MR Multitasking-based multi-dimensional assessment of cardiovascular system (MT-MACS) with extended spatial coverage and water-fat separation

Abstract

Purpose: To extend the MR MultiTasking-based Multidimensional Assessment of Cardiovascular System (MT-MACS) technique with larger spatial coverage and water-fat separation for comprehensive aortocardiac assessment.

Methods: MT-MACS adopts a low-rank tensor image model for 7D imaging, with three spatial dimensions for volumetric imaging, one cardiac motion dimension for cine imaging, one respiratory motion dimension for free-breathing imaging, one T2-prepared inversion recovery time dimension for multi-contrast assessment, and one T2*-decay time dimension for water-fat separation. Nine healthy subjects were recruited for the 3T study. Overall image quality was scored on bright-blood (BB), dark-blood (DB), and gray-blood (GB) contrasts using a 4-point scale (0-poor to 3-excellent) by two independent readers, and their interreader agreement was evaluated. Myocardial wall thickness and left ventricular ejection fraction (LVEF) were quantified on DB and BB contrasts, respectively. The agreement in these metrics between MT-MACS and conventional breath-held, electrocardiography-triggered 2D sequences were evaluated.

Results: MT-MACS provides both water-only and fat-only images with excellent image quality (average score = 3.725/3.780/3.835/3.890 for BB/DB/GB/fat-only images) and moderate to high interreader agreement (weighted Cohen's kappa value = 0.727/0.668/1.000/1.000 for BB/DB/GB/fat-only images). There were good to excellent agreements in myocardial wall thickness measurements (intraclass correlation coefficients [ICC] = 0.781/0.929/0.680/0.878 for left atria/left ventricle/right atria/right ventricle) and LVEF quantification (ICC = 0.716) between MT-MACS and 2D references. All measurements were within the literature range of healthy subjects.

Conclusion: The refined MT-MACS technique provides multi-contrast, phase-resolved, and water-fat imaging of the aortocardiac systems and allows evaluation of anatomy and function. Clinical validation is warranted.

Keywords: MR Multitasking; multi-contrast imaging; multi-dimensional imaging; phase-resolved imaging; water-fat separation; whole-heart MR.

FIGURE 1

Pulse sequence diagram for the MT‐MACS technique and corresponding k‐space sampling pattern. A, T2‐prepared inversion recovery (T2IR) magnetization preparations are applied at constant intervals followed by dual‐echo stack‐of‐stars FLASH readouts. Following each T2IR preparation module, RF pulse flip angles are 3° for the first 300 segments, and 1° for the next 200 segments. Auxiliary data are interleaved with imaging data every six segments. B, Simplified illustration of k‐space sampling strategy. The auxiliary data are collected at the 0° radial spoke of the center partition. For the imaging data, randomized partition‐encoding reordering following a variable‐density Gaussian distribution with the highest density at the center partition adopted in this sequence.

FIGURE 2

Example MT‐MACS images of the ventricular chambers (coronal view and short axis [SAX] view) and thoracic aorta at the mid‐diastolic end‐expiration phase generated from a 28‐y‐old female subject (Subject 1) and a 65‐y‐old male subject (Subject 2). Water‐only images with multiple contrast weightings, including BB, DB, and GB, and fat‐only images are displayed for each slice orientation.

FIGURE 3

Quantification of myocardial wall thickness of the LA/ LV/ RA/ RV. A, Graphic illustration of measuring the myocardial wall thickness of the LA/LV/RA/RV in a 40‐y‐old male subject. Myocardial wall thickness of each cardiac chamber was measured at the same location on the DB images of MT‐MACS at the mid‐diastolic end‐expiratory phase and corresponding 2D T2‐weighted turbo spin‐echo images with matched location and slice thickness. B, Bland–Altman plots and ICCs comparing measurement agreements between these two imaging techniques.

FIGURE 4

Quantification of LVEF. A, Graphic illustration of measuring LVEF in a 39‐y‐old male subject. The blood‐myocardium boundary was manually contoured in each slice of the MT‐MACS BB images and corresponding 2D cine balanced SSFP images with matched location and slice thickness for both the end‐diastolic and end‐systolic phases. B, Linear regression, ICCs, and Bland–Altman analyses comparing the measurement results acquired by these two imaging techniques.