Density of organic matrix of native mineralized bone measured by water- and fat-suppressed proton projection MRI

Abstract

Water- and fat-suppressed projection MR imaging (WASPI) utilizes the large difference between the proton T2*s of the solid organic matrix and the fluid constituents of bone to suppress the fluid signals while preserving solid matrix signals. The solid constituents include collagen and some molecularly immobile water and exhibit very short T2*. The fluid constituents include mobile water and fat, with long T2*. In WASPI, chemical shift selective low-power π/2 pulses excite mobile water and fat magnetization which is subsequently dephased by gradient pulses, while the magnetization of collagen and immobile water remains mostly in the z-direction. Additional selective π pulses in alternate scans further cancel the residual water and fat magnetization. Following water and fat suppression, the matrix signal is excited by a short hard pulse and the free induction decay acquired in the presence of a gradient in a 3D projection method. WASPI was implemented on a 4.7 T MR imaging system and tested on phantoms and bone specimens, enabling excellent visualization of bone matrix. The bone matrix signal per unit volume of bovine trabecular specimens was measured by this MR technique and compared with that determined by chemical analysis. This method could be used in combination with bone mineral density measurement by solid state 31P projection MRI to determine the degree of bone mineralization. Magn Reson Med 50:59-68, 2003. © 2003 Wiley-Liss, Inc.