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J W Wallis

Publications and source records attributed to J W Wallis.

17 recordsLinked to original sources
PubMed1992

Clinically important characteristics of maximum-likelihood reconstruction.

SPECT images of a Jaszczak rod phantom, a single-slice Hoffman brain phantom and a uniform water-bath were acquired. Simulated noisy bar phantoms incorporating depth-dependent attenuation and blur were produced and compared to simulations with depth-independent attenuation and blur, as is the case in PET. Following iterative maximum-likelihood reconstruction, regularization was performed with use of Gaussian filters. While correction for attenuation is achieved in approximately 10 iterations, spatial resolution in the SPECT reconstructions, quantified by contrast in the bar simulations and by visual inspection of the real data, was highly nonuniform, being poorest at the center and improving toward the periphery. Image resolution continued to improve well beyond 50 iterations when regularization was applied that maintained a constant signal-to-noise ratio. Contrast in the simulated PET data also improved with increasing iterations, but the PET data showed uniform contrast throughout the transaxial slices at all numbers of iterations.

Algorithms↗
PubMed1992

Fast maximum-likelihood reconstruction.

A significant problem in application of maximum-likelihood reconstruction to SPECT and PET studies is the very long computation time required for this iterative algorithm. An approach is presented in which the basic calculations involved in projection and backprojection are performed only once and stored in computer memory, thus significantly reducing the computation time.

Algorithms↗
PubMed1991

Gated cardiac blood pool studies in atrial fibrillation: role of cycle length windowing.

Cycle length windowing is gaining increasing acceptance in gated blood pool imaging of patients with atrial fibrillation (AF). The goals of this study were: to assess differences of ejection fraction (EF) in AF with and without windowing and to determine how EF varied with cycle length in patients with AF. Twenty patients with AF were prospectively studied by gated blood pool imaging, with simultaneous collection in each patient of 5-7 studies with cycle length windows spanning the cycle length histogram. Each window accepted beats of only a narrow range of cycle lengths. EF was determined for each of the narrow cycle length windows as well as for the entire gated blood pool study without cycle length windowing. For every patient an average of the windowed EFs was compared with the non-windowed EF. EF values were similar (mean windowed: 46.6; non-windowed: 45.5; P = 0.16), and there was a good correlation between the two techniques (r = 0.97). The data were then examined for a relationship of EF with cycle length. The difference from average windowed EF (delta EF) was calculated for each window and plotted vs. the cycle length of the center of each window. No predictable linear or nonlinear relationship of delta EF with window position was observed. Lack of predictable variation of EF with cycle length is likely due to lack of a predictable amount of ventricular filling for a given cycle length, as the amount of diastolic filling in AF depends on the random cycle length of the preceding beat.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial Fibrillation↗
PubMed1991

Three-dimensional display in nuclear medicine and radiology.

As three-dimensional displays become more common in nuclear medicine and radiology, it is important to know when they should be employed. Selection of the appropriate three-dimensional rendering method requires an understanding of the techniques and both their advantages and limitations. This review article is intended to provide an introduction to three-dimensional rendering techniques. Methods in surface mapping, surface rendering, and volume rendering are presented, with discussion of specific clinical applications in nuclear medicine, computed tomography, and magnetic resonance imaging.

Diagnostic Imaging↗
PubMed1990

The clinical role of radionuclide imaging in cardiac transplantation.

Cardiac transplantation is developing into a routine therapy with widespread availability. Nuclear medicine studies play an important role in evaluating potential transplant recipients and in providing documentation of the severity of ventricular dysfunction and the presence of ischemic but viable myocardium. Nuclear cardiology has increased our knowledge of the physiology of cardiac transplants and is now playing an increasing adjunctive role in management. Radionuclide ventriculography performed early after transplantation can identify patients both with abnormal biventricular function resulting from preservation injury and with isolated right ventricular dysfunction. It also provides important functional data that complements the cardiac biopsy in allograft rejection. New techniques such as 111In antimyosin antibodies and 111In lymphocytes have potential value for the accurate, noninvasive diagnosis of rejection. PET promises to improve the ability to diagnose coronary atherosclerosis of the allograft.

Coronary Disease↗
PubMed1990

Design and use of PET tomographs: the effect of slice spacing.

With modern positron tomographs producing 14, 21, or more transaxial slices, the effects of slice spacing on quantitative reconstruction and three-dimensional displays must be evaluated. This analysis can be approached in terms of the partial volume effect, quantified by the recovery coefficient, or in terms of sampling theory leading to the concept of aliasing. The axial recovery coefficient varies as a function of the position of an object in relation to the slices, with greater variability for larger slice spacings and finer axial resolutions. The aliased image power varies in the same way. The variability in the recovery coefficient and aliasing increase when smaller objects are imaged. Tomographs should be designed with slice spacing approximately half the full-width at half-maximum axial resolution of the tomograph; finer spacing does not appear to confer significant advantages. Thus, quantification and display in positron tomography depend on slice spacing, resolution, and object size.

