Whole knee joint T1 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-1 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-1) in both short and long T-2 tissues, which was then used to correct the 3D UTE-Cones variable flip angle (VFA) fitting to generate accurate T-1 maps. Numerical simulation was carried out to investigate the accuracy of T-1 measurement for a range of T-2 values, excitation pulse durations, and B-1 errors. Then, the 3D UTE-Cones AFI-VFA method was applied to healthy volunteers (N = 16) to quantify the T-1 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-1 measurements (error < 1%) when the tissue T-2 is longer than 1 ms and a 150 mu s 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-1 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-1 measurement of the major tissues in whole knee joints on a clinical 3T scanner.