Pediatric Cardiology

, Volume 36, Issue 8, pp 1761–1773 | Cite as

Aortic Measurements in Patients with Aortopathy are Larger and More Reproducible by Cardiac Magnetic Resonance Compared with Echocardiography

  • Atosa Nejatian
  • Johan Yu
  • Tal Geva
  • Matthew T. White
  • Ashwin Prakash
Original Article

Abstract

Accurate and reproducible aortic measurements are essential in aortopathy patients. Transthoracic echocardiography (TTE) is commonly used but has several limitations. Cardiac magnetic resonance (CMR) can offset these limitations but has not been directly compared with TTE. We compared the reproducibility of CMR and TTE measurements at multiple aortic levels. Patients with a connective tissue disorder (CTD) or bicommissural aortic valve (BAV) (n = 41; 22 CTD, 19 BAV; mean age 18.8 ± 8.9 years) with TTE and CMR imaging performed within 3 months of each other were randomly selected. Two blinded observers measured the aorta at multiple anatomic levels. Intra- and interobserver variability and agreement between techniques were assessed. Aortic root diameter measurements by TTE and CMR were equally reproducible (% error 4–10 %), but TTE measurements were systematically smaller by 5–7 % (p < 0.0001). Systematic differences were larger in BAV (11–12 %, p < 0.0001) due to root asymmetry. CMR measurements of aortic root cross-sectional area were feasible and highly reproducible (% error 5–8 %). Compared with CMR, ascending aorta measurements by TTE were less reproducible, especially in BAV (% error 21–24 vs. 6–7 %, p = 0.01). Distal aortic measurements by TTE were 14–29 % smaller and had poor reproducibility compared with CMR (% error 24–42 vs. 9–10 %; p < 0.0001). CMR measurement of the largest aortic root dimension is more reliable than TTE, especially when the root is asymmetric. Measurements of the thoracic aorta distal to the root by CMR are more accurate and reproducible than by TTE. These data support a role for CMR in aortopathy patients.

Keywords

Connective tissue disorder Marfan syndrome Bicuspid aortic valve CMR Echocardiography Aortic dilation 

Background

Patients with aortopathy associated with bicommissural aortic valve (BAV) and connective tissue disorders (CTDs) such as Marfan syndrome, Loeys–Dietz syndrome, and Ehlers–Danlos syndrome are at risk of aortic dilatation, occasionally leading to dissection, rupture, and death. These complications can be prevented by a timely surgical replacement of the affected aortic segment [1, 6, 8, 12, 13]. Aortic dilation commonly involves the aortic root in CTDs and the ascending aorta in BAV, but can also affect the more distal aorta (e.g., the isthmus, transverse arch, and descending aorta). Since the risk of aortic complications and the decision to perform elective surgery depend on the largest aortic dimension [5], an accurate and reproducible noninvasive technique for measuring aortic size is an essential element of clinical surveillance in these patients. Transthoracic echocardiography (TTE) is the modality most frequently used, since it is widely available, inexpensive, and easy to use [11]. However, image quality can be limited by poor acoustic windows, especially while imaging the more distal aorta. Cardiovascular magnetic resonance (CMR) offers several theoretical advantages over TTE, including more reliable and higher quality imaging and lack of dependence on patient size or acoustic windows [14]. However, CMR is more expensive and less readily available [11]. Although it has been hypothesized that CMR measurements may be more reproducible and reliable [17], a head-to-head comparison with TTE has not been performed along the entire aortic length. Such comparisons are difficult because there is no widely accepted “gold-standard” technique. Further, several different measurements of the aortic root are possible on CMR and there is lack of consensus and standardization regarding the most optimal measurement, especially in BAV patients [4].

In this study, we retrospectively compared multilevel aortic measurements on TTE and CMR in a random sample of aortopathy patients with respect to their intraobserver and interobserver variability, and their ability to measure the largest aortic size. In addition, we compared multiple diameter and cross-sectional area measurements of the aortic root by CMR in order to identify the largest and most reliable measurements.

Material and Methods

We conducted a retrospective analysis of existing clinical and imaging data at a single institution. The Boston Children’s Hospital Committee on Clinical Investigations approved review of the medical records and waived the requirement for informed consent. Demographic and clinical data were abstracted from the medical records. TTE and CMR images were retrieved from institutional digital image archives.

