Performance of the Tachyon Time-of-Flight PET Camera

We have constructed and characterized a time-of-flight Positron Emission Tomography (TOF PET) camera called the Tachyon. The Tachyon is a single-ring Lutetium Oxyorthosilicate (LSO) based camera designed to obtain significantly better timing resolution than the ~ 550 ps found in present commercial TOF cameras, in order to quantify the benefit of improved TOF resolution for clinically relevant tasks. The Tachyon's detector module is optimized for timing by coupling the 6.15 × 25 mm² side of 6.15 × 6.15 × 25 mm³ LSO scintillator crystals onto a 1-inch diameter Hamamatsu R-9800 PMT with a super-bialkali photocathode. We characterized the camera according to the NEMA NU 2-2012 standard, measuring the energy resolution, timing resolution, spatial resolution, noise equivalent count rates and sensitivity. The Tachyon achieved a coincidence timing resolution of 314 ps +/- ps FWHM over all crystal-crystal combinations. Experiments were performed with the NEMA body phantom to assess the imaging performance improvement over non-TOF PET. The results show that at a matched contrast, incorporating 314 ps TOF reduces the standard deviation of the contrast by a factor of about 2.3.

A detector response function design in pinhole SPECT including geometrical calibration

Clinical single photon emission computed tomography (SPECT) equipped with pinhole collimators have a magnification factor that results in high spatial resolution images for small animal imaging. Using Monte Carlo simulations to model the acquisition process and the propagation of the photons from their point of emission to their detection point then integrating the model into an iterative reconstruction algorithm improves the signal-to-noise ratio, the contrast and the spatial resolution in the reconstructed images. However, pinhole SPECT systems are known to be very sensitive to geometrical misalignments. Geometrical misalignments are defined as the radial or axial shift of the collimator pinhole and/or twist and tilt of the detector heads and are introduced in the system each time the collimation device is changed (pinhole to parallel holes or vice versa). In this work, we present a flexible detector response function table (DRFT) design that takes into account the geometrical misalignments and avoids performing new Monte Carlo simulations for each exam in order to calculate a geometrical study-dependent system matrix. The utilization of the DRFT for the calculation of the system matrix speeds up its computation time by two orders of magnitude making it acceptable for preclinical and clinical applications.

Apollo-Soyuz light-flash observations

While dark adapted, two Apollo-Soyuz astronauts saw eighty-two light flash events during a complete 51 degrees orbit which passed near the north magnetic pole and through the South Atlantic Anomaly. The frequency of events at the polar parts of the orbit is 25 times that noted in equatorial latitudes and no increased frequency was noted in the South Atlantic Anomaly at the 225-km altitude. The expected flux of heavy particles at the northern and southern points is 1-2 min-1 per eye, and the efficiency for seeing HZE particles which were below the Cerenkov threshold is 50%.

Effects of temporal modelling on the statistical uncertainty of spatiotemporal distributions estimated directly from dynamic SPECT projections

Artefacts can result when reconstructing a dynamic image sequence from inconsistent single photon emission computed tomography (SPECT) projection data acquired by a slowly rotating gantry. The artefacts can lead to biases in kinetic parameters estimated from time-activity curves generated by overlaying volumes of interest on the images. Insufficient sampling and truncation of projections by cone-beam collimators can cause additional artefacts. To overcome these sources of bias in conventional image based dynamic data analysis, we have been investigating the estimation of time-activity curves and kinetic model parameters directly from dynamic SPECT projection data by modelling the spatial and temporal distribution of the radiopharmaceutical throughout the projected field of view. In the present work, we perform Monte Carlo simulations to study the effects of the temporal modelling on the statistical variability of the reconstructed spatiotemporal distributions. The simulations utilize fast methods for fully four-dimensional (4D) direct estimation of spatiotemporal distributions and their statistical uncertainties, using a spatial segmentation and temporal B-splines. The simulation results suggest that there is benefit in modelling higher orders of temporal spline continuity. In addition, the accuracy of the time modelling can be increased substantially without unduly increasing the statistical uncertainty, by using relatively fine initial time sampling to capture rapidly changing activity distributions.

Theoretical study of lesion detectability of MAP reconstruction using computer observers

The low signal-to-noise ratio (SNR) in emission data has stimulated the development of statistical image reconstruction methods based on the maximum a posteriori (MAP) principle. Experimental examples have shown that statistical methods improve image quality compared to the conventional filtered backprojection (FBP) method. However, these results depend on isolated data sets. Here we study the lesion detectability of MAP reconstruction theoretically, using computer observers. These theoretical results can be applied to different object structures. They show that for a quadratic smoothing prior, the lesion detectability using the prewhitening observer is independent of the smoothing parameter and the neighborhood of the prior, while the nonprewhitening observer exhibits an optimum smoothing point. We also compare the results to those of FBP reconstruction. The comparison shows that for ideal positron emission tomography (PET) systems (where data are true line integrals of the tracer distribution) the MAP reconstruction has a higher SNR for lesion detection than FBP reconstruction due to the modeling of the Poisson noise. For realistic systems, MAP reconstruction further benefits from accurately modeling the physical photon detection process in PET.

