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A J Dwyer

Publications and source records attributed to A J Dwyer.

At least 19 recordsLinked to original sources
PubMed1998

Polypoid lesions of airways: early experience with computer-assisted detection by using virtual bronchoscopy and surface curvature.

PURPOSE: To test the application of a technique developed by the authors for the computer-assisted diagnosis of polypoid airway lesions from surface rendered virtual bronchoscopic reconstructions. MATERIALS AND METHODS: A computer algorithm was developed to detect polypoid airway lesions by means of segmentation of the bronchial surface with curvature classification. This method was tested with a bronchial phantom, five cadaveric lung specimens, and virtual bronchoscopic studies in 16 patients. RESULTS: For the patient studies, the sensitivity and specificity of the method were 47%-88% and 58%-89%, respectively, depending on the value of an adjustable parameter (the mean curvature threshold). The sensitivity increased (by 20% to 34%) when only lesions larger than 5 mm in diameter were considered. CONCLUSION: With this method, polypoid airway lesions can be detected automatically, although false-positive diagnoses present an important limitation.

Adult↗
PubMed1997

Advantages of polytetrafluoroethylene arteriovenous loops in the thigh for hemodialysis access.

BACKGROUND: The upper thigh is an alternative but infrequently used site to the forearm for placement of subcutaneous polytetrafluoroethylene (PTFE) arteriovenous conduits in patients requiring hemodialysis for end stage renal failure. This site has the great advantage of easier accessibility for self-cannulation. METHODS: The outcome was reviewed for 74 PTFE loops placed in 61 patients between 1985 and 1991. RESULTS: Mean loop survival time was 99.8 weeks (SD 78.0) when patients with early failure (<1 week), and those patients whose loops functioned adequately until transplantation or death were excluded. Infection occurred in 12 of 74 loops. CONCLUSIONS: Thigh PTFE loops provide satisfactory medium- to long-term vascular access for hemodialysis although, like all other forms of access currently available, they fall short of the ideal for prolonged dialysis.

Arteriovenous Anastomosis↗
PubMed1995

Decreased cerebral glucose metabolism in patients with brain tumors: an effect of corticosteroids.

The authors measured cerebral glucose metabolism (CMRglu) using [18F]fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) in patients with brain tumors to evaluate the effect of exogenous corticosteroids (in this instance, dexamethasone) on glucose metabolism. Fifty-six FDG-PET studies obtained in 45 patients with unilateral supratentorial brain tumors were analyzed. Patients with brain tumors were divided into three groups: 1) patients with cushingoid symptoms, who had been treated with combinations of radiotherapy and chemotherapy taking oral dexamethasone; 2) patients not taking dexamethasone but treated with radiotherapy; and 3) patients not taking dexamethasone who had not been treated with radiotherapy. Serial FDG-PET scans were obtained in eight of the cushingoid patients. Glucose metabolism was measured in the contralateral cerebral and ipsilateral cerebellar hemispheres in patients and compared to measurements taken from 19 normal volunteers. The authors found that in the cushingoid brain tumor patients there was a marked reduction in CMRglu compared to normal volunteers and other brain tumor patients (Kruskal-Wallis test; p 0.001). In the majority of patients who had serial FDG-PET scans, there was a decline in glucose metabolism over time and in one patient, in whom dexamethasone was reduced in dosage, there was a subsequent increase in CMRglu. The authors conclude that there is a generalized reduction in CMRglu in brain tumor patients taking dexamethasone compared to other brain tumor patients and normal volunteers, and that this effect is independent of radiotherapy, concurrent anticonvulsant medication, and transhemispheric functional disconnection (transhemispheric diaschisis).

Adult↗
PubMed1993

Neuroimaging of juvenile pilocytic astrocytomas: an enigma.

PURPOSE: To present the imaging, metabolic, and clinical data obtained in five patients with juvenile pilocytic astrocytomas (JPAs) and discuss the paradoxical neuroimaging findings. MATERIALS AND METHODS: Five patients with JPAs who had undergone structural imaging and fluorine-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) were studied. Normalized glucose utilization rates (GURs) in the tumor were compared with GURs in histopathologically verified low-grade astrocytomas and high-grade tumors. RESULTS: All JPAs enhanced markedly after administration of contrast medium. Their glucose metabolism was significantly (P < .001) higher than that of low-grade astrocytomas and was similar to that of anaplastic astrocytomas. All patients with JPAs were in stable condition and showed no evidence of disease progression despite contrast enhancement and high tumoral glucose metabolism. CONCLUSION: The paradoxical FDG PET findings and enhancement at structural imaging might reflect the unusual vascularity of pilocytic tumors, and the increased GUR might be related to expression of the glucose transporter.

Adolescent↗
PubMed1992

Hydrated clearance of gadolinium-DTPA as a measurement of glomerular filtration rate.