Subjects

Patients >1 year of age with BAV or CTD who had not undergone aortic surgery and had CMR and TTE performed within 3 months of each other from 2005 through 2013 at our institution were eligible for inclusion. Patients were excluded: (1) if they did not have at least one available image to measure each aortic segment on TTE and CMR or (2) if they had a transcatheter or surgical cardiac procedure between CMR and TTE examinations. Patients were not excluded based on image quality alone, as long as all segments were measurable. From the pool of eligible patients, subjects were randomly selected for inclusion to form roughly equal-sized groups (~20 per group) of CTD and BAV patients based on prior sample size calculations. This sample size yields 80 % power to detect a 2.3 mm difference in aortic dimension between the TTE and CMR measurements (intraobserver or interobserver), assuming a standard deviation of 5 mm for the mean difference and using a paired t test with a 0.05 two-sided significance level; the detectible difference is 3.3 mm within each disease group (BAV or CTD).

TTE

Echocardiography was performed using commercial scanners (iE33 or Sonos 7500, Philips Healthcare, Best, The Netherlands), and images were digitally archived and retrieved using commercial software (Merge Cardio, Merge Healthcare, Chicago, IL). Aortic measurements were made using electronic calipers perpendicular to the direction of blood flow on the peak-systolic frame from inner edge to inner edge according to the recommendations of the American Society of Echocardiography [9]. Diameters of the aortic root and the ascending aorta (AAO) were measured on a parasternal long-axis view (Fig. 1) with imaging of the AAO often requiring change in transducer position to optimize visualization. In addition to the peak-systolic measurements, aortic root diameter at end diastole was also measured. Diameters of the distal transverse arch, isthmus, and proximal thoracic descending aorta (DAO) 1 cm distal to the isthmus were measured from the suprasternal view. Finally, the abdominal DAO was measured 1 cm distal to the diaphragmatic plane from the subxiphoid short-axis view.
Fig. 1

TTE measurements. a. Aortic root diameter on parasternal long-axis view; b. ascending aorta diameter on modified parasternal long-axis view to optimize visualization of the AAO; c. distal transverse arch (a), isthmus (b), and thoracic descending aorta 1 cm distal to the isthmus measured on a suprasternal view (c); d. abdominal descending aorta 1 cm distal to the diaphragm

CMR

Imaging was performed using a 1.5 T whole-body scanner (Achieva, Philips Healthcare, Best, The Netherlands). Electrocardiogram-gated cine steady-state free precession (SSFP) imaging was performed during breath-holding in the short axis of the aortic root, short axis of the AAO, and in two orthogonal long-axis planes across the left ventricular outflow tract (LVOT) (Fig. 2) using the following imaging parameters: echo time 1.5–2 ms, repetition time 2.8–4.0 ms, flip angle 45°, turbo factor 10–20, and 30 reconstructed images per cardiac cycle. Three-dimensional (3D) magnetic resonance angiography (MRA) was performed using a non-gated gradient echo sequence during breath-hold after intravenous administration of 0.1–0.2 mmol/kg of gadopentate dimeglumine (Magnevist, Berlex, Seattle, WA). Images were analyzed on a commercial workstation (Extended Workstation, Philips Healthcare, Best, The Netherlands).

The aortic root was measured on two orthogonal cine SSFP LVOT planes (anterior–posterior and lateral) as well as on a short-axis cine SSFP plane across the aortic root at peak systole (Fig. 2 a–c). On the short-axis plane, diameter measurements were made in all possible combinations of sinus-to-sinus and sinus-to-commissure pairs. All measurements were made from inner edge to inner edge. During measurement, the short-axis images were cross-referenced to the two orthogonal long-axis views to help select the slice with the largest root dimension while correcting for through-plane motion during the cardiac cycle. In BAV with significant cusp fusion, leaflet morphology was ignored and the individual commissures and sinuses were identified based on the contour of wall of the sinuses (Fig. 3). In cases where systolic flow turbulence made visualization of the commissures difficult, the diastolic frame at the corresponding level was referenced to help identify the commissures and sinuses. In addition to diameter measurements, aortic root cross-sectional area (CSA) was measured by planimetry of the peak-systolic frame on short-axis cine SSFP images at the levels of the largest root dimension in each phase of the cardiac cycle.
Fig. 2

CMR measurements of the aortic root and ascending aorta. a. Anterior–posterior aortic root diameter on oblique sagittal long-axis cine SSFP image; b. lateral aortic root diameter on oblique coronal long-axis cine SSFP image; c. all possible sinus–sinus and sinus–commissure measurements of the aortic root on short-axis cine SSFP image; d. orthogonal measurements of the ascending aorta on short-axis cine SSFP image

Fig. 3

CMR assessment of aortic root in a patient with BAV. The commissures are identified by following the contour of the aortic sinuses. Identification of commissure on the systolic frame (b) is facilitated by referring to the diastolic frame (a) at the same level, which is less affected by flow-related dephasing