Kinetic analysis of 125I-iodorotenone as a deposited myocardial flow tracer: comparison with 99mTc-sestamibi

The goal of this investigation was to assess the accuracy of 7'-Z-[125I]iodorotenone (125I-iodorotenone) as a new deposited myocardial flow tracer and compare the results with those for 99mTc-sestamibi.

METHODS

The kinetics of these two flow tracers were evaluated in 25 isolated, erythrocyte- and albumin-perfused rabbit hearts over a flow range relevant to patients. The two flow tracers and a vascular reference tracer (131I-albumin) were introduced simultaneously as a compact bolus through a port just above the aortic cannula in the absence of tracer recirculation. Myocardial extraction, retention, washout, and uptake parameters were computed from the venous outflow curves using the multiple-indicator dilution technique and spectral analysis.

RESULTS

The extraction of 125I-iodorotenone was much higher than the extraction of 99mTc-sestamibi (0.84 +/- 0.05 vs. 0.48 +/- 0.10, respectively, P < 0.001). 125I-iodorotenone extraction was also less affected by flow than was 99mTc-sestamibi (P < 0.001). Net retention of 125I-iodorotenone was significantly greater than 99mTc-sestamibi net retention at 1 min (0.77 +/- 0.08 vs. 0.41 +/- 0.11, respectively, P < 0.001) and 26 min (0.46 +/- 0.13 vs. 0.27 +/- 0.11, respectively, P < 0.001) after tracer injection. Flow had less effect on 125I-iodorotenone net retention than on 99mTc-sestamibi net retention 1 min after tracer injection (P < 0.04). However, at 26 min, flow had an equivalent effect on the retention of both flow tracers (P < 0.4). The relationship between 125I-iodorotenone and 99mTc-sestamibi washout was complex and depended on elapsed time after isotope introduction and perfusion rate. Reflecting the favorable extraction and retention characteristics of 125I-iodorotenone, both its maximum myocardial uptake and its 26-min uptake were more closely related to flow than were those of 99mTc-sestamibi (P < 0.001 for both comparisons).

CONCLUSION

The extraction and retention of 125I-iodorotenone were greater than those of 99mTc-sestamibi, making 125I-iodorotenone the superior flow tracer in the isolated rabbit heart.

Direct least-squares estimation of spatiotemporal distributions from dynamic SPECT projections using a spatial segmentation and temporal B-splines

Artifacts can result when reconstructing a dynamic image sequence from inconsistent, as well as insufficient and truncated, cone beam single photon emission computed tomography (SPECT) projection data acquired by a slowly rotating gantry. The artifacts can lead to biases in kinetic model parameters estimated from time-activity curves generated by overlaying volumes of interest on the images. However, the biases in time-activity curve estimates and subsequent kinetic parameter estimates can be reduced significantly by first modeling the spatial and temporal distribution of the radiopharmaceutical throughout the projected field of view, and then estimating the time-activity curves directly from the projections. This approach is potentially useful for clinical SPECT studies involving slowly rotating gantries, particularly those using a single-detector system or body contouring orbits with a multidetector system. We have implemented computationally efficient methods for fully four-dimensional (4-D) direct estimation of spatiotemporal distributions from dynamic SPECT projection data. Temporal B-splines providing various orders of temporal continuity, as well as various time samplings, were used to model the time-activity curves for segmented blood pool and tissue volumes in simulated cone beam and parallel beam cardiac data acquisitions. Least-squares estimates of time-activity curves were obtained quickly using a workstation. Given faithful spatial modeling, accurate curve estimates were obtained using cubic, quadratic, or linear B-splines and a relatively rapid time sampling during initial tracer uptake. From these curves, kinetic parameters were estimated accurately for noiseless data and with some bias for noisy data. A preliminary study of spatial segmentation errors showed that spatial model mismatch adversely affected quantitative accuracy, but also resulted in structured errors (projected model versus raw data) that were easily detected in our simulations. This suggests iterative refinement of the spatial model to reduce structured errors as an area of future research.

List-mode maximum-likelihood reconstruction applied to positron emission mammography (PEM) with irregular sampling

We present a preliminary study of list-mode likelihood reconstruction of images for a rectangular positron emission tomograph (PET) specifically designed to image the human breast. The prospective device consists of small arrays of scintillation crystals for which depth of interaction is estimated. Except in very rare instances, the number of annihilation events detected is expected to be far less than the number of distinguishable events. If one were to histogram the acquired data, most histogram bins would remain vacant. Therefore, it seems natural to investigate the efficacy of processing events one at a time rather than processing the data in histogram format. From a reconstruction perspective, the new tomograph presents a challenge in that the rectangular geometry leads to irregular radial and angular sampling, and the field of view extends completely to the detector faces. Simulations are presented that indicate that the proposed tomograph can detect 8-mm-diameter spherical tumors with a tumor-to-background tracer density ratio of 3:1 using realistic image acquisition parameters. Spherical tumors of 4-mm diameter are near the limit of detectability with the image acquisition parameters used. Expressions are presented to estimate the loss of image contrast due to Compton scattering.

