Gradient Echo EPI

General considerations

• Up to 200,000 images can be acquired within a single EPI multi-phase series.
• When using the head coil, axial, axial oblique, coronal, and coronal oblique planes automatically have the phase and frequency swapped, in comparison to non-EPI scans. This is to lessen the presentation of geometric distortion and to reduce the potential for peripheral nerve stimulation.
• On wide bore systems, consider setting Max # Slices Optimization to 1 to increase the number of slices per acquisition at the expense of increased echo space and geometric distortion.
• There are two type-in PSD options that revert the echo spacing and dynamic phase correction method back to an earlier software version. Scan selections: 2D Mode, Echo Planar Imaging family, Gradient Echo EPI pulse or Spin Echo EPI pulse, type-in text field epira3 or epiRTra3. Use epiRTra3 for fMRI scans - you must have fMRI Imaging Option selected for this type-in. Open the fMRI screen (RXM parameters screen) and complete all fields.
• Both type-in PSDs result in a new User CV:
  • Phase Correction (CV8): Legacy = 0, New = 1. Select 0 if you want to use the earlier software version of phase correction.

Scan parameters

• Bandwidth: As the RBw increases, SNR decreases, chemical shift artifact decreases, minimum TE decreases (which means the ESP decreases). As ESP decreases, geometric distortion decreases.
  • 64 kHz RBw is used with 4 to 8 shots.
  • RBw > 64 kHz depends on balancing ESP and resolution demands.
  • Single-shot EPI uses the largest RBw possible.
  • When Ramp Sampling User CV is turned on, the RBw is automatically set.
• FOV: Large FOVs decrease resolution, increase SNR, and decrease ESP.
• Frequency: As the frequency matrix increases, the ESP increases.
  • The shortest possible ESP is desirable for a single-shot acquisition. Therefore, as the number of shots decrease, consider increasing the FOV.
  • Typically on a single shot, keep the frequency matrix as low as possible to keep the ESP as short as possible. Finding the right balance between ESP and resolution is critical.
  • Typically a 256 frequency matrix is used with 8 shots or more and RBw 32 to 64 kHz. For 512 frequency matrix, increase the shots and RBw.
• Phase: As phase matrix increases, the resolution increases, and the # of slices decrease, but the scan time does not change, (scan time = shots × TR). An EPI protocol is the only instance where phase may be larger than the frequency value.
• Phase Correct: Always select Phase Correct with EPI scans so that the system can run a “reference” scan prior to data acquisition.
  • If a 1 NEX acquisition is programmed, the reference scan can take as long as the EPI scan, but it is imperative to run Phase Correct in order to have optimum image quality.
  • The reference scan automatically occurs after a successful prescan. It makes calculations and corrections for placing the echo underneath the frequency gradient.
• Phase FOV: As phase FOV decreases, geometric distortion decreases.
• Shots: As the number of shots increase, the susceptibility artifacts decrease, and the scan time increases.
• TE: If a TE less than Minimum Full is selected, the system collects the minimum lines of k-space (65% of k-space) plus a required number of overscans. These images are labeled with an Effective TE. The benefit of only partially filling k-space is more slices.

User CVs

Click the Advance tab to view the available User CVs. The CVs may vary based on the field strength and selected scan and imaging parameters.

• Ramp Sampling
• Spectral-Spatial FatSAT
  • Consider setting Spectral-Spatial FatSAT to 1 to improve fMRI stability when Chem SAT is set to Fat.

User CVs for GRE EPI type in: epira3

• Phase Correction
• Ramp Sampling

User CVs for GRE EPI type in: epiRTra3

• Phase Correction
• Ramp Sampling

Post-process tasks