| Blood-oxygen-level-dependent functional magnetic resonance imaging(BOLD fMRI)can be utilized to study the hemodynamic behavior and metabolism of skeletal muscle.However it has not been used for functional study of peripheral artery disease(PAD)in a clinically relevant low-intensity exercise paradigm.Twelve healthy controls were studied at 3T using gradient echo T2*-weighted echo planar imaging(EPI)under an exercise paradigm(20 plantar flexions/min for 14 min)with low work load(2 kg),simulating normal walking.BOLD signal intensity time-courses in lateral gastrocnemius,medial gastrocnemius,soleus and tibialis anterior were calculated and analyzed.Along with MRI studies,basic physiology measurements(blood pressure,heart rate)were also taken during exercise.Subjective pain rating and perceive exaction were collected after exercise.The characteristics of dynamic BOLD signal change during and after exercise with different work intensity were studied and analyzed.Our data shows that signal intensity can be measured simultaneously in several muscle groups during exercise,and different muscle groups have similar temporal behavior of BOLD signal change.The increase of blood flow oxygen extraction by the working skeletal muscle will lead to a negative BOLD signal change:BOLD signal intensity decreases below baseline during exercise(p<0.05),and approaches back towards baseline after exercise in different muscle groups.The amplitude of such negative BOLD change is likely to be affected by work load.To determine the effect of work load to BOLD signal change,6 out of our 12 healthy cohort underwent the same exercise paradigm but with a higher work load(6 kg).Besides MRI,physiology measurements and subjective ratings,Doppler ultrasound and near infrared spectroscopic data were collected as reference.The result demonstrates that BOLD signal intensity decreases during plantar flexion and this effect is augmented at higher exercise intensities(p<0.05).BOLD MRI has great potential to quantify skeletal muscle oxygenation with high temporal and high spatial resolution.To testify the clinical application of this low-intensity exercise paradigm,9 patients with peripheral arterial disease(PAD)attended the low intensity(2 kg work load)exercise study.MRI data along with physiology measurements,pain rating,perceive exaction data were collection.Correlations were studied between BOLD signal and basic physiology changes as well as pain rating and perceive exaction.During exercise and recovery periods,most calf muscles showed significant BOLD signal changes comparing to the baseline(p<0.05).In general,there was a signal decrease(negative BOLD)after onset for all subjects.A significant signal decrease(p<0.05)during exercise was observed in soleus and tibialis anterior in healthy group.Such negative BOLD was also observed significantly in soleus and lateral gastrocnemius in PADs,but only during the early stage of exercise(first 2-3 min).In PADs,after the initial negative-BOLD phase,signal gradually increased above the baseline in lateral gastrocnemius during and after exercise.BOLD signal in lateral gastrocnemius of PADs was significantly stronger than that in healthy group during the later period of exercise(p<0.05).With our low-intensity exercise paradigm,dynamic BOLD signal measurement can be used to demonstrate different muscle hemodynamic behaviors in healthy and PADs.Negative BOLD signal change was found in healthy controls,while in PAD patients,lateral gastrocnemius showed positive BOLD signal change during the later phase of exercise,accompanied with higher heart rate,mean arterial pressure(MAP),subjective pain and fatigue rating than the healthy controls.The positive BOLD in these exercising muscles in PAD is likely due to the poor oxygen utilization in the tissue.The BOLD signal temporal behavior varied significantly across subjects in both groups,with significantly larger inter-subject variability within the PADs comparing to the healthy,which may reflect heterogeneous disease stage of the PAD cohort. |
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