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Noll An Overview of Echo Planar Imaging (EPI) Douglas C. Noll, Ph.D. Depts. of Biomedical Engineering and Radiology University of Michigan, Ann Arbor

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Noll Acknowledgements Scott Peltier, Alberto VazquezScott Peltier, Alberto Vazquez University of MichiganUniversity of Michigan Drs. S. Lalith Talagala and Fernando Boada (University of Pittsburgh)Drs. S. Lalith Talagala and Fernando Boada (University of Pittsburgh) SMRT and ISMRMSMRT and ISMRM

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Noll Objectives Explain how EPI images are acquired and createdExplain how EPI images are acquired and created Describe hardware requirementsDescribe hardware requirements Describe EPI variants, terminology, and parametersDescribe EPI variants, terminology, and parameters Demonstrate common EPI artifactsDemonstrate common EPI artifacts Summarize applications of EPISummarize applications of EPI

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Noll Outline Pulse sequence basicsPulse sequence basics LocalizationLocalization Variants on EPIVariants on EPI EPI ParametersEPI Parameters EPI ArtifactsEPI Artifacts EPI HardwareEPI Hardware ApplicationsApplications

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Noll Pulse Sequences Two Major AspectsTwo Major Aspects –Contrast (Spin Preparation) What kind of contrast does the image have? What is the TR, TE, Flip Angle, V enc ? –Localization (Image Acquisition) How is the image acquired? How is “k-space” sampled?

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Noll Pulse Sequences Spin Preparation (contrast)Spin Preparation (contrast) –Spin Echo (T1, T2, Density) –Gradient Echo –Inversion Recovery –Diffusion –Velocity Encoding Image Acquisition Method (localization, k-space sampling)Image Acquisition Method (localization, k-space sampling) –Spin-warp –EPI –RARE, FSE, etc.

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Noll Localization vs. Contrast In many cases, the localization method and the contrast weighting are independent.In many cases, the localization method and the contrast weighting are independent. –For example, the spin-warp method can be used for T1, T2, or nearly any other kind of contrast. –T2-weighted images can be acquired with spin- warp, EPI and RARE pulse sequences.

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Noll Localization vs. Contrast But, some localization methods are better than others at some kinds of contrast.But, some localization methods are better than others at some kinds of contrast. –For example, RARE (FSE) is not very good at generating short-TR, T1-weighted images. In general, however, we can think about localization methods and contrast separately.In general, however, we can think about localization methods and contrast separately.

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Noll Imaging Basics EPI is an image localization method.EPI is an image localization method. –Two-dimensional localization. To understand EPI, we will start at the beginning with one-dimensional localization.To understand EPI, we will start at the beginning with one-dimensional localization. –Here we “image” in 1D - the x-direction. (e.g. the L-R direction) We start with the simplest form of localization called “frequency encoding.”We start with the simplest form of localization called “frequency encoding.”

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Noll 1D Pulse Sequence

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Noll 1D Localization We acquire data while the x-gradient (Gx) is turned on and has a constant strength.We acquire data while the x-gradient (Gx) is turned on and has a constant strength. Recall that a gradient makes the magnetic field vary in a particular direction.Recall that a gradient makes the magnetic field vary in a particular direction. In this case, having a positive x-gradient implies that the farther we move along in the x-direction (e.g. the farther right we move) the magnetic field will increase.In this case, having a positive x-gradient implies that the farther we move along in the x-direction (e.g. the farther right we move) the magnetic field will increase.

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Noll 1D Pulse Sequence

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Noll Frequency Encoding A fundamental property of nuclear spins says that the frequency at which they precess (or emit signals) is proportional to the magnetic field strength:A fundamental property of nuclear spins says that the frequency at which they precess (or emit signals) is proportional to the magnetic field strength: This says that precession frequency now increases as we move along the x-direction (e.g. as we move rightwards).This says that precession frequency now increases as we move along the x-direction (e.g. as we move rightwards). f = B - The Larmor Relationship

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Noll Frequency Encoding

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Noll Fourier Transforms The last part of this story is the Fourier transform.The last part of this story is the Fourier transform. The Fourier transform is the computer program that breaks down each MR signal into its frequency components.The Fourier transform is the computer program that breaks down each MR signal into its frequency components. If we plot the strength of each frequency, it will form a representation (or image) of the object in one-dimension.If we plot the strength of each frequency, it will form a representation (or image) of the object in one-dimension.

