Imaging: Developing Techniques

Abstract

A number of imaging techniques are being investigated by physicists and engineers as possible tools in the diagnosis of human disease; they include magnetic source imaging, applied‐potential tomography, optical methods and microwave imaging. Physicians have yet to evaluate their performance to any great extent.

Keywords: magnetoencephalography; applied potential tomography; thermography; optical imaging; microwave imaging

Figure 1.

The typical current paths that can be detected by a magnetic source imaging system. Other current directions are, of course, sensed but with varying sensitivity, and dependence on gradiometer orientation if an array of detectors is used. Two typical (rather extended) possible current and field patterns are shown, together with detector coils. The coil detects the current at A, but does not detect that at B.

Figure 2.

The principal forms of gradiometer used in magnetic source imaging: (a) the axial version; (b) the transverse form. In both cases, flux such as that at A results in signal, while that at B does not (assuming, in case (b), that the flux comes from a remote, if false, source).

Figure 3.

Modern 306 channel magnetic source imaging system, showing the cryostat located over the patient's head. Courtesy of Neuromag Ltd, Helsinki, Finland.

Figure 4.

Typical data from a magnetic source imaging experiment on a volunteer. (a) Pattern of signals detailed from the array of SQUIDs (superconducting quantum interference devices) that are located as shown in (b) (small blocks). After processing, these signals can be used to reconstruct the position of the two dipole currents, as shown in (b), with the field contours plotted at 50‐fT (5 × 10−14‐T) intervals. The current dipoles, which are the result of the modelling, are shown with their locations overlaid on the magnetic resonance image (c), and as arrows on (b). Reproduced with permission from Young .

Figure 5.

Principles of a system of applied potential tomography, showing the major components that are needed. A single radiofrequency (RF) power amplifier is switched to power the electrodes, while a parallel detector channel is used with each. The computer is the central component of the system. ADC, analog to digital converter; VDU, visual display unit.

Figure 6.

Three‐dimensional ventilation impedance images at RV, FRC and PTV referenced to expected uniform ventilation at TLC. Images are displayed from four ‘slices’. The anterior of the thorax is at the top of the image, and the left side is on the right of the image. RV, residual volumes; FRC, functional residual capacity; PTV, partial tidal volume; TLC, total lung capacity. Courtesy of Professor B. H. Brown and Dr. P. Metherall, University of Sheffield, UK.

Figure 7.

Plots from near‐infrared experiments on two regions in adult human calf muscle superficially cooled by a pair of pads through which cold water was passed. Tissue temperatures were measured optically at points in the leg adjacent to the sides of the measurements. These were (a) at point 1, underneath one of the cooling pads in muscle; and (b) at point 3, also in muscle but at the top of the leg, away from the cooling pads. Myoglobin and haemoglobin are not distinguishable in this experiment, and three curves are plotted in each case, for the (blue) oxy‐ and (green) deoxy‐ moieties, and for (red) temperature. Reproduced with permission from (1996) Magnetic Resonance in Medicine36: 372.

Figure 8.

Operation of the time‐of‐flight method of optical imaging. Essentially the process, is analogous to that of a computed tomographic scanner, with a possible tan beam method of detection in which the optical system is swept round the subject's body. In (a), the paths of three light rays (A, B, C) are shown. None follows as long a route as the average ray actually does in tissue; however, A is more typical than the other two, with C being the nearest to optimum. It is rays such as these that arrive first, in the window marked on (b), showing the signal detected from an ideally square laser pulse.

Figure 9.

Operation of a possible microwave imaging system. The principal components of the system have distinct analogies with those used for applied potential tomography (APT) (Figure ). Small antennae replace the electrodes of APT.

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References

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Young, Ian R(Apr 2001) Imaging: Developing Techniques. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0002342]