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An Overview of Echo Planar Imaging An Overview of Echo Planar Imaging (EPI)(EPI)

Douglas C. Noll, Ph.D.Douglas C. Noll, Ph.D.

Depts. of Biomedical Engineering and RadiologyDepts. of Biomedical Engineering and Radiology

University of Michigan, Ann ArborUniversity of Michigan, Ann Arbor

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AcknowledgementsAcknowledgements

• Scott Peltier, Alberto VazquezScott Peltier, Alberto Vazquez• University of MichiganUniversity of Michigan• Drs. S. Lalith Talagala and Fernando Boada Drs. S. Lalith Talagala and Fernando Boada

(University of Pittsburgh) (University of Pittsburgh) • SMRT and ISMRMSMRT and ISMRM

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ObjectivesObjectives

• Explain how EPI images are acquired and Explain how EPI images are acquired and createdcreated

• Describe hardware requirementsDescribe hardware requirements• Describe EPI variants, terminology, and Describe EPI variants, terminology, and

parametersparameters• Demonstrate common EPI artifactsDemonstrate common EPI artifacts• Summarize applications of EPISummarize applications of EPI

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OutlineOutline

• 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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Pulse SequencesPulse Sequences

• Two Major AspectsTwo Major Aspects– Contrast (Spin Preparation)Contrast (Spin Preparation)

What kind of contrast does the image have?What kind of contrast does the image have?What is the TR, TE, Flip Angle, VWhat is the TR, TE, Flip Angle, Vencenc??

– Localization (Image Acquisition)Localization (Image Acquisition)

How is the image acquired? How is the image acquired? How is “k-space” sampled?How is “k-space” sampled?

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Pulse SequencesPulse Sequences

• Spin Preparation (contrast)Spin Preparation (contrast)– Spin Echo (T1, T2, Density)Spin Echo (T1, T2, Density)– Gradient EchoGradient Echo– Inversion RecoveryInversion Recovery– DiffusionDiffusion– Velocity EncodingVelocity Encoding

• Image Acquisition Method (localization, Image Acquisition Method (localization, k-space sampling)k-space sampling)– Spin-warpSpin-warp– EPIEPI– RARE, FSE, etc.RARE, FSE, etc.

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Localization vs. ContrastLocalization vs. Contrast

• In many cases, the localization method and In many cases, the localization method and the contrast weighting are independent.the contrast weighting are independent.

– For example, the spin-warp method can be used For example, the spin-warp method can be used for T1, T2, or nearly any other kind of contrast. for T1, T2, or nearly any other kind of contrast.

– T2-weighted images can be acquired with spin-T2-weighted images can be acquired with spin-warp, EPI and RARE pulse sequences.warp, EPI and RARE pulse sequences.

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Localization vs. ContrastLocalization vs. Contrast

• But, some localization methods are better But, some localization methods are better than others at some kinds of contrast.than others at some kinds of contrast.

– For example, RARE (FSE) is not very good at For example, RARE (FSE) is not very good at generating short-TR, T1-weighted images. generating short-TR, T1-weighted images.

• In general, however, we can think about In general, however, we can think about localization methods and contrast separately.localization methods and contrast separately.

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Imaging BasicsImaging Basics

• EPI is an image localization method.EPI is an image localization method.– Two-dimensional localization.Two-dimensional localization.

• To understand EPI, we will start at the To understand EPI, we will start at the beginning with one-dimensional localization.beginning with one-dimensional localization.– Here we “image” in 1D - the x-direction.Here we “image” in 1D - the x-direction.

(e.g. the L-R direction)(e.g. the L-R direction)

• We start with the simplest form of localization We start with the simplest form of localization called “frequency encoding.”called “frequency encoding.”

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1D Pulse Sequence1D Pulse Sequence

RF

Gx

Data Acq.

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1D Localization1D Localization

• We acquire data while the x-gradient (Gx) is We acquire data while the x-gradient (Gx) is turned on and has a constant strength.turned on and has a constant strength.

• Recall that a gradient makes the magnetic Recall that a gradient makes the magnetic field vary in a particular direction.field vary in a particular direction.

• In this case, having a positive x-gradient In this case, having a positive x-gradient implies that the farther we move along in the implies that the farther we move along in the x-direction (e.g. the farther right we move) the x-direction (e.g. the farther right we move) the magnetic field will increase. magnetic field will increase.

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1D Pulse Sequence1D Pulse Sequence

RF

Gx

Data Acq.

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Frequency EncodingFrequency Encoding

• A fundamental property of nuclear spins says A fundamental property of nuclear spins says that the frequency at which they precess (or that the frequency at which they precess (or emit signals) is proportional to the magnetic emit signals) is proportional to the magnetic field strength:field strength:

• This says that precession frequency now This says that precession frequency now increases as we move along the x-direction increases as we move along the x-direction (e.g. as we move rightwards).(e.g. as we move rightwards).

f = f = B - B - The Larmor RelationshipThe Larmor Relationship

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Frequency EncodingFrequency Encoding

x Position

BMag. FieldStrengthLow Frequency

High Frequency

x Position

High Frequency

Low Frequency

Object

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Fourier TransformsFourier Transforms

• The last part of this story is the Fourier The last part of this story is the Fourier transform.transform.

• The Fourier transform is the computer The Fourier transform is the computer program that breaks down each MR signal program that breaks down each MR signal into its frequency components.into its frequency components.

• If we plot the strength of each frequency, it If we plot the strength of each frequency, it will form a representation (or image) of the will form a representation (or image) of the object in one-dimension.object in one-dimension.

