Reliability of absolute and relative measurement of total body nitrogen by the 14N(n, 2n)13N reaction.

Measurements of total body nitrogen by in vivo neutron activation analysis using the 14N(n, 2n)13N reaction, may conveniently be calibrated using phantoms in which nitrogen is uniformly distributed. However, the response of the method varies considerably in the thickest body sections and in order to validate the calibration, it is necessary also to consider the spatial distributions both of nitrogen and of the major interfering element, oxygen. The effects of these spatial distributions have been calculated for the experimental conditions obtained at the East Kilbride facility for total body in vivo neutron activation analysis. The response for total body nitrogen was found to depend strongly on body weight (in the rang e 40 to 100 kg) but only weakly on total body content of adipose tissue (0 to 65% of body weight). It differed little from the response calculated for the nitrogen distributed uniformly within the body. The calculation was tested successfully against measured changes in the net response for nitrogen resulting from 'homogenisation' of inhomogeneous phantoms.

Simplification of simultaneous measurement of the total body content of five elements using only one 14 MeV neutron generator.

Measurement of body composition by total body in vivo neutron activation analysis is a valuable tool for clinical research but use of this technique has been largely restricted to medical units near existing neutron irradiation facilities. A facility for total body in vivo neutron activation analysis was purpose-built at low cost and has performed well. The methods of scanned bilateral neutron irradiation and scanned bilateral measurement of induced activity have been subject to continuous development. It has been found possible to dispense with one of the original complement of two 14 MeV neutron generators, without significantly impairing measurement of any of the elements for which the system was designed (phosphorus, calcium, nitrogen, chlorine and sodium). Thus a hospital department with access to a whole-body counter capable of measuring total body 40K can now extend its investigation of body composition to these five important elements at a cost of 75 m2 of floor space and less than 30 000 pounds.

Trans-thoracic fluid shifts and endocrine responses to 6 degrees head-down tilt.

A tomographic method of measuring electrical impedance known as Applied Potential Tomography (APT) has been used to image the impedance changes within the thoraxes of 8 healthy volunteers (4 male, 4 female) during 4-h periods of 6 degrees head-down tilt (HDT). A large decrease in impedance, reflecting an increase in thoracic fluid, was apparent within 1 min of tilting, peaked after 45 min, and was maintained throughout, although during the 4 h there was an 8% return towards baseline resistivity. Resistivity changes were most obvious in the region of the lungs. Simultaneous measurements of the key fluid regulating hormones revealed a significant increase in atrial natriuretic peptide (ANP) and a significant decrease in angiotensin II (AII) and aldosterone. There was no significant difference in plasma antidiuretic hormone level. These results illustrate the dynamic nature of fluid shifts during HDT, the spatial distribution of the fluid within the thorax and the associated endocrine responses.

Monitoring the thoracic sagittal curvature in kyphoscoliosis with surface topography: a trend analysis of 57 patients.

Records of Quantec measurements of tlie kypliotic curvature of the back were reviewed for all patients attending the children's orthopaedic clinic who were referred for back shape measurements. Of these, 57 children had five or more preoperative visits allowing trends to be calculated. Linear trends were found in 30 of the patients, with gradients ranging from 1.1 degree/yr to 7.2(0)1/yr. On average, the scatter of measurements about the trend line, or about the mean value in the other 27 cases, compared well with that expected from repeatability studies but the amount of scatter varied from one patient to another. This may well be due to sampling. Where such measurements are monitored for evidence of change in an individual patient, the possibility of larger than average scatter about any emerging trend should be considered.

The effect of the skull of low-birthweight neonates on applied potential tomography imaging of centralised resistivity changes.