Analytical propagation of errors in dynamic SPECT: estimators, degrading factors, bias and noise

Dynamic SPECT is a relatively new technique that may potentially benefit many imaging applications. Though similar to dynamic PET, the accuracy and precision of dynamic SPECT parameter estimates are degraded by factors that differ from those encountered in PET. In this work we formulate a methodology for analytically studying the propagation of errors from dynamic projection data to kinetic parameter estimates. This methodology is used to study the relationships between reconstruction estimators, image degrading factors, bias and statistical noise for the application of dynamic cardiac imaging with 99mTc-teboroxime. Dynamic data were simulated for a torso phantom, and the effects of attenuation, detector response and scatter were successively included to produce several data sets. The data were reconstructed to obtain both weighted and unweighted least squares solutions, and the kinetic rate parameters for a two-compartment model were estimated. The expected values and standard deviations describing the statistical distribution of parameters that would be estimated from noisy data were calculated analytically. The results of this analysis present several interesting implications for dynamic SPECT. Statistically weighted estimators performed only marginally better than unweighted ones, implying that more computationally efficient unweighted estimators may be appropriate. This also suggests that it may be beneficial to focus future research efforts upon regularization methods with beneficial bias-variance trade-offs. Other aspects of the study describe the fundamental limits of the bias variance trade-off regarding physical degrading factors and their compensation. The results characterize the effects of attenuation, detector response and scatter, and they are intended to guide future research into dynamic SPECT reconstruction and compensation methods.

Estimating glucose metabolism using glucose analogs and two tracer kinetic models in isolated rabbit heart

The purpose of this investigation was to 1) evaluate the relative accuracy of the Sokoloff and Patlak tracer kinetic models in estimating glucose metabolic rate (GMR) in the presence and absence of insulin; 2) evaluate the effect of nutritional state on the lumped constant (LC); and 3) compare the kinetics of 2-fluoro-2-deoxy-D-[14C]glucose (FDG) and 2-deoxy-D-[3H]glucose (DG) membrane transport and phosphorylation. The experimental preparation was the isolated, red blood cell-albumin-perfused rabbit heart. Our results showed that both tracer kinetic models provided GMR estimates that correlated well with the Fick method (for FDG, R = 0. 84 and 0.91 for the Sokoloff and Patlak models, respectively); nutritional state did not affect the LC; and FDG and DG have different transport and/or phosphorylation parameters. We also observed that 1) the addition of a fourth compartment to the Sokoloff model reduced the mean squared error between measured and modeled data by a factor of 7.4; 2) a longer time (21.8 min) was required to obtain a linear phase of the Patlak plot than is allowed in clinical studies; and 3) accurate GMR estimates were obtained only by using different LCs reflecting insulin's presence or absence. Our results indicate potential sources of error in the use of FDG and positron emission tomography to quantify GMR in patients.

Kinetic parameter estimation from SPECT cone-beam projection measurements

Kinetic parameters are commonly estimated from dynamically acquired nuclear medicine data by first reconstructing a dynamic sequence of images and subsequently fitting the parameters to time-activity curves generated from regions of interest overlaid upon the image sequence. Biased estimates can result from images reconstructed using inconsistent projections of a time-varying distribution of radiopharmaceutical acquired by a rotating SPECT system. If the SPECT data are acquired using cone-beam collimators wherein the gantry rotates so that the focal point of the collimators always remains in a plane, additional biases can arise from images reconstructed using insufficient, as well as truncated, projection samples. To overcome these problems we have investigated the estimation of kinetic parameters directly from SPECT cone-beam projection data by modelling the data acquisition process. To accomplish this it was necessary to parametrize the spatial and temporal distribution of the radiopharmaceutical within the SPECT field of view. In a simulated chest image volume, kinetic parameters were estimated for simple one-compartment models for four myocardial regions of interest. Myocardial uptake and washout parameters estimated by conventional analysis of noiseless simulated cone-beam data had biases ranging between 3-26% and 0-28%, respectively. Parameters estimated directly from the noiseless projection data were unbiased as expected, since the model used for fitting was faithful to the simulation. Statistical uncertainties of parameter estimates for 10,000,000 events ranged between 0.2-9% for the uptake parameters and between 0.3-6% for the washout parameters.

An investigation into the effect of input function shape and image acquisition interval on estimates of washin for dynamic cardiac SPECT

Dynamic cardiac SPECT and PET can be used to measure myocardial perfusion by estimating the kinetic rate constant describing the washing of radioactive-labelled tracers from the blood to the extravascular myocardial tissue. Because of differences in photon statistics and data acquisition techniques, protocols which produce optimal estimates of the washin for dynamic cardiac PET may give suboptimal estimates if applied in dynamic cardiac SPECT. Two important factors in the estimation of washin are the shape of the tracer input function and the image acquisition interval. This study uses computer simulations to investigate the effect of varying the tracer infusion length and image acquisition interval on the bias and variance of estimates of washin obtained with dynamic cardiac SPECT and 99mTc-labelled teboroxime. Bias in parameter estimates can be introduced by aliasing, integration of the time-varying radioactivity by the detector, and detector motion. This bias can be reduced by decreasing the acquisition interval and using a longer-duration input function. However, this results in poor photon statistics, which generate large variance, and can also introduce bias in the estimates of the washin. Our studies indicate that better estimates of the washin are obtained by using an acquisition interval that is of sufficient duration to obtain adequate photon statistics even if this is at the expense of temporal resolution. The increase in bias caused by using a 10 or 20 s acquisition interval instead of a 5 s acquisition interval is minimal when compared with the reduction in variance. Variance in estimates is also reduced by using a sharp input function, resulting in higher peak counts during washin. It is also shown that the variance of estimates of the washin increases generally when faster kinetics are observed. This variance can, however, be reduced by using longer acquisition intervals.