The AAO was measured on a short-axis cine SSFP image during peak systole (Fig. 2d). The imaging plane was planned perpendicular to the largest segment of the vessel identified on localizing images. The diameters of the distal transverse arch, isthmus, proximal thoracic DAO, and abdominal DAO were measured from multiplanar reformatted images from gadolinium 3D MRA using the thinnest possible slice and adjusting the image contrast and brightness such that the chest wall was just visible and lung tissue remained dark (Fig. 4).
Fig. 4

CMR measurements of distal aorta on Gd-MRA imaging. a. Measurements were performed at the distal transverse arch (a), isthmus (b), thoracic DAO 1 cm distal to the isthmus (c), and abdominal DAO 1 cm distal to the diaphragm (d); b. example of multiplanar reformatting of the 3D dataset to obtain a cross-sectional view of the distal transverse arch; c. this view was used to measure the diameters of the distal arch in orthogonal planes

Training and Sequence of Measurements

All study measurements were performed by two blinded fourth-year medical students. Prior to commencing the study measurements, both observers underwent a 2-week period of intensive training (~60 h) under the supervision of an expert in CMR and TTE. Following a demonstration of measurement techniques by the supervising physician, both observers independently practiced all required measurements on 48 TTE and 37 CMR datasets on patients with diagnoses other than aortopathy (excluded from the study). The supervisor provided feedback on these practice measurements, and at the end of the training period, the competence of each observer was confirmed by direct observation of measurements made on a CMR and TTE dataset each. To assess interobserver reproducibility, the two observers independently performed TTE and CMR measurements on all subjects. To assess intraobserver reproducibility, one observer repeated the measurements after an interval of 2 weeks in the same order as the first round of measurements.

Statistical Analysis

Intra- and interobserver reproducibility for TTE and CMR were assessed using the intraclass correlation coefficient (ICC) [3] and the Bland–Altman technique [2]. The ICC indicates the proportion of variability explained by subject differences in contrast to observer differences or random error, and a higher value denotes higher reproducibility [3]. For the Bland–Altman analysis, the reproducibility for each pair of measurements was expressed as ±1.96 standard deviation (SD) of the intra- or interobserver difference (reproducibility coefficient or RC). The RC reflects the maximum expected within-patient difference that can be attributed to observer variability, and a lower value denotes higher reproducibility [2]. The RC was also expressed as a percentage of the mean value of the parameter of interest (referred to as the % error). Comparisons of RC between TTE and CMR measurements at the same aortic location or between CMR (or TTE) measures at different aortic locations were performed using a bootstrap procedure. In this procedure, 5000 random samples of size n = 41 (the total sample size) were obtained with replacement from the study data, and the observed RC difference in the study data was compared with a null distribution of RC differences (generated from the 5000 bootstrap samples); the p value is defined as the proportion of bootstrap samples with a greater RC difference than that observed in the study data. Agreement between TTE and CMR measurements was investigated using a paired t test. Statistical analysis was performed using commercially available software (SPSS v. 22.0, Armonk, NY and, R v. 3.1.2, Vienna, Austria). The threshold for statistical significance was set at p < 0.05.

Results

Subjects

Clinical characteristics of the 41 study subjects are summarized in Table 1. Patients underwent clinically indicated TTE and CMR examinations in no specific order with a median interval of 5 days (range 0–29 days). The study group consisted mostly of young adults with either BAV (46 %) or CTD (54 %). The mean ages of patients with CTDs or BAV were similar (20.2 ± 2 vs. 17.3 ± 1 years, p = 0.3). The most common morphology of BAV was underdevelopment of the intercoronary (right–left) commissure (n = 10, 50 %), followed by right/non-commissure (n = 9, 45 %), and left/non-commissure (n = 1, 5 %). Mean values of aortic measurements by TTE and CMR obtained by observer 1 are summarized in Table 2.
Table 1

Patient characteristics

Median age (years)

17.4 (5–48)

Males

28 (68 %)

Diagnosis

 Bicommissural aortic valve

19 (46 %)

 Connective tissue disorder

22 (54 %)

  Marfan syndrome

16 (41 %)

  Loeys–Dietz syndrome

3 (7 %)

  Nonspecific connective tissue disorder

2 (4 %)

  Ehlers–Danlos syndrome

1 (2 %)

Data expressed as number (range or percentage)

Table 2

Mean values for aortic measurements by TTE and CMR

Parameter

All patients (n = 41)

BAV (n = 19)

CTD (n = 22)

Aortic root long-axis diameter in systole (mm)

