Page 1 of 9 An MRI-compatible lower-leg exercising device for assessing pulmonary arterial pressure Colleen Farrell, Amy Lenz, Val Maharaj, Deborah Yagow Department of Biomedical Engineering University of Wisconsin Madison Client Alejandro Roldán Advisor: Professor Naomi Chesler 5 May 2010 Abstract Our client plans to use magnetic resonance imaging (MRI) to study pulmonary blood flow before and during exercise in patients with pulmonary vascular disease. Our goal is to design an MRI-compatible exercise device that increases pulmonary artery blood flow during exercise. To do so, we performed exercise testing and motion capture analysis to determine the best motion. A leg-press type device was then manufactured to match that motion. Initial data showed that use of the device to exercise in the MRI scanner raised heart rate by 36 BPM and increased arterial cross-sectional by 60 mm2. In the future, our client hopes to use scan data such as these to determine whether moderate exercise will benefit patients with pulmonary vascular diseases. Keywords: pulmonary artery pressure, MRI, exercise 1. Introduction Pulmonary Hypertension (PH) is a condition in which the blood vessels of the lungs constrict or stiffen, thickening the vessel walls and causing a rise in pulmonary vascular resistance (PVR) that leads to unsafe pressure increases in the pulmonary arteries. Pulmonary pressure cannot be measured with a simple pressure cuff, as systemic pressure is commonly found. It is commonly estimated invasively with right heart catheterization or non-invasively with Doppler ultrasound measurement of the tricuspid regurgitant jet velocity. The tricuspid regurgitant jet is formed from an insufficiency of the tricuspid valve separating the right atrium and right ventricle of the heart. As the ventricle contracts, a stream of blood leaks back through the valve into the atrium and the velocity of this jet can be used with the modified Bernoulli equation to estimate the blood pressure of the pulmonary artery (Yock and Popp,Page 2 of 9 1984). PH can be of unknown cause, called Primary PH, but is more commonly a result of emphysema, Chronic Obstructive Pulmonary Disease (COPD), HIV, heart defects, or only appears with exercise (Primary Pulmonary Hypertension News, 2009). A PH patient’s quality of life is often severely compromised as side effects of PH may include fatigue, weight changes, shortness of breath, dizziness, heart palpitations, and many need oxygen tanks at all times. There is no cure for PH, but the goal is to reduce the high pulmonary pressures these patients experience as well as treating these symptoms. Previous studies have exercised subjects outside of the Magnetic Resonance Imaging (MRI) bore. In Holverda et al. patients completed 3 minutes of cycling in supine position at 40% of maximal workload (2009).When the exercise is complete the subject must be positioned for proper imaging of the heart. Correct subject placement can take up to 5 minutes, in which time the effects of the exercise can decrease as the subject lies still. This loss of hemodynamic changes decreases the effectiveness of the study. Often researchers use a 6 minute walk to exercise hypertensive patients (Mereles et al., 2006). However, limitations of the 6-minute walk test as a measure of exercise capacity have been identified and has led to MR imaging becoming the standard for assessment of right heart structure and function (McCullagh, B. 2010). It has also been established that resting pulmonary hemodynamics may not reflect exercise capacity and that the response of vasculature is very different at rest compared to exercise; however, the hemodynamics during exercise are much more clinically significant. The focus of this study was to design a device to allow a subject to exercise while positioned within the MRI bore and be immediately scanned following exercise. The goal is to determine the effectiveness of an MRI-compatible exercise device for increasing pulmonary artery blood flow. Flaws of Competitive Devices and Previous Studies The MRI compatible products made by Lode BV are the main competition to our device. The Lode Ergometers are utilized by many studies (Niezen et al., 1998, O’Connor et al., 2004), but there are several short comings of these products. Pricing for these ergometers are near $50,000, limiting availability to many investigators. Often these ergometers are not compatible with all brands of MR scanners. For example, the MRI Ergometer Pedal is not compatible with any GE scanners, and the MRI Ergometer Push/Pull is not compatible with GE’s new scanners, the Signa® HDx 3.0T and Signa® HDxt 3.0T. The devices are not compatible because the bore size of GE scanners is smaller than Phillips and Siemens, thus decreasing the range of movement necessary to operate the device. Figure 1: Tricuspid Regurgitant jet. The top image shows the tricuspid valve. The bottom image shows the tricuspid jet velocity. The white waveform along the bottom half of the image charts the magnitude of the jet velocity against time (Ultrasound acquired image).Page 3 of 9 In an early study utilizing the Lode Ergometer cycling motion and a Phillips NT 15 MRI scanner, the subject’s thorax was immobilized by straps, shoulder pads, and sand bags that were placed beside the subjects (Niezen, 1998). This study also required the patient to be brought out of the scanner after baseline imaging to strap their feet into the ergometer and then move them back into the bore for exercise. Other studies have demonstrated in their study limitations that doing a supine exercise test was not feasible during an MRI (Holverda et al., 2009). Due to previous flaws and holes of research, we wanted to build a device to collect high quality data on right ventricular and pulmonary vascular function in patients during exercise. This device could be used with any MRI scanner and would allow the radiologist to scan the patient during exercise. 2. Methods Device Design Initial exercise testing was done to determine what exercises raise heart rate and blood pressure the most. It was initially determined that a cycling
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