1. 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

2. Diagnosis: The correct answer is (b), the motion correction algorithm has failed for circular object that is stationary.

3. Discussion
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).

4. References
JG Pipe, Motion correction with PROPELLER MRI: application to head motion and free-breathing cardiac imaging. Magn Reson Med Nov; 42(5):963-9
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