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Biomedical subjects

G A Wright

Publications and source records attributed to G A Wright.

At least 19 recordsLinked to original sources
PubMed2002

A comparison of nutritional regulation in solitarious- and gregarious-phase nymphs of the desert locust Schistocerca gregaria.

Nutritional regulatory responses were compared for the cryptic 'solitarious' and the conspicuously coloured, aggregating 'gregarious' phases of the desert locust Schistocerca gregaria. The desert locust has the genetic potential to exist in either phase, changing between them within a lifetime and epigenetically across generations. Our aim was to compare final-instar nymphs of the two phases with respect to key nutritional variables, including (i) points of regulated intake (the 'intake target') for protein and carbohydrate, (ii) the nature of trade-offs between over-eating nutrients in excess and under-eating those in deficit when fed nutritionally unbalanced foods, (iii) diet-related patterns of nutrient utilisation, and (iv) the performance consequences of eating nutritionally unbalanced diets. When provided with pairs of nutritionally unbalanced but complementary foods, both phases regulated their intake of protein and carbohydrate to a similar point. However, when confined to foods that were of unbalanced protein to carbohydrate ratio, gregarious nymphs ate more than solitarious insects. Both phases regulated protein growth, but gregarious insects did so to a lower adult body protein content and converted ingested protein to growth less efficiently. When fed a food high in carbohydrate and low in protein, gregarious nymphs deposited more body lipid and survived less well than did solitarious insects. Solitarious nymphs developed more quickly than gregarious nymphs except on the two most extremely unbalanced diets, on which development time was similar. The results are discussed with respect to the different nutritional ecologies of the two phases and used to develop the hypothesis that animals have evolved to trade-off the cost of eating excess of a nutritionally unbalanced diet against the probability of encountering foods of complementary composition in the future.

Animal Nutritional Physiological Phenomena↗
PubMed2001

Partial discrete Fourier transform (PDFT) multiband encoding.

For conventional multiband encoding techniques such as Hadamard encoding, scan time scales linearly with the number of slices encoded simultaneously. In this work, a new multiband encoding technique called partial discrete Fourier transform (PDFT) encoding is introduced, which overcomes this restriction. This technique incorporates the principle of partial Fourier imaging, allowing the tradeoff of SNR and imaging time without changing the number of slices. The theory behind PDFT encoding and its inherent sensitivity to phase errors are outlined. The theory was validated through simulations, showing that phase errors result in degraded slice localization. The feasibility of PDFT encoding of 12 slices was tested with experimental excitation profile measurements and heart images of a human subject using commercial MRI equipment. Imaging time was reduced to 66% with SNR reduced to 82%. Magn Reson Med 45:118-127, 2001.

Computer Simulation↗
PubMed2001

Monitoring blood oxygen state in muscle microcirculation with transverse relaxation.

Oxygen uptake from the microcirculation is a direct measure of tissue function. Magnetic resonance is capable of detecting differences between oxygenated and deoxygenated blood due to the paramagnetic properties of deoxyhemoglobin. At the level of the microcirculation, however, imaging methods cannot directly visualize the vessels. Instead, bulk MR parameters are investigated for their ability to monitor blood oxygen saturation (%O(2)) changes in the microcirculation of tissue, specifically skeletal muscle. Experiments in an in vitro model verified the feasibility of detecting changes in exponential decay signals, and also verified the prediction of only two distinct decay components. Experiments in a rabbit model demonstrate that T(2)' and monoexponential T(2) decay are not sensitive to blood oxygen changes, but that the long-T(2) component in a biexponential fit is correlated to the blood oxygen state. Assuming a two-pool model for water protons in muscle, and with knowledge of the T(2)-%O(2) relation, estimates of the microcirculation blood oxygen state can be made with some reasonable assumptions. Magn Reson Med 45:662-672, 2001.

Animals↗
PubMed2001

Are ejection fractions from gated SPECT perfusion studies clinically useful? A comparison with radionuclide ventriculography.

Gated SPECT (GSPECT) was evaluated for the measurement of left ventricular ejection fraction (LVEF) by comparing with equilibrium gated radionuclide ventriculography (RNVG). A total of 99 subjects underwent GSPECT and RNVG imaging. All studies were acquired in list mode with GSPECT studies processed to give 16- and 8-frames per R-R interval, and RNVG studies 24 frames per R-R interval. The Cedars-Sinai QGS software was used to calculate ejection fraction from GSPECT studies. RNVG studies were processed using a manually drawn single region of interest technique. Comparison of LVEF from GSPECT with RNVG yielded correlation coefficients of 0.82 and 0.81 for 16- and 8-frame GSPECT studies respectively. The mean 95% prediction interval was 33 +/- 11 percentage points for both 16- and 8-frame studies, indicating a great disparity between predicted ejection fraction values from GSPECT and actual RNVG values. Subgroup analysis of 29 patients with pathological Q-wave evidence of myocardial infarction demonstrated a poorer correlation coefficient of r = 0.69. Subgroup analysis of 32 patients with end-diastolic volumes < 100 ml demonstrated a poorer correlation coefficient of r = 0.32. Ejection fractions calculated from 16- and 8-frame studies showed a correlation of 0.99 with a mean 95% prediction interval of 8.7 +/- 0.04 percentage points. The 8-frame studies underestimated LVEF by 3.6 +/- 2.3% compared to the 16-frame studies. In conclusion, left ventricular ejection fractions calculated using the QGS algorithm from GSPECT studies are inadequate for use in clinical practice.

