T₂, T_1ρ and T_RAFF4 detect early regenerative changes in mouse ischemic skeletal muscle

Abstract

The identification of areas with regenerative potential in ischemic tissues would allow the targeting of treatments supporting tissue recovery. The regeneration process involves the activation of several cellular and molecular responses which could be detected using magnetic resonance imaging (MRI). However, to date, magnetic resonance (MR) relaxation parameters have received little attention in the diagnosis and follow-up of limb ischemia. The purpose of this study was to evaluate the feasibility of different MRI relaxation and diffusion tensor imaging parameters in the detection of areas showing early signs of regeneration in ischemic mouse skeletal muscles. T and T relaxation time constants, together with T , T and diffusion tensor imaging, were evaluated to differentiate areas of regeneration in a mouse hind limb ischemia model before and 0, 1, 4, 7, 14 and 30 days after ischemia. All the measured relaxation times were longer in the areas of early regeneration compared with normal muscle tissue. The relaxation times increased after ischemia in the ischemic muscles, reaching a maximum at 4-7 days after occlusion, coinciding with the appearance of early signs of regeneration. Fractional anisotropy decreased significantly (p < 0.05) on days 1-4, whereas mean diffusivity, λ and λ decreased later, starting at day 7 after ischemia compared with the pre-operational time point. The percentages of areas with different tissue morphologies were determined based on histological analysis of the ischemic muscle cross-sections, and correlations between the percentages obtained and different relaxation times were calculated. The highest correlation between relaxation times and histology was achieved with T , T and T (R = 0.96, R = 0.92 and R = 0.84, respectively, p < 0.01) Early regenerative changes were visible using T , T and T MR relaxation time constants in skeletal muscle after ischemia. These markers could potentially be used for the identification of targets for therapies supporting muscle regeneration after ischemic injury.

Monitoring of diffusion properties and transverse relaxation time of mouse ischaemic muscle after administration of human mesenchymal stromal cells derived from adipose tissue

“…At day 14, both relaxation and diffusion parameters in the ligated limb were close to the values typical for the intact leg, although some indexes were still significantly changed with respect to the other side. The published results indicate that complete normalization of the diffusion and relaxation parameters should be expected within 3‐4 weeks after artery ligation . However, a direct comparison of our results with the reported data is difficult due to the variability in mice strains, mice age and diversity of surgical procedures of artery ligation.…”

“…This decay reflects intramuscular water content and its organization. Although it is possible to observe four exponential nature of this decay in the normal muscle using specialized techniques, global T2 changes determined from the mono‐exponential fitting of the MR signal acquired using standard multiecho sequences are usually reported …”

“…Although it is possible to observe four exponential nature of this decay in the normal muscle using specialized techniques, global T2 changes determined from the mono-exponential fitting of the MR signal acquired using standard multiecho sequences are usually reported. 16,[18][19][20][21] Our results revealed a spatial heterogeneity of T2 alterations after femoral artery ligation in the control group, in agreement with the study of Zaccagini et al, 20 While T2 was found to be elevated at days 1-7 in the anterior and medial ROIs (median T2 rel > 1.35 at all time points, with a maximum around day 3), the posterior ROI was characterized by a slight T2 increase at day 1 only (median T2 rel value of 1.15). The T2 elevation observed by Zhang et al 21 already 2 hours after ligation was explained as resulting from oedema, while the huge increases around day 3 were associated with the influx of inflammatory cells to the tissue.…”

“…The parameters obtained from relaxation and diffusion imaging of ischaemic hindlimbs were found to change dynamically during degeneration and regeneration processes after femoral artery ligation [18][19][20][21] and provided a useful insight into the therapeutic effects of human and mouse endothelial progenitor cells in mouse models of hindlimb ischaemia. 22,23 Recently, our group has reported that M2 macrophages are involved in the repair process of ischaemic muscle tissue after transplantation of hADSC in immunocompetent mice.…”

“…The parameters obtained from relaxation and diffusion imaging of ischaemic hindlimbs were found to change dynamically during degeneration and regeneration processes after femoral artery ligation and provided a useful insight into the therapeutic effects of human and mouse endothelial progenitor cells in mouse models of hindlimb ischaemia …”