Free-breathing abdominal T1 mapping using an optimized MR fingerprinting sequence

Abstract

In this work, we propose a free-breathing magnetic resonance fingerprinting (MRF) method that can be used to obtain B₁⁺ -robust quantitative T₁ maps of the abdomen in a clinically acceptable time. A three-dimensional MRF sequence with a radial stack-of-stars trajectory was implemented, and its k-space acquisition ordering was adjusted to improve motion-robustness in the context of MRF. The flip angle pattern was optimized using the Cramér-Rao Lower Bound, and the encoding efficiency of sequences with 300, 600, 900 and 1800 flip angles was evaluated. To validate the sequence, a movable multicompartment phantom was developed. Reference multiparametric maps were acquired under stationary conditions using a previously validated MRF method. Periodic motion of the phantom was used to investigate the motion-robustness of the proposed sequence. The best performing sequence length (600 flip angles) was used to image the abdomen during a free-breathing volunteer scan. When using a series of 600 or more flip angles, the estimated T₁ values in the stationary phantom showed good agreement with the reference scan. Phantom experiments revealed that motion-related artifacts can appear in the quantitative maps and confirmed that a motion-robust k-space ordering is essential. The in vivo scan demonstrated that the proposed sequence can produce clean parameter maps while the subject breathes freely. Using this sequence, it is possible to generate B₁⁺ -robust quantitative maps of T₁ and B₁⁺ next to M₀ -weighted images under free-breathing conditions at a clinically usable resolution within 5 min.

Keywords: abdominal imaging, Cramér-Rao lower bound, magnetic resonance fingerprinting, quantitative imaging, respiratory motion.

FIGURE 1

Proposed motion-robust 3D MRF sequence. (A), The data are acquired using a repeated flip angle sequence, each generating a fingerprint encoding T₁ and B₁⁺. Every repetition is preceded by a nonselective adiabatic inversion pulse. Each repetition is indicated with a separate color. (B), In the normal ordering, each fingerprint sequence consists of M·N TRs. Each TR index generates a separate k-space. (C), In the motion-robust ordering, each fingerprint sequence consists of M sets, containing N TRs each. All readouts acquired during the TR indices corresponding to one set are grouped together to form a single k-space. (D), Readout lines acquired for the normal ordering. Every TR, the readout angle is incremented with the golden angle (Δφ = 111.25 degrees), while the partition index stays the same. This results in a single undersampled k-space for every TR index. All lines of the same color are acquired sequentially during the same repetition. (E), Readout lines acquired for the motion-robust ordering. Every TR, the readout angle stays the same, but the partition index is increased. Note how readouts in adjacent partitions with the same angle are acquired successively in the motion-robust ordering, while in the normal ordering, the next partition is sampled during the next repetition of the sequence

FIGURE 2

Experimental setup. (A), The phantom placed on a movable cart. This cart is placed on a ramp. Under the influence of gravity, the cart moves to the right, while a rope connected to a motor can pull the cart to the left. (B), The 18-channel body coil that was used to acquire the data was placed over the movable phantom

FIGURE 3

Results of the flip angle optimization. (A), Optimized flip angle patterns for 300, 600, 900 and 1800 TRs (left to right, top to bottom). Note the smoothly varying flip angles within each set. Indicated are the main T₁-encoding part (red arrows), the main B₁⁺-encoding part (green arrows) and the delay (light blue arrows). (B), Normalized fingerprints for different combinations of T₁ and B₁⁺ values for the same four flip angle patterns

FIGURE 4

T₁ maps of the phantom scan without (left) and with (right) motion, for both ordering schemes, and for all four sequence lengths for which the flip angles were optimized. For the maps with motion, the motion speeds with the most severe artifacts were selected for each sequence separately. Note the severe motion-related artifacts visible when using the normal ordering, which are greatly reduced by using the motion-robust ordering

FIGURE 5

Correlation plots between the estimated T₁ values (vertical axis) and the corresponding T₁ values from the reference scan (horizontal axis) for each map in Figure 4. Each point indicates the mean T₁ values of both scans within one single tube, with the standard deviation depicted as error bars. The dashed line is the identity line. Note the increased deviation from the identity line for the normal ordering compared with the motion-robust ordering

FIGURE 6

M₀-weighted (top row), quantitative T₁ (middle row) and quantitative B₁⁺ (bottom row) maps of the phantom, both without (left) and with (right) motion, and with both ordering schemes, using the optimized sequence with 600 flip angles and three shots. Note the motion-related artifacts in the parameter maps acquired with the normal ordering

FIGURE 7

In vivo M₀-weighted (top row), quantitative T₁ (middle row) and quantitative B₁⁺ (bottom row) maps of all six volunteers for the normal ordering and the motion-robust ordering, using the optimized sequence with 600 flip angles and three shots. The motion-robust ordering reveals more details in the T₁ map and removes the motion-related artifacts visible in the maps of all three parameters. Areas with strong artifacts are highlighted with yellow ROIs

FIGURE 8

In vivo M₀-weighted, (top row) quantitative T₁ (middle row) and quantitative B₁⁺ (bottom row) maps of volunteer 4, as estimated by four different reconstruction methods. The first two columns correspond to the 3D MRF sequence with the normal ordering and the motion-robust ordering, respectively, as in Figure 7. For the third column, the motion-robust ordering was used, but the relative B₁⁺ value in the dictionary was fixed to 1. The top figure in the fourth column shows the estimated T₁ map from the DESPOT1 sequence. Notice the severe B₁⁺ artifacts when using this last method. The bottom figure in the fourth column shows the difference map between the T₁ maps when using the motion-robust ordering, with and without fixing B₁⁺ during matching. Notice that the difference map resembles the B₁⁺ pattern

FIGURE 9

In vivo quantitative M₀ (top row), T₁ (middle row) and B₁⁺ (bottom row) maps of volunteer 6, at a resolution of 1.3 mm × 1.3 mm × 3.0 mm (left two columns) and 1.0 mm × 1.0 mm × 3.0 mm (right two columns). Each image consists of an axial, a coronal and a sagittal view. Notice the motion artifacts when using the normal ordering at both resolutions