Adult↗
PubMed2001

Intracranial arteriovenous malformations: real-time auto-triggered elliptic centric-ordered 3D gadolinium-enhanced MR angiography--initial assessment.

Auto-triggered elliptic centric-ordered three-dimensional (3D) gadolinium-enhanced magnetic resonance (MR) angiography was compared with 3D multiple overlapping thin-slab acquisition time-of-flight (TOF) MR angiography in the evaluation of intracranial arteriovenous malformations (AVMs) in 10 patients. Intraarterial digital subtraction angiography (DSA) was the reference standard. Gadolinium-enhanced MR angiograms were found to be equivalent to DSA images in AVM component depiction in 70%--90% of cases and were consistently superior to TOF MR angiograms.

Adolescent↗
PubMed2000

Quantitative gated SPECT myocardial perfusion imaging with 201Tl: an assessment of the limitations.

Gated SPECT (GSPECT) perfusion imaging has been increasing in popularity both with 99Tc(m) agents and 201Tl. However, both higher activities than administered in the UK and multi-headed cameras are often used. The aim of this study was to assess GSPECT imaging using lower activities of 201Tl with a single-headed camera. Seventy patients underwent stress and redistribution GSPECT imaging after a mean injected activity of 62 +/- 7 MBq 201Tl. These patients also underwent radionuclide ventriculography (RNVG) imaging. The Cedars Sinai Quantitative Gated SPECT (QGS) package was used to calculate left ventricular ejection fraction (LVEF) from the GSPECT studies. Comparison of ejection fractions calculated using GSPECT with those calculated using RNVG yielded a correlation coefficient of 0.70 for the stress studies and 0.71 for the redistribution studies. The width of the mean 95% prediction interval ranged from 22 to 74 percentage points for the stress studies and 22 to 86 percentage points for the redistribution studies. Ejection fractions calculated from stress and redistribution GSPECT studies showed a correlation of 0.80 with a mean 95% prediction interval of 42.6 +/- 0.4 percentage points. In conclusion, left ventricular ejection fractions calculated using the QGS algorithm from 201Tl GSPECT studies are inadequate for use in clinical practice.

Adult↗
PubMed2000

Prostate cancer: MR imaging and thermometry during microwave thermal ablation-initial experience.

Percutaneous interstitial microwave thermoablation of locally recurrent prostate carcinoma was continually guided with magnetic resonance (MR) imaging. Phase images and data were obtained with a rapid gradient-echo technique and were used to derive tissue temperature change on the basis of proton-resonance shift. Thermally devitalized regions correlated well with the phase image findings. MR imaging-derived temperatures were linearly related to the fluoroptic tissue temperatures. MR imaging can be used to guide thermoablation.

Aged↗
PubMed1999

Differentiation of hepatic malignancies from hemangiomas and cysts by T2 relaxation times: early experience with multiply refocused four-echo imaging at 1.5 T.

The purpose of this study was to examine hepatic lesions with a sequence designed to yield improved T2 measurements and evaluate the clinical utility of these measurements in distinguishing malignant from benign disease. Using a modified Carr-Purcell sequence incorporating features designed to compensate for imperfections in the imaging system, including a train of refocusing pulses emitted in an MLEV pattern oriented in composite fashion along all three coordinate axes, and a single spatially selective pulse placed immediately before a spiral readout, 14 benign lesions and 13 malignant lesions were evaluated prospectively with a conventional 1.5 T imager. The maximum, minimum, and mean T2 values of malignant lesions, hemangiomas, and cysts exceeded corresponding published values from spin-echo and echoplanar studies. The mean T2 value of the malignant lesions differed significantly (P < 0.0001) from those of hemangiomas and cysts. All malignant lesions and all benign lesions were distinguishable by their T2 values, which had ranges of no greater than 118.6 msec and no less than 134.3 msec, respectively. This early experience suggests that improved T2 measurements can facilitate the differentiation of hepatic malignancies from hemangiomas and cysts.

Carcinoma, Hepatocellular↗
PubMed1999

Tracking oxygen effects on MR signal in blood and skeletal muscle during hyperoxia exposure.

