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Biomedical subjects

W H Wong

Publications and source records attributed to W H Wong.

At least 19 recordsLinked to original sources
PubMed1991

Bayesian image processing in magnetic resonance imaging.

In the past several years, image processing techniques based on Bayesian models have received considerable attention. In our earlier work, we developed a novel Bayesian approach which was primarily aimed at the processing and reconstruction of images in positron emission tomography. In this paper, we describe how the technique has been adopted to process magnetic resonance images in order to reduce noise and artifacts, thereby improving image quality. In this framework, the image is assumed to be a statistical variable whose posterior probability density conditional on the observed image is modeled by the product of the likelihood function of the observed data with a prior density based our prior knowledge. A Gibbs random field incorporating local continuity information and with edge-detection capability is used as the prior model. Based on the formalism of the posterior density, we can compute an estimate of the image using an iterative technique. We have implemented this technique and applied it to phantom and clinical images. Our results indicate that the approach works reasonably well for reducing noise, enhancing edges, and removing ringing artifact.

Algorithms↗
PubMed1991

Altered cerebral blood flow and glucose metabolism in patients with liver disease and minimal encephalopathy.

We measured CBF and the CMRglc in normal controls and in patients with severe liver disease and evidence for minimal hepatic encephalopathy using positron emission tomography. Regions were defined in frontal, temporal, parietal, and visual cortex; the thalamus; the caudate; the cerebellum; and the white matter along with a whole-slice value obtained at the level of the thalamus. There was no difference in whole-slice CBF and CMRglc values. Individual regional values were normalized to the whole-slice value and subjected to a two-way repeated measures analysis of variance. When normalized CBF and CMRglc values for regions were compared between groups, significant differences were demonstrated (F = 5.650, p = 0.00014 and F = 4.58, p = 0.0073, respectively). These pattern differences were due to higher CBF and CMRglc in the cerebellum, thalamus, and caudate in patients and lower values in the cortex. Standardized coefficients extracted from a discriminant function analysis permitted correct group assignment for 95.5% of the CBF studies and for 92.9% of the CMRglc studies. The similarity of the altered pattern of cerebral metabolism and flow in our patients to that seen in rats subjected to portacaval shunts or ammonia infusions suggests that this toxin may alter flow and metabolism and that this, in turn, causes the clinical expression of encephalopathy.

Brain↗
PubMed1991

Cerebral ammonia metabolism in patients with severe liver disease and minimal hepatic encephalopathy.

Cerebral ammonia metabolism was studied in five control subjects and five patients with severe liver disease exhibiting minimal hepatic encephalopathy. The arterial ammonia concentration in the control subjects was 30 +/- 7 mumol/L (mean +/- SD) and 55 +/- 13 mumol/L in the patients (p less than 0.01). In the normal subjects, the whole-brain values for cerebral blood flow, cerebral metabolic rate for ammonia, and the permeability-surface area product for ammonia were 0.58 +/- 0.12 ml g-1 min-1 0.35 +/- 0.15 mumol 100 g-1 min-1, and 0.13 +/- 0.03 ml g-1 min-1, respectively. In the patients, the respective values were 0.46 +/- 0.16 ml g-1 min-1 (not different from control), 0.91 +/- 0.36 mumol 100 g-1 min-1 (p less than 0.025), and 0.22 +/- 0.07 ml g-1 min-1 (p less than 0.05). The increased permeability-surface area product of the blood-brain barrier permits ammonia to diffuse across the blood-brain barrier into the brain more freely than normal. This may cause ammonia-induced encephalopathy even though arterial ammonia levels are normal or near normal and explain the emergence of toxin hypersensitivity as liver disease progresses. Greater emphasis on early detection of encephalopathy and aggressive treatment of minimal hyperammonemia may retard the development of ammonia-induced complications of severe liver disease.

Adult↗
PubMed1991

Performance standards in positron emission tomography.

A standard set of performance measurements is proposed for use with positron emission tomographs. This set of measurements has been developed jointly by the Computer and Instrumentation Council of the Society of Nuclear Medicine and the National Electrical Manufacturers Association. The measurements include tests of spatial resolution, scatter fraction, sensitivity, count rate losses and randoms, uniformity, scatter correction, attenuation correction, and count rate linearity correction.

Humans↗
PubMed1990

Temporal analysis of myocardial glucose metabolism by 2-[18F]fluoro-2-deoxy-D-glucose.

To assess kinetic changes of myocardial glucose metabolism after physiological interventions, we perfused isolated working rat hearts with glucose and 2-[18F]fluoro-2-deoxy-D-glucose (2-FDG). Tissue uptake of 2-FDG and the input function were measured on-line by external detection. The fractional rate of 2-FDG phosphorylation was determined by graphical analysis of time-activity curves. The steady-state uptake of 2-FDG was linear with time, and the tracer was retained predominantly in its phosphorylated form. Tissue accumulation of 2-FDG decreased with a reduction in work load and with the addition of competing substrates. Insulin caused a significant increase in 2-FDG accumulation in hearts from fasted but not from fed animals. We conclude that in the isolated working rat heart there is rapid adjustment of exogenous substrate utilization and that most interventions known to alter glucose metabolism induce parallel changes in 2-FDG uptake. Qualitative differences in the in vitro response to insulin may be affected by the presence of either endogenous insulin or glycogen.

Animals↗
PubMed1990

Design and performance of POSICAM 6.5 BGO positron camera.

A high-resolution, whole-body positron camera, POSICAM 6.5 BGO, has been designed, built, and tested; results from it are presented. The camera utilizes 1,320 BGO crystals and 720 PMTS in a staggered geometry to produce high resolution of 5.8 mm FWHM and 21 image planes simultaneously. The axial resolution of the camera is measured at 11.9 mm at the center. High axial sampling is achieved with 5.125 mm separation of the image planes such that three-dimensional imaging of an object can be carried out in a single scan. Recovery of volumetric distribution of radioactivity and object dimensions in axial and sagittal views is demonstrated by imaging spherical objects 13 mm to 39 mm in diameter.

