Last year, ultra-high-field 7T MRI was approved by the FDA and the European Union for clinical use in routine daily practice. The scientific community has great expectations from this technology, on the one hand due to the high spatial resolution that can be achieved and, on the other hand, due to the much-improved visualization of paramagnetic substances on susceptibility sequences.

Which organs will benefit most from this type of imaging?

Definitely the brain, due to its anatomic configuration and the fact that it is a near-motionless organ. Furthermore, the brain also allows all types of advanced imaging such as DTI, functional imaging,¹ and spectroscopy.² Conversely, 7T spine imaging is still in its early stages and requires more time and coil development so as to improve signal-to-noise ratio and decrease artifacts in order to achieve the same image quality as 3T MRI.

Several questions may arise concerning this type of MR scanner: Is it easily applicable in a clinical setting? Which patients will be eligible to be scanned at 7T? What are the side effects? Which clinical indications are appropriate—all indications or only for patients who previously underwent 3T MRI that failed to answer the clinical question? Will we need to learn a new neuroradiologic semiology?

At present, the clinical indications that seem to benefit most from this type of imaging are epilepsy,^1,3,4 aneurysms,⁵ multiple sclerosis,⁶ ischemia,⁷ arterial wall imaging,⁸ cavernomas,⁹ and dementia.¹⁰

The most common side effects seem to be vertigo, flashing lights, and a metallic taste in the mouth.

Currently, some clinical papers concerning hot topics in neuroradiologic imaging are starting to appear in specialized literature.

There are great expectations for 7T MRI concerning advances in new semiology, in the understanding of histopathologic mechanisms, and in detecting new findings, which may improve diagnostic accuracy and therefore change treatment and prognosis of some conditions.

New imaging signs are emerging and facilitating the detection and diagnosis of different pathologies such as the intracortical black line sign in type IIb focal cortical dysplasia,¹¹ vascular distribution in polymicrogyria, venous structures associated with tubers in tuberous sclerosis complex,^3,4 venous angioarchitecture of cerebral cavernous malformations,^9,12 or concentric enhancing foci of the ipsilateral vessel wall to the treated artery after thrombosuction⁸ compared with the contralateral side.

What is the reality?

7T MR imaging requires highly specialized engineering support in order to optimize the sequences, as well as to understand and minimize artifacts. Currently, this type of scanner is almost entirely reserved for research settings and cannot be found in hospitals, nor is it used in daily clinical practice. This makes it difficult for patients to access this technology and most of the research is carried out on volunteers and selected patients. Despite the current situation, we expect that this type of imaging will soar when 7T MR scanners are installed in university hospitals.

As opposed to MR scanners at lower magnetic field strengths, 7T MRI will allow clinical and functional protocols to coexist in harmony and intertwine, which should result in faster and more effective imaging.¹³

Limitations

Limitations of 7T MRI relate mainly to long image acquisition time, elevated specific absorption rate (SAR)—particularly in spin-echo‐based sequences, which limits the number of slices—and also issues relating to metallic devices and artifacts.

The first limitation could be overcome by using multiarray coils in combination with parallel imaging. Decreased SAR could be achieved with the implementation of adiabatic pulses and lower flip angles.

There are only a few metallic devices that have been tested with this type of scanner, and there are still many more that need testing for 7T compatibility.¹⁴

Artifacts of inhomogeneity are mostly seen on 3D FLAIR and DWI. Issues of geometric distortion and loss of signal, particularly at the level of the cerebellum and temporal lobes, have been almost completely resolved with the use of dielectric pads.

What is the future?

More and more patients will undergo 7T MRI, but in my opinion, this type of imaging will not be used as much as first intention as it will for patients for whom the diagnosis could not be determined with lower magnetic field strengths. There is hope that the highly specific emergent imaging signs of 7T MRI will approach features seen only on histopathology until now. As we have seen, more and more abnormalities, for example, of vascular structures, are becoming apparent that in themselves have no impact on the life of the patient. However, this technology also allows visualization of anatomic structures that until now could not be analyzed with current imaging techniques. We believe this will probably facilitate surgical technique and lead to reduced patient morbidity.

Finally, due to side effects, 7T MRI will probably be the highest field strength that we will ever use in clinical daily routine.

References

1. Grouiller F, Jorge J, Pittau F, et al. Presurgical brain mapping in epilepsy using simultaneous EEG and functional MRI at ultra-high field: feasibility and first results. MAGMA 2016;29:605-16
2. Vargas MI, Martelli P, Xin L, et al. Clinical neuroimaging using 7 T MRI: challenges and prospects. J Neuroimaging 2018;28:5-13
3. De Ciantis A, Barkovich AJ, Cosottini M, et al. Ultra-high-field MR imaging in polymicrogyria and epilepsy. AJNR Am J Neuroradiol 2015;36:309-16
4. Pittau F, Baud MO, Jorge J, et al. MP2RAGE and susceptibility-weighted imaging in lesional epilepsy at 7T. J Neuroimaging 2018;28:365-69
5. Sato T, Matsushige T, Chen B, et al. Wall contrast enhancement of thrombosed intracranial aneurysms at 7T MRI. AJNR Am J Neuroradiol 2019;40:1106-11
6. Maranzano J, Dadar M, Rudko DA, et al. Comparison of multiple sclerosis cortical lesion types detected by multicontrast 3T and 7T MRI. AJNR Am J Neuroradiol 2019;40:1162-69
7. Fujimoto K, Uwano I, Sasaki M, et al. Acetazolamide-loaded dynamic 7T MR quantitative susceptibility mapping in major cerebral artery steno-occlusive disease: comparison with PET. AJNR Am J Neuroradiol 2020;41:785-91
8. Lindenholz A, van der Schaaf IC, van der Kolk AG, et al. MRI vessel wall imaging after intra-arterial treatment for acute ischemic stroke. AJNR Am J Neuroradiol 2020;41:624-31
9. Dammann P, Barth M, Zhu Y, et al. Susceptibility weighted magnetic resonance imaging of cerebral cavernous malformations: prospects, drawbacks, and first experience at ultra-high field strength (7-Tesla) magnetic resonance imaging. Neurosurg Focus 2010;29:E5
10. McKiernan EF, O'Brien JT. 7T MRI for neurodegenerative dementias in vivo: a systematic review of the literature. J Neurol Neurosurg Psychiatry 2017;88:564-74
11. Bartolini E, Cosottini M, Costagli M, et al. Ultra-high-field targeted imaging of focal cortical dysplasia: the intracortical black line sign in type IIb. AJNR Am J Neuroradiol 2019;40:2137-42
12. Dammann P, Wrede K, Zhu Y, et al. Correlation of the venous angioarchitecture of multiple cerebral cavernous malformations with familial or sporadic disease: a susceptibility-weighted imaging study with 7-Tesla MRI. J Neurosurg 2017;126:570-77
13. Clarke WT, Mougin O, Driver ID, et al. Multi-site harmonization of 7 tesla MRI neuroimaging protocols. Neuroimage 2020;206:116335
14. Hoff MN, McKinney A IV, Shellock FG, et al. Safety considerations of 7-T MRI in clinical practice. Radiology 2019;292:509-18

Image from: Lindenholz A, van der Schaaf IC, van der Kolk AG, et al. MRI vessel wall imaging after intra-arterial treatment for acute ischemic stroke.