Blood and muscle T1 and T2 relaxivity was examined under normoxic (air; 20.8% O2) and hyperoxic (100% O2) conditions to determine whether the oxygenation state of blood in the large vessels and in the microcirculation can be monitored in vivo. The femoral artery/vein and the soleus and gastrocnemius muscles were examined in healthy human male volunteers. Arterial blood T1 decreased with hyperoxia, while venous blood T2 increased, due to increased dissolved O2 and decreased deoxyhemoglobin, respectively. A biexponential T2 model of muscle is proposed, where the short T2 component reflects primarily the intracellular and interstitial compartments (in fast exchange), and the long T2 reflects blood. In this model, the long T2 component increased with hyperoxia exposure. This was more evident in slow twitch (soleus) than in fast twitch (gastrocnemius) muscle. It is concluded that changes in the long T2 component reflect change in the microcirculation oxygenation state.

Adult↗
PubMed1999

Coronary venous oximetry using MRI.

Based on the Fick law, coronary venous blood oxygen measurements have value for assessing functional parameters such as the coronary flow reserve. At present, the application of this measure is restricted by its invasive nature. This report describes the design and testing of a noninvasive coronary venous blood oxygen measurement using MRI, with a preliminary focus on the coronary sinus. After design optimization including a four-coil phased array and an optimal set of data acquisition parameters, quality tests indicate measurement precision on the order of the gold standard optical measurement (3%O(2)). Comparative studies using catheter sampling suggest reasonable accuracy (3 subjects), with variability dominated by sampling location uncertainty ( approximately 7%O(2)). Intravenous dipyridamole (5 subjects) induces significant changes in sinus blood oxygenation (22 +/- 9% O(2)), corresponding to flow reserves of 1.8 +/- 0.4, suggesting the potential for clinical utility. Underestimation of flow reserve is dominated by right atrial mixing and the systemic effects of dipyridamole. Magn Reson Med 42:837-848, 1999.

Calibration↗
PubMed1999

In vivo flow-independent T2 measurements of superior mesenteric vein blood in diagnosis of chronic mesenteric ischemia: a preliminary evaluation.

RATIONALE AND OBJECTIVES: The authors attempted to determine whether the T2 relaxation time of superior mesenteric vein (SMV) blood would decrease in patients with chronic mesenteric ischemia after a meal. MATERIALS AND METHODS: Thirty-two patients without chronic mesenteric ischemia and eight patients with symptomatic chronic mesenteric ischemia underwent magnetic resonance (MR) imaging. All examinations were performed with a 1.5-T unit, a modified Carr-Purcell-Meiboom-Gill sequence, final section-selective pulse of 180 degrees, and spiral readout gradients. Measurements of SMV blood T2 were obtained after at least 6 hours of fasting and 15 and 35 minutes after ingestion of 240 mL of a liquid nutritional supplement. Maximal change of the SMV blood T2 was expressed as a percentage of the fasting T2 in all patients. RESULTS: In control patients, SMV blood T2 increased postprandially by 9.4% +/- 1.3 (95% confidence level; range, 6.8%-11.9%) (data range, -7.3% to 25.6%) compared with fasting T2. In symptomatic patients, SMV blood T2 decreased postprandially by 15.8% +/- 2.2 (95% confidence level; range, -20.1% to -10.7%) (data range, -7.9% to -25.3%). The difference between the two groups was statistically significant (P < .0001 by Student unpaired t test). CONCLUSION: Measurement of SMV blood T2 is a promising test for chronic mesenteric ischemia diagnosis. Therefore, conversion of T2 measurements to estimate oxygen saturation may not be necessary for all cases of this clinical indication.

Adult↗
PubMed1999

Effects of a thirteen-week inhalation exposure to ethyl tertiary butyl ether on fischer-344 rats and CD-1 mice.

The 1990 Clean Air Act Amendments require that oxygenates be added to automotive fuels to reduce emissions of carbon monoxide and hydrocarbons. One potential oxygenate is the aliphatic ether ethyl tertiary butyl ether (ETBE). Our objective was to provide data on the potential toxic effects of ETBE. Male and female Fisher 344 rats and CD-1 mice were exposed to 0 (control), 500, 1750, or 5000 ppm of ETBE for 6 h/day and 5 days/wk over a 13-week period. ETBE exposure had no effect on mortality and body weight with the exception of an increase in body weights of the female rats in the 5000-ppm group. No major changes in clinical pathology parameters were noted for either rats or mice exposed to ETBE for 6 (rats only) or 13 weeks. Liver weights increased with increasing ETBE-exposure concentration for both sexes of rats and mice. Increases in kidney, adrenal, and heart (females only) weights were noted in rats. Degenerative changes in testicular seminiferous tubules were observed in male rats exposed to 1750 and 5000 ppm but were not seen in mice. This testicular lesion has not been reported previously for aliphatic ethers. Increases in the incidence of regenerative foci, rates of renal cell proliferation, and alpha2u-globulin containing protein droplets were noted in the kidneys of all treated male rats. These lesions are associated with the male rat-specific syndrome of alpha2u-globulin nephropathy. Increases in the incidence of centrilobular hepatocyte hypertrophy and rates of hepatocyte cell proliferation were seen in the livers of male and female mice in the 5000-ppm group, consistent with a mitogenic response to ETBE. These two target organs for ETBE toxicity, mouse liver and male rat kidney, have also been reported for methyl tertiary butyl ether and unleaded gasoline.

