Biomedical subjects

A Madden

Publications and source records attributed to A Madden.

6 recordsLinked to original sources

PubMed1992

Radial diffusion coefficient mapping.

The two-dimensional mapping of the effective diffusion coefficient of water in tissues may provide a useful method of tissue characterization to complement T1 and T2 relaxation time studies for diagnostic purposes. Current diffusion techniques rely on the application of large gradient strengths and long echo times to achieve the required sensitivity. This, in turn, limits the applicability of the technique to tissues having long T2s with rapid water diffusion. In addition, the inherent directionality of these methods results in only the partial encoding of diffusion information. A modified diffusion sequence is presented, radial diffusion mapping (RAD), which provides enhanced sensitivity diffusion maps by employing gradient sensitization in three orthogonal directions. Results in both phantoms and volunteers are presented, together with an investigation of the effects of T2 on the measurement accuracy. Using RAD, a two- to five-fold improvement in sensitivity was achieved, thus significantly enhancing the dynamic range of the method and allowing more accurate in vivo diffusion measurements to be carried out.

Acetone↗

PubMed1991

The unstable ST segment early after thrombolysis for acute infarction and its usefulness as a marker of recurrent coronary occlusion.

To investigate the incidence of early recurrent ST elevation after intravenous thrombolytic therapy for acute myocardial infarction, 12-lead electrocardiograms were continuously monitored for 571 +/- 326 minutes in 31 patients presenting within 4 hours of symptom onset. The study group comprised 9 women and 22 men (mean age +/- standard deviation 53 +/- 12 years), with ST elevation (anterior in 15, inferior in 16) on the initial electrocardiogram, who were given either tissue plasminogen activator (22 patients) or streptokinase (9 patients). Angiography was performed in 30 of 31 patients at 7 to 10 days. Early (less than 3 hours) resolution of ST elevation occurred in 19 patients (61%) at a median of 94 minutes (interquartile range 57 to 113) after thrombolysis, whereas 12 (39%) had no or late (greater than 6 hours) resolution. Eleven of the 19 with early resolution (58%) had either transient (5 patients) or sustained (6 patients) recurrences of ST elevation. Recurrent ST elevation was equal to or more than the initial peak elevation in 9 of 11 patients, and greater than 75% of initial peak in 2. A total of 25 episodes of recurrent ST elevation were observed in the 11 patients (19 transient and 6 sustained episodes), of which 8 (32%) were silent. The proportion of silent episodes was similar for transient (35%) and sustained (33%) recurrences. All patients with sustained recurrent ST elevation had at least 1 preceding transient recurrence. The median duration of transient recurrent ST elevation was 43 minutes (28 to 63).(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary Angiography↗

PubMed1991

The water resonance as an alternative pH reference: relevance to in vivo 31P NMR localized spectroscopy studies.

pH measurements require a suitable pH reference within the 31P NMR spectrum with respect to which the chemical shift of Pi, and hence pH, may be calculated. In muscle spectra PCr is prominent and provides a reference frequency. However, recent localized tumor studies have reported the absence of PCr, for example, in breast tumors. The use of the alpha-, beta-, and gamma-ATP peaks as suitable references has been suggested, but the position of the beta- and gamma-ATP peaks is dependent upon the intracellular Mg2+ concentration. The alpha-ATP is not affected by ionic concentrations; however, it contains UDPG and NAD+, the presence of which can lead to peak-shape distortion. This paper considers the use of the H2O resonance from the proton spectrum used for shimming as a suitable pH reference, provided this is also localized to the same region of interest, using a sequence giving rise to eddy current effects comparable to those of the 31P NMR sequence. Localized in vivo measurements in the muscle and brain of volunteers indicate good agreement between the proton and phosphorus chemical shifts, allowing the PCr position to be predicted to within 0.01 ppm in all cases.

Body Water↗

PubMed1991

A quantitative analysis of the accuracy of in vivo pH measurements with 31P NMR spectroscopy: assessment of pH measurement methodology.

31P NMR spectroscopy provides a means of monitoring intracellular pH (pHi). In order to determine the significance of observed pH changes, an assessment of the measurement uncertainty is required. This paper considers the possible sources of error involved, firstly in chemical shift assignment and, secondly, in relating chemical shift to pH. A statistical analysis of these errors is presented and an expression for the accuracy of pH measurements obtained using in vivo 31P NMR spectroscopy is derived. Computer simulations and clinical measurements are considered to determine the likely error in clinical measurements. A comparison of pH measurement methodology using a centre-of-gravity calculation, Gaussian and Lorentzian fitting is also presented and discussed.

Humans↗

PubMed1990

The effect of non-protein liquid meals on the hepatic venous pressure gradient in patients with cirrhosis.

It has been suggested that protein feeding increases portal pressure in cirrhotic patients, but that carbohydrate and fat have little effect. We examined the relationship between feeding and portal pressure, using different liquid test meals (250 or 500 ml non-protein, 250 ml protein-containing, 500 ml water), in 29 alcoholic patients with cirrhosis and portal hypertension. The mean hepatic venous pressure gradient (HVPG) increased significantly 30 min after the protein meal (10% increase; p = 0.009) and returned to basal levels at 60 min. The mean HVPG also increased significantly after the non-protein meal: after 500 ml the increase was 23% at 30 min (p = 0.046) and 17% at 60 min (p = 0.12); and after 250 ml it was 15% at 30 min (p = 0.012) and 7% at 60 min (p = 0.05). Ingestion of 500 ml water caused a small, non-significant, increase in mean HVPG. Plasma glucagon levels increased significantly at 30 and 60 min after the protein meal, but did not change significantly after the non-protein meal or water. Both protein-containing and non-protein meals significantly elevate HVPG in alcoholic patients with cirrhosis and portal hypertension.

Adult↗

PubMed1990

An assessment of the sensitivity of in vivo 31P nuclear magnetic resonance spectroscopy as a means of detecting pH heterogeneity in tumours: a simulation study.

The presence of hypoxia and low pH in tumours is known to influence several treatment modalities including radiotherapy, chemotherapy and hyperthermia. Hypoxic and acidic regions have been demonstrated in tumours using pH and pO2 microelectrodes. The technique has shown marked heterogeneity within individual tumours. A non-invasive measure of intracellular pH is provided by 31P nuclear magnetic resonance (NMR) spectroscopy from determination of the chemical shift of inorganic phosphate, and is being increasingly applied to the study of human tumours in vivo. Based upon the assumption that hypoxic cells are also acidic, we have assessed the ability of a whole-body NMR spectrometer to detect acidic subpopulations within a tumour using simulated tumour spectra. In these simulations the size of the acidic subpopulation, assigned pH values from 5.0 to 7.0, was varied between 5% and 50% of the neutral "control" population. Gaussian noise was added to the simulated spectra giving signal-to-noise ratios for the neutral control inorganic phosphate peak of 3, 5 and 7, which are typical of values encountered in human tumour measurements. From the results of the simulations it seems unlikely that human hypoxic, and presumably acidic, cell fractions, typically 1-15%, will be detected by this method in the presence of signal-to-noise levels characteristic of current equipment. Therefore, whole-body in vivo 31P NMR spectroscopy may fail to identify significant pH heterogeneity within human tumours. Improvements in signal-to-noise and line separation owing to improvements in technique and higher field strength instruments may improve sensitivity to heterogeneous populations.

Cell Hypoxia↗