Consolidation of common parameters from multiple fits in dynamic PET data analysis

In dynamic positron emission tomography (PET) data analysis, regions of interest (ROI's) are analyzed by fitting a parametric model to the time-activity curve acquired after a radio-labeled tracer has been introduced into the patient's bloodstream. This procedure can be carried out for multiple ROI's and/or multiple injections of the same or a different radiopharmaceutical. The approach presented here takes advantage of prior knowledge that some of the parameters of those multiple fits are the same. Reduction of the total number of parameters to be estimated results in smaller statistical uncertainty for all parameter estimates, especially those common to multiple fits.

A methodology for specifying PET VOI's using multimodality techniques

Volume-of-interest (VOI) extraction for radionuclide and anatomical measurements requires correct identification and delineation of the anatomical feature being studied. We have developed a toolset for specifying three-dimensional (3-D) VOI's on a multislice positron emission tomography (PET) dataset. The software is particularly suited for specifying cerebral cortex VOI's which represent a particular gyrus or deep brain structure. A registered 3-D magnetic resonance image (MRI) dataset is used to provide high-resolution anatomical information, both as oblique two-dimensional (2-D) sections and as volume renderings of a segmented cortical surface. VOI's are specified indirectly in two dimensions by drawing a stack of 2-D regions on the MRI data. The regions are tiled together to form closed triangular mesh surface models, which are subsequently transformed into the observation space of the PET scanner. Quantification by this method allows calculation of radionuclide activity in the VOI's, as well as their statistical uncertainties and correlations. The methodology for this type of analysis and validation results are presented.

Consequences of using a simplified kinetic model for dynamic PET data

We compared a physiological model of 82Rb kinetics in the myocardium with two reduced-order models to determine their usefulness in assessing physiological parameters from dynamic PET data.

METHODS

A three-compartment model of 82Rb in the myocardium was used to simulate kinetic PET ROI data. Simulations were generated for eight different blood-flow rates reflecting the physiological range of interest. Two reduced-order models commonly used with myocardial PET studies were fit to the simulated data, and parameters of the reduced-order models were compared with the physiological parameters. Then all three models were fit to the simulated data with noise added. Monte Carlo simulations were used to evaluate and compare the diagnostic utility of the reduced-order models. A description length criterion was used to assess goodness of fit for each model. Finally, fits to simulated data were compared with fits to actual dynamic PET data.

RESULTS

Fits of the reduced-order models to the three-compartment model noise-free simulated data produced model misspecification artifacts, such as flow parameter bias and systematic variation with flow in estimates of nonflow parameters. Monte Carlo simulations showed some of the parameter estimates for the two-compartment model to be highly variable at PET noise levels. Fits to actual PET data showed similar variability. One-compartment model estimates of the flow parameter at high and low flow were separated by several s.d.s for both the simulated and the real data. With the two-compartment model, the separation was about one s.d., making it difficult to differentiate a high and a low flow in a single experiment. Fixing nonflow parameters reduced flow parameter variability in the two-compartment model and did not significantly affect variability in the one-compartment model. Goodness of fit indicated that, at realistic noise levels, both reduced-order models fit the simulated data at least as well as the three-compartment model that generated the data.

CONCLUSION

The one-compartment reduced-order model of 82Rb dynamic PET data can be used effectively to compare myocardial blood-flow rates at rest and stress levels. The two-compartment model can differentiate flow only if a priori values are used for nonflow parameters.

Kinetic analysis of rubidium and thallium as deposited myocardial blood flow tracers in isolated rabbit heart

Evaluation of myocardial perfusion with tracers such as thallium and rubidium is based on the assumption that tissue tracer content is proportional to flow. The purpose of this study was to evaluate the relationship between flow and tissue tracer content of 201Tl and 83Rb in the isolated perfused rabbit heart. 83Rb (86-day half-life), an isotope that is not used clinically, was used as a subsitute for 82Rb (76-s half-life) to improve the accuracy and precision of data acquisition. The multiple indicator-dilution technique was employed with two independent computational approaches. The first approach explicitly deconvolved 201Tl and 83Rb venous concentration curves by the intravascular reference tracer curve. The second approach used a conventional analysis. Both approaches showed that there was more early washout of 83Rb than 201Tl and that the heart retained 201Tl better than 83Rb within 2 min after isotope introduction. These data indicate that 201Tl is a better perfusion tracer than 83Rb in the isolated rabbit heart.

