7-Tesla MRI of the brain in a research subject with bilateral, total knee replacement implants: Case report and proposed safety guidelines

Perivascular Spaces, Glymphatic System and MR

The importance of the perivascular space (PVS) as one of the imaging markers of cerebral small vessel disease (CSVD) has been widely appreciated by the neuroradiologists. The PVS surrounds the small blood vessels in the brain and has a signal consistent with the cerebrospinal fluid (CSF) on MR. In a variety of physio-pathological statuses, the PVS may expand. The discovery of the cerebral glymphatic system has provided a revolutionary perspective to elucidate its pathophysiological mechanisms. Research on the function and pathogenesis of this system has become a prevalent topic among neuroradiologists. It is now believed that this system carries out the similar functions as the lymphatic system in other parts of the body and plays an important role in the removal of metabolic waste and the maintenance of homeostatic fluid circulation in the brain. In this article, we will briefly describe the composition of the cerebral glymphatic system, the influencing factors, the MR manifestations of the PVS and the related imaging technological advances. The aim of this research is to provide a reference for future clinical studies of the PVS and glymphatic system.

Neuroimaging at 3T vs 7T: Is It Really Worth It?

Food and Drug Administration approval of 7T MR imaging allows ultrahigh-field neuroimaging to extend from the research realm into the clinical realm. Increased signal is clinically advantageous for smaller voxels and thereby high spatial resolution imaging, with additional advantages of increased tissue contrast. Susceptibility, time-of-flight signal, and blood oxygen level-dependent signal also have favorable clinical benefit from 7T. This article provides a survey of clinical cases showcasing some advantages of 7T.

Safety for Human MR Scanners at 7T

After introduction of the first human 7 tesla (7T) system in 1999, 7T MR systems have been employed as one of the most advanced platforms for human MR research for more than 20 years. Currently, two 7T MR models are approved for clinical use in the U.S.A. The approval facilitated introduction of the 7T system, summing up to around 100 worldwide. The approval in Japan is much awaited. As a clinical MR scanner, the 7T MR system is drawing attention in terms of safety.Several large-sized studies on bioeffects have been reported for vertigo, dizziness, motion disturbances, nausea, and others. Such effects might also be found in MR workers and researchers. Frequency and severity of reported bioeffects will be presented and discussed, including their variances. The high resonance frequency and shorter RF wavelength of 7T increase the concern about the safety. Homogeneous RF pulse excitation is difficult even for the brain, and a multi-channel parallel transmit (pTx) system is considered mandatory. However, pTx may create a hot spot, which makes the estimation of specific absorption rate (SAR) to be difficult. The stronger magnetic field of 7T causes a large force of displacement and heating on metallic implants or devices, and the scan of patients with them should not be conducted at 7T. However, there are some opinions that such patients might be scanned even at 7T, if certain criteria are met. This article provides a brief review on the effect of the static magnetic field on humans (MR subjects, workers, and researchers) and neurons, in addition to scan sound, SAR, and metal implants and devices. Understanding and avoiding adverse effects will contribute to the reduction in safety risks and the prevention of incidents.

Perivascular Space Imaging at Ultrahigh Field MR Imaging

The recent Food and Drug Administration approval of 7 T MR imaging scanners for clinical use has introduced the possibility to study the brain not only in physiologic but also in pathologic conditions at ultrahigh field (UHF). Because UHF MR imaging offers higher signal-to-noise ratio and spatial resolution compared with lower field clinical scanners, the benefits of UHF MR imaging are particularly evident for imaging small anatomic structures, such as the cerebral perivascular spaces (PVS). In this article, the authors describe the application of UHF MR imaging for the investigation of PVS.

[Safety of implants in high field and ultrahigh field MRI]

BACKGROUND

It can be expected that the number of 7 T MRI systems for clinical use will increase in the future. On the other hand, almost no medical implant has been labeled MR conditional for 7 T, so far, leaving the question of implant safety unanswered to the MR operator.

METHODS

In principle, the same interactions between magnetizable and electric conductive material apply at 7 T as known at lower magnetic field strengths. However, there are a few important differences that need to be taken into account to perform a profound risk-benefit analysis. After a more general introduction of technical differences between 3 and 7 T systems, the article will focus mainly on safety assessments with regard to interactions between implant and radiofrequency (RF) transmit fields. In addition, strategies to ensure access at 7 T will be discussed.

RESULTS OF PRACTICAL RELEVANCE

Besides hazards due to the magnetic force which can be up to 2.3 times stronger at 7 T compared to 3 T, increased risks of RF-induced tissue heating are the most critical aspects. The resonant-length of an implant at 7 T is about 5 cm. Other than at 3 T, MR systems at 7 T are less standardized. Especially with regard to the RF transmit coil and transmission methods used, substantial differences need to be expected. Hence, it is important to critically question published safety assessments of implants and to have a thorough discussion about how this relates to the individual exposure scenario. For nonmagnetic implants without a dedicated 7 T safety evaluation, but which are 3 T MR conditional and have a certain minimum distance to the RF transmit coil, a consensus recommendation from the national network German Ultrahigh Field Imaging (GUFI) may be helpful.