Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

purely resistive system is the ability to turn off the magnetic field in an emergency. Thedisadvantages include high electricity costs and relatively poor uniformity/homogeneityof the field (e.g., 30 to 50 ppm in a 10 cm 3 volume). Systems using an electromagnetsolid core design overcome many of the disadvantages of the air core resistive magnetdesign; most significantly, the "open" design enables claustrophobic patients to toleratethe examination. Fringe fields of the main magnet are also better confined.Superconductive magnets typically use an air core electromagnet configuration(there are some solid core superconductive magnets), and consist of a large cylinderof approximately 1 m in diameter and 2 to 3 m in depth, wrapped with a long, continuousstrand of superconducting wire. Superconductivity is a characteristic of certainmetals (e.g., niobium-titanium alloys) that exhibit no resistance to electric currentwhen kept at extremely low temperatures, with liquid helium (boiling point of4°K) required as the coolant. After initialization ("ramp up") by an external electricsource, current continues to flow as long as the wire temperature is kept at liquidhelium temperatures. Higher temperatures cause the loss of superconductivity, andresistance heating "quenches" the magnetic field. Superconductive magnets achievehigh field strengths (0.3- to 3.0-T clinical systems are common, and 4.0- to 7.0-Tclinical large bore magnets are used in research). In addition, high field uniformityof 1 ppm in a 40-cm 3 volume is typical. The several disadvantages of superconductingmagnets include high initial capital and siting costs, cryogen costs (mostmagnets have refrigeration units to minimize liquid helium losses), difficulty inturning off the main magnetic field in an emergency, and extensive fringe fields.Uncontrolled quenching can cause explosive boiling of the liquid helium and severedamage to the magnet windings. Despite these disadvantages, the superconductivemagnet is by far the most widely used magnet type for clinical imaging.A cross section of the internal superconducting magnet components showsintegral parts of the magnet system, including gradient, shim, and RF coils, as wellas the cryogenic liquid containment vessels (Fig. 15-40).VacuumInsulationFIGURE 15-40. Cross-sectional view of the superconducting magnet shows thecoils for the magnet, shims, RF,and gradients. Surrounding the superconductorcoil assembly is the liquid helium bath and surrounding wrap of liquid nitrogencontainers with vacuum insulation barriers. In modern systems with refrigeration,the liquid nitrogen containers are unnecessary.