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Biomedical subjects

T G Turkington

Publications and source records attributed to T G Turkington.

At least 19 recordsLinked to original sources
PubMed2001

Introduction to PET instrumentation.

OBJECTIVE: The purpose of this paper is to introduce technologists to the basic principles of PET imaging and to the instrumentation used to acquire PET data. PET imaging is currently being done on a variety of imaging system types, and the technologist will be introduced to these systems and learn about the basic physical image-degrading factors in PET. After reading this article, the technologist should be able to describe the basics of coincidence imaging, identify at least 3 physical degrading factors in PET, and describe 2 different types of PET scanning systems.

Humans↗
PubMed2000

Attenuation correction in hybrid positron emission tomography.

Attenuation effects are more severe for coincidence imaging than for single-photon imaging. The capability to measure and correct attenuation now exists with dedicated positron emission tomography (PET) scanners. Attenuation correction may or may not improve lesion detection in various situations, but it definitely produces a more realistic radioactivity distribution and is essential for quantitation, which is an important PET capability. For hybrid PET systems, though, which are relatively new and in which neither the performance nor the cost of the scanner can be compromised much compared with the conventional nuclear medicine device alone, attenuation correction is still novel. Just as the entire modality of PET imaging on hybrid gamma cameras has expanded very rapidly, the capability to achieve attenuation correction has quickly followed. Both radioactive source-based and x-ray-based systems exist that provide adequate maps for attenuation correction, and the x-ray systems go even further to provide anatomic detail to aid in image interpretation.

Fluorodeoxyglucose F18↗
PubMed2000

Transmission scanning system for a gamma camera coincidence scanner.

UNLABELLED: The goal of this research was to develop and evaluate a practical transmission scanning system for attenuation correction on a 2-head gamma camera coincidence scanner. METHODS: The transmission system operates in singles mode and uses point sources of 137Cs that emit 662-keV gamma-radiation. Each point source is inserted between existing septa that are normally used to provide an approximately 2-dimensional emission acquisition geometry. The sources are placed along a line parallel to the axis of rotation near the edge of 1 camera. Data are acquired with the opposing camera. The septa provide axial collimation for the sources so that the transmission system operates in a 2-dimensional offset fanbeam geometry. Camera energy and spatial resolution were measured at 511 and 662 keV. Sensitivity was measured at 662 keV. The effects on axial resolution of adding supplemental collimation to the septa were shown. The system was calibrated and tested using a resolution (rod) phantom and a uniformity phantom. Torso phantom data were acquired. Patient transmission and emission scans were obtained. Postinjection transmission data were used to correct patient emission data. RESULTS: The camera resolution at postinjection counting rates was 11.7% full width at half maximum (FWHM) for 662-keV gamma-rays. Intrinsic spatial resolution was 2.7 mm (FWHM) at 662 keV. The sensitivity of the system was 280 Hz/MBq using five 74-MBq sources of 137Cs in the transmission geometry, with supplemental collimation added to the septa to improve axial resolution. The transaxial resolution of the system was such that the smallest rods (6-mm diameter and 12-mm spacing) were well resolved in a reconstructed resolution-phantom image. The corrected patient emission scans were free of attenuation-induced artifacts. CONCLUSION: An easily implemented transmission system for a 2-head gamma camera coincidence scanner that can be used for postinjection transmission scanning has been developed.

Cesium Radioisotopes↗
PubMed1999

Adult age differences in the functional neuroanatomy of verbal recognition memory.

Adult age differences are frequently observed in the performance of memory tasks, but the changes in neural function mediating these differences are largely unknown. We used (H2)15O positron emission tomography (PET) to measure changes in regional cerebral blood flow (rCBF) during Encoding, Baseline, and Retrieval conditions of a recognition memory task. Twelve young adults (20-29 years) and 12 older adults (62-79 years) participated. During each task condition, participants made a two-choice manual response to each of 64 words. Analyses of the performance data yielded evidence of age-related slowing of encoding and retrieval processes, and an age-related decline in the accuracy of yes/no recognition (d'). The rCBF activation associated with both encoding and retrieval was greater for older adults than for young adults, but this pattern was more clearly evident for memory retrieval. For young adults, rCBF activation during retrieval occurred primarily in right prefrontal cortex, whereas older adults exhibited a more bilateral pattern of prefrontal activation. Regression analyses predicting reaction time in the memory task from regional PET counts confirmed that the neural system mediating memory retrieval is more widely distributed for older adults than for young adults. Both age groups exhibited some decrease in rCBF activation in the second half of the test session, relative to the first half. The practice-related decrease in rCBF activation was more prominent for young adults, suggesting that the older adults' recruitment of additional neural systems reflects a more continual allocation of attention to support task performance.

