Accelerating whole-heart 3D T2 mapping: Impact of undersampling strategies and reconstruction techniques

Single breath-hold three-dimensional whole-heart T2 mapping with low-rank plus sparse reconstruction

The purpose of the current study was to develop and evaluate a three-dimensional single Breath-hOLd cardiac T₂ mapping sequence (3D BOLT) with low-rank plus sparse (L + S) reconstruction for rapid whole-heart T₂ measurement. 3D BOLT collects three highly accelerated electrocardiogram-triggered volumes with whole-heart coverage, all within a single 12-heartbeat breath-hold. Saturation pulses are performed every heartbeat to prepare longitudinal magnetization before T₂ preparation (T₂ -prep) or readout, and the echo time of T₂ -prep is varied per volume for variable T₂ weighting. Accelerated volumes are reconstructed jointly by an L + S algorithm. 3D BOLT was optimized and validated against gradient spin echo (GraSE) and a previously published approach (three-dimensional free-breathing cardiac T₂ mapping [3DFBT2]) in both phantoms and human subjects (11 healthy subjects and 10 patients). The repeatability of 3D BOLT was validated on healthy subjects. Retrospective experiments indicated that 3D BOLT with 4.2-fold acceleration achieved T₂ measurements comparable with those obtained with fully sampled data. T₂ measured in phantoms using 3D BOLT demonstrated good accuracy and precision compared with the reference (R²  > 0.99). All in vivo imaging was successful and the average left ventricle T₂ s measured by GraSE, 3DFBT2, and 3D BOLT were comparable and consistent for all healthy subjects (47.0 ± 2.3 vs. 47.7 ± 2.7 vs. 48.4 ± 1.8 ms) and patients (50.8 ± 3.0 vs. 48.6 ± 3.9 vs. 49.1 ± 3.7 ms), respectively. Myocardial T₂ measured by 3D BOLT had excellent agreement with 3DFBT2 and there was no significant difference in mean, standard deviation, and coefficient of variation. 3D BOLT showed excellent repeatability (intraclass correlation coefficient: 0.938). The proposed 3D BOLT achieved whole-heart T₂ mapping in a single breath-hold with good accuracy, precision, and repeatability on T₂ measurements.

T2 mapping in myocardial disease: a comprehensive review

Cardiovascular magnetic resonance (CMR) is considered the gold standard imaging modality for myocardial tissue characterization. Elevated transverse relaxation time (T2) is specific for increased myocardial water content, increased free water, and is used as an index of myocardial edema. The strengths of quantitative T2 mapping lie in the accurate characterization of myocardial edema, and the early detection of reversible myocardial disease without the use of contrast agents or ionizing radiation. Quantitative T2 mapping overcomes the limitations of T2-weighted imaging for reliable assessment of diffuse myocardial edema and can be used to diagnose, stage, and monitor myocardial injury. Strong evidence supports the clinical use of T2 mapping in acute myocardial infarction, myocarditis, heart transplant rejection, and dilated cardiomyopathy. Accumulating data support the utility of T2 mapping for the assessment of other cardiomyopathies, rheumatologic conditions with cardiac involvement, and monitoring for cancer therapy-related cardiac injury. Importantly, elevated T2 relaxation time may be the first sign of myocardial injury in many diseases and oftentimes precedes symptoms, changes in ejection fraction, and irreversible myocardial remodeling. This comprehensive review discusses the technical considerations and clinical roles of myocardial T2 mapping with an emphasis on expanding the impact of this unique, noninvasive tissue parameter.

A revisit of the k-space filtering effects of magnetization-prepared 3D FLASH and balanced SSFP acquisitions: Analytical characterization of the point spread functions

PURPOSE

3D FLASH and balanced SSFP (bSSFP) are increasingly used in quantitative MRI after contrast preparation. The acquired k-space data are modulated by T₁ relaxation (or additional T₂ for bSSFP). Three separate sequence parameters including the number of phase-encoding steps per shot (N), flip angle (FA), and TR have made the transient state of rapid gradient echo (GRE) imaging difficult for analysis and optimization. Here we aim to analytically characterize the k-space filtering effect of magnetization-prepared FLASH and bSSFP with the point spread functions (PSF).

METHODS

The amplitude effect is characterized with the peak magnitude of the PSF, i.e. PSF(0), which, due to their approaching from transient state to steady-state for the GRE acquisitions, obeys a linear (with a slope and an intercept, not proportional) relationship with the prepared longitudinal magnetization (M_prep). The blurring effect is characterized by the FWHM of the PSF. The magnetization-prepared acquisition-dependent image contrast efficiency is characterized with the relative contrast-to-noise ratio (CNR) per unit time (ruCNR).

RESULTS

The slope of PSF(0) characterizes the relative contrast between different M_prep levels. The intercept of PSF(0) could lead to quantification bias for magnetization-prepared imaging. FLASH and bSSFP experience very little blurring effect, which is to the contrary of conventional fast spin echo (FSE). Analytical selections of N, FA, and TR are provided to optimize ruCNR for different scenarios.

CONCLUSIONS

PSFs of the FLASH and bSSFP acquisitions are analytically derived and numerically validated, and compared with the FSE acquisition, thus providing a useful tool for optimizing magnetization-prepared GRE acquisitions.