Strategies for extraction of quantitative data from volumetric dynamic cardiac positron emission tomography data

The ability of positron emission tomography (PET) to serve as a useful myocardial perfusion indicator is well established. We describe a methodology for obtaining reliable quantitative kinetic parameters from dynamic cardiac PET data. Reconstructed images of the myocardium are subdivided into three-dimensional volumes of interest which are used to obtain quantitative measures of myocardial perfusion over physiologically meaningful anatomical regions. The quantitation technique rigorously models the uncertainty of estimated parameters while compensating for effects such as patient motion and partial volumes to arrive at model parameters with well-established confidence intervals.

Equivalent methods to analyse dynamic experiments in which the input function is noisy

A comparison is made between two methods of parameter estimation for analysis of dynamic experiments in which the input function is noisy. Noise in the input function leads to uncertainties in the calculated model-predicted values, and therefore the covariance matrix of the residuals is a function of the model parameters. Statistical uncertainties in the model-predicted values significantly change the nature of the fitting process and the quality of the results. The initial method uses a weighted least-squares criterion where the weighting matrix is the inverse of the full covariance matrix of the residuals, incorporating both the noise in the output data and the noise in the input function. The methodology was applied to dynamic emission tomography studies of the heart, where the blood (input) and tissue (output) tracer concentrations at each time are derived from two regions of interest in the same tomographic section. The second method introduces additional parameters to describe the input function, and adds terms to the weighted sum of squares which comprise the criterion. Instead of only summing the weighted terms to account for differences between the model and the output function, there is a second set of terms to account for the differences between the model and the input function. The two methods have different theoretical bases and appear to optimize different criteria, but it is shown here that they are equivalent to one another. The criterion which they minimize is the same under certain matrix invertibility constraints, which must be satisfied to ensure the stability of either method.

Temporal lobe metabolic differences in medication-free outpatients with schizophrenia via the PET-600

Regional cerebral glucose metabolic rates (rCMRglc) were compared in 18 unmedicated outpatients with schizophrenia and 11 normal controls using high resolution positron emission tomography (PET) and the tracer [F-18]-2-fluoro-2D-deoxyglucose (FDG). From previous work we expected to see abnormal hippocampal rCMRglc in the patients, but no striatal abnormalities. Trial-by-trial Stroop cognitive task, which has been shown to activate the anterior cingulate, was performed within a day of the PET study. As our patients performed abnormally on the Stroop we tested for a correlation between the anterior cingulate rCMRglc and Stroop performance. We found no whole slice cortical average glucose metabolic abnormalities. As, predicted we found abnormally decreased left hippocampal rCMRglc in the patients. No striatal or cingulate rCMRglc abnormalities were noted in patients, but they demonstrated a highly positive correlation between anterior and cingulate rCMRglc and Stroop facilitation. Patients with higher Stroop interference had more prominent hippocampal metabolic decreases. These localized temporal lobe abnormalities could account for some of the patient's positive symptoms and are consistent with recent findings in the literature.

Variability and reproducibility of rubidium-82 kinetic parameters in the myocardium of the anesthetized canine

Kinetic analysis of 82Rb dynamic PET data produces quantitative measures which could be used to evaluate ischemic heart disease. These measures have the potential to generate objective comparisons of different patients or the same patient at different times. To achieve this potential, it is essential to determine the variability and reproducibility of the kinetic parameters.

METHODS

A total of 48 82Rb dynamic PET datasets were acquired from two pure bred beagles. Each animal underwent eight 82Rb PET studies with essentially the same protocol for three successive weeks. Data were acquired with the Donner 600-Crystal Positron Tomograph (PET600). In each week, single-slice dynamic 82Rb PET datasets were collected with the animal at rest at three different gantry positions separated by 5 mm. Additional dataset were collected after dipyridamole infusion and after administration of aminophylline to induce a return to rest. A two-compartment kinetic model with correction for myocardial vasculature and spillover from the left ventricular blood pool was used to analyze the dynamic datasets. Model parameters for uptake (k1), washout (k2) and vascular fraction (fv) were estimated in 11-14 myocardial regions of interest (ROIs) using a weighted least-squares criterion. Statistical fluctuation due to the PET acquisition process was minimized by using a relatively high 82Rb dose (about 30 mCi) to take advantage of the high count rate capacity of the PET600.

RESULTS

The variation in mean k1, where the mean is taken over the myocardial ROIs was 10%-20% (Dog 1) and 15%-50% (Dog 2) among the rest studies conducted on the same date. Similar variation was evident in comparing studies in the same animal for different weeks.

CONCLUSION

Spatial and temporal variation in estimates of the uptake rate (k1) of 82Rb in the resting myocardium of the anesthetized canine are small in relation to the functional increase in k1 following dipyridamole infusion.

In vivo NMR spectral parameter estimation: a comparison between time and frequency domain methods

We have compared various methods of in vivo NMR spectral parameter estimation, namely a nonlinear fit of the free induction decay signal in the time domain (NLTD), a nonlinear fit of the fast Fourier transform of the FID data in the frequency domain using either a continuous Lorentzian model (NLLM) or a Fourier-sampled model (NLFM), and a time-domain linear prediction method using singular value decomposition (LPSVD). Monte Carlo simulations of 31P and 13C in vivo experiments were used to assess the bias and statistical uncertainties of spectral parameters obtained with each method. In the 31P case, all methods appear to be equivalent except the LPSVD method that led to significantly biased peak amplitudes (up to 28%). In the 13C case, the only methods able to recover the glycogen peak were the NLTD method and its equivalent in the frequency domain (NLFM). In both the 31P and the 13C cases simulations demonstrated that 256 data points were sufficient. These results demonstrate the feasibility and the robustness of a nonlinear fit of the FID data in the time domain, and we illustrate this on 31P and 13C data sets obtained in humans.

