MR-compatible treadmill for exercise stress cardiac magnetic resonance imaging

Abstract

This article describes an MR-safe treadmill that enables cardiovascular exercise stress testing adjacent to the MRI system, facilitating cardiac MR imaging immediately following exercise stress. The treadmill was constructed of nonferromagnetic components utilizing a hydraulic power system. Computer control ensured precise execution of the standard Bruce treadmill protocol commonly used for cardiovascular exercise stress testing. The treadmill demonstrated no evidence of ferromagnetic attraction and did not affect image quality. Treadmill performance met design specifications both inside and outside the MRI environment. Ten healthy volunteers performed the Bruce protocol with the treadmill positioned adjacent to the MRI table. Upon reaching peak stress (98 ± 8% of age-predicted maximum heart rate), the subjects lay down directly on the MRI table, a cardiac array coil was placed, an intravenous line connected, and stress cine and perfusion imaging performed. Cine imaging commenced on average within 24 ± 4 s and was completed within 40 ± 7 s of the end of exercise. Subject heart rates were 86 ± 9% of age-predicted maximum heart rate at the start of imaging and 81 ± 9% of age-predicted maximum heart rate upon completion of cine imaging. The MRI-compatible treadmill was shown to operate safely and effectively in the MRI environment.

Figure 1

Hydraulic and control schematic for MR-compatible treadmill. An electric motor driven pump outside of MRI room provides power to a hydraulic motor and cylinder mounted on the treadmill. Motor controller and computer are located outside of the MRI room. Speed and elevation feedback is transmitted through fiber-optic cables. Emergency stop (E-Stop) signal is also transmitted via optical fiber.

Figure 2

The hydraulic power pack located in the equipment room adjacent to the MRI room. An electric motor driven pump forces water through hoses run through a waveguide to the treadmill located in the MRI room. Motor controller provides feedback control of treadmill belt speed.

Figure 3

MRI-compatible hydraulic drive and elevation systems mounted to the front end of the treadmill. Metal components are manufactured from either aluminium or non-ferromagnetic 316 grade stainless steel.

Figure 4

Exam room layout for exercise stress MRI. The treadmill, treadmill control console, ECG display, and blood pressure meter are all within the room adjacent to the MRI system. Vacuum mattresses on patient table are used to facilitate accurate repositioning of the patient following exercise.

Figure 5

Cine images obtained at rest and immediately post-exercise stress. End systolic frames are shown for the same slices positions at rest (left column) and stress (right column). Increased myocardial contraction at peak systole is seen in the stress images compared to the rest images. In this subject, resting cine images were acquired at a heart rate of 59 beats per minute, and stress cine images at 156 beats per minute.

Figure 6

Myocardial perfusion images obtained at rest (top row) and immediately post-exercise stress (bottom row). Difference in heart rate causes each slice to be sampled at a different point in the cardiac cycle between rest and stress.