Adult↗
PubMed1999

Multimodality nuclear medicine imaging in three-dimensional radiation treatment planning for lung cancer: challenges and prospects.

The purpose of this study was to determine the utility of quantitative single photon emission computed tomography (SPECT) lung perfusion scans and F-18 fluorodeoxyglucose positron emission computed tomography (PET) during X-ray computed tomography (CT)-based treatment planning for patients with lung cancer. Pre-radiotherapy SPECT (n = 104) and PET (n = 35) images were available to the clinician to assist in radiation field design for patients with bronchogenic cancer. The SPECT and PET scans were registered with anatomic information derived from CT. The information from SPECT and PET provides the treatment planner with functional data not seen with CT. SPECT yields three-dimensional (3D) lung perfusion maps. PET provides 3D metabolic images that assist in tumor localization. The impact of the nuclear medicine images on the treatment planning process was assessed by determining the frequency, type, and extent of changes to plans. Pre-radiotherapy SPECT scans were used to modify 11 (11%) treatment plans; primarily altering beam angles to avoid highly functioning tissue. Fifty (48%) SPECT datasets were judged to be 'potentially useful' due to the detection of hypoperfused regions of the lungs, but were not used during treatment planning. PET data influenced 34% (12 of 35) of the treatment plans examined, and resulted in enlarging portions of the beam aperture (margins) up to 15 mm. Challenges associated with image quality and registration arise when utilizing nuclear medicine data in the treatment planning process. Initial implementation of advanced SPECT image reconstruction techniques that are not typically used in the clinic suggests that the reconstruction method may influence dose response data derived from the SPECT images and improve image registration with CT. The use of nuclear medicine transmission computed tomography (TCT) for both SPECT and PET is presented as a possible tool to reconstruct more accurate emission images and to aid in the registration of emission data with the planning CT. Nuclear medicine imaging techniques appear to be a potentially valuable tool during radiotherapy treatment planning for patients with lung cancer. The utilization of accurate nuclear medicine image reconstruction techniques and TCT may improve the treatment planning process.

Humans↗
PubMed1999

Calculation of attenuation factors from combined singles and coincidence emission projections.

We have developed a simple method for determining coincidence attenuation-correction factors C (the inverse of the total attenuation factors) from collimated singles (SPECT) and coincidence [positron emission tomography (PET)] projections without transmission data. Attenuation-correction factor estimates are determined for individual lines of response (LOR's) independently. The required data can be acquired using a gamma-camera system with coincidence capabilities. A first-order approximation (R) of C for an LOR is given by the product of the singles count rates, taken at each end of the LOR divided by the square of the coincidence count rate. The method was tested using simulated singles and coincidence projections starting with emission and attenuation maps from patient PET scans. Noise and resolution effects were modeled in separate studies. In the noise-free, high-resolution simulations, a scatter plot of the C values versus the corresponding R values for all LOR's produces a well-defined trajectory with little variance. Values of lnR were reconstructed into good quality attenuation maps that compare favorably with the originals. We conclude that the method works well on ideal data. The introduction of noise results in degraded images. In a simulated patient study, lung and outer body boundaries were visible in images produced with 3.2 x 10(4) coincidence counts.

Computer Simulation↗
PubMed1999

Regional cerebral blood flow and depersonalization after tetrahydrocannabinol administration.

OBJECTIVE: The aim of this study was to examine the relationship between depersonalization induced by tetrahydrocannabinol (THC), and regional brain activation. METHOD: Cerebral blood flow (CBF) was measured by means of positron emission tomography (PET) in 59 normal right-handed volunteers before and following intravenous infusions of THC. RESULTS: After THC, CBF showed a global increase which was more marked in the right hemisphere, frontal lobes and anterior cingulate. CONCLUSION: Regression analyses showed positive correlations between the right frontal and anterior cingulate and depersonalization.

Adult↗
PubMed1999

FDG imaging of lung nodules: a phantom study comparing SPECT, camera-based PET, and dedicated PET.

