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Monday Case of the Day
Physics
Authors: Qi Peng and Andres Rahal
Department of Radiology, UT Health Science Center at San Antonio
History:
63 yr old female underwent 3T MRI on
the abdomen using a multi-channel
torso coil. One of the coronal images
acquired with single-shot TSE (or
HASTE) sequence has a vessel-like
artifact within the spine (red arrow).
Challenge: Explain the artifact and
specify ways to avoid it.
Coronal section of a single-shot FSE MRI
Monday Physics Case of the Day Answers (1 of 4)
Qi Peng and Andres Rahal
Department of Radiology, UT Health Science Center at San Antonio
Answers: It is an aliasing artifact from a scan using a parallel imaging technique (e.g.,
SENSE) wrapped into the FOV from the right side of the patient. The HASTE
sequence used a parallel imaging technique (e.g., SENSE) to decrease
acquisition time, therefore the artifact is also at the center in addition to the left
side of the patient. The prescribed FOV has to be increased to avoid this
artifact.
Monday Physics Case of the Day Answers (2 of 4)
Qi Peng and Andres Rahal
Department of Radiology, UT Health Science Center at San Antonio
Findings:
1. FOV coverage on the patient right
side is not enough (red arrow).
2. Imaging fold-over direction is left to
right as indicated by the aliasing
artifact on the patient’s left shoulder
(yellow arrow).
3. There is an aliasing artifact on the
patient left side (green arrow) folded
from signal on the right side,
although most of it is removed by
the geometric correction algorithm of
the 3T scanner.
Monday Physics Case of the Day Answers (3 of 4)
Qi Peng and Andres Rahal
Department of Radiology, UT Health Science Center at San Antonio
Discussion:
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Advanced imaging protocols are more widely used clinically on high field
magnets (e.g., 1.5T and 3.0T) due to the inherent high SNR can be used to
speed up data acquisition using parallel imaging techniques, which lower SNR.
To use parallel imaging, a multi-channel coil has to be used. Multi-channel
coils also improves SNR, which also justifies the use of parallel imaging.
Parallel imaging techniques generally under-sample k-space profiles along
phase-encoding direction. A “reduction factor” of 2 means only half of the kspace phase profiles are acquired.
The phase-encoding direction is also the direction that may suffer from aliasing
artifacts if prescribed FOV is smaller than needed.
For 2D coronal abdomen MR imaging, the phase-encoding direction is usually
left to right due to its smaller dimension.
Normally aliasing happens at the edge on the other side of the image. This
may not be true anymore if parallel imaging techniques are employed. For
example, aliasing also happens close to the center of the image if the
reduction factor is 2.
It is always advisable to prescribe large enough FOV to avoid aliasing artifacts
overlapping the organ or pathology of interest, or to use some other antialiasing techniques. This is particularly true when parallel imaging is used.
Monday Physics Case of the Day Answers (4 of 4)
Qi Peng and Andres Rahal
Department of Radiology, UT Health Science Center at San Antonio
Discussion:
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Another image obtained from the
same patient with even smaller FOV
also with a reduction factor of 2
generated more aliasing artifacts.
The artifacts are worsened by the lack
of field homogeneity on the edge of
the main magnetic field and aliasing
results in superimposition of signals of
different phases that alternatively add
and cancel (Moire fringe pattern).
Note both the center and the edge of
the image suffer from these artifacts.