 TTE (anterior–posterior)

36.3 ± 8

32.1 ± 7

40.2 ± 7

 CMR (anterior–posterior)

36.9 ± 8

32.2 ± 7

40.6 ± 7

 CMR (lateral)

38.5 ± 7

35.4 ± 6

41.5 ± 6

 Mean difference between largest long-axis CMR and TTE

2.7 (7 %)

4.2 (12 %)

1.3 (3 %)

Aortic root short axis by CMR in systole (mm)

   

 Left/non-sinus

37.0 ± 6

34.3 ± 6

39.5 ± 5

 Right/left sinus

34.0 ± 7

30.3 ± 7

37.6 ± 6

 Right/non-sinus

35.0 ± 8

31.3 ± 7

38.5 ± 6

 Right sinus to left/non-commissure

35.9 ± 8

32.0 ± 7

39.6 ± 6

 Non-sinus to right/left commissure

36.7 ± 7

33.9 ± 7

39.4 ± 6

 Left sinus to right/non-commissure

36.8 ± 7

33.3 ± 6

40.1 ± 6

 Mean difference between largest short-axis CMR and long-axis TTE

2.0 (5 %)

3.8 (11 %)

0.3 (1 %)

 Aortic root cross-sectional area by CMR in systole (cm2)

11.8 ± 4

9.8 ± 4

13.7 ± 4

Ascending aorta diameter in systole (mm)

 TTE (anterior–posterior)

33.6 ± 8

36.0 ± 8

31.4 ± 8

 CMR (anterior–posterior)

33.7 ± 8

36.7 ± 9

30.9 ± 7

 CMR (lateral)

34.8 ± 9

37.7 ± 9

32.1 ± 8

 Mean difference between largest CMR and TTE

1.1 (3 %)

1.8 (5 %)

0.5 (2 %)

Distal transverse arch diameter (mm)

   

 TTE diameter (anterior–posterior)

17.9 ± 4

18.1 ± 4

17.7 ± 4

 CMR diameter on ungated MRA (anterior–posterior)

21.0 ± 4

21.5 ± 5

20.6 ± 4

 CMR diameter on ungated MRA (lateral)

20.0 ± 4

20.6 ± 4

19.5 ± 3

 Mean difference between largest CMR and TTE (mm)

2.9 (14 %)

2.6 (12 %)

3.1 (15 %)

Isthmus diameter (mm)

 TTE diameter (anterior–posterior)

14.5 ± 4

14.0 ± 4

14.9 ± 3

 CMR diameter on ungated MRA (anterior–posterior)

19.8 ± 3

19.8 ± 4

19.9 ± 3

 CMR diameter on ungated MRA (lateral)

19.2 ± 3

19.1 ± 4

19.4 ± 3

 Mean difference between largest CMR and TTE measurements

5.7 (29 %)

6.0 (30 %)

5.5 (28 %)

Thoracic descending aorta diameter (mm)

 TTE diameter (anterior–posterior)

15.5 ± 4

15.1 ± 4

15.9 ± 4

 CMR diameter on ungated MRA (anterior–posterior)

21.4 ± 4

20.6 ± 4

22.1 ± 5

 CMR diameter on ungated MRA (lateral)

21.0 ± 4

20.3 ± 4

21.6 ± 5

 Mean difference between largest CMR and TTE measurements (mm)

5.7 (27 %)

5.3 (26 %)

6.1 (28 %)

Abdominal descending aorta diameter (mm)

 TTE diameter (anterior–posterior)

15.0 ± 3

15.1 ± 3

14.9 ± 3

 CMR diameter on ungated MRA (anterior–posterior)

15.8 ± 3

16.0 ± 3

15.9 ± 3

 CMR diameter on ungated MRA (lateral)

16.2 ± 3

16.3 ± 3

16.1 ± 2

 Mean difference between largest CMR and TTE (mm)

1.4 (9 %)

1.6 (10 %)

1.3 (8 %)

BAV, bicommissural aortic valve; CMR, cardiac magnetic resonance; CTD, connective tissue disorder

Aortic Root

As shown in Table 3 and Fig. 4, both TTE and CMR demonstrated comparably high intra- and interobserver reproducibility in measuring aortic root diameter with <10 % error for both modalities (ICC ≥ 0.98). However, CMR measurements of the largest diameter on long-axis or short-axis images were systematically larger compared with TTE by 2–2.7 mm (5–7 %) (p < 0.0001). In patients with BAV, CMR measurements were 3.8–4.2 mm (11–12 %) larger than by TTE (p < 0.0001) (Table 2). This systematic difference was related to asymmetry of the sinuses of Valsalva with the lateral dimension being 7 % larger (11 % for BAV) than the antero-posterior dimension on long-axis CMR images (p = 0.0001).
Table 3

