Evaluation of Intracranial Aneurysms with 7 T versus 1.5 T MR Angiography and Digital Subtraction Angiography

Purpose: The approximately 4.7 times higher signal at 7 T compared with that at 1.5 T may improve image quality of clinical MR-angiograms and should therefore provide a superior visualization of intracranial arteries and aneurysms. The purpose of this study was to determine whether 7 T ToF-MR-angiography is superior to 1.5 T ToF-MR-angiography in the detection and evaluation of intracranial aneurysms. Materials and Methods: 10 patients with suspected or known intracranial aneurysms underwent 7 T ToF-MRA (Magnetom 7 T, Siemens, Erlangen, Germany), 1.5 T ToF-MRA (Magnetom Espree or Sonata; Siemens, Erlangen, Germany) and DSA (Infinix NB, Toshiba, Tochigi, Japan). 7 T images were performed by using an 8-channel head coil (Rapid, Wuerzburg, Germany), which was individually tuned on the patient's head. For 1.5 T imaging a quadrature transmit/receive head coil or a 12-channel head coil (Siemens, Erlangen Germany) was used. The interval between all three examinations was not longer than two days. Image analysis of source images and maximum intensity projections was done by two neuroradiologists blinded to clinical data and results of previous angiograms. They compared the image quality of aneurysms (dome, neck, origin) and different vessel segments (C 1, C 2, M 1–3, P 1–3, AICA, PICA) in both ToF-MRAs by using a five-point scale. Digital subtraction angiography (DSA) was used as a reference standard. A Wilcoxon signed rank test was used to compare the mean values of image quality determined by both readers. Results: Twelve intracranial aneurysms in ten patients depicted by conventional angiography were also identified in 7 T and 1.5 T ToF-MR-angiography by both readers. Ten of twelve intracranial aneurysms were located in the anterior circulation and the remaining two aneurysms were detected in the posterior circulation. Three aneurysms were small (0–5mm), five were medium (6–10mm), four were large (11–25 mm). Intracranial 3D-ToF-MRA at 7 T offers superior image quality compared with 1.5 T, particularly in the depiction of aneurysmal dome ( P <0.046) and the A 1-segment of the anterior cerebral artery ( P <0.058). Close to the skull base the delineation of the posterior inferior cerebral artery was inferior at 7 T compared with 1.5 T (P<0.009), mainly due to susceptibility artifacts. The mean image quality of M 2-segment of the middle cerebral artery ( P <0.039) and the P 3-segment of the posterior cerebral artery ( P <0.047) was higher at the lower field strength in comparison to 7 T. Conclusion: ToF-MR-angiography at 7 T is feasible for the detection of intracranial aneurysms. Our preliminary results indicate that ultrahigh field ToF-MRA at 7 T offers superior image quality of the aneurysmal dome and the A 1-segment of the anterior cerebral artery compared with that of 1.5 T-ToF-MR-angiography. Further investigations and optimized head coils are needed to convert the signal gain of ultrahigh field MRI into a significant improvement of the overall image quality of ToF-MRAs. Fig.1: 45-year old, female patient with an asymptomatic, saccular aneurysm of the right internal carotid artery. Lateral DSA (left), maximum intensity projec-tion of a 1.5 T ToF-MRA (middle) and a 7 T ToF-MRA (right). The distal middle cerebral artery branches are better visualized on the 7 T MR-angiogram.