An Overview of Echo Planar Imaging (EPI)

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

2. Acknowledgements • Scott Peltier, Alberto Vazquez • University of Michigan • Drs. S. Lalith Talagala and Fernando Boada (University of Pittsburgh) • SMRT and ISMRM

3. Objectives • Explain how EPI images are acquired and created • Describe hardware requirements • Describe EPI variants, terminology, and parameters • Demonstrate common EPI artifacts • Summarize applications of EPI

4. Outline • Pulse sequence basics • Localization • Variants on EPI • EPI Parameters • EPI Artifacts • EPI Hardware • Applications

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

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

7. Localization vs. Contrast • 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.

8. Localization vs. 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.

9. Imaging Basics • EPI is an image localization method. • Two-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.”

10. 1D Pulse Sequence

11. 1D Localization • 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. • 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.

12. 1D Pulse Sequence

13. 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: • This says that precession frequency now increases as we move along the x-direction (e.g. as we move rightwards). f = gB - The Larmor Relationship

14. Frequency Encoding

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

16. Fourier Transforms

17. Alternate Method for 1D Localization • 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. • At different locations spins will precess at different frequencies, but only during the times that the gradient is “on.”

18. Alternate Method for 1D Localization

19. On/Off Gradients in 1D Localization • In the case previously described, the spins precessed smoothly. • 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. • At each step, the spatial information has been encoded into the phase. • This is a form of “phase encoding.”

20. Stop-Action Movement of Magnetization Sample 1 Sample 2 Sample 3

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

22. Different 1D Localization Methods

23. Alternate Method #2 for 1D Localization • 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. • Spins precess for a particular length of time and then a single sample is acquired.

24. Alternate Method #2 for 1D Localization

25. Phase Encoding in 1D • 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. • Again, spatial information has been encoded into the phase of spin. • Another form of “phase encoding.”

26. Phase Encoding in 1D Phase Encode 0 Phase Encode 1 Phase Encode 2

27. Three Methods for 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). • The Fourier transform creates the 1D image.

28. Three Methods for 1D Localization FrequencyEncoding PhaseEncoding Method #1 PhaseEncodingMethod #2

29. 2D Localization • 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 : • Frequency encoding in one direction (e.g. Left-Right) • Phase encoding in the other direction (e.g. Anterior-Posterior)

30. 2D Localization - Spin Warp Frequency Encoding(in x direction) Phase EncodingMethod #2(in y direction)

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

32. Spin-Warp Pulse Sequence