Tanya D. Khokhlova

Focused array transducer for two-dimensional optoacoustic tomography

Optoacoustic (OA) imaging utilizes short laser pulses to create acoustic sources in tissue and time resolved detection of generated pressure profiles for image reconstruction. The ultrasonic transients provide information on the distribution of optical absorption coefficient that can be useful for early cancer diagnostics. In this work a new design of wide-band array transducer is developed and tested. The array consists of 32 focused piezo-elements made of PVDF slabs imposed on a cylindrical surface. A single array element response to an OA signal coming from arbitrarily located point source is investigated theoretically and experimentally. The measured signals correspond well to numerically calculated ones. Focal zone maps of the elements with aperture angles 30 degrees and 60 degrees are presented and discussed; the resolution in direction perpendicular to the imaging plane is determined. Point spread function of the whole array is calculated using experimentally obtained signals from the sources located at different distances from the array. Backprojection algorithm is employed for reconstruction of the optoacoustic images. It is shown that the spatial resolution of the images yielded by the proposed array increases significantly compared to previous transducer designs.

Opto-acoustic imaging system for early breast cancer diagnostics: experimental and numerical studies

Optoacoustic (OA) imaging is based on the generation of thermoelastic stress waves by heating an object in an optically heterogeneous medium with a short laser pulse. The stress waves contain information on the distribution of structures with enhanced optical absorption that can be used for early cancer diagnostics. This technique has already been applied in-vivo for breast cancer diagnostics and yielded higher contrast of obtained images than that of X-ray or ultrasonic images. The resolution was comparable with that yielded by ultrasonic imaging. Therefore, OA imaging is a very promising technique and it is being rapidly developed. Research in the area is now mostly targeted to the development of OA wave detection systems and image reconstruction algorithms. In this work a new design of receiving array transducer, that allows to enhance image resolution is proposed. The array consists of 64 focused piezo-elements made of PVDF slabs imposed on a spherical surface. Resolution yielded by the array in different directions is determined. Several tissue irradiation geometries and laser wavelengths are considered for optimization of the OA image contrast. Obtained results are used for maximum imaging depth studies. All the investigations include both numerical modelling and experiment.

Focused array transducer for optoacoustic tomography

Optoacoustic (OA) imaging is based on the generation of thermoelastic stress waves by heating an object in an optically heterogeneous medium with a short laser pulse. The stress waves contain information on the distribution of structures with preferential optical absorption that can be used for early cancer diagnostics. In this work a new design of array transducer is proposed and different characteristics of the array are discussed. The array consists of 64 focused piezo-elements made of PVDF slabs imposed on a spherical surface. Resolution yielded by the array in different directions is determined and discussed. Numerical modeling is performed for optimization of tissue irradiation conditions, determination of contrast of the OA images and maximum imaging depth.

A multimodal evaluation of boiling histotripsy lesion properties in ex vivo bovine liver

New types of high intensity focused ultrasound (HIFU) therapy aiming at mechanical homogenization of tissue has shown great promise, namely, cavitation-cloud histotripsy and boiling histotripsy (BH). BH uses millisecond-long bursts of HIFU waves containing shocks to repeatedly induce boiling at the focus; the interaction of incident HIFU waves with vapor bubbles homogenizes tissue. In this study, degassed ex vivo bovine liver samples were sonicated using a 256-element 1.2 MHz array of a clinical MR-HIFU system. The BH lesions were produced using 10-ms long pulses with 80 MPa shocks in situ and pulse repetition frequencies (PRFs) of 1-10 Hz to cover a range of effects from pure mechanical homogenization to thermal ablation. Individual lesions were generated for the multimodal analysis of the lesion including ultrastructure (electron microscopy), molecular (biochemistry), and microstructure (histological) methods. The extent of homogenization and thermal denaturation was evaluated for each lesion. The resul...

Erosion of soft tissue by focused ultrasound-induced streaming

Mechanical erosion of soft tissues into subcellular debris has been demonstrated with pulsed high-intensity focused ultrasound, facilitated by either boiling or cavitation bubbles. In this work, we propose acoustic streaming as a primary cause of tissue erosion at a tissue-fluid interface. Bovine liver tissue and polyacrylamide gels were sonicated in a degassed water bath, with the focus positioned at the tissue/fluid interface. Pulses with duration between 1 to 104 μs and constant duty cycle of 0.5% were applied from a 1 MHz transducer generating focal pressures |p- | ≤17 MPa and p+ ≤90 MPa. Results showed a strongly nonlinear change in erosion with pulse duration, being greatest for pulse lengths between 50-500 μs. For longer pulses (>1 ms), high-speed videos showed streaming velocities >10 m/s. Moreover, lesions >1cm in depth were produced in tissue phantoms even with a single pulse and tissue disruption was evident where no bubbles were observed at the tissue-fluid interface. Enhancement of erosion wa...

A training phantom for ultrasound-guided boiling histotripsy therapy

Boiling histotripsy (BH) uses millisecond-long focused ultrasound pulses with shocks to mechanically disrupt targeted tissue under real-time ultrasound monitoring. However, adipose tissue and ribs can interfere with BH therapy through aberration, absorption, and diffraction. Here we introduce a robust abdominal wall phantom that includes fat, muscle, and rib layers for demonstrating the use of BH and investigating the impact of anatomic structures on treatment success. The skin is a silicone sheet; the fat and muscle layers are poly-vinyl alcohol phantoms with irregular-shaped walls; the ribs are 3D-printed sections from a human model anatomically relevant to liver or kidney treatments. The target is a transparent alginate or polyacrylamide gel that allows visualization of the lesion. The pieces are assembled in a water-filled container providing coupling between layers and allowing components to be shifted in position relative to the transducer. A BH transducer (1.2 MHz, 12.5 cm focal length, f# = 1) con...