2D Imaging (interleaving is the name of the game)

• The principle of how we encode data in 2D is the same as 3D data, but there are some hugely important differences

• Assume the desired $T_{R}$ is long (for contrast reasons). For a 3D experiment, you only get one excitation per $T_{R}$.

• However, if you are treating the same volume as a set of 2D slices, you can probably excite each slice in the volume during the same $T_{R}$. Think of each slice as a separate MRI experiment, effectively running concurrently.

  
  
  
  
  
  
  
  

• therefore, for the 2D case $T_{T} = N_{y} \cdot T_{R}$, and you will have gotten effective coverage of the volume. However, if you tried a 3D acquisition, it would take you $T_{T} = N_{slices}\cdot N_{y} \cdot T_{R}$ to get the same coverage.

• disadvantage is you are stuck with the slices you collected, it is tricky to look at them in any other way. can only get slices so thin with slice-select methods.

• caveat: slice-to-slice cross talk does exist, so frequently interleave slice excitation, i.e., excite every other slice do odd slices, then do even slices to minimize artifacts. artifacts $\equiv$ something in an image that shouldn't be.

• keep in mind that sequence diagrams only show experiment for a single slice, repetition of a 2D experiment for many slices during a single $T_{R}$ is generally implied