7 Tesla and Beyond: Advanced Methods and Clinical Applications in Magnetic Resonance Imaging

Abstract

Ultrahigh magnetic fields offer significantly higher signal-to-noise ratio, and several magnetic resonance applications additionally benefit from a higher contrast-to-noise ratio, with static magnetic field strengths of B0 ≥ 7 T currently being referred to as ultrahigh fields (UHFs). The advantages of UHF can be used to resolve structures more precisely or to visualize physiological/pathophysiological effects that would be difficult or even impossible to detect at lower field strengths. However, with these advantages also come challenges, such as inhomogeneities applying standard radiofrequency excitation techniques, higher energy deposition in the human body, and enhanced B0 field inhomogeneities. The advantages but also the challenges of UHF as well as promising advanced methodological developments and clinical applications that particularly benefit from UHF are discussed in this review article.

FIGURE 1

Sodium (²³Na) MRI at 1.5 (A), 3 (B), and 7 T (C). Images show that the SNR increases markedly with B₀. Sequence parameters: TE (1.5 and 3 T), 0.2 milliseconds; TE (7 T), 0.5 milliseconds; TR, 50 milliseconds; flip angle α, 77 degrees; nominal spatial resolution, 4 mm³; acquisition time, 10 minutes 50 seconds. Reused with permission from John Wiley & Sons, Kraff et al.

FIGURE 2

Susceptibility, R₁, R₂* maps, proton density-, T₂-weighted images, and color-coded diffusion anisotropy maps (left to right and top to bottom) of the same transverse slice in one healthy volunteer showing detailed anatomical substructures in the midbrain at 7 T and 3 T. A sagittal R₁ image indicates the slice location. Histologic myelin stain additionally shown for anatomical correlation (myelin stain reproduced from http://www.brains.rad.msu.edu and http://brainmuseum.org, supported by the US National Science Foundation). The cerebral aqueduct (CA), crus cerebri (corticobulbar fibers [cb], corticospinal fibers [cs], corticopontine fibers [cp]), the central tegmental tract (ctt), the mammillary body (MB), the medial lemniscus (ml), the medial longitudinal fasciculus (mlf), the optic tract (opt), the periaqueductal gray (PAG), the red nuclei (RN), the spinothalamic tract (stt), the substantia nigra (SN), the superiorcolliculus (SC), and the third ventricle (3 V) are indicated. The trochlear nuclei (TN) can only be clearly delineated in the histology stain. For better clarity, bilateral structures are only indicated monolaterally and at 7 T images. Reused with permission from John Wiley & Sons, Straub et al.

FIGURE 3

Axial T₂-weighted images of an MS patient acquired at 3 T (A) and 7 T (C) and corresponding axial SWI scans acquired at 3 T (B) and 7 T (D). Note the central vessels and iron deposits (rims) within MS lesions in SWI scans (B and D, white arrows). In T₂-weighted images, central vessels are challenging to depict (A and C, white arrows). Reused with permission from Wolters Kluwer Health, Inc, Springer et al.

FIGURE 4

Magnetic resonance images of the distal tibia acquired in vivo with FIESTA-C at 7 T (A) and 3 T (B) are depicted. The enhanced visualization of trabecular bone structure at 7 T is well demonstrated by these images. Please note that, although the same sequence is used, chemical shift and susceptibility artifacts are enhanced at 7 T, shown by thicker appearing trabeculae and at the muscle/fat interface. Also, the image contrast and signal of muscle and fat are different at 7 T and 3 T. Reused with permission from Wolters Kluwer Health, Inc, Krug et al.

FIGURE 5

Three-dimensional VIBE MRI at 1.5 T (A), 3 T (B), and 7 T (C) in the same subject. Seven Tesla 3D VIBE MRI demonstrated diagnostic potential by means of pathology detection, as it revealed a second hemorrhaged renal cyst (dashed arrow C₁) not displayed at lower field strengths. In the second row, arrows show a further very small renal cyst in the same subject, which is also best visible at 7 T (C₂). Reproduced with permission, open access, Laader et al.

FIGURE 6

Axial intracellular and extracellular pH maps in 3 patients with glioma merged with nonenhanced T₁-weighted MPRAGE (black/blue boxes: manually segmented tumor ROIs within the solid tumor compartment). Reused with permission from John Wiley & Sons, Korzowski et al.

FIGURE 7

Sample spectra overlaid with the LCModel fit (red color) at 3 T (left column) and 7 T (right column) from 3 different locations: occipital lobe (top), frontal lobe (middle), and parietal lobe (bottom). For instance, note the tNAA signal at 7 T. The better spectral resolution allows differentiation of NAA and NAAG at 7 T, but not at 3 T (measurement parameters: excitation flip angle, 45 degrees; field of view, 220 × 220 mm²; 64 × 64 matrix; TR, 600 milliseconds; TE*, 1.5 milliseconds; bandwidth, 6000 Hz; 2048 points; and 10-mm slice thickness). Reused with permission from Wolters Kluwer Health, Inc, Gruber et al.

FIGURE 8

Relaxation-compensated APT CEST-MRI at 7 T and clinical MR mammography. Patient 1: high grade/patient 2: intermediate grade intraductal breast cancer of no special type. In both patients, a strong gadolinium enhancement can be observed at clinical MRI: gadolinium-enhanced (Gdce), fat-saturated T₁-weighted MRI after administration of a standard dose (0.1 mmol/kg body weight) of gadobutrol (TR, 28 milliseconds; TE, 4.76 milliseconds; slice thickness, 1.1 mm; flip angle, 25; field of view, 360; matrix, 320) and subtraction MRI. In addition, T₂-weighted TSE (1c, 2c) and the APT_AREX contrasts at 7 T are shown. All breast cancers could be clearly identified on the APT_AREX contrast. APT signal hyperintensities showed a distinct morphological correlation with the contrast-enhanced MR images. Reproduced with permission from Elsevier, Loi et al.