Multislice localized parallel excitation for EPI applications in humans

In this work, the opportunities and challenges for the use of parallel transmission in combination with 2D RF pulses designed on EPI‐based excitation trajectories for diffusion‐weighted imaging (DWI) with reduced FOV are presented and analyzed in detail. The use of localized excitation allows for shortening of the EPI read‐out, which is especially important for EPI applications outside of the brain. DWI is chosen as a practically important and relevant example demonstrating the key aspects of 2D spatial selection. The properties of accelerated pulses are explored experimentally in phantoms for two different schemes, in which the thickness of the excited limited slices is encoded either along the frequency or phase encoding directions of the excitation trajectory. The feasibility of application of parallel transmission for MR imaging in humans is analyzed based on several pilot experiments. Although the parallel transmission acceleration is demonstrated to work in some examples in the spinal cord and abdomen, the results also uncover a number of challenges. Nonetheless, the reduction of FOV in the phase encoding direction of the read‐out train along with the associated substantial shortening of the minimum echo train length and reduction of geometric distortions motivates further search for an advantageous use of the parallel transmit technology in EPI applications. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 153–173, 2015     

Experimental Analgesic Nephropathy

The renal and other organ damage induced by mild analgesics in animals under acute or subacute conditions is not specific to analgesics; it may have a relationship to acute poisoning in man but is not related to human nephropathy attributed to analgesic abuse. Chronic experiments with single or mixed analgesics conducted under stringent scientific conditions are negative if the dose levels are not lethal. Dehydration combined with high dose levels of analgesics produces papillary necrosis in rats but such experiments are not relevant to human analgesic nephropathy. The dependence of phenacetiri metabolism on the dose level administered, demonstrated by the increased formation of potentially toxic metabolites, has great importance for the understanding of analgesic nephropathy in man. This type of nephropathy seems to be specific for humans; the immunologic theory affirming the role of the antigen of p-phenetidine is the most plausible but needs further confirmation. Results in the field of analgesic-induced experimental kidney damage indicate that had pathologists and clinicians not drawn attention to the problem of nephropathy caused by analgesic abuse, the toxicologists would never have suspected, forseen, or predicted the appearence of such a syndrome.     

Medication safety in rehabilitation medicine

Rehabilitation medicine is practiced in a variety of settings. Physiatrists are an integral part of the care provided in many of these settings and are often consulted to provide diagnostic and therapeutic services and expertise to individuals with a variety of diagnoses. In this role, it is imperative that physiatrists have a working knowledge of various medications as well as the principles of medication safety. This article provides a foundation in the general and specific aspects of medication safety as they apply to the practice of rehabilitation medicine.     

Safety considerations during cardiac and pulmonary rehabilitation program

As more and more patients with cardiac and pulmonary diseases are living longer lives, the need for cardio-pulmonary rehabilitation continues to grow. The goal of this article is to provide clinicians of rehabilitation medicine with an overview of the safety concerns and strategies to implement in the rehabilitation of patients with cardiac and/or pulmonary disorder.     

Non‐invasive determination of tissue thermal parameters from high intensity focused ultrasound treatment monitored by volumetric MRI thermometry

A method is proposed for estimating the perfusion rate, thermal diffusivity, and the absorption coefficient that influence the local temperature during high intensity focused ultrasound (HIFU) thermotherapy procedures. For this purpose, HIFU heating experiments (N = 100) were performed ex vivo on perfused porcine kidney (N = 5) under different flow conditions. The resulting spatio‐temporal temperature variations were measured non‐invasively by rapid volumetric MR‐temperature imaging. The bio‐heat transfer (BHT) model was adapted to describe the spatio‐temporal evolution of tissue temperature in the cortex. Absorption and perfusion coefficients were determined by fitting the integrated thermal load (spatial integration of the thermal maps) curves in time with an analytical solution of the BHT equation proposed for single point HIFU heating. Thermal diffusivity was determined independently by analyzing the spatial spread of the temperature in time during the cooling period. Absorption coefficient and thermal diffusivity were found to be independent of flow, with mean and average values of 11.0 ± 1.85 mm³ · K · J^?1 and 0.172 ± 0.003 mm² · s^?1, respectively. A linear dependence of the calculated perfusion rate with flow was observed with a slope of 9.20 ± 0.75 mm^?3. The perfusion was found to act as a scaling term with respect to temperature but with no effect on the spatial spread of temperature which only depends on the thermal diffusivity. All results were in excellent agreement with the BHT model, indicating that this model is suitable to predict the evolution of temperature in perfused organs. This quantitative approach allows for determination of tissue thermal parameters with excellent precision (within 10%) and may thus help in quantifying the influence of perfusion during MR guided high intensity focused ultrasound (MRgHIFU). Copyright © 2009 John Wiley & Sons, Ltd.     

