Diffusion weighted hyperpolarized 129 Xe MRI of the lung with 2D and 3D (FLORET) spiral

Abstract

Purpose: To enable efficient hyperpolarized ¹²⁹ Xe diffusion imaging using 2D and 3D (Fermat Looped, ORthogonally Encoded Trajectories, FLORET) spiral sequences and demonstrate that ¹²⁹ Xe ADCs obtained using these sequences are comparable to those obtained using a conventional, 2D gradient-recalled echo (GRE) sequence.

Theory and methods: Diffusion-weighted ¹²⁹ Xe MRI (b-values = 0, 7.5, 15 s/cm² ) was performed in four healthy volunteers and one subject with lymphangioleiomyomatosis using slice-selective 2D-GRE (scan time = 15 s), slice-selective 2D-Spiral (4 s), and 3D-FLORET (16 s) sequences. Experimental SNRs from b-value = 0 images ( $\mathrm{SNR}{0}_{\mathrm{EX}}$ ) and mean ADC values were compared across sequences. In two healthy subjects, a second b = 0 image was acquired using the 2D-Spiral sequence to map flip angle and correct RF-induced, hyperpolarized signal decay at the voxel level, thus improving regional ADC estimates.

Results: Diffusion-weighted images from spiral sequences displayed image quality comparable to 2D-GRE and produced sufficient $\mathrm{SNR}{0}_{\mathrm{EX}}$ (16.8 ± 3.8 for 2D-GRE, 21.2 ± 3.5 for 2D-Spiral, 20.4 ± 3.5 for FLORET) to accurately calculate ADC. Whole-lung means and SDs of ADC obtained via spiral were not significantly different (P > 0.54) from those obtained via 2D-GRE. Finally, 2D-Spiral images were corrected for signal decay, which resulted in a whole-lung mean ADC decrease of ˜15%, relative to uncorrected images.

Conclusions: Relative to GRE, efficient spiral sequences allow ¹²⁹ Xe diffusion images to be acquired with isotropic lung coverage (3D), higher $\mathrm{SNR}$ (2D and 3D), and three-fold faster (2D) within a single breath-hold. In turn, shortened breath-holds enable flip-angle mapping, and thus, allow RF-induced signal decay to be corrected, increasing ADC accuracy.

Keywords: ADC; FLORET; GRE; diffusion; hyperpolarized 129Xe; spiral.

FIGURE 1

¹²⁹Xe diffusion sequences with bipolar diffusion encoding gradients (Δ = δ = 3.5 ms) placed between excitation and acquisition. b‐values were acquired sequentially (b = 0 to b _max, inner green loop) before progressing to the next line of k‐space (middle blue loop). A, 2D‐GRE pulse sequence. All lines of k‐space for all b‐values were acquired before progressing to the next slice (outer purple loop). B, 2D‐Spiral sequence. Archimedean spiral (uniform density) readout was implemented with the same diffusion encoding and acquisition loop order as (A). C, FLORET using one‐hub (linear ordering). b‐values were acquired first (green inner loop) before moving to the next projection (blue outer loop).

FIGURE 2

Theoretical signal‐to‐noise ratio, $\mathit{SNR}{0}_{\mathit{TH}}$ as a function of the flip angle, $\alpha$, using parameters in Table 1 and Eq. 3 (2D‐GRE) and Eq. 5 (2D‐Spiral and FLORET). For each sequence, $\mathit{SNR}{0}_{\mathit{TH}}$ is expected to be maximized at specific flip angle (color‐matched triangles above curves), with FLORET displaying decreased $\mathit{SNR}{0}_{\mathit{TH}}$, relative to the 2D‐GRE. 2D‐Spiral is expected to produce substantially higher $\mathit{SNR}{0}_{\mathit{TH}}$ than either 2D‐GRE or FLORET. For all three sequences, the maximum achievable $\mathit{SNR}{0}_{\mathit{TH}}$ only slightly exceeded (i.e., <4%) the signal expected using ${\alpha }_{\mathit{\text{global}}}$ as defined in Eq. 10 (dashed vertical lines).

