Comparative Study of Patient Comfort at 7T and 3T MRI

1. Susan C. Fulton, Silvina G. Horovitz, Jeff H. Duyn Advanced MRI section, LFMI, NINDS, National Institutes of Health, Bethesda, MD, USA Introduction: Imaging at high field strength such as 7T offers higher SNR, which can be used to increase spatial resolution, but may increase patient discomfort in several ways. For example, moving too rapidly in the high magnetic field may cause sensations of dizziness, nausea, metallic taste, and perception of light flashes. Since whole body 7T imaging was only recently classified by the FDA as not a significant health risk, we set out to determine whether subjects scanned in the 7T magnet were experiencing any more discomfort than those scanned in a clinical 3T magnet. Toward this goal, we performed exit interviews on healthy volunteers scanned in the 7T and 3T magnets, asking a variety of questions to gauge several aspects of their comfort levels. Comparative Study of Patient Comfort at 7T and 3T MRI Methods: Scanner: GE 7T Excite (bore 60cm diam x 335cm length) and GE 3T LX (bore 60cm diam x 262cm length). The gradient performance of both scanners was 150mT/m/s slew rate and 40mT/m maximum gradient amplitude. Volunteers: For the 7T, a total of 186 scans were performed on 48 different healthy volunteers, with 25 volunteers undergoing multiple scans. For the 3T, a total of 100 scans were performed on 43 different healthy volunteers. Exit Interviews:In the exit interview, volunteers were asked to score their general comfort level before, during and after the scan on a scale of one to five, one being the least comfortable and five the most comfortable. Interview Topics:Subjects were also asked whether they felt certain sensations during the scan, such as nervousness, muscle stimulation, heat, visual changes, unusual taste, burning, claustrophobia, sleepiness, dizziness, headache, nausea, tingling, pain and cold. In addition, they were asked if they would be willing to return for another scan. Since anxiety may have been higher on the first visit, we analyzed successive visits separately. Modifications to the Study:The 7T scanner is a new system and several adjustments were made to the setup over the course of the study. Early in the study, we suspected that slowing the patient table would reduce reports of dizziness. The table speed was 40mm/s for the first 78 scans (Jun 04 – May 05), and was slowed to 20mm/s for the remaining 108 scans (Jun 05 –Dec 05). After scan 138 (Sep 05 - Dec 05), a custom designed 16-channel head coil was used (Nova Medical, Wakefield, MA), earlier experiments were performed with a GE transmit-receive coil. For a 3 month period, the fan and gradient chillers were not functioning properly. Figure 2: Reported sensations for volunteers that were scanned at least 3 times at 7T. Results: Overall comfort:This was consistently high in all interviews. On a scale of 1 to 5 (with 5 being the most comfortable), subjects in the 3T rated overall comfort level as 4.2 during the scan and 4.5 immediately afterwards. The 7T resulted in similar ratings of 4.3 and 4.6, respectively. Pain: Reports of pain increased after switching to the custom made 16-channel coil, which had may have had insufficient padding. This agrees with data from the 3T, on which a similar 16-channel coil was used for all scans reported here. Adding padding to the 16-channel coil resulted in fewer reports of pain. Dizziness:Before slowing the table speed, patient dizziness was reported in 20%of the cases (Figure 1). Slowing the table speed reduced this to 6%. When separating the first visit from subsequent ones, a decrease in reported sensations was observed, which could be related to subject acclimation. As shown in Figure 2, dizziness reporting decreased from about 20% on the 1st visit to 10% by the 3rd visit. Taste sensation:Succeeding visits and slower table speed both resulted in reduced reports of unusual taste sensation (see Figures 1 and 2). We are further investigating this effect to sort out whether this trend is just due to acclimation. Cold/Heat Sensation:Several volunteers reported feeling cold during their first and second scans, then reported feeling hot during their third scan (see Figure 2). Sensation of cold is common during an MRI scan, often caused by the bore ventilation fan. Increased reports of heat coincided with a malfunction of the fan and gradient chillers. Cold was generally reported again once these systems were repaired. Muscle Stimulation:Muscle stimulation does not seem to follow a trend but instead was related to the particular pulse sequence employed. Sleepiness:Volunteers often report feeling drowsy during scanning in both the 3T and the 7T magnets. Willingness to Return:In response to the question of whether they will return, the ‘yes’ responses for the 7T decreased slightly from 98% after the 1st visit to 95% after the 2nd, but was 100% after the 3rd visit. For 3T, the exit interviews were not separated into successive visits, but 97.5% of all subjects reported that they would return. Conclusion: The exit interviews used in this study allowed us to identify and monitor areas of patient discomfort affecting the volunteer’s experience. This knowledge has led us to improve the scan set up, such as reducing patient table speed and increasing padding in the 16 channel coil. There were no significant differences in reported sensations of discomfort between the 3T and 7T scanners, and overall comfort was consistently high in both scanners. Figure 1: Summary of sensations reported by all volunteers at both 3T and 7T. e-mail: fultons@mail.nih.gov http://amri.ninds.nih.gov