Special characteristics and potential of single photon emission computed tomography in the brain

Single photon emission tomographic techniques for evaluation of the brain have as their major advantage the ability to employ readily available radionuclides, such as technetium-99m. With present radiopharmaceuticals, single photon emmision tomography of the brain primarily provides morphological information that may be complimentary to standard gamma camera images. Particular areas of assistance have included detection of basal lesions, delineation of multiplicity of lesions, definition of medial extent of abnormalities, clear separation of skull and intracranial abnormalities, and perhaps improved lesion characterization. Overall, however, the reported improvement in sensitivity has been relatively small. To optimally utilize the tomographic and quantiative capabilities of single emission tomography, new classes of radiopharmaceuticals must be developed that can penetrate the blood-brain barrier and provide information on CNS function and pathophysiology. If such radiopharmaceuticals can be labeled with single emission radionuclides, this technique has the potential to provide critically important information. The ultimate outcome of single emission tomographic techniques for the study of the brain may depend on radiopharmaceutical advances.

Multiple photon coincidence tomography

Coincidence scanning devices that measure the distribution of radioisotopes emitting multiple photons in nuclear cascade decays offer a possible supplementary approach to tomography in nuclear medicine. Design factors that serve to determine resolution, sensitivity, and statistical noise for the multiphoton coincidence linear scanner (MCLS), the total organ kinetic imaging monitor (TOKIM), and related systems have been well studied. Focused collimator coincidence scanner (FCCS) systems are capable of extremely high resolution--spherical cold lesions of less than 0.2 cu cm volume being easily detectable. Although FCCS scan speeds are too slow for imaging of large organs, scan times for small organs or for the rescanning of suspicious or ambigous regions appearing on conventional scans are well within practical clinical limits. In view of recent developments in the on-site cyclotron production of short-lived radioisotopes and the current interest in high resolution localization of neurologic receptors in vivo, FCCS systems may prove also to be of value in basic neurophysiologic studies.

In vivo simulation of thallium-201 myocardial scintigraphy by seven-pinhole emission tomography

Seven-pinhole emission tomography has been studied under conditions that simulate clinical myocardial imaging with thallium-201, and is compared with planar imaging with a heart phantom. The seven-pinhole technique produces reconstructed images that offer a tomographic presentation of the object but do not quantitatively represent true cross sections of the object's activity distribution. Tomography produces significantly greater image contrast than planar imaging, even when maximal background subtraction is used to enhance the planar images. Two quantitative limitations of seven-pinhole tomography are noted for a simulated 24-g, 1.5-cm-thick complete transmural infarct: (a) the defect's activity concentration is not accurately reconstructed, and (b) it propagates longitudinally into some reconstructed planes that do not contain it. The imaging limitations of seven-pinhole tomography under the conditions studied are shown to be consistent on several camera/computer/software configurations.

Radioisotope section scanning

Radioisotope section scanning, a relatively new imaging technique, can be regarded as "in vivo autoradiography." It permits, via computer utilization, reconstruction of a three-dimensional image of the distribution of a radioactive tracer within the body. Rapid progress in the design of instrumentation has allowed for initial clinical trials to be carried out. Worldwide, some 30 centers are currently engaged in research in this field, simultaneously ascertaining the physical performance characteristics and figures of merit of the different apparatuses and clarifying the most promising areas of clinical application. This paper is concerned only with radioisotope section scanning via standard radionuclides and radiopharmaceuticals. It does not address itself to work involving positron-emitting radionuclides. Data are given on the physical parameters and areas of clinical application of several types of available equipment.

Theoretical limitations in the immunodiagnostic imaging of cancer with computed tomography and nuclear scanning

In order to help assess the feasibility of using immunologically tagged agents to render tumors detectable with current computed tomographic and nuclear scanners, mathematical formulations were developed to determine the theoretical limits of tumor detection relative to size and depth of the lesions. The results of our analysis suggest that visualization with computed tomography of a tiny tumor (1 sq mm, cross-sectional area) would require binding in the order of 2 x 10(5) iodine atoms/antigenic site, while imaging of a very large (900-sq mm) tumor would require approximately 10(4) atoms/site. Very low energy scanners might reduce these discouraging estimates by an order of 10(2). The immunological imaging of tumors with nuclear scanning appears quite feasible from our formulations, as has been demonstrated by others clinically. Small (1-sq cm) and deep (greater than or equal to 5-cm) tumors appear detectable with uptake ratios of the order of 5 or higher, which seem to be attainable currently. Smaller and deeper tumors require much higher uptake ratios to be detected.

