Synopsis

Volumetric T₁ mapping in the abdomen is desirable for whole liver assessment of hepatic diseases. In case of breath-hold imaging, accurate but time-consuming methods that sample the relaxation curve (IR or Look-Locker) are restricted to few slices only. To address these limitations, sparse Cartesian sampling with spatiotemporal incoherence is utilized to render 3-D Look-Locker within a single breath-hold possible. We demonstrate feasibility in both phantom and in-vivo measurements. The proposed method shows high agreement with a 2-D reference acquisition and enables an accurate mapping for a wide T₁ range, including very low values due to its high temporal resolution.

Introduction

Abdominal T₁ mapping can help diagnosing and staging hepatic diseases such as liver cirrhosis¹. Yet, accurate methods that are based on sampling the relaxation curve are usually limited to a few slices in case of breath-hold imaging. Common 3-D techniques are often based on a variable-flip-angle approach, which is B₁ sensitive, even when complemented with an additional B₁ mapping acquisition and correction². Look-Locker sequences are considered more accurate albeit time-consuming, restricting volumetric Look-Locker to static imaging only³. Sparse sampling with incoherence in both space and time can alleviate this problem. To this end, an existing 3-D CINE sequence prototype^4,5 was extended to support inversion pulses and FLASH contrast.

We investigate whether the increased signal of 3-D acquisitions in combination with a sparse spatiotemporally incoherent sampling of the relaxation curve in high temporal resolution can yield T₁ values in a wide range with high accuracy. To our knowledge, this is the first application of whole-liver T₁ mapping and 3-D Cartesian Look-Locker within a breath-hold. Experiments include both phantom and in-vivo measurements.

Materials and Methods

A Look-Locker T₁-mapping scheme with continuous sampling after an initial inversion pulse was used. We utilized a multi-TI CINE protocol with an IR-FLASH sequence featuring adiabatic inversion⁴. Time points in the reconstruction were assigned to contrasts after inversion pulses. For sufficiently high spatiotemporal sampling density, k-space segmentation with multiple inversions and therebetween a wait-time for free relaxation was introduced.

A variable-density spiral spokes pattern ensured Cartesian sampling with a high temporal resolution⁵ (~100ms), which allows to determine very low T₁ values. For improved k-space coverage, multiple spiral arms were sampled in each shot and the set of spokes was rotated successively by the golden angle per shot and TI (Figure 1). Time-resolved reconstructions were performed using a FISTA algorithm⁶, which incorporates wavelet regularization in both space and time domain. 40 iterations with spatial/temporal regularization weights of 0.0006/0.007 were used. T₁ maps were obtained using a phase-corrected multi-step parameter fitting utilizing a smoothed flip angle map.

A 3 T MR scanner (MAGNETOM Skyra, Siemens Healthcare, Erlangen, Germany) was used for all experiments. An 18-channel body coil was used for volunteers, a 20-channel head coil for phantom experiments. Reconstructions were compared against 2-D multi-slice reference measurements of a prototypical LL sequence by means of ROI mean and standard deviation:

In-vivo: axial slices from a 6-slice 2-D acquisition were compared against corresponding slices in 3-D

Phantom: assessment based on the T1–array of the NIST phantom⁷

3-D imaging parameters: FoV = 365x255x150mm³, matrix = 160x94x30, TR = 2.4ms, TE = 1ms, flip-angle = 6°, bandwidth = 1563Hz/Px, 19 TIs, ΔTI = 102ms, net acceleration = 15.4, acquisition window = 2s, wait time = 3.8s, 4 inversions.

2-D imaging parameters: FoV = 380x308mm², matrix size = 192x125 (1mm² interpolated), slice thickness = 5mm, TR = 3ms, TE = 1.3ms, flip-angle = 8°, bandwidth = 1530 Hz/px, 16 TIs, ΔTI = 225ms (2x acceleration), scan time = 22.8s (6 slices).

Results and Discussion

The 3-D+t image reconstructions took less than a minute using the scanner graphics hardware while T₁ mapping on the CPU (not parallelized) required 5–10s per slice. In-vivo results in Figure 2 show axial slices and a coronal reformation from the 3-D acquisition (A-C) in comparison to the 2-D reference (D,E). Figure 3 illustrates the signal recovery of the 3-D and 2-D acquisition in comparison. Labeled ROIs used for the in-vivo quantitative evaluation, which is presented in Table 1, are shown. Table 2 summarizes the results of the phantom evaluation.

Average hepatic T1 values of 799±32 and 810±44ms between the 2-D reference and 3-D show very high agreement in vivo. The phantom comparison shows excellent agreement with reference values for both methods in a wide range. Yet, only the 3-D acquisition with its short ΔTI, allowed determining phantom tubes with T₁ values as low as 60ms accurately. While the visual appearance between 2-D and 3-D is quite different due to image resolution, the 3-D acquisition, despite high acceleration, is hardly affected by artifacts and allows delineating most vessels and anatomical structures.

Conclusion

Acknowledgements

References

[1] Haimerl M, Verloh N, Zeman F, et al. Assessment of Clinical Signs of Liver Cirrhosis Using T1 Mapping on Gd-EOB-DTPA-Enhanced 3T MRI. PLoS ONE. 2014;8(12):e85658

[2] Deoni S. High-resolution T1 mapping of the brain at 3T with driven equilibrium single pulse observation of T1 with high-speed incorporation of RF field inhomogeneities (DESPOT1-HIFI). J Magn Reson Imaging. 2007;26(4):1106-1111

[3] Henderson E, McKinnon G, Lee, TY, et al. A Fast 3D Look-Locker Method for Volumetric T1. Mapping Magn Reson Imaging 1999;17(8):1163-1171

[4] Stalder AF, Speier P, Zenge M, et al. Cardiac Multi-Contrast CINE: Real-Time Inversion-Recovery Balanced Steady-State Free Precession Imaging with Compressed-Sensing and Motion-Propagation. In: Proceedings of the 22nd Annual Meeting of ISMRM. 2014.#431

[5] Wetzl J, Lugauer F, Schmidt M, et al. Free-Breathing, Self-Navigated Isotropic 3-D CINE Imaging of the Whole Heart Using Cartesian Sampling. In: Proceedings of the 24th Annual Meeting of ISMRM. 2016.#411

[6] Liu J, Rapin J, Chang T, et al. Dynamic Cardiac MRI Reconstruction with Weighted Redundant Haar Wavelets. In: Proceedings of the 20th Annual Meeting of ISMRM. 2012.#178

[7] Keenan K, Stupic K, Boss M, et al. Multi-Site, Multi-Vendor Comparison of T1 Measurement Using ISMRM/NIST System Phantom. In: Proceedings of the 24th Annual Meeting of ISMRM. 2016.#3290