Evolution of UHF Body Imaging in the Human Torso at 7T

Abstract

The potential value of ultrahigh field (UHF) magnetic resonance imaging (MRI) and spectroscopy to biomedical research and in clinical applications drives the development of technologies to overcome its many challenges. The increased difficulties of imaging the human torso compared with the head include its overall size, the dimensions and location of its anatomic targets, the increased prevalence and magnitude of physiologic effects, the limited availability of tailored RF coils, and the necessary transmit chain hardware. Tackling these issues involves addressing notoriously inhomogeneous transmit B 1 ( B 1 + ) fields, limitations in peak B 1 + , larger spatial variations of the static magnetic field B 0 , and patient safety issues related to implants and local RF power deposition. However, as research institutions and vendors continue to innovate, the potential gains are beginning to be realized. Solutions overcoming the unique challenges associated with imaging the human torso are reviewed as are current studies capitalizing on the benefits of UHF in several anatomies and applications. As the field progresses, strategies associated with the RF system architecture, calibration methods, RF pulse optimization, and power monitoring need to be further integrated into the MRI systems making what are currently complex processes more streamlined. Meanwhile, the UHF MRI community must seize the opportunity to build upon what have been so far proof of principle and feasibility studies and begin to further explore the true impact in both research and the clinic.

Ultra-high-field MR in Prostate cancer: Feasibility and Potential

“…Coil designs with multiple independent transmit elements allow for parallel transmission to manipulate the transmit field distribution. These coils are most effectively used when coupled with a system equipped with parallel transmit hardware which has been previously described in the context of UHF body imaging [ 14 ]. The possibility to modulate the phase and/or magnitude of each transmit element in an array enables so-called B 1 + shimming.…”

“…To perform these studies, it was important to consider the increased R 2 * (1/T 2 *) relaxivity of Gadolinium based contrast agents at 7 T. While the R 1 (1/T 1 ) relaxivity, which gives the desired enhancement for DCE MRI, remains relatively constant with increasing field strength, the R 2 * relaxivity was shown to increase nearly fourfold at 7 T compared to 3 T [ 50 ] thus dominating the observed signal changes especially in the blood where the concentrations are the highest. To avoid biasing the interpretations of these data, either the time-varying T 2 * needs to be calculated and corrected for through a multi-TE acquisition, or ultra-short TE acquisitions can be used to mitigate the T 2 * effects [ 14 ].…”

Metamaterial Liner for MRI Excitation—Part 2: Design and Performance at 4.7T

“…Thus, in the MM the SAR is redistributed towards the neck and legs and reduced at the periphery of the torso and arms. In high (>1.5 T) ultra-high (>4.7 T) field MRI the application of many rapid imaging sequences is greatly limited by local SAR constraints [1], [35]. Thus, the significantly reduced local SAR of the MM liner design would improve the temporal imaging efficiency (i.e., allowing faster image acquisition).…”

On the way to routine cardiac MRI at 7 Tesla - a pilot study on consecutive 84 examinations

“…Magnetic resonance Imaging at ultrahigh field strength (UHF) has demonstrated its capability in brain imaging and in other organ systems [1][2][3][4]. Cardiac magnetic resonance (CMR) imaging is a valuable non-invasive tool for both the diagnostic evaluation of patients with cardiac pathologies and for research on cardiovascular disease.…”