PURPOSE: To evaluate 2-[fluorine-18]fluoro-2-deoxy-D-glucose (FDG) imaging of simulated lung nodules in a realistic chest phantom by using attenuation-corrected and non-attenuation-corrected 511-keV single photon emission computed tomography (SPECT), camera-based positron emission tomography (PET), and dedicated PET imaging. MATERIALS AND METHODS: Spheres with diameters of 6, 10, 13, and 22 mm were placed in the lungs of an anthropomorphic chest phantom to simulate nodules. The lungs, nodules, chest wall, and mediastinum were filled with fluorine-18 activities based on the average radionuclide concentrations in those structures from analysis of attenuation-corrected dedicated FDG PET scans. The image sets were evaluated visually and quantitatively by using contrast and signal-to-noise ratios. RESULTS: Attenuation correction reduced the artificially high apparent uptake in the lungs, restored the spherical shape to the nodules, and provided an accurate outer body contour with appropriate intensity. Dedicated PET depicted all four nodules, camera-based PET depicted the three largest nodules, and SPECT depicted the two largest nodules. Lesion contrast was better on the attenuation-corrected images than on the non-attenuation-corrected images. The signal-to-noise ratio generally was improved with attenuation correction. CONCLUSION: Attenuation correction results in many changes in the images and improves lesion detection.

Fluorodeoxyglucose F18↗
PubMed1999

Aging and recognition memory: changes in regional cerebral blood flow associated with components of reaction time distributions.

We used H(2)15O positron emission tomography (PET) to measure age-related changes in regional cerebral blood flow (rCBF) during a verbal recognition memory task. Twelve young adults (20 to 29 years) and 12 older adults (62 to 79 years) participated. Separate PET scans were conducted during Encoding, Baseline, and Retrieval conditions. Each of the conditions involved viewing a series of 64 words and making a two-choice response manually. The complete reaction time (RT) distributions in each task condition were characterized in terms of an ex-Gaussian model (convolution of exponential and Gaussian functions). Parameter estimates were obtained for the mean of the exponential component (tau), representing a task-specific decision process and the mean of the Gaussian component (mu) representing residual sensory coding and response processes. Independently of age group, both tau and mu were higher in the Encoding and Retrieval conditions than in the Baseline condition, and tau was higher during Retrieval than during Encoding. Age-related slowing in task performance was evident primarily in mu. For young adults, rCBF activation in the right prefrontal cortex, in the Retrieval condition, was correlated positively with mu but not with tau. For older adults, rCBF changes (both increases and decreases) in several cortical regions were correlated with both mu and tau. The data suggest that the attentional demands of this task are relatively greater for older adults and consequently lead to the recruitment of additional neural systems during task performance.

Adult↗
PubMed1999

Gangliogliomas: characterization by registered positron emission tomography-MR images.

OBJECTIVE: The purpose of this study was to correlate 18F-fluorodeoxyglucose positron emission tomography (PET) and MR imaging features of cerebral gangliogliomas before and after PET-MR image registration. CONCLUSION: After registration of PET and MR images, all six gangliogliomas in our series were shown to have heterogeneous metabolic activity. Areas of hypermetabolic activity were seen in all lesions. In five of the six cases, PET-MR image registration provided information regarding tumor metabolism that was not available on nonregistered hard-copy examinations.

Adolescent↗
PubMed1998

Cerebellar activity and disturbed time sense after THC.

Because marijuana continues to be the most commonly used illicit drug, its effects on the brain function are of major interest. We utilized positron emission tomography (PET) and magnetic resonance imaging (MRI) to study the effects of delta-9-tetrahydrocannabinol (THC) infusion on brain blood flow and its behavioral correlates in 46 volunteers. Consistent with previous reports, there was a significant increase in cortical and cerebellar blood flow following THC, but not all subjects showed this effect. Those who showed a decrease in cerebellar CBF also had a significant alteration in time sense. The relationship between decreased cerebellar flow and impaired time sense is of interest because the cerebellum has been linked to an internal timing system.

Adult↗
PubMed1998

Quantification of myocardial perfusion by MRI after coronary occlusion.

The objectives of this study were to define the relationship between the first order constant of Gd-DTPA transfer (K1) and the myocardial blood flow (MBF) at rest and to compare it with an equivalent relationship obtained for positron emission tomography (PET). In a canine model of permanent coronary occlusion (n = 4), myocardial and blood time concentration curves obtained by 13N-ammonia PET and Gd-DTPA-enhanced MRI were fitted by a one-compartment model to determine K1. A linear relationship was observed between MRI-derived K1 and MBF measured by microspheres (K1 = 0.88 x flow -0.015, R = 0.95), which compares favorably with the equivalent relationship derived from PET (K1 = 0.74 x flow +0.16, R = 0.88). The results of this preliminary study suggest that, at rest and distal to a permanently occluded coronary artery, myocardial perfusion quantification by MRI is possible and can challenge PET.

Animals↗
PubMed1997

Marijuana intoxication and brain activation in marijuana smokers.