Comparison of reproducibility of systolic aortic root measurements by TTE and CMR

 

All patients (n = 41)

Reproducibility coefficient (% error)

ICC

BAV (n = 19)

Reproducibility coefficient (% error)

ICC

CTD (n = 22)

Reproducibility coefficient (% error)

ICC

Diameter on long-axis images (mm)

TTE (anterior–posterior)

 Intraobserver

1.9 (5 %)

0.99

2.1 (6 %)

0.99

1.7 (4 %)

0.99

 Interobserver

3.1 (9 %)

0.98

3.1 (10 %)

0.97

2.9 (7 %)

0.98

CMR (anterior–posterior)

 Intraobserver

2.6 (7 %)

0.99

2.5 (8 %)

0.98

2.6 (7 %)

0.99

 Interobserver

3.3 (9 %)

0.98

2.8 (9 %)

0.98

3.3 (9 %)

0.98

CMR (lateral)

 Intraobserver

2.0 (5 %)

0.99

2.1 (6 %)

0.99

1.7 (4 %)

0.99

 Interobserver

3.1 (9 %)

0.98

3.1 (10 %)

0.97

2.9 (7 %)

0.98

Diameter on short-axis CMR images (mm)

Left/non-sinus

 Intraobserver

2.3 (6 %)

0.97

3.0 (9 %)

0.96

1.6 (4 %)

0.96

 Interobserver

3.9 (10 %)

0.95

3.5 (10 %)

0.96

4.2 (11 %)

0.93

Right/left sinus

 Intraobserver

2.6 (8 %)

0.98

3.2 (10 %)

0.97

2.0 (5 %)

0.98

 Interobserver

3.9 (12 %)

0.96

4.2 (14 %)

0.95

3.3 (9 %)

0.96

Right/non-sinus

 Intraobserver

2.0 (6 %)

0.99

2.6 (8 %)

0.98

1.2 (3 %)

0.99

 Interobserver

3.8 (11 %)

0.97

4.4 (14 %)

0.96

2.3 (8 %)

0.96

Right sinus to left/non-commissure

 Intraobserver

1.9 (5 %)

0.99

1.7 (5 %)

0.99

2.1 (5 %)

0.99

 Interobserver

2.5 (8 %)

0.99

3.4 (11 %)

0.97

2.3 (6 %)

0.98

Non-sinus to right/left commissure

 Intraobserver

1.6 (4 %)

0.99

1.5 (4 %)

0.99

1.8 (5 %)

0.99

 Interobserver

2.5 (7 %)

0.98

2.4 (7 %)

0.99

2.4 (6 %)

0.98

Left sinus to right/non-commissure

 Intraobserver

1.6 (4 %)

0.99

1.7 (5 %)

0.99

1.6 (4 %)

0.99

 Interobserver

2.7 (7 %)

0.98

3.1 (9 %)

0.97

2.2 (6 %

0.98

Cross-sectional area on CMR short-axis images(cm2)

 Intraobserver

0.6 (5 %)

0.99

0.5 (5 %)

0.99

0.7 (5 %)

0.99

 Interobserver

0.9 (8 %)

0.99

1.1 (11 %)

0.99

0.7 (5 %)

0.99

BAV, bicommissural aortic valve; CMR, cardiac magnetic resonance; CTD, connective tissue disorder; ICC, intraclass correlation coefficient; reproducibility coefficient = 1.96 × SD of mean difference; TTE, transthoracic echocardiography;  % error = reproducibility coefficient × 100/population mean value for parameter

Among the various long- and short-axis CMR measurements of the aortic root, the reproducibility was excellent with no significant difference between specific diameters. However, there were prominent differences between the measurements related to root asymmetry with a 13 % (4.9 mm) mean difference between the largest and smallest diameters (p < 0.0001) (Table 4). The measurement from left sinus to non-sinus was most frequently the largest, while the measurement from right sinus to left sinus was often the smallest.

