The particularities of 3D acquisitions are:

  • excitation of a complete volume at each repetition (volume = « thick slice »), rather than one thin slice
  • spatial encoding in 3D by adding phase encoding in the 3rd dimension in relation to the phase and frequency encodings used in 2D imaging
  • multiplication of the number of repetitions of a factor equal to the number of « slices» (partitions) in the third dimension to fill all the 3D k-space
  • reconstruction by 3D Fourier transform.

 

 

 

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All this has consequences for:

 

  • acquisition time: given the large amount of acquired data needed to fill the 3D k-space, either very short TR sequences (echo gradient type) are used, or faster k-space filling methods.
  • the amount of signal: at each repetition, the signal comes from the whole volume, rather than a single slice. Therefore more signal is recorded, with fewer noise. The partitions can be finer than the classic 2D slices, because the signal to noise ratio is better compared to a slice of the same thickness acquired in 2D.
  • spatial resolution: the entire volume of interest is explored, with no interval between partitions, allowing fine section and multiplanar rconstructions.
  • artifacts: because of the two phase encodings, wraparound and truncation artifacts can be seen in two different directions.

 

 

 

 

 

Duration of a 3D imaging sequence

 

Duration = TR ∙ NPy ∙ NPz ∙ Nex

 

With

 

  • TR = Repetition time
  • NPy = Number of encoding steps in the y-axis
  • NPz= Number of encoding steps in the z-axis
  • Nex= Number of excitations