R Stafford

Transurethral ultrasound applicators with directional heating patterns for prostate thermal therapy: In vivo evaluation using magnetic resonance thermometry

A catheter-based transurethral ultrasound applicator with angularly directional heating patterns has been designed for prostate thermal therapy and evaluated in canine prostate in vivo using MRI to monitor and assess performance. The ultrasound transducer array (3.5 mm diameter tubular transducers, 180 degrees active sectors, approximately 7.5 MHz) was integrated to a flexible delivery catheter (4 mm OD), and encapsulated within an expandable balloon (35 mm x 10 mm OD, 80 ml min(-1) ambient water) for coupling and cooling of the prostatic urethra. These devices were used to thermally coagulate targeted portions of the canine prostate (n = 2) while using MR thermal imaging (MRTI) to monitor the therapy. MRI was also used for target definition, positioning of the applicator, and evaluation of target viability post-therapy. MRTI was based upon the complex phase-difference mapping technique using an interleaved gradient echo-planar imaging sequence with lipid suppression. MRTI derived temperature distributions, thermal dose exposures, T1-contrast enhanced MR images, and histology of sectioned prostates were used to define destroyed tissue zones and characterize the three-dimensional heating patterns. The ultrasound applicators produced approximately 180 degrees directed zones of thermal coagulation within targeted tissue which extended 15-20 mm radially to the outer boundary of the prostate within 15 min. Transducer activation lengths of 17 mm and 24 mm produced contiguous zones of coagulation extending axially approximately 18 mm and approximately 25 mm from base to apex, respectively. Peak temperatures around 90 degrees C were measured, with approximately 50 degrees C-52 degrees C corresponding to outer boundary t43 = 240 min at approximately 15 min treatment time. These devices are MRI compatible, and when coupled with multiplanar MRTI provide a means for selectively controlling the length and sector angle of therapeutic thermal treatment in the prostate.

Magnetic resonance temperature imaging validation of a bioheat transfer model for laser‐induced thermal therapy

Purpose: Magnetic resonance‐guided laser‐induced thermal therapy (MRgLITT) is currently undergoing initial safety and feasibility clinical studies for the treatment of intracranial lesions in humans. As studies progress towards evaluation of treatment efficacy, predictive computational models may play an important role for prospective 3D treatment planning. The current work critically evaluates a computational model of laser induced bioheat transfer against retrospective multiplanar MR thermal imaging (MRTI) in a canine model of the MRgLITT procedure in the brain. Methods: A 3D finite element model of the bioheat transfer that couples Pennes equation to a diffusion theory approximation of light transport in tissue is used. The laser source is modelled conformal with the applicator geometry. Dirichlet boundary conditions are used to model the temperature of the actively cooled catheter. The MRgLITT procedure was performed on n = 4 canines using a 1‐cm diffusing tip 15‐W diode laser (980 nm). A weighted norm is used as the metric of comparison between the spatiotemporal MR‐derived temperature estimates and model prediction. Results: The normalised error history between the computational models and MRTI was within 1–4 standard deviations of MRTI noise. Active cooling models indicate that the applicator temperature has a strong effect on the maximum temperature reached, but does not significantly decrease the tissue temperature away from the active tip. Conclusions: Results demonstrate the computational model of the bioheat transfer may provide a reasonable approximation of the laser–tissue interaction, which could be useful for treatment planning, but cannot readily replace MR temperature imaging in a complex environment such as the brain.

Development of a computational paradigm for laser treatment of cancer

The goal of this project is to develop a dynamic data-driven planning and control system for laser treatment of cancer. The research includes (1) development of a general mathematical framework and a family of mathematical and computational models of bio-heat transfer, tissue damage, and tumor viability, (2) dynamic calibration, verification and validation processes based on laboratory and clinical data and simulated response, and (3) design of effective thermo-therapeutic protocols using model predictions. At the core of the proposed systems is the adaptive-feedback control of mathematical and computational models based on a posteriori estimates of errors in key quantities of interest, and modern Magnetic Resonance Temperature Imaging (MRTI), and diode laser devices to monitor treatment of tumors in laboratory animals. This approach enables an automated systematic model selection process based on acceptance criteria determined a priori. The methodologies to be implemented involve uncertainty quantification methods designed to provide an innovative, data-driven, patient-specific approach to effective cancer treatment.

