In vivo field-cycling relaxometry using an insert coil for magnetic field offset

Purpose
The T1 of tissue has a strong dependence on the measurement magnetic field strength. T1-dispersion could be a useful contrast parameter, but is unavailable to clinical MR systems which operate at fixed magnetic field strength. The purpose of this work was to implement a removable insert magnet coil for field-cycling T1-dispersion measurements on a vertical-field MRI scanner, by offsetting the static field over a volume of interest.

Methods
An insert magnet coil was constructed for use with a whole-body sized 59 milli-Tesla (mT) vertical-field, permanent-magnet based imager. The coil has diameter 38 cm and thickness 6.1 cm and a homogeneous region (± 5%) of 5 cm DSV, offset by 5 cm from the coil surface. Surface radiofrequency (RF) coils were also constructed.

Results
The insert coil was used in conjunction with a surface RF coil and a volume-localized inversion-recovery pulse sequence to plot T1-dispersion in a human volunteer's forearm over a range of field strengths from 1 mT to 70 mT.

Conclusion
T1-dispersion measurements were demonstrated on a fixed-field MRI scanner, using an insert coil. This demonstrates the feasibility of relaxation dispersion measurements on an otherwise conventional MR imager, facilitating the exploitation of T1-dispersion contrast for enhanced diagnosis. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.