What is the normal thoracic aorta diameter on imaging in adults, including typical ranges for the ascending, arch, and descending segments and how age, sex, and body size affect these values?

Normal Thoracic Aorta Size

The normal ascending aorta measures approximately 34 mm in men and 32 mm in women, with upper normal limits of 42 mm in men and 39 mm in women; the descending thoracic aorta measures approximately 26 mm in men and 23 mm in women, with these values increasing 0.12-0.29 mm per year with age. 1, 2

Sex-Specific Normal Ranges

Ascending Aorta and Aortic Root

• Men: Mean diameter 34.1 ± 3.9 mm, with upper normal limit (mean + 2 SD) of approximately 42 mm 1, 2
• Women: Mean diameter 31.9 ± 3.5 mm, with upper normal limit of approximately 39 mm 1, 2
• The aortic root ranges from 3.63-3.91 cm in men and 3.50-3.72 cm in women 1, 3

Descending Thoracic Aorta

• Men: Mean diameter 25.8 ± 3.0 mm at mid-descending level 1, 4
• Women: Mean diameter 23.1 ± 2.6 mm at mid-descending level 1, 4
• At the diaphragmatic level: 2.43-2.69 cm in men and 2.40-2.44 cm in women 1, 4

Aortic Arch

• Normal range 2.6-2.9 cm in adults, with men having 1-3 mm larger diameters than women 2

Age-Related Changes

Aortic diameter increases progressively with age at all levels measured. 1, 3

• The rate of increase is 0.12-0.29 mm per year at each measured level 1, 4
• A practical formula for upper normal limit of ascending aorta: D(mm) = 31 + 0.16 × age 5
• For descending aorta: D(mm) = 21 + 0.16 × age 5
• This means a 20-year-old has an upper limit of 34 mm for ascending aorta, while an 80-year-old has 44 mm 5

Body Size Adjustments

Body mass index and body surface area significantly affect aortic diameter and should be considered when evaluating measurements. 1, 3

• Each unit increase in BMI increases aortic diameter by approximately 0.27 mm 1, 4
• Indexing to height or body surface area improves accuracy, particularly in patients with Marfan syndrome or bicuspid aortic valve 1, 3
• Aortic root dilatation in male adults can be suspected when indexed diameter/BSA exceeds 22 mm/m² 3

Measurement Technique Considerations

CT imaging is the gold standard for measuring thoracic aortic diameters, and measurements must be perpendicular to the axis of blood flow. 2, 4

• Oblique imaging planes significantly overestimate true diameter and can lead to unnecessary intervention 4, 3
• The aorta gradually tapers from the sinuses of Valsalva distally, so measurements at different levels are not directly comparable 1, 4
• Different imaging modalities yield different values: CT/MRI measurements (sinus to commissure) are generally smaller than echocardiographic measurements (sinus to sinus) 3

Pathologic Thresholds

Aortic ectasia is defined as diameters exceeding 2 SD above the mean but less than 150% of normal, while aneurysm is defined as ≥150% of normal diameter. 1, 2

• Ascending aorta aneurysm: ≥5.0 cm (approximately 150% of normal) 1, 2
• Descending aorta aneurysm: ≥4.0 cm (approximately 150% of normal) 1, 2
• An ascending aorta exceeding the diameter at the sinuses of Valsalva is considered enlarged, even if both measurements are within normal range 1, 3

High-Risk Intervention Thresholds

• General population: ≥5.5 cm warrants consideration for intervention 1
• Connective tissue disorders (Marfan, Ehlers-Danlos): ≥5.0 cm prompts consideration of prophylactic repair 1
• Growth rate >0.5 cm per year should prompt evaluation for intervention 2

Common Pitfalls to Avoid

• Failing to account for patient age: A 40 mm ascending aorta may be normal in an 80-year-old but abnormal in a 20-year-old 5
• Ignoring sex differences: Men have consistently larger diameters (1-3 mm) than women across all aortic levels 1, 2, 4
• Using oblique measurements: Always measure perpendicular to blood flow axis to avoid overestimation 2, 4
• Not considering body habitus: Small-statured individuals may have pathologic dilatation despite "normal" absolute measurements 1, 3