An investigation is presented into the likely effects of the neonatal skull on impedance images produced by applied potential tomography (APT) by imaging impedance changes inside the skull of a human infant of occipito-frontal circumference 30 cm. Measurements have been made with the skull immersed in a tank of saline and electrodes fixed to the perimeter of the tank. Sensitivity measurements have been assessed for imaging a small target close to the centre of the skull as compared with images produced without the skull. The results obtained compare favourably with measurements on a more realistic model of the neonatal head constructed by filling the skull with agar jelly to leave only a thin exterior coating of jelly to simulate the scalp. These experiments suggest that in the central region of the head of a neonate, measured changes by the APT technique are about 44% of that expected from a homogeneous phantom, but that this might vary from 32% to 55% at different points in the image in a very complex manner.

An assessment of dynamic images by applied potential tomography for monitoring pulmonary perfusion.

Applied potential tomography (APT) images can be collected at a rate of 24 per second and data collection can be synchronised with the ECG. Images thus obtained from a thoracic plane allow the spatial separation of impedance changes originating in the heart, aorta and lungs and have raised the possibility of detecting pulmonary perfusion abnormalities from the cardiac-related impedance changes in the lungs. We have recently started a study to compare isotope perfusion scans with APT images and present here a few initial examples which suggest that further investigation of this field may prove rewarding.

Noise and spatial resolution of a real-time electrical impedance tomograph.

The Sheffield real-time electrical impedance tomograph produces 25 images per second, using 16 electrodes with adjacent-pair current drive and non-iterative image reconstruction. We describe the data acquisition timing of this instrument and present quantitative measurements of its signal-to-noise ratio and spatial resolution.

Blood flow imaging using electrical impedance tomography.

It is shown that a real-time electrical impedance tomography (EIT) system can be used to image the flow of saline through the human vascular system. A 10 ml bolus of 0.9% saline injected intravenously distal to an EIT imaging plane allows venous flow to be observed. Measurements on a cylindrical tank with flow along axial conductive tubes have been used to establish that the area under a concentration against time curve can be obtained from the EIT images and used to determine the flow rate down the tube. In vivo results show that flow images of the venous system in a limb can be obtained and that there is adequate sensitivity to follow the passage of a saline bolus though the cardiac chambers.

Local blood volume changes in women with pelvic congestion measured by applied potential tomography.

1. Applied potential tomography is a new, non-invasive technology for observing changes in blood volume. We have used it to study 12 women with lower abdominal pain caused by pelvic congestion, and 15 control subjects. 2. A significant increase in blood volume of 1.8% was observed in the pelvis of women with congestion when changing from the supine to the erect position, and of 2.7% in the control subjects (P less than 0.0002). The difference between the groups was not significant. 3. The distribution of the area over which blood volume changes took place was significantly different between the two groups (P less than 0.002). More of the posterolateral part of the pelvis was involved in women with pelvic congestion than in the control subjects. 4. Applied potential tomography distinguishes between normal women and those having pelvic congestion with a specificity of 87% and a sensitivity of 75%.

Monitoring body fluid distribution in microgravity using impedance tomography (APT (applied potential tomography)).

For an astronaut, the excitement of going into orbit is accompanied by a shift of 1 to 1.5 l of fluid from the legs into the upper body. Information on the way the redistributed fluid is handled by the body is very useful to space physiologists studying the process of adaptation to zero-gravity. Applied potential tomography (APT) can be used to image changes in fluid distribution. To ensure that the technique was capable of measuring fluid shifts induced by changing gravitational forces on the body, a standard Sheffield APT system was used to study several subjects during the eight ESA parabolic flight campaign. The results clearly demonstrated the feasibility of using APT for monitoring fluid redistribution during space flight. A battery-powered, body-worn APT system has now been developed for use in space. The equipment was tested on the eleventh parabolic flight campaign. The data collected with the miniaturised system was comparable to that obtained in the earlier experiment. Ergonomic tests indicated that the equipment is no more difficult to operate and maintain under weightless conditions than on earth. The system is undergoing space qualification tests in Munich. If no problems arise it will be used by German astronauts on missions to MIR and Skylab.