QF1-006 - ECG-free myocardial T1/ECV mapping in small animals at high heart rates using MR Multitasking: A feasibility study in a HFpEF rat model

Background: Cardiac magnetic resonance (CMR) T1 mapping before and after gadolinium contrast injection allows extracellular volume fraction (ECV) quantification, which can be used to characterize focal or diffuse myocardial fibrosis. However, it is technically challenging to acquire high-quality T1 and ECV maps in small animals for preclinical research because of high heart rates ( >300 bpm) and high respiration rates (~60 cpm).

In this study, we developed an ECG-free, free-breathing MR Multitasking [1] T1 mapping method on a 9.4T small animal MRI system. The feasibility of characterizing diffuse myocardial fibrosis was tested in a rat model with heart failure with preserved ejection fraction (HFpEF).

Methods: Fig. 1(a) shows the imaging workflow of the study. The sequence design of Multitasking T1 mapping is illustrated in Fig. 1(b), with the following imaging parameters: matrix size = 128 x 128, FOV = 40 x 40 mm, voxel size = 0.31 x 0.31 x 1.5 mm, flip angle = 5°, TR/TE = 7.0/2.4 ms, recovery period (time between adjacent IR pulses) = 2.9 s. In each Multitasking T1 mapping module, 85 IR preparation pulses were applied, resulting in a scan time of 4 min 10 s. The total exam time for one rat was around 25 minutes. After the scan, hematocrit (HCT) level was measured for ECV calculation.

Multitasking image reconstruction was similar to [2], except that in this work, pre- and post-Gd images were reconstructed jointly to ensure co-registration and improve SNR. Images at diastole were used for analysis. ECV values were calculated as (1-HCT) times the ratio of ΔR1 in the septal myocardium to ΔR1 in the aorta. The LV blood pool was not used because of the signal loss resulting from washout effect in this single-slice setup.

The Dahl salt-sensitive rat model was used, as in [3]. Nine healthy rats and nine high-salt fed rats diagnosed with HFpEF were imaged. All MRI data were acquired on a 9.4 T Bruker BioSpec preclinical system with a single-channel volume coil.

Results: Fig. 2(a-c) shows representative T1 and ECV maps from control group and HFpEF group. Welch’s t-test showed that ECV values were significantly higher in the HFpEF group (22.4%±2.5%) compared to those in the control group (18.0%±2.1%), p < 0.005, as displayed in Fig. 2(d).

Conclusion: ECG-free, free-breathing CMR Multitasking T1 mapping shows excellent T1 and ECV maps in a high heart rate rat model. The elevated ECV values found in in the HFpEF group are consistent to previous human studies. This technique can be a powerful tool for myocardial tissue characterization in small animal models.