Technetium (99mTc)-diethylene triamine pentaacetic acid (DTPA) hydrated clearance studies are accurate for determining GFR but require special facilities for handling and measuring samples. We investigated the potential of a non-radioactive paramagnetic analog, Gadolinium (Gd)-diethylene triamine pentaacetic acid (DTPA), an approved NMR contrast agent, as a glomerular filtration marker. Instead of relying on the radioactivity of technetium, this test is based on the fact that gadolinium induces alterations in the NMR T1 relaxation times in blood and urine samples. Ninety patients underwent simultaneous determinations of GFR using 1 mCi of Tc-DTPA and 0.05 mmol/kg Gd-DTPA (Berlex Labs) IV. The patients were hydrated with oral and intravenous fluid. Following a one hour equilibrium period, three or four consecutive urine collections were obtained; plasma samples were acquired at the beginning and end of each approximately 20-minute interval. 99mTc-DTPA radioactivity was determined with a scintillation counter. T1 relaxation times were measured on a 10 MHz NMR spectrometer. These were converted to Gd-DTPA concentration by comparison with standard solutions. The Gd-DTPA derived GFR closely approximated the 99mTc-DTPA derived GFR which ranged from 15 to 147 ml/min. The equation and correlation coefficient of the regression line is y = 1.04 x -2.2, r = 0.94. Thus, Gd-DTPA is a safe, non-radioactive indicator of GFR that may provide an alternative renal clearance method for clinical studies of progressive renal disease and nephrotoxicity.

Contrast Media↗
PubMed1992

Pulmonary metastases: MR imaging with surgical correlation--a prospective study.

The sensitivity of magnetic resonance (MR) imaging for detection of pulmonary metastases in 11 patients scheduled for thoracotomy and curative resection of metastases was evaluated with a prospective, controlled study. MR imaging performed at 0.5 T was compared with chest radiography, computed tomography (CT), and thoracotomy in 12 cases. (One patient had two separate occurrences of pulmonary metastases.) All images were interpreted in blinded fashion. When all MR sequences were interpreted together, MR imaging enabled correct identification of all patients with pulmonary nodules (100%). CT enabled detection of at least one nodule in all 12 cases (100%) by design; the sensitivity of chest radiography was only 64%. For individual nodules, MR imaging was at least as sensitive as CT (P2 less than .25 [two-sided value]) and significantly more sensitive than chest radiography (P2 less than .01). Among all MR sequences, short inversion time inversion-recovery sequences had the highest sensitivity for detection of individual nodules (82%).

Adult↗
PubMed1992

Distinction between hyperaldosteronism due to bilateral hyperplasia and unilateral aldosteronoma: reliability of CT.

Hyperaldosteronism due to a unilateral adenoma must be distinguished from hyperaldosteronism due to bilateral hyperplasia to enable the proper choice between surgical treatment (for adenoma) or medical treatment (for hyperplasia). To compare the efficacy of computed tomography (CT) and adrenal venous sampling, both examinations were performed in 24 patients with primary aldosteronism. All patients with a diagnosis of adenoma based on findings at venous sampling underwent adrenalectomy. The CT-based diagnosis was unilateral aldosteronoma in 17 patients and hyperplasia in seven patients. On the basis of venous sampling, unilateral adenoma was diagnosed in 22 patients; this diagnosis was confirmed by means of unilateral adrenalectomy in 21 patients. The most common error was diagnosis of hyperplasia based on the presence of bilateral nodules on CT scans: In six of seven patients with such a diagnosis, venous sampling and subsequent surgery revealed a unilateral adenoma. In hyperaldosteronism with multiple bilateral nodules, CT cannot reliably permit distinction between hyperplasia and adenoma.

Adenoma↗
PubMed1992

Mapping of brain tumor metabolites with proton MR spectroscopic imaging: clinical relevance.

Brain tumor metabolism was studied with hydrogen-1 magnetic resonance spectroscopy and positron emission tomography with fluorine-18 fluorodeoxyglucose in 50 patients. N-acetylaspartate (NAA) was generally decreased in tumors and radiation necrosis but was somewhat preserved at neoplasm margins. Choline was increased in most solid tumors. Solid high-grade gliomas had higher normalized choline values than did solid low-grade gliomas (P < .02), but the normalized choline value was not a discriminator of tumor grade, since necrotic high-grade lesions had reduced choline values. Serial studies in one case showed an increase in choline as the glioma underwent malignant degeneration. Choline values were lower in chronic radiation necrosis than in solid anaplastic tumors (P < .001). In two cases studied before and after treatment, clinical improvement and a reduction in choline followed therapy. Lactate is more likely to be found in high-grade gliomas, but its presence is not a reliable indicator of malignancy.

Adolescent↗
PubMed1991

Matchmaking and McNemar in the comparison of diagnostic modalities.

Comparative studies of radiologic techniques commonly yield matched data due to the ease and desirability of performing all of the techniques on each of the patients. The two-by-two matched data array and the McNemar analysis provide a succinct format for the presentation and proper analysis of matched comparisons of binary (positive and negative) test results. When comparing tests, it is essential not to rely on just the statistical significance of the differences in sensitivities (or specificities); the magnitude of the differences must also be assessed. Confidence intervals provide a useful form of estimation by providing a range of plausible values and an indication of the precision of the estimate. The matched data array also indicates the complementarity of the diagnostic tests being compared.