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Noll Fourier Transforms

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Noll Alternate Method for 1D Localization In the case just described, the “frequency encoding” gradient was constant.In the case just described, the “frequency encoding” gradient was constant. –At different locations spins precessed at different frequencies. –This was true as long as the gradient was “on.” We now look at an alternate situation where the gradient is turned “on” and “off” rapidly.We now look at an alternate situation where the gradient is turned “on” and “off” rapidly. –At different locations spins will precess at different frequencies, but only during the times that the gradient is “on.”

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Noll Alternate Method for 1D Localization

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Noll On/Off Gradients in 1D Localization In the case previously described, the spins precessed smoothly.In the case previously described, the spins precessed smoothly. In this case, the spins precess in a “stop- action” or jerky motion.In this case, the spins precess in a “stop- action” or jerky motion. What is different here is that we sample the MR signal while it has stopped precessing.What is different here is that we sample the MR signal while it has stopped precessing. –At each step, the spatial information has been encoded into the phase. –This is a form of “phase encoding.”

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Noll Stop-Action Movement of Magnetization Sample 1 Sample 2 Sample 3

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Noll Different 1D Localization Methods Upper - smooth precession at different frequencies. (frequency encoding)Upper - smooth precession at different frequencies. (frequency encoding) Lower - precession in small steps, phase contains location info. (phase encoding).Lower - precession in small steps, phase contains location info. (phase encoding). Sampled data is the same (if we neglect T2).Sampled data is the same (if we neglect T2). The Fourier transform creates the 1D image.The Fourier transform creates the 1D image.

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Noll Different 1D Localization Methods

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Noll Alternate Method #2 for 1D Localization In the above cases, gradients were turned on and samples were acquired following a single RF excitation pulse.In the above cases, gradients were turned on and samples were acquired following a single RF excitation pulse. –At different locations spins precessed at different frequencies. –Motion was either smooth or “stop-action.” We now look at a situation where a single sample is acquired after each RF pulse.We now look at a situation where a single sample is acquired after each RF pulse. –Spins precess for a particular length of time and then a single sample is acquired.

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Noll Alternate Method #2 for 1D Localization

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Noll Phase Encoding in 1D Again, spins precess only as long as gradient is turned “on.”Again, spins precess only as long as gradient is turned “on.” If we look spins after each step (sample location), the precession will again appear as “stop-action” motion.If we look spins after each step (sample location), the precession will again appear as “stop-action” motion. Again, spatial information has been encoded into the phase of spin.Again, spatial information has been encoded into the phase of spin. –Another form of “phase encoding.”

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Noll Phase Encoding in 1D Phase Encode 0 Phase Encode 1 Phase Encode 2

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Noll Three Methods for 1D Localization 1D Localization:1D Localization: –Frequency encoding –Phase encoding following a single RF pulse –A single phase encode following each of many RF pulses Sampled data is the same (if we neglect T2).Sampled data is the same (if we neglect T2). The Fourier transform creates the 1D image.The Fourier transform creates the 1D image.

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Noll Three Methods for 1D Localization Frequency Encoding Phase Encoding Method #1 Phase Encoding Method #2

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Noll 2D Localization In general, we will combine two 1D localization methods to create localization in two dimensions (2D).In general, we will combine two 1D localization methods to create localization in two dimensions (2D). The spin-warp method (used in almost all anatomical MRI) is a combination of :The spin-warp method (used in almost all anatomical MRI) is a combination of : –Frequency encoding in one direction (e.g. Left- Right) –Phase encoding in the other direction (e.g. Anterior-Posterior)

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Noll 2D Localization - Spin Warp Frequency Encoding (in x direction) Phase Encoding Method #2 (in y direction)

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Noll Spin-Warp Imaging For each RF pulse:For each RF pulse: –Frequency encoding is performed in one direction –A single phase encoding value is obtained With each additional RF pulse:With each additional RF pulse: –The phase encoding value is incremented –The phase encoding steps still has the appearance of “stop-action” motion

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Noll Spin-Warp Pulse Sequence

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