Biomedical subjects

J L Lancaster

Publications and source records attributed to J L Lancaster.

At least 37 records · Page 2Linked to original sources

PubMed2000

Using measured 30-150 kVp polychromatic tungsten x-ray spectra to determine ion chamber calibration factors, Nx (Gy C(-1)).

Two methods for determining ion chamber calibration factors (Nx) are presented for polychromatic tungsten x-ray beams whose spectra differ from beams with known Nx. Both methods take advantage of known x-ray fluence and kerma spectral distributions. In the first method, the x-ray tube potential is unchanged and spectra of differing filtration are measured. A primary standard ion chamber with known Nx for one beam is used to calculate the x-ray fluence spectrum of a second beam. Accurate air energy absorption coefficients are applied to the x-ray fluence spectra of the second beam to calculate actual air kerma and Nx. In the second method, two beams of differing tube potential and filtration with known Nx are used to bracket a beam of unknown Nx. A heuristically derived Nx interpolation scheme based on spectral characteristics of all three beams is described. Both methods are validated. Both methods improve accuracy over the current half value layer Nx estimating technique.

Aluminum↗

PubMed2000

Modification and benchmarking of MCNP for low-energy tungsten spectra.

The MCNP Monte Carlo radiation transport code was modified for diagnostic medical physics applications. In particular, the modified code was thoroughly benchmarked for the production of polychromatic tungsten x-ray spectra in the 30-150 kV range. Validating the modified code for coupled electron-photon transport with benchmark spectra was supplemented with independent electron-only and photon-only transport benchmarks. Major revisions to the code included the proper treatment of characteristic K x-ray production and scoring, new impact ionization cross sections, and new bremsstrahlung cross sections. Minor revisions included updated photon cross sections, electron-electron bremsstrahlung production, and K x-ray yield. The modified MCNP code is benchmarked to electron backscatter factors, x-ray spectra production, and primary and scatter photon transport.

Algorithms↗

PubMed2000

An improved method for rapid objective measurement of gamma camera resolution.

An improved method for an easy, rapid measurement of the intrinsic spatial resolution of a gamma camera is presented. A simplified model was previously developed based solely on mean and standard deviation measurements taken from a region of interest in bar pattern images. This led to an estimate of the modulation transfer function and the full width at half maximum (FWHM) of a line spread function (LSF). The improved method involved expanding the simplified model to incorporate input modulation, square wave input versus the assumed sinusoidal input, aperture (pixel) size, and scatter from the plastic in the bar pattern. The input square wave modulation was calculated to be unity for the typical bar patterns used for gamma camera quality control assessment. For the typical range of bar sizes available, in combination with the typical resolving capabilities of gamma cameras, the sinusoidal approximation of the bar pattern was found to be valid (<1% contribution to the measured resolution from higher harmonic frequencies present in a square wave input with an effective input modulation greater than unity by a factor of 4/pi). The aperture correction factor was calculated for numerous bar and pixel size combinations. Applying the aperture correction factor results in an improvement in the accuracy of the calculated FWHM values, especially for large apertures (pixel sizes). For a camera with a specified FWHM value of 3.5 mm, the simplified model predicts values ranging from 3.2 to 4.1 mm, when the acquisition matrix varies from 128(2) to 512(2). When the expanded model is used with the aperture correction applied, this range was reduced to 3.6-3.9 mm. The scatter correction further improved the calculated FWHM (from 3.6 to 3.8 mm). It is suggested that the expanded model should be used when more accurate measurements are desired, such as in acceptance testing.

Algorithms↗

PubMed2000

Measurement and validation of benchmark-quality thick-target tungsten X-ray spectra below 150 kVp.