Diminished glucose transport in Alzheimer's disease: dynamic PET studies

Dynamic positron emission tomography with [18F]fluorodeoxyglucose was used in six patients with Alzheimer's disease (AD) and seven healthy age-matched control subjects to estimate the kinetic parameters K1*, k2*, and k3* that describe glucose transport and phosphorylation. A high-resolution tomograph was used to acquire brain uptake data in one tomographic plane, and a radial artery catheter connected to a plastic scintillator was used to acquire arterial input data. A nonlinear iterative least-squares fitting procedure that included terms for the vascular fraction and time delay to the peripheral sampling site was used to fit a three-compartment model to the brain data. Regions studied included frontal, temporal, occipital, and the entire cortex and subcortical white matter. The values obtained for the individual rate constants and regional CMRglc (rCMRglc; calculated using regional values of the rate constants) were higher than those reported previously. A significant (p less than 0.05) decrease was found in K1* in frontal and temporal cortex in the AD patients compared with the controls, with values of 0.157 and 0.161 ml/g/min in frontal and temporal cortex, respectively, of controls and 0.127 and 0.126 ml/g/min in frontal and temporal cortex of the AD patients. rCMRglc was also significantly (p less than 0.02) lower in the AD patients than controls in all cortical brain regions. Lower values of k3* were found in all brain regions in the AD patients, although these were not statistically significant. These findings provide evidence of an in vivo abnormality of forward glucose transport in AD.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical evaluation of a high-resolution (2.6-mm) positron emission tomography

The intrinsic resolution of the Donner 600-crystal positron emission tomograph (PET 600) is 2.6 mm full width at half maximum (FWHM) in-plane and 6 mm FWHM axially. More than 100 patients with glioma, radiation necrosis, Alzheimer disease, or epilepsy have been studied with this system. Approximately 1 million events are acquired in 15 minutes, starting 1 hour after injection of 10 mCi (370 MBq) of fluorine-18-fluorodeoxyglucose. Normal structures as small as the superior colliculi and the external capsule have been resolved. Improved separation of the cortical ribbon from adjacent white matter has allowed more accurate determination of cortical metabolic rate. In two of 15 patients undergoing evaluation for recurrent glioma, the PET 600 images showed tumor uptake that was not apparent on a lower-resolution study. A high-activity orbiting transmission source with electronic collimation allows accurate, short-duration transmission measurements to be made after radiopharmaceutical administration. The anatomic detail seen on the transmission images can be used for reproducible patient positioning with an accuracy of 1-2 mm perpendicular to the image plane. These findings demonstrate the practicality and clinical effectiveness of high-resolution positron emission tomography.

Regional cerebral glucose transport and utilization in Alzheimer's disease

We performed dynamic positron emission tomographic (PET) studies of glucose utilization, using (18F) 2-fluoro-2-deoxy-D-glucose (FDG), in patients with probable Alzheimer's disease (AD) and healthy age-matched controls, to evaluate blood-brain-barrier glucose transport and glucose utilization rates in the disease. We found no significant differences in rate constants for glucose transport (k1 and k2) and phosphorylation (k3), nor for the vascular fraction (fv), between the 2 groups, although k3 and fv were relatively depressed in temporal cortex in AD. Absolute rates of glucose use were depressed in temporal and parietal cortex, and relative rCMRglc rates were lower in frontal, temporal, parietal, and occipital cortices. These data suggest that in AD bidirectional glucose transport is intact, and that temporal-parietal hypometabolism is present upon a background of widespread cortical metabolic impairment.

Kinetic data analysis with a noisy input function

Methods of parameter estimation are proposed for the analysis of dynamic experiments in which the input function is noisy. Noise in the input function leads to uncertainties in the calculated model-predicted values, and therefore the covariance matrix of the residuals is a function of the model parameters. These statistical uncertainties in the model-predicted values significantly change the nature of the fitting process and the quality of the results. The proposed optimisation methods use weighted least-squares criteria, and three choices for the weighting matrix are considered. The proposed weighting matrices, in order of complexity are: (1) the identity matrix (no weighting), (2) the covariance matrix of the data (ignoring the noise in the input function), and (3) the full covariance matrix of the residuals (incorporating both the noise in the data and the noise in the input function). The methodology is applied to dynamic emission tomography studies of the heart, where the blood (input) and tissue tracer concentrations at each time are derived from two regions of interest in the same tomographic slice. Computer stimulations of compartmental systems show that parameters and their covariance matrix are more accurately estimated when the full covariance matrix of the residuals is used as a weighting matrix rather than either of the other two methods. For the practical example considered, parameter bias was increased by a factor of at least four when the noise in the input function was ignored, and one parameter had a bias of 24% when the unweighted least-squares criterion was used.

