Thorough explanation of Geometry Parameters in the Simulator. This list will be presented if copy custom parameters is selected, when right clicking on a sequence to copy slice parameters.
1. FOV Read (Frequency Direction)
Definition: Field of View in the readout (frequency encoding) direction determines the physical size of the imaging area along the frequency axis. The frequency axis is where spatial information is encoded by varying the Larmor frequency of the spins.
Effect: A smaller FOV increases resolution but may cause aliasing if anatomy extends beyond the boundary. The increase in resolution is due to the same number of pixels being spread over a smaller physical area.
Tip: Adjust according to anatomical coverage needed — avoid truncating critical structures. Also, consider the impact on signal-to-noise ratio (SNR) – a smaller FOV with the same matrix size means smaller voxels, which generally reduces SNR.
2. FOV Phase (Arrow Lines)
Definition: Field of View in the phase encoding direction — shown as arrow lines during slice planning.
Effect: Like FOV Read, it defines anatomical coverage but in the orthogonal direction. (to the frequency encoding direction within the slice).
Tip: A smaller FOV Phase reduces scan time only if the number of phase encoding steps (Matrix Phase) is also reduced proportionally. If the Matrix Phase remains the same, a smaller FOV Phase would not directly reduce scan time, but it would increase resolution in the phase direction. However, the most common reason to reduce FOV Phase is to reduce the number of phase encoding steps and thus scan time, which does increase the risk of wrap-around (foldover) artifacts. A key point: reducing the number of phase encoding steps directly reduces scan time.
3. Foldover Direction
Definition: The direction in which phase encoding is applied — can be right-left (RL), anterior-posterior (AP), or foot-head (FH).
Effect: Determines where aliasing appears if the anatomy extends beyond the FOV in the phase encoding direction.
Tip: Change foldover direction to shift aliasing away from the area of interest or use Foldover Suppression (also known as "No Phase Wrap" or "Oversampling").
Note: When copying slice parameters from a native sequence, Corsmed MRI Simulator embeds this parameter within the Center Slice Position. You will have to change it to the desired parameter, if the copied foldover direction is not the one intended for use. Open a sequence by double clicking it, then look for the Geometry Tab and select Foldover Direction.
4. Matrix Read
Definition: Number of pixels in the readout direction. Higher values increase resolution and scan time.
Effect: Higher values increase resolution in the read direction (smaller pixel size). It does not directly increase scan time for a 2D sequence, as the readout is sampled during the same acquisition window. However, a higher Matrix Read often implies a need for higher receiver bandwidth, which can impact SNR. For 3D acquisitions, it will increase scan time.
Tip: Use high readout matrices for detailed imaging (e.g., brain), but balance against SNR (higher bandwidth to accommodate more points can reduce SNR) and scan duration, as there may be potential longer minimum echo times (TE) if the readout window needs to be extended.
5. Matrix Phase
Definition: Number of pixels in the phase encoding direction. This is also often referred to as "number of phase encoding steps" or "number of lines in k-space.”
Effect: More phase lines = better resolution but longer scan time and potential for more motion artifacts. Each phase encoding step requires a new excitation, thus directly increasing scan time.
Tip: Keep practical — e.g., 256 or 320 — unless high detail is essential. Higher phase matrix values are very susceptible to motion, as each line of k-space represents a different point in time during the acquisition.
6. Slice Thickness
Definition: The measured width of each slice. This is determined by the bandwidth of the RF pulse and the strength of the slice-select gradient.
Effect: Thicker slices = higher SNR (because more spins contribute to the signal) and faster scans (fewer slices needed for coverage), but less spatial detail along the slice-select direction.
Tip: Use thinner slices for small structures (e.g., pituitary gland) or when planning 3D reconstructions (where isotropic voxels are often desired).
7. Slice Gap
Definition: The distance between adjacent slices.
Effect: Helps reduce crosstalk (interference between adjacent slices due to imperfect slice profiles) but leaves gaps in coverage.
Tip: Use small gaps (or zero) for detailed surveys; interleaved mode helps mitigate crosstalk. Interleaving acquires non-adjacent slices first, then fills in the gaps, allowing more time for excited spins to decay before the adjacent slice is excited.
8. Slice Number
Definition: Number of slices acquired in a scan.
Effect: More slices = longer scans (assuming 2D acquisition), but bigger coverage. For 3D acquisitions, "slice number" might be more accurately thought of as "partitions" or "slabs," and increasing them significantly increases scan time.
Tip: Plan based on anatomy; more slices are needed for full coverage of large areas like spine or abdomen.
9. In-Plane Orientation
Definition: Direction of the imaging plane — sagittal, coronal, or transverse, or oblique.
Effect: Determines the anatomical view.
Tip: Always align slices to match clinical question — e.g., sagittal for spine, coronal for sinuses. Also, for optimal diagnostic quality, align to minimise partial volume averaging of structures of interest.
Note: When copying slice parameters from a native sequence, Corsmed MRI Simulator embeds this parameter within the Center Slice Position. You will have to change it to the desired parameter, if the copied in-plane orientation is not the one intended for use. Move the yellow cutlines grabbing them from the white boxes (there’s no correspondent parameter field to populate, but a movement to be made with the mouse).
10. Center of the Slice (Center Position)
Definition: The center of the slice (also called center position) is not only the geometric midpoint of the slice plane or group being acquired, but also the intersection point between the slice plane and the scanner's isocenter (the origin of the coordinate system in the bore of the MRI magnet, where the magnetic field is most homogeneous and gradient distortions are minimal).
Effect: The center of the slice is aligned with a specific anatomical landmark and centered to cover the entire anatomy of interest, since it's the exact 3D location (X, Y, Z in scanner coordinates) where the selected slice thickness is centered. This is where:
- The phase encoding, frequency encoding, and slice selection gradients meet.
- The RF pulse is maximally tuned to excite protons within the slice.
Tip: Always center over the target anatomy to maximise resolution and signal over the region of interest, reduce the risk of excluding relevant anatomy and maintain proper alignement across sequences (especially in multi-planar imaging).
Note: When copying this parameter over, Corsmed MRI Simulator also copies the in-plane orientation and the foldover direction from the native sequence.
11. Angulation of the Slice
Definition: Rotation of the imaging plane relative to the scanner’s standard axes (X, Y, Z).
Effect: Allows alignment with anatomical structures (e.g., spinal curvature). This minimises partial volume artifacts and improves visualisation of pathology.
Tip: Use oblique angles to match complex anatomy — improves anatomical accuracy and diagnostic confidence.
12. Saturation Bands
Definition: Pre-saturation slabs placed outside the FOV to nullify signal from moving tissue. They work by applying an RF pulse and a gradient to a specific region before the main excitation, spoiling the magnetisation in that region.
Effect: Reduce motion or flow artifacts (e.g., from vessels, swallowing, or breathing motion) that would otherwise wrap into the image or cause ghosting.
Tip: Use on superior or inferior edges of FOV to suppress flow in arteries or veins. Can also be used to suppress fat signal or CSF signal in specific areas without using full fat suppression techniques.
13. Slice Groups (Multiple Stacks)
Definition: Multiple independently positioned and angled groups of slices.
Effect: Allows imaging of non-contiguous or tilted structures in one experiment without re-prescribing each individual slice.
Tip: Useful for knees, bilateral joints (TMJ), or curved anatomy like the spine (e.g., separate cervical, thoracic, lumbar stacks). This significantly improves workflow efficiency.