Coherence‐induced artifacts in large‐flip‐angle steady‐state spin‐echo imaging

Abstract

High-resolution imaging of trabecular bone aimed at analyzing the bone's microarchitecture is preferably performed with spinecho-type pulse sequences. Unlike gradient echoes, spin-echoes are immune to artifactual broadening of trabeculae caused by local static field gradients near the bone-bone marrow interface and signal loss from chemical shift dephasing at kspace center. However, the previously practiced 3D fast largeangle spin-echo (FLASE) pulse sequence was found to be prone to a low-frequency modulation artifact in both the readout and slice direction. The artifact is caused by deviations in the effective flip angle of the nonselective 180°pulse, which converts a fraction of the phase-encoded transverse magnetization to longitudinal magnetization. The latter recurs as transverse magnetization in the subsequent pulse sequence cycle forming a spurious stimulated echo. There is growing evidence that the mechanical competence of trabecular bone (the type of bone in which the majority of osteoporotic fractures occur), and thus its resistance to fracture, is significantly determined by the bone's architectural make-up (1-5). In vivo MR microimaging has proven its potential for assessing the bone's 3D structure. Nevertheless, obtaining reproducible structural parameters strongly depends on achieving artifact-free images in a resolution regime on the order of 100 -150 m (6).The magnetic properties of bone and the chemical composition of bone marrow pose stringent requirements on pulse sequences appropriate for MR microimaging of trabecular bone. Strong susceptibility-induced gradients as well as the multiple spectral components of triacyl glycerides in fatty bone marrow cause dephasing that diminishes the marrow signal. At the interface with the trabeculae the susceptibility-induced local gradients lead to signal loss that causes the trabeculae to appear artifactually thickened in the reconstructed image (7). In a spin-echo sequence, the signal loss caused by local susceptibilityinduced gradients is fully recovered and the isochromats from the various spectral components of fat are rephased at k-space center. These two reasons make spin-echo type sequences particularly well suited for imaging trabecular bone. However, patient comfort and the large number of pulse sequence cycles inherent to 3D imaging put limits on the allowable repetition time, TR.Two previous publications from this laboratory introduced 3D FLASE (fast large-angle spin echo), a pulse sequence optimized for the purpose of imaging trabecular bone (8,9). The pulse sequence has been used in prior trabecular bone imaging studies (5,10) and is an integral element of the virtual bone biopsy (6). 3D FLASE is a large-flip-angle, 3D spin-echo sequence, shown in Fig. 1a). The fundamental principle of FLASE is the dual function of the 180°pulse that serves both for phase-reversal and for restoration of the inverted longitudinal magnetization created by the initial excitation pulse. A slice-selective excitation with a flip angle of 140°, used in 3D FLASE give...

Structural and mechanical parameters of trabecular bone estimated from in vivo high‐resolution magnetic resonance images at 3 tesla field strength

“…Through-slice signal fluctuation attributed to a specific stimulated echo was minimized by changing the sign of the crusher moment relative to that of the slice-encoding gradient. We found that the subject’s ankle size and position within the coil can lead to inter-scan variations in B1 inhomogeneity (the major source of stimulated echo artifacts due to deviations of the phase-reversal pulse from 180°) (13). The presence of such artifacts can thus impair reproducibility of quantitative measures.…”

“…In MRI, detection of changes in these parameters is mired by image artifacts, involuntary subject motion, and volume misalignment between time-points which limit the achievable reproducibility. Image artifacts associated with TB imaging include susceptibility-induced thickening (12), stimulated echo artifacts (13), and aliasing (14). Advances in acquisition and image processing have led to the development of the “virtual bone biopsy”, a μMRI based suite of tools that provides detailed quantitative information on TB architecture at surrogate skeletal sites (15).…”

Quantitative MRI for the assessment of bone structure and function

“…The imperfect 1808 pulse converts a fraction of the phase-encoded transverse magnetization to longitudinal magnetization that recurs as transverse magnetization in the subsequent pulse sequence cycle, forming a spurious stimulated echo (89). A modification in the location of the encoding gradients remedies the problem.…”

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

“…Notice the banding artifact (vertical streaking) in Fig. 6c, caused by stimulated echoes (14) resulting from imperfections in the nonselective phase‐reversal pulses. As mentioned above, the longer repetition time of FLADE diminishes this kind of artifact.…”

“…First, the large flip angle (>90°) 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 crusher gradients (14). Third, the low receiver bandwidth required for good SNR performance causes broadening of the point spread function in the readout direction.…”