Magnetic resonance imaging (MRI) has been a vital tool to assess cerebrovascular lesions. In addition to 1.5T and 3T MRI currently being the most widely used medical imaging modalities in clinical settings, ultra-high field MRI at 7Tesla (7T) has been studied extensively over the past few years. 7T MRI is characterized by high signal-to-noise and contrast-to-noise ratios. The clinical application of 7T MRI has been investigated in the field of structural brain anatomy and used to locate lesions. In 7T MRI, better spatial resolution of targeted lesions1 and better anatomical patterns of the substantia nigra2 have been previously achieved. Regarding cerebrovascular diseases, some pilot studies have shown the advantages of 7T MRI in depicting aneurysms, small perforating arteries and microbleeds. In this paper, the current status of 7T MRI is presented along with its future direction in cerebrovascular diseases.
The changes in the T1 and T2 relaxation times of brain tissues in high magnetic field strength MRI at 7T have exhibited better contrast. An increased T1 relaxation time can result in a difference in excitation and saturation between moving blood and stationary tissues. Stationary tissues become more excited and saturated with increasing exposure to radiofrequency pulses in longer T1 relaxation times. However, moving blood becomes less excited and saturated, which is attributed to a high signal.3 Consequently, a more sharply delineated image of the vessel can be obtained. Monninghoff et al4 reported that 7T MRI shows a better depiction of the dome and neck of an aneurysm than that of 1.5T MRI (Figure 1). A clear depiction of the anterior choroidal artery (AchoA) and perforating arteries from the posterior communicating artery (PCoA) also has been achieved with 7T MRI (Figure 2).5 Accordingly, 7T MRI has the potential to distinguish between a true aneurysm and an infundibulum which is frequently observed in the AchoA and PCoA. Additionally, a clear depiction of the perforating artery can be beneficial in planning the treatment modality. Longer T1 weighted images can also provide more spatial details of the vessel wall, and consequently, its stenotic nature can be assessed.3
The high magnetic susceptibility of paramagnetic substances such as iron deposits or blood degradation products allow for better detection of iron deposited in vessel walls and microbleeds in prolonged T2 suppression (Figure 3). Such characteristics of 7T MRI allow for another view to assess the hemorrhage risk of aneurysms and moyamoya disease (MMD).
The discrepancy between the low rupture risk of a small, unruptured aneurysm and the relatively high proportion of small aneurysms in subarachnoid hemorrhage requires another image study to reveal more characteristics of the unstable aneurysm beyond its size and location. Aneurysm wall vulnerability could be a promising method to resolve such a discrepancy. Hasan et al6 reported that early reuptake of ferumoxytol, which is ultra small super paramagnetic particles of iron oxide in MRI, may show the instability of an aneurysm. They postulated that the degree of inflammation assessed with ferumoxytol reuptake by macrophages in the wall of the aneurysm could be an indicator for active inflammation of the aneurysm. Accordingly, 7T MRI could produce a more prominent low signal in a T2 weighted image due to its enhanced susceptibility in detecting iron oxide.
Microbleeds indicate a bleeding-prone microangiopathy. Patients who underwent intravenous thrombolysis or warfarin therapy tended to have higher bleeding rates. Biessels et al7 reported that there was a clear anatomical relationship between microbleeds and penetrating arteries in patients with hypertensive hemorrhage shown by 7T MRI. The technical advantages of 7T MRI such as short echo times are a more accurate method to identify microbleeds. For MMD patients, radiographic clues to assess future hemorrhages remain undetermined. Kuroda et al8 reported that a microbleeds in the initial MRI was a significant risk factor for subsequent hemorrhages in their prospective study. Consequently, a more accurate association between microbleeds and MMD-related hemorrhages can be revealed by the enhanced susceptibility shown in 7T MRI.
However, 7T MRI remains technically demanding in terms of having an inhomogeneous transmit field and pronounced artifacts close to the skull base and the sinuses or from metals, and due to the limitation of the specific absorption rate (SAR). An inhomogeneous transmit field can distort a brain image based on the location due to different pulse angles and can cause SAR restrictions. In particular, peripheral lesions such as temporal or cerebellar can be impinged.3 Enhanced susceptibility artifacts due to air or metal between different tissues and in the brain parenchyma can also be a challenge. Therefore, 7T MRI is not appropriate for evaluating post-operative patients who have clips, coils, stents or CranioFix. Another concern of 7T MRI is safety considerations and imaging parameters. The safe use of ultra-high magnetic field MRI at 7T in patients with metal implants has not been proven yet. Thus, future studies should demonstrate the safety of 7T MRI in these patients. Additionally, further studies to determine optimal imaging parameters are necessary.
In summary, 7T MRI has the technical advantage of producing high spatial resolution images compared to that of 1.5T or 3T MRI. Although promising clinical applications of 7T MRI have been reported in some pilot studies, large comparative studies with 1.5T or 3T MRI are still necessary. In particular, the diagnostic ability to detect an unstable aneurysm wall, and to determine the nature of stenosies, and microbleeds could provide new insight into current treatment modalities.
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