Highly Accelerated, Intravascular T1, T2, and Proton Density Mapping with Linear Algebraic Modeling and Sensitivity Profile Correction at 3T
Guan Wang1,2, Yi Zhang2, Shashank Sathyanarayana Hegde2,3, and Paul A. Bottomley2

1Dept. of Electrical & Computer Engineering, Johns Hopkins University, Baltimore, MD, United States, 2Russell H. Morgan Dept. of Radiology & Radiological Sciences, Johns Hopkins University, Baltimore, MD, United States, 3(currently) Philips Innovation Campus, Bangalore, India

Synopsis

Vessel wall MRI with intravascular (IV) detectors can produce superior local signal-to-noise ratios (SNR) and generate high-resolution T1, T2, and proton density (PD) maps that could be used to automatically classify atherosclerotic lesion stage. However, long acquisition times potentially limit multi-parametric mapping. Here, for the first time, spectroscopy with linear algebraic modeling (SLAM) is applied to yield accurate compartment-average T1, T2 and PD measures at least 10 times faster compared to a standard full k-space reconstructed MIX-TSE sequence at 3T. Simple phase and magnitude sensitivity corrections are incorporated into the SLAM reconstruction to compensate for IV detector non-uniformity.

PURPOSE

Vessel wall MRI with intravascular (IV) detectors can produce superior local signal-to-noise ratios (SNR) [1] and high resolution (<200μm) T1, T2, and proton density (PD) maps [2]. Combined with machine learning-based automatic atherosclerotic lesion classification, up to 97% area-under-curve (AUC) receiver operating characteristic (ROC) analyses is potentially achievable [2]. However, long acquisition times are potentially limiting for clinical T1, T2 and PD mapping, and increase the likelihood of motion artifacts. Recently, spectroscopy with linear algebraic modeling (SLAM) was developed for MRS and CEST to reduce scan-times up to 120-fold by delivering compartment-average signals reconstructed from a small subset of those k-space acquisitions having the highest SNR [3-4]. Here, for the first time, SLAM is applied to yield compartment-average T1, T2 and PD measures up to 10-fold faster than a standard “MIX” sequence [5]. The phase and amplitude of the sensitivity profiles of the IV coils as receivers, are incorporated into the SLAM reconstruction to compensate for intra-compartmental non-uniformity. SLAM measurements in human vessel specimens are compared with those from conventional full k-space reconstruction acquired at 3T.

METHODS

IV MRI was performed on freshly autopsied human iliac artery specimens immersed in saline and placed in a Philips 3T Achieva scanner with a loopless antenna in the lumen. Reference data were acquired with an axial 2D MIX-turbo spin-echo (TSE) sequence, interleaved with inversion recovery (IR; scan-time=2.3 min; 8 echoes; voxel=0.3x0.3x5mm3; FOV=55mm; TSE-factor=16, TI=370ms, TRSE/TRIR=760/1920ms, TE1/TE2/TE3/TE4= 25/65/105/145ms). The T1, T2 and PD values of the compartments were computed as in [5]. Compartment-average T1, T2 and PD were measured in compartments that were manually segmented from anatomical images (fat=F; lesion=L; vessel fluid contents=W1; smooth vessel-wall muscle=SM, surrounding tissue=W2) and reconstructed using: (i) the full k-space data set; and (ii) SLAM applied to the central k-space data with enough outer k-space data discarded to achieve acceleration factors of R≤10. During reconstruction, the magnitude and phase of the IV coil’s sensitivity were corrected by 1/r (r=distance from the coil center; Fig.1a), and an azimuthal phase map linearly varying from 0-2π (Fig.1b), respectively. Compartment average T1, T2 and PD values from SLAM were compared with conventional Fourier transform (FT) MRI.

RESULTS

Magnitude and phase inhomogeneities of IV-MRI were successfully eliminated by the sensitivity profile corrections (Fig.1c-d). A segmented FOV from a diseased vessel (myelodysplastic syndrome) is exemplified in Fig.2. The 10-fold accelerated SLAM parametric maps show high consistency with the regular FT-MIX maps in Fig.3. SLAM results for the vessel wall are plotted against R in Fig. 4, with error-bands indicating compartment mean ±standard deviations (SD) measured in the FT-MIX maps. For R≤10, SLAM T1, T2 and PD measurements in all compartments fall within the mean±SD of the FT results. In both lesion compartments (L1, L2) and F, and with R=10, errors in the three parameters are ≤ 0.5%(±4%) compared to the FT mean. Even in W1 and W2, where the SD of T1 and PD are ≥30% in the FT maps, SLAM T1 and PD agree with the FT means within ≤6%±6%.

DISCUSSION

SLAM reconstruction can be combined with IV MRI to produce accurate compartment average T1, T2, and PD values at least R=10 times faster. In principle, R can be increased until the number of phase-encoding steps equals the number of compartments, which in Fig. 2 would yield R=30. While the SLAM assumption of signal uniformity within compartments [3] is adequately satisfied by applying simple sensitivity profile corrections, these may not suffice for large R in tiny compartments with large phase variations. Nevertheless, SLAM offers a means of dramatically reducing scan-times in high-resolution multi-parametric IV-MRI to ≤10 sec, even in regions of low SNR. In vivo SLAM IV-MRI measurements of T1, T2 and PD could enable fast and reliable high-resolution characterization of vessel disease, and in conjunction with automated disease classification [2], permit automatic real-time high-resolution detection of cardiovascular disease.

Acknowledgements

We thank Parag Karmarkar for help with the experiments and insightful discussions. This work was supported by NIH grant R01EB007829.

References

1. El-Sharkawy AM, Qian D, Bottomley PA: The performance of interventional loopless MRI antennae at higher magnetic field strengths. Med Phys 2008, 35:1995-2006.

2. Wang G, Erturk MA, Hegde SS, Bottomley PA: Automated classification of vessel disease based on high-resolution intravascular multi-parametric mapping MRI. International Society of Magnetic Resonance in Medicine 2015, 1659.

3. Zhang Y, Gabr RE, Schär M, Weiss RG, Bottomley PA: Magnetic resonance Spectroscopy with Linear Algebraic Modeling (SLAM) for higher speed and sensitivity. Journal of Magnetic Resonance 2012, 218:66-76.

4. Zhang Y, Gabr RE, Zhou J, Weiss RG, Bottomley PA: Highly-accelerated quantitative 2D and 3D localized spectroscopy with linear algebraic modeling (SLAM) and sensitivity encoding. Journal of Magnetic Resonance 2013, 237:125-138.

5. Cuppen J: RLSQ: T1, T2, and ρ calculations, combining ratios and least squares. Magnetic resonance in medicine 1987, 5:513-524.

Figures

Simulated magnitude (a) & phase (b) of IV receiver coil’s sensitivity profile. IV MRI magnitude(c) & phase(d) after correction-based on (a,b).

Anatomical compartment masks overlaid on vessel IV-MRI.

Color-coded MIX T1(ms; a, c), and PD(% vs. water; b, d) maps calculated using: (a, b) IV MRIs reconstructed from the full k-space data; and (c, d) using a SLAM reconstruction based on 10% of the k-space data. The coil magnitude and phase corrections were incorporated in the reconstructions.

SLAM T1, T2 values vs. R. The error bands denote compartment mean±SD derived from the FT maps. SLAM and FT measures of T1, T2 and PD (not shown) agree.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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