Ascending Aortic Nomograms derived from Contrast Enhanced Magnetic Resonance Angiography: Preliminary Values for Sex, Body Surface Area and Age Related Variability
Lara Mrak1, Christopher J. François1, Sonja Kinner1,2, and Mark L. Schiebler1

1Radiology, UW-Madison, Madison, WI, United States, 2University Hospital Essen, Essen, Germany

Synopsis

The size of the ascending aorta is of critical importance for patient survival as it is directly related to the likelihood of aortic dissection and risk of death. The traditional imaging method that has been used for this measurement has been computed tomography of the chest. With many more children and young adults now being imaged with contrast enhanced magnetic resonance angiography examinations (CE-MRA) , it is useful to know what the range of variability is for the normal aorta on these exams. We present here age and sex specific values for nomograms of the ascending aorta derived from non-gated CE-MRA examinations that are indexed to body surface area.

Introduction

There is a progressive enlargement of the ascending aorta with age. Males have larger ascending aortas than females. The size of the normal aorta needs to be indexed to the patient’s body surface area m2 (BSA), as it is well known that larger individuals have larger aortas. Aortic size nomogram tables have been developed that show the risk of complications from aortic rupture based on size and the BSA. ( Ref 1). For example, an individual with a BSA of 1.8 m2 with an ascending aorta larger than 5.0 cm has a risk of complications from aortic dissection of 8%/year. (Ref 1)

Purpose

We sought to generate sex and age specific nomograms of the orthogonal and direct axial diameters of the ascending aorta that are indexed to the BSA from ungated contrast-enhanced magnetic resonance angiography (CE-MRA) exams.

Methods

This was a HIPAA compliant and IRB approved retrospective study from a data set of over 113 CE-MRA examinations initially performed for the primary evaluation of suspected pulmonary embolism. The 113 subjects, (60 females: 53 males) were evenly distributed into the following age groups: (A) < 45 years; (B) 45-54; (C) 55-64; and (D) > 65 years. The CE-MRA exam was performed at 1.5T ( GE Healthcare, Waukesha, WI) using a post contrast ( gadobenate dimeglumine at 0.1mmol/kg, Bracco Diagnostics) fluoro-triggered, 3D elliptical-centric fast SPGR sequence performed pre-, during-, and post-injection. (Ref 2) Axial (Ref 3) and double-oblique (orthogonal)(Ref 4) measurements of the ascending aorta (AscA) diameter were performed directly on a PACS workstation. Measurements were made at the level of the main right pulmonary artery (Figures 1 and 2). One investigator measured the AscA three times in all 113 patients and a second investigator performed the same measurements three times in a subset of 21 patients. Inter- and intra-observer variability between measurements was quantified using Bland-Altman analysis. Plots for the aortic dimensions were generated for each age group and sex followed by the combined results for each sex plotting the aortic dimensions indexed to BSA by age.

Results

We found the inter-observer variability to be 0.34mm ± 3.365 mm (2 SD) for the axial measurements and 0.12 mm ± 3.13 (2 SD) for orthogonal measurements. While the intra-observer variability was 0.20mm ± 2.81mm (2 SD) for axial measurements and 0.58mm ± 2.61 mm (2 SD) for orthogonal measurements. The inter-test variability between the axial and orthogonal measurements showed a bias of -0.60 mm ± 2.43 mm( 2 SD) (Figure 3). The combined nomogram for all of the females with AscA dimension mm /BSA m2 shows a R2 value of 0.21 (Figure 4), while the male indexed nomogram shows a R2 value of 0.107 (Figure 5).

Discussion

We confirm the findings of Bireley et al (Ref 4) showing that orthogonal measurements of the AscA are less variable for a single observer than the direct axial measurements. As expected, we found that males have larger AscA dimensions than women; even after indexing to BSA. We show that there is good interobserver agreement as well as reasonable intraobserver agreement for these measurements made free hand on the PACS work station without the use of post processing to determine the aortic wall. These nomograms can serve as a basis for determining the relative importance of an individual’s aortic dimensions using non-gated breath hold CE-MRA examinations.

Disclosure

Cardiopulmonary MRA is an off label use of gadolinium based contrast agents.

Acknowledgements

The authors wish to thank GE Healthcare, Bracco Diagnostics and the Departmental Research and Development Fund.

References

1. Davies Ann Thorac Surg 2006;81:169–77

2. Schiebler JMRI 2013; 38:914-925

3. Wolak JACC 2008; 1 (2): 200-208

4. Bireley JMRI 2007; 26: 1480-1485

Figures

Figure 1: Axial plane AP (vertical white line) and transverse measurement (horizontal white line) of the ascending aorta at the level of the maximum size of the right main pulmonary artery. These two measurements were averaged to get the axial ascending aortic diameter.

Figure 2: Generation of orthogonal planes to measure the ascending aorta by using the PACS workstation to get parallel to the center-line of the ascending aorta at the level of the right main pulmonary artery. The two white lines on the orthogonal axial image (top right) represent where the AP and transverse measurements of the aorta were taken.

Figure 3: Bland-Altman plot of the inter test variability for the average of three measurements of the ascending aorta in the axial plane (non-angled) and average of the three measurements in the orthogonal plane (N-113). This shows good agreement between these two locations for measuring the ascending aorta with a negative bias of -0.60 mm. The orthogonal measurements are thus on average 0.6 mm smaller than the direct axial measurements.

Figure 4: Plot of the orthogonal ascending aortic diameter in mm/BSA m2 versus age for females (N=60).

Figure 5: Plot of the orthogonal ascending aortic diameter in mm/ BSA m2 versus age for males (N=53).



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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