Pulse-height distributions of two constant potential X-ray tubes with fixed anode tungsten targets were measured and unfolded. The measurements employed quantitative alignment of the beam, the use of two different semiconductor detectors (high-purity germanium and cadmium-zinc-telluride), two different ion chamber systems with beam-specific calibration factors, and various filter and tube potential combinations. Monte Carlo response matrices were generated for each detector for unfolding the pulse-height distributions into spectra incident on the detectors. These response matrices were validated for the low error bars assigned to the data. A significant aspect of the validation of spectra, and a detailed characterization of the X-ray tubes, involved measuring filtered and unfiltered beams at multiple tube potentials (30-150 kVp). Full corrections to ion chamber readings were employed to convert normalized fluence spectra into absolute fluence spectra. The characterization of fixed anode pitting and its dominance over exit window plating and/or detector dead layer was determined. An Appendix of tabulated benchmark spectra with assigned error ranges was developed for future reference.

Benchmarking↗

PubMed2000

Bromocriptine: a novel approach to the treatment of type 2 diabetes.

OBJECTIVE: In vertebrates, body fat stores and insulin action are controlled by the temporal interaction of circadian neuroendocrine oscillations. Bromocriptine modulates neurotransmitter action in the brain and has been shown to improve glucose tolerance and insulin resistance in animal models of obesity and diabetes. We studied the effect of a quick-release bromocriptine formulation on glucose homeostasis and insulin sensitivity in obese type 2 diabetic subjects. RESEARCH DESIGN AND METHODS: There were 22 obese subjects with type 2 diabetes randomized to receive a quick-release formulation of bromocriptine (n = 15) or placebo (n = 7) in a 16-week double-blind study. Subjects were prescribed a weight-maintaining diet to exclude any effect of changes in body weight on the primary outcome measurements. Fasting plasma glucose concentration and HbA(1c) were measured at 2- to 4-week intervals during treatment. Body composition (underwater weighing), body fat distribution (magnetic resonance imaging), oral glucose tolerance (oral glucose tolerance test [OGTT]), insulin-mediated glucose disposal, and endogenous glucose production (2-step euglycemic insulin clamp, 40 and 160 mU x min(-1) x m(-2)) were measured before and after treatment. RESULTS: No changes in body weight or body composition occurred during the study in either placebo- or bromocriptine-treated subjects. Bromocriptine significantly reduced HbA(1c) (from 8.7 to 8.1%, P = 0.009) and fasting plasma glucose (from 190 to 172 mg/dl, P = 0.02) levels, whereas these variables increased during placebo treatment (from 8.5 to 9.1%, NS, and from 187 to 223 mg/dl, P = 0.02, respectively). The differences in HbA(1c) (delta = 1.2%, P = 0.01) and fasting glucose (delta = 54 mg/dl, P < 0.001) levels between the bromocriptine and placebo group at 16 weeks were highly significant. The mean plasma glucose concentration during OGTT was significantly reduced by bromocriptine (from 294 to 272 mg/dl, P = 0.005), whereas it increased in the placebo group. No change in glucose disposal occurred during the first step of the insulin clamp in either the bromocriptine- or placebo-treated group. During the second insulin clamp step, bromocriptine improved total glucose disposal from 6.8 to 8.4 mg x min(-1) kg(-1) fat-free mass (FFM) (P = 0.01) and nonoxidative glucose disposal from 3.3 to 4.3 mg min(-1) x kg(-1) FFM (P < 0.05), whereas both of these variables deteriorated significantly (P < or = 0.02) in the placebo group. CONCLUSIONS: Bromocriptine improves glycemic control and glucose tolerance in obese type 2 diabetic patients. Both reductions in fasting and postprandial plasma glucose levels appear to contribute to the improvement in glucose tolerance. The bromocriptine-induced improvement in glycemic control is associated with enhanced maximally stimulated insulin-mediated glucose disposal.

Abdomen↗

PubMed1999

Functional volumes modeling: scaling for group size in averaged images.

Functional volumes modeling (FVM) is a statistical construct for metanalytic modeling of the locations of brain functional areas as spatial probability distributions. FV models have a variety of applications, in particular, to serve as spatially explicit predictions of the Talairach-space locations of functional activations, thereby allowing voxel-based analyses to be hypothesis testing rather than hypothesis generating. As image averaging is often applied in the analysis of functional images, an important feature of FVM is that a model can be scaled to accommodate any degree of intersubject image averaging in the data set to which the model is applied. In this report, the group-size scaling properties of FVM were tested. This was done by: (1) scaling a previously constructed FV model of the mouth representation of primary motor cortex (M1-mouth) to accommodate various degrees of averaging (number of subjects per image = n = 1, 2, 5, 10), and (2) comparing FVM-predicted spatial probability contours to location-distributions observed in averaged images of varying n composed from randomly sampling a 30-subject validation data set.

