Ex vivo imaging of subacute myocardial infarction with ultra-short echo time 3D quantitative T1- and T1ρ -mapping magnetic resonance imaging in mice.

Aims: The aim of this study was to develop an ultra-short echo time 3D magnetic resonance imaging (MRI) method for imaging subacute myocardial infarction (MI) quantitatively and in an accelerated way. Here, we present novel 3D T₁- and T_1ρ -weighted Multi-Band SWeep Imaging with Fourier Transform and Compressed Sensing (MB-SWIFT-CS) imaging of subacute MI in mice hearts ex vivo.

Methods and results: Relaxation time-weighted and under-sampled 3D MB-SWIFT-CS MRI were tested with manganese chloride (MnCl₂) phantom and mice MI model. MI was induced in C57BL mice, and the hearts were collected 7 days after MI and then fixated. The hearts were imaged with T₁ and adiabatic T_1ρ relaxation time-weighted 3D MB-SWIFT-CS MRI, and the contrast-weighted image series were estimated with a locally low-rank regularized subspace constrained reconstruction. The quantitative parameter maps, T₁ and T_1ρ , were then obtained by performing non-linear least squares signal fitting on the image estimates. For comparison, the hearts were also imaged using 2D fast spin echo-based T₂ and T_1ρ mapping methods. The relaxation rates varied linearly with the MnCl₂ concentration, and the T₁ and T_1ρ relaxation time values were elevated in the damaged areas. The ischaemic areas could be observed visually in the 3D T₁, 3D T_1ρ , and 2D MRI maps. The scar tissue formation in the anterior wall of the left ventricle and inflammation in the septum were confirmed by histology, which is in line with the results of MRI.

Conclusion: MI with early fibrosis, increased inflammatory activity, and interstitial oedema were determined simultaneously with T₁ and T_1ρ relaxation time constants within the myocardium by using the 3D MB-SWIFT-CS method, allowing quantitative isotropic 3D assessment of the entire myocardium.