Improved respiratory navigator gating for thoracic 4D flow MRI.
BACKGROUND
Thoracic and abdominal 4D flow MRI is typically acquired in combination with navigator respiration control which can result in highly variable scan efficiency (Seff) and thus total scan time due to inter-individual variability in breathing patterns. The aim of this study was to test the feasibility of an improved respiratory control strategy based on diaphragm navigator gating with fixed Seff, respiratory driven phase encoding, and a navigator training phase.


METHODS
4D flow MRI of the thoracic aorta was performed in 10 healthy subjects at 1.5T and 3T systems for the in-vivo assessment of aortic time-resolved 3D blood flow velocities. For each subject, four 4D flow scans (1: conventional navigator gating, 2-4: new implementation with fixed Seff =60%, 80% and 100%) were acquired. Data analysis included semi-quantitative evaluation of image quality of the 4D flow magnitude images (image quality grading on a four point scale), 3D segmentation of the thoracic aorta, and voxel-by-voxel comparisons of systolic 3D flow velocity vector fields between scans.


RESULTS
Conventional navigator gating resulted in variable Seff=74±13% (range=56%-100%) due to inter-individual variability of respiration patterns. For scans 2-4, the new navigator implementation was able to achieve predictable total scan times with stable Seff, only depending on heart rate. Semi- and fully quantitative analysis of image quality in 4D flow magnitude images was similar for the new navigator scheme compared to conventional navigator gating. For aortic systolic 3D velocities, good agreement was found between all new navigator settings (scan 2-4) with the conventional navigator gating (scan 1) with best performance for Seff=80% (mean difference=-0.01 m/s; limits of agreement=0.23 m/s, Pearson's ρ=0.89, p<0.001). No significant differences for image quality or 3D systolic velocities were found for 1.5T compared to 3T.


CONCLUSIONS
The findings of this study demonstrate the feasibility of the new navigator scheme to acquire 4D flow data with more predictable scan time while maintaining image quality and 3D velocity information, which may prove beneficial for clinical applications.
Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA.
Pim
P
van Ooij
Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA.
Edouard
E
Semaan
Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA.
Susanne
S
Schnell
Siemens Healthcare, Chicago, IL, USA.
Shivraman
S
Giri
Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA.
Zoran
Z
Stankovic
Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA.
James
J
Carr
Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA.
Alex J
AJ
Barker
Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA; Department of Biomedical Engineering, McCormick School of Engineering, Northwestern University, Chicago, IL, USA. Electronic address: mmarkl@northwestren.edu.
Michael
M
Markl
D000328:Adult; D000368:Aged; D000465:Algorithms; D001013:Aorta, Thoracic / Q000033:anatomy & histology / Q000502:physiology*; D001783:Blood Flow Velocity / Q000502:physiology*; D005240:Feasibility Studies; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods; D021621:Imaging, Three-Dimensional / Q000379:methods*; D018810:Magnetic Resonance Angiography / Q000379:methods*; D008297:Male; D008875:Middle Aged; D015203:Reproducibility of Results; D055425:Respiratory-Gated Imaging Techniques / Q000379:methods*; D012680:Sensitivity and Specificity; D055815:Young Adult
4D flow MRI; Aorta; Navigator gating; Respiration control