Peripheral artery disease (PAD), generally a manifestation of atherosclerosis in vessels supplying the lower limbs, is diagnosed based on a reduction in the ratio of blood pressures measured in the ankle and brachial artery, known as the ankle-brachial index (ABI)¹. An ABI less than 0.9 signifies the presence of macrovascular stenoses, but does not specify spatial location, is not sensitive to microvascular impairment, and does not reflect improvements due to pharmacologic or exercise therapy^2,3. Recent studies suggest that functional deficits present in the vasculature of patients with PAD may be detected through dynamic imaging studies^4-7. Similar to cardiac stress testing, peripheral vascular function is interrogated by measuring the kinetics of recovery following induced ischemia (during reactive hyperemia), which is blunted and delayed in patients with PAD compared to their healthy peers. This has been reported from independent studies measuring muscle perfusion⁴, dynamics of venous oxygen saturation (SvO₂)⁵, or relative changes in skeletal muscle T₂*⁶, which is related to tissue oxygenation and perfusion, and more recently using PIVOT, a combined method to measure perfusion, SvO₂, and T₂* simultaneously⁸. The purpose of this study was to evaluate whether a supervised exercise intervention resulted in changes in MRI-measured vascular function. 136 patients (86 male, 68±8 years old) with PAD were recruited to participate in the study and underwent one screening visit and two testing visits (study design shown in Figure 1). The treadmill-walking test employed the Gardner protocol⁹, which requires patients to walk on a treadmill at 2 mph initially with 0% incline and the incline is increased by 2% every 2 minutes. Patients indicate when they first experience pain (claudication onset time, COT), and when the pain limits their ability to walk (peak walking time, PWT). The MRI protocol consisted of dynamic imaging using PIVOT¹⁰ (see Figure 2 for pulse sequence diagram) continuously throughout one minute of baseline, five minutes of proximal arterial occlusion, and six minutes of post-ischemic recovery. Data were acquired with an 8-ch Tx/Rx knee coil at 3T. Ischemia was induced by inflating a cuff around the thigh to 75 mmHg above the systolic blood pressure. All PIVOT measures were quantified with 2-second temporal resolution. Perfusion was calculated in regions of interest (ROIs) in the gastrocnemius, soleus, peroneus, and tibialis anterior muscles, and was averaged across all ROIs (leg) and peak hyperemic flow (PHF) and time to peak perfusion (TTP_perf) were recorded. SvO₂ was measured in the larger posterior tibial vein, and washout time (time to minimum SvO₂) and upslope (maximum slope of recovery) were recorded. T₂* was quantified in the soleus, normalized to the average baseline value, and the relative T₂*_max and time to peak T₂* (TTP_T2*) were recorded. Example images and time courses are shown in Figure 3. Paired t-tests were used to assess the change from baseline to follow-up, and the Holm-Bonferroni correction was applied to maintain a family-wise error rate of 0.05. 112/136 patients returned for their follow-up visit 119±28 days after the baseline visit (19 lost to follow-up, 5 presently enrolled). From the initial testing visit, significant correlations were detected between ABI and reactive hyperemia response time across all patients, however no correlation was observed between ABI and reactive hyperemia response magnitude or between ABI and indices of functional disease impairment measured by treadmill-walking test (Figure 4). From baseline to follow-up, COT and PWT significantly increased in the SER group (paired t-test, p<0.0001 for both). COT significantly increased in the SMC group as well (paired t-test, p<0.01). From the MRI data, no significant differences were detected between the SER and SMC groups at baseline or follow-up for any metric. In the SER group, the change in peak perfusion between baseline and follow-up significantly increased in the peroneus (p<0.05) and when averaged across the whole leg (p<0.05) (Figure 5).This preliminary analysis showed that patients enrolled in a supervised exercise intervention had a significant increase in peak perfusion in the peroneus and across the entire cross-section of the leg. This response may be attributed to increased angiogenesis induced by exercise. Moreover, since the reactive hyperemia response time is highly correlated with ABI as seen in Figure 4 (which is primarily a marker of macrovascular disease), it would make sense that changes in response time are not observed as the stenoses are relatively fixed. Future analyses will explore whether changes in the MRI-measures of vascular function are correlated with changes in physiologic response as measured by the treadmill-walking tests, blood markers of angiogenesis, and patient quality of life questionnaires.