Review of neural network applications in medical imaging and signal processing.

The current applications of neural networks to in vivo medical imaging and signal processing are reviewed. As is evident from the literature neural networks have already been used for a wide variety of tasks within medicine. As this trend is expected to continue this review contains a description of recent studies to provide an appreciation of the problems associated with implementing neural networks for medical imaging and signal processing.

Regional cerebral blood flow velocity changes after indomethacin infusion in preterm infants.

Cerebral blood flow velocity was assessed during infusion of indomethacin over 30 minutes. Eleven preterm infants with symptomatic patent ductus arteriosus were studied on 12 occasions. Indomethacin infusion was associated with a significant reduction in time averaged mean velocity (TAMV), peak systolic velocity (PSV), and end diastolic velocity in both the anterior cerebral artery and middle cerebral artery. The fall in the TAMV and PSV was gradual with maximal change 30-40 minutes after the start of the infusion. It was concluded that administration of indomethacin by slow infusion produces haemodynamic alterations to the cerebral circulation comparable in magnitude with changes described with bolus administration. Indomethacin remains a useful and effective treatment for patent ductus arteriosus in preterm infants, but should continue to be used with caution.

Neural networks for electrical impedance tomography image characterisation.

The Southampton electrical impedance tomography (EIT) system used a Sheffield data acquisition unit and a PC based 'Harlequin' transputer card to reconstruct and display images of the distribution of internal conductivity within the thorax. The system produces real-time images relating to both cardiac and pulmonary function. As a first step towards diagnosis using these images neural nets have been applied to the identification of regions of interest in the EIT images for which some activity with time, such as ventricular ejection, is sought. This paper addresses the use of a back-projection network to identify characteristic regions within the images. The network facilitates the production of automated real-time activity plots by defining their effective extent in the images of specific organs. The application is novel within the medical imaging field as the aim is to use neural networks for real-time image analysis.

Pulmonary perfusion and ventricular ejection imaging by frequency domain filtering of EIT (electrical impedance tomography) images.

While EIT images can produce clinically useful qualitative information, the extraction of quantitative data is essential in clinical monitoring. In the case of imaging of the thorax the parameters available relate to cardiac activity and pulmonary perfusion. Imaging the relatively small changes in the resistivity of the lungs due to pulmonary perfusion in the presence of noise and the larger ventilation component is difficult. Suggested solutions involve multiple time averaging of cardiac gated data or reconstructed images. The required number of data frames for this type of processing is large (at least 100 cardiac cycles). Because the ventilation and perfusion components of the resistivity signals are well separated in the frequency domain, they can be differentiated by filtering. We report the results of this analysis which requires a data collection period of typically 15 s.

The resolution of transcranial Doppler scanning: a method for in vitro evaluation.

The circle of Willis may be examined using low frequency pulsed Doppler ultrasound transmitted through the thin squamous temporal bone or 'acoustic windows' of the skull. The thickness and shape of these windows affect the characteristics of the ultrasound beam and partly determine the spatial resolution of transcranial Doppler scanning. We have studied the effects of temporal bone on the ultrasound beam and sample volume characteristics of the EME Transcan 2 MHz transducer using a stepper motor, computer-controlled ultrasound plotting system. The lateral and axial spatial dimensions of the sample volume, and a direct estimate of the lateral resolution of the system, were determined at various depths of interest using Doppler targets. The results in this initial study of a small number of specimens indicates a decrease in sample width, and hence an increase in lateral resolution, presumably due to the lensing effect of the skull cusping. The estimated lateral resolution of the system is of the order of 4 mm.

The spectral expansion of a head model in electrical impedance tomography.

This paper examines whether electrical impedance tomography (EIT) can provide information of use to magneto-encephalographic modelling (MEG). The EIT image domain is expanded in a complete set of orthogonal basis images, the number of which is given by the number of independent measurements (104 for 16 electrodes). They are ordered according to their sensitivity to data noise, with more centrally located features and higher spatial frequency components appearing towards the higher terms in the series, in the case of uniform resistivity distribution. This indicates that the resolution can be improved at the expense of degrading signal-to-noise in the reconstructed image. Applied to an approximate model of the head, the technique generates a set of basis images that emphasises central features of the head.

Transcranial Doppler ultrasound studies of cerebral autoregulation and subarachnoid hemorrhage in the rabbit.

