PubMed Health⌕ Search

Biomedical subjects

D Townsend

Publications and source records attributed to D Townsend.

At least 19 recordsLinked to original sources
PubMed2004

The roaming atom: straying from the reaction path in formaldehyde decomposition.

We present a combined experimental and theoretical investigation of formaldehyde (H2CO) dissociation to H2 and CO at energies just above the threshold for competing H elimination. High-resolution state-resolved imaging measurements of the CO velocity distributions reveal two dissociation pathways. The first proceeds through a well-established transition state to produce rotationally excited CO and vibrationally cold H2. The second dissociation pathway yields rotationally cold CO in conjunction with highly vibrationally excited H2. Quasi-classical trajectory calculations performed on a global potential energy surface for H2CO suggest that this second channel represents an intramolecular hydrogen abstraction mechanism: One hydrogen atom explores large regions of the potential energy surface before bonding with the second H atom, bypassing the saddle point entirely.

Journal Article↗
PubMed2001

Comparison of 3-D reconstruction with 3D-OSEM and with FORE+OSEM for PET.

The combination of Fourier rebinning (FORE) and the ordered subsets expectation-maximization (OSEM), a fast statistical algorithm, appears as a promising alternative to the fully three-dimensional (3-D) iterative approach for clinical positron emission tomography (PET) data. In this paper, we evaluated the properties of FORE+OSEM and compared it with fully 3-D OSEM using both simulations and data acquired by commercial scanners. The aim is to determine to what extent the speed advantage of FORE+OSEM is paid for by a possible degradation of image quality in the case of noisy clinical PET data. A forward- and back-projection pair based on a line integral model was used in two-dimensional OSEM and 3-D OSEM (3D-OSEM) instead of a system matrix. Different variants of both approaches have been studied with simulations in terms of contrast-noise tradeoff. Two variants--FORE+OSEM with attenuation weighting (AW) [FORE+OSEM(AW)] and 3D-OSEM with attenuation-normalization weighting (ANSP) and a shifted-Poisson (SP) model [3D-OSEM(ANSP)]--were compared with measured phantom data and patient data. Based on the results from both simulations and measured data, we conclude that: 1) both attenuation (-normalization) weighting and the SP model improve the image quality but slow down the convergence and 2) despite its approximate nature, FORE+OSEM does not show apparent image degradation compared with 3D-OSEM for data with a noise level typical of a whole-body FDG scan.

Algorithms↗
PubMed2001

Prisoners with HIV/AIDS: a participatory learning and action initiative in Malaysia.

Rapid participatory research and project development is possible within a tightly controlled social context such as a prison. Having gained access, based on trust and mutual respect, external agents may then facilitate significant change. Given adequate support, incarcerated people with HIV/AIDS and limited medical access may be able to develop mutual care, social support and income-generating activities. In the Malaysian context, we estimated in 1998 that up to one-quarter of prisoners with HIV had indicators of significant disease. We estimated that significant indicators remained unrevealed among between one-half and two-thirds of these. Given prevailing conditions, these would probably only be amenable to peer-based care.

Female↗
PubMed2000

7. Survey of Results of Whole Body Imaging Using the PET/CT at the University of Pittsburgh Medical Center PET Facility.

Purpose: At the University Of Pittsburgh Medical Center, over 100 oncology studies have been performed using a combined PET/CT scanner. The scanner is a prototype, which combines clinical PET and clinical CT imaging in a single unit. The sensitivity achieved using three-dimensional PET imaging as well as the use of the CT for attenuation correction and image fusion make the device ideal for clinical oncology. Clinical indications imaged on the PET/CT scanner include, but are not limited to, tumor staging, solitary pulmonary nodule evaluation, and evaluation of tumor reoccurrence in melanoma, lymphoma, colorectal cancer, lung cancer, pancreatic cancer, head and neck cancer, and renal cancer.Methods: For all studies, seven millicuries of F(18)-fluorodeoxyglucose is injected and a forty-five minute uptake period is allowed prior to positioning the patient in the scanner. A helical CT scan is acquired over the region, or regions of interest followed by a multi-bed whole body PET scan for the same axial extent. The CT scan is used to correct the PET data for attenuation. The entire imaging session lasts 1-1.5 hours depending on the number of beds acquired, and is generally well tolerated by the patient.Results and Conclusion: Based on our experience in over 100 studies, combined PET/CT imaging offers significant advantages, including more accurate localization of focal uptake, distinction of pathology from normal physiological uptake, and improvements in evaluating therapy. These benefits will be illustrated with a number of representative, fully documented studies.

