To develop and implement a rapid imaging method for estimating T₂* changes in the muscles of the lower leg, following brief single maximum voluntary contractions at 3 T.Muscle function impairment can result from reduced blood supply and tissue oxygenation, as for example in patients with peripheral arterial occlusive disease (PAOD).¹ Assessing the microvascular function in such diseases may help to better understand the factors responsible for their onset and progression. Microvascular function in the skeletal muscle can be assessed through blood oxygenation level dependent (BOLD) MRI signal changes after performing a brief exercise or following a period of induced ischemia.² Multiple factors influence BOLD signals, oxygenation and blood volume being the largest,³ and it has been shown that the muscle BOLD responses are different at long echo times (i.e. TE: 45 ms) compared to short echo times (TE: 6 ms).⁴ Acquiring multiple echoes for mapping effective relaxation time constant (T₂*) values better reflects multiple components that influence small vessel function. The time course of T₂* changes has been reported previously in reactive hyperemia paradigms.^5,6 However, mild to moderate pain during cuff protocols has been reported, particularly by PAOD patients.^7,8 It would therefore be useful to establish a method to measure exercise induced T₂* changes that can be more easily tolerated by these patient populations. In this work, we developed and implemented a rapid echo-planar imaging (EPI) method to map T₂* changes, following a single maximum voluntary contraction on a 3 T whole body clinical scanner.We recruited 7 healthy subjects (three men and four women, age: 33.5 ± 6.3 years, BMI 22.8 ± 4.1 kg/m², mean ± standard deviation). We performed MRI measurements on a 3 T MRI whole body scanner (MAGNETOM Prisma, Siemens Healthcare, Erlangen, Germany) using a 15-channel transmit/receive knee coil (QED, Quality Electrodynamics LLC, Cleveland, OH). The subject’s right foot was secured in a boot attached to a pedal of a custom-built foot device. The footplate angle was fixed at 120^o for an isometric exercise. We acquired images for 10 min, during which time subjects performed a 1-s duration maximal voluntary plantar flexion every 90 s. We mapped T₂* dynamically, using a modified single-shot echo-planar imaging sequence that continuously excited and acquired one slice for a given TR with varied TE after each excitation. The data acquisition parameters are, TR 100 ms, 10 EPI images with varied TE from 7.2 ms to 52.2 ms per s, bandwidth-per-pixel 2242 Hz, iPat = 3, 25 cm field-of-view, 1 cm slice thickness, 64×64 acquisition matrix, 21^o flip angle. We acquired anatomical images to identify the largest cross-section area of the posterior muscles of the lower leg that we also used to identify large blood vessels and exclude them from T₂* fitting in the BOLD experiment. We calculated T₂* values in the BOLD images pixel-wise using custom written MATLAB code, and calculated four parameters in regions of interest in the soleus muscle. These parameters are: 1) Pre-exercise T₂*, 2) Peak post-contraction T₂*, 3) Peak T₂* percent change, 4) time to peak (TTP). Sample images are shown in Figures 1.A and B. A time course of T₂* in the soleus muscle during a 10-min acquisition is shown in Figure 1.C. We did not perform T₂* fitting on the set of EPI images during the contraction. After contraction, T₂* increased rapidly with an average TTP of 13.8 ± 4.1 s (mean ± SD) across subjects. T₂* at rest was 25.4 ± 1.5 ms and increased by 3.8 ± 1.8%. The results across seven subjects are summarized in Table 1.We developed and implemented a rapid (1-s temporal resolution) EPI sequence for mapping T₂* changes following single brief contractions. Exercise induced BOLD is more physiologically relevant in disease conditions like PAOD, since the control of blood flow from the terminal arterioles is greatly important during the initial stages of exercise. Therefore initial responses to exercise reflect the function of the microvasculature, which is captured with single contraction protocols.^9,10 A limitation of our method is that it only allowed moderate in plane resolution (3.9 x 3.9 mm²). Higher in plane resolution may improve our ability to exclude voxels containing big vessels. We developed and implemented a rapid (1-s temporal resolution) EPI sequence for mapping T₂* changes following single brief contractions at 3 T. This development will allow us to study microvascular function in patients with PAOD, who cannot tolerate cuff protocols.