Characterization of lower limb muscle activation patterns during walking and running with Intravoxel Incoherent Motion (IVIM) MR perfusion imaging

Background: The distribution of energy use among different lower limb muscles during walking and running is not well understood. Local blood flow within skeletal muscle tissue depends on its metabolic activity during activation. The non-invasive magnetic resonance microvascular perfusion method Intravoxel Incoherent Motion (IVIM) is able to quantify muscle activation. Purpose: To non-invasively determine quantitative changes in local microvascular perfusion and blood flow via IVIM in order to characterize specific muscle activation of the lower limb at rest, during walking and during running. Methods: 3 T MR IVIM diffusion-weighted images of N = 16 lower extremities (bilateral imaging of n = 8 healthy volunteers; mean age 27.5 +/- 5.7 years) were acquired at rest and immediately after walking and running for 15 min, respectively. A transverse monopolar pulsed gradient fat suppressed spin echo EPI sequence was used (9 b-values from 0 to 1000s/mm(2), 3 orthogonal directions). Anatomical transverse T1-weighted turbo SE images were acquired at rest. Muscles at the pelvis, thigh and lower leg were segmented. IVIM perfusion parameters f, D* and fD* and the diffusion coefficient D were obtained after standard two-steps fitting of the IVIM bi-exponential signal equation. Descriptive statistics, t-tests, Pearson's correlations and Partial Spearman correlations were used for statistical analyses. Results: The microvascular blood flow (fD*) increased significantly and stepwise from rest (1.65 +/- 0.83*10(-3) mm(2)/s) to walking (1.99 +/- 0.80*10(-3) mm(2)/s, P < 0.001) and running (2.18 +/- 0.98*10(-3) mm(2)/s, P < 0.001). The perfusion increase was most pronounced for lower leg and feet muscles (P < 0.001). Hamstring muscles showed a higher microvascular perfusion increase than quadriceps muscles (P < 0.05). A higher increase of the heartrate from walking to running correlated significantly with a lower increase of fD* from walking to running (R = -0.16, P = 0.001). Conclusion: IVIM MRI quantitatively measures local microvascular muscle perfusion to detect muscle activation patterns through walking and running. A redistribution of blood flow towards the lower leg was observed during running as compared to walking.