20 cm in diameter.

L-[4-11C]aspartic acid: enzymatic synthesis, myocardial uptake, and metabolism.

Sterile, pyrogen-free L-[4-11C]aspartic acid was prepared from 11CO2 using phosphoenolpyruvate carboxylase and glutamic/oxaloacetic acid transaminase immobilized on Sepharose supports to determine if it is a useful indicator for in vivo, noninvasive determination of myocardial metabolism. An intracoronary bolus injection of L-[4-11C]aspartic acid into dog myocardium showed a triexponential clearance curve with maximal production of 11CO2 100 s after injection. Inactivation of myocardial transaminase activity modified the tracer clearance and inhibited the production of 11CO2. Positron-computed tomography imaging showed that the 11C activities retained in rhesus monkey myocardium are higher than those observed in dog heart after intravenous injection of L-[4-11C]aspartic acid. These findings demonstrated the rapid incorporation of the carbon skeleton of L-aspartic acid into the tricarboxylic acid cycle after enzymatic transamination in myocardium and suggested that L-[4-11C]aspartic acid could be of value for in vivo, noninvasive assessment of local myocardial metabolism.

A BOUNDARY METHOD FOR ATTENUATION CORRECTION IN POSITRON COMPUTED TOMOGRAPHY

A new method for attenuation correction in positron computed tomography (PCT) has been developed, and it can improve the quality of PCT images. The method requires a short transmission scan by the PCT system. Then boundaries between tissues with significantly different attenuation coefficients are determined from the transmission image by edge-finding techniques. Attenuation correction factors(ACF) are then calculated using these boundaries and the average attenuation coefficients within the enclosed regions. The method has been tested on computer-simulated data, on scans of phantoms, and on patient studies, and has been found effective in reducing the random noise in transmission measurements and in providing more accurate ACFs than the method using geometric attenuation correction. As a result, transmission scan times can be shortened, inconvenience to patients is reduced, and PCT images are improved.

Tomographic mapping of human cerebral metabolism: normal unstimulated state.

Positron computed tomography (PCT) and F-18 2-fluorodeoxyglucose (FDG) were used to study the living human brain in coronal and transverse sections. Scans taken at 1-cm intervals in both directions clearly delineated the cerebral substructures (e.g., cortical areas, basal ganglia, internal capsule, and cerebellum). A compartmental model was used to calculate local cerebral metabolic rates for glucose from the images. Values agreed well with previous global and local human values and demonstrated metabolic symmetry of the left and right hemispheres at rest. Baseline anatomic maps such as these are a necessary prelude to the study of human brain metabolism in stimulated and pathologic states.

Positron computed tomography studies of cerebral glucose metabolism in man: Theory and application in nuclear medicine

The capability of positron computed tomography (PCT) to delineate the substructures of the brain and its facility for accurately measuring the local tissue radioactivity concentration allow the application of tracer kinetic models for the study of local cerebral function in man. This principle and an adaptation of the 14C-deoxyglucose (DG) model of Sokoloff et al. with 18F-2-fluoro-deoxy-D-glucose (FDG) is being used at UCLA. Brookhaven National Laboratory, University of Pennsylvania, NIH, and the Massachusetts General Hospital to determine the local cerebral glucose metabolic rate (LCMRGIc) in normal man at rest and during sensory activation and the changes that occur in patients with a variety of cerebral disorders. Kinetic studies with PCT have been employed to measure the rate constants of the model in different gray and white matter structures of the brain in both normal and ischemic states. The precision of the method in normals has been shown to be about +/- 5% for 1.5-2.0 sq cm regions of the brain. Studies in normals have yielded values for hemispheric CMRGIc that are in agreement with measurement using the Kety-Schmidt technique and LCMRGIc values in agreement with values in monkeys using DG autoradiography. Studies in volunteers subjected to visual and auditory stimulation are demonstrating the potential of this technique for investigating the human brain's response to different stimuli. STudies in patients with stroke show excellent correlation between the degree, extent, and particular structures involved and the clinical symptoms. The method consistently detected hypometabolism in cortical, thalamic, and striatal tissues that were dysfunctional due to deactivation or damage but which appeared normal on x-ray CT. Studies in patients with partial epilepsy have shown hypometabolic zones that highly correlated anatomically with interictal EEG spike foci and were associated with normal x-ray CT studies in 77% of the patients studied. The studies on epilepsy at UCLA have resulted in the integration of the LCMRGIc study into the clinical workup of patients with partial epilepsy that are candidates for surgical resection of their epileptogenic focus (effective June 1979). Studies on Huntington's chorea, Parkinson's disease, aphasia, dementia, schizophrenia, and tumors are in early stage of investigation but also are providing exciting new results. Further studies are needed to determine the role of the local function information obtained with the PCT-FDG method in elucidating the basic mechanism and the potential to aid in improving the approach to medical therapy.

