Image Processing Reference
In-Depth Information
original MR image
corrected MR image
(a)
original MR image
Intensity
before correction
440
430
420
x
410
400
390
corrected MR image
Intensity
after correction
400
395
390
385
380
x
375
370
365
(b)
FIGURE 5.1
(a) Top row: Severe intensity distortions in breast MR mammography (from
data provided by M. Brady, University of Oxford) are due to the RF surface coil and
strongly hamper visual assessment. The corrected image (right) can clearly be better
assessed. (b) Bottom row: The correction of subtle intensity distortions is visualized on a
head MR slice from a multiple sclerosis patient (from data provided by the BIOMORPH
consortium [EU BIOMED 2]). The original image and the corrected image shown in the
first column are visually similar. An optimal threshold-based segmentation of the gray
matter (second column) is greatly improved after correction. The clear inhomogeneity of
the intensity values along a reference line that lies fully within the same tissue type (white
matter) has also been removed by the correction (third column). The reference line is
indicated as a yellow line in the corrected MR image.
electrical properties of the subject. Here is a list of the most common sources of
MRI intensity inhomogeneity:
1.
Static magnetic field
inhomogeneities are created by the technical
difficulties to create a perfectly uniform field, as well as by ferromag-
netic objects in the imaged object. This leads to both intensity inhomo-
geneities as well as spatial distortions.
B
0
B
inhomogeneities can be
0
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