MRI of the knee at 7.0 Tesla

Measurement protocols which have been optimized for MRI at field strengths of 1.5 T or 3 T cannot be directly transferred to 7 T. Specific absorption rate limitations, different tissue relaxation times, as well as new image artifacts require adjustments of the sequence parameters. The goal of our study was to investigate and optimize various sequences for 7 T imaging of the knee.

Starting with sequences used on a standard 1.5 T scanner, the parameters were modified to obtain optimal image contrast, maximum coverage, and the highest spatial resolution within a reasonable acquisition time. All sequences were optimized in two healthy volunteers and then tested in 10 patients with various pathologies. High-resolution 7 T images with several SE and GRE sequences were acquired and compared to 1.5 T images.

A comparison of 1.5 T and 7 T images clearly shows the advantage of MRI at higher field strengths, especially the higher SNR which could be translated into higher spatial resolution. The MEDIC sequence appears to be very well suited for the assessment of cartilage pathologies at 7 T. Using the DESS sequence, full coverage of the knee can be obtained with a very high resolution of 0.4 x 0.4 x 1.0 mm(3) within 7 minutes. Despite optimization of the STIR sequence parameters, bone marrow edema is better visualized at 1.5 T compared to 7 T. The PD TSE renders excellent image quality at 7 T. The total acquisition time of the 7 T protocol is approximately 40 minutes.

Gradient echo sequences provide excellent image contrast at very high spatial resolution in a reasonable scan time. However, not all sequences used at 1.5 T are currently well suited for high-field imaging, in particular SAR-intensive sequences. Imaging of meniscal tears and lesions of the cruciate ligaments may benefit from the higher spatial resolution. The most favorable clinical indication for knee examinations at 7 T currently appears to be cartilage imaging.