Journal Article↗
PubMed2000

20. The Case for PET/CT. Experience at the University of Pittsburgh.

Purpose: Whole-body positron emission tomography (PET) with [F-18]fluorodeoxyglucose (FDG) is an important tool in the management of patients with cancer. While the sensitivity of FDG PET for tumor localization is most often reported to be >/= 85%, the specificity is frequently found to be lower. One limitation of whole-body PET imaging is the confounding effect of normal physiologic FDG accumulation in kidneys, ureters, bladder, stomach and bowel. Also, highly metabolically active tissues such as brain and muscle can mask detection of adjacent abnormalities. Interpretation of functional PET images can be improved by correlation with anatomic imaging, e.g. CT or MRI. The utility of both visual comparison and retrospective fusion of PET with previously acquired morphologic studies may be, however, limited by the time interval between image acquisition and patient positioning differences.Results: In our experience with over 150 examinations using a unique combined PET/CT scanner, we have frequently found that the direct registered images were critical to correct study interpretation. In this work, we review the beneficial results of a combined PET/CT tomograph for diagnosis/staging and localization of malignancy. We will further present specific examples of clinical questions uniquely addressed by PET/CT, and their impact on patient management.Conclusion: Acquisition of co-registered PET and CT images in the same scanning session may enable physicians to more precisely discriminate physiologic uptake and tumor. Our initial experience suggests that this combined PET/CT device may improve the accuracy of PET, and in so doing, enhance the value of diagnostic PET in oncologic applications.

Journal Article↗
PubMed2000

Detection of dentine caries using the oblique lateral radiograph.

OBJECTIVES: The aim of the study was to evaluate the level of agreement between bitewing radiographs and oblique lateral radiographs for the diagnosis of dental caries extending into dentine, and to draw conclusions from the results as to which view is more useful. SAMPLE: Radiographs were selected from patients attending an orthodontic assessment clinic: 53 patients, 52% male, 48% female, aged 6-18 years old. Total 105 bitewings and 105 oblique lateral radiographs. METHODS: Contemporaneous bitewing and oblique radiographs taken over the period 1987-97 were independently assessed by two dentists for the presence of caries extending into dentine. RESULTS: The agreement between the oblique lateral and the bitewing radiographs for the presence of approximal caries expressed in the form of Cohen's Kappa gave values of 0.64 for observer 1 and 0.53 for observer 2. For occlusal caries, values were 0.72 for observer 1 and 0.56 for observer 2. Kappa values for the level of agreement between observers and for the same observer on different occasions were within the range 0.5-0.95 for both types of film. CONCLUSION: The Kappa values for the agreement between the two views as well as the intra- and inter-observer agreement and reproducibility of the oblique lateral view all fall in the range K = 0.5-0.75. Results suggested a fair to good agreement in diagnoses made using the two types of film. However, levels were lower than estimates of reproducibility reported for bitewing films alone.

Adolescent↗
PubMed1999

Exact rebinning methods for three-dimensional PET.

The high computational cost of data processing in volume PET imaging is still hindering the routine application of this successful technique, especially in the case of dynamic studies. This paper describes two new algorithms based on an exact rebinning equation, which can be applied to accelerate the processing of three-dimensional (3-D) PET data. The first algorithm, FOREPROJ, is a fast-forward projection algorithm that allows calculation of the 3-D attenuation correction factors (ACF's) directly from a two-dimensional (2-D) transmission scan, without first reconstructing the attenuation map and then performing a 3-D forward projection. The use of FOREPROJ speeds up the estimation of the 3-D ACF's by more than a factor five. The second algorithm, FOREX, is a rebinning algorithm that is also more than five times faster, compared to the standard reprojection algorithm (3DRP) and does not suffer from the image distortions generated by the even faster approximate Fourier rebinning (FORE) method at large axial apertures. However, FOREX is probably not required by most existing scanners, as the axial apertures are not large enough to show improvements over FORE with clinical data. Both algorithms have been implemented and applied to data simulated for a scanner with a large axial aperture (30 degrees), and also to data acquired with the ECAT HR and the ECAT HR+ scanners. Results demonstrate the excellent accuracy achieved by these algorithms and the important speedup when the sinogram sizes are powers of two.

Algorithms↗
PubMed1998

A method for the assessment of the functional neuroanatomy of human sleep using FDG PET.

