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Tuesday Case of the Day
Physics
Authors: Dustin K. Ragan, PhD1 and Charles E. Willis, PhD, DABR2
1Department
2U.T.
History:
of Pediatrics, Washington University, St. Louis, MO
M. D. Anderson Cancer Center, Houston, TX
Substantial signal dropout was observed in arterial spin label (ASL)
images, acquired at 3T in a young female patient three-months after
mild traumatic brain injury
Figure 2. No artifacts are
displayed on the DTI
images
Figure 1. Examples of the artifact (red arrows)
which was only observed in this one sequence
and this one patient
The likely cause of the artifact is:
A.Malfunctioning receive coil
B.Transmit (B1) inhomogeneity
C.Susceptibility-related T2* dephasing
D.Magnetic field (B0) inhomogeneity
Findings:
Anatomical and DTI images did not display signal loss
1. The same coil was used to acquire all images, but signal loss is only visible in one
sequence
2. DTI images are also typically acquired with an EPI-readout, which would also be
sensitivity signal dropout from magnetic susceptibility in the brain
3. Artifact is located at the periphery of the brain
Figure 3: ASL image with artifact
Figure 4: DTI image without artifact
Figure 5: T1-weighted
anatomical image
Diagnosis:
D. Magnetic field inhomogeneity
Discussion:
Pseudocontinous ASL uses a preparatory train of RF pulses
positioned at the neck along with (usually) an EPI readout to acquire
images of cerebral blood flow
Coil problems would manifest across all of the acquired images.
Therefore, choice A. Malfunctioning receive coil is incorrect.
Transmit inhomogeneity typically produces shading on the interior of
the images, not at the periphery. Therefore, choice B. Transmit (B1)
inhomogeneity is incorrect.
Susceptibility-related dephasing is common in EPI acquisitions, which
is used in both ASL and DTI acquisitions. Because only the ASL
images were affected, choice C. Susceptibility-related dephasing is
incorrect. Also, large susceptibility artifacts are relatively rare in mild
TBI, so it is inconsistent with the patient’s presentation.
The patient was wearing a shirt with iron-based glitter up to the shoulder.
This distorted the position of the labeling pulses, causing them to saturate
brain tissue instead of blood.
Figure 6: Susceptibility
artifacts present on both
ASL and DTI images
(Magnetic field inhomogeneties in the brain can distort both ASL and DTI,
as in the figure, but only ASL is sensitive to the neck)
Discussion:
The gradient strengths used in ASL labeling are relatively weak,
which amplifies the distorting effect of field inhomogeneities
ΔB=Gx
Magnetic field offset
Strong gradient
A magnetic field shift produces a
much larger distortion in the
presence of a weak gradient than
a strong one
Gradient strengths used slice
selection are around ~40 mT/m;
those used in labeling are ~10
mT/m
Weak gradient
Physical position
As a result, the ASL labeling
pulses are highly susceptible to
position shifts
RF pulses are not localized and
can unintentionally affect the
entire sensitive volume of the coil
References/Bibliography:
Jahanian H, et al. B0 field inhomogeneity considerations in pseudo-continuous
arterial spin labelng (pCASL): effects on tagging efficiency and
correction strategy. NMR in Biomedicine 24: 1202-1209. 2011.
Haacke M., et al. Magnetic Field Inhomogeneity Effects and T2* Dephasing, In
Magnetic Resonance Imaging: Physical Principles and Sequence
Design. p. 569-617. New York, John Wiley & Sons.