Dhiraj Arora

Model-predictive control of hyperthermia treatments

A model-predictive controller (MPC) of the thermal dose in hyperthermia cancer treatments has been developed and evaluated using simulations with one-point and one-dimensional models of a tumor. The developed controller is the first effort in: 1) the application of feedback control to pulsed, high-temperature hyperthermia treatments; 2) the direct control of the treatment thermal dose rather than the treatment temperatures; and 3) the application of MPC to hyperthermia. treatments. Simulations were performed with different blood flow rates in the tumor and constraints on temperatures in normal tissues. The results demonstrate that 1) thermal dose can be controlled in the presence of plant-model mismatch and 2) constraints on the maximum allowable temperatures in normal tissue and/or the pulsed power magnitude can be directly incorporated into MPC and met while delivering the desired thermal dose to the tumor. For relatively high blood flow rates and low transducer surface intensities-factors that limit the range of temperature variations in the tumor, the linear MPC, obtained by piece-wise linearization of the dose-temperature relationship, provides an adequate performance. For large temperature variations, the development of nonlinear MPC is necessary.

Direct thermal dose control of constrained focused ultrasound treatments: phantom and in vivo evaluation

The first treatment control system that explicitly and automatically balances the efficacy and safety goals of noninvasive thermal therapies is described, and its performance is evaluated in phantoms and in vivo using ultrasound heating with a fixed, focused transducer. The treatment efficacy is quantified in terms of thermal dose delivered to the target. The developed feedback thermal dose controller has a cascade structure with the main nonlinear dose controller continuously generating the reference temperature trajectory for the secondary, constrained, model predictive temperature controller. The control system ensures thermal safety of the normal tissue by automatically complying with user-specified constraints on the maximum allowable normal tissue temperatures. To reflect hardware limitations and to prevent cavitation, constraints on the maximum transducer power can also be imposed. It is shown that the developed controller can be used to achieve the minimum-time delivery of the desired thermal dose to the target without violating safety constraints, which is a novel and clinically desirable feature. The developed controller is model based, and requires patient- and site-specific models for its operation. These models were obtained during pre-treatment identification experiments. In our implementation, predictive models, internally used by the automatic treatment controller, are dynamically updated each time new temperature measurements become available. The adaptability of internal models safeguards against adverse effects of modelling errors, and ensures robust performance of the control system in the presence of a priori unknown treatment disturbances. The successful validation with two experimental models of considerably different thermal and ultrasound properties suggests the applicability of the developed treatment control system to different anatomical sites.

MR thermometry-based feedback control of efficacy and safety in minimum-time thermal therapies: Phantom and in-vivo evaluations

The experimental validation of a model-based, thermal therapy control system which automatically and simultaneously achieves the specified efficacy and safety objectives of the treatment is reported. MR-thermometry measurements are used in real-time to control the power of a stationary, focused ultrasound transducer in order to achieve the desired treatment outcome in minimum time without violating the imposed safety constraints. Treatment efficacy is quantified in terms of the thermal dose delivered to the target. Normal tissue safety is ensured by automatically maintaining normal tissue temperature below the imposed limit in the user-specified locations. To reflect hardware limitations, constraints on the maximum applied power are also imposed. At the pretreatment stage, MR imaging and thermometry are used to localize the treatment target and identify thermal and actuation models. The results of phantom and canine experiments demonstrate that spatially-distributed, real-time MR temperature measurements enhance ones ability to robustly achieve the desired treatment outcome in minimum time without violating safety constraints. Post-treatment evaluation of the outcome using T2-weighted images of canine muscle showed good spatial correlation between the sonicated area and thermally damaged tissue.

Model predictive control of ultrasound hyperthermia treatments of cancer

A model predictive controller (MPC) of the thermal dose in hyperthermia cancer treatments has been developed and evaluated using simulations with one-point and one-dimensional models of a tumor. The developed controller is the first effort in (a) the application of feedback control to pulsed, high temperature hyperthermia treatments, (b) the direct control of the treatment thermal dose rather than the treatment temperatures, and (c) the application of MPC to hyperthermia treatments. Simulations were performed with different blood flow rates in the tumor and constraints on temperatures in normal tissues. The results demonstrate that (a) thermal dose can be controlled in the presence of plant-model mismatch, (b) constraints on the maximum allowable temperatures in normal tissue and/or the pulsed power magnitude can be directly incorporated into MPC and met while delivering the desired thermal dose to the tumor.

Constrained Predictive Control of Thermal Therapies for Minimum-Time Delivery of Thermal Dose

A method for time-optimal, direct control of thermal dose in thermal therapies is developed and experimentally validated using a focused ultrasound transducer and a phantom patient. State constraint on the maximum allowable temperature in a selected spatial location is imposed to prevent damage to critical normal tissues. A saturation constraint on the ultrasound power is imposed to reflect hardware limitations. It is shown that to achieve the minimum time treatment it is necessary to control the therapy with either saturated ultrasound power or active normal-tissue temperature constraints. The special cases for which the necessary condition are also sufficient for time optimality are also established. The model-based treatment control system is then designed that ensures that the necessary condition for time optimal treatment is satisfied throughout the treatment. During validation experiments, the ultrasound specific absorption rate and thermal response models of the phantom, needed for the operation of the designed treatment control system, were identified using temperature measurements. The performance of the treatment control system during the experiments demonstrates that the proposed approach is effective at delivering the desired thermal dose in a near-minimum time without violating safety constraints imposed in healthy tissues.

Nonlinear and model predictive control of thermal dose in high temperature therapies

A novel technique for direct control of thermal dose in thermal therapies is developed and evaluated. The developed controller has a cascade structure with a linear constrained model predictive temperature controller in the inner loop. The temperature controller manipulates the intensity of the ultrasound power transducer with saturation constraints, which noninvasively heats the spatially distributed target. The reference temperature trajectories for the predictive controller are dynamically generated by the nonlinear thermal dose controller in the outer loop. Simulations using a one-dimensional model of a tumor with constraints on the maximum allowable temperature in normal tissues, and a pulsed power ultrasound source were performed to ascertain the robustness and effectiveness of the proposed method. The results demonstrate that the proposed control structure is effective at delivering the desired thermal dose in the minimum time without violating constraints on the maximum allowable temperature in healthy tissue.

Constrained model-predictive thermal dose control for MRI-guided ultrasound thermal treatments (Invited Paper)

Ultrahigh resolution OCT using broadband light sources achieves improved axial image resolutions of ~2-3 um compared to standard 10 um resolution OCT used in current commercial instruments. High-speed OCT using Fourier/spectral domain detection enables dramatic increases in imaging speeds. 3D OCT retinal imaging is performed in human subjects using high-speed, ultrahigh resolution OCT, and the concept of an OCT fundus image is introduced. Three-dimensional data and high quality cross-sectional images of retinal pathologies are presented. These results show that 3D OCT may be used to improve coverage of the retina, precision of cross-sectional image registration, quality of cross-sectional images, and visualization of subtle changes in retinal topography. 3D OCT imaging and mapping promise to help elucidate the structural changes associated with retinal disease as well as to improve early diagnosis and monitoring of disease progression and response to treatment.