Equipment Design↗
PubMed1988

PET camera performance design evaluation for BGO and BaF2 scintillators (non-time-of-flight).

Bismuth germanate (BGO) and barium fluoride (BaF2) scintillators are presently used in positron emission tomegraphy (PET) cameras. This study evaluates the important PET performance parameters of image resolution, true sensitivity, scatter background, accidental background, and realistic maximum radioactivity in the field of view for both BGO and BaF2 in an identical non-time-of-flight, whole-body camera configuration. These performance parameters are evaluated for three phantoms simulating (a) the head, (b) whole-body cross-section and (c) heart/kidney. This study finds that the high stopping power of BGO yields higher sensitivity, higher resolution, less vignetting, better immunity from scattered gamma, and lower accident/true ratio at any dose level. The faster BaF2 timing acceptance window is traded off by its vulnerability to noncoincidental scatter-gamma which increases accidental coincidences. The BGO is found to be a better choice for non-time-of-flight systems especially with large objects which produce a lot of noncoincidental scatter-gammas. This study also found that the practical diminishing return maximum activity within the field-of-view is approximately 20-25 mCi for existing conventional cameras.

Barium↗
PubMed1986

Noninvasive assessment of coronary stenoses by myocardial perfusion imaging during pharmacologic coronary vasodilation. VIII. Clinical feasibility of positron cardiac imaging without a cyclotron using generator-produced rubidium-82.

The purpose of this study was to determine the clinical feasibility of diagnosing significant coronary artery disease by positron imaging of myocardial perfusion without a cyclotron, using generator-produced rubidium-82 (82Rb). Fifty patients underwent positron emission tomography of the entire heart using a multislice positron camera and intravenous 82Rb or nitrogen-13 ammonia (13NH3) before and after intravenous dipyridamole combined with handgrip stress. Images were read by two observers blinded as to clinical or arteriographic data. Automated quantitative coronary arteriography was obtained for the arteriographic determination of coronary flow reserve, previously demonstrated to be a single integrated measure of stenosis severity accounting for all its geometric dimensions of length, absolute diameter, percent narrowing and asymmetry by quantitative analysis of cine films. Significant coronary artery disease was defined as an arteriographically determined coronary flow reserve of less than 3.0 based on all stenosis dimensions. Any single geometric measure of stenosis severity alone was an inadequate reference standard for comparison with perfusion images. Sensitivity of identifying patients with coronary artery disease having an arteriographically determined coronary flow reserve of less than 3.0 was 95% by positron imaging with a specificity of 100%. The single case that was missed, studied with 13NH3, had a 43% diameter narrowing of a small ramus intermedius off the left coronary artery with no significant narrowing of the major coronary arteries. Positron emission tomography of myocardial perfusion before and after intravenous dipyridamole combined with handgrip stress utilizing generator-produced 82Rb provides sensitive and specific diagnosis of reduced coronary flow reserve due to coronary artery disease in humans.

Coronary Angiography↗
PubMed1986

Positron imaging of myocardial infarction with rubidium-82.

Positron imaging provides tomographic images of regional myocardial perfusion but has required an on-site cyclotron. Rubidium-82 (82Rb) is a short-lived (T1/2 = 75 sec) positron emitter available from a generator. In order to determine the feasibility for its use to image acute myocardial infarction, 18 patients with transmural infarctions who had coronary arteriography were given 30-40 mCi of 82Rb intravenously and positron tomographic imaging was carried out within 96 hr after onset of symptoms. Nine simultaneous transaxial slices were obtained for each patient with a positron camera. Images were also reconstructed in a long-axis, short-axis, and three-dimensional display. One study could not be interpreted because of excessive lung activity. Fourteen normals were also studied. The infarct related artery determined by angiography was correctly diagnosed by positron imaging in all 17 patients as were all three prior infarcts by readers blinded to the clinical data. No defects were observed in normals or in noninfarcted myocardial regions. This study indicates that 82Rb should be useful for perfusion imaging in patients with acute myocardial infarction. The short half-life of 82Rb should make it ideal for providing serial assessment of perfusion in patients undergoing thrombolytic therapy.

Acute Disease↗
PubMed1976

Blood flow and venous distensibility in the upper and lower extremities of newborn infants.

The electrocapacitance plethysmograph was utilized to measure peripheral blood flow and venous distensibility in 17 newborn infants and 20 adults. Measurements were made in the upper and lower extremities in each subject under identical environmental conditions. Blood flow in the forearm and calf were found to be significantly higher in infants than adults. In infants there were no significant differences in the blood flow between the upper and lower extremities. In contrast, in the adults, the blood flow was significantly higher in the upper than in the lower extremities. Similarly, venous distensibility was observed to be higher in infants than in adults. While no significant differences were observed in the venous distensibility between the upper and lower extremities in infants, the venous distensibility was found to be higher in the forearm than in the leg in adults.

Adult↗
PubMed1975

Determination of serum thyroxine after dissociation from thyroxine-binding globulin in alkaline solution and absorption on dextran-coated charcoal.

A competitive protein-binding procedure for determining serum thyroxine is described in which thyroxine is described in which thyroxine is first dissociated from serum thyroxine-binding globulins in an alkaline solution. After binding equilibration, the unbound thyroxine is separated by dextran-coated charcoal. The procedure takes only 100 mul of serum, requires no evaporation, and is sensitive and reproducible. Results by this procedure correlate well with those for a generally accepted method.

Charcoal↗