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Fourier TransformsFourier Transforms

MR SignalFourierTransform

x Position

High Frequency

Low Frequency

Object

1D Image

time

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Alternate Method for 1D LocalizationAlternate Method for 1D Localization

• In the case just described, the “frequency In the case just described, the “frequency encoding” gradient was constant.encoding” gradient was constant.– At different locations spins precessed at different At different locations spins precessed at different

frequencies.frequencies.– This was true as long as the gradient was “on.”This was true as long as the gradient was “on.”

• We now look at an alternate situation where We now look at an alternate situation where the gradient is turned “on” and “off” rapidly.the gradient is turned “on” and “off” rapidly.– At different locations spins will precess at different At different locations spins will precess at different

frequencies, but only during the times that the frequencies, but only during the times that the gradient is “on.”gradient is “on.”

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Alternate Method for 1D LocalizationAlternate Method for 1D Localization

RF

Gx

Data Acq.

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On/Off Gradients in 1D LocalizationOn/Off Gradients in 1D Localization

• In the case previously described, the spins In the case previously described, the spins precessed smoothly. precessed smoothly.

• In this case, the spins precess in a “stop-In this case, the spins precess in a “stop-action” or jerky motion. action” or jerky motion.

• What is different here is that we sample the What is different here is that we sample the MR signal while it has stopped precessing.MR signal while it has stopped precessing.– At each step, the spatial information has been At each step, the spatial information has been

encoded into the phase.encoded into the phase.– This is a form of “phase encoding.”This is a form of “phase encoding.”

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Stop-Action Movement of MagnetizationStop-Action Movement of Magnetization

M

STOP

STOP

Sample 1Sample 1 Sample 2Sample 2 Sample 3Sample 3

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Different 1D Localization MethodsDifferent 1D Localization Methods

• Upper - smooth precession at different Upper - smooth precession at different frequencies. (frequencies. (frequency encodingfrequency encoding))

• Lower - precession in small steps, phase Lower - precession in small steps, phase contains location info. (contains location info. (phase encodingphase 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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Different 1D Localization MethodsDifferent 1D Localization Methods

RF

Gx

Data Acq.

RF

Gx

Data Acq.

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Alternate Method #2 for 1D LocalizationAlternate Method #2 for 1D Localization

• In the above cases, gradients were turned on In the above cases, gradients were turned on and samples were acquired following a single and samples were acquired following a single RF excitation pulse.RF excitation pulse.– At different locations spins precessed at different At different locations spins precessed at different

frequencies.frequencies.– Motion was either smooth or “stop-action.”Motion was either smooth or “stop-action.”

• We now look at a situation where a single We now look at a situation where a single sample is acquired after each RF pulse.sample is acquired after each RF pulse.– Spins precess for a particular length of time and Spins precess for a particular length of time and

then a single sample is acquired.then a single sample is acquired.

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Alternate Method #2 for 1D LocalizationAlternate Method #2 for 1D Localization

RF

Gx

Data Acq.

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Phase Encoding in 1DPhase Encoding in 1D

• Again, spins precess only as long as gradient Again, spins precess only as long as gradient is turned “on.”is turned “on.”

• If we look spins after each step (sample If we look spins after each step (sample location), the precession will again appear as location), the precession will again appear as “stop-action” motion.“stop-action” motion.

• Again, spatial information has been encoded Again, spatial information has been encoded into the phase of spin.into the phase of spin.– Another form of “phase encoding.”Another form of “phase encoding.”

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Phase Encoding in 1DPhase Encoding in 1D

Phase Phase Encode 0Encode 0

Phase Phase Encode 1Encode 1

Phase Phase Encode 2Encode 2

M

STOP

STOP

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Three Methods for 1D LocalizationThree Methods for 1D Localization

• 1D Localization:1D Localization:– Frequency encodingFrequency encoding– Phase encoding following a single RF pulsePhase encoding following a single RF pulse– A single phase encode following each of many RF A single phase encode following each of many RF

pulsespulses

• 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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Three Methods for 1D LocalizationThree Methods for 1D Localization

RF

Gx

Data Acq.

RF

Gx

Data Acq.

RF

Gx

Data Acq.

FrequencyFrequencyEncodingEncoding

PhasePhaseEncodingEncodingMethod #1Method #1

PhasePhaseEncodingEncodingMethod #2Method #2

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2D Localization2D Localization

• In general, we will combine two 1D In general, we will combine two 1D localization methods to create localization in localization methods to create localization in two dimensions (2D).two dimensions (2D).

• The spin-warp method (used in almost all The spin-warp method (used in almost all anatomical MRI) is a combination of :anatomical MRI) is a combination of :– Frequency encoding in one direction (e.g. Left-Frequency encoding in one direction (e.g. Left-

Right)Right)– Phase encoding in the other direction (e.g. Phase encoding in the other direction (e.g.

Anterior-Posterior)Anterior-Posterior)

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2D Localization - Spin Warp2D Localization - Spin WarpRF

Gx

Data Acq.

Frequency EncodingFrequency Encoding(in x direction)(in x direction)

Phase EncodingPhase EncodingMethod #2Method #2(in y direction)(in y direction)

Gx

Gy

Data Acq.

RF

RF

Data Acq.

Gy

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Spin-Warp ImagingSpin-Warp Imaging

• For each RF pulse: For each RF pulse: – Frequency encoding is performed in one direction Frequency encoding is performed in one direction – A single phase encoding value is obtainedA single phase encoding value is obtained

• With each additional RF pulse:With each additional RF pulse:– The phase encoding value is incrementedThe phase encoding value is incremented– The phase encoding steps still has the The phase encoding steps still has the

appearance of “stop-action” motionappearance of “stop-action” motion

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Spin-Warp Pulse SequenceSpin-Warp Pulse Sequence

Gx

Gy

Data Acq.

RF

PhaseEnc.

Freq.Enc.