Biomedical subjects

Tomi Huttunen

Publications and source records attributed to Tomi Huttunen.

6 recordsLinked to original sources

PubMed2006

Determination of heterogeneous thermal parameters using ultrasound induced heating and MR thermal mapping.

In this paper, a method for the determination of spatially varying thermal conductivity and perfusion coefficients of tissue is proposed. The temperature evolution in tissue is modelled with the Pennes bioheat equation. The main motivation here is a model-based optimal control for ultrasound surgery, in which the tissue properties are needed when the treatment is planned. The overview of the method is as follows. The same ultrasound transducers, which are eventually used for the treatment, are used to inflict small temperature changes in tissue. This temperature evolution is monitored using MR thermal imaging, and the tissue properties are then estimated on the basis of these measurements. Furthermore, an approach to choose transducer excitations for the determination procedure is also considered. The purpose of this paper is to introduce a method and therefore simulations are used to verify the method. Furthermore, computations are accomplished in a 2D spatial domain.

Body Temperature↗

PubMed2005

Scanning path optimization for ultrasound surgery.

One of the problems in ultrasound surgery is the long treatment times when large tumour volumes are sonicated. Large tumours are usually treated by scanning the tumour volume using a sequence of individual focus points. During the scanning, it is possible that surrounding healthy tissue suffers from undesired temperature rise. The selection of the scanning path so that the tumour volume is treated as fast as possible while temperature rise in healthy tissue is minimized would increase the efficiency of ultrasound surgery. The main purpose of this paper is to develop a computationally efficient method which optimizes the scanning path. The optimization algorithm is based on the minimum time formulation of the optimal control theory. The developed algorithm uses quadratic cost criteria to obtain the desired thermal dose in the tumour region. The derived method is evaluated with numerical simulations in 3D which are applied to ultrasound surgery of the breast in simplified geometry. Results from the simulations show that the treatment time as well as the total applied energy can be decreased from 16% to 43% as compared to standard sonication. The robustness of the optimized scanning path is studied by varying the perfusion and absorption in the tumour region.

Body Temperature↗

PubMed2005

A full-wave Helmholtz model for continuous-wave ultrasound transmission.

A full-wave Helmholtz model of continuous-wave (CW) ultrasound fields may offer several attractive features over widely used partial-wave approximations. For example, many full-wave techniques can be easily adjusted for complex geometries, and multiple reflections of sound are automatically taken into account in the model. To date, however, the full-wave modeling of CW fields in general 3D geometries has been avoided due to the large computational cost associated with the numerical approximation of the Helmholtz equation. Recent developments in computing capacity together with improvements in finite element type modeling techniques are making possible wave simulations in 3D geometries which reach over tens of wavelengths. The aim of this study is to investigate the feasibility of a full-wave solution of the 3D Helmholtz equation for modeling of continuous-wave ultrasound fields in an inhomogeneous medium. The numerical approximation of the Helmholtz equation is computed using the ultraweak variational formulation (UWVF) method. In addition, an inverse problem technique is utilized to reconstruct the velocity distribution on the transducer which is used to model the sound source in the UWVF scheme. The modeling method is verified by comparing simulated and measured fields in the case of transmission of 531 kHz CW fields through layered plastic plates. The comparison shows a reasonable agreement between simulations and measurements at low angles of incidence but, due to mode conversion, the Helmholtz model becomes insufficient for simulating ultrasound fields in plates at large angles of incidence.

Algorithms↗

PubMed2004

Simulation study for thermal dose optimization in ultrasound surgery of the breast.

In this paper the previously published optimization algorithm for thermal dose optimization is tested with numerical simulations. The simulations concern the thermal dose optimization in ultrasound surgery of the breast. The optimization algorithm is extended by setting the inequality constraint approximations to temperature in healthy tissue as well as in tumor region. In addition, the simulations are accomplished in realistic 3D geometry with varying thermal parameters. Another topic of the paper is to show the potential of the hemispherical phased array applicator for ultrasound surgery of the breast. With such an applicator larger tissue volumes can be treated with shorter time as compared to single element transducers. In simulations the geometrical focus of the applicator was placed mechanically in the middle of the treatable region. The whole tumor region was then scanned electrically by changing the phase of the emitted wave from individual elements. The simulations indicate that a feasible treatment plan can be achieved. In simulated cases the desired thermal dose was achieved for tumors with diameter from 1.5 to 2.4 cm, depending on the position of tumor. The maximum temperature limitations of 45 degrees C in healthy region and 80 degrees C in tumor region can be maintained in all simulations.

Algorithms↗

PubMed2003

Thermal dose optimization method for ultrasound surgery.

In this paper, a model-based optimization method is derived to control the thermal dose in biological tissues for ultrasound surgery. The optimization method uses the bioheat equation as a system model and quadratic cost criteria for the desired thermal dose. Time-harmonic quasi-stationary ultrasound fields are used as the heat source. In this method the optimal phase and the amplitude trajectories are found directly by minimizing the associated cost function. The approach also allows for maximum input amplitude constraints. The method is based on the Hamiltonian form of the system and results in a large dimensional nonlinear optimization problem which is solved with a gradient-type iterative scheme. The performance of the optimization method is tested with 2D simulations and it is shown that the approach is able to yield a feasible nominal solution. This nominal evolution would then eventually be sought to be maintained with the help of a feedback controller during the actual sonication.

Algorithms↗

PubMed2003

Modeling of anomalies due to hydrophones in continuous-wave ultrasound fields.

Needle and spot-poled membrane hydrophones using polyvinylidene fluoride (PVDF) sensors are widely used for characterization of biomedical ultrasound fields. It is known that, in measurements of continuous-wave (CW) fields, standing waves may be generated between the transducer and the hydrophone, distorting the field and possibly alternating the signal of the hydrophone. This study uses a three-dimensional, full-wave method to computationally simulate the distortion in the CW field caused by needle and membrane hydrophones. The physical model used in simulations is based on the linear time-harmonic wave equation, which therefore neglects the effects of nonlinear wave propagation. The significance of the distortion is examined by comparing fields emitted by 0.5-5.0 MHz planar circular transducers in the absence and presence of the hydrophones. In addition, the effect of the field distortions on the signal of the hydrophones is studied with simulated measurements. The simulations showed an observable standing wave pattern between the source and the needle hydrophone if the diameter of the needle was larger than a half of the wavelength. However, the standing waves had no clear effect on the signal of the hydrophone. The presence of membrane hydrophone in the CW field generated notable standing waves. Furthermore, the standing waves caused a periodic distortion to the signal of the membrane hydrophone.

Artifacts↗