Table of contents

Eight different modifications of the same single tissue compartment model to measure myocardial blood flow, based on inhalation of ¹⁵O-labelled CO₂ and positron emission tomography, were assessed in both dogs and human normal volunteers. Several models provided results with the same degree of accuracy in dogs. However, a number of these models gave poorer results in humans. It was established that the model containing components for blood flow, fraction of water exchanging tissue and spill-over arterial blood volume provided the most accurate and reproducible results. This model contains inherent corrections for the limited spatial resolution of positron emission tomographs. For ease of computation, linearisation of the operational (fitting) equation was tested, but found not so be satisfactory. The left atrium was slightly better than the left ventricle for determining the arterial input function. Inclusion of the blood volume term in the fitting procedure was significantly better than subtracting blood volume prior to analysis, both in terms of accuracy and precision.

During cardiac systole the stroke volume of blood which is ejected from the right ventricle increases the pulmonary blood volume. This systolic pulmonary blood volume increase is measured by using ECG gated chest scintigrams obtained with ⁹⁹Tc^m-labelled red blood cells. The total radiation counts in a region of interest that includes either the right or the left lung increases during systole. On the average, the increase of the total pulmonary systolic radiation was found to be 62% of the systolic radiation decrease in the left ventricle region of interest. The discrepancy between the value of the systolic pulmonary blood volume increase and that of stroke volume is attributed to blood flow from the lungs into the left atrium during the cardiac systole period.

The reproducibility of two methods of measuring broadband ultrasonic attenuation (BUA) in the calcaneus have been studied. An improvement in reproducibility in vivo from 9.6% to 2.8% between old and new techniques has been observed. Measurements of the calcaneus using BUA were correlated with measurements of bone mineral density measured by dual energy X-ray absorptiometry in the lumbar spine, femur and total body and bone mineral content in the distal and proximal forearm measured by single photon absorptiometry. For the older BUA technique the correlation coefficients ranged between r=0.27 and r=0.34. For the newer BUA technique the correlation coefficients ranged between r=0.49 and r=0.62 and were all significant (P<0.001).

A new technique for the quantitative analysis of labelled leucocyte images from patients with inflammatory bowel disease is described. The method involves the computer generation of a 'background' image which, after appropriate registration, is subtracted from the patient's image to leave a residue which represents abnormal uptake in the bowel. Quantification of the residual activity yields a scan score which can be related to the level of disease activity in patients with Crohn's disease. In 54 investigations on 33 patients the scan scores correctly agreed with a clinical assessment of disease activity in 16 of 20 cases with inactive disease and 32 out of 34 cases with active disease. Most of the discrepancies reflected inaccuracies in the clinical assessment of activity rather than shortcomings of the imaging technique.

The 2-pool urea kinetic model has been developed analytically and applied to the description of the observed increase in blood levels of urea following dialysis (urea rebound), assuming that the dialyser urea clearance K<0.4 X where X is the urea mass transfer coefficient between the intracellular and extracellular pools (volumes V₁, V₂ respectively). Urea generation was also neglected. Measurements were made in a group of six children suffering from chronic renal failure. From the model X, the efficiency of dialysis, and the equilibrium urea concentration C_infinity were estimated in the presence of urea rebound using a blood urea measurement taken 90 min following start of dialysis, in addition to the conventional samples taken immediately pre- and post-dialysis. The results indicated that C_infinity was estimated to within 10% of the true equilibrium urea concentration. The error in the estimate of dialysis efficiency based on a single pool model was reduced by at least 50% using the model. The model may be applied clinically to the estimation of dialysis efficiency in the presence of significant urea rebound.

The possible use of impedance measurement with scalp electrodes to detect cerebral ischaemia non-invasively was investigated in the anaesthetised rat. Global cerebral ischaemia was induced by diathermy of both vertebral arteries and reversible occlusion of the carotid arteries. Impedance was measured at 50 kHz by a four electrode method. With cortical electrodes in various positions reversible impedance increases of 15-60% were recorded during episodes of cerebral ischaemia which lasted for 5-30 min. With electrodes placed in overlying positions in the scalp, impedance increases had a similar time course but their amplitudes were about 10-20% of the increases measured on the cortex. These were not due solely to changes in temperature which accompanied cerebral ischaemia or local changes in the scalp. These findings suggest that it may be possible to image cerebral ischaemia in human subjects non-invasively with electrical impedance tomography and scalp electrodes.

The possible use of impedance measurement with scalp electrodes to detect intracranial events non-invasively was investigated by measuring the localised impedance changes during cortical spreading depression (CSD) in anaesthetised rats. Impedance was measured over ipsilateral fronto-parietal cortex by four electrode method operating at 50 kHz with electrodes spaced 0.5 mm apart. Cortical impedance increased by 39% of the resting level during CSD. With scalp electrodes placed on abraded skin, an unexpected impedance decrease of 0.8% occurred, which correlated spatially and temporally with CSD. CSD was accompanied by a small rise in temperature; when scalp temperature was held constants warming the scalp, no impedance change greater than baseline variability (+or-0.1% of the resting impedance level) was observed. The non-invasive detection of CSD in migraine in humans may be possible by measuring the characteristic temperature-relate-impedance changes with electrical impedance tomography and scalp electrodes.

The performance of a prototype impedance imaging (electrical impedance tomography, EIT) system using cortical or scalp electrodes has been assessed in a model of global cerebral ischaemia in the anaesthetised rat. In preliminary calibration experiments using a circular array of electrodes around a tank of saline, the centre of a polythene rod could be localised with a mean error of 4% of the tank diameter, and two such rods could be discriminated when separated by 22% of the tank diameter. Cerebral ischaemia was produced by diathermy of the vertebral arteries and reversible occlusion of the common carotid arteries for 15 min. A bimodal impedance increase of about 50-200% was recorded with a ring of cortical electrodes in the fronto-occipital plane. With a similarly orientated ring of scalp electrodes, a unimodal impedance increase of about 10% was observed. In both cases, impedance reversed after cerebral reperfusion. Similar but irreversible changes were observed post mortem.