Fat-water separation with alternating repetition time balanced SSFP

Abstract

Balanced SSFP achieves high SNR efficiency, but suffers from bright fat signal. In this work, a multiple-acquisition fat-water separation technique using alternating repetition time (ATR) balanced SSFP is proposed. The SSFP profile can be modified using alternating repetition times and appropriate phase cycling to yield two spectra where fat and water are in-phase and out-of-phase, respectively. The signal homogeneity and the broad width of the created in-phase and out-of-phase profiles lead to signal cancellation over a broad stop-band. The stop-band suppression is achieved for a wide range of flip angles and tissue parameters. This property, coupled with the inherent flexibility of ATR SSFP in repetition time selection, makes the method a good candidate for fat-suppressed SSFP imaging. The proposed method can be tailored to achieve a smaller residual stop-band signal or a decreased sensitivity to field inhomogeneity depending on application-specific needs.

Figure 1

a: The in-phase $({\psi }_2^i=45°)$ and out-of-phase $({\psi }_2^i=135°)$ magnetization profiles for the frequency range [-600 200] Hz, α = 60°, and TR1/TR2/TE = 3.45/1.15/1.725 ms assuming T1/T2 = 1000/200 ms. Note the phase difference between the out-of-phase and in-phase profiles at -220 Hz. b: The summation and subtraction of the in-phase and out-of-phase profiles after compensation for the phase difference yield the water-only and fat-only spectra respectively. c: The in-phase $({\psi }_2^i=45°)$ and out-of-phase $({\psi }_2^o=-45°)$ magnetization profiles. d: The corresponding water-only and fat-only spectra.

Figure 2

The ratio of the average pass-band signal to the average stop-band signal was simulated for a range of T1/T2 ratios, flip angles and various suppression methods: (displayed in logarithmic scale) a: FS-ATR, b: LCSSFP, c: ${\psi }_2^{i,o}=(45°,-45°)$ and d: ${\psi }_2^{i,o}=(45°,135°)$. The proposed method $({\psi }_2^{i,o}=(45°,135°))$ achieves better suppression than LCSSFP for the whole range of simulation parameters except in the small vicinity of α = 30°. The performance of the proposed method improves with increasing flip angle. The signal ratio of FS-ATR is lower compared to the multiple-acquisition methods.

Figure 3

Phantom images were acquired with out-of-phase and in-phase 3D ATR SSFP profiles, where off-resonance was generated with a linear field gradient across the readout direction (horizontal). Water images were reconstructed from the two sets of in-phase and out-of-phase images with the corresponding ψ₂ pairs, ${\psi }_2^{i,o}=(45°,135°)$ and ${\psi }_2^{i,o}=(45°,-45°)$. The ${\psi }_2^{i,o}=(45°,-45°)$ pair creates a broader stop-band.

Figure 4

Comparison of FS-ATR and the proposed method at two different field strengths: a: 1.5 T, b: 3 T. a: Coronal and axial slices for the FS-ATR and ${\psi }_2^{i,o}=(45°,135°)$ acquisitions at 1.5 T are displayed along with the corresponding MIPs. The arrows point to the region where the proposed method achieves better fat suppression than the FS-ATR method. Improved fat suppression of the proposed method results in superior depiction of the vasculature in the MIPs. However, regions with visible residual fat signal still exist in the images produced with the proposed method as a result of the remnant stop-band signal. b: Sagittal and coronal thin slab MIPs of the calf acquisitions at 3 T are displayed for the FS-ATR (ψ₂ = 180°), ${\psi }_2^{i,o}=(135°,225°)$ and ${\psi }_2^{i,o}=(45°,-45°)$ methods. The ${\psi }_2^{i,o}=(135°,225°)$ combination achieves better fat suppression than FS-ATR. However, the range of off-resonant frequency variation limits the amount of fat suppression. The ${\psi }_2^{i,o}=(45°,-45°)$ combination achieves the highest level of fat suppression due to its broad stop-band.

Figure 5

Sagittal and axial calf images are shown for LCSSFP, and the ${\psi }_2^{i,o}=(45°,135°)$ and ${\psi }_2^{i,o}=(45°,-45°)$ pairs. The ${\psi }_2^{i,o}=(45°,-45°)$ method achieves the highest level of suppression, at the expense of a lower SNR.