Flow phenomena
Introduction
This section refers mainly to the Artefact problems subheading discussed under the Image optimization heading considered for each examination in Part 2. The most common flow phenomena are summarized in Table 4.1. Only a brief overview is provided here. For a more detailed explanation, please refer to Chapter 6 of MRI in Practice or an equivalent text.
Table 4.1 Artefacts and their remedies
Artefact | Remedy | Penalty of remedy |
Truncation | Increase phase encodings | Increases scan time |
Use more than one NEX/NSA | Increases scan time | |
Phase mismapping | Respiratory compensation | May lose slices |
Gating | TR variable | |
Pre-saturation | May lose slices | |
GMN | Increases minimum TE | |
Immobilize patient | None | |
Use antispasmodic agent | Costly, invasive | |
Sedation | Invasive, requires monitoring | |
Chemical shift | Increase bandwidth | Decreases TE |
Reduce FOV | Reduces SNR | |
Use chemical saturation | Reduces SNR | |
Chemical misregistration | Set TE at multiple of periodicity | None |
Aliasing | Oversampling (frequency) | None |
Oversampling (phase) | None or increase in scan time depending on system | |
Enlarge FOV | Reduces resolution | |
Zipper | Call engineer | Irate engineer! |
Magnetic susceptibility | Use SE | Not flow sensitive |
Remove metal where possible | None | |
Shading | Load coil properly | None |
Crosstalk | None | None |
Cross excitation | Interleaving of slice acquisition | Doubles the scan time |
Squaring off of RF pulses | Reduces SNR |
The most common types of flow phenomena are:
- TOF (not to be confused with TOF-MRA)
- entry slice phenomenon
- intra-voxel dephasing.
Time of flight
TOF phenomenon occurs because nuclei that move through the slice may receive only one of the RF pulses applied. In GRE sequences, the gradient rephasing is not slice selective, so nuclei produce signal as long as they have been excited at some point and are rephased by the gradient. In a SE sequence, a nucleus may receive the excitation pulse but then exit the slice before the 180° rephasing pulse can be applied. Conversely, it may not be present in the slice when the excitation pulse is applied, and then enter the slice to receive only the 180° pulse. Under these circumstances, the nucleus does not produce a signal. In SE sequences, TOF effects cause either a signal loss or signal enhancement from flowing nuclei, and they are compensated for by using pre-saturation pulses placed between the origin of the flow and the FOV.
Entry slice phenomenon

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