The authors describe a method for Doppler ultrasound recording of flow velocity in the basilar artery of normal rabbits and rabbits with experimental subarachnoid hemorrhage (SAH). With this transcranial Doppler (TCD) model, clinical assumptions regarding flow velocity/cerebral blood flow (CBF) relationships, autoregulatory responses, and Doppler spectral waveform analysis can be tested under controlled conditions and compared with established methods of CBF measurement (hydrogen clearance). The time course of changes in flow velocity following SAH (cerebral vasospasm) is successfully demonstrated using the experimental TCD method. There are significant differences in the flow velocity and CBF responses to hypercapnia, hypocapnia, and trimethaphan-induced hypotension which indicate that TCD cannot be considered a simple alternative to CBF measurement for the study of cerebrovascular reactivity and cerebral autoregulation.

A transputer implemented algorithm for electrical impedance tomography.

Electrical impedance tomography (EIT) offers a non-invasive technique of great scope for producing clinically important information in a number of medical applications. Initial work by Brown used an isopotential approach to solving the complex backprojection problem. In this study a less restrictive algorithm for EIT has been developed based on the work of Yorkey and of Kaczmarz. Since considerably more processing is required this has been implemented on a transputer in Occam 2 with the aim of achieving real-time imaging. Data were collected, using a Sheffield prototype EIT system, from a test object and a human thorax. Image processing used both the 'Sheffield' and the Yorkey/Kaczmarz algorithms. Our initial results indicate that the images generated using the latter approach were more representative of the source impedance distributions.

Strain-gauge fontanometry--an advance in non-invasive intracranial pressure measurement.

The validation of a fontanometer based on the strain-gauge principle is described. Fontanelle pressures correlated well with invasively determined cerebrospinal fluid (CSF) pressures in a group of neonates and infants with hydrocephalus. This device, which can be zeroed in vivo, has the additional advantage of a fast response time and allows reliable measurements of spontaneous pulsations and induced changes in CSF pressure. Unlike previously reported devices, the accuracy of this transducer was shown to be independent of external forces.

Impact of duplex scanning on carotid endarterectomy.

Duplex ultrasound permits safe and accurate assessment of the extracranial vasculature. This paper reports the change in patterns of referral to a specialized vascular unit following its introduction; increased referrals were seen in all specialties except neurology. A widely available and reliable duplex service has revealed more extracranial vascular disease than was previously recognized, and has increased referrals for carotid endarterectomy, thereby increasing surgical workload.

Estimation of aortic and pulmonary blood flow in a paediatric population using Doppler ultrasound: an in-vitro study.

The noninvasive measurement of blood flow using ultrasound has been attempted by numerous workers. In order to evaluate the technique of duplex scanning applied to the problem of noninvasive assessment of aortic and pulmonary blood flow in a paediatric population with ventricular septal defect, we attempted an in-vitro simulation of these flow conditions. The factors which have been considered in the design of the apparatus include the availability of a range of tube diameters, an aortic-"like" waveform, laminar flow, variable downstream peripheral resistance, a compliance factor and viscosity of the flow medium. The ultrasound probe beam characteristics, sample volume size and shape were also determined. From the results of the beam plots it was apparent that the region of sensitivity of the CW Doppler probes were inappropriate for the measurement of blood flow in neonates. In order to simulate physiological parameters in a paediatric population as closely as possible, five tubes of diameters ranging from 9-18 mm were used, in each tube a range of flow values from 0.8 L min-1 to 5 L min-1 were measured. The flow values were measured by both "bucket and stopwatch" and duplex scanner for both steady and pulsatile flow conditions. The results are presented as correlations between the direct and noninvasive pulsed Doppler assessment of flow measurement and are in the range 0.92-0.99 significant at p less than .001. A discussion of the reliability of flow measurements made using a conventional duplex scanner is given.

The validation of duplex scanning and continuous wave Doppler imaging: a comparison with conventional angiography.

Colour-coded continuous wave (CW) Doppler imaging and duplex scanning have been assessed prospectively in comparison with biplanar angiography for their accuracy in detecting significant arterial disease in the extracranial circulation. Of 96 comparisons with biplanar angiography, the sensitivity of Doppler imaging was 90% and specificity 98% in the detection of greater than 50% internal carotid stenosis and 86 and 100%, respectively, in the diagnosis of internal carotid occlusion. Of 85 comparisons with biplanar angiography, the sensitivity of duplex scanning was 93% and specificity 98% in the detection of greater than 50% internal carotid stenosis and 92 and 100%, respectively, in the diagnosis of internal carotid occlusion. The value of the peak systolic Doppler shift frequency of the internal carotid artery signal has proved to be the most reliable indicator of greater than 50% stenosis and is utilised in conjunction with a periorbital examination. It is concluded that both Doppler imaging and duplex scanning are effective screening techniques for the presence of significant (greater than 50%) internal carotid artery disease.

