Introduction

HCM is usually detected at a stage when clinical symptoms appear, by which point the heart has already experienced irreversible damage on a microstructural level, described as myocardial remodelling [1]. Most efforts to detect and assess these changes non-invasively involve cardiac MR (CMR) [1]. T₁ relaxation time mapping and late gadolinium enhancement (LGE) are currently recommended for CMR based HCM risk assessment [2]. Myocardial T₂^* mapping presents a viable alternative to detect preclinical signs of HCM [3]. Myocardial T₂^* was reported to be prolonged in HCM patients. The transient T₂^* difference between diastole and systole was found to be diminished in HCM patients. The mechanistic or biophysical underlying of these clinical observations is unexplored so far. We hypothesize that myocardial T₂^* mapping in a humanized mouse model of HCM will provide a framework for deciphering the mechanistic or biophysical underlying of the early T₂^* changes in HCM. To test our hypothesis, we implemented a data acquisition scheme that enables retrospective data sorting and full cardiac cycle coverage for T₂^* mapping in healthy mice and in a humanized mouse model of HCM at 9.4 T.

Methods

Figure 1 illustrates the data acquisition scheme used for full cardiac cycle coverage T₂^* mapping. Continuous acquisition of T₂^* weighted multi-echo GRE data was accompanied by simultaneous tracking of cardiac activity using ECG and pulse oximetry. For retrospective gating the cardiac cycle was divided into 10 phases. For image reconstruction, the time points of the MR k-space lines were matched to the corresponding cardiac phases. T₂^* mapping was conducted using a 9.4T animal scanner (Biospec 94/20, Bruker, Germany) with the following parameters: TR=14 ms, TE=1.5-11.1 ms, echo spacing=1.6 ms (7 TEs), repetition: 300 (acquired continuously to fill 300 k-spaces). Conventional multi gradient echo (MGE) imaging (TR=35 ms, TE=1.5-31.9 ms, echo spacing=1.6 ms, 20 TEs) was used as a reference. For evaluation of the retrospectively sorted T₂^* mapping scheme, Ferumoxytol phantom was used: c=50 µg/mL Ferumoxytol in 2% agarose poured in 1.5 mL Eppendorf tube, further diluted to 25 µg/mL. For the in vivo study, a mouse model (Mybpc3-KI) with a point mutation in Mybpc3 gene was used [4]. Four Mybpc3-KI and four wildtype controls (all in C57BL/6J background and 6 weeks old) were examined. The Mybpc3-KI (HCM) mice didn’t show HCM symptoms but a slight left ventricular hypertrophy at the time of the experiment. B₀ shimming was performed to minimize macroscopic magnetic field inhomogeneities. We used a 4-element surface coil (model T114262 V3; Bruker Biospin, Germany). Short axis views were used to acquire whole cardiac cycle coverage CINE images (adapted FLASH, cardiac frames=10, TE/TR=2.1/10ms, FA=20°, FOV=20x20mm², matrix=192x192, thickness=0.8mm) and T₂^*-maps (MGE, cardiac frames=7, TE/TR=2.1/20ms, FA=10°, FOV=35x35mm², matrix=128x128, thickness=0.8mm).

Results

Evaluation of the implemented T₂^* mapping approach demonstrated accordance between the retrospective sorting-based approach and the conventional MGE. Figure 2 highlights this agreement by summarizing the T₂^* results obtained for the 50 and 25 µg/ml Fe phantom. The in vivo study provided left ventricle myocardium from both HCM and control mice during several heart cycles. Mean myocardial T₂^* (averaged over the entire cardiac cycle) of T₂^*_Ctrl= 9.52 ± 0.63 (T₂^*_systole = 9.08 ± 0.46 ms, T₂^* _diastole = 9.82 ± 0.57 ms) and T₂^*_HCM = 10.0 ± 0.23 ms (T₂^*_systole = 9.82 ± 0.22 ms, T₂^* _diastole = 10.12 ± 0.16 ms). Our findings show a T₂^* prolongation in HCM mice compared to healthy control (Figure 3). A close examination of T₂^* throughout the cardiac cycle revealed a transient change in myocardial T₂^*. T₂^* was decreased during systole and increased during diastole (Figure 4). The differences of (ΔT₂^* = T₂^*_diastole -T₂^*_systole) was found to be decreased in HCM mice (ΔT₂^*=1.03 ± 0.03) compared to healthy controls (ΔT₂^*=1.08 ± 0.08) (p<0.05).

Discussion and conclusion

This work demonstrates the feasibility of retrospective gating based T₂^* mapping in mice with a heart rate of up to 600 bpm. Unlike prospective triggering, our approach facilitates for the first time T₂^* mapping with whole cardiac cycle coverage. Our findings demonstrate an increase of myocardial T₂^* in HCM, which accords with previous human studies [3]. Although the first instinct would be to attribute this observation to a higher washout of deoxygenated haemoglobin (deoxyHb), it stands to reason that the T₂^* increase could be induced by a decreased myocardial blood volume fraction shadowing the excess of deoxyHb [3]. Alterations in the dynamic T₂^* change between systole and diastole could reflect early signs of microvascular dysfunction consistent with early progression of HCM. To conclude, our study provides an important foundation for gaining a better understanding of the pathophysiological and biophysical underpinning of microstructural changes in the HCM.

References

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2. Maron, B.J., et al., Diagnosis and Evaluation of Hypertrophic Cardiomyopathy: JACC State-of-the-Art Review. J Am Coll Cardiol, 2022. 79(4): p. 372-389.

3. Huelnhagen, T., et al., Myocardial Effective Transverse Relaxation Time T 2(*) is Elevated in Hypertrophic Cardiomyopathy: A 7.0 T Magnetic Resonance Imaging Study. Sci Rep, 2018. 8(1): p. 3974.

4. Vignier, N., et al., Nonsense-mediated mRNA decay and ubiquitin-proteasome system regulate cardiac myosin-binding protein C mutant levels in cardiomyopathic mice. Circ Res, 2009. 105(3): p. 239-48.