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Biomedical subjects

J B Weaver

Publications and source records attributed to J B Weaver.

At least 37 records · Page 2Linked to original sources
PubMed1995

The apparent diffusion constant measured by MRI correlates with pO2 in a RIF-1 tumor.

As tissue oxygen tension (pO2) is an important variable in cancer therapy, it would be of major clinical benefit to be able to measure pO2 noninvasively. Current methods for determining pO2 in clinical settings are limited to superficial tumors. The authors measured the apparent diffusion constant (ADC) in an implanted murine fibrosarcoma (RIF-1) using magnetic resonance imaging and correlated the ADC with tissue pO2 measured by electron paramagnetic resonance oximetry. The ADC correlates significantly with tissue pO2 in this tumor (r = 0.89; n = 14) and so may provide a noninvasive index of pO2 in tumors.

Animals↗
PubMed1995

High resolution renal diffusion imaging using a modified steady-state free precession sequence.

A modified steady-state free precession (SSFP) diffusion sequence is proposed for high resolution renal imaging. A pair of bipolar diffusion gradients was used to minimize the errors in measured apparent diffusion coefficient (ADC) caused by variations in T1, T2, and RF flip angle that have been observed with previously employed SSFP diffusion sequences. Motion sensitivity was reduced by the use of compensated gradients, frame-by-frame averaging, and a repetition time of 22 ms, which for a single-acquisition 128 x 128 image requires only 3 s. High resolution was achieved by signal averaging. The modified sequence was applied to in vivo diffusion measurements. In six normal rat kidneys the ADCs (mean +/- SD; x 10(-3) mm2/s) of the cortex, outer medulla, and inner medulla were 2.28 +/- 0.05, 2.38 +/- 0.10, and 2.95 +/- 0.05, respectively. The technique requires relatively large gradients to achieve adequate diffusion weighting.

1-Propanol↗
PubMed1995

Efficient calculation of the principal components of imaging data.

Principal components analysis (PCA) of images is important in many applications such as positron emission tomography and functional magnetic resonance imaging. PCA is difficult for image data because the correlation matrix is very large. We present a direct method of calculating the PCA of the voxels from the small matrix expressing the correlations between images, instead of the larger matrix representing the correlations between voxels. The method is fast and accurate. It is faster and requires less memory than a singular value decomposition, although it is less accurate. It is much faster and more accurate than iterative and other approximate methods developed for this problem.

Algorithms↗
PubMed1995

Fetal mortality associated with cholestasis of pregnancy and the potential benefit of therapy with ursodeoxycholic acid.

Cholestasis of pregnancy is associated with increased fetal morbidity and mortality and should be treated actively. The significance attached to pruritus in pregnancy is often minimal, but it is a cardinal symptom of cholestasis of pregnancy, which may have no other clinical features. Eight women with previous cholestasis of pregnancy were referred to The Liver Unit within a 12 month period for advice concerning future pregnancies. Thirteen pregnancies had been affected by cholestasis of pregnancy and 12 had been treated expectantly with resultant perinatal morbidity or mortality in 11 (one normal delivery), including; eight stillbirths, two premature deliveries with fetal distress (one died in perinatal period), and an emergency caesarean section for fetal distress. The other pregnancy was treated actively and delivery was uncomplicated. Subsequently, three of these cases with recurrent cholestasis of pregnancy were referred while pregnant. In each, cholestasis developed with severe pruritus, gross increase of serum bile acids, and deranged liver tests. Each was treated with the choleretic agent ursodeoxycholic acid, with rapid clinical improvement and resolution of deranged biochemistry. In conclusion, cholestasis of pregnancy continues to be treated expectantly despite its association with increased morbidity and mortality and evidence suggesting improved prognosis with active treatment and the potential of reducing the associated perinatal mortality. In an uncontrolled series of three patients with cholestasis of pregnancy, ursodeoxycholic acid seemed to provide safe and effective therapy.

Adult↗
PubMed1994

The adverse influence of pregnancy upon sulphation: a clue to the pathogenesis of intrahepatic cholestasis of pregnancy?