Accelerated point spread function mapping using signal modeling for accurate echo‐planar imaging geometric distortion correction

Echo‐planar imaging is a fast and commonly used magnetic resonance imaging technique with applications in diffusion weighted and functional MRI. Fast data acquisition in echo‐planar imaging is accomplished by the extended readout, which also introduces sensitivity to off‐resonance effects such as amplitude of static (polarizing) field inhomogeneities and eddy‐currents. These off‐resonance effects produce geometric distortions in the corresponding echo‐planar images. To correct for these distortions, an acceleration of point spread function (PSF) acquisition using a special sampling pattern is presented in this work. The proposed technique allows for reliable and fully automated distortion correction of echo‐planar images at a field strength of 3 T. Additionally, a new approach to visualize and determine the distortions in a hybrid (x, y, k_PSF) three‐dimensional space is proposed. The accuracy and robustness of the proposed technique is demonstrated in phantom and in vivo experiments. The accuracy of the presented method here is compared to previous techniques for echo‐planar imaging distortion correction such as PLACE. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Three-dimensional arbitrary voxel shapes in spectroscopy with submillisecond TEs

A novel spectroscopic method for submillisecond TEs and three‐dimensional arbitrarily shaped voxels was developed and applied to phantom and in vivo measurements, with additional parallel excitation (PEX) implementation. A segmented spherical shell excitation trajectory was used in combination with appropriate radiofrequency weights for target selection in three dimensions. Measurements in a two‐compartment phantom realized a TE of 955 µs, excellent spectral quality and comparable signal‐to‐noise ratios between accelerated (R = 2) and nonaccelerated modes. The two‐compartment model allowed a comparison of the spectral suppression qualities of the method and, although outer volume signals were suppressed by factors of 1434 and 2246 compared with the theoretical unsuppressed case for the clinical and PEX modes, respectively, incomplete suppression of the outer volume (935 cm³ compared with a target volume of 5.86 cm³) resulted in a spectral contamination of 10.2% and 6.5% compared with the total signal. The method was also demonstrated in vivo in human brain on a clinical system at TE = 935 µs with good signal‐to‐noise ratio and spatial and spectral selection, and included LCModel relative quantification analysis. Eight metabolites showed significant fitting accuracy, including aspartate, N‐acetylaspartylglutamate, glutathione and glutamate. Copyright © 2012 John Wiley & Sons, Ltd.     

Rapid motion correction in MR‐guided high‐intensity focused ultrasound heating using real‐time ultrasound echo information

The objective of this study was to evaluate the feasibility of integrating real‐time ultrasound echo guidance in MR‐guided high‐intensity focused ultrasound (HIFU) heating of mobile targets in order to reduce latency between displacement analysis and HIFU treatment. Experiments on a moving phantom were carried out with MRI‐guided HIFU during continuous one‐dimensional ultrasound echo detection using separate HIFU and ultrasound imaging transducers. Excellent correspondence was found between MR‐ and ultrasound‐detected displacements. Real‐time ultrasound echo‐based target tracking during MR‐guided HIFU heating is shown with the dimensions of the heated area similar to those obtained for a static target. This work demonstrates that the combination of the two modalities opens up perspectives for motion correction in MRI‐guided HIFU with negligible latency. Copyright © 2010 John Wiley & Sons, Ltd.     

MR thermometry for monitoring tumor ablation

Local thermal therapies are increasingly used in the clinic for tissue ablation. During energy deposition, the actual tissue temperature is difficult to estimate since physiological processes may modify local heat conduction and energy absorption. Blood flow may increase during temperature increase and thus change heat conduction. In order to improve the therapeutic efficiency and the safety of the intervention, mapping of temperature and thermal dose appear to offer the best strategy to optimize such interventions and to provide therapy endpoints. MRI can be used to monitor local temperature changes during thermal therapies. On-line availability of dynamic temperature mapping allows prediction of tissue death during the intervention based on semi-empirical thermal dose calculations. Much progress has been made recently in MR thermometry research, and some applications are appearing in the clinic. In this paper, the principles of MRI temperature mapping are described with special emphasis on methods employing the temperature dependency of the water proton resonance frequency. Then, the prospects and requirements for widespread applications of MR thermometry in the clinic are evaluated.