What is a gradient echo, and how does it differ from an FID? |
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In the previous Q&A we discussed the free induction decay (FID) signal that can be observed immediately after RF excitation is complete. The FID has an underlying frequency of a sine wave oscillating at the Larmor frequency damped by exponential decay with time constant T2* ("T2-star"). T2* reflects the effects of true T2 due to molecular mechanisms as well as phase dispersion due to magnetic field inhom*ogeneities.
Agradient echo (GRE)is simply a clever manipulation of the FID signal that begins by applying an external dephasing gradient field across the specimen or tissue. This gradient (produced by special coils hidden within the magnet housing) causes a calibrated change in local magnetic fields and hence alters the resonance frequencies slightlyacross the specimen. This results in accelerated dephasing and 'squelching/ scrambling' of the FID. In step 2, the process is reversed. A rephasing gradient is applied with the same strength but opposite polarity to the dephasing gradient, reversing/ undoing the phase scramble. A small GRE has been generated! Note that the rephasing gradient has only refocused spins scrambled by the dephasing gradient itself. T2 and T2* processes are unaffected. |
First step in GRE formation. A gradient is applied across the specimen, resulting in accelerated dephasing and squelching of the FID.
Second step in GRE formation. A rephasing gradient is applied (opposite in polarity to the dephasing gradient). This reverses the phase shifts induced by the dephasing gradient and resurrects the FID as GRE. Note that T2* decay continues unabated. |
The illustrations on this page serve as our first introduction to theMR pulse-timing diagram. This is a graphical representation of when RF-pulses are applied, MR signals are generated, and gradients are turned on and off. Time is on the horizontal axis. Here the gradients are shown in green as rectangular "lobes". An upwardly projecting lobe means the gradient is turned on with positive polarity; a downward lobe means the gradient in turned on in the opposite direction. The height of the rectangle is proportional to the strength of the applied gradient and hence the spread of frequencies it induces. The width of each rectangle is the time that the gradient is applied. The area under each gradient lobe therefore reflects (frequency x time) and hence the net phase accumulated by spins in due to the gradient alone.
Pulse timing diagram for a GRE signal generation showing (−) and (+) gradient lobes that dephase and rephase spins respectively. The lowest line shows the phase changes of four spins in different spatial locations subjected to the (−) and (+) gradients. The peak of the gradient echo occurs when the net phase shift among spins is zero. | Note that in our diagramthe upward gradient has been left on twice as long as the dephase gradient.This is the optimal design when the gradient is used for frequency-encoding purposes, but is not strictly necessary as aGRE will form after only one (+) lobe. The lower portion of this diagram shows the phases of four spins subjected to dephase and rephase gradients. The peak echo occurs when the net phase shift among spins is zero, which happens when the (+) area under the rephase gradient matches the (−)area under the dephase gradient. |
The rephasing of spins by gradient reversal is often illustrated by analogy to runners on a track, such as thetortoise and hare shown right. The fast hare represents spins precessing rapidly (and accumulating phase) by virtue of their location in a stronger portion of the gradient; the tortoise represents more slowly precessing spins in a weaker part of the gradient. The fast hare travels much farther initially (corresponding to a larger phase accumulation). The reversal of direction half way through the race corresponds to the gradients being applied with opposite polarities (rephase lobes). The hare again runs faster but in the opposite direction, having more distance to make up. Finally both return to the starting line at the same time (equivalent to net phase shift = 0). |
(Reprinted courtesy of Berlex Imaging) |
Gradient echoes are also referred to as gradient-recalled echoes or field echoes.
References
Bernstein MA, King KF, Zhou XJ. Handbook of MRI Pulse Sequences. Oxford: Elsevier, 2004, pp 267-297. (Although missing newer sequences developed in the last 10 years, this text is a classic/must read for anyone wishing to understand the details of RF and pulse sequence design)
Elster AD. Gradient echo imaging: techniques and acronyms. Radiology 1993; 186:1-8.
Winker ML, Ortendahl DA, Mills TC et al. Characteristics of partial flip angle and gradient reversal MR imaging. Radiology 1988;166:17-26.
©2024 AD Elster,ELSTER LLC |
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