Adrenal scintigraphy: comparison of the Anger tomoigraphic scanner and the large-field gamma camera: concise communication

To evaluate observer performance in adrenal scintigraphy with two different imaging instruments and to assess the requirement for computer processing in routine adrenal imaging, gamma-camera images (analog and computer processed) and Anger tomoscanner studies were independently interpreted by three observers. Thirty-eight pairs of images were used for the analysis of the receiver operating characteristic (ROC) curve in comparing the tomoscanner with the large-field gamma camera, and 44 pairs of images were used for the comparison of the gamma camera analog images with computer-processed images. ROC analysis demonstrated a higher detection rate for the tomographic images. The computer-processed images did not improve adrenal detection. When each case was considered individually, the tomoiscanner was preferred in 37% of cases, and in 50% of cases, both instruments were considered to be equaly valuable.

Quantification of myocardial infarction by computer-assisted positron emission tomography

The accuracy of three-dimensional transverse section positron emission imaging for quantification of myocardial infarction size was validated and compared with the accuracy of two dimensional planar positron imaging. After induction of acute anterior myocardial infarction in anaesthetised dogs, gallium-68 albumin microspheres were injected into the left atrium. Planar and transverse section images of the thorax were obtained with a multicrystal positron camera. After staining with tetrazolium tetrachloride injected intravenously, the hearts were excised, sectioned manually, and planar imaging repeated. Each myocardial infarction was clearly delineated by transverse section imaging with high contrast ratios (mean 0.68 +/- 0.02 SEM); planar imaging identified seven of nine infarcts but with lower contrast ratios (0.24 +/- 0.04; P < 0.001). The volume of infarcted myocardium determined from transverse section images correlated well with true infarct volume (r = 0.94); whereas planar images poorly predicted infarct size (r = 0.63). Thus, computer-assisted transverse section positron imaging provides in vivo localisation of microsphere distribution for improved radioisotopic quantification of myocardial infarction.

Effects of stroke on local cerebral metabolism and perfusion: mapping by emission computed tomography of 18FDG and 13NH3

By means of emission computed tomography (ECT), we used 18F-fluorodeoxyglucose (18FDG) and 13N-ammonia (13NH3) as indicators of abnormalities in local cerebral glucose utilization (LCMRglc) and relative perfusion, respectively. The ECAT positron tomograph was used to scan normal control subject and 10 stroke patients at various times during recovery. In normal subjects, mean CMRglc was 5.28 +/- 0.76 mg per 100 gm tissue per minute (mean +/- SD; N = 8). In patients with stroke, mean CMRglc in the contralateral hemisphere was moderately decreased during the first week, profoundly depressed in irreversible coma, and normal after clinical recovery. Quantification was restricted by incomplete understanding of tracer behavior in diseased brain, but relative local distributions of 18FDG and 13NH3 trapping qualitatively reflected the increases and decreases as well as coupling and uncoupling expected for local alterations in glucose utilization and perfusion in stroke. Early after cerebrovascular occlusion there was a greater decrease in local trapping of 13NH3, than 18FDG within the infarct, probably because of increased anaerobic glycolysis. Otherwise, 18FDG was a more sensitive indicator of cerebral dysfunction than was 13NH3. Hypometabolism, due to deactivation or minimal damage, was demonstrated with the 18FDG scan in deep structures and broad zones of cerebral cortex that appeared normal on x-ray computed tomography and technetium 99m pertechnetate scans. In its present state of development, the 18FDG ECT method should aid in defining the location and extent of altered brain in studies of disordered function after stroke. With improved knowledge of tracer behaviour in diseased brain, the method has promise for mapping the response to therapeutic intervention and increasing our understanding of how the human brain responds to stroke.

Utilization of the computed tomography scanner in interstitial dosimetry

A method is described by which the isodose distribution of interstitial implants can be superimposed on the image of a computed tomography (CT) scan. This technique decreases the uncertainty about the delivered dose, and thus enhances clinical judgment. CT scans of implants in the head and neck area, cervix, vagina, rectum, and urethra are regularly obtained.

