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Magnetic resonance (MR) scanning began to make an impact in the clinical practice setting in the mid 1970s. At introduction the most common systems operated at a field strength of 0.6T and there was credible doubt that more powerful magnets would be feasible, particularly for whole-body (beyond the brain) imaging needs. Eventually, technological advances made high-field MR imaging practical, and systems operating at 1.5T have become the current clinical benchmark. New systems sold today at lower field strengths are open designs designed for larger or claustrophobic patients. While there once was a significant market for closed systems operating at 1 .OT due to lower cost, the decreasing cost differential with 1.5T and market demands have all but eliminated these systems from new purchase considerations.
Over the last several years, systems operating at higher fields have become more prevalent, particularly at research centers. An informal survey of the market reveals that approximately 250 wholebody-capable MR systems are currently in operation with the majority of recent installations in the clinical practice setting. Market data in late 2004 indicate that 3T systems make up 25% of new high-field MR purchases.
Fueling the shift in interest from 1.5 to 3T from primarily a research device to clinical practice validates that what was once considered very high-field MR (3T) is practical, feasible, and, indeed, currently or potentially superior to 1.5T for clinical indications throughout the body. The driving forces behind this increased penetration of 3T scanners into the clinical setting include reduced concerns over surface coil availability, radiofrequency (RF) deposition limit, higher ambient noise, system homogeneity, increased magnetic susceptibility and chemical shift effects, and reduced tissue contrast. Also, this shift results from the documentation of incremental benefits of 3T over 1.5T with respect to image quality and efficiency.
Specific absorption rate
Specific absorption rate (SAR) is a measure of energy deposited by an RF field in a given mass of tissue. SAR is established by the International Electrotechnical Commission (IEC) to not exceed 8 watts per kg (W/kg) of tissue for any 5-minute period or 4 W/kg for a whole body averaged over 15 minutes.1 Dissipation of RF energy in the body can result in tissue heating. The doubling of field from 1.5T to 3T leads to a quadrupling...