Steady-state free precession imaging in the presence of motion: application for improved visualization of the cerebrospinal fluid.

Abstract

The authors discuss the appropriate FISP (fast imaging with steady-state precession) sequence structure to maintain constant phase at the radio-frequency pulse in the presence of motion. They present preliminary results of its application to head and spine imaging in an effort to maintain contrast between the cerebrospinal fluid (CSF) and the soft tissue. In the usual application of these FISP-like sequences, the gradient structure is modified to avoid unwanted signal (and contrast) variations due to field inhomogeneities. This change makes the signal sensitive to motion with a resulting decrease in signal intensity for moving tissue. The expected high contrast at large flip angles for tissues with low T1/T2 ratios such as CSF is not obtained. The technique discussed here overcomes the effects of field inhomogeneities and compensates for moving spins so that the transverse steady-state equilibrium and hence high contrast are obtained simultaneously.

A simple geometrical description of the TrueFISP ideal transient and steady‐state signal

“…, [12] a result also known from previous publications (14,16,18,19), which reflects the often-cited dependence of the TrueFISP steady-state signal on the ratio T 2 /T 1 .…”

“…[10] This expression appears rather compact in comparison but, as shown in Fig. 6a, it appears to be a suitable approximation for the exact formulation found in the literature (11)(12)(13)(14)(15)(16)(17)(18)(19), here adapted to sequences with alternating phase:…”

“…With the maximal signal normalized to 1 and extrapolated to the time t ϭ 0, the signal decay can be expected to start at S(0) ϭ sin , which will then converge toward the steady-state signal. This signal evolution can be expressed as [18] where M SS,Ќ can be approximated by Eq. [10], the ideal zenith angle is defined by Eq.…”

“…Hence, ideally no positiondependent dephasing takes place so that it is appropriate to describe the magnetization at a certain frequency by a single vector. With use of matrix notation, an analytic expression can be derived for the steady-state signal of TrueFISP sequences, which is a rather complex function of the parameters involved (T 1 , T 2 , M 0 , TR, flip angle, and frequency offset) (11)(12)(13)(14)(15)(16)(17)(18)(19). Recently, matrix-based mathematics were successfully used to assess the TrueFISP signal decay in the transient phase at off-resonance frequencies, applied in combination with suitable approximations and rather abstract mathematical procedures based on spinor formalism and perturbation theory (20,21).…”

Quantitative MRI for the assessment of bone structure and function

“…There is thus an additional mechanism of local signal attenuation in b-SSFP imaging of trabecular bone, unrelated to intra-voxel phase dispersion. These problems can be mitigated by combining data acquired with different RF phase (91,92), which shifts the center of the passband, as applied recently to b-SSFP imaging of TB (85). Although b-SSFP is often regarded as a most efficient pulse sequence, the need to combine images reduces its effectiveness.…”