In vivo MRI of submillisecond T(2) species with two-dimensional and three-dimensional radial sequences and applications to the measurement of cortical bone water

Abstract

Water in dense collagenous tissues such as tendons and ligaments, as well as water in cortical bone that occupies the spaces of the lacuno-canicular system or is tightly bound to collagen, is not ordinarily detectable by MRI. Water proton T(2) in these structures is generally less than 1 ms. Recent advances in instrumentation in conjunction with non-Cartesian imaging strategies now allow center of k-space to be scanned 100 micros or less after excitation. We examined the performance of two radial pulse sequences, a 2D sequence with half-pulse excitation and a new 3D hybrid sequence with variable-echo Cartesian encoding in the third dimension, on a whole-body 3 T scanner. Both pulse sequences used long-T(2) soft-tissue suppression pulses. The half-pulse slice profiles observed experimentally agreed well with those computed on the basis of a numerical solution of Bloch equations. The techniques yielded a signal-to-noise ratio of the order of 25 in 9 min scan time at a nominal voxel size of 0.58 x 0.58 x 8 mm(3) and 50-90 micros 'echo time' in the cortex of the tibial mid-shaft. With the use of an external reference, the water volume fraction of cortical bone in four subjects (mean +/- SD age 32.25 +/- 5.3 years) was found to be 22.5 +/- 2.7%, in good agreement with literature values.