Bertram J. Wilm

Reduced field‐of‐view MRI using outer volume suppression for spinal cord diffusion imaging

A spin‐echo single‐shot echo‐planar imaging (SS‐EPI) technique with a reduced field of view (FOV) in the phase‐encoding direction is presented that simultaneously reduces susceptibility effects and motion artifacts in diffusion‐weighted (DW) imaging (DWI) of the spinal cord at a high field strength (3T). To minimize aliasing, an outer volume suppression (OVS) sequence was implemented. Effective fat suppression was achieved with the use of a slice‐selection gradient‐reversal technique. The OVS was optimized by numerical simulations with respect to T1 relaxation times and B1 variations. The optimized sequence was evaluated in vitro and in vivo. In simulations the optimized OVS showed suppression to <0.25% and ∼3% in an optimal and worst‐case scenario, respectively. In vitro measurements showed a mean residual signal of <0.95% ± 0.42 for all suppressed areas. In vivo acquisition with 0.9 × 1.05 mm2 in‐plane resolution resulted in artifact‐free images. The short imaging time of this technique makes it promising for clinical studies. Magn Reson Med 57:625–630, 2007.

Higher order reconstruction for MRI in the presence of spatiotemporal field perturbations

Despite continuous hardware advances, MRI is frequently subject to field perturbations that are of higher than first order in space and thus violate the traditional k‐space picture of spatial encoding. Sources of higher order perturbations include eddy currents, concomitant fields, thermal drifts, and imperfections of higher order shim systems. In conventional MRI with Fourier reconstruction, they give rise to geometric distortions, blurring, artifacts, and error in quantitative data. This work describes an alternative approach in which the entire field evolution, including higher order effects, is accounted for by viewing image reconstruction as a generic inverse problem. The relevant field evolutions are measured with a third‐order NMR field camera. Algebraic reconstruction is then formulated such as to jointly minimize artifacts and noise in the resulting image. It is solved by an iterative conjugate‐gradient algorithm that uses explicit matrix‐vector multiplication to accommodate arbitrary net encoding. The feasibility and benefits of this approach are demonstrated by examples of diffusion imaging. In a phantom study, it is shown that higher order reconstruction largely overcomes variable image distortions that diffusion gradients induce in EPI data. In vivo experiments then demonstrate that the resulting geometric consistency permits straightforward tensor analysis without coregistration. Magn Reson Med, 2011.

A transmit/receive system for magnetic field monitoring of in vivo MRI.

Magnetic field monitoring with NMR probes has recently been introduced as a means of measuring the actual spatiotemporal magnetic field evolution during individual MR scans. Receive‐only NMR probes as used thus far for this purpose impose significant practical limitations due to radiofrequency (RF) interference with the actual MR experiment. In this work these limitations are overcome with a transmit/receive (T/R) monitoring system based on RF‐shielded NMR probes. The proposed system is largely autonomous and protected against RF contamination. As a consequence the field probes can be positioned freely and permit monitoring imaging procedures of arbitrary geometry and angulation. The T/R approach is also exploited to simplify probe manufacturing and remove constraints on material choices. Probe miniaturization permits monitoring imaging scans with nominal resolutions on the order of 400 μm. The added capabilities of the new probes and system are demonstrated by first in vivo results, obtained with monitored gradient‐echo and spin‐echo echo‐planar imaging (EPI) scans. Magn Reson Med, 2009.

Gradient system characterization by impulse response measurements with a dynamic field camera.

This work demonstrates a fast, sensitive method of characterizing the dynamic performance of MR gradient systems. The accuracy of gradient time‐courses is often compromised by field imperfections of various causes, including eddy currents and mechanical oscillations. Characterizing these perturbations is instrumental for corrections by pre‐emphasis or post hoc signal processing. Herein, a gradient chain is treated as a linear time‐invariant system, whose impulse response function is determined by measuring field responses to known gradient inputs. Triangular inputs are used to probe the system and response measurements are performed with a dynamic field camera consisting of NMR probes. In experiments on a whole‐body MR system, it is shown that the proposed method yields impulse response functions of high temporal and spectral resolution. Besides basic properties such as bandwidth and delay, it also captures subtle features such as mechanically induced field oscillations. For validation, measured response functions were used to predict gradient field evolutions, which was achieved with an error below 0.2%. The field camera used records responses of various spatial orders simultaneously, rendering the method suitable also for studying cross‐responses and dynamic shim systems. It thus holds promise for a range of applications, including pre‐emphasis optimization, quality assurance, and image reconstruction. Magn Reson Med, 2013.

Diffusion-weighted imaging of the entire spinal cord

In spite of their diagnostic potential, the poor quality of available diffusion‐weighted spinal cord images often restricts clinical application to cervical regions, and improved spatial resolution is highly desirable. To address these needs, a novel technique based on the combination of two recently presented reduced field‐of‐view approaches is proposed, enabling high‐resolution acquisition over the entire spinal cord. Field‐of‐view reduction is achieved by the application of non‐coplanar excitation and refocusing pulses combined with outer volume suppression for removal of unwanted transition zones. The non‐coplanar excitation is performed such that a gap‐less volume is acquired in a dedicated interleaved slice order within two repetition times. The resulting inner volume selectivity was evaluated in vitro. In vivo diffusion tensor imaging data on the cervical, thoracic and lumbar spinal cord were acquired in transverse orientation in each of four healthy subjects. An in‐plane resolution of 0.7 × 0.7 mm2 was achieved without notable aliasing, motion or susceptibility artifacts. The measured mean ± SD fractional anisotropy was 0.69 ± 0.11 in the thoracic spinal cord and 0.75 ± 0.07 and 0.63 ± 0.08 in cervical and lumbar white matter, respectively. Copyright

