T₁ and T₂ quantification from standard turbo spin echo images

Abstract

Purpose To extract longitudinal and transverse (T1 and T2) relaxation maps from standard MRI methods. Methods Bloch simulations were used to model relative signal amplitudes from standard turbo spin‐echo sequences: proton density weighted, T2‐weighted, and either T2‐weighted fluid attenuated inversion recovery or T1‐weighted images. Simulations over a range of expected parameter values yielded a look‐up table of relative signal intensities of these sequences. Weighted images and flip angle maps were acquired in 8 subjects at 3 T using both single and multislice acquisitions. The T1 and T2 maps were fit by comparing the weighted images to the look‐up table, given the measured flip angles. Results were compared with inversion recovery and multi‐echo spin‐echo experiments. Results A region analysis showed that relaxation maps computed from single‐slice proton density, T2 and T1 weighting provided a mean T1 error of 4% in gray matter and 11% in white matter, and a mean T2 error of 3% and 4%, respectively, in comparison to reference measurements. In multislice acquisitions that are optimized to reduce cross‐talk and incidental magnetization transfer, the mean T1 error was 7% in gray matter and 1% in white matter, and the mean T2 errors were 3% and 4%, respectively. The best T1 results were achieved using proton density, T2 and T1 weighting rather than the fluid attenuated inversion recovery, although T2 maps were largely unaffected by this choice. Incidental magnetization transfer reduced T1 accuracy in standard interleaved multislice acquisitions. Conclusion Through exact sequence modeling and separate flip angle measurement, T2 and T1 may be quantified from a turbo spin‐echo brain protocol with proton density, T2, and T1 weighting.

Analysis of magnetization transfer (MT) influence on quantitative mapping of T2 relaxation time

“…From a practical point of view, multi‐slice acquisitions are designed to alleviate MT DIR effects related to slice‐profile imperfections by interleaving the slice order. This means that instead of acquiring slices sequentially (i.e., ascending or descending order), the series of odd‐numbered slices are acquired first and then the even‐numbered slices …”

Characterization of B1+ field variation in brain at 3 T using 385 healthy individuals across the lifespan

“…Furthermore, for many quantitative MRI methods, flip‐angle precision in each voxel is needed. Examples include T 1 and T 2 mapping and quantitative magnetization transfer, in which substantial errors can arise without accurate flip‐angle knowledge and inclusion in the modeling process 6–11 . Therefore, a map can be beneficial to measure the exact flip angle in each voxel using one of several different methods, including double‐angle method, 12,13 Bloch‐Siegert shift, 14 actual flip‐angle imaging 15 or dual refocusing echo acquisition mode 16 …”

“…The lack of maps in many studies arises from the additional scan time required on the order of 1 min, although methods are becoming faster, 17,18 and the irrelevance of mapping to interpretation of standard weighted images currently dominating clinical practice. However, production of quantitative MRI maps from weighted images would require map knowledge 10,19 . When maps are not available, the use of large experimental data sets may aid in predicting maps to enable more accurate quantitative maps, particularly for maps derived from clinical or retrospective data 20 .…”

Improving parametric estimation in the brain from multispin‐echo sequences using a fusion bootstrap moves solver

“…Recently, other methodologies have been suggested that T 2 accuracy is greatly improved by matching the MSE signal to a precomputed EMC dictionary 13,14,16 . In many cases, T 1 estimation may also be of interest and accurate knowledge of the field is required to minimize estimation errors 21 . Although some studies have explored estimation with either separate scanning or by joint estimation, many are still limited to research‐only and/or specifically pulse‐programed sequences.…”

“…Thus, it would be of great interest to perform simultaneous multiparametric estimation (here focused on and T 2 ) with standard clinical protocols (such as the MSE). A similar idea was also explored showing quantification of T 1 and T 2 from standard turbo spin‐echo protocols 21,22 …”