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Optoacoustic tomography

Optoacoustic (OA) diagnostics of various media that appeared several decades ago has become a convenient measurement tool. Optoacoustic methods are used for measuring small optical absorptions as well as for detection, localization and assessment of parameters of inhomogeneities (defects) in studied materials, for example, in optically nontransparent (opaque) media.

When used in a therapeutic transparency window (600-1100 nm), the OA method enables one to probe soft biotissues at the depth up to 10 cm and detect with high resolution inhomogeneities differing from the surrounding tissue only by the optical absorption coefficient. Thus, the OA tomography complements the traditional tomographic diagnostic techniques such as ultrasonic scanning, fluoroscopy, and nuclear magnetic resonance that are sometimes insufficiently contrasted and selective in detecting pathological changes against the background of normal tissues.

A special experimental setup was created for the OA studies at the IAP RAS Laboratory of Ultrasonic Diagnostics. A pulsed Nd:YAG laser operating at the wavelength of 1064 nm with pulse repetition rate 10 Hz, pulse duration 30 ns, and pulse energy 25-30 mJ is used for heating. Acoustic radiation from the studied object after laser pumping is recorded by a system with the reception band of ultrasound radiation of 1.5-7.5 MHz. The design of the setup allows one to employ different methods for scanning studied samples, depending on the problem to be solved. The first two-dimensional OA tomograms of samples of bio-like media were obtained on this setup using the following techniques:

  • walk-around in the transmission mode;
  • a stationary ring of weakly-directed sensors with subsequent solution of the inverse problem (back-propagation);
  • sector scanning in the case of one-side access to the object.
Tomogram of gelatine phantom with two spherical absorbing inhomogeneities

Model objects of highly scattering media reliably reveal absorbing optical inhomogeneities 3-6 mm in size at the depths up to 40 mm with spatial resolution of about 0.5-1 mm and contrast of absorption of optical radiation by inhomogeneities and the surrounding medium not less than 1.5. Applicability of different scanning methods, capabilities of complementary use of the OA tomography and traditional methods of active ultrasound location, thermal effects arising at OA probing of biotissues by sources of heating pulse radiation are investigated theoretically and in experiments.

Results of the research may be used for development of techniques and facilities for early diagnosis of malignant tumors of soft tissues, including mammary gland.