Equipment Design↗
PubMed1990

The clinical utility of radionuclide ventriculography in cardiac transplantation.

To assess ventricular function in patients who have undergone cardiac transplantation, 247 radionuclide ventriculograms were performed on 94 patients. During the first three days after transplantation, 19% demonstrated left ventricular dysfunction and 41% showed isolated right ventricular dysfunction. In 95 cases, radionuclide ventriculography was performed within 24 hr of myocardial biopsy. A reduction in left ventricular ejection fraction to less than 50% was significantly more common with moderate-severe rejection than with mild rejection. In six instances in which there was discordance between ventriculography and biopsy, radionuclide ventriculography proved particularly useful: three cases showed severe left ventricular dysfunction despite only mild rejection by biopsy, and three cases with ventricular dysfunction from rejection were missed by the initial biopsy. Thus, radionuclide ventriculography can provide functional data in transplant patients that is complementary to myocardial biopsies since biopsy grade is a poor predictor of left ventricular function and biopsy can miss significant rejection.

Biopsy↗
PubMed1990

Three-dimensional functional images of myocardial oxygen consumption from positron tomography.

Images from positron emission tomography (PET) are usually presented as transaxial slices portraying tissue radioactivity. Studies can be difficult to interpret from transaxial images, and the temporal changes in tissue tracer concentrations which permit quantitative determinations of metabolism and perfusion are not displayed. We have developed a method to give quantitatively accurate three-dimensional images of myocardial oxygen consumption from serial images of the myocardial washout of carbon-11-acetate. Following i.v. bolus injection, data are collected for 20-30 min. The time-activity curves for each pixel in the transaxial slices are fit to a monoexponential function to determine the washout rate, which is directly related to the rate of myocardial oxygen utilization. Thus, functional images of myocardial oxygen consumption are produced for all seven slices of PET data. A previously developed method is then used to generate realistic and quantitatively accurate three-dimensional images.

Acetates↗
PubMed1990

Volume rendering in three-dimensional display of SPECT images.

Three-dimensional display utilizing volume rendering is valuable in the display of SPECT data. Volume rendering enhances continuity of structures compared to slice display, and cine display of volume rendered images facilitates understanding of spatial relationships. Image contrast and noise characteristics were analyzed for depth-weighted maximum activity projection, a form of volume rendering proposed by the authors for "hot spot" imaging in nuclear medicine. Contrast in the rendered images was nearly equal to that in slice display, and substantially improved compared to planar images. Image noise was reduced compared to both projection and slice display. Rendered images may be generated automatically in only a few minutes following filtered backprojection, permitting routine clinical use.

Bone and Bones↗
PubMed1989

A hyper-recombination mutation in S. cerevisiae identifies a novel eukaryotic topoisomerase.

A hyper-recombination mutation was isolated that causes an increase in recombination between short repeated delta sequences surrounding the SUP4-omicron gene in S. cerevisiae. The wild-type copy of this gene was cloned by complementation of one of its pleiotropic phenotypes, slow growth. DNA sequence of the clone revealed a 656 amino acid open reading frame capable of encoding a protein homologous to the bacterial type I topoisomerase. No homology was detected with previously identified eukaryotic topoisomerases. Construction of double mutants with either of the two known yeast topoisomerase genes revealed synergistic effects on growth suggesting overlapping functions. Expression of bacterial topoisomerase I in yeast can fully complement the slow growth defect of a null mutation. We have named this locus TOP3 and suggest that it defines a novel eukaryotic topoisomerase gene.

Amino Acid Sequence↗
PubMed1989

Interactive reconstruction in single-photon tomography.

A new method is described to allow interactive selection of the reconstruction filter at the time of interpretation of images from single-photon tomography. In the filtered back projection algorithm, the only part of the reconstruction process requiring user interaction is the selection of the window function. Since the ramp and window filters have different purposes, they can be separated, placing the window at the end of the reconstruction process as a three-dimensional filter. All stages of reconstruction except the window filtering are performed before the physician begins to interpret the study. The three-dimensional filtering is performed very rapidly with use of the Chebyshev convolution algorithm. A 64 x 64 x 64 pixel cube of data is filtered in 13-33 s using filters of 3-11 lengths. Smaller volumes of image data can be filtered in less than 1 s; thus, the user can interactively choose any desired filter for a given tomographic study at the time of interpretation of the images.

Algorithms↗
PubMed1989

Three-dimensional display of gated cardiac blood-pool studies.

There has been little interest in use of single photon tomography in gated cardiac blood-pool imaging. This fact arises most likely from two principal causes: the difficulty associated with interpretation of images presented as series of beating slices, and the formidable computational burdens involved in processing the data. We have addressed both of these issues. A new method, called volume rendering, is used to generate realistic and easily interpreted three-dimensional images of the cardiac blood pool. These images of the beating heart can be displayed in a continuously rotating cine format or viewed in any orientation selected by the observer. Total computation time for a 16-frame gated study, including filtered backprojection, spatial and temporal filtering, and volume rendering, is 82.7 min employing a 32-bit computer and an array processor. With greater use of the array processor it should be possible to reduce the time to approximately 40 min, thus making routine use of these three-dimensional images clinically feasible.

