Hybrid medical imaging: scanning thermoacoustic tomography

Scanning thermoacoustic tomography based on microwave-induced thermoacoustic waves studied. Two-dimensional images of /spl sim/50 mm thick biological tissue samples were obtained. The thermoacoustic signals were also simulated theoretically. The image resolution was significantly improved compared with purely microwave imaging

Scanning thermoacoustic tomography in biological tissue

Microwave-induced thermoacoustic tomography was explored to image biological tissue. Short microwave pulses irradiated tissue to generate acoustic waves by thermoelastic expansion. The microwave-induced thermoacoustic waves were detected with a focused ultrasonic transducer. Each time-domain signal from the ultrasonic transducer represented a one-dimensional image along the acoustic axis of the ultrasonic transducer similar to an ultrasonic A-scan. Scanning the system perpendicularly to the acoustic axis of the ultrasonic transducer would generate multi-dimensional images. Two-dimensional tomographic images of biological tissue were obtained with 3-GHz microwaves. The axial and lateral resolutions were characterized. The time-domain piezo-electric signal from the ultrasonic transducer in response to the thermoacoustic signal was simulated theoretically, and the theoretical result agreed with the experimental result very well

Combining microwave and ultrasound: scanning thermoacoustic tomography

Scanning thermoacoustic tomography based on microwave-induced thermoacoustic waves was studied. Two-dimensional images of /spl sim/50-mm thick biological tissue samples were obtained experimentally. The thermoacoustic signals were also simulated theoretically. The image resolution was significantly improved compared with purely microwave imaging

Frequency-swept ultrasound-modulated optical tomography of scattering media

A novel frequency-swept ultrasound-modulated optical tomography technique was developed to image scattering media. A frequency-swept ultrasonic wave was used to modulate the laser light passing through a scattering medium. The modulated light was received by an optical detector and was heterodyned with a reference frequency sweep. The heterodyned signal was recorded in the time domain and was then analyzed in the frequency domain to yield a one-dimensional image along the ultrasonic axis. Multiple one-dimensional images obtained at various positions perpendicular to the ultrasonic axis were combined to yield a two-dimensional tomographic image of the medium

Photoacoustic and thermoacoustic tomography: system development for biomedical applications

Photoacoustic tomography (PAT), as well as thermoacoustic tomography (TAT), utilize electromagnetic radiation in its visible, near infrared, microwave, and radiofrequency forms, respectively, to induce acoustic waves in biological tissues for imaging purposes. Combining the advantages of both the high image contrast that results from electromagnetic absorption and the high resolution of ultrasound imaging, these new imaging modalities could be the next successful imaging techniques in biomedical applications. Basic research on PAT and TAT, and the relevant physics, is presented in Chapter I. In Chapter II, we investigate the imaging mechanisms of TAT in terms of signal generation, propagation and detection. We present a theoretical analysis as well as simulations of such imaging characteristics as contrast and resolution, accompanied by experimental results from phantom and tissue samples. In Chapter III, we discuss the further application of TAT to the imaging of biological tissues. The microwave absorption difference in normal and cancerous breast tissues, as well as its influence on thermoacoustic wave generation and the resulting transducer response, is investigated over a wide range of electromagnetic frequencies and depths of tumor locations. In Chapter IV, we describe the mechanism of PAT and the algorithm used for image reconstruction. Because of the broad bandwidth of the laser-induced ultrasonic waves and the limited bandwidth of the single transducer, multiple ultrasonic transducers, each with a different central frequency, are employed for simultaneous detection. Chapter V further demonstrates PATÂs ability to image vascular structures in biological tissue based on bloodÂs strong light absorption capability. The photoacoustic images of rat brain tumors in this study clearly reveal the angiogenesis that is associated with tumors. In Chapter VI, we report on further developing PAT to image deeply embedded optical heterogeneity in biological tissues. The improved imaging ability is attributed to better penetration by NIR light, the use of the optical contrast agent ICG (indocyanine green) and a new detection scheme of a circular scanning configuration. Deep penetrating PAT, which is based on a tissueÂs intrinsic contrast using laser light of 532 nm green light and 1.06 µm near infrared light, is also presented

Photoacoustic microscopy with submicron resolution

We show that it is possible to obtain high optical contrast photoacoustic images of tissue with 0.55 μm transverse resolution. To achieve high sensitivity, we used a high NA (0.85), 125 MHz spherically focused ultrasonic transducer in a confocal arrangement with a high resolution optical objective (NA=0.6). Laser pulses of a few nJ in pulse energy with durations of 1.5 ns at a 20 KHz pulse repetition rate were used to generate photoacoustic waves. Although the penetration depth is limited to hundreds of microns by both optical scattering and ultrasonic absorption, the developed technique can compete with optical microscopy, for example, in quantitative spectral measurements, in microcirculation research, or in nanoparticle detection

Improving the image quality of photoacoustic tomography (PAT) by using a negative acoustic lens

Although a small point ultrasound transducer has a wide acceptance angle, its signal-to-noise (SNR) is low due to the high thermal-noise-induced electric voltages in the transducer, which is a result of its small active area. By contrast, a finite size flat transducer has high sensitivity (good SNR), but the acceptance angle is generally small, which limits its application in reconstruction-based photoacoustic tomography (PAT). In this paper, we report a negative lens concept to increase the acceptance angle for a flat transducer. We also provide phantom experiments that demonstrate this concept can greatly increase the detection region for PAT and without losing sensitivity

Microwave-induced thermoacoustic tomography using multi-sector scanning

A study of microwave-induced thermoacoustic tomography of inhomogeneous tissues using multi-sector scanning is presented. A short-pulsed microwave beam is used to irradiate the tissue samples. The microwave absorption excites time-resolved acoustic waves by thermoelastic expansion. The amplitudes of the acoustic waves are strongly related to locally absorbed microwave-energy density. The acoustic waves may propagate in all spatial directions. A focused ultrasonic transducer is employed to acquire temporal acoustic signals from multiple directions. Each detected signal is converted into a one-dimensional (1D) image along the acoustic axis of the transducer. The cross-sectional images of the tissue samples are calculated by combining all of the 1D images acquired in the same planes

Negative lens concept for photoacoustic tomography

Although a small point ultrasound transducer has a wide acceptance angle, its small active area leads to a high thermal-noise-induced electric voltage in the transducer, thus the sensitivity is low. By contrast, a finite-size flat transducer has high sensitivity, but the acceptance angle is small, which limits its application in reconstruction-based photoacoustic tomography (PAT). Here, we propose a negative lens concept to increase the acceptance angle of a flat transducer without losing sensitivity. Phantom experiments demonstrate that use of this concept greatly increases the detection region for PAT with high sensitivity