Multi-echo fat-water–separated imaging has been implemented with a variety of sequences and protocols for both two-dimensional and three-dimensional acquisitions. Sequences include spoiled gradient recalled echo, turbo spin-echo, and steady-state free precession (SSFP), and in specific protocols may include magnetization preparations such as double inversion recovery (DIR) for dark blood contrast and inversion recovery (IR) for late enhancement.
Cardiac imaging using multi-echo sequences is typically performed using an ECG-triggered, segmented acquisition. Breath-held, cine imaging produces fat-water–separated images at a number of cardiac phases. Using cine imaging with continuous radiofrequency drives the magnetization to steady state, and depending on the readout flip angle, has T1-weighted contrast, which helps distinguish the myocardial from adjacent blood pool. However, T1-weighted contrast, if uncorrected, may also lead to overestimation of fat [37
•]. While cine imaging is useful for visualizing intramyocardial fat and for characterizing suspected masses, continuous imaging is not compatible with magnetization preparations such as dark-blood or IR. Single cardiac phase acquisitions, typically in mid-diastole, may be used in situations in which magnetization preparation is desired. Single phase imaging acquires data on the approach to steady state, and thus has a somewhat higher SNR and reduced T1-weighting for a given readout flip angle, as compared with cine imaging. The reduced T1-weighting improves fat fraction estimates. The reduction in blood pool contrast may be mitigated by using magnetization preparation.
Fat-water imaging may be performed with and without a dark blood preparation, depending on the desired blood pool contrast (Fig. ). Dark blood DIR-prepared fat-water–separated imaging provides improved delineation of the myocardium. The thin right ventricular (RV) myocardium is difficult to discern in the bright blood images due to lack of sufficient contrast between RV myocardium and blood. However, the RV free wall is well depicted in the dark blood–prepared images. In this example, the measured T1-weighted contrast of between myocardium and blood (water image septal region) was 1:1.5, whereas for the dark blood protocol the contrast was greater than 4:1. This improves the ability to discern fatty infiltration of the thin-walled RV myocardium.
Dark blood–prepared fat-water–separated images (right) provide better delineation of thin-walled right ventricular myocardium compared with bright blood images (left)
Fat-water–separated imaging may be combined with phase-sensitive inversion recovery (PSIR) late enhancement to provide positive correlation between fibrosis and fat, which both appear bright post-contrast. The water and fat images are spatially registered since they are reconstructed from the same multi-echo dataset. Furthermore, chemical shift artifacts may be eliminated in reconstruction.
Multi-echo acquisitions are typically performed using an echo-train readout for improved efficiency. Interleaving with two or more shots may be used for reduced echo-spacing, particularly at higher field strength to reduce potential ambiguities due to higher chemical shift [38
]. Echo-train readout using monopolar readout with gradient flyback is slightly less efficient than bipolar [33
] but avoids potential artifacts. The individual echo time images may be noisy as a result of large bandwidth, but the fat and water-separated images have improved SNR due to the effective signal averaging inherent in the reconstruction. The effective number of signals averaged approaches the number of echoes when the echo spacing is chosen judiciously [39
Examples of fat-water–separated images are presented for various clinical cases drawn from our site. Typical parameters for these examples are 1.5 T, single echo train acquisition using monopolar readout with gradient flyback, number of echoes typically 4, bandwidth approximately 1,000 Hz/pixel, and echo spacing on order of 2.2 to 2.5 ms with TR approximately 10 to 11 ms. Dark blood or IR preparations were used in specific protocols.