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 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 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.