1. 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

2. Noll Spin-Warp Pulse Sequence

3. Noll Spin-Warp Data Acquisition In 1D, the Fourier transform produced a 1D image.In 1D, the Fourier transform produced a 1D image. In 2D, the Fourier transform is applied in both the frequency and phase encoding directions.In 2D, the Fourier transform is applied in both the frequency and phase encoding directions. –This is called the 2D Fourier transform. Commonly we structure the samples in a 2D grid that we call “k-space.”Commonly we structure the samples in a 2D grid that we call “k-space.” –One line of k-space is acquired at a time.

4. Noll Spin-Warp Data Acquisition 2D Fourier Transform

5. Noll Echo-Planar Imaging As with spin-warp imaging, echo-planar imaging (EPI) is just the combination of two 1D localization methodsAs with spin-warp imaging, echo-planar imaging (EPI) is just the combination of two 1D localization methods EPI is also a combination of :EPI is also a combination of : –Frequency encoding in one direction (e.g. Left-Right) –Phase encoding in the other direction (e.g. Anterior-Posterior) EPI uses a different phase encoding method.EPI uses a different phase encoding method.

6. Noll Echo-Planar Imaging Frequency Encoding (in x direction) Phase Encoding Method #1 (in y direction)

7. Noll Echo-Planar Imaging For each RF pulse:For each RF pulse: –Frequency encoding is performed many times –All phase encoding steps are obtained –The entire image is acquired With each additional frequency encoding (each additional line in the k-space grid):With each additional frequency encoding (each additional line in the k-space grid): –The phase encoding value is incremented –The phase encoding steps still has the appearance of “stop-action” motion

8. Noll EPI Pulse Sequence

9. Noll EPI Data Acquisition As with Spin-Warp imaging, we put the acquired data for the frequency and phase encoding into the 2D grid called “k-space.As with Spin-Warp imaging, we put the acquired data for the frequency and phase encoding into the 2D grid called “k-space. Also, the 2D Fourier transform is used to create the image.Also, the 2D Fourier transform is used to create the image. In EPI, the data is filled into k-space in a rectangular “zig-zag”-like pattern.In EPI, the data is filled into k-space in a rectangular “zig-zag”-like pattern.

10. Noll EPI Data Acquisition

11. Noll EPI Imaging In summary, EPI data is in many ways like Spin-Warp imaging:In summary, EPI data is in many ways like Spin-Warp imaging: –They are combinations of two kinds of 1D localization. –They have both frequency and phase encoding. –Data are acquired on a 2D grid called k-space. –Images are reconstructed by a 2D Fourier transform.

12. Noll EPI Imaging It is also different from Spin-Warp Imaging:It is also different from Spin-Warp Imaging: –The image can be acquired with a single RF pulse. –The phase encoding steps all happen in rapid succession. –The frequency direction alternates in sign. –The time needed to acquire data after each RF pulse is very long. –Special hardware is required. These differences are the focus of the rest of this presentation.These differences are the focus of the rest of this presentation.

13. Noll Variants on EPI There are many variations on EPI.There are many variations on EPI. One technique that is useful for Spin-Warp imaging that also works for EPI is “Partial k- space” or “Half k-space” acquisitions.One technique that is useful for Spin-Warp imaging that also works for EPI is “Partial k- space” or “Half k-space” acquisitions. Like Spin-Warp imaging, this can be used to:Like Spin-Warp imaging, this can be used to: –Reduce echo-time. (phase) –Improve spatial resolution. (frequency)

14. Noll Full k-space Partial Phase Data Partial Frequency Data Partial k-space EPI

15. Noll Multi-shot EPI While possible to acquire an entire image with a single RF pulse (single-shot), it is sometimes necessary to use multiple shots.While possible to acquire an entire image with a single RF pulse (single-shot), it is sometimes necessary to use multiple shots. There are two common ways of doing this:There are two common ways of doing this: –Interleaving –Mosaic Multi-shot EPI is useful to:Multi-shot EPI is useful to: –Improve spatial resolution –Reduce artifacts

16. Noll Multi-shot EPI Interleaved EPI Mosaic EPI

17. Noll Methods Similar to EPI One method that has very similar properties to EPI is Spiral Imaging.One method that has very similar properties to EPI is Spiral Imaging. Like EPI:Like EPI: –All image data can be acquired in a single-shot. –Multi-shot variants also exist. –Many of the artifacts are similar. But:But: –Image reconstruction is complicated. –Some artifacts are different.

18. Noll Spiral Imaging Pulse Sequence k-Space Data

19. Noll EPI Parameters Many parameters are the same as in spin- warp imaging:Many parameters are the same as in spin- warp imaging: –SE vs. GRE or IR –TR, TE, Flip Angle, TI –FOV, matrix size, spatial resolution Some parameters require extra thought, however:Some parameters require extra thought, however: –If only a single image is acquired using single-shot EPI, the TR might be meaningless. (TR is infinite)

20. Noll Scan Time in EPI The scan time is most closely related to the “number of shots” and not matrix size.The scan time is most closely related to the “number of shots” and not matrix size. –Scan Time = (number of shots)*(TR) –Not (number of phase encodes)*(TR) Consider 64x64 single-shot EPI and 128x128 single-shot EPI - both are single-shot and take a single RF pulse to acquire an image.Consider 64x64 single-shot EPI and 128x128 single-shot EPI - both are single-shot and take a single RF pulse to acquire an image. If 128x128 has artifacts that are too severe, however, multi-shot EPI may be required.If 128x128 has artifacts that are too severe, however, multi-shot EPI may be required.