Physical attributes of single-photon tomography

Physical properties important for assessment of the potentials of emission computed tomography implemented by collimated detector systems include sensitivity, statistical and angular sampling requirements, attenuation compensation, resolution uniformity, and multisection design constraints. The limited angular range and sampling interval of coded aperture methods for longitudinal tomography impose severe limitations on quantitative imaging capabilities. These methods are discussed. Disadvantages of limited angular range are avoided by transverse section devices that have lower sensitivity than comparable positron devices. It is shown here, however, that the sensitivity for a single section device for head transverse section 2 cm thick can be congruent to 200 events sec-1 microCi-1 per axial cm for 2 x 2 cm resolution. This is 40% of that for a well-designed positron system of similar resolution. The problem of attenuation compensation for constant attenuation, as in brain imaging, is well understood and a fast algorithm of the convolution type gives excellent results if angular sampling is over 360 degrees. Though there is a need to move the single-photon detector array over wide angular distances for adequate sampling, dynamic ECT is possible for the measurement of biological washout kinetics of clinical importance, such as clearance studies in brain. Based on physical principles, experiences with low sensitivity devices, and the prospects for devices with sensitivity comparable to positron tomographs, single photon tomography has sound potentials for research and clinical studies of the adult brain and whole body in small subjects. Practical whole-body tomography in adults is limited to nonquantitative lesion detection.

Double-tracer, multiple-organ transaxial emission computed tomography: improved localization and characterization of radiotracer distributions

Transaxial emission computed tomography (ECT) was performed in dogs with experimentally induced abscesses, using 67Ga and several 99mTc-labeled organ-specific radiopharmaceuticals. Superimposition of 99mTc ECT images onto the 67Ga images gave the precise anatomic location of the abnormal foci of 67Ga uptake, correctly localizing a perinephric abscess, a splenic abscess, and a focus of osteomyelitis. This technique also permits further characterization of regions of decreased activity.

[Radionuclide determination of the state of the venous blood return to the heart in diagnosing pulmonary hypertension]

The method for quantitative evaluation of the transfer of 90mTc-albumin bolus to the heart through the vena cava superior system in scintigraphy on the commonly used gamma chambers with a computer PdP-11/40 is substantiated. It was established that in patients with pulmonary hypertension the average pulmonary circulation time is reduced and the bolus is retained at the level of the trunk of the pulmonary artery, whereas in diminished contractility of the myocardium of the right heart the rate of venous blood return to the heart is reduced and the bolus is retained at the level of the subclavian vein.

A spectral unfolding method to determine source depth distribution

The depth distribution in tissue of a gamma-emitting isotope can be estimated by observing the energy spectrum of photons emerging from the body. Monte Carlo methods were used to compute surface energy spectra for 99Tcm point sources at various depths. From these noise-free results a discrete response matrix was constructed to relate the input source depth to the corresponding output spectrum. Observed spectra with noise were then simulated independently by Monte Carlo calculations assuming various measurement times and source strengths. The source distribution was determined by solving the discrete inverse unfolding problem by the Method of Regularisation. The effective depth resolution is seen to depend strongly upon measurement time. The results show that limited depth resolution can be obtained from a single view of the object region.

Nuclear imaging: new developments

Tomographic images produced with very short-lived radioisotopes of basic biologic elements can provide a form of moving in vivo autoradiography that delineates the dynamic state of the body constituents. The nuclides must be used at the site of the cyclotron where they are produced, but the day may be at hand when such facilities will be common, at least in the larger teaching hospitals.

