Fast Low‐Angle Dual Spin‐Echo (FLADE): A new robust pulse sequence for structural imaging of trabecular bone

Abstract

Mechanical strength and fracture resistance of trabecular bone (TB) are largely determined by the structural arrangement of individual trabeculae. Fast 3D spin-echo approaches are preferable to gradient echoes in that they are less sensitive to local induced gradients at the bone/marrow interface caused by magnetic susceptibility difference between the two tissues. FLASE is a 3D pulse sequence that serves this purpose. Here, we present a new pulse sequence dubbed FLADE (fast lowangle dual spin-echo) that overcomes some of the limitations inherent to FLASE, such as sensitivity to artifactual stimulated echoes. The double-echo sequence features a flip angle <90 degrees allowing for TR Ͻ Ͻ T 1 . The second phase-reversal pulse has the dual function of creating a second echo and restoring inverted longitudinal magnetization. The prolonged TR, made possible by sampling only half of k z -space, is used to collect navigator echoes in adjacent slabs for sensing subpixel translational displacements. FLADE is shown to provide SNR comparable to FLASE while having narrower point-spread func- The recognition of the role of trabecular bone (TB) architecture as a determinant of mechanical competence and fracture resistance (1-3) independent of apparent density has given substantial impetus to the development of noninvasive techniques for measuring TB structural parameters (see, for example, (4 -6)). The clinical utility of imaging TB along with extraction of structural parameters has recently been demonstrated in a number of articles showing that fracture susceptibility is associated with the degree of integrity of the TB network (7,8). The technique has also proven to be applicable to the evaluation of treatment efficacy (9,10). However, obtaining images in a resolution regime on the order of 100 -150 m pixel size poses unique challenges not only in terms of SNR requirements (6).Bone is more diamagnetic than fatty marrow (the type of marrow prevailing in the distal extremities of the adult skeleton) by about 2.5-3 ppm (11). The induced inhomogeneous fields in the boundary region between bone and marrow cause intra-voxel phase dispersion, which, in turn, leads to artifactual broadening of the trabeculae (12). Further, since the marrow is spectrally heterogeneous, the various spectral components are not in phase at k-space center, leading to significant SNR loss in gradient-echo images (13). These problems are largely overcome with fast 3D spin-echo imaging techniques such as FLASE (Fast Large-Angle Spin-Echo) (13). The FLASE pulse sequence provides significant advantages over other fast spin-echo sequences, such as RASEE (12), for high resolution TB imaging. However, there are several aspects of FLASE that can lead to difficulties. First, the large flip angle (Ͼ90 o ) of the selective excitation pulse can be problematic in terms of the uniformity of the slice profile. Second, the short repetition time (much shorter than T 1 of the protons in fatty marrow) can cause stimulated echoes that are difficult to eliminate using crushe...

Validation of oxygen extraction fraction measurement by qBOLD technique

“…The spin echo occurs at the 11th of 31 gradient echoes, corresponding to 56 ms after the RF excitation pulse, and gradient echo spacing is 1.895 ms. To eliminate image wrapping artifacts along the slice selection direction, and to generate a uniform refocusing profile, the slice thickness of the excitation RF pulse was set at 9 mm while the slice thickness for the refocusing pulse is kept at 18 mm. For spoiled single spin echo sequence, the optimal flip angle for excitation RF pulse is cos −1 (−e − TR / T 1 ) (18), which is larger than 90°. In our study, it was experimentally determined to be 110°.…”

Quantitative MRI for the assessment of bone structure and function

“…In this case the optimum flip angle is less than 908 and the second refocusing pulse has the dual function of spin rephasing and restoration of the longitudinal magnetization inverted after the first phase-reversal pulse. The new pulse sequence, dubbed 'FLADE' (fast low-angle dual echo), has several advantages besides being more robust by being largely immune to artifacts from stimulated echoes (94).…”

Helmholtz-pair transmit coil with integrated receive array for high-resolution MRI of trabecular bone in the distal tibia at 7T

“…These gave a super-linear dependence of SNR with field strength, $\text{SNR}\propto {\text{B}}_0^{1.26}$, indicating possible g -factor effects. Then we computed an upper bound on the amount of T 1 saturation in the SNR measurements using an equation previously derived (27) for the normalized steady-state signal after an excitation pulse followed by a series of 180° refocusing pulses: $$S_n=\frac{1-{false(-1false)}^n{e}^{-\text{TR}/{\text{T}}_1}-2{\displaystyle \sum _{j=1}^n}{(-1)}^{n-j}{e}^{-false(\text{TR}-{\tau }_{j}false)/{\text{T}}_1}}{1-{false(-1false)}^{n}cos(\varphi )e^{-\text{TR}/{\text{T}}_1}}sin(\varphi )$$…”