S Meeson

Electrical impedance tomography with compensation for electrode positioning variations.

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.

The dependence of EIT images on the assumed initial conductivity distribution: a study of pelvic imaging.

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.

EIT data noise evaluation in the clinical environment

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.

The in vivo relationship between cross-sectional area and CT dose index in abdominal multidetector CT with automatic exposure control

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.

Nonlinear reconstruction constrained by image properties in electrical impedance tomography.

It is proposed that image quality, for example the degree of roughness, in electrical impedance tomography is the essential measure required to regularize nonlinear reconstruction. Most previously published work has addressed efficiency, stabilization and speed of reconstruction and has overlooked the targeted image qualities. The measure of quality adopted is the mean square gradient of the logarithm of resistivity which, in combination with the chi2 statistic as a measure of the fit to the data, is minimized by iteration until convergence to a stable image is achieved. This penalty function is invariant to the scale of the resistivity and to the interchange of resistivity and conductivity. The algorithm is tested on computer simulated data and on measurements from a cylindrical tank of electrolyte. The results demonstrate the increased image definition that it would be possible to achieve as data acquisition systems are improved. The images show how a reduction in resolution can be traded for reduced noise artefacts, by selecting an appropriate target chi2.

Justifying multidetector CT in abdominal sepsis: time for review?

The further development of multidetector row CT (MDCT) has led to changes in the application and examination technique, leading to a need to justify the level and frequency of radiation exposure associated with MDCT. A literature review of how the use of modern scanners has affected diagnosis was undertaken, followed by a year-long retrospective study of MDCT scans of patients presenting with symptoms of abdominal sepsis. The diagnostic accuracy of detecting causes of abdominal sepsis using this technology was sought. Scans were performed using a LightSpeed 16 system (GE Healthcare Medical Systems, Slough, UK and Milwaukee, WI). Clinical diagnoses were based upon surgical and histopathological findings, treatment outcome and follow-up scans. System dose parameters recorded were the dose-length product (DLP) and volume CT dose index. The literature on investigating suspected abdominal sepsis has not been updated significantly since the time of conventional CT. 94 patients were included in the study; causes of abdominal sepsis could be detected with a sensitivity of 0.95 and a specificity of 0.91. Repeat examination and cumulative exposure was a key finding. Patients with abscesses and acute pancreatitis had the highest number of scanner visits; patients with diverticular disease had the lowest number of visits, lowest cumulative DLP and shortest stay in hospital. Cumulative DLP was affected by scan length, number of scans and patient size. In conclusion, diagnostic accuracy data for MDCT scans using 16 slices confirm that CT remains a suitable modality for imaging abdominal sepsis but scope for dose constraint exists.

Preliminary findings from tests of a microwave applicator designed to treat Barrett's oesophagus.

Barretts oesophagus is considered to increase the risk of cancer 30-fold. Helical microwave antennas have been developed for ablative treatment of Barretts. A microwave balloon applicator was tested in an initial animal study using adult white pigs. For treatment, a balloon filled with tissue-equivalent material encapsulated the antenna. A range of different treatment temperatures and durations was used to investigate a range of thermal ablations of the oesophageal epithelium. Eight animals were investigated, five non-survival and three with a 1-week survival period. The balloon was fitted with an array of temperature sensors, which gave an indication of the treatment in situ and allowed modifications to be performed in real time. Temperature data were recorded from all four quadrants of the balloon throughout and test sites were collected and analysed histologically. All experiments were successfully completed without perforation, serious adverse effects or death. Sites of discrete ulceration were induced in the survival tests, whereas the non-survival tests yielded little reproducible tissue modification. Results suggested that an activation temperature of approximately 55 degrees C needed to be reached during the treatment for tissue damage to be induced. Once damage had been triggered the severity was related to the mean temperature attained during the treatment period. A mean temperature of 52 degrees C or more resulted in substantial damage, whilst a mean temperature of approximately 50 degrees C resulted in the desired surface damage with sparing of subjacent tissues.

Effect of insertion depth on helical antenna performance in a muscle-equivalent phantom.

Barretts oesophagus is considered to increase the risk of cancer 30 fold. A set of helical microwave antennas was designed to investigate their potential use in the thermal therapy of Barretts oesophagus. For treatment, a balloon filled with muscle-equivalent material encapsulates the antenna. The effects of insertion depth and coil-spacing on the thermal distribution produced by the antennas (20-35 mm) were characterized. The 35 mm helical antenna, with a coil-spacing of 3.6 mm resulted in uniform heating for an insertion depth of 40 mm. It was observed that the resultant temperature distribution produced, by the antennas, was dependent on the insertion depth within the phantom. For all antennas studied, deeper insertion resulted in two high intensity regions, approximately 1/4 and 3/4 along the antenna length. In contrast, shallow insertion resulted in predominant tip heating with undesirable heating at the phantom entry point. However, by manipulating the coil-spacing of the helix, uniform temperature profiles were achieved for a range of insertion depths.

Optimal filtering of EIT data in spectral expansion analysis

The signal-to-noise rations for some EIT measurements are very low, and for in vivo EIT measurements these are dependent on the electrode positioning and the distance from the current drive. The effect of removing noisy measurements to produce higher-fidelity images was investigated for the case of gastric emptying data. A consequence of this filtering was the reduction in the size of the sensitivity matrix and its subsequent singular-value decomposition. Several different filters were tested and for each of these spectral expansion regularization filter was optimized using a chi 2 test. Filtering out the measurement made by the spinal electrode, where the spinal bone barrier lies directly in the current path to the stomach, produced improved images by reducing the artefact content in the spinal sector of the conductivity map. For stomach imaging little useful information is produced by the spinal electrode, and the benefits of filtering dominate. However artefact images may be generated. In contrast consistent small improvements were produced by filtering out some of the weakest signals.

Measuring the rate of change of haemodynamic response at the onset of exercise in normal limbs and those with intermittent claudication

Patients with claudication have an inadequate haemodynamic response to exercise. Blood flow response will not only have a magnitude, but also a rate of change. There is scope for investigating these parameters, as manipulation of the factors which control them may benefit work to improve the treatment for claudication. This work compares the responses for patients with one normal limb and one with intermittent claudication. A custom-built ergometer allows unilateral, infragenicular plantar flexion exercise, whilst common femoral artery blood flow can be measured continuously by Duplex ultrasound. This apparatus was used to measure blood flow before, at the onset of and during a 5 W square-wave exercise stimulus in 15 patients. The claudicant group had a mean steady-state gain that was approximately half that of the normal group at around 170 ml min(-1) (p < 0.001) and a response time that was much shorter (p = 0.006). A mean response time of 21.0 +/- 1.4 s was achieved in claudicant limbs compared to 31.8 +/- 2.9 s in normals. However, the average rate of change of blood flow during this response time was estimated to still be greater for the normal group, at 431.7 +/- 47.1 ml min(-2), than for the claudicant group. The differences in magnitude and rate of change of limb blood flow response to exercise in claudicants were significant and may have implications for the treatment of claudication.