Fat and water separation in balanced steady‐state free precession using the Dixon method

Abstract

The advantage of increased signal-to-noise ratio (SNR) efficiency in balanced steady-state free precession (SSFP) imaging (also denoted as true fast imaging in steady-state precession (TrueFISP), balanced fast field-echo (FFE), or fast imaging employing steady-state acquisition (FIESTA) by various manufacturers) has made this technique attractive for clinical applications. Examples of such applications include (but certainly are not limited to) cardiac imaging (1,2), angiography (3,4), gastrointestinal imaging (5,6), and fetal imaging (7). In certain situations, the signal from fat protons is a major source of interference that hinders our ability to interpret the image unambiguously. This is understood because fat has a higher T 2 /T 1 value compared to parenchymal tissues, which corresponds to bright steady-state signals on SSFP images (8). Therefore, for SSFP imaging applications intended to highlight fluids with large T 2 /T 1 values, such as angiography, myelography, and MR cholangiopancreatography (MRCP), it is essential to eliminate the fat signals.Fat suppression in SSFP imaging can be accomplished by using frequency-selective RF pulses in every TR, similarly to the conventional approach used in spin-echo imaging (5). This method increases TRs that are ordinarily short in generic SSFP sequences, and thus increases total scan time by a noticeable factor. Alternatively, magnetization preparation during the steady state, which refers to the addition of one fat-suppression pulse every several TRs, has also been shown to be effective (9). The latter method is advantageous in that the scan time is not significantly increased, which is beneficial for 3D examinations.Other methods, such as linear combination SSFP (10) and fluctuating equilibrium MR (11), have been proposed that make use of the SSFP spectral profiles manipulated by different RF phase schemes to selectively reconstruct different spectral species. For 2D imaging, the use of a single fat-suppression RF pulse followed by a centric-ordered SSFP readout should serve the same purpose well, with the exception that the resulting image contrast is inevitably altered to proton-density weighting due to the transient-state signal behavior (12).In a recent work, it was shown that SSFP images exhibit spin-echo-like behavior, such that spin isochromats at similar resonant frequencies show phase coherence at either 0°or 180°relative to the RF pulses at the time TR/2, the nominal TE in SSFP imaging (13). For off-resonance species, such as fat relative to water, the SSFP angle (i.e., the precession phase angle for the spin isochromats within one TR in the rotating frame) can be manipulated by adjusting the center reference frequency, which in turn determines the directional location for phase coherence in the rotating frame (13). This property leads naturally to the use of in-phase and out-of-phase images for Dixon addition/subtraction to achieve fat-water separation in SSFP imaging (14). In this study, we demonstrate the feasibility of separating fat and water sig...

Dixon techniques for water and fat imaging

“…Dixon techniques can be successfully implemented with many different pulse sequences, including fast spin echo (18,(45)(46)(47)(48), steady-state free precession (18,49), or even non-rectilinear (50,51) pulse sequences. However, the success of most Dixon techniques is in general dependent on the success of phase correction.…”

Non‐contrast enhanced MR angiography: Physical principles

“…Another clinical study reported significant improvement in image quality using respiratory navigator-gated bSSFP NCE-MRA instead of the breath-hold approach (93). The Dixon method bSSFP, where two bSSFP images acquired with different center frequency offsets are combined using the Dixon method (94), has been demonstrated to depict conspicuous vessels with good fat suppression for NCE-MRA in renal arteries (95) and arteries of the extremities (96). Also leveraging the frequency selectivity of bSSFP, fat-suppressed bSSFP NCE-MRA can be used in conjunction with an alternate TR bSSFP technique to reduce fat signal to image the arteries in the lower extremities (97).…”

Magnetic resonance tissue quantification using optimal bSSFP pulse-sequence design

“…Our method improves on conventional Dixon methods (Glover, 1991;Huang et al, 2004;Reeder et al, 2004;Vasanawala et al, 2000) since it − does not dictate a particular configuration of phases (e.g., in and out of phase) and look for pulse-sequence design variables to match this configuration, − takes all tissue parameters into account (relaxation constants, as well as resonance offset),…”