Adaptive Real-Time Bioheat Transfer Models for Computer-Driven MR-Guided Laser Induced Thermal Therapy

The treatment times of laser induced thermal therapies (LITT) guided by computational prediction are determined by the convergence behavior of partial differential equation (PDE)-constrained optimization problems. In this paper, we investigate the convergence behavior of a bioheat transfer constrained calibration problem to assess the feasibility of applying to real-time patient specific data. The calibration techniques utilize multiplanar thermal images obtained from the nondestructive in vivo heating of canine prostate. The calibration techniques attempt to adaptively recover the biothermal heterogeneities within the tissue on a patient-specific level and results in a formidable PDE constrained optimization problem to be solved in real time. A comprehensive calibration study is performed with both homogeneous and spatially heterogeneous biothermal model parameters with and without constitutive nonlinearities. Initial results presented here indicate that the calibration problems involving the inverse solution of thousands of model parameters can converge to a solution within three minutes and decrease the ||·||L 2 2 (0,T;L 2 (¿)) norm of the difference between computational prediction and the measured temperature values to a patient-specific regime.

A model evaluation study for treatment planning of laser-induced thermal therapy

Abstract A cross-validation analysis evaluating computer model prediction accuracy for a priori planning magnetic resonance-guided laser-induced thermal therapy (MRgLITT) procedures in treating focal diseased brain tissue is presented. Two mathematical models are considered. (1) A spectral element discretisation of the transient Pennes bioheat transfer equation is implemented to predict the laser-induced heating in perfused tissue. (2) A closed-form algorithm for predicting the steady-state heat transfer from a linear superposition of analytic point source heating functions is also considered. Prediction accuracy is retrospectively evaluated via leave-one-out cross-validation (LOOCV). Modelling predictions are quantitatively evaluated in terms of a Dice similarity coefficient (DSC) between the simulated thermal dose and thermal dose information contained within N = 22 MR thermometry datasets. During LOOCV analysis, the transient model’s DSC mean and median are 0.7323 and 0.8001 respectively, with 15 of 22 DSC values exceeding the success criterion of DSC ≥ 0.7. The steady-state model’s DSC mean and median are 0.6431 and 0.6770 respectively, with 10 of 22 passing. A one-sample, one-sided Wilcoxon signed-rank test indicates that the transient finite element method model achieves the prediction success criteria, DSC ≥ 0.7, at a statistically significant level.

TU‐B‐I‐617‐01: High Field MRI — Technology, Applications, Safety, and Limitations

Signal-to-noise ratio in conventional magnetic resonance imaging (MRI) is inextricably tied to the static magnetic field strength (B0). Until recently, most clinical MRI scanners operated at field strengths at or below 1.5 Tesla. However, due to technological advancements in magnet design and shielding, which ease siting requirements, 3 Tesla clinical scanners are now enjoying wide commercial availability and there is a push for even higher field whole body scanners (7–9Tesla) throughout the industry. The drive towards high-field MRI is fueled by the benefits of potentially higher signal-to-noise ratios, contrast-to-noise ratios, and spectral resolution. In many cases, these benefits translate directly into higher spatial and/or temporal resolution than previously possible with MRI at lower fields as well as the ability to explore new territory, such as molecular imaging. There are, however, very real technological, physical and safety limitations that must be navigated and may limit the full realization of these benefits at high-field. Technology issues include homogeneity of the static and radiofrequency magnetic fields, higher gradient coil performance and linearity, and the design of robust radiofrequency array coils for signal reception. At high-field, physics concerns include changes in relaxation kinetics, increased susceptibility effects and other changes in contrast mechanisms. Safety limitations include higher power radiofrequency pulses and the potential for tissue heating or coil burns, stimulation effects from stronger, faster switching gradients and physiological effects of motion within the high-field environment and, most prominently, the potential dangers associated with the main magnetic field, such as ferromagnetic projectiles in the scan room and effects on implanted medical devices, many of which have yet to be evaluated at fields above 1.5 Tesla. Ultimately, design of protocols and acquisition methods that account for these limitations need to be pursued in order to reap the benefits of high-field MRI without compromising patient safety. Many MR imaging techniques have already seen demonstrable improvement at higher fields and have driven the development and distribution of high-field systems. Techniques in functional magnetic resonance imaging relying on blood-oxygen level dependent contrast mechanisms, techniques in angiography and techniques in dynamic susceptibility contrast perfusion imaging all benefit from higher fields. Changes in relaxation kinetics can provide enhanced contrast for angiography and arterial spin labeling techniques. Additionally, proton MR spectroscopy methods for brain and body imaging benefit from the higher spectral resolution of high field as do multi-nuclear techniques. This course will review the technology and physics behind the emerging high-field systems (3–9 Tesla) with emphasis on the commercially available and widespread 3 Tesla systems. Major applications of high-field MRI will be addressed with an eye on the future. Aspects of safety in high-field MRI will also be covered, paying particular attention to how safety considerations may influence the development and implementation of patient protocols at higher fields. Educational Objectives: 1. Introduction to relevant high-field technology, physics and techniques. 2. Understand benefits and limitations of high-field MRI. 3. Understand safety concerns associated with high-field MRI systems. 4. Awareness of some of the most relevant applications of high-field MR imaging.