External assessment of myocardial metabolism with C-11 palmitate in vivo

The externally detected rate of clearance of C-11 palmitate ([11C]palmitic acid) from isolated hearts varies directly with CO2 production from neutral lipids and with physiological indexes of myocardial oxygen consumption. The present study was performed to determine whether myocardial metabolism could be quantified noninvasively in vivo in a fashion analogous to that in the isolated heart. Opened chest, male rabbits were injected with C-11 palmitate (100-200 muCi) and coincidence counts were detected externally with two NaI(TI) crystals so placed that their colinear field of view encompassed the heart. The monoexponential rate of clearance of tracer--obtained from the portion of the residue-detection curve reflecting metabolism of fatty acid incorporated into neutral lipids--correlated directly with induced changes in tension-time index after injections into the left atrium (r = 0.96, n = 12), right atrium (r = 0.86, n = 14), and ear vein (r = 0.93, n = 14). Clearance of labeled palmitate from the vascular pool within the field of detection (determined with both C-14 palmitate and red blood cells labeled with C15O-hemoglobin) was rapid and did not significantly affect measurements of palmitate clearance from the heart itself.

[Measures of regional cerebral blood deficit by inhalation of xenon 133: clinical applications]

Multiple measurements of regional cerebral blood flow (rCBF) after 133 xenon inhalation allowed us to compare results obtained during different forms of migraine and during transient ischaemic attacks, as opposed to a group of normal controls. In normal healthy volunteers (N = 65) classified in decades, there is a stepwise reduction in cortical blood flow (F1) with advancing age (r = .97, p < .0001). The reduction in the slow component of blood flow (F2 congruent to white matter flow) is not significant. In common and classical form of migraine (N = 23)F1 is signifcantly increased during the cephalalgic phase and during the two consectuvie days (p < .01). On the contrary, in accompanied migraine (N = 17), F1 is significantly decreased during the first four days (p < .001) and to a lesser extent from the 4th to the 10th day (N = 5; p < .05). In transient ischaemic attacks (T.I.A.; N = 12) F1 is significantly increased until three weeks after the attack (p < .01). The differences in rCBF in accompanied migraine versus T.I.A. appears to help in the differential diagnosis of clinically difficult cases.

[Positron emission computed tomography and cerebrovascular disorders: significance and principles of the technic, first applications in cerebrovascular diseases]

Positron emission computed tomography is a new technic allowing to map the functional state (metabolism and circulation) of a given human organ. By using the coincidence detection of annihilation radiation emitted by positron emitting radionuclides and applying the three-dimensional reconstruction technic to the regional radionuclide distribution in the tissue, it yields an image of the distribution of a previously administered radionuclide in any desired transverse section of the body and allows through already developed mathematical models the precise measurement of local values of metabolism and circulation. This approach appears to be especially promising for the study of the nomal and diseased brain. It circumvents many of the problems of conventional radionuclides and detection systems. Principles, interest and first results of the technic in relation to cerebrovascular problems are presented.

Regional cerebral blod flow studied by xenon-133. Intra-arterial injection studies and inhalation studies using emission tomography

A survey of the Xenon-133 techniques for measurement of regional cerebral blood flow, rCBF, in man is presented. The intra-arterial Xe-133 injection method is very sensitive for detecting even small hyperemic areas, but cannot "see" smaller ischemic areas. The Xe-133 inhalation (or i.v. inj.) technique is insensitive both to hyperemia and ischemia yielding essentially only a mean flow value. A new rapidly moving single photon tomograph following D. Kuhl's principle is presented applicable to Xe-133. Preliminary clinical data show that this technique is able to detect ischemic areas both with Xe-133 intra-arterial injection and with Xe-133 inhalation. The practical and economic advantages of Xe-133 or Xe-127 tomography over positron tomography for rCBF are discussed.

Positron imaging feasibility studies: selective tumor concentration of 3H-thymidine, 3H-uridine, and 14C-2-deoxyglucose

The potential usefulness of substrates for glycolysis and nucleic acid synthesis as tumor imaging agents was compared to that of 67Ga-citrate. In separate experiments, 3H-thymidine, 3H-uridine, 14C-2-deoxyglucose, and 67Ga-citrate were injected intraveneously into BALB/c mice with solid subcutaneous EMT-6 sarcomas. For the 3H- and 14C-labeled substrates, absolute uptakes in tumor and tumor-to-blood ratios were as high 1 hour after injection as the comparable maximum values achieved for 67Ga-citrate after 48 hours. These studies suggest that positron-labeled analogues of thymidine, uridine, and 2-deoxyglucose should be useful radiopharmaceuticals for tumor imaging by positron-emission tomography.

