B₁-corrected breast T₁ mapping at ultralow field

This study aims to develop an efficient T₁ mapping approach that is free from systematic bias caused by B₁ inhomogeneity in ultralow-field (ULF) MRI and to determine the T₁ values (without fat suppression) of breast tissues at 6.5 mT.

We describe here a method for ULF T₁ mapping using a variable flip angle approach for both mapping T₁ and B₁. The B₁ mapping is used to correct the T₁ maps-and, unique to operation in the ULF regime, a B₁ map acquired in a phantom can be applied to the in vivo T₁ mapping data, reducing in vivo scan time. The T₁ mapping method was validated in vitro with copper sulphate phantoms at seven different concentrations. A breast phantom was scanned to test the T₁ mapping protocol. Four healthy volunteers were recruited for T₁ mapping to obtain in vivo T₁ values of breast tissues. No fat suppression was implemented in this preliminary study.

The copper sulphate phantom T₁ map measurements have less than 13% deviation from the reference T₁ values, and less than 6% deviation when the reference T₁ was greater than 50 ms. Four healthy volunteers were scanned for T₁ mapping. In these in vivo breast T₁ maps, the 25th to 75th percentiles ranged from 79 to 243 ms at 6.5 mT.

Variable flip angle method can be used for both B₁ and T₁ mapping at ULFs, which enables efficient in vivo T₁ mapping for ULF MRI.