Echo-Planar Imaging (EPI)
What is echo-planar imaging (EPI)? Is this the same as Fast Spin Echo (FSE)?
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Advances in gradient and digital data acquisition technology have made it possible to obtain individual MR slices in the time frame of 50-100 msec, thus minimizing the effects of patient motion. Generically, this acquisition mode is known as echo-planar imaging (EPI).
Although EPI may seem like a recent development, the technique is among the oldest methods of spatial localization in MRI, first described by Mansfield in 1977. Mansfield's group produced the first biologic EPI images of a beating rabbit heart in 1981 and of an infant human heart in 1983.
One of their early EPI techniques (MBEST) which became relatively popular in the 1980's is diagrammed below. Following a spin-preparation module (which might be as simple as a single RF-pulse), a strong switched frequency-encoding gradient was applied simultaneously with an intermittently "blipped" low-magnitude phase-encoding gradient. Gradient echoes (GREs) were collected with each oscillation of the readout (frequency) gradient.
One of their early EPI techniques (MBEST) which became relatively popular in the 1980's is diagrammed below. Following a spin-preparation module (which might be as simple as a single RF-pulse), a strong switched frequency-encoding gradient was applied simultaneously with an intermittently "blipped" low-magnitude phase-encoding gradient. Gradient echoes (GREs) were collected with each oscillation of the readout (frequency) gradient.
Modulus-blipped echo-planar single-pulse technique (MBEST), one of the early EPI sequences.
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The end result was a "zig-zag" traversal of k-space. The first downward-going frequency-and phase-encoding lobes imparted initial negative phase shifts to all spins, moving the signal from the middle of k-space to the far left bottom corner to a point we have previously identified as (−kxmax,−kymax).
Subsequent positive and negative frequency-encoding lobes swept k-space from left-to-right and right-to-left respectively. Meanwhile, the blipped low-amplitude phase-encoding gradient pulses produced a step-wise increase along the ky-axis. |
Zig-zag traversal of k-space from MBEST
Frequency-encoding is along horizontal axis; phase-encoding is along the vertical. |
As originally defined, echo planar imaging referred to a sequence in which data from all of k-space for an entire 2D plane was collected following a single RF-excitation pulse. More recently the term has been expanded to include any rapid gradient-echo or spin-echo sequence in which k-space is traversed in one or a small number of excitations. In the modern lexicon these are termed single shot EPI and multi-shot EPI, respectively.
Some additional vendor-specific terminology accompanies these concepts. The number of k-space lines (echoes) collected in a single shot is called the "Echo Train Length (ETL)" by GE and Canon; the "EPI factor" by SIemens and Philips; and the "shot factor" by Hitachi. The regions of k-space divided up by each shot of a multi-shot sequence are called segments. Hence it is common to refer to lines (or views) per segment as a measure of the intensity of k-space coverage.
In a very broad sense, a fast (turbo) spin-echo sequence with a very high turbo factor/ETL could be considered a multi-shot EPI sequence. The HASTE/SS-FSE technique is an example of this.
References
Mansfield P. Snap-shot MRI. Nobel Lecture, 8 Dec 2003 from www.nobelprize.org Mansfield P. Multi-planar image formation using NMR spin echoes. J Phys C: Solid State Phys 1977; 10:L55-L58.
Ordidge R. The development of echo-planar imaging (EPI):1977-1982. Magn Reson Mater Phys Biol Med 1999;9:117-121.
Poustchi-Amin M, Mirowitz SA, Brown JJ, et al. Principles and applications of echo-planar imaging: a review for the general radiologist. Radiographics May, 2001 (online only).
Redzian R, Mansfield P, Doyle M, et al. Real-time nuclear magnetic resonance clinical imaging in paediatrics. Lancet 1983; 322:1281-2. (First human EPI image).
Stehling MK, Turner R, Mansfield P. Echo-planar imaging: magnetic resonance imaging in a fraction of a second. Science 1991; 254:43-50.
Mansfield P. Snap-shot MRI. Nobel Lecture, 8 Dec 2003 from www.nobelprize.org Mansfield P. Multi-planar image formation using NMR spin echoes. J Phys C: Solid State Phys 1977; 10:L55-L58.
Ordidge R. The development of echo-planar imaging (EPI):1977-1982. Magn Reson Mater Phys Biol Med 1999;9:117-121.
Poustchi-Amin M, Mirowitz SA, Brown JJ, et al. Principles and applications of echo-planar imaging: a review for the general radiologist. Radiographics May, 2001 (online only).
Redzian R, Mansfield P, Doyle M, et al. Real-time nuclear magnetic resonance clinical imaging in paediatrics. Lancet 1983; 322:1281-2. (First human EPI image).
Stehling MK, Turner R, Mansfield P. Echo-planar imaging: magnetic resonance imaging in a fraction of a second. Science 1991; 254:43-50.