3D isotropic high‐resolution diffusion‐weighted MRI of the whole brain with a motion‐corrected steady‐state free precession sequence

The main obstacle to high‐resolution (<1.5 mm isotropic) 3D diffusion‐weighted MRI is the differential motion‐induced phase error from shot‐to‐shot. In this work, the phase error is addressed with a hybrid 3D navigator approach that corrects motion‐induced phase in two ways. In the first, rigid‐body motion is corrected for every shot. In the second, repeatable nonrigid‐body pulsation is corrected for each portion of the cardiac cycle. These phase error corrections were implemented with a 3D diffusion‐weighted steady‐ state free precession pulse sequence and were shown to mitigate signal dropouts caused by shot‐to‐shot phase inconsistencies compared to a standard gridding reconstruction in healthy volunteers. The proposed approach resulted in diffusion contrast more similar to the contrast observed in the reference echo‐planer imaging scans than reconstruction of the same data without correction. Fractional anisotropy and Color fractional anisotropy maps generated with phase‐corrected data were also shown to be more similar to echo‐planer imaging reference scans than those generated without phase correction. Magn Reson Med 70:466–478, 2013. © 2012 Wiley Periodicals, Inc.     

Fast diffusion‐weighted steady state free precession imaging of in vivo knee cartilage

Quantification of molecular diffusion with steady state free precession (SSFP) is complicated by the fact that diffusion effects accumulate over several repetition times (TR) leading to complex signal dependencies on transverse and longitudinal magnetization paths. This issue is commonly addressed by setting TR > T₂, yielding strong attenuation of all higher modes, except of the shortest ones. As a result, signal attenuation from diffusion becomes T₂ independent but signal‐to‐noise ratio (SNR) and sequence efficiency are remarkably poor. In this work, we present a new approach for fast in vivo steady state free precession diffusion‐weighted imaging of cartilage with TR << T₂ offering a considerable increase in signal‐to‐noise ratio and sequence efficiency. At a first glance, prominent coupling between magnetization paths seems to complicate quantification issues in this limit, however, it is observed that diffusion effects become rather T₂(ΔD ～ 1/10 ΔT₂) but not T₁ independent (ΔD ～ 1/2 ΔT₁) for low flip angles α ～ 10 ? 15°. As a result, fast high‐resolution (0.35 × 0.35 ? 0.50 × 0.50 mm² in‐plane resolution) quantitative diffusion‐weighted imaging of human articular cartilage is demonstrated at 3.0 T in a clinical setup using estimated T₁ and T₂ or a combination of measured T₁ and estimated T₂ values. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Absolute MR thermometry using time‐domain analysis of multi‐gradient‐echo magnitude images

MRI allows for absolute temperature measurements in substances containing two spectral resonances of which the frequency difference Δf(T) is related to absolute temperature. This frequency difference can be extracted from spectroscopic data. An image‐based MR technique that allows for the acquisition of spectroscopic data at high temporal and spatial resolution is the multi‐gradient‐echo sequence. In this work, the application of the multi‐gradient‐echo sequence for MR thermometry purposes was further developed. We investigated the possibility of postprocessing the multi‐gradient‐echo data into absolute temperature maps, using time‐domain analysis of the magnitude of the multi‐gradient‐echo signals. In this approach, instead of an indirect computation of Δf(T) from separately found frequencies, Δf(T) is a direct output parameter. In vitro experiments were performed to provide proof of concept for retrieving absolute temperature maps from the time‐domain analysis of multi‐gradient‐echo magnitude images. It is shown that this technique is insensitive to both field drift and local field disturbances. Furthermore, ex vivo bone marrow experiments were performed, using the fat resonance as a reference for absolute temperature mapping. It is shown that the postprocessing based on the magnitude signal in the time domain allows for the determination of Δf(T) in bone marrow. Magn Reson Med 64:239–248, 2010. © 2010 Wiley‐Liss, Inc.