Distortion-free imaging: A double encoding method (DIADEM) combined with multiband imaging for rapid distortion-free high-resolution diffusion imaging on a compact 3T with high-performance gradients

Abstract

Background: Distortion-free, high-resolution diffusion imaging using DIADEM (Distortion-free Imaging: A Double Encoding Method), proposed recently, has great potential for clinical applications. However, it can suffer from prolonged scan times and its reliability for quantitative diffusion imaging has not been evaluated.

Purpose: To investigate the clinical feasibility of DIADEM-based high-resolution diffusion imaging on a novel compact 3T (C3T) by evaluating the reliability of quantitative diffusion measurements and utilizing both the high-performance gradients (80 mT/m, 700 T/m/s) and the sequence optimization with the navigator acquisition window reduction and simultaneous multislice (multiband) imaging.

Study type: Prospective feasibility study.

Phantom/subjects: Diffusion quality control phantom scans to evaluate the reliability of quantitative diffusion measurements; 36 normal control scans for B₀ -field mapping; six healthy and two patient subject scans with a brain tumor for comparisons of diffusion and anatomical imaging.

Field strength/sequence: 3T; the standard single-shot echo-planar-imaging (EPI), multishot DIADEM diffusion, and anatomical (2D-FSE [fast-spin-echo], 2D-FLAIR [fluid-attenuated-inversion-recovery], and 3D-MPRAGE [magnetization prepared rapid acquisition gradient echo]) imaging.

Assessment: The scan time reduction, the reliability of quantitative diffusion measurements, and the clinical efficacy for high-resolution diffusion imaging in healthy control and brain tumor volunteers.

Statistical test: Bland-Altman analysis.

Results: The scan time for high in-plane (0.86 mm² ) resolution, distortion-free, and whole brain diffusion imaging were reduced from 10 to 5 minutes with the sequence optimizations. All of the mean apparent diffusion coefficient (ADC) values in phantom were within the 95% confidence interval in the Bland-Altman plot. The proposed acquisition with a total off-resonance coverage of 597.2 Hz wider than the expected bandwidth of 500 Hz in human brain could yield a distortion-free image without foldover artifacts. Compared with EPI, therefore, this approach allowed direct image matching with the anatomical images and enabled improved delineation of the tumor boundaries.

Data conclusion: The proposed high-resolution diffusion imaging approach is clinically feasible on C3T due to a combination of hardware and sequence improvements.

Level of evidence: 3 TECHNICAL EFFICACY: Stage 1 J. Magn. Reson. Imaging 2020;51:296-310.

Keywords: DIADEM; compact 3T; diffusion imaging; distortion-free; geometric distortion; multishot EPI.

Figure 1.

A multi-band DIADEM diffusion sequence diagram with a navigator-echo (a) and corresponding k-space (b). The navigator-echo (NE) acquisition window is reduced by four times compared to the signal acquisition window. Note that spin-warp phase-encoded and corresponding rewinder gradients, see Δk_s, (blue) are applied before and after the spin-warp phase-encoded signal acquisition, and multi-band acquisition scheme (green) is added on the slice selection axis during the signal and the navigator echo acquisition. In (b), a summarized description of the k-space including the important dimensions and various parameters applied in this method is presented. The abbreviations are: FOV: field of view, rFOV: reduced FOV, rR: reduced resolution, esp: echo spacing, PI: parallel imaging.

Figure 2.

The off-resonance frequency range within masked brain volume in the standard space. Each individual off-resonance range on the whole-body 3T (WB3T, a) and the compact 3T (C3T, b), and the group data (c) are shown. In addition, an averaged off-resonance frequency map for the data for the entire group, including C3T and WB3T data is presented in (d).

Figure 3.

Diffusion quality control phantom measurements using EPI and DIADEM: Non-diffusion weighted image (DWI) and sum of all 6 diffusion weighted images (isoDWI) and corresponding ADC map were shown in the left. The intrinsic (i.e., acquired) image resolution is 0.86×0.86×4 mm³ (Table 1–5 and 1–8). On the right and top, the mean and standard deviation of ADC values measured from seven vials (shown in the zoomed image) were shown. Note that the seven vials from 1^st to 7^th contain the PVP of 0%, 0%, 10%, 20%, 30%, 40%, and 50%, respectively. As a reference, gray dashed lines show the standard deviation (SD) of ADC values measured on thirteen 3T MR scanners at 11 different sites in a previous study. On the right and bottom, a Bland–Altman plot shows agreement of the mean ADC values in between this (i.e. both EPI and DIADEM) and the previous study. The mean and the 95% confidence interval (±1.96 SD) lines were drawn based on the results from the previous study.

Figure 4.