Although sleep is characterized by relative behavioral inactivity, cortical activity is known to cycle in well-defined periods across this state. Cognitive function during sleep has been difficult to define, although disturbances in sleep are known to result from, and to cause, various human pathologies, including neuropsychiatric disorders. Assessment of brain function in humans (related to cognitive operations) during sleep has been limited, until recently, to surface electrophysiologic recordings that limit analysis of regional function, particularly in deep structures. The current report describes one method of assessing human forebrain activation during sleep using the [18F]2-fluoro-2-deoxy-d-glucose ([18F]FDG) method and positron emission tomography (PET) measures of regional cerebral glucose utilization. In comparison with other functional brain imaging techniques (e.g., assessment of blood flow or functional magnetic resonance imaging), this method offers the advantage of a more naturalistic study of sleep since subjects do not have to sleep in a scanning device. This leads to a higher rate of successful completion of studies. The primary disadvantage of this method is the decreased temporal resolution necessitating assessments of global sleep states (e.g., REM or NREM) as opposed to assessing events within a sleep state (e.g., sleep spindles or rapid eye movements).

Electroencephalography↗
PubMed1998

Skeletal muscle glycolytic capacity and phosphofructokinase regulation in horses with polysaccharide storage myopathy.

OBJECTIVE: To determine whether polysaccharide storage myopathy (PSSM) in Quarter Horses is attributable to a defect in glycolysis or in the allosteric regulation of phosphofructokinase (PFK) enzyme. ANIMALS: Muscle biopsy specimens were obtained from 6 Quarter Horses with PSSM and 8 Quarter Horse or Thoroughbred control horses. PROCEDURES: Maximal activity of glycogenolytic and glycolytic enzymes was determined spectrophotometrically. Maximal activity of PFK was determined for each horse at pH 8.0, and at pH 7.0 when variable concentrations of the activators, fructose 6 phosphate, fructose 2,6 bisphosphate, and adenosine monophosphate or inhibitors adenosine triphosphate and citrate were added to the reaction mixture. Relative activity was calculated as activity at pH 7/maximal PFK activity. RESULTS: Deficiencies in glycogenolytic or glycolytic enzyme activities were not evident in horses with PSSM. Differences between horses with PSSM and control horses in relative activity of PFK were not apparent for any of the activators or inhibitors used in the study. CONCLUSIONS: In a group of horses with PSSM, we were unable to detect a glycogenolytic or glycolytic enzyme deficiency or abnormality in the allosteric regulation of PFK. CLINICAL RELEVANCE: Although PSSM is clinically and histologically similar to glycogenolytic/glycolytic enzyme deficiencies in human beings and other animal species, abnormalities in this metabolic pathway are not present in horses with PSSM.

Adenosine Monophosphate↗
PubMed1996

Temporomandibular disorder: efficacy of an oral habit reversal treatment program.

Little attention has been directed at interventions that directly teach facial pain patients to detect, interrupt, and reverse the maladaptive oral habits and cognitions thought to be associated with muscle hyper-reactivity and pain. The present study tested a seven-session structured-group habit reversal treatment intervention for facial pain patients. Nine facial pain subjects participated in treatment groups and seven qualified subjects who did not participate in treatment served as a comparison group. Pre, post, and follow-up assessments included measures of pain and psychological functioning. Subjects in the treatment group improved on all measures of pain relative to those in the comparison group. These improvements were maintained or augmented at four month follow-up assessments suggesting that habit reversal training, particularly in a group format, may provide a cost-effective intervention for facial pain patients.

Adaptation, Psychological↗
PubMed1996

Effects of oxytocin on sperm transport in the pubertal rat.

Oxytocin is present in the mammalian testis where it increases contractility of seminiferous tubules in vitro and has been implicated in sperm transport. The present study investigated whether oxytocin affects the transport of spermatozoa from the testis in vivo. In rats, mature spermatozoa are first seen in the testis 42 days postpartum and arrive in the epididymis at about day 45. Male Wistar rats were given daily subcutaneous injections of either oxytocin (0.5 micrograms), the oxytocin antagonist des Gly-NH2d(CH2)5-[D-Tyr2,Thr4]OVT (0.2 micrograms) or saline from day 40 postpartum. Groups of six animals were killed 2 h after their last injection on days 43, 44, 45 and 46 postpartum. Testes were removed and fixed in Bouin's fluid for histological examination and the number of spermatozoa in the epididymides was counted. Spermatozoa were seen in the epididymis earlier in the oxytocin-treated rats (day 43) than in the control animals (day 44), and treatment with the antagonist delayed the appearance of spermatozoa in the epididymis until day 45. When the testes were examined, residual bodies, which were used as an indicator of spermiation, were seen only in one control animal before day 44. Residual bodies were seen in the testes of all oxytocin-treated rats on day 43 but were not detected until day 45 in the oxytocin antagonist-treated rats. These data show that in rats oxytocin can affect the arrival of spermatozoa in the epididymis. Although this may be due in part to effects on tubal transport or the secretion of tubular fluid, these findings suggest that the peptide may affect spermiation.

Animals↗
PubMed1995

Capital decisions. Where is the smart money being invested?

