Gradient-echo pulse sequence

The preparation module of the pulse sequence consists of an excitation pulse which is termed the alpha (a) pulse (Fig.1). It tilts the magnetization by a flip anglea which is typically between 0 and 90. In the special case where a = 90 the sequence is identical to the so-called partial saturation or saturation recovery pulse sequence. The flip angle can also be slowly increased during data acquisition (variable flip angle approach: tilted optimized nonsaturation excitation TONE ). Then, the data are not acquired in a steady state, where z-magnetization recovery and destruction by ad-pulses are balanced, but rather such that the z-magnetization is "used up" during imaging by tilting a little more of the remaining z-magnetization into the xy-plane for each acquired imaging line.

The readout- or acquisition module occurs during a free induction decay FID , during which the read gradient is turned on such that localization of the signal in the readout direction is possible. To accomplish this, the data are sampled during a gradient-echo (hence the name), which is achieved by properly dephasing the spins before they are rephased by an equal but opposite gradient to generate the echo (Fig. 1) when the areas under the negative and positive gradients are equal.

The end module is either absent or additional gradients, or radiofrequency pulses are introduced. This is done with the aim to spoil (see spoiler gradient) any remaining xy-magnetization or to refocus the xy-magnetization at the moment when the spin system is subject ot the next a pulse.

In standard GRE imaging, this basic pulse sequence is repeated as many times as image lines have to be acquired. As a result of the short repetition time, the z-magnetization cannot fully recover and after a few initial a-pulses there is an equilibrium established between z-magnetization recovery and z-magnetization reduction due to the a-pulses. Ultrafast GRE sequences are obtained if the repetition time TR is so short that image acquisition lasts less than one second and typically less than 500 ms. Such sequences are often labelled with the prefix "turbo" (turboFLASH, turboFFE, turboGRASS, see table).

If multiple lines are acquired after a single a pulse, the pulse sequence is a type of gradient-echo echo planar imaging EPI pulse sequence. In cardiac gating studies it is possible to assign consecutive lines either to different images, yielding a multiphase sequence (see cine MR imaging) with as many images as lines, or the lines are grouped together into segments and assigned to the same image. The overall time to acquire such a segment has to be small compared to the RR-interval of the cardiac cycle, i. e. 50 ms, and hence contains typically 8 to 16 image lines. This strategy any remaining xy-magnetization after the readout module, which is the case for short repetition times.As a result, only z-magnetization remains during a subsequent excitation. The signal intensity of a GRE sequence is given by

I µr M_z0 e^-TE/T2*(1-e^-TR/T1)sina/(1- e^-TR/T1cosa)

where r is the proton density, M_z0 the main magnetization, TE the echo-, TR the repetition time, and T1 and T2* the respective relaxation times. Note that for TE < < T2* and large, the above expression yields relatively high signal and becomes independent of T2*, hence the term T1-weighting. For TR > > T1, the angle a maximizing signal intensity is given by the Ernst angle.

Refocused GRE sequences use a refocusing gradient in the phase encoding direction during the end module to maximize (refocus) remaining xy-magnetization at the time when the next excitation is due, while the other two gradients are, in any case, balanced. When the next excitation pulse is sent into the system with an opposed phase, it tilts the magnetization in the -a direction. As a result the z-magnetization is again partly tilted into the xy-plane ( Fig.2: for graphic simplicity, only the x- but not the y-axis is shown), while the remaining xy-magnetization is tilted partly into the z-direction. The rules of vector addition show (Fig. 2) that by this the z-magnetization components add, while the xy-magnetization components have opposite signs and therefore the xy-magnetization is reduced. The signal intensity I is given by

I µr M_z0 sina/{1 + T1/T2* - (T1/T2* - 1) cosa)}

for TE < < T2*. It is obvious that the smaller T2*, the less important are the contributions of the xy-magnetization to the z-magnetization and the signal. Obviously an additional condition is that if TR > > T2*, the xy-magnetization also disappears owing to dephasing. Hence, spoiled and refocused GRE images are most different for relatively long T2* values which exist e.g. in the brain. When T2* is relatively long and TR is chosen in the same range, the z-magnetization component in the refocused sequences is larger, but the xy-component smaller than in the spoiled sequences with consequently a reduced image signal.

Contrast-enhanced GRE sequences provide T2 contrast but have a relatively poor signal to noise ratio. Repetitive RF pulses with small flip angles a together with appropriate gradient profiles lead to the superposition of two resonance signals (Fig.3). The first signal A is due to the free induction decay FID observed after the first and all ensuing RF excitations. The second is a resonance signal B obtained as a result of a spin-echo generated by the the second and all ensuing RF-pulses. Hence it is absent after the first excitation, is a result of the free induction decay of the second to last RF-excitation and has a TE which is almost 2TR (Fig. 3). For this echo to occur the gradients have to be such that they are completely symmetrical relative to the half time between two RF-pulses, a condition which makes it difficult to integrate this pulse sequence into a multiple slice imaging technique. Note that the signal B not only contains echo contributions from free induction decay n-2 but also from n-4, n-6 etc, but obviously weakened by T2-decay. Since the echo is generated by a RF-pulse it is truly T2- rather than T2*-weighted. Correspondingly it is href="../Volume I/inversion recovery IR pulse sequence.html#Fig.1">inversion recovery (IR) pulse sequence (I), Fig. 1

GRE techniques except for some of the "magnetization prepared" techniques show high vascular signal intensities because the magnetization of the inflowing spins coming from the outside of the repetitively excited volume, is much higher than that of the stationary spins (flow effects).

Gradient-echo pulse sequence, Table 1

Generic and brand names of various gradient-echo pulse sequences

GvS

GvS