OBJECTIVE AND METHOD: The acute effects of delta9 tetrahydrocannabinol (THC) on cerebral blood flow (CBF) were studied in human subjects. Regional CBF was measured with 15O-water and Positron Emission Tomography (PET) in 32 volunteers with a history of exposure to marijuana. Scans were performed before and after intravenous (I.V.) infusion of either of two doses of THC or a placebo, given under double blind conditions. RESULTS: THC but not placebo increased CBF especially in the frontal regions bilaterally, insula and cingulate gyrus and sub-cortical regions with somewhat greater effects in the right hemisphere. While most regions showed significant change at 60 minutes for the lower dose group, the higher dose group had significant change at 30 and 60 minutes. There was a highly significant change in the anterior/posterior ratio for the two THC groups reflecting minimal change in occipital flow but significant increases in frontal flow. Self ratings of THC intoxication showed significant effects, and regression analysis indicated it correlated most markedly with the right frontal region. CONCLUSION: Behavioral manifestations of marijuana intoxication may be associated with increased functional activity of the brain especially the frontal cortex, insula and cingulate gyrus.

Adult↗
PubMed1997

Fully Bayesian estimation of Gibbs hyperparameters for emission computed tomography data.

In recent years, many investigators have proposed Gibbs prior models to regularize images reconstructed from emission computed tomography data. Unfortunately, hyperparameters used to specify Gibbs priors can greatly influence the degree of regularity imposed by such priors and, as a result, numerous procedures have been proposed to estimate hyperparameter values from observed image data. Many of these procedures attempt to maximize the joint posterior distribution on the image scene. To implement these methods, approximations to the joint posterior densities are required, because the dependence of the Gibbs partition function on the hyperparameter values is unknown. In this paper, we use recent results in Markov chain Monte Carlo (MCMC) sampling to estimate the relative values of Gibbs partition functions and using these values, sample from joint posterior distributions on image scenes. This allows for a fully Bayesian procedure which does not fix the hyperparameters at some estimated or specified value, but enables uncertainty about these values to be propagated through to the estimated intensities. We utilize realizations from the posterior distribution for determining credible regions for the intensity of the emission source. We consider two different Markov random field (MRF) models-the power model and a line-site model. As applications we estimate the posterior distribution of source intensities from computer simulated data as well as data collected from a physical single photon emission computed tomography (SPECT) phantom.

Algorithms↗
PubMed1996

Adult age differences in regional cerebral blood flow during visual world identification: evidence from H215O PET.

We used H215O PET to investigate adult age differences in regional cerebral blood flow (rCBF) during the performance of a visual word identification task. The study participants were 20 healthy, right-handed men: 10 young adults between 18 and 27 years of age, and 10 older adults between 63 and 75 years of age. The word identification task comprised six blocks of test trials representing four task conditions; subjects responded manually. The task conditions varied with regard to whether semantic retrieval was required (e.g., word/nonword discrimination vs simple response to each stimulus) and with regard to the difficulty of visual encoding (e.g., words presented normally vs words with asterisks inserted between adjacent letters). Each subject performed all six trial blocks, concurrently with each of six H215O PET scans. Analyses of quantitative CBF data obtained from the arterial time-activity curve demonstrated a significant age-related decline in global CBF rate. Analyses of the changes in rCBF between task conditions indicated that retrieval of semantic information sufficient to distinguish words from nonwords is mediated by a ventral occipitotemporal cortical pathway. Specific areas within this pathway were also associated with visual encoding processes. Several rCBF activations were significantly greater for young adults than for older adults, indicating an age-related decline in processing efficiency within this ventral occipitotemporal pathway. Although the performance data demonstrated a greater age-related slowing for visual encoding than for semantic retrieval, these age-related performance changes were not associated with corresponding changes in rCBF activation.

Adolescent↗
PubMed1995

Quantitation of 211At in small volumes for evaluation of targeted radiotherapy in animal models.

We have evaluated SPECT and two planar imaging methods, geometric mean (GM) and buildup factor (BF), for their potential to quantitate in vivo 211At distributions in rat spinal subarachnoid spaces using phantom studies. The use of medium-energy collimators and the small diameter (3 mm) of the subarachnoid space complicate quantitation. Net activities from distributions in various backgrounds were obtained using a large region of interest with background subtraction. Results showed quantitation accuracy within 10% for SPECT and BF in low backgrounds increasing to 25% at higher background levels while GM errors ranged from 20 to 45%. We have also obtained images of [211At]astatide distributions, administered intrathecally, in rats.

Animals↗