To explore whether cross-sectional area measurements by planimetry are reliable in asymmetric aortic roots, we studied the reproducibility of these measurements. Analysis on intra- and interobserver variability found that aortic root cross-sectional area is highly reproducible with 5–8 % error and an ICC of 0.99.
Table 4

Comparison of CMR and TTE aortic root measurements

 

All patients (n = 41)

BAV (n = 19)

CTD (n = 22)

Largest long-axis CMR measurement (most frequent)

Lateral (33/41)

Lateral (17/19)

Lateral (16/22)

Smallest long-axis CMR measurement (most frequent)

Anterior–posterior (33/41)

Anterior–posterior (17/19)

Anterior–posterior (16/22)

Largest short-axis CMR measurement (in order of frequency)

Left/non (21/41)

Left/non (11/19)

Left/non (10/22)

Right comm (8/41)

Right comm (3/19)

Right comm (5/22)

Left comm (6/41)

Non-comm (3/19)

Left comm (4/22)

Non-comm (4/41)

Left comm (2/19)

Right/left (1/22)

Right/left (1/41)

 

Right/non (1/22)

Right/non (1/41)

 

Non-comm (1/22)

Smallest short-axis CMR measurement (in order of frequency)

Right/left (19/41)

Right/left (11/19)

Right/non (9/22)

Right/non (14/41)

Right/non (5/19)

Right/left (8/22)

Right comm (4/41)

Left comm (1/19)

Right comm (3/22)

Non-comm (2/41)

Left/non (1/19)

Non-comm (2/22)

Left/comm (1/41)

Right comm (1/19)

 

Left/non (1/41)

  

Largest long-axis CMR diameter (mm)

39.2 ± 7

35.9 ± 7

42.1 ± 6

Smallest long-axis CMR diameter (mm)

36.4 ± 7

31.0 ± 6

40.5 ± 7

Largest short-axis CMR diameter (mm)

38.4 ± 7

35.5 ± 7

41.0 ± 6

Smallest short-axis CMR diameter (mm)

33.5 ± 7

29.6 ± 6

37.1 ± 6

TTE diameter (mm)

36.4 ± 8

32.1 ± 7

40.7 ± 6

Mean difference between largest and smallest long-axis CMR diameters (mm)

2.8 (7 %)

4.1 (11 %)

1.5 (3.5 %)

Mean difference between largest and smallest short-axis CMR diameters (mm)

4.9 (13 %)

5.9 (16 %)

4.0 (10 %)

Mean difference between largest long-axis CMR and TTE diameters (mm)

2.7 (7 %)

4.2 (12 %)

1.4 (3 %)

Mean difference between largest short-axis CMR and TTE diameters (mm)

2.0 (5 %)

3.8 (11 %)

0.4 (1 %)

BAV, bicommissural aortic valve; CMR, cardiac magnetic resonance; CTD, connective tissue disorder; TTE, transthoracic echocardiography. Comm = commissure directly opposite to the sinus being measured

Ascending Aorta

As shown in Tables 2 and 5, while there was reasonable agreement between TTE and CMR measurements of AAO diameters, the reproducibility of the TTE measurements was suboptimal, especially in BAV patients (17 % error overall, 21–24 % for BAV). This translated into limits of agreement of 6–7 mm in measuring the AAO diameter by TTE. In contrast, CMR measurements were highly reproducible (6–7 % error) with limits of agreement of 2–3 mm (p = 0.01 compared with TTE).
Table 5

Comparison of reproducibility of aortic measurements distal to the aortic root by TTE and CMR

 

All patients (n = 41)

Reproducibility coefficient (% error)

ICC

BAV (n = 19)

Reproducibility coefficient (% error)

ICC

CTD (n = 22)

Reproducibility coefficient (% error)

ICC

Ascending aorta diameter (mm)

TTE (anterior–posterior)

 Intraobserver

5.6 (17 %)

0.95

7.5 (24 %)

0.90

2.6 (7 %)

0.99

 Interobserver

5.7 (17 %)

0.94

6.6 (21 %)

0.91

3.8 (11 %)

0.97

CMR (anterior–posterior)

 Intraobserver

1.9 (6 %)

0.99

1.9 (6 %)

0.99

1.9 (5 %)

0.99

 Interobserver

2.2 (7 %)

0.99

2.2 (7 %)

0.99

2.3 (6 %)

0.99

Distal transverse arch diameter (mm)

TTE (superior–inferior)

 Intraobserver

4.6 (25 %)

0.86

4.3 (24 %)

0.86

4.9 (27 %)

0.87

 Interobserver

7.5 (42 %)

0.64

4.4 (25 %)

0.85

9.8 (54 %)

0.50

CMR (superior–inferior)

 Intraobserver

1.9 (9 %)

0.98

1.9 (9 %)

0.97

1.9 (9 %)

0.98

 Interobserver

1.0 (9 %)

0.97

2.1 (10 %)

0.96

1.6 (8 %)

0.98

Isthmus diameter (mm)

TTE (anterior–posterior)

 Intraobserver

6.0 (42 %)

0.76

6.2 (41 %)