Brain Mapping↗

PubMed1999

Accurate high-speed spatial normalization using an octree method.

The goal of regional spatial normalization is to remove anatomical differences between individual three-dimensional (3-D) brain images by warping them to match features of a standard brain atlas. Full-resolution volumetric spatial normalization methods use a high-degree-of-freedom coordinate transform, called a deformation field, for this task. Processing to fit features at the limiting resolution of a 3-D MR image volume is computationally intensive, limiting broad use of full-resolution regional spatial normalization. A highly efficient method, designed using an octree decomposition and analysis scheme, is presented to resolve the speed problem while targeting accuracy comparable to current volumetric methods. Translation and scaling capabilities of octree spatial normalization (OSN) were tested using computer models of solid objects (cubes and spheres). Boundary mismatch between transformed and target objects was zero for cubes and less than 1% for spheres. Regional independence of warping was tested using brain models consisting of a homogenous brain volume with one internal homogenous region (lateral ventricle). Boundary mismatch improved with successively smaller octant-level processing and approached levels of less than 1% for the brain and 5% for the lateral ventricle. Five 3-D MR brain images were transformed to a target 3-D brain image to assess boundary matching. Residual boundary mismatch was approximately 4% for the brain and 8% for the lateral ventricle, not as good as with homogeneous brain models, but similar to other results. Total processing time for OSN with a 256(3) brain image (1-mm voxel spacing) was less than 10 min.

Algorithms↗

PubMed1999

Topical aminoglycosides in the management of active mucosal chronic suppurative otitis media.

Debate has currently re-emerged following a renewed warning issued from the Committee on the Safety of Medicines (CSM) regarding the relative risk of ototoxicity from the use of aminoglycoside-containing drops in patients with tympanic membrane perforations. We present the findings of a survey of ENT consultants, questioning their views and current practice, and we add to the debate by means of a review and discussion of the literature.

Administration, Topical↗

PubMed1999

Adult presentation of a tracheoesophageal fistula with co-existing laryngeal cleft.

Congenital posterior laryngeal clefts are rare anomalies, being first described by Richter in 1792, and first operated upon successfully by Pettersson in 1955. Most clefts present within the first few months of life, with only a few cases being documented as presenting in adulthood. We present the case of a 50-year-old lady presenting with a previously undiagnosed posterior laryngeal cleft with an associated tracheoesophageal fistula. The defect was graded as 2 on the Benjamin and Inglis classification system of posterior laryngeal clefts. Following a literature review, we believe that our patient is the oldest to present with this congenital defect.

Female↗

PubMed1999

Paediatric tympanoplasty.

Much debate exists over the management of mucosal chronic suppurative otitis media in children, with the majority of it centred around the correct timing to perform either a myringoplasty (an operative repair of the tympanic membrane) or type I tympanoplasty (reconstruction of the tympanic membrane when there is an intact and mobile ossicular chain). Further discussion will use the term tympanoplasty to mean both of the above definitions. We present the findings of a recent survey of UK ENT consultants questioning their opinions on various management aspects of mucosal CSOM in the paediatric population. We also present an extensive review of the literature to provide us with published evidence in order to analyse the results of the questionnaire.

Adolescent↗

PubMed1999

Systemic absorption of gentamicin in the management of active mucosal chronic otitis media.

