Diffusion tensor MR imaging and fiber tractography: technical considerations

Abstract

This second article of the 2-part review builds on the theoretic background provided by the first article to cover the major technical factors that affect image quality in diffusion imaging, including the acquisition sequence, magnet field strength, gradient amplitude, and slew rate as well as multichannel radio-frequency coils and parallel imaging. The sources of many common diffusion image artifacts are also explored in detail. The emphasis is on optimizing these technical factors for state-of-the-art diffusion-weighted imaging and diffusion tensor imaging (DTI) based on the best available evidence in the literature. An overview of current methods for quantitative analysis of DTI data and fiber tractography in clinical research is also provided.

Fig 1.

Shift of DWIs due to eddy currents. A, The b = 0 s/mm² image with the brain-CSF interface outlined (yellow). B, Corresponding DWI with the same outline (red), unchanged in position compared with A, shows a shift of the brain anteriorly, most easily seen at the ventricular margins and at the occipital lobes. C, Another DWI with the diffusion gradient pointing in a different direction than in B shows a different degree of anterior shift.

Fig 2.

Combined DWI image shows pronounced artifacts at the anterior temporal lobes and around the superior cerebellar vermis (black lines) due to mechanical vibration.

Fig 3.

Pulse sequence diagrams show the benefits of parallel imaging for DWI. At an acceleration factor of R = 2, the echo-train length for the single-shot EPI acquisition is only half as long. This is reflected in a shorter readout time (t_acq) and allows the echo train to be better centered at the peak of the spin-echo, improving SNR, decreasing T2 and T2* contrast blurring, and reducing off-resonance artifacts that cause geometric distortions. The shorter readout time also enables a reduction of TE, further improving SNR and reducing geometric distortion. However, the use of parallel imaging results in an intrinsic loss of SNR that may offset the aforementioned SNR gains. RF indicates radio-frequency.

Fig 4.

Parallel imaging ameliorates susceptibility-induced geometric distortions and T2 and T2* contrast blurring in 3T DWI performed with a single-shot echo-planar sequence. A, The b = 1000 s/mm² DWI image acquired at 3T without parallel imaging shows warping of the pons and anterior temporal lobes. There is also signal-intensity void with adjacent regions of signal-intensity pileup in the temporal lobes. These are typical artifacts encountered with 3T ssEPI DWI due to susceptibility effects from the adjacent air-filled mastoid sinuses and sphenoid sinus. B, The b = 1000 s/mm² 3T DWI image acquired at the same axial level with ASSET parallel imaging (R = 2) demonstrates reduced foreshortening of the pons and reduced warping and signal-intensity distortions in the temporal lobes. There is also mitigation of contrast blurring, seen as improved definition of the cerebellar fissures and folia as well as better gray-white matter differentiation in the occipital lobes.

Fig 5.

3T-versus-7T DTI with 36 diffusion-encoding directions at b = 3000 s/mm² and 2.0 × 2.0 × 2.0 mm isotropic voxel resolution. Directionally encoded color FA maps at the axial level of cingulum bundles and the callosal striations are shown for 3T (A) and for 7T (B) in a healthy adult volunteer. Both scanners were equipped with 40 mT/m gradients and 8-channel phased-array head coils, and ASSET parallel imaging was used with an acceleration factor of 2. The standard DTI color conventions are used, with red representing left-right fiber orientation, green representing anteroposterior, and blue representing craniocaudal. With this combination of high spatial resolution and very strong diffusion weighting, the 3T image appears grainy because of inadequate SNR. However, with identical scanning parameters, the additional SNR at 7T produces a higher quality image. Parallel imaging is essential for SS-EPI at ultra-high field to combat the increased susceptibility artifacts as well as the signal intensity loss and contrast blurring due to shorter T2 ad T2* relaxation times.

Fig 6.

Unfolding artifacts from the globes in ASSET-accelerated DWI. A, The b = 0 s/mm² image acquired at 3T with an ASSET acceleration factor of R = 2 shows unfolding artifacts from the distorted high-signal-intensity globes appearing as dark bands in the occipital regions (black arrows) as well as a bright band in the anterior left temporal lobe (black arrowhead). B, The unfolding artifacts are not apparent on the combined DWI image because the globes contain fluid with high diffusivity; therefore, the globes signal intensity is suppressed by the diffusion gradients. C, However, the unfolding artifacts are again apparent on the ADC map (white arrows and arrowhead) because the b = 0 s/mm² image is required for ADC calculation (equation 5, Part I).