Biomedical Engineering Reference
In-Depth Information
9
Although a direct compar-
ison is difficult, it is fair to say that localization accuracy of MRI has tended to
improve since its advent.
Specific characteristics of each artifact generation mechanism and some cor-
rection methods available will now be discussed. Some correction schemes are
designed to correct for both gradient nonlinearity and field inhomogeneity;
these are discussed in each respective section.
MR images with 3 mm and 5 mm slice thicknesses.
5.3.2
Geometric Distortion Due to Gradient Field Nonlinearity
Gradient field nonlinearity is a consequence of limitations and imperfec-
tions in the design of the gradient coils.
24
Following the nomenclature pro-
posed by Sumanaweera, the geometric distortions resulting from gradient
field nonlinearity are the
barrel
distortion (2D and 3D), the
potato chip
effect
(slice selection, 2D) and the
effect (2D). As gradient field nonlinearity
is solely dependent on geometry of gradient coils, its effects are constant in
time and independent of the imaging sequence used (for a given gradient).
Furthermore, the barrel effect is independent of gradient strength.
bow-tie
23
14
Walton
et al. have more recently shown that geometric distortions due to gradient
nonlinearity are smaller in 3D acquisitions than 2D, as predicted by
Sumanaweera.
24
This is because a weak slice selection gradient is used to
excite the whole volume, therefore reducing considerably the magnitude of
the potato chip effect.
5.3.2.1
Correction
The distortions due to gradient nonlinearity, and in particular the barrel effect,
can be corrected by applying a theoretically or experimentally derived correc-
tion field.
In practice, many modern scanners incorporate such a
scheme in an automatic image reconstruction mechanism, e.g., General Elec-
tric's
12,14,17,22,25
15
In our experience with frame
based MRI stereotaxy, care must be taken as such correction schemes can fail
under specific circumstances; see Lemieux et al.
Gradwarp
; see reference 8 in Sumanaweera.
26
and Figure 5.1. The details of
these correction schemes are generally not published and, depending on the
manufacturer, it may be possible to switch off the correction temporarily. Fur-
thermore, it is often impossible to determine when such correction schemes
were used in each specific study published. This would require, for example,
that the authors state the version of the scanner software used, which is rarely
the case (see also Sumanaweera et al.
24
).
5.3.3
Geometric Distortion Due to Field Inhomogeneity
Field inhomogeneity has three possible origins: imperfection of the field gen-
erated by the magnet, eddy currents induced in the conducting structures of
the scanner by the switching gradients, and spatial variations in the magnetic
susceptibility within the imaged volume.
4,15
Although the latter cannot be
