Philip VanBaren

Two-dimensional temperature estimation using diagnostic ultrasound

A two-dimensional temperature estimation method was developed based on the detection of shifts in echo location of backscattered ultrasound from a region of tissue undergoing thermal therapy. The echo shifts are due to the combination of the local temperature dependence of speed of sound and thermal expansion in the heated region. A linear relationship between these shifts and the underlying tissue temperature rise is derived from first principles and experimentally validated. The echo shifts are estimated from the correlation of successive backscattered ultrasound frames, and the axial derivative of the accumulated echo shifts is shown to be proportional to the temperature rise. Sharp lateral gradients in the temperature distribution introduce ripple on the estimates of the echo shifts due to a thermo-acoustic lens effect. This ripple can be effectively reduced by filtering the echo shifts along the axial and lateral directions upon differentiation. However, this is achieved at the expense of spatial resolution. Experimental evaluation of the accuracy (0.5/spl deg/C) and spatial resolution (2 mm) of the algorithm in tissue mimicking phantoms was conducted using a diagnostic ultrasound imaging scanner and a therapeutic ultrasound unit. The estimated temperature maps were overlaid on the gray-scale ultrasound images to illustrate the applicability of this technique for image guidance of focused ultrasound thermal therapy.

Noninvasive real-time multipoint temperature control for ultrasound phased array treatments

A method for noninvasively estimating spatiotemporal temperature changes in samples using diagnostic ultrasound, and using these as inputs to a multipoint ultrasound phased array temperature controller, is presented in this paper. This method is based on a linear relationship between the apparent tissue echo pattern displacements and temperature, as seen along A-lines acquired with diagnostic ultrasound when the sample is heated by external heating fields. The proportionality constant between echo displacement and temperature is determined by the local change in speed of sound due to temperature and the linear coefficient of thermal expansion of the material. Accurate estimation of the displacements and proportionality constant yields accurate calibrated high-resolution (1 mm spatial, sub-/spl deg/C) noninvasive sample temperature estimates. These are used as inputs to a multipoint temperature controller, capable of controlling ultrasound phased array treatments in real-time. Phantom and in vitro results show that the noninvasively estimated temperature values can effectively be used to control ultrasound hyperthermia treatments, almost replacing invasive thermocouple measurements. The mathematical background and assumptions of the noninvasive temperature estimator and the controller are presented in this paper, together with experimental results showing estimator and controller performance and limitations. To the best of our knowledge, this paper presents the first demonstration of real-time treatment control based entirely on noninvasive temperature estimates.

A hybrid computational model for ultrasound phased-array heating in presence of strongly scattering obstacles

A computationally efficient hybrid ray-physical optics (HRPO) model is presented for the analysis and synthesis of multiple-focus ultrasound heating patterns through the human rib cage. In particular, a ray method is used to propagate the ultrasound fields from the source to the frontal plane of the rib cage. The physical-optics integration method is then employed to obtain the intensity pattern inside the rib cage. The solution of the matrix system is carried out by using the pseudo inverse technique to synthesize the desired heating pattern. The proposed technique guides the fields through the intercostal spacings between the solid ribs and, thus, minimal intensity levels are observed over the solid ribs. This simulation model allows for the design and optimization of large-aperture phased-array applicator systems for noninvasive ablative thermal surgery in the heart and liver through the rib cage.

Multipoint temperature control during hyperthermia treatments: theory and simulation

A real-time multipoint feedback temperature control system has been designed and implemented with an ultrasound phased-array applicator for hyperthermia. The control parameters are the total power available from the supply and the dwell times at a sequence of preselected heating patterns. Thermocouple measurements are assumed for temperature feedback. The spatial operator linking available heating patterns to temperature measurements is measured at the outset of the treatment and can be remeasured on line an adaptive implementation. A significant advantage of this approach is that the controller does not require a priori knowledge of either the placement of the thermocouples or the power distribution of the ultrasound heating patterns. Furthermore, the control loop uses a proportional integral (PI) gain in conjunction with a singular value decomposition (SVD) of the spatial transfer operator. This approach is advantageous for robust implementation and is shown to properly balance the power applied to the individual patterns. The controller also deals with saturation in the inputs without integrator windup and, therefore, without temperature overshoot. Here, the authors present the theoretical formulation and representative simulation results of the proposed controller. The control algorithm has been verified experimentally, both in vitro and in vivo. A subsequent paper describing these results and the practical implementation of the controller will follow.<<ETX>>

Dynamic focusing in ultrasound hyperthermia treatments using implantable hydrophone arrays

A prototype 16-element needle hydrophone array has been designed, fabricated and characterized. The primary use of this array is to provide acoustic feedback during ultrasound hyperthermia treatments. This feedback can be used to compensate for patient motion and tissue inhomogeneities by controlling the phased array driving patterns. It can also be used in adaptive dynamic focusing, a procedure which enables the phased array to focus at points away from specified control points. The hydrophone array consists of a PVDF sheet, which covers a silicon substrate carrier that contains the signal electrodes of the individual acoustic sensors. A complete description of the hydrophone array and its characteristics is given in this paper. The aberration correction and motion compensation algorithms are also described, and some experimental results are shown. Finally, a Taylor series based adaptive dynamic focusing method for phased arrays based on a set of discrete hydrophone array measurements is described. This algorithm does not require any prior knowledge of the applicator geometry and all the parameters needed for correction can be measured directly at the hydrophone array sensor locations.<<ETX>>