Percutaneous MRI-guided laser thermal therapy in canine prostate

Prostate cancer is the most common cancer in American men excluding skin cancer, and approximately 230,000 cases of prostate cancer will be diagnosed in the U.S. in 2004. In the non-surgical treatment of localized prostate cancer, fiberoptically delivered interstitial laser thermal therapy may be ideal for treating discrete tumors with minimal invasiveness. Real-time magnetic resonance imaging can be used to compute temperature changes based on the proton resonance frequency (PRF) shift, and two-dimensional maps of temperature rise and chronic thermal damage can be constructed in order to control laser therapy. In this work, we describe an MRI-compatible percutaneous grid template and localization and planning software for precise placement of minimally invasive laser catheters to effect a target ablation zone. We evaluated the accuracy of the catheter placement, and we present our preliminary experience with percutaneous MRI-guided feedback controlled laser ablation in a canine prostate model. Histological analysis is used to assess the effectiveness and accuracy of treatment visualization.

Using Cyber-Infrastructure for Dynamic Data Driven Laser Treatment of Cancer

Hyperthermia based cancer treatments are used to increase the susceptibility of cancerous tissue to subsequent radiation or chemotherapy treatments, and in the case in which a tumor exists as a well-defined region, higher intensity heat sources may be used to ablate the tissue. Utilizing the guidance of real-time treatment data while applying a laser heat source has the potential to provide unprecedented control over the outcome of the treatment process [6,12]. The goals of this work are to provide a working snapshot of the current system architecture developed to provide a real-time finite element solution of the problems of calibration, optimal heat source control, and goal-oriented error estimation applied the equations of bioheat transfer and demonstrate that current finite element technology, parallel computer architecture, peer-to-peer data transfer infrastructure, and thermal imaging modalities are capable of inducing a precise computer controlled temperature field within the biological domain.

SU‐E‐J‐210: Characterizing Tissue Equivalent Materials for the Development of a Dual MRI‐CT Heterogeneous Anthropomorphic Phantom Designed Specifically for MRI Guided Radiotherapy Systems

Purpose: MRI guided radiotherapy (MRIgRT) is an emerging technology which will eventually require a proficient quality auditing system. Due to different principles in which MR and CT acquire images, there is a need for a multi-imaging-modality, end-to-end QA phantom for MRIgRT. The purpose of this study is to identify lung, soft tissue, and tumor equivalent substitutes that share similar human-like CT and MR properties (i.e. Hounsfield units and relaxation times). Methods: Materials of interested such as common CT QA phantom materials, and other proprietary gels/silicones from Polytek, SmoothOn, and CompositeOne were first scanned on a GE 1.5T Signa HDxT MR. Materials that could be seen on both T1-weighted and T2-weighted images were then scanned on a GE Lightspeed RT16 CT simulator and a GE Discovery 750HD CT scanner and their HU values were then measured. The materials with matching HU values of lung (−500 to −700HU), muscle (+40HU) and soft tissue (+100 to +300HU) were further scanned on GE 1.5T Signa HDx to measure their T1 and T2 relaxation times from varying parameters of TI and TE. Results: Materials that could be visualized on T1-weighted and T2-weighted images from a 1.5T MR unit and had an appropriate average CT number, −650, −685, 46,169, and 168 HUs were: compressed cork saturated with water, Polytek Platsil™ Gel-00 combined with mini styrofoam balls, radiotherapy bolus material, SmoothOn Dragon-Skin™ and SmoothOn Ecoflex™, respectively. Conclusion: Post processing analysis is currently being performed to accurately map T1 and T2 values for each material tested. From previous MR visualization and CT examinations it is expected that Dragon-Skin™, Ecoflex™ and bolus will have values consistent with tissue and tumor substitutes. We also expect compressed cork statured with water, and Polytek™-styrofoam combination to have approximate T1 and T2 values suitable for lung-equivalent materials.

MRI-guided laser thermal therapy in the prostate: Preliminary results

Minimally invasive thermal therapies for the treatment of prostate cancer offer potential to reduce cost, treatment time, and patient trauma. A drawback to such therapies is that it is often difficult or impossible to know the exact volume of which is being destroyed. In this work, we report on the use of magnetic resonance (MR) thermal imaging to provide real-time feedback control over laser interstitial thermal therapy (LITT) in an in vivo canine prostate model.