Monday Case of the Day
Physics
Authors: Yong Zhou Ph.D.
Radiology Service, Spectrum Health, Grand Rapids, MI
History: A pediatric seizure patient (<4 m, female) underwent routine head
MRI scan on a 1.5 T scanner.
Imaging sequence & parameters:
Axial T2W 2D PROPELLER sequence
FOV: 20 cm
Slice Thickness: 5 mm (6.5 mm spacing)
Matrix: 320 x 320
TR/TE: 3000/131
NEX: 1.5
ETL: 28
Pixel Bandwidth: 122 Hz
What is the most likely cause of the
image artifact in these images?
(a) Patient motion
(b) Motion correction failure
(c) Imperfection of RF slice profile
(d) Field inhomogeneity
Diagnosis:
The correct answer is (b), the
motion correction algorithm has failed for circular
object that is stationary.
Discussion
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PROPELLER type of imaging technique is widely used in
neuro imaging to correct for in-plane motion-induced artifact.
(Ref 1 & 2)
In some images of pediatric patients, certain slices in
Propeller images look blurred. It’s due to the limitation of the
motion correction algorithm when the object has rotational
symmetry, such as the vertex of the pediatric head. This
phenomenon is also demonstrated in the scan of stationary
cylindrical or spherical phantom (A on the right). This rarely
occurs in adult brain images.
The main reason of this type of artifact is the limitation of the
image reconstruction algorithm. When object is rotationally
invariant (circular with little internal structure), the
corresponding k-space has similar invariance. This causes
the failure in filtering the data due to the lack of internal
reference, and induces motion-like artifact.
The possible solutions are:
(1) Use fiducial markers in the regions where this type of
artifact may occur (B on the right)
(2) Scan with the traditional rect-linear Fourier based
sequence (assuming motion is minimal).
(3) Change to higher spatial resolution and change ETL.
A
B
Figure: Stationary cylindrical
phantom generates motion-like
artifact (A). An externally
fiducial marker (red arrow) can
provide a reference that breaks
the rotational symmetry and
eliminates the artifact (B).
References
1. JG Pipe, Motion correction with PROPELLER MRI: application to
head motion and free-breathing cardiac imaging. Magn Reson
Med. 1999 Nov; 42(5):963-9
2. KP Forbes et al, Brain imaging in the unsedated pediatric patient:
comparison of periodically rotated overlapping parallel lines with
enhanced reconstruction and single-shot fast spin-echo
sequences. AJNR Am J Neuroradiol 2003 May; 24(5): 794-8