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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
PubMed2006

MRI relaxation fluctuations in acute reperfused hemorrhagic infarction.

MRI evaluations of intramyocardial hemorrhage in acute infarction have relied on T(2) and T(2)(*) shortening only. We propose a more comprehensive evaluation of hemorrhagic infarction based on the concept that fluctuations in T(2) and T(1) relaxation in acute reperfused infarction will reflect transient edema and hemoglobin oxidative denaturation to uncompartmentalized methemoglobin. Anteroapical infarction was created via percutaneous balloon in young swine (22-25 kg, N = 12). T(2), T(1), diastolic wall thickness (DWT), and the Gd-DTPA partition coefficient (lambda) were measured on days 0, 2, and 7. DWT was elevated at 1 hr postreperfusion (128% +/- 53%, P = 0.0001), and alleviated on days 2 and 7 (48% +/- 10%, P = 0.008; 53% +/- 24%, P = 0.003). T(2) and T(1) elevations were coincident with early edema (DeltaT(2) = 55% +/- 24%, P < 0.0001; DeltaT(1) = 27% +/- 18%, P < 0.04). T(2) and T(1) were nearly normal on day 2 (DeltaT(2) = 8% +/- 8%, P = 0.27; DeltaT(1) = 0% +/- 1%, P = 0.65). On day 7, T(2) increased while T(1) decreased (DeltaT(2) = 27% +/- 16%, P = 0.005; DeltaT(1) = -14% +/- 10%, P = 0.02). Lambda was elevated by >150% at all time points (P < or = 0.002). Histology verified hemorrhagic injury. T(1) and T(2) fluctuations are consistent with transient edema, as well as hemoglobin oxidative denaturation to decompartmentalized methemoglobin. This methodological development may broaden our understanding of hemorrhagic microvascular injury and improve its detection in clinical populations.

Acute Disease↗
PubMed2006

T2 fluctuations in ischemic and post-ischemic viable porcine myocardium in vivo.

T2 relaxation can augment delayed-enhancement viability imaging because it is sensitive to tissue edema and microcirculatory oxygen state. We demonstrate the T2 'signatures' of sub-lethal ischemia and stunning in porcine myocardium perfused by the distal left anterior descending artery, by imaging during percutaneous balloon occlusion for 25 minutes and subsequent reperfusion (n = 9). Muscle displayed ischemic dysfunction and partial post-ischemic functional recovery (p < or = 0.0004), concommitant with an elevated post-ischemic T2 (deltaT2 = 27 +/- 18%, p = 0.005). TTC staining verified muscle viability. The T2 fluctuations may reflect hyperemia and tissue cellular edema in accord with the known pathophysiology of ischemic and post-ischemic yet viable muscle.

Animals↗
PubMed2005

Application of MCBEND to PBMR shielding analysis.

Shielding analysis of an early design of Pebble Bed Modular Reactor (PBMR) has been carried out by using the Monte Carlo code MCBEND. The issues of concern were damage to the core barrel and the reactor pressure vessel (RPV), activation of the core barrel, RPV, top plate and bottom plate, and also burn-up of boron in the control layer underneath the core. The analysis below the core was complicated due to the presence of the de-fuelling chute, which meant that multiplication had to be taken into account. The analysis of boron burn-up was particularly challenging and was tackled using a combination of MCBEND and the criticality code MONK in the depletion mode. The application of MCBEND to the shielding analysis of the PBMR is described, with particular attention being paid to the regions below the core.

Algorithms↗
PubMed2004

In vitro generation and stability of the lactokinin beta-lactoglobulin fragment (142-148).

The objectives of this study were to investigate the generation of beta-lactoglobulin fragment (142-148) (beta-LG f(142-148) during the hydrolysis of whey proteins, and the in vitro stability of this fragment upon incubation with gastrointestinal and serum proteinases and peptidases. An enzyme immunoassay (EIA) protocol was developed for the quantification of beta-LG f(142-148) in whey protein hydrolysates and in human blood serum. The minimum detection limit was 3 ng/mL. The level of the peptide in whey protein hydrolysates was influenced by the degree of hydrolysis (DH). As expected, highest levels of this peptide were found in hydrolysates generated with trypsin. Sequential incubation of hydrolysates at different DH values with pepsin and Corolase PP, to simulate gastrointestinal digestion, generally resulted in the degradation of beta-LG f(142-148) as determined by EIA. Reversed-phase HPLC and angiotensin-I-converting enzyme (ACE) activity assays demonstrated that synthetic beta-LG f(142-148) was rapidly degraded upon incubation with human serum. Furthermore, beta-LG f(142-148) could not be detected by EIA in the sera of 2 human volunteers following its oral ingestion or in sera from these volunteers subsequently spiked with beta-LG f(142-148). These in vitro results indicate that beta-LG f(142-148) is probably not sufficiently stable to gastrointestinal and serum proteinases and peptidases to act as an hypotensive agent in humans following oral ingestion. The in vitro methodology described herein has general application in evaluating the hypotensive potential of food protein-derived ACE inhibitory peptides.

Angiotensin-Converting Enzyme Inhibitors↗
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↗