Rahel Heule1, Carl Ganter2, Oliver Bieri1. 1. Division of Radiological Physics, Department of Radiology, University of Basel Hospital, Basel, Switzerland. 2. Department of Radiology, Klinikum rechts der Isar, Technische Universität München, Munich, Germany.
Abstract
PURPOSE: Variable flip angle (VFA) T1 quantification using three-dimensional (3D) radiofrequency (RF) spoiled gradient echo imaging offers the acquisition of whole-brain T1 maps in clinically acceptable times. However, conventional VFA T1 relaxometry is biased by incomplete spoiling (i.e., residual T2 dependency). A new postprocessing approach is proposed to overcome this T2-related bias. METHODS: T1 is quantified from the signal ratio of two spoiled gradient echo (SPGR) images acquired at different flip angles using an analytical solution for the RF-spoiled steady-state signal in combination with a global T2 guess. T1 accuracy is evaluated from simulations and in vivo 3D SPGR imaging of the human brain at 3 Tesla. RESULTS: The simulations demonstrated that the sensitivity of VFA T1 mapping to T2 can considerably be reduced using a global T2 guess. The method proved to deliver reliable and accurate T1 values in vivo for white and gray matter in good agreement with inversion recovery reference measurements. CONCLUSION: Based on a global T2 estimate, the accuracy of VFA T1 relaxometry in the human brain can substantially be improved compared with conventional approaches which rely on the generally wrong assumption of ideal spoiling.
PURPOSE: Variable flip angle (VFA) T1 quantification using three-dimensional (3D) radiofrequency (RF) spoiled gradient echo imaging offers the acquisition of whole-brain T1 maps in clinically acceptable times. However, conventional VFA T1 relaxometry is biased by incomplete spoiling (i.e., residual T2 dependency). A new postprocessing approach is proposed to overcome this T2-related bias. METHODS: T1 is quantified from the signal ratio of two spoiled gradient echo (SPGR) images acquired at different flip angles using an analytical solution for the RF-spoiled steady-state signal in combination with a global T2 guess. T1 accuracy is evaluated from simulations and in vivo 3D SPGR imaging of the human brain at 3 Tesla. RESULTS: The simulations demonstrated that the sensitivity of VFA T1 mapping to T2 can considerably be reduced using a global T2 guess. The method proved to deliver reliable and accurate T1 values in vivo for white and gray matter in good agreement with inversion recovery reference measurements. CONCLUSION: Based on a global T2 estimate, the accuracy of VFA T1 relaxometry in the human brain can substantially be improved compared with conventional approaches which rely on the generally wrong assumption of ideal spoiling.
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