Methodologic factors affecting PET measurements of cerebral glucose metabolism

Measurements of cerebral glucose utilization rates in similar populations of human subjects under similar conditions vary considerably because of methods used in data collection and analysis. Using data acquired in two patients, we evaluated the effects of time schedule of data collection, region of interest size, method of attenuation correction, and input function shape on LCMRglu determined by dynamic positron emission tomographic scanning and calculation of rate constants. These different strategies of data acquisition and analysis produced variations of 3 to 14% in calculated LCMRglu. These factors, in conjunction with the well described effects of instrument resolution and sensitivity may account for data discrepancies in the literature.

Dynamic PET data analysis

A general method for estimating the precision of parameters resulting from the use of various experimental designs (rate of injection and rate of tomographic data collection) in emission tomography studies is proposed. The sensitivity matrix of the study model and an estimate of the statistical uncertainty of the tomographic data are used to compute the covariance matrix of the parameters. The determinant of this covariance matrix (proportional to the total volume of uncertainty of the model parameters) serves as a criterion to be minimized. The method is applied to a three-compartment, three-transfer rate constant for glucose metabolism using dynamic positron emission tomography, and a comparison of various current protocols is made with simulated data. The results show that higher rates of injection and higher rates of tomographic data collection at early times lead to smaller statistical uncertainties for the estimates of rate constants. However, for the range of rate constants encountered in practice, differences are insignificant when an initial scan duration less than 30 s is used, without regarding the injection duration.

An attenuated projector-backprojector for iterative SPECT reconstruction

A new ray-driven projector-backprojector which can easily be adapted for hardware implementation is described and simulated in software. The projector-backprojector discretely models the attenuated Radon transform of a source distributed within an attenuating medium as line integrals of discrete pixels, obtained using the standard sampling technique of averaging the emission source or attenuation distribution over small square regions. Attenuation factors are calculated for each pixel during the projection and backprojection operations instead of using precalculated values. The calculation of the factors requires a specification of the attenuation distribution, estimated either from an assumed constant distribution and an approximate body outline or from transmission measurements. The distribution of attenuation coefficients is stored in memory for efficient access during the projection and backprojection operations. The reconstruction of the source distribution is obtained by using a conjugate gradient or SIRT type iterative algorithm which requires one projection and one backprojection operation for each iteration.

Dead time correction and counting statistics for positron tomography

A correction for loss of events due to dead time in dynamic positron emission tomography (PET) is presented. The model employs a paralysing dead time to describe the behaviour of a tomograph over the range of event rates normally encountered in patient studies (up to 200 000 events/s per detector layer). The Donner 280-crystal positron tomograph has a dead time of 1.8 microseconds/event for observed count rates less than 200 000 events/s. The dead time correction factor is 1.8 at 180 000 events/s. The correction is applied to projection data and region of interest analysis of dynamic PET studies, and formulae for the covariances between corrected projection data and between counts in regions of interest in different images from the same dynamic study are established. At 180 000 events/s, the variance of the actual (corrected) number of events in a region containing 3.34 X 10(5) actual events is predicted from the model to be 3.86 X 10(6) (events)2, more than 10 times the variance that would be expected from a naive assumption of Poisson statistics. These statistical results are verified experimentally. An error of 25% is observed in myocardial flow if dead time compensation is not applied, showing the necessity for this correction.

A new fast algorithm for the evaluation of regions of interest and statistical uncertainty in computed tomography

A new algorithm for region of interest evaluation in computed tomography has been developed. Region of interest evaluation is a technique used to improve quantitation of the tomographic imaging process by summing (or averaging) the reconstructed quantity throughout a volume of particular significance. An important application of this procedure arises in the analysis of dynamic emission computed tomographic data, in which the uptake and clearance of radiotracers are used to determine the blood flow and/or physiologic function of tissue within the significant volume. The new algorithm replaces the conventional technique of repeated image reconstructions with one in which projected regions are convolved and then used to form multiple vector inner products with the raw tomographic data sets. Quantitation of regions of interest is made without the need for reconstruction of tomographic images. The computational advantage of the new algorithm over conventional methods is between a factor of 20 and a factor of 500 for typical applications encountered in medical science studies. The greatest benefit of the new algorithm (and the motivation for its development) is the ease with which the statistical uncertainty of the result is computed. The entire covariance matrix for the evaluation of regions of interest can be calculated with relatively few operations.

Instrumentation for positron emission tomography

Positron emission tomography with a spatial resolution of 2 mm full width at half maximum for quantitation in regions of interest 4 mm in diameter will become possible with the development of detectors that achieve ultrahigh resolution. Improved resolution will be possible using solid-state photodetectors for crystal identification or photomultiplier tubes with many small electron multipliers . Temporal resolution of 2 seconds and gating of cyclic events can be accomplished if statistical requirements are met. The major physical considerations in achieving high-resolution positron emission tomography are the degradation in resolution resulting from positron range, emission angle, parallax error, detector sampling density, the sensitivity of various detector materials and packing schemes, and the trade off between temporal resolution and statistical accuracy. The accuracy of data required for physiological models depends primarily on the fidelity of spatial sampling independent of statistical constraints.

