Low Cost Magnetic Resonance Imaging-Compatible Stepper Exercise Device for Use in Cardiac Stress Tests

Cardiovascular disease is the leading cause of death in developed nations, including the United States [1]. According to the Centers for Disease Control and Prevention (CDC), there were 595,444 deaths due to heart disease alone in 2010 [2]. Assessment of cardiac function during exercise can be used as a diagnostic tool in the investigation of a wide range of cardiovascular diseases [1]. Several cardiovascular pathologies, such as myocardial ischemia, may only be apparent when the heart rate and cardiac output (CO) are higher than at rest [3,4]. For this reason, diagnostic stress tests, in which a subject’s heart rate and CO are increased by either pharmacological stimulation or physical exercise, are often conducted. Pharmacological stress tests have been shown to be as diagnostically useful as exercise stress tests and safe, provided that trained professionals closely monitor vital signs and resuscitation equipment is available [5,6]. These tests can be performed with different noninvasive imaging techniques but MRI is the reference standard for noninvasive measurement of global and local cardiac function as it provides three dimensional imaging data with high spatial and temporal resolution [7–9]. Using MR images, changes in flow, ventricular volume, and cardiac output can be measured throughout the cardiac cycle [10,11]. In addition, cardiac perfusion can be assessed [12]. Indices of cardiac function from MRI-based pharmacological stress tests are strong predictors of myocardial infarction morbidity and mortality [13]. However, advantages to performing an exercise stress test are that it more closely resembles cardiovascular stress experienced during daily life, is better tolerated by patients, and is more cost effective [14]. Physical exercise stress is commonly utilized in conjunction with echocardiography to evaluate cardiac function and performance [3,15]. In the context of MRI imaging, physical exercise stress poses significant challenges due to the limited space in the cylindrical cavity of the MR machine in which imaging takes place (the bore), the horizontal position of the subject during imaging, incompatibility of standard exercise equipment with the strong magnetic fields required for MR imaging and the sensitivity of MRI to motion artifacts. Therefore, most MRI-based physical exercise stress tests to date have used a paradigm in which exercise is performed outside of the MRI bore with a bicycle or treadmill; when the target level of exercise is reached, the subject stops exercising, relocates to a supine position on the MR table and is advanced into the bore for imaging [16,17]. The time lapse between exercising and imaging often allows for the subject’s heart rate and CO to recover, especially for subjects with close to normal cardiac function. Therefore, exercise devices that can be used while the subject’s torso remains in the MRI bore address a significant unmet clinical need. Herein, we describe the design and construction of low-cost exercise device that is compatible with both exercise and cardiac MR imaging and pilot data from 5 healthy subjects that demonstrate its ability to induce exercise stress.