0.69

5.9 (42 %)

0.81

 Interobserver

4.9 (34 %)

0.80

3.5 (23 %)

0.88

6.0 (42 %)

0.75

CMR (anterior–posterior)

 Intraobserver

1.7 (9 %)

0.97

1.9 (10 %)

0.96

1.6 (8 %)

0.98

 Interobserver

2.1 (10 %)

0.95

2.3 (11 %)

0.93

1.4 (7 %)

0.98

Thoracic descending aorta diameter (mm)

TTE (anterior–posterior)

 Intraobserver

3.8 (24 %)

0.88

4.6 (29 %)

0.82

2.8 (19 %)

0.94

 Interobserver

4.6 (30 %)

0.81

5.0 (32 %)

0.76

4.2 (28 %)

0.85

CMR (anterior–posterior)

 Intraobserver

2.3 (11 %)

0.96

2.6 (12 %)

0.95

2.0 (10 %)

0.96

 Interobserver

2.4 (11 %)

0.96

2.8 (13 %)

0.95

1.9 (9 %)

0.97

Abdominal descending aorta diameter (mm)

TTE (anterior–posterior)

 Intraobserver

3.3 (22 %)

0.83

3.2 (21 %)

0.86

3.6 (24 %)

0.81

 Interobserver

3.5 (23 %)

0.81

3.6 (24 %)

0.81

3.5 (23 %)

0.81

CMR (anterior–posterior)

 Intraobserver

1.8 (12 %)

0.94

2.0 (12 %)

0.93

1.5 (9 %)

0.95

 Interobserver

1.9 (12 %)

0.93

1.5 (9 %)

0.96

2.3 (14 %)

0.95

BAV, bicommissural aortic valve; CMR, cardiac magnetic resonance; CTD, connective tissue disorder; ICC, intraclass correlation coefficient; reproducibility coefficient = 1.96 × SD of mean difference; TTE, transthoracic echocardiography;  % error = reproducibility coefficient × 100/population mean value for parameter

Aortic Isthmus

As shown in Table 5 and Fig. 5, measurements of the aortic isthmus by TTE showed poor reproducibility (34–42 % error; ICC 0.76–0.80) and, on average, TTE diameter measurement were smaller by 5.7 mm (29 %, p < 0.0001) compared with CMR (Table 2). In contrast, CMR measurements were highly reproducible (<10 % error; ICC > 0.95, p < 0.0001 compared with TTE).
Fig. 5

Bland–Altman diagrams comparing the interobserver variability of TTE and CMR measurements across the length of the aorta. In each diagram, the central solid red line represents the bias between observers, while the two dashed blue lines denote the 95 % limits of agreement. AAO ascending aorta; Tr. Arch transverse arch, DAO descending aorta

Transverse Aortic Arch and Descending Aorta

As shown in Table 5 and Fig. 5, TTE measurements of the transverse aortic arch and the thoracic or abdominal descending aorta showed poor reproducibility (23–42 % error; ICC 0.64–0.88) and, on average, TTE diameter measurements were smaller by 1.4–5.7 mm (14–29 %) compared with CMR. In contrast, CMR measurements were highly reproducible (<10 % error; ICC > 0.95) (Table 4).

Discussion

In this head-to-head comparison of TTE and CMR in aortopathy patients, CMR demonstrated several advantages over TTE. CMR yielded more reliable measurements of the largest aortic root diameter, especially in patients with asymmetric aortic roots and was able to directly measure aortic root cross-sectional area, which proved to be highly a reproducible parameter. Furthermore, CMR provided a more reproducible measurement of the AAO diameter, especially in BAV patients. Importantly, TTE diameter measurement of the aortic arch, isthmus, and descending aorta were systematically smaller compared with CMR and showed high observer variability. In contrast, measurements of the distal aorta by CMR were highly reproducible.

Both TTE and CMR are commonly utilized in serial follow-up of patients with aortopathy and as such, there is no widely acceptable gold-standard technique. A head-to-head comparison of TTE and CMR along the entire aortic length has not been previously reported, and prior studies have been confined to measurements of the aortic root and/or AAO [15, 17].

