Flow phenomena and artefacts

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
Truncation	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
Magnetic susceptibility	Remove metal where possible	None
Shading	Load coil properly	None
Crosstalk	None	None
Cross excitation	Interleaving of slice acquisition	Doubles the scan time
Cross excitation	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.