Synopsis
Noninvasive imaging can improve risk
stratification in carotid atherosclerosis. Multi-sequence MRI allows
visualization of plaque burden and components, while PET can be used to study
inflammation. Combining the two modalities with hybrid PET/MRI in a one-stop
shop approach may improve assessment of vulnerable plaque.
The goal of this work was to optimize a 3D
multi-sequence carotic PET/MRI protocol including 3D MPRAGE, 3D SPACE pre- and
post-contrast, and UTE, using simulations and optimization in healthy volunteers.
Feasibility of the protocol was demonstrated in
a patient suffering from carotid atherosclerosis (>70% stenosis).
Background
While in current clinical practice the decision
to perform surgical removal of the atherosclerotic plaque (carotid
endarterectomy) is mainly based on the degree of stenosis and symptomatology,
increasing evidence shows that assessment of atherosclerotic plaque with
noninvasive imaging can improve risk stratification1.
Both PET and MRI have independently been used
to assess molecular and morphological features that determine plaque
vulnerability. Multi-sequence MRI allows visualization of overall plaque
burden, intra-plaque hemorrhage (IPH), lipid-rich necrotic core (LRNC), fibrous
cap status, ulcerations and calcifications, while PET can be used to study
plaque inflammation.
Combining the strengths of both modalities with
hybrid PET/MRI imaging in a one-stop shop approach may improve differentiation
between vulnerable and stable plaques. Recently, the feasibility of
simultaneous PET/MRI of the carotid arteries has been demonstrated in several
studies where MRI was mainly used for anatomical reference and attenuation
correction2-7. Recently, a more comprehensive MRI protocol was reported
on a PET/MRI system including 3D-TOF MR angiography, 2D T2w, and pre-
and post-contrast 2D T1w MRI to visualize plaque components8.
IPH has proven to be an important plaque feature that predicts recurrent clinical
events9,10. It has been shown that 3D MPRAGE provides higher
detection rates for IPH with less false positives than other T1w
sequences11,12 and is able to differentiate IPH from the LRNC13.
Other 3D techniques (e.g. SPACE) are emerging for carotid plaque imaging and benefit
from higher resolution in slice direction, possibility of isotropic resolution
to allow multi-planar reconstruction (MPR) and larger anatomical coverage14,15.
Purpose
To develop
and optimize a protocol for simultaneous PET/MRI of the carotid artery
including 3D (SPACE), a dedicated IPH (MPRAGE) sequence and UTE attenuation
correction to enable comprehensive atherosclerotic plaque imaging.
Methods
Permission for this study was granted by the
local ethics committee and written informed consent was obtained from all
subjects.
To determine the optimal shot interval (TRshot)
for MPRAGE, simulations were performed as previously described by Zhu et al.16
using Matlab (R2013b). The protocol was then tested in volunteers on the 3T
Siemens Biograph mMR using a head-neck coil (Siemens) in combination with a
4-channel special purpose coil on one side of the neck (Siemens). To ensure
black-blood in the entire field-of-view (FOV), for most subjects it was
necessary to shift the table 50 mm off-center in head direction during MPRAGE. Thereby,
inflowing blood from the heart and aorta was moved into the FOV of the body transmit
coil and effectively inverted by the global inversion pulse (Figure 2).
3D SPACE was optimized for acquisition time
(4:48 min) by restricting the anterior-posterior FOV to 30 mm, which was
sufficient to encompass internal and external carotid arteries on both sides.
One patient scheduled for carotid endarterectomy
was recruited for 18F-FDG PET/MRI of the symptomatic plaque. While
the patient was on the table, 2 MBq/kg bodyweight 18F-FDG was
injected and the multi-sequence MRI (Table 1) was subsequently performed. After
the pre-contrast scans, 0.1 mmol/kg bodyweight gadobutrol (Bayer) was intravenously
injected. T1w TSE and 3D
SPACE were repeated at 6 min and 10 min post-injection, respectively. 45
minutes after 18F-FDG injection, the PET acquisition (15 min, single
bed position) was started. During PET, two MR sequences for attenuation
correction were performed: a system-standard two-point Dixon and a 3D dual-echo
UTE (TE=0.07/2.46ms). The latter may be used to correct for bone and coil
material2,4.
Results & discussion
From the MPRAGE simulations, the optimal TR
shot
was determined to be 800ms with a turbo factor of 44, which was restricted by
the system to be equal to the number of slices. This TR
shot yielded
maximum signal of the vessel wall and IPH, acceptable IPH-to-wall (±2) and blood-to-wall (±0.5) ratios in a reasonably short
acquisition time (3:35 min) (Figure 1). Figure 2 shows the MPRAGE images from
the patient, where IPH can be clearly identified. The other multi-sequence images
are presented in Figure 3. 3D SPACE axial MPR images (Figure 3.B1-2) had a slightly
lower in-plane resolution compared to 2D T
1w TSE (Figure 3.A1-2),
with similar contrast.
18F-FDG PET revealed no significant
18F-FDG
uptake (target-to-background ratio 1.08, reference jugular vein), indicating that
there was no detectable active inflammation (Figure 3.E). The TE=0.07ms UTE was
able to detect the foam padding and circuitry of the special purpose coil
(Figure 4), which enables attenuation correction of the RF coil
4.
Conclusion
We demonstrated that PET/MRI of the carotid
artery with a comprehensive 3D multi-sequence approach is feasible and enables visualization
of various plaque components including IPH. The protocol described here may be easily
adapted for use with other PET tracers.
Acknowledgements
This research was funded by grants from
Academisch Fonds, Maastricht University Medical Center, and Stichting de Weijerhorst.References
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