A prospective study was performed on patients with active mucosal chronic otitis media who were being treated with the gentamicin-containing preparation Gentisone HC. In 27 patients plasma gentamicin levels were measured. Detectable levels were found in 7/27 (26%). Pre- and post-treatment audiometry was performed on 16 patients. There was no statistically significant difference in the change of the mean bone conduction thresholds as a result of treatment, between the treatment and control ears (P = 0.07, Wilcoxon signed Ranks Test at 95% C.I.). We conclude that there is evidence of systemic absorption of gentamicin that would ultimately be absorbed into the perilymph. Gentamicin is known to be ototoxic affecting the vestibular system in lower doses and the cochlea in high doses, hence audiometric assessment is not an appropriate screen for ototoxicity when using topical gentamicin-containing drops. Alternative topical preparations should be further investigated.

Absorption↗

PubMed1999

Reciprocal limbic-cortical function and negative mood: converging PET findings in depression and normal sadness.

OBJECTIVE: Theories of human behavior from Plato to Freud have repeatedly emphasized links between emotion and reason, a relationship now commonly attributed to pathways connecting phylogenetically "old" and "new" brain regions. Expanding on this theory, this study examined functional interactions between specific limbic and neocortical regions accompanying normal and disease-associated shifts in negative mood state. METHOD: Regions of concordant functional change accompanying provocation of transient sadness in healthy volunteers and resolution of chronic dysphoric symptoms in depressed patients were examined with two positron emission tomography techniques: [15O]water and [18F]fluorodeoxyglucose, respectively. RESULTS: With sadness, increases in limbic-paralimbic blood flow (subgenual cingulate, anterior insula) and decreases in neocortical regions (right dorsolateral prefrontal, inferior parietal) were identified. With recovery from depression, the reverse pattern, involving the same regions, was seen--limbic metabolic decreases and neocortical increases. A significant inverse correlation between subgenual cingulate and right dorsolateral prefrontal activity was also demonstrated in both conditions. CONCLUSIONS: Reciprocal changes involving subgenual cingulate and right prefrontal cortex occur with both transient and chronic changes in negative mood. The presence and maintenance of functional reciprocity between these regions with shifts in mood in either direction suggests that these regional interactions are obligatory and probably mediate the well-recognized relationships between mood and attention seen in both normal and pathological conditions. The bidirectional nature of this limbic-cortical reciprocity provides additional evidence of potential mechanisms mediating cognitive ("top-down"), pharmacological (mixed), and surgical ("bottom-up") treatments of mood disorders such as depression.

Affect↗

PubMed1999

Otological complications following basal skull fractures.

A retrospective review is presented of 50 patients, with a basal skull fracture, who had radiological or clinical evidence of temporal bone involvement. The study looks at the incidence of significant otological symptoms reported during the admission and we review the subsequent patient management. We propose that all patients presenting with a head injury, and one or more of a number of symptoms characteristic of a temporal bone fracture, should automatically have an otological assessment. This will facilitate the early detection and subsequent follow up of potentially correctable middle ear disorders. We review the literature to highlight the high incidence of complications associated with these fractures.

Adolescent↗

PubMed1999

Global spatial normalization of human brain using convex hulls.

UNLABELLED: Global spatial normalization transforms a brain image so that its principal global spatial features (position, orientation and dimensions) match those of a standard or atlas brain, supporting consistent analysis and referencing of brain locations. The convex hull (CH), derived from the brain's surface, was selected as the basis for automating and standardizing global spatial normalization. The accuracy and precision of CH global spatial normalization of PET and MR brain images were evaluated in normal human subjects. METHODS: Software was developed to extract CHs of brain surfaces from tomographic brain images. Pelizzari's hat-to-head least-square-error surface-fitting method was modified to fit individual CHs (hats) to a template CH (head) and calculate a nine-parameter coordinate transformation to perform spatial normalization. A template CH was refined using MR images from 12 subjects to optimize global spatial feature conformance to the 1988 Talairach Atlas brain. The template was tested in 12 additional subjects. Three major performance characteristics were evaluated: (a) quality of spatial normalization with anatomical MR images, (b) optimal threshold for PET and (c) quality of spatial normalization for functional PET images. RESULTS: As a surface model of the human brain, the CH was shown to be highly consistent across subjects and imaging modalities. In MR images (n = 24), mean errors for anterior and posterior commissures generally were <1 mm, with SDs < 1.5 mm. Mean brain-dimension errors generally were <1.3 mm, and bounding limits were within 1-2 mm of the Talairach Atlas values. The optimal threshold for defining brain boundaries in both 18F-fluorodeoxyglucose (n = 8) and 15O-water (n = 12) PET images was 40% of the brain maximum value. The accuracy of global spatial normalization of PET images was shown to be similar to that of MR images. CONCLUSION: The global features of CH-spatially normalized brain images (position, orientation and size) were consistently transformed to match the Talairach Atlas in both MR and PET images. The CH method supports intermodality and intersubject global spatial normalization of tomographic brain images.

