Ultra-high field magnets for whole-body MRI

Abstract

For whole-body MRI, an ultra-high field (UHF) magnet is currently defined as a system operating at 7 T or above. Over 70 UHF magnets have been built, all with the same technical approach originally developed by Magnex Scientific Ltd. The preferred coil configuration is a compensated solenoid. In this case, the majority of the field is generated by a simple long solenoid that stretches the entire length of the magnet. Additional coils are wound on a separate former outside the main windings with the purpose of balancing the homogeneity. Most of the magnets currently in operation are passively shielded systems where the magnet is surrounded by a steel box of 200–870 tonnes of carbon steel. More recently actively shielded magnets have been built for operation at 7 T; in this case the stray field is controlled by with reverse turns wound on a separate former outside the primary coils. Protection against quench damage is much more complex with an actively shielded magnet design due to the requirement to prevent the stray field from increasing during a quench. In the case of the 7 T 900 magnet this controlled by combining some of the screening coils into each section of the protection circuit. Correction of the field variations caused by manufacturing tolerances and environmental effects are made with a combination of superconducting shims and passive shims. Modern UHF magnets operate in zero boil-off mode with the use of cryocoolers with cooling capacity at 4.2 K. Although there are no cryogen costs associated with normal operation UHF magnets require a significant volume (10 000–20 000 l) of liquid helium for the cool-down. Liquid helium is expensive therefore new methods of cool-down using high-power cryocoolers are being implemented to reduce the requirement.

Ultra-High Field NMR and MRI—The Role of Magnet Technology to Increase Sensitivity and Specificity

“…Research at 3 T started in the late 1980s (3T/80 cm, Detroit), on passively shielded prototype systems with fast gradients and local transmit-receive coils. Today, MRI is widely used for noninvasive imaging of internal body structures, providing high soft tissues contrast, at field strengths up to 3 T for clinical routine and 7 T, 9.4 T and 10.5 T (a 11.7 T for head only MRI scanner was designed, however, at the time of writing this magnet was out of order due to magnet quench) for research only [55][56][57][58][59][60][61][62][63].…”

“…By 2015, about 60 % of all clinical MRI volume sales were achieved for 1.5 T, 34% for 3 T systems (slowly replacing current 1.5 T scanners), and a rather stable 6 % for low field scanners (<0.5 T). Although a few 4 T and 4.7 T prototype magnets have been installed since 1988, stronger magnets have been operating at 7 T for UHF research only [57,58,61,66,67]. Thus, 7 T human whole body scanners are currently only being installed in selected high end (clinical) research units (with about 70 systems installed so far).…”

A novel passive shimming scheme using explicit control of magnetic field qualities with minimal use of ferromagnetic materials

“…In addition, it is difficult to operate the high‐order shim coils due to the magnetic field coupling in their superconducting states. Thus, ultra‐high‐field magnets generally use the superconducting shim coils to offset the low‐order harmonics, and the remnant harmonic terms with small amplitudes are eliminated by the passive shimming technique 12,13 …”

“…[7][8][9] The permanent magnet, superconducting magnet of 1.5T, 3T, and some 7T MRI systems mainly apply the passive shimming operation to reach the target magnetic field homogeneity, 10,11 and the shim coils are reserved for dynamic field adjustment. Even for the ultrahigh field MRI superconducting magnet systems such as 9.4T, 10.5T, 11.75T, etc., it is challenging to abandon passive shimming to elevate the magnetic field quality, [12][13][14][15][16] although these high-end systems are required to use fewer iron pieces to avoid accompanying side effects such as eddy current, temperature drift, etc. For ultra-high-field MRI superconducting magnet systems, the superconducting shim coils may have the main contribution to the magnetic field homogeneity.…”

Conduction-cooled ReBCO coils for the wind converter: from laboratory to field test

“…For some applications, both are valid. For instance, magnetic resonance imaging (MRI) magnets can benefit from an increase in magnetic field, since this can increase the signal-to-noise ratio (SNR) and lead to an improved spectral resolution [22,23]. This requires a central magnetic field of more than 7 T. The disadvantages, however, include a larger stray field and an increase in costs.…”