SAR and RF Power Deposition
SAR and RF Power Deposition At 3 T the Specific Absorption Ratio (SAR), a
measure of RF energy deposited in the body, increases by a factor of four, and has an undesirable impact on essential image characteristics including contrast, acquisition time, and resolution. The SAR thresholds as per Food and Drug Administration guidelines are 2 W/kg in the normal mode and 4 W/kg in the first level-controlled mode.
Magnetic Field Distortions
Susceptibility effects are greater at 3 T. In body imaging these may arise from surgical clips or hardware or from air-soft tissue interfaces that arise from the lungs, gastrointestinal tract or irregular skin surfaces.
Fields-of-view in excess of 35-40 cm are commonly used for body imaging in the z-direction, placing high demand on static magnetic field homogeneity over a large area. Field homogeneity is significantly more challenging at 3 T.
Motion from respiration, heart and bowel peristalsis may cause marked image deterioration. At 3 T fast acquisition techniques may be limited by SAR concerns.
Sequences have been optimized at 1.5 T to achieve a balance between speed of acquisition and image contrast. At 3 T, image contrast may suffer from a combination of compromises arising from SAR limitations. In addition, adjustments are required
to adapt to the longer T1 and shorter T2 of soft tissues at 3 T.
RF signal generated at 3 T is four times stronger than at 1.5 T. However, a simultaneous increase in noise by a factor of two results in a net SNR ratio increase of only two.A challenge is to be able to realize this theoretical benefit as the increase in SNR
at 3 T is balanced against losses from adjustments for limitations related to SAR.
This phenomenon is seen as a loss of signal when the transmitted wavelength approaches the size of the body structure . For a circular body habitus there is loss of signal at the periphery, while for an oval body habitus the signal loss occurs at the periphery of the short axis.To a certain extent this can be corrected using phantom-derived algorithms like body-tuned CLEAR (Philips Medical Systems).
Alustiza JM, Artetxe J, Castiella A. MR quantification
of hepatic iron concentration. Radiology