In this article, a robust methodology for in vivo T 1 mapping is presented. The approach combines a gold standard scanning procedure with a novel fitting procedure. Fitting complex data to a five-parameter model ensures accuracy and precision of the T 1 estimation. A reduced-dimension nonlinear least squares method is proposed. This method turns the complicated multiparameter minimization into a straightforward one-dimensional search. As the range of possible T 1 values is known, a global grid search can be used, ensuring that a global optimal solution is found. When only magnitude data are available, the algorithm is adapted to concurrently restore polarity. The performance of the new algorithm is demonstrated in simulations and phantom experiments. The new algorithm is as accurate and precise as the conventionally used Levenberg-Marquardt algorithm but much faster. This gain in speed makes the use of the fiveparameter model viable. In addition, the new algorithm does not require initialization of the search parameters. Finally, the methodology is applied in vivo to conventional brain imaging and to skin imaging. INTRODUCTIONThe T 1 parameter is an intrinsic MR property of tissue, and mapping T 1 in vivo is useful in several ways. First, knowledge of T 1 helps in optimizing the MR protocol, e.g., by setting the Ernst angle appropriately. In addition, it provides a tool to evaluate contrast uptake, blood perfusion and volume, as well as disease progression during a longitudinal study. Furthermore, it is often desirable to compare T 1 measurements across subjects and across scanners. Although there are many techniques for T 1 mapping (1), there is also a wide range of reported T 1 values in tissue (2), an inconsistency that raises the issue of reproducibility and standardization. The gold standard for T 1 mapping was developed from NMR experiments performed in the late 1940s (3,4). The method is known as inversion recovery T 1 mapping (IR), and it consists of inverting the longitudinal magnetization M z and sampling the MR signal as it recovers with an exponential recovery time T 1 . Different models have been used for T 1 mapping (1). With all models, the fit is traditionally performed using a Levenberg-Marquardt (LM) algorithm (5). Many methods have been proposed to speed up the scanning and fitting procedures, at the expense of accuracy and precision.In this article, we first justify the need for a fourparameter model when accurate T 1 mapping is desired. We show that this model is equivalent in terms of accuracy and precision of the T 1 estimation to a more general five-parameter model. We propose to solve a nonlinear least squares (NLS) problem to fit complex data to the five-parameter model. The problem is reduced to a search over one dimension, which substantially decreases the computational complexity. When only magnitude data are available, the algorithm is adapted to concurrently restore polarity. We perform Monte-Carlo simulations to compare the proposed algorithms to the conventional LM algorithm and...

Purpose Elongated conductors, such as pacemaker leads, neurostimulator leads, and conductive guidewires used for interventional procedures, can couple to the MRI radiofrequency (RF) transmit field, potentially causing dangerous tissue heating. The purpose of this work is to demonstrate the feasibility of using parallel transmit to control induced RF currents in elongated conductors, thereby reducing the RF heating hazard. Methods Phantom experiments were performed on a four-channel parallel transmit system at 1.5T. Parallel transmit “null mode” excitations that induce minimal wire current were designed using coupling measurements derived from axial B1+ maps. The resulting current reduction performance was evaluated with B1+ maps, current sensor measurements, and fluoroptic temperature probe measurements. Results Null mode excitations reduced the maximum coupling mode current by factors ranging from 2–80. For the straight wire experiment, a current null imposed at a single wire location was sufficient to reduce tip heating below detectable levels. For longer insertion lengths and a curved geometry, imposing current nulls at two wire locations resulted in more distributed current reduction along the wire length. Conclusion Parallel transmit can be used to create excitations that induce minimal RF current in elongated conductors, thereby decreasing the RF heating risk, while still allowing visualization of the surrounding volume.

Longitudinal, remote monitoring of motor symptoms in Parkinson’s disease (PD) could enable more precise treatment decisions. We developed the Motor fluctuations Monitor for Parkinson’s Disease (MM4PD), an ambulatory monitoring system that used smartwatch inertial sensors to continuously track fluctuations in resting tremor and dyskinesia. We designed and validated MM4PD in 343 participants with PD, including a longitudinal study of up to 6 months in a 225-subject cohort. MM4PD measurements correlated to clinical evaluations of tremor severity (ρ = 0.80) and mapped to expert ratings of dyskinesia presence (P < 0.001) during in-clinic tasks. MM4PD captured symptom changes in response to treatment that matched the clinician’s expectations in 94% of evaluated subjects. In the remaining 6% of cases, symptom data from MM4PD identified opportunities to make improvements in pharmacologic strategy. These results demonstrate the promise of MM4PD as a tool to support patient-clinician communication, medication titration, and clinical trial design.

Purpose The development of catheters and guidewires that are safe from radiofrequency (RF)-induced heating and clearly visible against background tissue is a major challenge in interventional MRI. An interventional imaging approach using a toroidal transmit-receive (transceive) coil is presented. This toroidal transceiver allows controlled, low levels of RF current to flow in the catheter/guidewire for visualization, and can be used with conductive interventional devices that have a localized low-impedance tip contact. Methods Toroidal transceivers were built, and phantom experiments were performed to quantify transmit power levels required for device visibility and to detect heating hazards. Imaging experiments in a pig cadaver tested the extendibility to higher field strength and non-phantom settings. A photonically-powered optically-coupled toroidal current sensor for monitoring induced RF currents was built, calibrated, and tested using an independent image-based current estimation method. Results Results indicate that high-SNR visualization is achievable using milliwatts of transmit power—power levels orders of magnitude lower than levels that induce measurable heating in phantom tests. Agreement between image-based current estimates and RF current sensor measurements validates sensor accuracy. Conclusion The toroidal transceiver, integrated with power and current sensing, could offer a promising platform for safe and effective interventional device visualization.

Purpose The concept of an “RF Safety Prescreen” is investigated, wherein dangerous interactions between RF fields used in MRI, and conductive implants in patients are detected through impedance changes in the RF coil. Theory The behavior of coupled oscillators is reviewed, and the resulting, observable impedance changes are discussed. Methods A birdcage coil is loaded with a static head phantom and a wire phantom with a wire close to its resonant length, the shape, position and orientation of which can be changed. Interactions are probed with a current sensor and network analyzer. Results Impedance spectra show dramatic, unmistakable splitting in cases of strong coupling, and strong correlation is observed between induced current and scattering parameters. Conclusions The feasibility of a new, low-power prescreening technique has been demonstrated in a simple phantom experiment, which can unambiguously detect resonant interactions between an implanted wire and an imaging coil. A new technique has also been presented which can detect parallel transmit null modes for the wire.