High-resolution diffusion imaging on the standard whole-body (a) and the compact 3T MRI (b and c) acquired using single-shot EPI (a and b) and the proposed multi-shot DIADEM approach (c). The susceptibility-induced off-resonance effect during EPI and DIADEM acquisitions on the compact 3T are shown as a masked frequency map covering brain areas in (d). A slice from each non-DWI (upper), DWI image (middle), and color-coded FA map (bottom) enlarged from a dashed rectangular area shown in (a) was chosen for demonstration. In the color-coded FA maps (bottom row), green, red, and blue colors represent the dominant diffusion along the anterior-to-posterior (AP), right-to-left (RL), superior-to-inferior (SI) directions, respectively. Yellow and white arrows indicate the loss of spatial resolution caused by eddy-current- and susceptibility-induced geometric distortions. The intrinsic image resolution is 0.94(AP)×0.94(RL)×4(SI) mm³ (Table 1–1-3).

Figure 5.

The proposed diffusion (a-d) and conventional anatomical images (e-f) with a high in-plane resolution. A slice from non-DWI (a), isoDWI obtained by the summation of all 6 DWIs (b), color-coded FA map (c), and MD map (d) are displayed. The corresponding slices from the anatomical 2D-FSE (e) and FLAIR data (f) are shown for comparison. The intrinsic image resolution is 0.86×0.86×4 mm³ for both diffusion (Table 1–5) and anatomical data (Table 1–9 and 1–10).

Figure 6.

Whole brain diffusion imaging with an isotropic resolution of 1.4 mm³. For demonstration purpose, reformatted coronal, reformatted sagittal, and axial slices of non-DWI (a), isoDWI (b), FA (c), and a color-coded FA map (e) selected from a 3D brain volume. The imaging protocols are shown in Table 1–4.

Figure 7.

Comparison of color-coded FA maps between single-band and multi-band DIADEM. Two representative slices in in-vivo brainstem (first column) and cerebrum (second column) were chosen for demonstration. A color-coded vector map was overlaid on the FA map to demonstrate the cortical diffusion anisotropy and its radial diffusion orientation in in-vivo human brain acquired with single-band and multi-band DIADEM (third column). The intrinsic (i.e., acquired) image resolution is 0.86×0.86×4 mm³ (Table 1–5 and 1–6).

Figure 8.

Comparison of ADC maps between single-band (SB) and multi-band (MB) DIADEM. As two representative slices, central (first column, top) and superior slices (second column, top) were chosen for demonstration. Histograms (third column, top) and Bland–Altman plots (bottom row) of the ADC difference between single-band and multi-band DIADEM in white matter areas in all slices were shown. In each Bland-Altman plot, a matched and random sample of 10% (~7000 voxels) from the entire white matter area (FA>0.4 and ADC<10×10⁻⁴ mm²/s) was compared for better demonstration.

Figure 9.

DWI acquired with single-shot EPI and high spatial resolution distortion-free imaging (DIADEM) in slices with compressed (a-f) and stretched distortion (g-l) in a patient with a meningioma superior to the right orbit with the standard EPI-based non-DWI (a) and trace (b) images and DIADEM-based non-DWI (c), isoDWI (d), and MD (e) images with a corresponding image from MPRAGE (f). The off-resonance frequency maps corresponding to the slices with compressed (a-f) and stretched distortion (g-l) are shown in (m) and (n), respectively. The full FOV (upper row) and zoomed images (bottom row) are shown to demonstrate the geometrical difference between EPI (a-b and g-h) and DIADEM images (c-e and i-k) more clearly. Note that considerable distortion occurred on the EPI (arrow in a and g), which was not present when the DIADEM was used (curved arrow in c and i). Bright signal intensity seen in orbital fat surrounding the eyes is shown in the reformatted MPRAGE image (yellow arrow in f). The image resolutions for EPI, DIADEM, and MPRAGE are 1.72×0.86×4 (Table 1–8), 0.86×0.86×2.7 (Table 1–5), and 1×1.2×1 mm³ (Table 1–11).

Figure 10.

Mass effect from a left vestibular schwannoma that deforms and displaces the brainstem as demonstrated with full FOV (upper row) and zoomed images (bottom row) including MPRAGE (a) and DIADEM with isoDWI (b), ADC (c), FA (d), and color-coded FA (e). A corresponding color-coded FA map obtained from a healthy volunteer is displayed in (f) for comparison. Note that identical imaging parameters in the scan for the healthy volunteer were applied, except for the reduced NE acquisition window. The acquired image resolutions for DIADEM and reformatted MPRAGE are 0.86×0.86×4 (Table 1–5) and 1×1.2×1 mm³ (Table 1–11).