Image-guided noninvasive surgery with ultrasound phased arrays

The current status and the future needs of image-guided ultrasound phased array systems for noninvasive surgery are addressed. Preliminary results from an integrated image- guided therapeutic phased array system for noninvasive surgical applications currently being developed at our laboratory are shown. The therapeutic array utilizes piezo- composite transducer technology and operates (therapeutically) at 1 and 2 MHz. It consists of 64 elements on a spherical shell with a geometric center at 100 mm from its apex. The array was shown to be capable of producing well defined thermal lesions in tissue media at depths from 40 to 60 mm and to scan therapeutic foci up to +/- 15 mm from its geometric center. Image guidance is provided by a modified diagnostic ultrasound scanner which, in addition to providing standard B-scan images of the target region, provides real-time images of the temperature rise due to the therapeutic beam. The temperature information is obtained using a correlation based algorithm for echo displacement estimation, which can be directly related to local variation in tissue temperature due to the therapeutic beam. A complete description of the combined imaging/therapy system is given. Furthermore, illustrative examples of noninvasive real-time image-guided tissue ablation, temperature estimation, and temperature control are presented and discussed.

Motion compensation algorithm for noninvasive two-dimensional temperature estimation using diagnostic pulse-echo ultrasound

The feasibility of real-time non-invasive spatio-temporal temperature estimation from pulse-echo diagnostic ultrasound data has been previously demonstrated in stationary phantoms. The method is based on first estimating the axial shifts of the RF-echo data due to local changes in the speed of sound and thermal expansion in the propagating medium, and then differentiating these estimates along axial direction to obtain the temperature rise map. In a clinical setup, however, translation, rotation and deformation affect the estimates. In this paper we introduce an algorithm to compensate for tissue translation and uniform deformation along the axial and lateral directions of the ultrasound imaging plane. This is achieved by separating the components of the time-shift map due to temperature rise (a local effect, occurring within the vicinity of the heated region) from the component due to translation and deformation (effect observed over a larger region). A rubber phantom experiment was designed where high intensity focused ultrasound was used to generate localized heating while motion was applied to the phantom and/or imaging transducer. Temperature profiles were successfully estimated while the phantom was translated by 30 mm and axially deformed by 13%.

Quantitative analysis and applications of non-invasive temperature estimation using diagnostic ultrasound

A quantitative analysis of a two-dimensional non-invasive real-time temperature-change estimation algorithm based on diagnostic backscattered ultrasound RF-data is presented in this paper. An experimental system consisting of a therapeutic ultrasound unit and an ultrasound imaging unit was used to quantitatively characterize the accuracy, spatial resolution and ripple artifacts of the temperature estimates in a rubber phantom heating experiment. The ripple is shown to be a consequence of the thermo-acoustic lens effect resulting from local changes of the speed of sound in heated regions. Non-invasive temperature estimates were used as inputs to a multipoint ultrasound phased array temperature controller, for a long duration hyperthermia experiment. The applicability of this method to tissue media, as well as its major limitations are discussed.

Combined imaging and therapy with piezocomposite phased arrays

Recently, the authors have developed temperature estimation algorithms based on signal processing of pulse-echo ultrasound radio frequency (RF) data at diagnostic levels. A modified commercially available scanner was used to obtain 2D temperature estimates that were color coded and overlaid on real-time B-scan images to show the location and extent of the therapeutic beam with respect to the target as well as any critical structures in the treatment region. Recent advances in piezocomposite transducer technology have allowed the development of high power phased arrays for high-intensity focused ultrasound (HIFU) generation with excellent cross coupling characteristics. Furthermore, fractional bandwidths of 30-60 % are now possible with such arrays. Therefore, the authors have investigated the feasibility of utilizing a 64-element piezocomposite therapeutic array in an imaging mode. The array is concave on a spherical shell with a radius of curvature of 100 mm with elements arranged in a linear configuration (2 mm/spl times/50 mm). The array operates at a center frequency of 1.25 MHz and has a fractional bandwidth of 37%. Wire target images as well as images from a tissue mimicking cyst phantom were reconstructed using a synthetic aperture technique. Excellent image quality was achieved in a region extending 70 mm axially and 60 mm laterally and centered around the geometric center of the array. Furthermore, the authors have experimentally established that the speckle generated by this array can be used to extract temperature information based on our previously described algorithm. This was demonstrated by using a 4/spl times/64 matrix switch and a high speed digitizer to acquire image frames during a slow heating experiment.

Non-invasive spatio-temporal temperature estimation using diagnostic ultrasound

A method for non-invasively estimating tissue temperatures using 2D diagnostic ultrasound imaging arrays is presented in this paper. It is based on a linear relationship between the apparent speckle pattern displacements and temperature, as seen on acquired A-lines when the sample is heated by external heating fields. The proportionality constant between speckle displacement and temperature is determined by the differential change in the speed of sound due to temperature and the linear coefficient of thermal expansion of the material. Accurate estimation of the displacements and proportionality constant translates into accurate, calibrated, high-resolution (2 mm spatial, sub-/spl deg/C) noninvasive 2D spatio-temporal sample temperature estimates. The mathematical background of this method and experimental results are shown.