Diagnostic Imaging↗
PubMed1991

Detection of ACTH-producing bronchial carcinoid tumors: MR imaging vs CT.

Adrenocorticotrophic hormone (ACTH)-producing bronchial carcinoid tumors tend to occur in the middle third of the lung adjacent to pulmonary vessels. Because they cause signs and symptoms when quite small (by virtue of their ACTH production), they may not be detected by CT. MR imaging was performed in 10 consecutive patients with surgically proved ACTH-producing bronchial carcinoid tumors in order to test the ability of MR to clarify equivocal or indeterminate findings on CT examinations. All bronchial carcinoid tumors had high signal intensity on T2-weighted and short-inversion-time inversion-recovery images, facilitating their distinction from pulmonary vasculature. In eight patients, the CT and MR images were equivalent in the detection of bronchial carcinoid tumors. In two patients, MR showed tumors in the middle third of the lung that were equivocal on CT. MR imaging may distinguish small bronchial carcinoid tumors from adjacent pulmonary vessels in the central third of the lung at a time when the CT study is nondiagnostic or equivocal.

Adrenocorticotropic Hormone↗
PubMed1990

von Hippel-Lindau disease: radiologic screening for visceral manifestations.

The visceral manifestations of von Hippel-Lindau (VHL) disease can cause significant morbidity and mortality. The authors prospectively screened 37 persons from a single kindred. Twenty-five subjects underwent abdominal ultrasound (US), contrast material-enhanced abdominal computed tomography (CT), and nonenhanced abdominal magnetic resonance (MR) imaging. Eight subjects younger than 16 years of age underwent abdominal US and MR imaging only. Scrotal US was employed in 25 male patients. Eleven subjects had renal cysts or tumors. Contrast-enhanced CT depicted renal abnormalities in 10 of these subjects, US in seven, and MR imaging in nine. Among 12 subjects with pancreatic cysts or tumors, CT showed pancreatic abnormalities in all 12, US in nine, and MR imaging in nine. Three subjects (mean age, 34.5 years) had renal tumors, and three had pancreatic masses. Scrotal US revealed epididymal cystadenomas in seven subjects; two of these tumors were surgically verified. A combination of contrast-enhanced CT and scrotal US in male patients appears to be the best way to screen for visceral manifestations of VHL disease.

Adolescent↗
PubMed1990

Detection of malignant bone tumors: MR imaging vs scintigraphy.

One hundred six patients with a known or suspected diagnosis of bone cancer (11 patients with biopsy-proved primary tumors, 95 patients with metastatic disease) were evaluated with scintigraphy and MR imaging to determine the relative sensitivity of each technique in the detection of bone disease. MR imaging was performed at 0.5 T as part of the entry evaluation into Intramural Research Board protocols (30%), for evaluation of cord compression, or because of an equivocal scintigram. MR was performed with T1-weighted (e.g., 300-500/10-20 [TR/TE]), T2-weighted (e.g., 2000/80) spin-echo (SE), and a short-TI inversion recovery (STIR) pulse sequence. Scintigrams were performed with 99mTc-methylene diphosphonate. A retrospective analysis showed that in 30 (28%) of 106 patients, MR imaging performed over a limited region of interest revealed a focal abnormality consistent with tumor that was not observed on scintigraphy. Only one patient had an abnormality on scintigraphy, caused by a metastasis, that was not found on MR images. In 73 (69%) of the 106 patients, the results of MR imaging and scintigraphy were equivalent; in 41 cases results of both techniques were normal. A McNemar analysis of the discordant cases showed MR imaging to be more sensitive than scintigraphy was (p less than .001). Our results suggest that although MR imaging has a greater sensitivity in detecting focal disease, scintigraphy is still the most useful screening test for evaluating the entire skeleton. MR imaging should be reserved for clarification of scintigraphic findings when suspicion is high for tumor.

Adolescent↗
PubMed1989

Renal calculi after shock wave lithotripsy: US evaluation with an in vitro phantom.

Fragments of renal calculi from patients who had undergone extracorporeal shock wave lithotripsy (ESWL) were embedded in an in vitro tissue-equivalent ultrasound (US) phantom to assess (a) the accuracy with which such fragments could be measured; (b) the influence of stone composition, stone depth, and transducer frequency; and (c) the effect of fragment "clumping" on size determinations and acoustic shadowing. Fragments as small as 0.8 mm in diameter could be detected at 7.5 MHz, although a practical limitation of 1-2 mm for lower transducer frequencies (3.5 and 5.0 MHz) was observed. Fragment diameter tended to be overestimated with US by a mean of 1 mm. Transducers of 5 and 7.5 MHz were more accurate in determination of size than 3.5-MHz transducers. Struvite fragments were less echogenic and were therefore measured more accurately, but smaller struvite fragments were less detectable than other fragment types. Fragment "clumping" led to significant overestimation of residual stone size even when small fragments were separated by as much as the distance equal to the diameter of one fragment. This study demonstrates that even under the best circumstances the disadvantages of US in assessing residual stone disease are significant.

Humans↗