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.
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
- 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