Nuclear magnetic resonance tomography of the brain

Nuclear magnetic resonance (NMR) has long promised to provide a risk-free alternative method for both structural and functional tomography of the human body, since the original small scale demonstration of its principles by Lauterbur. The considerable technical problems involved in scaling up a successful 8 cm diameter line-scan NMR imaging apparatus have been overcome, and 128 x 128 element NMR tomograms of electronically definable planes less than 1 cm thick through the living brain can be produced routinely in total time of less than 2.5 min. The method used is briefly described, and it is compared with radionuclide emission tomography, which it most resembles. The contrast mechanism in the images and the hazard are also briefly discussed.

An automated method for the alignment of image pairs

The computer comparison of two images of the same organ requires proper alignment of the images before further computer processing. This alignment can be achieved by (a) fixing patient position during the study, (b) alignment methods using analytical transformations, or (c) operator interaction. We propose an automated method based upon the cross-correlation between projections of the images. With fast Fourier transforms, the algorithm becomes computationally cheap.

ECG-gated emission computed tomography of the cardiac blood pool

ECG-gated cross-sectional images of the cardiac blood pool were produced using a specially constructed emission computed tomographic scanner. A pair of large-field-of-view cameras were mounted in opposition in a gantry that rotates 360 degrees about the patient. The coordinates of each detected event, the output of a physiological synchronizer, and the position of the camera heads were input to a dedicated minicomputer which was used to produce the images. Display as a movie permitted evaluation of regional and global wall motion in cross section without the disadvantages of superimposed blood pools as obtained in nontomographic views.

[Myocardial scintigraphy]

Myocardial scintigraphy may be performed at rest or after stress testing using potassium-like cations, labeled metabolites or infarct-fixing tracers. It is an in vivo method of studying myocardial ischaemia and infarction and may provide diagnostic information when other techniques fail. The diagnostic possibilities of myocardial scintigraphy have been increased by the introduction of tomographic techniques and new radioisotopes with very short half lives.

Noninvasive determination of local cerebral metabolic rate of glucose in man

A method for the determination of local cerebral metabolic rates of glucose (LCMRGlc) in normal man is described. The method employs [18F]2-fluoro-2-deoxy-D-glucose (FDG) and emission-computed tomography (ECT). FDG was injected intravenously as a bolus. Radioactivities in separate brain regions were measured with ECT. Plasma FDG concentration following injection was measured from blood samples. A mathematical model that describes the kinetics of FDG transports was employed to determine the transport rate constants of FDG and to convert the radioactivity measurements to metabolic rates. The model has taken into account the possible dephosphorylation reaction from FDG-6-PO4 (FDG-6-P) to free FDG in brain tissues. Experiments were performed in 13 normal volunteers. The rate constants of FDG in man were found to be comparable to those of deoxyglucose in rat and in rhesus monkey. The average LCMRGlc in gray and in white matter were found to be 7.30 +/- 1.18 (SD) and 3.41 +/- 0.64 mg/min per 100 g brain tissue, respectively. The subject-to-subject variation of LCMRGlc as measured by the present method was comparable to those of other methods that measure whole-brain CMRGlc.

Fully three-dimensional positron emission tomography

Fully three-dimensional positron emission tomography is considered and a reconstruction algorithm derived. The reconstruction problem is formulated mathematically as a three-dimensional convolution integral of a point spread function with an unknown positron activity distribution and is solved by Fourier transform methods. Performance of the algorithm is evaluated using both simulated phantom data produced by a Monte Carlo computer program and phantom data obtained from the University of Chicago/Searle Positron Camera. It is concluded that the method is computationally feasible and results in accurate reconstructions.

Myocardial positron tomography with manganese-52m

The biodistribution of radiomanganese (54Mn) was studied in mice, rats, and dogs. Disappearance of radioactivity from the blood was extremely rapid, with a half-time of approximately 0.8 minutes. This resulted in very favorable myocardium-to-blood ratios, even at early times after administration. The myocardial uptake in dogs was greater than 3% at three and 15 minutes, with myocardium-to-blood ratios of about 40:1 at 15 minutes. Positron tomograms obtained with 52mMn clearly demonstrated regional myocardial perfusion. There was good correlation (r = 0.89) of microsphere-to-radiomanganese distribution in the infarcted dog heart.

