Coronary artery disease (CAD) is the leading cause of death in the United States. Early detection provides the opportunity to initiate medical treatment and lifestyle changes that can save lives. Cardiac imaging and treadmill exercise stress testing are the primary noninvasive methods of diagnosing CAD. Echocardiography and single photon emission computed tomography (SPECT) are imaging methods combined with exercise stress testing clinically; however both suffer from limitations in image quality and diagnostic ability. In addition, SPECT involves the injection of a radioisotope. Magnetic resonance imaging (MRI) can provide a comprehensive cardiac examination (function and perfusion) with higher quality images without the use of radiation.
The challenges of the MRI environment have prevented the successful combination of treadmill exercise stress and MRI. Standard treadmills contain ferromagnetic components and use electromagnetic motors. The strong magnetic field of the MRI environment can turn ferromagnetic objects into projectile hazards and can disrupt the operation of electromagnetic motors. A treadmill exercise cardiac magnetic resonance imaging (CMR) system requires a completely MR compatible treadmill which is able to sit safely in the MR environment and not affect MRI image quality. The treadmill must be capable of automatically running the Bruce treadmill protocol and allow for the acquisition of function images within 60 s of peak stress, according to American Heart Association (AHA) guidelines.
The feasibility of the exercise stress CMR method was tested using a “semi-compatible” treadmill that could be placed in the outer corner of the MRI room. Ten cardiac patients successfully completed the study with an average time to start function imaging of 71±7 s.
A MRI compatible treadmill was developed, which used water hydraulics to power the drive and elevation systems. This design allows a traditional electromagnetic motor to be placed outside of the MR environment, and power is transferred through fluid-filled hoses running through an opening in the MRI room to MR compatible hydraulic actuators located on the front of the treadmill. A hydraulic motor powers the treadbelt through a shaft and pulley system. A hydraulic cylinder acts on a lever arm to raise and lower the treadmill elevation. All structural components were fabricated from nonferromagnetic aluminum and stainless steel.
MR safety tests performed with a powerful hand magnet confirmed only a small amount of ferromagnetic components in the drive bearings and cylinder tie rods. Substitute components have been located for replacement, but the system was deemed safe under supervision. MRI system diagnostic testing confirmed that the presence of the treadmill system in the room does not affect image quality, and tests comparing treadmill performance inside and outside of the MRI room confirmed that the MR environment did not significantly affect treadmill performance (no worse than 1.6% difference at any stage). The treadmill was tested in the MR environment under subject load, and was able to successfully operate through the range of the Bruce protocol. Three subjects performed maximal exercise testing, and function imaging was successfully completed in an average of 45 s, meeting the 60 s AHA guideline.