Steady-state versus spoiled gradient echo imaging in Bright Blood Cardiac MRI Sequences

In gradient echo (GRE) imaging, the TR is often shorter than the T2 of most tissues, and the transverse magnetization will not have fully decayed before the next RF pulse. Thus, there will be residual transverse magnetization that adds T2 contrast (in addition to T1 contrast) to the image. This additional T2 contrast is undesirable for many applications, as the T1 and T2 contrast may be competitive. For example, a liver lesion that is hypointense on T1 and hyperintense on T2 may be isointense with both T1 and T2 weighting. To achieve T1 weighting with a short TR GRE sequence, spoiling the residual transverse magnetization is necessary. This spoiling can be accomplished with an RF pulse or gradients. The majority of fast GRE sequences used in noncardiac clinical MRI are spoiled.

In steady-state GRE sequences, spoiling is not performed, and residual transverse magnetization is retained. The retained residual transverse magnetization increases the signal-to-noise ratio (SNR) of steady-state sequences relative to spoiled sequences. The image contrast will depend on the T2-to-T1 ratio. As stated previously, this is undesirable for many applications. In steady-state sequences, only fluid and fat will have a high signal (fluid and fat have comparable T1 and T2 times, while in most other tissues, T2 time is much shorter than T1 time). However, in bright blood cardiac MRI, hyperintense blood relative to other tissues is exactly what is needed; thus, steady-state GRE sequences are optimal for cine cardiac imaging (cMRI).

The sequences used in cardiac imaging are balanced SSFP sequences. Different trade names for these sequences are TrueFISP (Siemens), FIESTA (GE), and balanced FFE (Phillips). These sequences are very fast and have a high SNR, but the T2-to-T1 image contrast limits the role of these sequences for noncardiac applications.

SSFP cine MRI has largely replaced spoiled GRE cine MRI for evaluation of cardiac function. SSFP sequences do not depend on flow; they have a higher SNR; and they are faster. Spoiled GRE sequences are T1 weighted and depend on through plane flow enhancement (similar to time-of-flight MR angiography) to generate contrast. The blood may become saturated if the flow is slow or the TR is short. Thus, spoiled GRE cine MRI does not allow for the use of very low TRs, because there is not enough time for saturated blood to be replaced by unsaturated blood between excitation pulses.

With SSFP sequences, blood signal is dependent on intrinsic contrast rather than inflow effects, and TR can be as short as possible. SSFP cine MRI can be almost 3 times as fast as spoiled GRE cine MRI. In addition, the SSFP sequence has a higher SNR due to the residual transverse magnetization. This is particularly true at low TRs. With spoiled GRE sequences, SNR decreases with decreasing TR. With SSFP sequences, SNR is high even at low TRs, because residual transverse magnetization increases with shorter TRs.