Model predictive control for MR-HIFU-mediated, uniform hyperthermia.

Purpose: In local hyperthermia, precise temperature control throughout the entire target region is key for swift, safe, and effective treatment. In this article, we present a model predictive control (MPC) algorithm providing voxel-level temperature control in magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) and assess the improvement in performance it provides over the current state of the art. Materials and methods: The influence of model detail on the prediction quality and runtime of the controller is evaluated and a tissue mimicking phantom is characterized using the resulting model. Next, potential problems arising from modeling errors are evaluated in silico and in the characterized phantom. Finally, the controller's performance is compared to the current state-of-the-art hyperthermia controller in side-by-side experiments. Results: Modeling diffusion by heat exchange between four neighboring voxels achieves high predictive performance and results in runtimes suited for real-time control. Erroneous model parameters deteriorate the MPC's performance. Using models derived from thermometry data acquired during low powered test sonications, however, high control performance is achieved. In a direct comparison with the state-of-the-art hyperthermia controller, the MPC produces smaller tracking errors and tighter temperature distributions, both in a homogeneous target and near a localized heat sink. Conclusion: Using thermal models deduced from low-powered test sonications, the proposed MPC algorithm provides good performance in phantoms. In direct comparison to the current state-of-the-art hyperthermia controller, MPC performs better due to the more finely tuned heating patterns and therefore constitutes an important step toward stable, uniform hyperthermia.

Modelling the temperature evolution of bone under high intensity focused ultrasound.

Magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) has been clinically shown to be effective for palliative pain management in patients suffering from skeletal metastasis. The underlying mechanism is supposed to be periosteal denervation caused by ablative temperatures reached through ultrasound heating of the cortex. The challenge is exact temperature control during sonication as MR-based thermometry approaches for bone tissue are currently not available. Thus, in contrast to the MR-HIFU ablation of soft tissue, a thermometry feedback to the HIFU is lacking, and the treatment of bone metastasis is entirely based on temperature information acquired in the soft tissue adjacent to the bone surface. However, heating of the adjacent tissue depends on the exact sonication protocol and requires extensive modelling to estimate the actual temperature of the cortex. Here we develop a computational model to calculate the spatial temperature evolution in bone and the adjacent tissue during sonication. First, a ray-tracing technique is used to compute the heat production in each spatial point serving as a source term for the second part, where the actual temperature is calculated as a function of space and time by solving the Pennes bio-heat equation. Importantly, our model includes shear waves that arise at the bone interface as well as all geometrical considerations of transducer and bone geometry. The model was compared with a theoretical approach based on the far field approximation and an MR-HIFU experiment using a bone phantom. Furthermore, we investigated the contribution of shear waves to the heat production and resulting temperatures in bone. The temperature evolution predicted by our model was in accordance with the far field approximation and agreed well with the experimental data obtained in phantoms. Our model allows the simulation of the HIFU treatments of bone metastasis in patients and can be extended to a planning tool prior to MR-HIFU treatments.

Simultaneous T1 measurements and proton resonance frequency shift based thermometry using variable flip angles.

A method is presented which allows precise temperature and longitudinal (T(1)) relaxation time measurements with high spatial and temporal resolution. This is achieved by combining dynamic variable flip angle based T(1) relaxation mapping with proton resonance frequency shift based thermometry. Herein, dynamic T(1) mapping is either used as a complementary measure of temperature or for the detection of T(1) contrast agent release. For the first application, the temperature evolution during a high-intensity focused ultrasound tissue ablation experiment was measured in both, porcine fat and muscle, simultaneously. In this application, temperature accuracies of 2.5 K for T(1)-based thermometry in fat and 1.2 K for proton resonance frequency shift-based thermometry in muscle were observed. The second application relates to MR-guidance of high-intensity focused ultrasound-induced local drug delivery by means of thermo-sensitive liposomes labeled with a T(1) contrast agent (Gd-HPDO3A). When the measured temperature exceeded the phase transition temperature of the liposomes, T(1) was observed to decrease with a good temporal and spatial correlation due to the release of Gd-HPDO3A. The presented results demonstrate the feasibility of the proposed method for two important applications in MR-guided noninvasive therapy. It offers a high temporal resolution when compared with interleaved Look-Locker based T(1) mapping techniques and thus represents an interesting candidate for simultaneous real-time monitoring of T(1) and temperature changes.

Ultrasound-mediated intracellular drug delivery using microbubbles and temperature-sensitive liposomes.