TOMOGRAPHIC MEASUREMENT OF LOCAL CEREBRAL GLUCOSE METABOLIC RATE IN HUMANS WITH (F-18)2-FLUORO-2-DEOXY-D-GLUCOSE

Tracer techniques and quantitative autoradiographic and tissue counting models for measurement of metabolic rates were combined with positron computed tomography (PCT) and (F-18)2-fluoro-2-deoxy-D-glucose (FDG) for the measurement of local cerebral metabolic rate for glucose (LCMRGlc) in humans. A three-compartment model, which incorporates hydrolysis of FDG-6-PO4 to FDG, was developed for the measure of kinetic constants and calculation of LCMRGlc. Our model is an extension of that developed by Sokoloff et al. Although small, hydrolysis of FDG-6-PO4 was found to be significant. A PCT system, the ECAT, was used to determine the rate constants, lumped constant, and stability of the model in human beings. The data indicate that cerebral FDG-6-PO4 in humans increases for about 90 minutes, plateaus, and then slowly decreases. After 10 minutes, cerebral blood FDG activity levels were found to be a minor fraction of tissue activity. Precursor pool turnover rate, distribution volumes, and red blood cell-plasma concentration ratios were determined. Reproducibility (precision) of LCMRGlc measurements (approximate 2 cm2 regions) was +/- 5.5% over a 5-hour period. The replacement of arterial blood sampling with venous sampling was validated.

Design Considerations in Positron Computed Tomography (PCT)

PCT system design is complex and involves optimization and tradeoffs in numerous design parameters. A design approach or system evaluation in terms of FWHM resolution and gross efficiency is insufficientand mist. leading. There are many sources of noise in PCT other than statistical which must be minimized. When they become competitive with statistical noise, then further increases in efficiency will not produce significant improvements in performance and image signal/noise. At this time, circumferential systems are more optimal than planar arrays for PCT. The advantages and disadvantages of the hexagon and circle need to be analytically determined in more detail. The importance of positron emission computed tomography (PCT) has grown over the past five years. The principle objectives of this technique is to combine the noninvasive, analytical measurement of tissue radionuclide concentration of labeled compounds which trace a physiologic process in a known manner, and physiologic models that allow the calculation of physioloqic variables of interest. This approach is referred to as Physiologic Tomography, PT1 PT requires tomographs capable of analytical measurements in man rather than qualitative grey scale imaging of activity distributions. This requires more careful and detailed system design to both control the fundamental factors which can compromise this objective, and to maximize the image signal-to-noise ratio (S/N). This paper will discuss some of the physical aspects, sources of noise, design factors, and design alternatives in PCT.

Analysis of a Cylindrical Hybrid Positron Camera with Bismuth Germanate (BGO) Scintillation Crystals

Theoretical design studies of a multilayer hybrid computed tomographic system which could perform both X-ray transmission computed tomography (XCT) and positron computed tomography (PCT) are presented. Use of BGO scintillation crystals makes it possible to achieve the highest spatial resolution and coincident data collection efficiency possible with the present technology. In the new hybrid system, the multilayer positioning is achieved by use of one dimensional Anger principle with BGO crystals. The ultimate resolution and counting rate attainable with the system is discussed in reference to the true to random ratio. Present study indicates that the resolution and efficiency that can be achieved in PCT mode are 5-6 mm fwhm with efficiency gain of over a hundred in comparison to the existing NaI(Tl) ring system at UCSD.