To decrease scan times, it is possible to reduce repetition times to values less than T2. This complicates imaging, as there will still be some residual transverse magnetization at the end of each T R period, which will persist into and can be refocussed in the next T R period. Most often this fast imaging is conducted in the steady-state, and leads to two broad classifications based on how the residual transverse magnetization is handled. In the first, steady-state free precession (SSFP), the residual transverse magnetization is intentionally maintained and “refocussed”. This leads to an extremely efficient use of the available magnetization, though the signal will in general be given by a complex equation [40].
The other alternative is to “spoil”, or set to zero, the transverse magnetization at the end of each T R period. Setting the transverse magnetization to zero leads to a simpler expression for the theoretical signal, making qualitative and quantitative interpretation easier, but spoiling the magnetization is not always straightforward.
A simplistic approach would place a large gradient at the end of eachT R interval. This would effectively dephase the spins from the immediately preceding RF pulse and lead to zero netMxy, but repeated applications would lead to coherence pathways
where the net induced phase of the gradients is balanced. Alternatively, gradients with random [41] or linearly increasing amplitude [42,43] may be used. These techniques, however, are not sufficient. Random gradients will not lead to a true steady-state, leading to variations in signal fromT RtoT R. In conventional Cartesian acquisitions this will manifest itself as a ghosting artefact. Linearly increasing gradients on the other hand, may lead to a true steady-state, but with a spatially dependent spoiling efficiency [36], manifesting itself as a banding artefact.
The standard spoiling implementation on most scanners is based on radio fre- quency spoiling, using an RF phase that increases quadratically from T R to T R
combined with a constant amplitude crusher gradient [44]:
φn = φn−1+nφseed (1.25)
where φn is the phase of the nTH RF pulse, and φseed is the quadratic increment.
A quadratically increasing phase combined with a crusher gradient that creates a multiple of 2π phase variation across the imaging voxel ensures that the signal will enter steady-state. An appropriate choice of φseed (usually 117◦ or 123◦) [44] will
ensure that the accumulated coherence pathways will effectively cancel and give a signal that closely approximates a perfectly spoiled steady-state value.
The quadratically increasing phase has proven to be quite robust for a range of imaging applications, but it is not without its limitations. For quantitative imaging, the results may be highly sensitive to any deviation from ideal spoiling, and other values for φseed have been suggested for variable flip angle based T1 imaging [45, 46]. For the approach to steady-state, different values again for φseed are optimal,
depending on the circumstances [15,47].
Phase spoiling becomes even more complicated with sequences that are not a simple repetition of the same RF pulse. For the AFI flip angle imaging technique [26], which employs an interleaved T R acquisition scheme, the quadratically increasing phase needs to be modified significantly [32], and there is evidence that for the short repetition times and large flip angles typically involved, the only effective spoiling method requires damping signal via large gradients to cause losses due to diffusion [31,32].
For the above quantitative T1 and flip angle mapping techniques, where accuracy of the measured signal is essential, there may be some incentive to revisiting random spoiling. By combining randomized RF phase and gradient crusher amplitudes it is possible to produce a signal that will on average match the ideally spoiled value [16]. The random nature of the spoiling means that the magnetization is never truly in steady-state. Thus in conventional imaging, there will be random oscillations from
T RtoT R. These can manifest themselves as ghosting artefacts as in the early random spoiling implementations. In this case, however, they are overcome by employing a radial sampling trajectory, effectively oversampling the centre of k-space and thus averaging out the random fluctuations.
The primary motivation for quadratic RF spoiling is that it leads to magnetization in the steady-state. This produces no variation in magnetization from repetition to repetition and thus a good point spread function with no ghosting artefacts. In sequences that sample along the transient, quadratic spoiling is often still used, but now the motivation is somewhat different. If perfect spoiling were achieved there would be variation in signal from T R to T R by the nature of sampling during the transient phase. Even an optimized choice ofφspoilis not capable of matching a slowly
varying transient, and instead produces erratic deviation from the ideal transient. These deviations are consistent from transient to transient, but difficult to predict based on the physical and imaging parameters.