Adult↗

PubMed1999

Endoscopic dacryocystorhinostomy without silicone stenting.

Fifteen endoscopic dacryocystorhinostomies (DCRs) were performed without the use of silicone stents. These patients were followed up for an average of eight months. This procedure was successful in 87% of cases as measured by patients' relief of symptoms and endoscopic visualisation of a middle meatal ostium draining the lacrimal sac. Endoscopic DCR without silicone stenting compares favourably with other techniques described in the literature which use silicone stents. It does not have the disadvantage of complications associated with these stents.

Adult↗

PubMed1998

Beyond the single study: function/location metanalysis in cognitive neuroimaging.

Cognitive neuroimaging maps the brain locations of mental operations. This process is iterative, as no single study can fully characterize a mental operation or its brain location. This iterative discovery process, in combination with the location-reporting standard (i.e. spatial coordinates) of the cognitive neuroimaging community, has engendered a new form of metanalysis. Response locations from multiple studies have been analyzed collectively so as to better describe the spatial distribution of brain activations, with promising results. New hypotheses regarding elementary mental operations and their respective brain locations are being generated and refined via metanalysis. These hypotheses are being tested and confirmed by subsequent, prospective experiments. Function/location metanalysis is an important new tool for hypothesis generation in cognitive neuroimaging. This form of metanalysis is fundamentally different from the effect-size metanalyses prevalent in other literatures, with unique advantages and challenges.

Brain Mapping↗

PubMed1998

Rates of disagreement in imaging interpretation in a group of community hospitals.

RATIONALE AND OBJECTIVES: Prospective studies of radiologists' interpretations of selected radiographs reported 20-40 years ago indicated error rates of 30% and higher. The authors retrospectively evaluated the interpretations of groups of radiologists and determined a range of rates of disagreement in interpretation. Quality assessment or recredentialing may add to the importance of such studies in the future. MATERIALS AND METHODS: Over a 7-year period, a team of radiologists reviewed imaging interpretations in the radiology departments of six community hospitals. Each review, which lasted about 3 days, included evaluation of the interpretations of a 3%-4% sample of the images read by the radiologists at these hospitals. Reading errors were quantitated and evaluated qualitatively. RESULTS: In a review of over 11,000 images read by 35 radiologists, the authors found a 4.4% mean rate of interpretation disagreement; only one radiologist had a mean rate above 8%. Qualitative analysis of the interpretation errors revealed a mean rate of 3.0% of errors that were considered to be below an acceptable standard of care. Radiologists whose errors included a relatively high proportion of false-positive findings tended to make relatively fewer total errors. CONCLUSION: Rates of disagreement for a broad range of studies that radiologists interpret in a community hospital setting appear to be far lower than earlier studies on selective radiographs indicated.

Diagnostic Errors↗

PubMed1998

Papillary endothelial hyperplasia (Masson's tumour) of the maxillary sinus.

In 1923, Masson described a neoplastic process consisting of papillary hyperplasia of the endothelial cells, with a consequent obliteration of the vascular lumen, followed later by degenerative changes. Masson coined the term vegetant intravascular haemangioendothelioma, however, these days it is more commonly known as papillary endothelial hyperplasia (PEH), or by the pseudonym, Masson's tumour. Although relatively rare, there are numerous accounts of PEH in the literature, describing its predilection for the head and neck region. Our case report describes the finding of a PEH within the paranasal sinuses, a site not previously mentioned even in the largest of series found on literature search. We will then discuss the relevant histological features of the lesion, and its natural history.

Aged↗