Echo Planar Imaging (EPI)

Figure: The trajectory through k-space during one $T_{R }$period as traversed in an EPI sequence. The numbered lines correspond to the echos shown in fig. 2.8(a). Note the reversal of trajectory direction due to the switched phase encoding.

Figure 2.8: Echo-Planar Imaging: (a) a Gradient Echo scheme, also showing a continuous phase-encoding gradient. The echos are induced by the switching read-out gradient. (b) A Spin Echo scheme, here the read-out gradient switching refocuses the spin echo induced by the 180$^{\circ }$ pulse. Also shown is the use of blipped phase-encoding. Note that either continuous or blipped phase-encoding may be used with either SE or GE sequences, this representation is for brevity.

[Continuous, Gradient Echo] [Blipped, Spin Echo]

It is possible to sample k-space more efficiently than by acquiring a single line of k-space per RF excitation, as mentioned in section 2.2.2. Single-Shot EPI (SS-EPI) samples k-space in a single continuous trajectory (see figure 2.7). This is achieved by generating and reading all of the required echos from a single FID. The RF pulse is applied in the presence of the slice selection gradient (see fig. 2.8(a)) which is switched after the pulse in order to reverse the signal growth and leads to better slice selection [9]. The switched read-out lobes induce the echos. Due to the continuous phase-encoding gradient, each echo is acquired with a different degree of total phase accumulation. The reversal of the gradient in the x direction results in the continuous trajectory through k-space (left to right, followed by right to left etc). Instead of using continuous phase-encoding, it is also possible to apply a small gradient prior to each echo acquisition, so that the phase-encoding for each echo is constant (see fig. 2.8(b)).

When the data acquisition is completed within the order of magnitude of $T_{2 }$, a much more complete use is made of the spin magnetisations than other methods and the SNR (per unit time) is greater. Single-shot EPI however requires specialised (and expensive) gradient amplifiers in order to perform the required rapid gradient switching. Segmented/multishot imaging is also possible without such modification, where a subset of phase-encoding steps is acquired within each $T_{R }$period. True EPI uses magnetic gradients to induce the (gradient) echos, but it is also possible to use a 180$^{\circ }$ RF pulse to induce a spin echo, which is then refocused for read out using switched magnetic gradients (see fig. 2.8(b) for an example).