(SAR) The Specific Absorption Rate is defined as the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).
The SAR describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the MR signal. Inhomogeneity of the RF field leads to a local exposure where most of the absorbed energy is applied to one body region rather than the entire person, leading to the concept of a local SAR. Hot spots may occur in the exposed tissue, to avoid or at least minimize effects of such theoretical complications, the frequency and the power of the radio frequency irradiation should be kept at the lowest possible level. Averaging over the whole body leads to the global SAR.
It increases with field strength, radio frequency power and duty cycle, transmitter-coil type and body size. The doubling of the field strength from 1.5 Tesla (1.5T) to 3 Tesla (3T) leads to a quadrupling of SAR. In high and ultrahigh fields, some of the multiple echo, multiple-slice pulse sequences may create a higher SAR than recommended by the agencies. SAR can be reduced by lower flip angle and longer repetition times, which could potentially affect image contrast.
Normally no threatening increase in temperature could be shown. Even in high magnetic fields, the local temperature increases not more than 1°C. 2.1°C is the highest measured increase in skin temperature. Eddy currents may heat up implants and thus may cause local heating.

FDA SAR limits:

IEC (International Electrotechnical Commission) SAR limits of some European countries:
All limits are averaged over 6 minutes. (For more details: IEC 60601-2-33 (2002))

In most countries standard MRI systems are limited to a maximum SAR of 4 W/kg, so most scanning in level II is impossible.
For Level I, in addition to routine monitoring, particular caution must be exercised for patients who are sensitive to temperature increases or to RF energy.
For Japan different SAR limits are valid.

Manufactured by Esaote S.p.A.;
a low field open MRI scanner with permanent magnet for orthopedic use. The outstanding feature of this MRI system is a patient friendly design with 24 cm diameter, which allows the imaging of extremities and small body parts like shoulder MRI. The power consumption is around 1.3 kW and the needed minimum floor space is an area of 16 sq m.
At RSNA 2006 Hologic Inc. introduced a new dedicated extremity MRI scanner, the Opera. Manufactured by Esaote is the Opera a redesign of Esaote's 0.2 Tesla E-Scan XQ platform, which now enables complete imaging of all extremities, including hip and shoulder applications. 'Real-time positioning' reportedly speeds patient setup and reduces exam times.
Esaote North America and Hologic Inc are the U.S. distributors of this MRI device.

From GE Healthcare;
the Signa Infinity Magnetic Resonance system is a short bore, high performance, whole-body imaging system operating at 1.0 Tesla. The system can image in any orthogonal or oblique plane (including single and double axis oblique), using a wide variety of pulse sequences.

From Siemens Medical Systems;
Received FDA clearance in 2007.
The MAGNETOM Essenza is designed to combine high system performance with simple installation and power requirements to provide optimal operating costs for limited budgets. The standard system has up to 25 integrated coil elements and 8 independent radio frequency channels. Tim allows the combination of up to 4 different coils that reduce patient and coil repositioning.
The 1.5 Tesla system is designated for a complete range of clinical applications, including neurology, orthopedics, body imaging, angiography, cardiology, breast imaging, oncology and pediatric MRI.

From Siemens Medical Systems;
Received FDA clearance in 2010.
The MAGNETOM Aera is a patient friendly, comfortable 1.5 Tesla MRI system with advanced radio frequency chain.
The system is equipped with the Tim 4G and Dot system (Total imaging matrix + Day optimizing throughput), to enhance both productivity and image quality.
Tim 4G technology provides improved SNR. The standard system configuration of 48 radio frequency channels and 204 coil elements creates an imaging matrix that allows maximum use of coil elements at full field of view. Dot provides improved image consistency through new features like auto align, auto FoV and automatic bolus detection.