Sulphation of oestrogens and monohydroxy bile acids is important in attenuating their cholestatic potential. Thus, impairment of sulphation could lead to retention of cholestatic compounds and precipitate intrahepatic cholestasis in susceptible individuals. We tested the hypothesis that such a mechanism may be involved in the pathogenesis of intrahepatic cholestasis of pregnancy. In vivo and in vitro assessment of sulphation capacity was performed in patients with cholestasis of pregnancy, compared with control females on and off the oestrogen-containing oral contraceptive pill and control individuals during normal pregnancy and post partum, to assess the influence of high oestrogen states upon this metabolic pathway. During in vivo studies utilising paracetamol as a metabolic probe, the proportion of paracetamol sulphate and sulphate: glucuronide ratio were decreased in those with elevated oestrogens, whether the rise in oestrogens was endogenous, in pregnancy (paracetamol sulphate p < 0.05; paracetamol sulphate:glucuronide ratio p < 0.01), or exogenous, with the contraceptive pill (paracetamol sulphate p = 0.2; paracetamol sulphate:glucuronide ratio p < 0.001). In vitro, platelet sulphotransferase activity was measured, utilising phenol as substrate. Sulphotransferase activity decreased during pregnancy compared with repeat measurements post partum (p < 0.005) and compared with non-pregnant individuals (p < 0.05). In conclusion, we have shown that elevated oestrogens are associated with significant impairment in sulphation capacity. An imbalance of sulphation with glucuronidation provoked by high circulating oestrogen levels may be contributory in the pathogenesis of cholestasis of pregnancy.

Acetaminophen↗
PubMed1992

Wavelet-encoded MR imaging.

Wavelet encoding is presented and compared to phase encoding. In wavelet encoding a distribution of spins is excited by a slice selective RF pulse; for each repetition time the distribution excited has the profile of a wavelet at different scale and translation. The spin density can be reconstructed with an inverse wavelet transform. Wavelet encoding has three advantages over phase encoding: (1) there is no Gibb's ringing from partial volume effects, (2) the effective repetition time can be 36 times the repetition time for a 256 x 256 image, and (3) motion artifacts are local and dramatically reduced. Using wavelet encoding, a 256 x 256 T2-weighted projection image can be acquired in 33 s.

Artifacts↗
PubMed1991

Filtering noise from images with wavelet transforms.

A new method of filtering MR images is presented that uses wavelet transforms instead of Fourier transforms. The new filtering method does not reduce the sharpness of edges. However, the new method does eliminate any small structures that are similar in size to the noise eliminated. There are many possible extensions of the filter.

Artifacts↗
PubMed1991

Limited field of view spin echo MR imaging.

Several groups have reported using a method of limiting the field of view (FOV) where the slices excited by the 90 and 180 degree pulses are perpendicular. However, only one slice can be excited during each repetition time, so multislice imaging is not possible. We present a modification of this method that allows multislice imaging. The slices excited by the 90 degrees and 180 degrees pulses are at a small angle; the field of view is limited and multislice imaging is possible. The modifications also allow the center of the FOV to be offset to any position. We describe the conditions that yield optimal images for the given FOV, slice thickness, and interslice gap. Representative images demonstrating the features of the technique are presented. The technique can be used to reduce the number of phase-encoding steps resulting in reduced imaging time, or it can be used to increase the spatial resolution without increasing the imaging time.

Humans↗
PubMed1988

Simultaneous multislice acquisition of MR images.

The simultaneous multislice technique is a method of imaging multiple parallel slices with the number of echoes normally used to image a single slice. Images of 16 slices have been obtained from a single 128-echo acquisition. The distance between the slices can be decreased to approximately 15% of the field of view in the readout direction with the cost of significant image blurring. The image blurring is negligible when the distance between the slices approaches the field of view in the readout direction. The trade-offs are described, and equations and images are presented.

Magnetic Resonance Imaging↗
PubMed1988

An assessment of the value of biochemical fetoplacental function tests in detecting intrauterine growth retardation.

Four serum biochemical fetoplacental function tests (HPL, unconjugated and total oestriol and AFP) were used to screen for intrauterine growth retardation in 793 patients attending an antenatal clinic. The birthweight corrected for sex, birth ranking and maternal size was used as an index of growth. The sensitivity and specificity of each test was calculated at varying cut-off points and plotted as 'Receiver Operator Curves'. HPL is clearly better than the oestriols or AFP with 80% sensitivity in detecting birth weights of less than the 5th centile when the 25th HPL centile is used as the cut-off point. However, the sensitivity of HPL is sufficiently low to make the value of screening questionable, particularly since 40% of all growth retarded infants were in mothers who did not attend antenatal clinic for screening and 25% of all growth retarded infants were detected by clinical examination.

Female↗