Fast T1 measurement of cortical bone using 3D UTE actual flip angle imaging and single‐TR acquisition (3D UTE‐AFI‐STR)

Abstract

Purpose To describe a new method for accurate T₁ measurement of cortical bone that fits the data sets of both 3D UTE actual flip angle imaging (UTE‐AFI) and UTE with a single TR (UTE‐STR) simultaneously (UTE‐AFI‐STR). Theory and Methods To make both the constant values and longitudinal mapping functions in the signal equations for UTE‐AFI and UTE‐STR identical, the same RF pulses and flip angles were used. Therefore, there were three unknowns in the three equations. This was sufficient to fit the data. Numerical simulation as well as ex vivo and in vivo cortical bone studies were performed to validate the T₁ measurement accuracy with the UTE‐AFI‐STR method. The original UTE‐AFI variable TR (VTR) (ie, combined UTE‐AFI and UTE with VTR) and simultaneous fitting (sf) of UTE‐AFI and UTE‐VTR (sf‐UTE‐AFI‐VTR) methods were performed for comparison. Results The numerical simulation study showed that the UTE‐AFI‐STR method provided accurate value of T₁ when the SNR of the UTE‐STR image was higher than 40. The ex vivo study showed that the UTE‐AFI‐STR method measured the T₁ of cortical bone accurately, with difference ratios ranging from −5.0% to 0.4%. The in vivo study showed a mean T₁ of 246 ms with the UTE‐AFI‐STR method, and mean difference ratios of 2.4% and 5.0%, respectively, compared with the other two methods. Conclusion The 3D UTE‐AFI‐STR method provides accurate mapping of the T₁ of cortical bone with improved time efficiency compared with the UTE‐AFI‐VTR/sf‐UTE‐AFI‐VTR methods.