The purpose of this study is to demonstrate the feasibility of using a susceptibility-based MR method for measuring muscle oxygen extraction fraction (OEF) of foot.Peripheral vascular disease as seen in atherosclerosis and resulting peripheral arterial occlusive leads to hypoxia and consecutive claudication of the lower limbs skeletal muscles during effort, or in advanced stages of disease even at rest[1]. Muscle BOLD MRI has been used to evaluate foot muscle imaging of healthy volunteers and tissue oxygenation of patients with peripheral artery occlusive disease (PAOD)[1]. However, BOLD MRI is sensitive to many physiological perturbations, including tissue water content, inflammation and changes in tissue oxygenation[2]. More recently, an MR-based method combining asymmetric spin-echo (ASE) sequence and a susceptibility model was proposed for evaluating regional skeletal muscle oxygenation [3]. Our study want to prove the feasibility of using this susceptibility-based MRI technique to assess foot muscle OEF during resting state. Besides, to reduce susceptibility artifacts and distortion due to the relatively long echo train length (ETL) in single-shot ASE, a multi-echo acquisition scheme was employed.

Study population

The study protocol was approved by the local hospital’s institutional review board. Three healthy volunteers (mean age 23 ± 1 years, range 22-24) were recruited for foot imaging using a 3.0-T whole-body scanner (Achieva TX, Philips Healthcare).

MR Imaging

Before ASE scan, both sagital and axial T1-weighted anatomical image were obtained using a 16-channel foot coil. Unlike the conventional single-shot ASE, a muti-shot triple-echo ASE sequence with 20 varied echo shifts was implemented to acquire the source images for foot muscle OEF quantification. This modification significantly improved the signal-to-noise ratio and spatial resolution by reducing echo time and increasing matrix size. Scan parameters for the multi-shot ASE were as follows: TR = 2000 ms, TE1/TE2/TE3 = 40/56/72 ms, FOV = 240 × 250 mm², receiver bandwidth = 160 kHz, image size = 124×148(Table 1). The total acquisition time was approximately 4 min. The 180° pulse shift was varied from -10 ms to 9 ms with an increment of 1 ms. For comparison, conventional single-shot ASE scan was performed with identical geometric parameters, but the TE1/TE2/TE3 were limited to 55/75/95 ms due to relatively longer ETL(Table 1).

Data Analysis

Data analysis was performed by using self-developed Matlab (MathWorks Inc., Natick, MA, USA) routines. The estimation of muscle OEF and R2’ was derived from a theoretical model proposed by Yablonskiy and Haacke[4], which has been widely used in cereberal OEF measurement. A nonlinear least-squares curve fitting function was used to fit this model. Before analysis, all the ASE images were filtered using a Gaussian low-pass filter (kernel size = 3.0 × 3.0 mm²) to improve the signal-to-noise ratio (SNR).

A representative anatomic T1-weighted sagital image of foot is shown in Fig.1a. The spin echo image of multi-shot and single-shot ASE sequence are graghed in Fig.1d and Fig.1g separately. The corresponding R2 map, R2’ map and OEF map from multi-shot ASE sequence are displayed in the middle column of Fig.1, while the right column shows the corresponding R2 map, R2’ map and OEF map of single-shot ASE sequence. Fig.2 shows the results in the axial view of foot. It is obvious that multi-shot ASE method reduced artifacts and image distortion compared with single-shot ASE method. Besides, better SNR and higher spatial resolution were achieved with this multi-shot acquisition. The foot muscle OEF results from our experiments (0.38±0.16, 0.40±0.15, 0.41±0.15, 0.36±0.14 for axial view of multi-shot ASE, axial view of single-shot ASE, sagital view of multi-shot ASE and sagital view of single-shot ASE, respectively) are in line with the muscle OEF data in [3].To our knowledge, this is the first study to measure muscle OEF in foot with MRI-based method. The results demonstrate that our method is feasible in measuring foot muscle oxygenation. Potentially, it would be promising for evaluating foot oxygenation in patients with diabetes and peripheral artery disease.