Accurate T mapping of short T tissues using a three-dimensional ultrashort echo time cones actual flip angle imaging-variable repetition time (3D UTE-Cones AFI-VTR) method.

Purpose: To develop an accurate T measurement method for short T tissues using a combination of a 3-dimensional ultrashort echo time cones actual flip angle imaging technique and a variable repetition time technique (3D UTE-Cones AFI-VTR) on a clinical 3T scanner.

Methods: First, the longitudinal magnetization mapping function of the excitation pulse was obtained with the 3D UTE-Cones AFI method, which provided information about excitation efficiency and B inhomogeneity. Then, the derived mapping function was substituted into the VTR fitting to generate accurate T maps. Numerical simulation and phantom studies were carried out to compare the AFI-VTR method with a B -uncorrected VTR method, a B -uncorrected variable flip angle (VFA) method, and a B -corrected VFA method. Finally, the 3D UTE-Cones AFI-VTR method was applied to bovine bone samples (N = 6) and healthy volunteers (N = 3) to quantify the T of cortical bone.

Results: Numerical simulation and phantom studies showed that the 3D UTE-Cones AFI-VTR technique provides more accurate measurement of the T of short T tissues than the B -uncorrected VTR and VFA methods or the B -corrected VFA method. The proposed 3D UTE-Cones AFI-VTR method showed a mean T of 240 ± 25 ms for bovine cortical bone and 218 ± 10 ms for the tibial midshaft of human volunteers, respectively, at 3 T.

Conclusion: The 3D UTE-Cones AFI-VTR method can provide accurate T measurements of short T tissues such as cortical bone. Magn Reson Med 80:598-608, 2018. © 2018 International Society for Magnetic Resonance in Medicine.