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» MRI Safety

Exposures, limits, safe systems of work. This section explores safety issues in magnetic resonance imaging.

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What is SAR? How can it be calculated?

RF exposure is measured as either an increase in body temperature or as the Specific Absorption Rate (SAR). It depends on induced electric field, pulse duty cycle (D), tissue density, conductivity (sigma, σ) and the patients size. It can be calculated from the average forward power passing into the RF transmitting coil, and the body mass immersed in the RF transmitting field.

For a sphere of tissue of radius r, SAR is proportional to σr2B02α2D. Alpha (α) is the flip angle, and B0 is the strength of the external magnetic field.

The SAR (rate of energy deposition) is expressed per kilogram of body weight. Neglecting the effects of cooling, an SAR of 1 Wkg-1 applied for an hour would result in a temperature rise of about 1 °C.

Further reading on this topic:
Books: MHRA Guidelines p21, 29-31, MRI From Picture to Proton p190-191, Q&A in MRI p304-305
Online:, IMRSER

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