FIGURE 3

Comparison of b₀ images from a healthy female volunteer (47 y). Each row corresponds to the sequences used: top, 2D‐GRE; middle, 2D‐Spiral; and bottom, FLORET. Scan time was 15 s (0.69 ms per voxel) for 2D‐GRE, 4 s (0.16 ms per voxel) for 2D‐Spiral and 16 s (0.06 ms per voxel) for FLORET. Images obtained using all sequences showed qualitatively good agreement in depicting structural features.

FIGURE 4

Comparison of b₀ images from a subject with LAM (female, 51 y). Row corresponds to the sequences used: top, 2D‐GRE; middle, 2D‐Spiral; and bottom, 3D FLORET. Scan time was 15 s (0.69 ms per voxel) for 2D‐GRE, 4 s (0.16 ms per voxel) for 2D‐Spiral and 16 s (0.06 ms per voxel) for FLORET. All sequences showed minimal image artifacts and good correlation of structural features. For example, ventilation defects (red, yellow and green arrows) were consistently observed in the same regions.

FIGURE 5

$\mathit{SNR}{0}_{\mathit{EX}}$ and $\mathit{NSNR}{0}_{\mathit{EX}}$ across subjects and sequences. A, $\mathit{SNR}{0}_{\mathit{EX}}$ showed no significant difference between the three sequences (P > 0.15). B, No significant difference in $\mathit{NSNR}{0}_{\mathit{EX}}$ was observed between the 2D sequences (P = 0.56). However, FLORET displayed a significant increase (˜2.5‐fold) in $\mathit{NSNR}{0}_{\mathit{EX}}$ over both 2D‐GRE and 2D‐Spiral (P = 0.007).

FIGURE 6

Representative slices from ADC maps of all subjects and sequences: First column, 2D‐GRE; second column, 2D‐Spiral; and third column, FLORET. The fourth column shows corresponding ADC histograms (Pixel count [P.C.] is normalized to account for the large number of voxels [˜10‐fold larger than 2D‐GRE or 2D‐Spiral] acquired in FLORET). No obvious difference in mean ADC or distribution was observed between the three sequences. Note that, the 5 y old subject could not hold his breath to the end of the 2D‐GRE scan. Exhalation resulted in lower $\mathit{SNR}$, 0.5‐fold higher mean ADC, and a one‐fold wider ADC distribution.

FIGURE 7

ADC comparison across sequences. A, Mean ADC across subjects and sequences. No significant difference in mean ADC across all subjects between the 2D‐GRE and 2D‐spiral (P _mean  = 0.54) or FLORET (P _mean  = 0.69). B, Mean ADC increased linearly with age (R ²  > 0.82; P < 0.05) for healthy subjects (i.e., excluding the LAM subject) as expected and showed no significant difference in best fit slopes (P _rate  >0.82) between spiral and 2D‐GRE sequences. Additionally, the LAM subject displayed a nearly two‐fold higher mean ADC relative to a similarly aged healthy subject.

FIGURE 8

Correcting RF induced signal decay using 2D‐Spiral. A, Representative slice of the uncorrected b‐value images for a healthy subject (12 y old). B, Diffusion‐weighted images corrected using Eq. 9 and the flip angle map calculated from Eq. 8 (right side of panel) [The bright spot in the right lung is likely an airway]. C, Mean signal of the uncorrected, globally corrected and locally corrected images across the b‐values. D, ADC maps showing uncorrected, globally corrected, and locally corrected data. The corrections reduced the mean ADC by ˜22% and ˜12% with the global and local corrections, respectively. ADC difference map (global – local correction) is shown to the right and demonstrates overcorrection and an increase in mean ADC of 0.0033 ± 0.0016 cm²/s using global correction. E, Histograms of the uncorrected and globally and locally corrected ADC maps showing the ADC distribution is shifted to lower values and the mean is reduced by 0.007 and 0.004 cm²/s using the global and local corrections, respectively.