High‐resolution intracranial and cervical MRA at 3.0T: Technical considerations and initial experience

Abstract

Initial experience with intracranial and cervical MRA at 3.0T is reported. Phantom measurements (corrected for relaxation effects) show S/N (3.0T) ‫؍‬ 2.14 ؎ 0.08 ؋ S/N (1.5T) in identical-geometry head coils. A 3.0T 3DTOF intracranial imaging protocol with higher-order autoshimming was developed and compared to 1.5T 3DTOF in 12 patients with aneurysms. A comparison by two radiologists showed the 3.0T to be significantly better (P < 0.001) for visualization of the aneurysms. The feasibility of cervical and intracranial contrast enhanced MR angiography (CEMRA) at 3.0T is also examined. The relaxivity of the gadolinium contrast agent decreases by only about 4 -7% when the field strength is increased from 1.5 to 3.0T. Cervical 3.0T CEMRA was obtained in eight patients, two of whom had 1.5T studies available for direct comparison. Image comparison suggests 3.0T to be a favorable field strength for cervical CEMRA. Voxel volumes of 0.62-0.73 mm 3 (not including zero-filling) were readily achieved at 3.0T with the use of a single-channel transmitreceive head or cervical coil, a 25 mL bolus of gadoteridol, and a 3D pulse sequence with a 66% sampling efficiency. This spatial resolution allowed visualization of intracranial aneurysms, carotid dissections, and atherosclerotic disease including ulcerations. Potential drawbacks of 3.0T MRA are increased SAR and T* 2 dephasing compared to 1.5T. Image comparison suggests signal loss due to T* 2 dephasing will not be substantially more problematic than at 1.5T. The 1998 revision of the U.S. Food and Drug Administration guidelines (1) states that MRI systems with main static field strengths of 4.0T and less can qualify as nonsignificant risk devices. Moreover, with the advent of actively shielded magnet technology it is now feasible to site a 3.0T scanner in a clinical setting. Much of the prior use of 3.0T and higher field strength MRI systems, however, has been for pure or clinical research, mostly in the fields of functional brain MRI (e.g., 2,3) and spectroscopy (e.g., 4). This is quite logical, since fMRI and spectroscopy benefit not only from the increased S/N of 3.0T, but also from the linear dependence of magnetic susceptibility changes and chemical shift, respectively.We have applied our 3.0T system to both clinical research and routine clinical use. During the period October 1999 to May 2001, 2908 routine clinical brain exams were performed in an outpatient setting (5). The purpose of this article is twofold: 1) to provide an initial comparison of intracranial 3D time of flight (3DTOF) magnetic resonance angiography (MRA) at 1.5 and 3.0T; and 2) to establish the initial feasibility of 3D contrast-enhanced MR angiography (CEMRA) for the cervical and intracranial arteries at 3.0T. CEMRA is already a well-established technique at 1.5T and has been applied previously to study both intracranial aneurysms (6) and the cervical arteries (7). We examined whether the increased S/N available at 3.0T can translate into increased spatial resolution for CEMRA. We believe that the de...

Comparing real‐world advantages for the clinical neuroradiologist between a high field (3 T), a phased array (1.5 T) vs. a single‐channel 1.5‐T MR system

“…Specific absorption rate (SAR) increases with the square of the static field strength and could theoretically exceed levels allowed by the relevant regulatory bodies (19). T1 relaxation times increase with field strength necessitating longer imaging times to achieve the equivalent contrast in T1-weighting and to minimize saturation in T2-weighted imaging (16).…”

Imaging artifacts at 3.0T

“…Because the T1 of the unenhanced tissue typically increases more than the enhancing tissue, equivalent doses of Gd chelate often produce a greater effect at 3T than at 1.5T. This is greatly advantageous for MR angiography (MRA) (18,19,21), and has been used to reduce the contrast agent dose at 3T (32). At equivalent doses, however, increased dural enhancement is normal and should not be interpreted as a sign of a disease process, such as meningitis (Fig.…”

3.0‐Tesla MR angiography of intracranial aneurysms: Comparison of time‐of‐flight and contrast‐enhanced techniques

“…The imaging included both standard acquisitions (sagittal spin echo T1 and transaxial spin echo and fast spin echo T2), three-dimensional TOF MR angiography, and CE MR angiography using elliptical centric view order (7). The techniques used for the TOF and CE MR angiography have been previously published (8).…”

“…Details of the fluoroscopic CE MR angiography technique were described previously (8). With a two-dimensional fluoroscopic sequence, an axial monitoring plane consisting of a 6-mm-thick, 32 ϫ 32-mm field of view was established at the skull base.…”