Data Display↗
PubMed1987

99Tcm-MDP uptake by lymph nodes following tracer infiltration: clinical and laboratory evaluation.

Uptake of bone scanning agents in non-osseous sites has been described in a variety of pathologic conditions including tumor metastases. We have seen several patients in which such uptake was proximal and ipsilateral to the injection site of 99Tcm-methylene diphosphonate, apparently in normal lymph nodes. To further investigate this phenomenon, it was studied in a rat model. Activity in popliteal nodes ipsilateral to the injection site was over 60-fold greater in the animals that received subcutaneous (s.q.) footpad injection compared to femoral IV injection. Ipsilateral popliteal node activity in the s.q. group was 159 times that of contralateral popliteal nodes, with an ipsilateral node to liver ratio of 184:1. In summary, dramatically increased uptake of 99Tcm-MDP in normal lymph nodes ipsilateral and proximal to an extravasated injection has been demonstrated. An awareness of this phenomenon in the clinical setting can avoid confusion with pathologic forms of soft tissue uptake.

Aged↗
PubMed1986

Dynamic arrhythmia filtration for gated blood-pool imaging: validation against list mode technique.

Normal resting heart rate variations distort the diastolic portions of left ventricular time-activity curves, altering calculated measures of diastolic function such as peak filling rate. Diastolic filling parameters obtained by two methods of arrhythmia removal--list-mode acquisition and a new approach, dynamic arrhythmia filtration (DAF)--were compared. In DAF, data are evaluated for cycle length in real time and accepted or rejected immediately according to preset, operator-determined cycle-length criteria, eliminating the need for postprocessing of data and for large mass data storage. We prospectively determined EF, time to end-systole, peak filling rate, and time to peak filling on gated blood-pool studies of 25 patients. Identical camera and ECG data were sent simultaneously to two computers and analyzed using list mode and DAF, with an R-R interval of 5-6% of mean heart rate. Excellent correlation between list mode and DAF was seen in peak filling rate (r = 0.94), EF (r = 0.99), and time to end-systole (r = 0.97). Lesser correlation seen in time to peak filling (r = 0.65) may be due to inherent problems in measuring this parameter reliably, possibly lessening this parameter's clinical usefulness. DAF is less time consuming and less technically demanding than list mode, and provides results which correlate closely to those obtained by list mode.

Electrocardiography↗
PubMedDate not supplied

Biochemical tests for pheochromocytoma: strategies in hypertensive patients.

OBJECTIVE: To determine the optimal clinical strategy utilizing either 1) serum catecholamines, 2) urinary metanephrines, or 3) urinary vanillylmandelic acid measurements in the evaluation of hypertensive patients with suspected pheochromocytoma. DESIGN: Prospective clinical determination of test-operating characteristics. SETTING: Tertiary care university medical center. PATIENTS/PARTICIPANTS: 415 patients referred for evaluation of suspected pheochromocytoma. INTERVENTIONS: All subjects had measurements of the three above-mentioned biochemical tests with the diagnosis of pheochromocytoma established by tissue confirmation. RESULTS: All three biochemical tests were similar in sensitivity (0.70-0.75), specificity (0.90-0.95), and receiver operating characteristics. With an estimated disease prevalence of 5.9% in symptomatic hypertensive patients, the predictive value of any single negative test would be 98% in ruling out disease. CONCLUSIONS: All three biochemical tests have similar performance characteristics and only a single test need be performed to exclude pheochromocytoma in most symptomatic hypertensive patients in a primary care population.

3-Iodobenzylguanidine↗
PubMedDate not supplied

Cardiac nuclear medicine.

In this paper, the principal applications of nuclear medicine to studies of the heart are described. First, gated cardiac blood pool imaging is discussed, then thallium-201 myocardial imaging, myocardial infarct scintigraphy with 99mTc pyrophosphate, and evaluation of intracardiac shunts. In gated cardiac blood pool imaging, the patient's red blood cells are labeled with 99mTc. Images of the cardiac blood pool are then obtained in multiple projections and displayed in an endless-loop cine display. Quantitative indices of cardiac function are readily obtained, and a variety of functional images can be generated. Blood pool imaging may also be performed with use of a first-pass technique that yields similar information. Applications of blood pool imaging are discussed. The theory and techniques of planar and tomographic thallium-201 myocardial imaging are described, together with their application in the diagnosis of coronary artery disease. The prognostic value of thallium imaging is also examined. Myocardial infarct imaging with 99mTc pyrophosphate is described, and clinical indications are reviewed. Left-to-right cardiac shunts can be evaluated by following the first transit of a bolus of radiopharmaceutical through the lungs. Right-to-left shunts may be evaluated by injection of 99mTc macroaggregated albumin.

Blood↗