Can a transthoracic (TT) echocardiogram overestimate the size of the ascending aorta compared to a computed tomography (CT) angiogram?

Can Transthoracic Echocardiogram Overestimate Ascending Aorta Size Compared to CT Angiogram?

No, transthoracic echocardiography (TTE) typically underestimates—not overestimates—the maximum aortic root diameter when compared to CT angiography (CTA). 1

Direction of Measurement Discrepancy

The most recent and highest quality evidence demonstrates that:

• TTE consistently underestimates maximum aortic root diameter compared to both CTA and MRA in a retrospective study of 127 patients comparing all three modalities 1
• This underestimation occurs despite TTE being highly accurate for ascending aortic measurements when proper technique is used 2, 3

Why the Measurement Differences Occur

Measurement Convention Differences

The primary source of variation stems from different measurement conventions between modalities:

• TTE uses the leading edge-to-leading edge (L-L) convention, which measures from the outer anterior wall to the inner posterior wall 1
• CTA and MRA use the inner edge-to-inner edge (I-I) convention 1
• The L-L convention theoretically provides measurements 2-4 mm larger than I-I convention 1
• However, CTA and MRA measurements from sinus-to-commissure are generally smaller than echocardiographic sinus-to-sinus measurements 1

Technical Factors Contributing to TTE Underestimation

Several technical limitations explain why TTE underestimates rather than overestimates:

• Oblique imaging planes can cause measurement errors in all modalities, but TTE has more limited acoustic windows 1
• The distal ascending aorta and proximal arch represent a "blind spot" for TTE due to left mainstem bronchus interposition 1
• Cardiac motion artifacts are more problematic for TTE than for ECG-gated CTA 1
• TTE may miss the true maximum diameter if optimal imaging planes are not obtained 4

Magnitude of Differences

When comparing all measurement techniques across modalities:

• Mean differences range from 4.8 to 5.4 mm depending on the aortic level measured 4
• Maximum individual differences can reach 18 mm in patients with aortic pathology 4
• The best agreement occurs between TTE L-L edge measurements and CTA during mid-systole 4

Clinical Implications and Recommendations

When to Confirm TTE Findings with CTA

If TTE shows an increase of ≥3 mm per year in aortic diameters, confirmation by CTA or MRA should be obtained 1

This recommendation exists because:

• TTE may underestimate true aortic size
• Surgical decision-making requires accurate measurements
• The risk of underestimating aortic size is more dangerous than overestimating (could miss surgical thresholds)

Optimal Measurement Strategy

For accurate serial monitoring:

• Use the same imaging modality with the same measurement method over time (Class I recommendation) 1
• ECG-triggered CTA is recommended for comprehensive diagnosis and pre-surgical assessment, particularly for the aortic root and ascending aorta 1
• TTE remains the first-line imaging technique for initial evaluation (Class I recommendation) 1
• CTA or MRA should be used at least once in patients with aortic pathology to establish accurate baseline measurements 4

Common Pitfalls to Avoid

• Do not assume TTE overestimates aortic size—the evidence shows the opposite 1
• Do not rely solely on TTE for surgical planning in patients with borderline measurements near surgical thresholds 1
• Do not compare measurements between modalities without accounting for convention differences (L-L vs I-I edge) 1, 4
• Ensure measurements are perpendicular to the aortic long axis using double-oblique technique for CTA/MRA to avoid overestimation from oblique slices 1