In the clinical environment the reliable interpretation of EIT images depends on the quality of the data. In the electrically noisy hospital environment the system performance needs to be assessed for each clinical investigation. From the model of noise presented, a figure of merit for comparisons of system performance with a known standard, or with previous studies, can be generated. The method depends on calculating the variances of the differences in reciprocity measurements as a function of the distance between the current drive electrodes and the receive voltage electrodes. These measurements fit the noise model, with minimal interference from physiological variability, and permit a figure of merit to be calculated which is a representation of the noise at the point to the system. Typical figures of merit are 7.36 +/- 0.03 microV for a test card and 10.50 +/- 16 microV for subject data.

Ideally electrical impedance tomography (EIT) should not be oversensitive to electrode positions, but this conflicts with efforts to produce high-resolution images. Two procedures are presented that balance reducing the sensitivity to electrode position errors with generating practicable EIT images. The first provides a criterion based on electrode sensitivity for regularizing the reconstruction through spectral expansion. The main consequences of this are that smoother images are produced and the number of artefacts and their magnitude are generally reduced. The second modification uses the recorded data to compensate for electrode movements that have occurred after the reference data were measured. Image smoothness is used as the criterion for the readjustment. Computer simulation tests have shown that this modification produces improved image fidelity.

EIT measurements on humans are often made in regions of the body where the conductivity distribution is far from uniform. This paper addresses the problem of deriving accurate quantitative data in one such region: the conductivity changes associated with the accumulation of blood in the pelvic bowl. A computer map of the bone in the pelvic region was constructed, from which an appropriate reconstruction matrix was generated. Both computer simulations and tank tests were performed to assess whether this bone reconstruction matrix produced impedance images with closer fidelity to the measured object than images produced using a reconstruction based on a uniform conductivity distribution. As expected, images produced by the computer simulation indicated that the bone reconstruction matrix produced images of better fidelity than did the uniform reconstruction matrix. However, in the case of the tank data only a moderate improvement was achieved. The reconstruction matrix based on a uniform conductivity distribution was found to produce satisfactory images for both bone and near-uniform objects, but for regions further into the pelvic bowl, where the signal was lower, the uniform reconstruction matrix was less satisfactory.

The relationship between patient cross-sectional area and both volume CT dose index (CTDI) and dose length product was explored for abdominal CT in vivo, using a 16 multidetector row CT (MDCT) scanner with automatic exposure control. During a year-long retrospective survey of patients with MDCT for symptoms of abdominal sepsis, cross-sectional areas were estimated using customised ellipses at the level of the middle of vertebra L3. The relationship between cross-sectional area and the exposure parameters was explored. Scans were performed using a LightSpeed 16 (GE Healthcare Medical Systems, Milwaukee, WI) operated with tube current modulation. From a survey of 94 patients it was found that the CTDI increased with the increase in patient cross-sectional area. The relationship was logarithmic rather than linear, with a least-squares fit to the data (R(2) = 0.80). For abdominal CT the cross-sectional area gave a measure of patient size based on the region of the body to be exposed. Exposure parameters increased with increasing cross-sectional area and the greater radiation exposure of larger patients was partly a consequence of their size. Given increasing obesity levels we believe that cross-sectional area and scan length should be added to future dose surveys, allowing patient size to be considered as a factor of relevance when examining population doses.