Cardiac imaging in nuclear medicine

With the rapid improvement in instrumentation and radiopharmaceuticals, cardiovascular nuclear medicine has undergone dramatic growth. Radiotracer techniques for the estimation of myocardial blood flow, metabolism and cardiac hemodynamics have been accepted into routine clinical practice. These techniques are also providing sensitive tools to help us elucidate cardiac physiology and pathophysiology. This review explores a number of recent developments in this rapidly changing field.

Whole-body single-photon emission computed tomography using dual, large-field-of-view scintillation cameras

A whole-body single-photon emission computed tomography system (SPECT) consisting of two large-field-of-view scintillation cameras mounted on a rotatable gantry, a minicomputer and a display station has been designed, constructed and evaluated. In its usual mode of operation, eleven contiguous transverse sections, each 12.5 or 25 mm thick, are reconstructed from projection data acquired during a single, continuous 360 degree rotation lasting from 2 to 22 min. A generalised filtered and weighted backprojection algorithm is used to reconstruct data obtained with conventional parallel-hole collimators in the case of body scanning, or with specially designed fan beam collimators in the case of centrally positioned organs. A simple, yet effective, correction is used to compensate for the effects of gamma ray attenuation within the patient. In addition to providing transverse section images, the system is capable of simultaneous acquisition of opposed conventional scintigrams, the reconstruction of longitudinal section images, and the acquisition of gated cardiac transverse sections. Resolutions in the reconstructed images are typically 15 mm for body scans and 11 mm for brain scans, with only slight variations in sensitivity and resolution within the image. Phantoms and clinical data demonstrate that the SPECT system generates high quality section images while maintaining most of the flexibility of normal scintillation cameras, with the added advantage of dual heads.

Long-chain F-18 fatty acids for the study of regional metabolism in heart and liver; odd-even effects of metabolism in mice

In view of the potential usefulness of fluorine-tagged fatty acids in the study of regional metabolism in the heart and liver, the time courses of uptake and release of 9,10-[18F]fluorostearic acid, 2-[18F]fluorostearic acid, 16-[18F]fluorohexadecanoic acid, 17-[18F]fluoroheptadecanoic acid have been investigated in several organs of NMRI mice. Whereas 2-[18F]fluorostearic acid shows very little uptake in the heart muscle but an increasing accumulation in the liver, the fatty acids with the F-18 label in the middle or at the end of the carbon chain exhibit uptake and elimination behavior similar to that of the analogous C-11-labeled compounds. After rapid concentration in the heart within 1 min, clearance takes place with fast and slow components. 16-[18F]fluorohexadecanoic acid and 17-[18F]fluoroheptadecanoic acid have different half-times of elimination. These differences are also reflected by the fact that nearly all the activity present in the heart can be recovered as fluoride(F-18) in the case of 17-[18F]fluoroheptadecanoic acid, whereas practically no fluoride was found among the metabolites of 16-[18F]fluorohexadecanoic acid. Similar differences were observed for the F-18 activity in bone. The results can be interpreted in terms of the odd-even rule: beta oxidation of even-numbered fatty acids ends up with [18F]fluoroacetic acid, whereas the odd-numbered fatty acids give rise to beta-[18F]fluoropropionic acid. Only in the latter case does dehalogenation take place leading to free fluoride, whereas fluoroacetic acid undergoes further reactions in the citric acid cycle.

Cranial scintigraphy: value of adding emission computed tomographic sections to conventional pertechnetate images (512 cases)

A prospective trial was carried out to assess the contribution made by a radionuclide transverse-section view to conventional radionuclide scans of the brain. Each set of scans was reported by two independent teams of observers, but only one team viewed the tomography section. An abnormality rating was used to decide whether a set of scans was positive or normal. The reports for 512 patients were analyzed and compared with the final independent diagnosis. Greater accuracy of diagnosis with the section view was significant at the 0.1% level; the results showed that failure to detect tumors was almost halved while there was a 16% improvement for the detection of infarcts. Abnormality ratings were also used to plot ROC curves and rating curves; these showed that the addition of the tomographic view increased markedly the proportion of true positives without any increase in the proportion of false-positive reports.

Tomographic measurement of local cerebral glucose metabolic rate in humans with (F-18)2-fluoro-2-deoxy-D-glucose: validation of method

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.