Health care executives are faced with a welter of priorities when it comes to allocating their increasingly scarce capital: they need to build information systems, organize primary care physicians, and expand outpatient services. But what are the defining issues? We asked five executives to explain how their strategies are changing as a result of managed care growth and integration. What emerges is a snapshot of the new capital allocation reality.

Capital Expenditures↗
PubMed1995

Tyrosinase inhibition due to interaction of homocyst(e)ine with copper: the mechanism for reversible hypopigmentation in homocystinuria due to cystathionine beta-synthase deficiency.

Deficiency of cystathionine beta-synthase (CBS) is a genetic disorder of transsulfuration resulting in elevated plasma homocyst(e)ine and methionine and decreased cysteine. Affected patients have multisystem involvement, which may include light skin and hair. Reversible hypopigmentation in treated homocystinuric patients has been infrequently reported, and the mechanism is undefined. Two CBS-deficient homocystinuric patients manifested darkening of their hypopigmented hair following treatment that decreased plasma homocyst(e)ine. We hypothesized that homocyst(e)ine inhibits tyrosinase, the major pigment enzyme. The activity of tyrosinase extracted from pigmented human melanoma cells (MNT-1) that were grown in the presence of homocysteine was reduced in comparison to that extracted from cells grown without homocysteine. Copper sulfate restored homocyst(e)ine-inhibited tyrosinase activity when added to the culture cell media at a proportion of 1.25 mol of copper sulfate per 1 mol of DL-homocysteine. Holo-tyrosinase activity was inhibited by adding DL-homocysteine to the assay reaction mixture, and the addition of copper sulfate to the reaction mixture prevented this inhibition. Other tested compounds, L-cystine and betaine did not affect tyrosinase activity. Our data suggest that reversible hypopigmentation in homocystinuria is the result of tyrosinase inhibition by homocyst(e)ine and that the probable mechanism of this inhibition is the interaction of homocyst(e)ine with copper at the active site of tyrosinase.

Adolescent↗
PubMed1995

Standard views in cardiac multimodality tomographic imaging.

In cardiology, it is often necessary to acquire more than one type of image to investigate a given clinical problem of a single patient. Images obtained from different imaging modalities are usually recorded and displayed in different orientations, at different positions, and at different scale factors. It is then necessary for the physician to mentally integrate the image information from the different imaging modalities. This phenomenon is particularly true with tomographic imaging techniques that allow complete freedom of the acquisition plane. In particular, when comparing images obtained from ultrasound, computed tomography, magnetic resonance imaging, positron-emission tomography, and single-photon emission computed tomography. The purpose of this article is to propose a standard set of slice orientations that could be easily applied to all modalities. Such common views could greatly facilitate the user's perception of the regional abnormalities observed in the different imaging modalities. This standardization is certainly useful for clinical application but also for every research study that requires a comparative evaluation of the different imaging modalities. Although exact registration of the images from the different modalities requires sophisticated computer programs, the simple reference method in plane positioning proposed here based on plane orientation according to the cardiac geometry can certainly provide a practical and convenient method for the reasonably accurate image registration required for visual comparative studies.

Diagnostic Imaging↗
PubMed1994

Rotating positron tomographs revisited.

We have compared the performance of a PET scanner comprising two rotating arrays of detectors with that of the more conventional stationary-ring design. The same total number of detectors was used in each, and neither scanner had septa. For brain imaging, we find that the noise-equivalent count rate is greater for the rotating arrays by a factor of two. Rotating arrays have a sensitivity profile that peaks in the centre of the field of view, both axially and transaxially. In the transaxial plane, this effect offsets to a certain extent the decrease in the number of photons detected towards the centre of the brain due to self-absorption. We have also compared the performance of a rotating scanner to that of a full-ring scanner with the same number of rings. We find that a full-ring scanner with an axial extent of 16.2 cm (24 rings) is a factor of 3.5 more sensitive than a rotating scanner with 40% of the detectors and the same axial extent.

Brain↗
PubMed1994

Quantitative metabolic PET imaging of a plasma cell granuloma.

We report a patient who underwent surgical resection of two lung nodules that proved to be recurrent plasma cell granuloma, also known as inflammatory pseudotumor. Prior to surgery, positron emission tomography (PET) was performed with 18F-labeled fluoro-2-deoxy-D-glucose (18FDG) and rubidium-82 (82Rb). The 18FDG PET scan revealed that the nodules corresponded to two areas of intense uptake. PET imaging with 82Rb, the marker of flow, also showed intense uptake. Thus, PET demonstrated both a high degree of metabolic activity and increased perfusion. These features suggest a lesion with high cellular activity rather than a simple reparative process. The true nature of this lesion remains unknown.

Animals↗