In vitro and in silico assessment of RF‐induced heating around intracranial aneurysm clips at 7 Tesla

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Ultra‐high field (UHF) MRI at 7 Telsa (T) has demonstrated strong clinical benefits in neuroimaging 1, for example, for the diagnosis of tumorous entities 3 or the assessment of patients with multiple sclerosis 5. UHF MR angiography (MRA) also showed strong improvements in comparison to clinical field strengths at 1.5 or 3T 7, and the increased resolution enables the depiction of very small blood vessels (lenticulostriate arteries 10, perforating arteries 12, subcallosal artery 13), as well as small aneurysms 14 that may cause a subarachnoid hemorrhage. Nowadays, the gold standard for depiction of small aneurysms is invasive digital subtraction angiography. Aneurysms smaller than 3 mm are not reliably detectable with time‐of‐flight (TOF) MRA at clinical field strengths of 1.5 and 3T. In this regard, high‐resolution TOF MRA at 7T could very well become a noninvasive key clinical tool for the diagnosis of aneurysms and other cerebrovascular pathologies. Furthermore, MRI is the indicated follow‐up imaging modality for patients after aneurysm treatment, including patients with multimodal aneurysm therapy and clip implantation 22.
For most UHF sites, however, electrically conductive implants remain an absolute contraindication at 7T. UHF sites tend to be cautious with respect to the higher resonance frequency, the various transmit radiofrequency (RF) coils, and the fact that 7T MRI currently is not medically indicated. Nevertheless, some UHF sites already have performed dedicated safety tests for passive implants 25 or have included carefully selected subjects with implants 34. Because manufacturers only have verified MRI of patients with implanted aneurysm clips to be conditional for some clips in an MR environment up to 3T, a detailed compliance test for 7T is a prerequisite before any examination. Whereas the action of forces and torques on the aneurysm clip in the static magnetic field is reduced or completely avoided by nonferromagnetic materials such as titanium, RF‐induced heating due to of electric field (E‐field) elevations in the tissue close to the metallic clip is the major concern with respect to patient safety. Such field elevations typically depend strongly on the orientation of the elongated implant with respect to polarization of the E‐field. Because the position and orientation of clips inside the head can vary significantly, an investigation of the polarization dependency must be performed. This is particularly important in UHF MRI because field distribution and polarization are considerably nonuniform in the human head and, moreover, depend on the design of the local transmit coil and the size and tissue distribution of the head 36.
Previously published data concerning specific absorption rate (SAR) and temperature elevations near aneurysm clips at clinical field strengths 37 or at 7T 37 were not compared to the magnitude and polarization of the local field distribution. Hence, the data were obtained for random orientations of the clips and the results cannot be generalized. This study presents a comprehensive test procedure for RF‐safety assessment of an aneurysm clip at 7T that account for the polarization effects as well as the more complex coupling of the electromagnetic field with the human body.

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