Administration, Inhalation↗
PubMed1998

Water dynamics in human blood via combined measurements of T2 relaxation and diffusion in the presence of gadolinium.

An analytical model of tissue relaxation and restricted diffusion in human blood is presented. The blood tissue model is composed of three different compartments: red blood cells, plasma, and macromolecular protons. The relaxation rate constants and free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to relaxation and diffusion in the Carr-Purcell Meiboom-Gill and pulsed-field-gradient multispin echo experiments for this tissue model are derived. The model is fitted to the experimental data for human blood with various concentrations of Gadolinium contrast agent. The obtained model parameters are realistic. The validity and sensitivity of the model are also discussed.

Blood↗
PubMed1998

Partial volume effects on vascular T2 measurements.

Quantitative, vascular T2 measurements are of interest for applications such as MR oximetry. In the situation of a vessel with long T2 relaxation times embedded in tissue with relatively short T2 values, contamination of the blood signal from the surrounding tissue can bias T2 measurements. Limited data sampling and vessels running obliquely through the imaging slice can cause significant signal contamination. Using a model of these effects to predict the behavior of T2 measurements under a range of conditions, a set of parameters that provides the best combination of measurement accuracy with a signal-to-noise ratio as high as possible is proposed.

Artifacts↗
PubMed1998

Design of practical T2-selective RF excitation (TELEX) pulses.

Traditional T2-based imaging techniques are geared toward imaging long-T2 species. Traditional techniques are, therefore, not optimal in clinical situations where the information of interest lies in the short-T2 species. T2-selective RF excitation (TELEX) is a technique for obtaining a T2-based contrast that highlights short-T2 values while suppressing long-T2 values-opposite to traditional T2 contrast. Previously, TELEX has been demonstrated qualitatively to highlight only very short-T2 values (T2 approximately 0.001 s). When applied to longer T2 values (T2 > or = 0.01 s), TELEX becomes sensitive to deltaB0 non-uniformities. This restricts its application to problems in which the T2 of interest is very short. In this study, TELEX is characterized quantitatively. Furthermore, a bandwidth broadening scheme is developed that reduces the deltaB0 sensitivity of TELEX. This permits the technique to be applied to longer T2 values. The capabilities and limitations of a practical implementation of TELEX are discussed.

Humans↗
PubMed1998

Mesenteric ischemia due to hemorrhagic shock: MR imaging diagnosis and monitoring in a canine model.

PURPOSE: To test whether changes in the percentage of oxygenated hemoglobin (%HbO2) and blood flow in the superior mesenteric vein (SMV), as measured with magnetic resonance (MR) imaging in vivo, can be used to diagnose and monitor mesenteric ischemia due to hemorrhagic shock in a canine model. MATERIALS AND METHODS: Eight mongrel dogs (weight range, 20-30 kg) underwent fasting for 24 hours before the experiments. MR imaging measurements of SMV %HbO2 and volume flow rate were obtained at the resting state and after 5%, 10%, and 15% of the blood volume of the dogs had been removed sequentially, which led to a total blood volume depletion of 30%. In four dogs, resuscitation was performed with normal saline solution in a volume equal to the total volume of blood removed. RESULTS: SMV %HbO2 and SMV flow measurements at the different stages of blood removal were all significantly different (P < .05) from baseline measurements and from each other. After volume replacement with normal saline solution, SMV %HbO2 and flow were not significantly different (P > .05) from the baseline values. CONCLUSION: SMV %HbO2 and volume flow rate, as measured with MR imaging in vivo, can be used to diagnose and monitor mesenteric ischemia due to hemorrhagic shock in a canine model.

Animals↗
PubMed1998

Test bolus examination in the carotid artery at dynamic gadolinium-enhanced MR angiography.

At magnetic resonance angiography in nine patients, a test bolus of gadopentetate dimeglumine was injected to determine transit time from venous injection to arrival in the carotid arteries. Mean times to onset of enhancement were calculated (arterial, 15.7 seconds [range, 10-22 seconds]; venous, 23.4 seconds [range, 16-30 seconds]; optimal imaging time, 7.78 seconds [range, 6-10 seconds]). Transit time was calculated correctly with a test bolus examination.

Aged↗