Franck Girard, PhD Valentine L. Marcar, DPhil Franciszek Hennel, PhD Ernst Martin, MD Index terms: Magnetic resonance (MR), pulse sequences Magnetic resonance (MR), technology Radiology 2000; 216:900 –902 Abbreviation: RARE = rapid acquisition with relaxation enhancement 1 From the Department of Magnetic Resonance, University Children’s Hospi- tal Zurich, Steinwiestrasse 75, CH-8032 Zurich, Switzerland (F.G., V.L.M., E.M.) and the Commissariat a L’Energie Atom- ique—Service Hospitalier Fre ´de ´ric Joliot, Orsay, France (F.H.). Received January 4, 1999; revision requested February 23; final revision received December 28; ac- cepted January 12, 2000. Supported in part by grant 31-50734.97 from the Swiss National Science Foundation and grant BBW: 95.0314 from the Swiss Fed- eral Office for Education and Science. Address correspondence to F.G. (e-mail: frank.girard@kispi_unizh.ch). © RSNA, 2000 Author contributions: Guarantor of integrity of entire study, F.G.; study concepts, F.H.; study design, F.G.; definition of intellectual content, F.G.; literature research, F.H.; clinical studies, F.G., V.L.M.; experimental stud- ies, F.G., V.L.M.; data acquisition, F.G., V.L.M., E.M.; data analysis, F.G.; manu- script preparation and editing, F.G.; manuscript review, V.L.M. Anatomic MR Images Obtained with Silent Sequences 1 The authors evaluated silent mag- netic resonance (MR) imaging se- quences for their suitability in provid- ing high-spatial-resolution anatomic images that are of sufficient quality to be useful in a clinical setting. The authors compared the images ob- tained with a silent rapid acquisition with relaxation enhancement (RARE) sequence to its standard counterpart with respect to signal-to-noise ratio, distribution of gray level, and spatial resolution. No real differences were observed between the standard and the silent RARE MR images. Anatomic images were also acquired with a silent spin-echo sequence. Acoustic noise levels with the silent sequences were at least 22 dB (A-weighted scale) lower than those with standard sequences, without loss of image quality. Magnetic resonance (MR) images are pro- duced by applying radio-frequency pulses together with magnetic field gradients. The rapid switching of these gradients creates an induced Lorentz force, which in turn causes vibrations in the gradient coil structure. These vibrations are per- ceived as loud acoustic noise (1). Previous studies showed that levels around 80 dB on the A-weighted scale could be reached with standard rapid acquisition with re- laxation enhancement (RARE) sequences (2,3), and levels up to 115 dB are not uncommon with echo-planar imaging sequences (4). These acoustic noise levels occasionally approach the limits set by almost all safety boards, implying that ear protection should be used by patients and staff working near the magnet tube to avoid long-term hearing loss (5). The gradient acoustic noise during an MR im- aging examination is not only a major source of discomfort, it is the principle rea- son for the waking of children who had to be sedated, which invariably means the ex- amination must be abandoned. For anx- ious patients (6), those who experience claustrophobia, and children, this acoustic noise is an additional distress factor. In some instances, the gradient acous- tic noise also represents an obstacle to performing specific functional measure- ments, particularly those involving audi- tory stimuli (7–9). Finally, the acoustic noise level generated by a sequence can be expected to increase with increasing field strength (1). Reduction in the acous- tic noise level will become an important issue when field strength as high as 8 T is used for imaging purposes. Few useful solutions have been offered to overcome this problem. Acoustic noise- cancellation devices have been designed (10,11) and specific gradient coils have been constructed (12,13), but these ap- proaches are costly. From the point of view of adapting imaging sequences, little work has been done. One approach along these lines was the application of a reduced slew rate in functional MR imaging (14); another was the replacement of gradient pulsing with a mechanically rotated coil (15). The latter solution is currently restricted to use in the spin-echo sequence and may not be implemented with standard systems, espe- cially since use of a rotating coil may be considered a potential safety hazard. An approach to reduce the acoustic noise level is use of “silent” MR imaging sequences (1), which are based on the linear acoustic response of the gradient system (16) (ie, the imager acoustic noise spectrum is given by the product of the spectrum of the gradient waveform and the frequency response function of the system). Silent sequences make use of “soft” gradient waveforms that contain no frequencies for which the frequency response function is high. These meth- ods are implemented on the basis of the three rules of Hennel et al (1), that is, sinusoidal gradient slopes, maximum slope duration, and minimum number of slopes. We acquired images with silent RARE, 900