title: Aortic length measurements for pulse wave velocity calculation: manual 2D vs automated 3D centreline extraction.
abstract: BACKGROUND
  Pulse wave velocity (PWV) is a biomarker for the intrinsic stiffness of the aortic wall, and has been shown to be predictive for cardiovascular events. It can be assessed using cardiovascular magnetic resonance (CMR) from the delay between phase-contrast flow waveforms at two or more locations in the aorta, and the distance on CMR images between those locations. This study aimed to investigate the impact of different distance measurement methods on PWV. We present and evaluate an algorithm for automated centreline tracking in 3D images, and compare PWV calculations using distances derived from 3D images to those obtained from a conventional 2D oblique-sagittal image of the aorta.
  
  
  METHODS
  We included 35 patients from a twin cohort, and 20 post-coarctation repair patients. Phase-contrast flow was acquired in the ascending, descending and diaphragmatic aorta. A 3D centreline tracking algorithm is presented and evaluated on a subset of 30 subjects, on three CMR sequences: balanced steady-state free precession (SSFP), black-blood double inversion recovery turbo spin echo, and contrast-enhanced CMR angiography. Aortic lengths are subsequently compared between measurements from a 2D oblique-sagittal plane, and a 3D geometry.
  
  
  RESULTS
  The error in length of automated 3D centreline tracking compared with manual annotations ranged from 2.4 [1.8-4.3] mm (mean [IQR], black-blood) to 6.4 [4.7-8.9] mm (SSFP). The impact on PWV was below 0.5m/s (<5%). Differences between 2D and 3D centreline length were significant for the majority of our experiments (p < 0.05). Individual differences in PWV were larger than 0.5m/s in 15% of all cases (thoracic aorta) and 37% when studying the aortic arch only. Finally, the difference between end-diastolic and end-systolic 2D centreline lengths was statistically significant (p < 0.01), but resulted in small differences in PWV (0.08 [0.04 - 0.10]m/s).
  
  
  CONCLUSIONS
  Automatic aortic centreline tracking in three commonly used CMR sequences is possible with good accuracy. The 3D length obtained from such sequences can differ considerably from lengths obtained from a 2D oblique-sagittal plane, depending on aortic curvature, adequate planning of the oblique-sagittal plane, and patient motion between acquisitions. For accurate PWV measurements we recommend using 3D centrelines.
authors.affiliation: Department of Biomedical Engineering, Division of Imaging Sciences and Biomedical Engineering, King's College London, St Thomas' Hospital, 4th floor Lambeth Wing, Westminster Bridge Road, London, SE17EH, UK. arna.van_engelen@kcl.ac.uk.
authors.forename: Arna
authors.identifier: http://orcid.org/0000-0003-4145-0406
authors.initials: A
authors.lastname: van Engelen
authors.affiliation: Department of Cardiovascular Imaging, Division of Imaging Sciences and Biomedical Engineering, King's College London, St Thomas' Hospital, 4th floor Lambeth Wing, Westminster Bridge Road, London, SE17EH, UK.
authors.forename: Miguel
authors.initials: M
authors.lastname: Silva Vieira
authors.affiliation: Department of Cardiovascular Imaging, Division of Imaging Sciences and Biomedical Engineering, King's College London, St Thomas' Hospital, 4th floor Lambeth Wing, Westminster Bridge Road, London, SE17EH, UK.
authors.forename: Isma
authors.initials: I
authors.lastname: Rafiq
authors.affiliation: Department of Clinical Pharmacology, St Thomas' Hospital, Westminster Bridge Road, London, SE17EH, UK.
authors.forename: Marina
authors.initials: M
authors.lastname: Cecelja
authors.affiliation: Philips Healthcare, Guildford, UK.
authors.forename: Torben
authors.initials: T
authors.lastname: Schneider
authors.affiliation: HSDP Clinical Platforms, Philips HealthTech, Best, The Netherlands.
authors.forename: Hubrecht
authors.initials: H
authors.lastname: de Bliek
authors.affiliation: Department of Biomedical Engineering, Division of Imaging Sciences and Biomedical Engineering, King's College London, St Thomas' Hospital, 4th floor Lambeth Wing, Westminster Bridge Road, London, SE17EH, UK.|Department of Bioengineering and Surgery, University of Michigan, Ann Arbor, MI, USA.
authors.forename: C Alberto
authors.initials: CA
authors.lastname: Figueroa
authors.affiliation: Department of Cardiovascular Imaging, Division of Imaging Sciences and Biomedical Engineering, King's College London, St Thomas' Hospital, 4th floor Lambeth Wing, Westminster Bridge Road, London, SE17EH, UK.|Department of Pediatrics, Pediatric Cardiology, UT Southwestern Medical Center, Dallas, USA.
authors.forename: Tarique
authors.initials: T
authors.lastname: Hussain
authors.affiliation: Department of Biomedical Engineering, Division of Imaging Sciences and Biomedical Engineering, King's College London, St Thomas' Hospital, 4th floor Lambeth Wing, Westminster Bridge Road, London, SE17EH, UK.|Pontificia Universidad Católica de Chile, Escuela de Ingeniería, Santiago, Chile.
authors.forename: Rene M
authors.initials: RM
authors.lastname: Botnar
authors.affiliation: Department of Biomedical Engineering, Division of Imaging Sciences and Biomedical Engineering, King's College London, St Thomas' Hospital, 4th floor Lambeth Wing, Westminster Bridge Road, London, SE17EH, UK.
authors.forename: Jordi
authors.initials: J
authors.lastname: Alastruey
mesh_terms: D000328:Adult; D000368:Aged; D000465:Algorithms; D001011:Aorta / Q000000981:diagnostic imaging* / Q000503:physiopathology / Q000601:surgery; D001017:Aortic Coarctation / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery; D001331:Automation; D001783:Blood Flow Velocity; D016208:Databases, Factual; D005260:Female; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods*; D018810:Magnetic Resonance Angiography / Q000379:methods*; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D011237:Predictive Value of Tests; D063177:Pulse Wave Analysis / Q000379:methods*; D012039:Regional Blood Flow; D015203:Reproducibility of Results; D012189:Retrospective Studies; D059289:Vascular Stiffness; D055815:Young Adult
keywords: Aortic stiffness; Cardiovascular magnetic resonance; Centreline; Pulse wave velocity; Semi-automated tracking