Alina Psenicny1, Camila Munoz1, Aurélien Bustin1, Karl P Kunze2, Radhouene Neji1,2, Pier-Giorgio Masci1, René M Botnar1, and Claudia Prieto1
1Biomedical Engineering Department, School of Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom, 2MR Research Collaborations, Siemens Healthcare Limited, Frimley, United Kingdom
Synopsis
Simultaneous
18F-FDG PET-MR imaging has shown promise for improved diagnostic
accuracy of inflammatory cardiac diseases, such as cardiac sarcoidosis.
However, respiratory motion and mis-registration between free-breathing 3D PET
and conventional 2D breath-held MR images remain a challenge that has hindered clinical adoption of this technique. Here we introduce a 3x-accelerated free-breathing
motion-corrected 3D whole-heart T2-mapping prototype sequence, which provides characterisation
of myocardial inflammation while additionally providing non-rigid respiratory deformation
fields to correct simultaneously acquired PET data. Results from phantom,
healthy subjects and patients show that the method produces good-quality
high-resolution 3D T2 maps from an efficient scan of ~10 minutes.
Introduction
Simultaneous
18F-FDG PET-MR imaging has shown promise for improved diagnostic
accuracy of inflammatory cardiac diseases, such as cardiac sarcoidosis.1-2 PET-MR imaging
of cardiac sarcoidosis can provide information about myocardial function, the
pattern of injury and disease activity from a single scan. However, fusion and
interpretation of conventional cardiac MR data, usually acquired in 2D under
multiple breath holds, and simultaneously acquired 3D free-breathing PET data remain
a challenge due to misalignment and differences in geometry between both
modalities. Moreover, respiratory motion degrades both image quality and
quantification of PET and MR images, hindering the clinical adoption of this
technique.
Here we introduce an efficient
free-breathing 3D whole-heart T2-mapping sequence, which provides myocardial
inflammation characterisation and non-rigid respiratory motion fields to
correct simultaneously acquired PET data in a 3T hybrid PET-MR system. This is
achieved by extending a previously introduced approach for 3D translational
motion-corrected T2 mapping at 1.5T.3
The proposed sequence potentially enables complementary and fully co-registered motion-compensated
T2 mapping and 18F-FDG PET imaging for inflammatory
characterisation of cardiac disease.Methods
Acquisition & Reconstruction: A
free-breathing 3D whole-heart T2-mapping prototype sequence (Figure
1) was implemented on a 3T PET-MR
system (Biograph
mMR, Siemens Healthcare, Erlangen, Germany). Three spoiled gradient echo datasets
are acquired with an undersampled variable-density Cartesian trajectory4
with an acceleration factor of 3x and varying magnetisation preparation
schemes. A saturation pulse (SAT) is acquired at the start of each heartbeat to
reset the magnetisation and render the sequence heart-rate independent, followed
by a fat saturation pulse and T2-preparation pulses of different durations
(0, 28 and 55ms) immediately prior to the imaging sequence. Image navigators
(iNAVs)5 are integrated in the sequence to
enable 100% respiratory scan efficiency and predictable scan time. Acquired 3D data
are assigned to different respiratory positions (bins) based on the foot-head (FH)
motion estimated from the iNAVs. Images for each respiratory bin are
reconstructed with iterative SENSE6 and intra-bin FH and right-left (RL)
iNAV-based translational motion correction. 3D bin-to-bin non-rigid motion is estimated
from the respiratory-resolved bin images and incorporated in a motion-compensated
reconstruction framework7 with patch-based low-rank
regularization (HD-PROST3) to produce motion-compensated
datasets. T2-maps
are computed using dictionary-matching9.
The
non-rigid motion fields estimated with this approach can be used to
motion-correct both the MR and PET data to the same respiratory position8,
enabling direct fusion of both datasets for analysis and interpretation.
Imaging
& Analysis: The accuracy
of the proposed 3D T2 mapping was investigated in a phantom with
different agarose concentrations (0.8, 1, 1.5, 2, 3 and 5%) with the same
parameters as for the in-vivo scans (acquisition time = 3min43s), and compared
to a gold-standard 2D multi-echo spin-echo sequence (TR=10s, TE=12, 28 and
55ms, matrix size = 128x128, resolution 2x2x8mm3, acquisition time =
21min10s for each TE).
Six
healthy subjects (3 males, 29±2 years-old) and three patients with suspected cardiovascular
disease (2 males, 60±8 years-old) were scanned on a 3T PET-MR scanner using the
proposed non-rigid motion-compensated 3D T2 mapping sequence. Data
was acquired during mid-diastole using a subject-specific trigger delay, acquisition
window and saturation time (longest possible). For the healthy subjects,
imaging parameters included: coronal orientation, FOV=312x312x60-72mm3,
1.5mm3 isotropic resolution, bandwidth=670Hz/px, TR/TE=3.45/1.57ms,
flip angle=15°, acquisition time = 9.3±1.1min. Matching imaging parameters were
used for patients scans, with the exception of FOV=332x332x 64-72mm3 and
resolution (1.6mm3 isotropic) with acquisition time = 8.3±0.5min. In
both cases, image navigators were acquired using 14 low-flip angle (3°) lines
before the 3D acquisition using the same FOV. Conventional breath-held 2D T2
maps were acquired at apex, mid and base for comparison purposes (imaging parameters
for healthy subjects: resolution = 1.5x1.5x8mm3, T2-preparation
pulses = 0, 28, 55 ms, flip angle=12°, bandwidth=1155 Hz/px), imaging parameters
were the same for patients with exception of resolution = 1.9x1.9x8mm3.
3D T2 maps were reconstructed without
motion correction, translational motion correction only (TRMC) (FH and RL
motion estimated from the iNAVs) and non-rigid motion correction (NRMC).Results
Phantom
results show high correlation of the proposed approach with gold standard 2D T2-mapping
(R²=0.99, Figure 2). Improvements in T2-prepared images and final T2 map image quality
is seen with TRMC and further improvements with proposed NRMC as shown on Figure
3. Non-rigid motion fields are obtained with the proposed approach (Figure 3), which
can be used to correct simultaneously acquired PET data. The proposed approach
showed good agreement with conventional 2D T2 maps (Figure 4) while
providing whole-heart coverage (Figure 5) with high isotropic resolution.
In-vivo 3D T2 septal myocardium values were in agreement with
conventional 2D T2 mapping (Figure 4) both in patients (3D-T2map
= 40.4±2.2ms, 2D-T2map = 40.3±2.2ms) and healthy subjects (3D-T2map
= 39.0±1.4ms, 2D-T2map = 38.8±1.2ms).Conclusion
An
efficient free-breathing 3D whole-heart T2-mapping sequence, which provides
characterisation of myocardial inflammation and non-rigid respiratory motion
fields to correct simultaneously acquired PET data in a 3T hybrid PET-MR system
has been proposed. Preliminary MRI-only results showed that the proposed approach
achieves good image quality with T2 values comparable to reference
methods in phantom, healthy subjects and patients experiments. Future studies
will evaluate the proposed method in patients with cardiac sarcoidosis
undergoing PET-MR.Acknowledgements
This
work was supported by EPSRC (EP/L015226/1, EP/P032311/1, EP/P007619/1) and the Wellcome/EPSRC Centre for Medical
Engineering (NS/A000049/1).References
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