Aortic Root

The aortic root is the commonest site for aortic dilation in CTDs, and serial measurement of aortic root diameter by TTE is recommended to guide medical and surgical management [5, 6, 12]. However, TTE measurement of the aortic root is performed in a single anterior–posterior plane, while CMR allows measurements of orthogonal dimensions using both long- and short-axis imaging planes. It is well known that the aortic root can enlarge asymmetrically in patients with aortopathy [16]. Because the risk of aortic complications is closely related to aortic root size, measurement of the largest diameter is paramount. Our data show that because the largest root diameter is often in the lateral plane, TTE measurement of the anterior–posterior dimension alone can result in underestimation of maximal root size. Similarly, among various CMR measurements, long- and short-axis diameters in the lateral plane are often the largest and should be used in serial follow-up. The largest of these diameters should be reported while specifying how this measurement was made, to allow consistency on subsequent examinations. However, the large number of possible sinus-to-sinus and sinus-to-commissure measurements may introduce variability in serial measurements when reported by different observers. This variability can be minimized by (1) carefully documenting in the report how measurements were performed and by archiving screen shots of the measurements in the patient’s medical records and (2) by measuring aortic root cross-sectional area, which was found in this study to be highly reproducible. Further studies should investigate whether the use of aortic root cross-sectional area improves the prediction of major aortic complications.

Ascending Aorta

The AAO is a common site for dilation in patients with BAV, and measurement of aortic size is commonly performed using TTE. Our results show that although there is good agreement between TTE and CMR measurements of the AAO, the reproducibility of TTE measurements is suboptimal with intra- and interobserver differences in BAV patients up to 6–7 mm. While these differences may not be significant in patients with mild dilation, they may impact management in patients who have more severe dilation and are approaching the threshold for surgical intervention. CMR is a more reliable alternative in these patients, and its higher reliability may offset the higher cost by requiring less frequent examinations.

Distal Aorta

The more distal aortic segments, including the transverse arch, isthmus, and the descending aorta are uncommon sites of dilation. However, these aortic segments can be affected, especially in CTD patients with severe aortopathy. These sites are especially difficult to visualize by TTE due to poor echocardiographic windows in older and larger patients. Our results show that TTE measurements of these aortic segments have poor reproducibility and are subject to significant underestimation compared with CMR. Our findings are consistent with those of Lanzarini et al. [7] who, in a study on Turner syndrome patients, found good agreement between TTE and CMR measurements in the proximal aorta and worse agreement in the distal aorta. Periodic evaluation of the entire aorta is essential in the follow-up of CTD patients, and our data support the use of CMR to complement TTE in assessing the distal aorta.

Clinical Recommendations

In considering the optimal use of noninvasive imaging for surveillance of aortopathy patients, accuracy, reproducibility, and cost must be considered. Although CMR demonstrates a clear superiority over TTE in assessment of the aorta along its entire length, these advantages need to be balanced against its higher cost and more restricted availability. The use of CMR may be reasonable at initial diagnosis for baseline measurements and at infrequent intervals thereafter, whereas TTE can be used for routine follow-up. More frequent use of CMR may be considered in patients with poor TTE windows, those close to the surgical threshold, those in whom significant asymmetry of the root has been identified, and in patients with significant dilation distal to the aortic root. Furthermore, CMR may also provide information not obtainable by TTE, such as vertebral arterial tortuosity [10] and quantitative assessment of aortic valve regurgitation.

Limitations

Measurements were performed by fourth-year medical students who underwent stringent training followed by an evaluation of competence prior to study commencement. Although it is conceivable that their relative lack of experience may have influenced our results, the reproducibility values presented are similar or better than prior reports [15, 16, 17]. Because we excluded several patients who had prior cardiac surgery and those in whom all aortic segments could not be visualized by TTE, our results represent a best-case scenario with respect to TTE. While we evaluated intraobserver and interobserver measurement variability, the retrospective study design did not allow assessment of interstudy variability related to variable acquisition techniques between examinations.

Conclusions

In patients with aortopathy, CMR offers several advantages over TTE including more reliable assessment of the largest diameter and cross-sectional area in asymmetric aortic roots, more reproducible measurement of AAO diameter (especially in BAV patients), and significantly larger and more reliable measurements of the distal aorta. These data support a prominent role for CMR in the follow-up of aortopathy patients and may also be used for sample size calculations in clinical trials.

Notes

Conflict of interest

The authors declare that they have no conflict of interest.

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Copyright information

© Springer Science+Business Media New York 2015

Authors and Affiliations

  • Atosa Nejatian
    • 1
  • Johan Yu
    • 1
  • Tal Geva
    • 1
    • 2
  • Matthew T. White
    • 3
    • 4
  • Ashwin Prakash
    • 1
    • 2
  1. 1.Department of CardiologyBoston Children’s HospitalBostonUSA
  2. 2.Department of PediatricsHarvard Medical SchoolBostonUSA
  3. 3.Clinical Research CenterBoston Children’s HospitalBostonUSA
  4. 4.Department of PsychiatryHarvard Medical SchoolBostonUSA

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