SENSE‐DTI at 3 T

Abstract

While holding vast potential, diffusion tensor imaging (DTI) with single-excitation protocols still faces serious challenges. Limited spatial resolution, susceptibility to magnetic field inhomogeneity, and low signal-to-noise ratio (SNR) may be considered the most prominent limitations. It is demonstrated that all of these shortcomings can be effectively mitigated by the transition to parallel imaging technology and high magnetic field strength. Using the sensitivity encoding (SENSE) technique at 3 T, brain DTI was performed in nine healthy volunteers. Despite enhanced field inhomogeneity, parallel acquisition permitted both controlling geometric distortions and enhancing spatial resolution up to 0.8 mm in-plane. Heightened SNR requirements were met in part by high base sensitivity at 3 T. Diffusion tensor imaging (1,2) is a promising noninvasive method for studying white matter structure of the human brain in vivo. Based on the concept of anisotropic water diffusion across tissue, the measurement of 3D diffusion properties, as described by a local diffusion tensor, allows the characterization of the axonal architecture of white matter networks. For that purpose, a 3D tracking of axonal projections, known as fiber tracking (3-7), is required. However, the low SNR and the limited spatial resolution (8 -10) of the method severely impair its application. A serious resolution limit stems from the strong link between voxel size and SNR, the latter being inherently low due to diffusion weighting. Only improving the SNR of the initial diffusion-weighted (DW) images will enable better spatial resolution. Therefore, the use of high magnetic fields and the related SNR gain could considerably enhance the performance of DTI and fiber tracking.The calculation of the local diffusion tensor requires a set of DW images, acquired with diffusion gradients applied in at least six noncollinear directions, plus a reference image without diffusion weighting. The sequence most commonly used for DTI is spin-echo single-shot EPI (SE-sshEPI). It allows for whole brain coverage in an acceptable scan time and is insensitive to bulk motion due to its speed. Critical shortcomings of sshEPI are image blurring due to T* 2 decay during the EPI readout interval and off-resonance effects, caused by the long EPI echo train (11,12). Both effects scale with the main magnetic field B 0 , making the transition to higher field strength challenging. At 3 T, signal alteration and geometric distortion due to static resonance offset effects, e.g., in the vicinity of airtissue interfaces, are a serious problem when using sshEPIbased protocols.Recently, the potential of parallel imaging techniques, such as simultaneous acquisition of spatial harmonics (SMASH) (13) and sensitivity encoding (SENSE) (14), has been demonstrated for sshEPI in general (15), as well as for diffusion-weighted MRI (DWI) (16 -18) and DTI (19) at 1.5 T. Parallel imaging techniques were shown to significantly reduce EPI-related artifacts as a result of shortening the echo train by facto...

Optimizing signal‐to‐noise ratio of high‐resolution parallel single‐shot diffusion‐weighted echo‐planar imaging at ultrahigh field strengths

“…Figure 4 illustrates the rSNRs for different coil array sizes consisting of 16, 32, and 64 elements. As shown before (9,10), it can be observed at 7 T that the SNR of high-resolution single-shot SE DW EPI profits, under certain circumstances, from PI acquisition (Fig. 4a-c).…”

“…Nevertheless, it has been shown that PI can be beneficial for DW EPI in terms of SNR by allowing for a reduction of the echo time of the sequence which overcompensates for the SNR loss caused by applying PI (9,10). This effect is pronounced at low b-factors and high image resolutions, especially when no partial Fourier acquisition is applied.…”

“…Multishot approaches have been presented (4,5) which mitigate these artifacts but are less robust to motion and therefore require sophisticated phase correction methods (6). Alternatively, single-shot methods can be combined with parallel imaging (PI) techniques (7)(8)(9)(10) that permit shortening the readout train length and thereby reduce susceptibility-induced artifacts and T 2 * blurring. However, PI acceleration is limited by the inherent geometryfactor (g-factor) penalty that leads to spatially inhomogeneous noise amplification at high PI reduction factors (11).…”

Three‐dimensional fiber architecture of the nonpregnant human uterus determined ex vivo using magnetic resonance diffusion tensor imaging

“…The data were obtained from diffusion-weighted spin-echo measurements using two different protocols for DTI, namely, single-shot echo planar imaging (sshEPI) for scanning the entire uterus, and scans with enhanced in-plane resolution in the submillimeter realm with very low geometric distortions for the purpose of validation (two specimens). The parallel imaging technique Sensitivity Encoding (SENSE) (Pruessmann et al, 1999) was used in order to improve the quality of the images (Bammer et al, 2002;Jaermann et al, 2004).…”

High field MRI in the diagnosis of multiple sclerosis: high field–high yield?

“…These higher SNR values can be traded off in a different way: First of all, it allows an image acquisition in a faster way or with a higher spatial resolution (or a combination of both). Secondly, the higher SNR offers the chance to apply parallel imaging techniques which once again can reduce the scan time (due to shorter echo trains), but more importantly also reduces geometrical distortions which consequently lead to a better image quality [83][84][85]. The higher SNR can also be invested into higher spatial resolution which offers new possibilities for "fiber tracking" applications.…”