Acoustic thermotomography

Diagnostic procedures that do not damage the organism of the examined patient are attracting active attention of clinicians. Such techniques are especially important for diagnosing patients with high risk of radiation damage (infants, expectant mothers, patients with excess irradiation dose). In these cases application of traditional methods of visualization of internal organs (X-ray examination, NMR tomography, ultrasonic techniques) is undesirable; instead, passive methods using intrinsic radiation of the body should be employed. These are thermal and radio-thermal

imaging techniques. We can also include here the method of acoustic thermography elaborated at the IAP RAS Department of Radiophysical Methods in Medicine and the acoustic thermotomography aimed at investigating internal temperature distribution in a biological object.

Acoustic thermotomography (ATT) is based on the principles well-known in IR and radio thermometry, namely, reception of radiation generated by a heated body. Intensity of the radiation is proportional to the temperature and absorption coefficient of the medium (brightness temperature). The distinguishing feature of ATT is recording acoustic emission of heated objects at the frequencies of 1 to10 MHz (the traditional range for ultrasonic diagnostics) at 1.5-0.15 mm wavelengths, thus ensuring a high directivity pattern of the antenna (the use of submillimeter waves in radiothermometry is restricted by strong absorption). The first experiments on registering acoustic radiation of heated bodies were performed at the Institute of Radio Electronics of the USSR Academy of Sciences back in 1987. IAP RAS was a pioneer in conducting experiments on detecting heated bodies and in designing a scanning acoustic thermotomograph. The device is a hardware-software complex comprising a multielement scanning acoustic antenna array, a unit for signal processing, and a computer with the corresponding software for recording a signal, controlling antenna scanning, and for signal processing by a tomographic algorithm for constructing images of a heated object. Currently, the sensitivity of about 0,2^o at averaging time of 5 s was attained. The time needed for recording one section is 5 min.

The research team of the project designed a laboratory prototype of a multichannel acoustic thermotomograph that was used in experiments on detecting heated objects placed in different biological environment. The experiments aimed at studying different factors influencing accuracy of detection, sensitivity of the device, and others. The block-diagram of the experimental setup is shown in fig. 1a. Results of the experiments demonstrate that the developed devices allow localization of objects whose temperature differs from that of the surrounding medium by 1-2 K. The accuracy of reconstructing a coordinate, i.e., the difference between the true position of objects in the medium and their reconstructed position is 2- 4 mm . Characteristic tomographic imaging of objects obtained in experiments is illustrated in fig. 1b.

Fig. 1. ( а ) Diagram of the experimental setup for detecting heated bodies placed in biological media; (b) result of detection of the heated body placed in gelatine.

Capabilities of acoustic-brightness thermometry are such that it can solve two independent tasks of medical diagnostics. One of the principal tasks is detection of relatively small sections of biological tissues whose temperature is higher than that of the surrounding tissues. Receiving antenna arrays in the form of flat-surface piezoelectric plates operating in the zone of geometric acoustics are usually used in experiments on acoustic-brightness thermometry. The transverse spatial resolution sufficient for detecting temperature inhomogeneity is equal approximately to the antenna aperture.

The other task is monitoring of the temperature of a known region in space. In this case, a heated source may be detected by means of a focused antenna. The first acoustic thermotomograph with focused antenna was designed and fabricated in IAP RAS and was used in experiments on detecting heated sources in different biological media. A typical two-dimensional tomogram obtained as a result of scanning a studied medium by antenna focus is presented in fig. 2.

Fig. 2. Two-dimensional tomogram of a heated sample placed in water. The tomogram was obtained by antenna focus scanning in depth and along the transverse coordinate.

The experimental research verified that ATT is a good tool for investigation of internal temperature of different biological objects. If an appropriate technique is developed, ATT may be used for mass examination of population (for example, for detecting malignant growths at an early stage), as well as for monitoring efficiency of treatment (tumor hyperthermy, pharmacological procedures).