Regional cerebral metabolic alterations in dementia of the Alzheimer type: positron emission tomography with [18F]fluorodeoxyglucose

Alzheimer disease is the most common cause of dementia in adults. Despite recent advances in our understanding of its anatomy and chemistry, we remain largely ignorant of its pathogenesis, physiology, diagnosis, and treatment. Dynamic positron emission tomography using [18F]fluorodeoxyglucose (FDG) was performed on the Donner 280-crystal ring in 10 subjects with dementia of the Alzheimer type and six healthy age-matched controls. Ratios comparing mean counts per resolution element in frontal, temporoparietal, and entire cortex regions in brain sections 10 mm thick obtained 40-70 min following FDG injection showed relatively less FDG uptake in the temporoparietal cortex bilaterally in all the Alzheimer subjects (p less than 0.01). Left-right alterations were less prominent than the anteroposterior changes. This diminished uptake was due to lowered rates of FDG use and suggests that the metabolic effects of Alzheimer disease are most concentrated in the temporoparietal cortex. Positron emission tomography is a most powerful tool for the noninvasive in vivo assessment of cerebral pathophysiology in dementia.

Brain tumor evaluation using Rb-82 and positron emission tomography

Positron emission tomography of the brain with 75-sec rubidium-82 obtained from a portable generator (25-day Sr-82 leads to Rb-82) was used to evaluate the integrity of the blood-brain barrier (BBB) in patients with brain tumors. Rubidium is normally excluded from the central nervous system by the intact BBB, but when the BBB is disrupted by a tumor. Rb enters and pools in the extravascular spaces of the central nervous system. Since Rb is also rapidly cleared from the blood, a high tissue-to-blood ratio of the Rb-82 tracer is achieved in regions of BBB disruption after intravenous injection. With dynamic positron emission tomographic imaging, the extravasation of the Rb tracer can be evaluated independent of the intravascular Rb concentration, and very small changes in the BBB permeability can be detected. The results of our studies in eight patients show that this technique is a promising method for evaluation of the BBB integrity in brain-tumor patients.U

Quantitative potentials of dynamic emission computed tomography

Statistical uncertainties in emission computed tomography were simulated in 60 computer studies involving various numbers of events and distributions of activity. Previous studies have shown that for a uniform disc of activity of rms percentage of uncertainty per resolution cell is: 120 X (number of resolution cells)1/4 X (number of events per resolution cell)- 1/2. In this work we examined the more general situation where one or two regions of uniform activity are surrounded by a uniform background, and found that for an equal number of recorded events the uncertainties were reduced when the activity was concentrated in a portion of the field. The empirical relation rms % uncertainty in nt = 120(N)1/4(nt)-3/4, where nt is the number of events in an average target (organ) resolution cell and N is the total number of events recorded, satisfactorily described the relationships between uncertainties, contrast, total number of detected events, and number of resolution cells for all 60 computer studies. By means of this relation, we show the theoretical possibility of gated cardiac imaging with 20% uncertainty in 1 cm X 1 cm regions, and of 1-sec cerebral blood-flow images with 20% uncertainty in 2 cm X 2 cm regions.

The effects of a finite number of projection angles and finite lateral sampling of projections on the propagation of statistical errors in transverse section reconstruction

The dependence of noise amplification on the nuber of projection angles and on the lateral sampling interval of projections is presented. It is shown that about 1-5D/d angles and a sampling interval of about 0-5d are required in order that the data be efficiently utilized. D is the linear dimension of the reconstruction region and d is the linear dimension of the cells into which the reconstruction region is subdivided (resolution length). Values for noise amplification are given for various combinations of projection angles and lateral sampling intervals.

Emission computer assisted tomography with single-photon and positron annihilation photon emitters

Computed transverse section emission tomography using 99mTc with the Anger camera is compared to positron annihilation coincident detection using a ring of crystals and 68Ga. The single-photon system has a line spread function (LSF) of 9 mm full width at half maximum (FWHM) at the collimator and gives a transverse section reconstruction LSF of 11 mm FWHM with 144 views. The positron ring has a LSF of 6 mm at the center with a transverse section reconstruction LSF of 7.5 mm FWHM. Correction for uniformity of detector response and accurate center of rotation determination is essential in both techniques. The signal-to-noise ratio in a reconstruction is diminished by a factor of 1.2 x (number of resolution elements)1/4 over that expected from the average number of events per resolution element. Attenuation compensation causes more noise to appear in the center than the edge for both modes and an average increase in uncertainty of 30%. The effects of attenuation result in more loss of data for positron coincidence imaging than for single-photon imaging even at energies of 80 keV. For a 20-cm cylinder imaged in transverse section, only 20% of the positron annihilation events are not scattered; however, at 140 keV, 40% of the photons are not scattered. The relative crystal efficiency gives single-photon imaging an advantage of 5. On the other hand, the solid angle advantage of positron photon coincidence imaging is about 100 for the comparisons of this paper. Taking these factors into account, we find positron-computed section imaging has a tenfold increase in sensitivity over multiple-view imaging with the scintillation camera, which gives multiple sections but requires camera or patient rotation.