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

Abstract

To describe a new method for accurate T 1 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 T 1 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 T 1 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 T 1 of cortical bone accurately, with difference ratios ranging from −5.0% to 0.4%. The in vivo study showed a mean T 1 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 T 1 of cortical bone with improved time efficiency compared with the UTE-AFI-VTR/ sf-UTE-AFI-VTR methods.

Targeted magnetic resonance imaging (tMRI) of small changes in the T1 and spatial properties of normal or near normal appearing white and gray matter in disease of the brain using divided subtracted inversion recovery (dSIR) and divided reverse subtracted inversion recovery (drSIR) sequences

“…From Eq. [14] and Figures 9,12 , it is possible to calculate synthetic dSIR and drSIR images from T 1 maps produced from other sources such as MP2RAGE, MR fingerprinting and actual flip angle-ultrashort TE (UTE) ( 29 ). These provide flexibility for targeting and generating contrast using different TIs.…”

A preparation pulse for fast steady state approach in Actual Flip angle Imaging

“…AFI was originally proposed by Yarnykh as a method for a fast and low‐SAR B 1 computation and is also employed in the inhomogeneity correction for accurate T 1 mapping 20,22–27 …”

Diagnosis of Delayed Post-Hypoxic Leukoencephalopathy (Grinker’s Myelinopathy) with MRI Using Divided Subtracted Inversion Recovery (dSIR) Sequences: Time for Reappraisal of the Syndrome?

“…It is also possible to synthetically create narrower mD images from wider mD images. The T 1 maps may be produced by MR fingerprinting and other methods, such as Actual Flip Angle imaging [27], which do not involve an IR sequence so that the more general term T 1 -bipolar filter (T 1 -BLAIR) imaging may be preferred to describe imaging utilizing a T 1 -bipolar filter. The term includes direct acquisitions (such as with dSIR) as well as synthetic imaging utilizing T 1 maps generated in different ways.…”