The accuracy of duplex scanning in the evaluation of early carotid disease.

The accuracy of duplex scanning in 69 comparisons with biplanar angiography in the detection of early carotid disease has been assessed. The various criteria reported for the categorisation of less than 50% disease have been critically analysed. As part of the study, 50 internal carotid arteries of 25 young, presumed normal medical students of mean age 20 years have been examined. The results suggest that duplex scanning has the ability to grade less than 50% carotid disease into normal, 1-24% and 25-49% stenosis categories. The most sensitive indicator of early disease is obtained from the real-time B-scan. The waveform changes of the maximum frequency envelope were more sensitive than spectral broadening except where full spectral broadening was present. For confident assessment, the real-time B-scan and pulsed Doppler must always be used in conjunction.

Characterisation of carotid artery disease: comparison of duplex scanning with histology.

The ability of duplex scanning to characterise the component tissue of atheromatous lesions of the internal carotid artery and to detect ulceration has been examined in a histological study of 42 carotid endarterectomy specimens. The results suggest that the only component of atheromatous lesions of the internal carotid artery which can be characterised from the B-scan is calcification. The presence of ulceration, intraluminal and intramural thrombus, fibrous intimal thickening and necrosis are not related to the echogenic appearance of internal carotid stenoses and may not be detected reliably. B-mode imaging alone cannot reliably grade greater than 50% internal carotid stenoses, but when this is combined with pulsed Doppler in the technique of duplex scanning, accurate results may be obtained.

The role of continuous wave Doppler imaging in a vascular unit.

A continuous wave (CW) Doppler imaging system has been used in the assessment of patients with suspected carotid artery disease. In 137 comparisons with conventional angiography the sensitivity of CW Doppler Imaging was 91% and specificity 93% respectively in the detection of greater than 50% diameter internal carotid stenosis. The main diagnostic criterion for greater than 50% internal carotid stenosis was the presence of a peak internal carotid systolic Doppler shift frequency greater than 4 KHz. An examination protocol is recommended which combines CW Doppler imaging with common carotid waveform changes and the temporal artery compression test.

The reliability of Doppler ultrasound techniques in the assessment of carotid disease.

The reliability of continuous wave (CW) Doppler imaging and Duplex scanning in the assessment of carotid artery disease has been evaluated prospectively in comparison with biplanar angiography. Of 130 comparisons the sensitivity of Doppler imaging was 89% and specificity 99% in the detection of greater than or equal to 50% internal carotid stenosis, and 87% and 99% respectively in the diagnosis of internal carotid occlusion. Of 118 comparisons the sensitivity of Duplex scanning was 93% and specificity 98% in the detection of greater than or equal to 50% internal carotid stenosis, and 93% and 99% respectively in the diagnosis of internal carotid occlusion. CW Doppler imaging and Duplex scanning are accurate techniques in the diagnosis and categorisation of internal carotid artery disease. CW Doppler imaging is an effective screening technique for the presence of greater than or equal to 50% internal carotid artery stenosis. Duplex scanning also has the potential for improved detection of early internal carotid artery disease and study of the natural history of atherosclerosis.

Potential of SeHCAT retention as an indicator of terminal ileal involvement in inflammatory bowel disease.

The absorption of 75Se-23-selena-25-homotaurocholate (SeHCAT) was compared with vitamin-B12 absorption and conventional radiography in 44 patients with inflammatory bowel disease. The retention of SeHCAT was normal in 11 patients with ulcerative colitis but was abnormally low in 9 patients with terminal-ileal resection, 9 out of 14 patients with small-bowel Crohn's disease and in 2 out of 10 patients with Crohn's colitis. The 5 patients with small-bowel Crohn's disease and normal retention had either inactive disease or no radiological evidence of terminal ileal involvement. Measurements of the absorption of vitamin B12 did not discriminate between these groups, and there was very poor correlation between B12 and SeHCAT absorption (r = 0.506, P less than 0.05). There was extremely good correlation of SeHCAT retention measured using a wholebody counter with that measured using an uncollimated gamma camera (r = 0.96, P less than 0.001). The results suggest that SeHCAT retention may prove complementary to conventional methods of assessing small-bowel disease in patients with inflammatory bowel disease. As measurement by gamma camera is feasible, this test can be used in most departments of nuclear medicine.