A novel two-step protocol for intracellular drug delivery has been evaluated in vitro. As a first step TO-PRO-3 (a cell-impermeable dye that displays a strong fluorescence enhancement upon binding to nucleic acids) encapsulated in thermosensitive liposomes was released after heating to 42°C. A second step consisted of ultrasound-mediated local permeabilization of cell membrane allowing TO-PRO-3 internalization observable as nuclear staining. Only the combination of two consecutive steps - heating and sonication in the presence of SonoVue microbubbles led to the model drug TO-PRO-3 release from the thermosensitive liposomes and its intracellular uptake. This protocol is potentially beneficial for the intracellular delivery of cell impermeable drugs that suffer from rapid clearance and/or degradation in blood and are not intrinsically taken up by cells.

Influence of cell-internalization on relaxometric, optical and compositional properties of targeted paramagnetic quantum dot micelles.

Quantum dot micelles (pQDs) with a paramagnetic coating are promising nanoparticles for bimodal molecular imaging. Their bright fluorescence allows for optical detection, while their Gd payload enables visualization with contrast-enhanced MRI. A popular approach in molecular MRI is the targeting of abundantly expressed cell surface receptors. Ligand-receptor binding often results in cell internalization of the targeted contrast agent. The interpretation of molecular imaging with pQDs therefore requires knowledge about the consequences of cellular internalization for their relaxometric, optical and compositional properties. To study these, Cd-containing core-shell-shell QDs coated with a monolayer of lipids, of which 50 mol% was a Gd-containing lipid, were incubated with human umbilical vein-derived endothelial cells (HUVECs) for up to 24 h. α(ν) ß(3)-integrin targeted (RGD) and non-targeted (NT) pQDs were compared. pQDs uptake was monitored by fluorescence microscopy, FACS, ICP-MS, relaxometry and MRI. Cell-associated pQDs displayed longitudinal relaxation rates and fluorescent intensities which were linear with the cell-associated Gd and Cd concentrations, implying that the Gd and Cd uptake by HUVECs can be quantified using relaxometric and optical measurements, respectively. However, the Gd-to-Cd molar ratio in pellets of pQD-incubated cells was consistently higher than the Gd-to-Cd molar ratio of the pQDs as prepared. It is proposed that this increase in Gd-to-Cd molar ratio was due to non-specific lipid-transfer between the pQDs and the cellular membranes. These findings show that, in the case of contrast agents that are formed by non-covalent interactions, experimental procedures are needed with which representative components of the probes can be monitored.

Dendritic PARACEST contrast agents for magnetic resonance imaging.

MRI contrast agents based on chemical exchange-dependent saturation transfer (CEST), such as Yb(III)DOTAM complexes, are highly suitable for pH mapping. In this paper, the synthesis of Yb(III)DOTAM-functionalized poly(propylene imine) dendrimers is described. The applicability of these dendritic PARACEST MRI agents for pH mapping has been evaluated on a 7 T NMR spectrometer and on a 3 T clinical MRI scanner. As expected, based on the different numbers of exchangeable amide protons, the lowest detectable concentration of the first and third generation dendritic PARACEST agents is by a respective factor of about 4 and 16 lower than that of a mononuclear reference complex. The pH dependence of the CEST effect observed for these compounds depends on the generation of the poly(propylene imine) dendrimer. Upon going to higher generations of the Yb(III)DOTAM-terminated dendrimer, a shift of the maximum CEST effect towards lower pH values was observed. This allows for a fine-tuning of the responsive pH region by varying the dendritic framework.

Stability of Dilute Colloidal Silica Suspensions in the Vicinity of the Binodal Curve of the System 2-Butoxyethanol/Water

In the temperature-composition diagram of the system 2-butoxyethanol (abbreviated C4E1)/water, SiO2 [mass fraction of SiO2, y(SiO2) approximately 0.01; volume fraction phi(SiO2) approximately 10(-3)] there exists a flocculation temperature-composition curve running below the binodal curve for x > xc (x, mole fraction of C4E1; xc = 0.0598). The concentration of the colloidal particles is low enough to consider them an "impurity." In the region of the phase diagram bound by the binodal and the flocculation curve the suspension of the colloidal SiO2 particles is unstable and the particles flocculate reversibly. The difference between the temperature of phase separation and the flocculation temperature increases with increasing values of x - xc up to a maximum value of x (xmax). For x > xmax the suspensions are unstable at all temperatures studied. For x < xc the suspension of the SiO2 particles is stable up to the temperature of phase separation of the C4E1/water mixture. The flocculation curve is assumed to reflect the influence of local concentration fluctuations with long range correlations on the stability of the SiO2 suspensions. For x > xc these concentration fluctuations are water rich and interact with the hydrophilic surface of the colloidal SiO2 particles by forming an adsorption layer. This layer is assumed to modify the interparticle potential energy-distance curve and to trigger flocculation. For x < xc the concentration fluctuations are C4E1 rich and no adsorption layer is formed at the hydrophilic surface of the colloidal particles. Copyright 1997Academic Press