21. Noll Echo Time in EPI In EPI, it is often hard to achieve a short echo time.In EPI, it is often hard to achieve a short echo time. –The TE is defined as the time between the RF pulse and the acquisition of the center of k-space. –In single-shot EPI, this could be a long time (often a minimum TE of 15-20 ms). This can be addressed by doing a partial k- space acquisition in the phase encoding direction.This can be addressed by doing a partial k- space acquisition in the phase encoding direction. –This will allow much shorter TE’s (5-10 ms).

22. Noll Echo Time in EPI Full k-space Partial k-space

23. Noll Pulse Sequence Options in EPI Flow Compensation (Gradient Moment Nulling):Flow Compensation (Gradient Moment Nulling): –Flow Comp (GNM) is often not as effective with EPI due to the long echo times. –Partial k-space (phase) acquisitions reduce echo time and make this technique more effective. Spatial and chemical presaturation can also be used (fat saturation is nearly always used).Spatial and chemical presaturation can also be used (fat saturation is nearly always used). There are also a 3D (volume) versions of EPI.There are also a 3D (volume) versions of EPI.

24. Noll T1 Weighting in EPI In EPI, short TE’s are difficult to obtain and the TR is often very long.In EPI, short TE’s are difficult to obtain and the TR is often very long. –EPI is not well suited to T1-weighted imaging with the usual short TR pulse sequences. On the other hand, one shot (or a small number of shots) is required for an image.On the other hand, one shot (or a small number of shots) is required for an image. –EPI is well-suited to inversion recovery T1- weighted imaging.

25. Noll Artifacts in EPI The ability to acquire images very rapidly is the strength of the EPI method.The ability to acquire images very rapidly is the strength of the EPI method. As a result, artifacts resulting from subject motion are nearly non-existent when imaging with single-shot EPI.As a result, artifacts resulting from subject motion are nearly non-existent when imaging with single-shot EPI. Ghosting artifacts resulting from pulsatile blood flow are also extremely rare with single- shot EPI.Ghosting artifacts resulting from pulsatile blood flow are also extremely rare with single- shot EPI.

26. Noll Artifacts in EPI There are however, several kinds of image artifacts that are very different from those seen in spin-warp imaging:There are however, several kinds of image artifacts that are very different from those seen in spin-warp imaging: –“N/2” or Nyquist ghosting –Distortions from magnetic field inhomogeneity –Chemical shift and susceptibility artifacts

27. Noll N/2 Ghosting N/2 (“N over 2”) or Nyquist ghosting artifacts are unique to EPI.N/2 (“N over 2”) or Nyquist ghosting artifacts are unique to EPI. –Caused by imperfections in the image acquisition. There are two distinct kinds:There are two distinct kinds: –Even and Odd Ghosts “Ghost tuning” procedures can reduce or eliminate these ghosts.“Ghost tuning” procedures can reduce or eliminate these ghosts. –Tuning can be done for each day, subject, or scan. –Might also be done automatically (with prescan).

28. Noll N/2 Ghosting Even Ghost Odd Ghost Original Image

29. Noll Distortions from Inhomogeneity EPI is very sensitive to center frequency adjustments and inhomogeneities.EPI is very sensitive to center frequency adjustments and inhomogeneities. For a misadjusted center frequency, the image is shifted in the “phase” direction.For a misadjusted center frequency, the image is shifted in the “phase” direction. –Careful prescan tuning is necessary. For misadjusted shims, the image can be twisted, stretched or squeezed.For misadjusted shims, the image can be twisted, stretched or squeezed. –Shimming is often necessary (esp. at high fields).

30. Noll Distortions from Inhomogeneity Center Frequency Misadjustment Original Image “X” shim (L/R shim) “Y” shim (A/P shim)

31. Noll Fat and Susceptibility Artifacts In EPI, unsuppressed fat is often shifted 2 cm or more.In EPI, unsuppressed fat is often shifted 2 cm or more. –Fat suppression (Fat Sat) is always required. At areas of high magnetic susceptibility, a “piling-up” artifacts is often seen.At areas of high magnetic susceptibility, a “piling-up” artifacts is often seen. –Prevalent near frontal sinuses, above ears, etc. –Pulse sequence parameters can affect this: Interleaving usually reduces this artifact.Interleaving usually reduces this artifact. Increasing resolution in the frequency direction often worsens the artifact.Increasing resolution in the frequency direction often worsens the artifact.

32. Noll Fat and Susceptibility Artifacts Fat Artifact Susceptibility(“piling-up”)Artifact Original Image

33. Noll EPI Hardware EPI is an extremely rapid and useful imaging method.EPI is an extremely rapid and useful imaging method. It does, however, require some special, high performance hardware. Why?It does, however, require some special, high performance hardware. Why? –In spin-warp, we acquire a small piece of data for an image with each RF pulse. –However in EPI, we try to acquire all of the data for an image with a single RF pulse.

34. Noll Spin-Warp vs. EPI Pulse Sequences EPISpin-Warp Many acquisitions to make a one image. One acquisition to make one image.

35. Noll T2 Decay and Acquisition Time In spin-warp imaging, only a single phase encode need to be acquired.In spin-warp imaging, only a single phase encode need to be acquired. –Only takes a short time. In EPI, all phase encode lines need to be acquired.In EPI, all phase encode lines need to be acquired. –Takes longer. –Without special hardware - 100 ms to 1 second. T2 decay reduces signal throughout data acquisition.T2 decay reduces signal throughout data acquisition.

36. Noll T2 Decay and Acquisition Time Signal decays away during acquisition. Data Acq. takes longer.