Time efficient whole-brain coverage with MR Fingerprinting using slice-interleaved echo-planar-imaging

Abstract

Magnetic resonance fingerprinting (MRF) is a promising method for fast simultaneous quantification of multiple tissue parameters. The objective of this study is to improve the coverage of MRF based on echo-planar imaging (MRF-EPI) by using a slice-interleaved acquisition scheme. For this, the MRF-EPI is modified to acquire several slices in a randomized interleaved manner, increasing the effective repetition time of the spoiled gradient echo readout acquisition in each slice. Per-slice matching of the signal-trace to a precomputed dictionary allows the generation of T₁ and T₂* maps with integrated B₁⁺ correction. Subsequent compensation for the coil sensitivity profile and normalization to the cerebrospinal fluid additionally allows for quantitative proton density (PD) mapping. Numerical simulations are performed to optimize the number of interleaved slices. Quantification accuracy is validated in phantom scans and feasibility is demonstrated in-vivo. Numerical simulations suggest the acquisition of four slices as a trade-off between quantification precision and scan-time. Phantom results indicate good agreement with reference measurements (Difference T₁: -2.4 ± 1.1%, T₂*: -0.5 ± 2.5%, PD: -0.5 ± 7.2%). In-vivo whole-brain coverage of T₁, T₂* and PD with 32 slices was acquired within 3:36 minutes, resulting in parameter maps of high visual quality and comparable performance with single-slice MRF-EPI at 4-fold scan-time reduction.

Figure 1

(A) Schematic diagram of the slice-interleaved MRF-EPI comprising multiple consecutive slice groups, each acquiring four slices with multiple EPI readouts; slice order is randomized within each measurement. Profiles of the repetition time (B) echo time (C) and flip angle variations (D) used for the proposed MRF measurements.

Figure 2

(A) Average normalized noise amplification and (B) dictionary matching precision of a Monte-Carlo simulation for a range of T₁ (1000–2500 ms) and T₂* (50–70 ms) values in dependence of the number of baseline images. Both graphs show only minor variations between 40–160 baseline images, corresponding to 1–4 slices.

Figure 3

(A) T₁ and T₂* maps obtained in phantom measurements using slice-interleaved MRF-EPI and the respective reference method IR-TSE and GRE. (B) Comparison of T₁, T₂* and proton density (PD) values of slice-interleaved MRF-EPI with the reference methods showing nearly identical average quantification in all phantoms. (C) Bland-Altman plot showing good agreement between the slice-interleaved MRF-EPI (T1_SI, T2*_SI) and the single-slice MRF-EPI (T1_SS, T2*_SS).

Figure 4

T₁, T₂* and proton density maps acquired with slice-interleaved MRF-EPI, 32 slices with a resolution of 1 × 1 × 3 mm were measured within a total measurement time of 3:36 minutes.

Figure 5

Exemplary in vivo T₁ and T₂* maps acquired in one subject with slice-interleaved and single-slice MRF-EPI. Both techniques achieve visually comparable image quality, with good white/grey matter delineation in the T₁ maps and susceptibility contrast weigthing in the T₂* maps. Intracranial calcification is clearly depicted by signal dropout in the T₂* map of both sequences (white arrow).

Figure 6

(A) T₁, T₂*, semi quantitative M₀, corrected proton density and field bias map of an MS patient with clearly visible lesions (white arrow), (B) Exemplary fingerprints from manually drawn ROIs in grey matter (GM), white matter (WM) and a lesion.