- Vascular MR benefits from the increased sensitivity at ultra-high magnetic field strength.

- High resolution intracranial 7T TOF-MRA enables unsurpassed non-invasive visualization of vascular pathologies.

- Depending on the availability of body transmit/receive RF coils, 7T vascular MR imaging is now being extended from intracranial to body applications.

To overcome the general technical and methodological challenges associated with 7 Tesla UHF MR to assess the increased sensitivity for MRA of the intracranial vasculature, liver, renal and peripheral arteries.Several comparison studies performed at different magnetic field strength have shown that MR angiography (MRA) benefits from the increased sensitivity provided by ultra-high field (UHF) strength MRI systems [1]. Especially the time-of-flight (TOF) MRA techniques profit from high sensitivity and excellent background signal suppression, leading to very high spatial resolution MR angiograms in intracranial applications providing submillimeter resolution [1,2]. While non-enhanced intracranial MR imaging and MRA at 7T using TOF, due to the availability of commercial radiofrequency (RF) head coils, can be considered a widespread technique among the UHF sites, 7T MRA of the body vasculature or of the peripheral arteries, however, is still hampered by numerous technical challenges.High-field MRA of the intracranial vessels has been performed on whole-body MRI systems with 7T magnetic field strength (e.g. MAGNETOM 7T, Siemens Healthcare GmbH). For signal excitation (Tx) and reception (Rx), custom-built 8-channel and 16-channel Tx/Rx RF head coils [3] or a commercially available 1/32-channel RF head coil (Nova Medical, USA) have been used in these studies. High spatial resolution intracranial MRA was acquired with TOF sequences with adapted imaging parameters to yield a spatial resolution of 0.2x0.2x0.4 mm3. Modifications of the RF pulses used in 7T TOF MRA have been suggested to improve venous saturation and to reduce the RF power to stay within the constraints of the specific absorption rate (SAR) [4,5]. The availability of head RF coils has enabled intracranial 7T TOF-MRA applications in various studies on patients with cerebral arteriovenous malformations (AVM) [6] and intracranial aneurysms [7]. In all of these studies the excellent spatial resolution enabled visualization of vascular fine structures and pathologies [5-7] (Fig. 1).While intracranial MRA at 7T has demonstrated excellent image quality, the transition of UHF MRA to other vascular regions in the human body is technically challenging. The resonance frequency at 7T is relatively high with 300 MHz and the associated wavelength in the human body, thus, can be considered relatively short. This leads to well-known RF inhomogeneities when UHF MR body-imaging is pursued [8]. Ultra-high field MRA of the body vasculature is, thus, strongly dependent on the availability and development of custom-built multi-channel Tx/Rx RF body coils and appropriate B1-shimming systems for RF signal homogenization in the body [9]. Initial studies using an 8-channel Tx/Rx RF system with B1-shimming capabilities in combination with a 2x 4-channel Tx/Rx RF body array coil and with using a TOF sequence were able to demonstrate non-enhanced 7T MRA of the renal arteries [10] and of the liver vessels [11]. Although RF signal inhomogeneities remain, these studies can be considered a first important step to apply 7T MRA in non-neuro vascular regions (Fig. 2). Further studies will have to concentrate on the methodological challenges such as RF transmit systems, RF coils, B1-shimming algorithms and adaption of MRA sequences to the specific needs of UHF MRA.Vascular imaging of the peripheral arteries at 7T adds another set of technical challenges to the ones mentioned above for body MRA. Peripheral UHF MRA requires not only multichannel transmit/receive RF body coils and B1-shimming, but additionally a method for stepwise motion of the patient table to acquire MRA data over a large field-of-view (FOV). In two recent studies investigating the technical feasibility of peripheral MRA at 7T [12,13], a 16-channel Tx/Rx RF body coil has been combined with an effective method for B1-shimming [9], and with a rolling table platform for manual patient repositioning [14]. For acquisition of MRA data on patients with peripheral arterial occlusive disease (PAOD), a T1-weighted fast gradient echo sequence (2D Turbo-FLASH) has been used. Data acquisition was gated by using a phonocardiogram. The resulting spatial resolution was in the range of 1.5 mm3 isotropic. The acquisition time for this multi-step examination to cover the peripheral arteries from the pelvis down to the feet accumulated to rather long 30 min per patient. Nevertheless, the concept proved technical feasible and the resulting peripheral MR angiograms obtained at 7T compared well with the intraindividual 1.5T contrast-enhanced MRA data sets serving as standard of reference [12,13] (Fig. 3).The high sensitivity inherent to UHF MR imaging can be used to achieve 7T MRA data of the intracranial vasculature and pathologies with unsurpassed high spatial resolution and vessel detail. The translation from intracranial UHF MRA to body MRA applications, however, is currently limited by numerous technical challenges. Initial studies providing encouraging first results of 7T body and peripheral MRA on patients have demonstrated that these hurdles can be overcome. This can be seen as a first step to further assess the clinical potential of vascular MR at 7T.