Assessing the functionality of muscle fibers is essential to monitor both pathological and physiological processes, such as in athletes, or in patients with muscular diseases. Structural and functional muscle information can be obtained with MRI using oxygenation¹ or contraction imaging². Phase contrast (PC)^2,3 or spin tag (ST) techniques⁴ are commonly used to image muscle motion. Contraction can be assessed using real-time methods⁵, possibly in combination with a mechanical restriction to the movement to improve repeatability⁶, but synchronization and standardization are very challenging. Here, we propose a dedicated setup to accurately measure muscle contraction with high temporal resolution using MRI in combination with a commercial electrical muscle stimulator (EMS), hence allowing a direct assessment of muscle kinematics.A commercially-available EMS device was used to induce involuntary periodic muscle contraction of the vastus lateralis muscle (VL) synchronized with high-temporal-resolution cine phase contrast (PC) MRI acquisition at 3T. The EMS device produced a monopolar square wave with adjustable frequency and pulse duration (set to 150 pulses per second and 300 μs, respectively, see Figure 1). A three-directional gradient echo PC velocity encoding sequence was applied for three different stimulation settings. MR acquisitions were performed on a parasagittal slice with a spatial resolution of 2.3 x 2.3 x 5 mm³ and a temporal resolution of 42 ms. The velocity encoding had a VENC of 25 cm/ s (TR/ TE = 10.6/ 7.21 ms, bandwidth/ pixel = 400 Hz / Px, flip angle = 10°, matrix = 96 x 128, FOV= 225 x 300 mm²,1 k-space line per segment, acquisition time 5 min) and 94 temporal phases were acquired. The proposed method was evaluated in five male volunteers at varying levels of stimulation (10, 14, 18-20 mA). Maximum contraction speed was calculated offline, and the strain tensors were extracted from the velocity fields.

The proposed MRI acquisition produced artifact-free velocity and strain maps and the data were consistent across all volunteers. We observed two different phases with different velocity vectors close to the beginning and end of the contraction, respectively, (see Figure 2). For maximum stimulation, at the time point of release, the velocity was in general much higher and approximately double as compared to the one at the beginning of the contraction period (see Figure 3). There were also apparent differences in the velocity directions for the proximal and the distal part of the VL, being reflected in the derived strain maps as well (see Figure 4). Moreover, the distal part of the muscle appeared to be activated earlier in time.

At different stimulation levels, all derived quantitative parameters varied substantially and appeared to follow an approximate power-law. At 18 mA, the maximum contraction speed was 3.77±0.78 cm/s with a maximum principal strain of 0.37±0.14, whereas at 10 mA, no contraction was induced.

The purpose of this study was to introduce a method for assessing functional and dynamic muscle properties with MRI in a controlled way by inducing involuntary muscle contraction with an EMS device. We chose to use a widely available commercial EMS device and open hardware and software in order to make our setup easily reproducible. Furthermore, the generation of the trigger from the stimulation signal itself allows the usage of a wide range of triggered MRI sequences, including conventional cardiovascular sequences, usually available on commercial scanners with little or no modification.

The proposed method was successful in producing accurate velocity and strain maps of the vastus lateralis muscle. All considered quantitative parameters resulting from the proposed method showed a prominent dependency on the stimulation current, which was consistently observed across all subjects.

MRI of EMS-controlled involuntary muscle contraction is feasible and allows offline calculation of velocity and strain maps that significantly depend on the stimulation current used. The main advantages of this method is that it is non invasive, utilizes a commercially available electrical muscle stimulator and simple open-source hardware and offers simultaneous acquisition of synchronized MRI images with controlled muscle contraction.