From the beginning of MRI use, one source of its noninvasive nature and general appeal was its safety. The modality is not, however, entirely free of risk: the radiofrequency energy that reaches the patient causes heating.1 The amount of energy absorbed by the patient is measured as the specific absorption rate (SAR). The US Food and Drug Administration2 limits the maximum SAR to the amount required to raise the body’s core temperature by 1degreeC.
The risk of exceeding the SAR limit increases at higher field strengths and possesses SAR 4 times higher than at 1,5T, for example. This can be especially problematic, particularly with the use of popular spin echo-based pulse sequences like rapid acquisition with relaxation enhancement (RARE), also known as fast-spin echo or turbo-spin echo (TSE), and half-Fourier acquisition single-shot turbo-spin echo (HASTE). In these sequences, each echo is preceded by an RF pulse, which corrects for heterogeneity of the magnetic field and for chemical shifting. This results in long echo trains that can, at fields greater than 1.5T, easily exceed SAR limits.
Hennig and Scheffler3 of the section of medical physics, department of diagnostic radiology, University of Freiburg, Germany, have succeeded in preserving the utility and safety of TSE and HASTE with more high-field magnets using a new spin-refocusing strategy employing echoes of echoes (hyperechoes). In their experience, hyperecho multishot TSE imaging with an echo-train length of 12 to 25 preserves the signal-to-noise ratio of the conventional sequences but reduces the SAR by 70% to 80%. Even more impressive results were obtained with single-shot HASTE imaging, where the SAR reduction was 90% or more. These researchers expect hyperecho protocols to give MRI practitioners new freedom in the design of pulse sequences and to give physicists new insight into molecular behavior in magnetic fields.
– Judith Gunn Bronson, MS
- Bottomley PA, Edelstein WA. Power deposition in whole-body NMR imaging. Med Phys. 1981;8:510-512.
- US Food and Drug Administration Center for Devices and Radiological Health. Guidance for the Submission of Premarket Notifications for Magnetic Resonance Imaging Devices. Rockville, Md: FDA; 1998.
- Hennig J, Scheffler K. Hyperechoes. Magn Reson Med. 2001;46:6-12.