Whole knee joint T values measured in vivo at 3T by combined 3D ultrashort echo time cones actual flip angle and variable flip angle methods.

Purpose: To measure T relaxations for the major tissues in whole knee joints on a clinical 3T scanner.

Methods: The 3D UTE-Cones actual flip angle imaging (AFI) method was used to map the transmission radiofrequency field (B ) in both short and long T tissues, which was then used to correct the 3D UTE-Cones variable flip angle (VFA) fitting to generate accurate T maps. Numerical simulation was carried out to investigate the accuracy of T measurement for a range of T values, excitation pulse durations, and B errors. Then, the 3D UTE-Cones AFI-VFA method was applied to healthy volunteers (N = 16) to quantify the T of knee tissues including cartilage, meniscus, quadriceps tendon, patellar tendon, anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), marrow, and muscles at 3T.

Results: Numerical simulation showed that the 3D UTE-Cones AFI-VFA technique can provide accurate T measurements (error <1%) when the tissue T is longer than 1 ms and a 150 μs excitation RF pulse is used and therefore is suitable for most knee joint tissues. The proposed 3D UTE-Cones AFI-VFA method showed an average T of 1098 ± 67 ms for cartilage, 833 ± 47 ms for meniscus, 800 ± 66 ms for quadriceps tendon, 656 ± 43 ms for patellar tendon, 873 ± 38 ms for ACL, 832 ± 49 ms for PCL, 379 ± 18 ms for marrow, and 1393 ± 46 ms for muscles.

Conclusion: The 3D UTE-Cones AFI-VFA method allows volumetric T measurement of the major tissues in whole knee joints on a clinical 3T scanner.