MIDA: A Multimodal Imaging-Based Detailed Anatomical Model of the Human Head and Neck

Computational modeling and simulations are increasingly being used to complement experimental testing for analysis of safety and efficacy of medical devices. Multiple voxel- and surface-based whole- and partial-body models have been proposed in the literature, typically with spatial resolution in the range of 1–2 mm and with 10–50 different tissue types resolved. We have developed a multimodal imaging-based detailed anatomical model of the human head and neck, named “MIDA”. The model was obtained by integrating three different magnetic resonance imaging (MRI) modalities, the parameters of which were tailored to enhance the signals of specific tissues: i) structural T1- and T2-weighted MRIs; a specific heavily T2-weighted MRI slab with high nerve contrast optimized to enhance the structures of the ear and eye; ii) magnetic resonance angiography (MRA) data to image the vasculature, and iii) diffusion tensor imaging (DTI) to obtain information on anisotropy and fiber orientation. The unique multimodal high-resolution approach allowed resolving 153 structures, including several distinct muscles, bones and skull layers, arteries and veins, nerves, as well as salivary glands. The model offers also a detailed characterization of eyes, ears, and deep brain structures. A special automatic atlas-based segmentation procedure was adopted to include a detailed map of the nuclei of the thalamus and midbrain into the head model. The suitability of the model to simulations involving different numerical methods, discretization approaches, as well as DTI-based tensorial electrical conductivity, was examined in a case-study, in which the electric field was generated by transcranial alternating current stimulation. The voxel- and the surface-based versions of the models are freely available to the scientific community.

High-resolution diffusion tensor imaging of prostate cancer using a reduced FOV technique.

OBJECTIVE Diffusion tensor imaging (DTI) offers the promise of improved tumor localization in prostate cancer but the technique suffers from susceptibility-induced artifacts that limit the achievable resolution. The present work employs a reduced field-of-view technique that enables high-resolution DTI of the prostate at 3T. Feasibility of the approach is demonstrated in a clinical study including 26 patients and 14 controls. MATERIALS AND METHODS Reduced field-of-view acquisition was established by non-coplanar application of the excitation and the refocusing pulse in conjunction with outer volume suppression. Accuracy for cancer detection of apparent diffusion coefficient (ADC) mapping and T2-weighted imaging was calculated and compared with reference to the findings of trans-rectal ultrasound-guided octant biopsy. Mean ADCs and fractional anisotropy (FA) values in the patients with positive and negative biopsies were compared to each other and to the controls. RESULTS Fine anatomical details were successfully depicted on the ADC maps with sub-millimeter resolution. Accuracy for prostate cancer detection was 73.5% for ADC maps and 71% for T2-weighted images, respectively. Cohens kappa (κ=0.48) indicated moderate agreement of the two methods. The mean ADCs were significantly lower, the FA values higher, in the patients with positive biopsy than in the patients with negative biopsy and the controls. Monte Carlo simulations showed that the FA values, but not the ADCs, were slightly overestimated. Bootstrap analysis revealed that the ADC, but not the FA value, is a highly repeatable marker. CONCLUSION In conclusion, the present work introduces a new approach for high-resolution DTI of the prostate enabling a more accurate detection of focal tumors especially useful in screening populations or as a potential navigator for image-guided biopsy.

Real-time feedback for spatiotemporal field stabilization in MR systems

MR imaging and spectroscopy require a highly stable, uniform background field. The field stability is typically limited by hardware imperfections, external perturbations, or field fluctuations of physiological origin. The purpose of the present work is to address these issues by introducing spatiotemporal field stabilization based on real‐time sensing and feedback control.

Retrospective correction of physiological field fluctuations in high‐field brain MRI using concurrent field monitoring

Magnetic field fluctuations caused by subject motion, such as breathing or limb motion, can degrade image quality in brain MRI, especially at high field strengths. The purpose of this study was to investigate the feasibility of retrospectively correcting for such physiological field perturbations based on concurrent field monitoring.

SELOVS: Brain MRSI localization based on highly selective T1- and B1-insensitive outer-volume suppression at 3T

In vivo, high‐field MR spectroscopic imaging (MRSI) profits from signal‐to‐noise ratio (SNR) gain and increased spectral resolution. However, bandwidth limitations of slice‐selective excitation and refocusing pulses lead to strong chemical‐shift displacement at high field strength when using conventional MRSI localization based on PRESS. Consequential metabolic information, particularly of border regions such as cortical brain tissue, is distorted. In addition, lipid contamination remains a major confound. To address these problems it is proposed to abandon PRESS selection and rely on a novel scheme of highly selective T1‐ and B1‐insensitive outer‐volume suppression in combination with slice‐selective spin‐echo acquisition for brain MRSI. Multiple cycles of overlapping suppression slabs are applied with flip angles optimized to account for tissue‐dependent T1 relaxation times and band crossings. Broadband frequency modulated saturation pulses with polynomial phase‐response are utilized in order to minimize chemical‐shift displacement. Efficacy of the outer‐volume suppression sequence was simulated and evaluated in vitro and in vivo. Brain MRSI localization at 3T was significantly improved and reliable suppression of short‐range lipid contamination enabled, leading to substantial enhancement of spectral quality, particularly in cortical tissue. Hence, the new method holds potential to expand the applicability of high‐field MRSI to the entire brain. Magn Reson Med, 2007.