Ultrasound Screening of AAA: Diameter Thresholds and Measurement by AIUM 2025

Clinical Task of Ultrasound

The revision of the AIUM Practice Parameter 2025 describes two closely related but different scenarios: screening an asymptomatic adult with risk factors and diagnostic examination in the presence of symptoms, a palpable pulsatile mass, known aneurysm, or post-intervention monitoring. In both cases, the main measurable parameter is the maximum outer diameter of the abdominal aorta. Ultrasound should not replace clinical decision-making regarding surgical strategy, but it must accurately document the presence, size, extent, and anatomy of the aneurysmal segment.

AAA Diagnostic Threshold

In the AIUM practical protocol, the threshold for an abdominal aortic aneurysm is a maximum diameter of ≥3.0 cm. An additional criterion is a local increase in diameter of at least 1.5 times the expected normal diameter of the corresponding segment. This is important in cases of focal deformation, pronounced asymmetry, or comparison with an adjacent unchanged section.

Diameter Measurement Technique

Calipers are placed on the outer contours of the wall—from outer edge to outer edge. Measurement is performed perpendicular to the long axis of the vessel. In a tortuous aorta, measurement should not be taken on an oblique section: such a section overestimates the diameter and may lead to false classification as AAA.

The protocol records anterior-posterior and transverse dimensions in longitudinal and transverse projections, selecting the maximum reproducible outer diameter. If the wall is calcified or partially shadowed, measurement should be performed on the frame where the outer contour is most convincing; if uncertain, the limitation must be reflected in the report.

Standard Scanning Volume

The examination is conducted from the proximal section of the abdominal aorta to the bifurcation. The assessment includes longitudinal and transverse sections of the proximal, middle, and distal sections. If an aneurysm is detected, the scan is expanded: its upper and lower boundaries, relation to visceral landmarks, bifurcation, common iliac arteries, presence of mural thrombus, and free fluid, if visible, are described.

Color and spectral Doppler are not substitutes for B-mode diameter measurement. Doppler is useful for confirming a patent lumen, distinguishing the vessel from adjacent structures, assessing turbulent flow, identifying residual lumen against thrombus, or evaluating postoperative changes.

Screening: Minimal Response

Screening ultrasound must answer one main question: is there enlargement of the abdominal aorta to the AAA threshold. The minimally required information is whether the aorta is visualized throughout its accessible length, the maximum outer diameter, and the level of measurement. A negative screening formulation is correct only with sufficient visualization of the distal section and bifurcation, as infrarenal localization is most common.

If the study is limited by meteorism, obesity, postoperative changes, tenderness, or dressings, the result should not be issued as a complete negative screening. The report should indicate which segments were not assessed and recommend alternative imaging or repeat ultrasound based on clinical indications.

Diagnostic Ultrasound for Suspected AAA

In cases of abdominal, back, or flank pain, hypotension, palpable pulsatile mass, or known aneurysm, the examination is diagnostic. Besides the diameter, the shape of the aneurysm, maximum extent, presence of thrombus, involvement of bifurcation and iliac arteries, and signs of complications within the capabilities of ultrasound are important. A normal or limited ultrasound protocol does not exclude retroperitoneal bleeding; with clinical suspicion of rupture, urgent routing according to local protocol, usually with CT angiography in a stable patient, is required.

Common Errors

• Measuring the inner lumen instead of the outer diameter: with mural thrombus, this underestimates the size.
• Oblique transverse section in tortuosity: it overestimates the diameter.
• Reporting only the proximal section: the distal aorta and bifurcation should be assessed if technically possible.
• Using color Doppler as a contour for calipers: the flow boundary is not equal to the outer wall.
• Lack of a phrase about limitations when part of the aorta is not visualized.

What Should Be in the Protocol

An optimal report contains the indication, quality of visualization, maximum outer diameter, plane and level of measurement, description of the aneurysm if the diameter is ≥3.0 cm, condition of the bifurcation and iliac arteries in the accessible zone. For dynamics, the current maximum outer diameter should be compared with the previous study, if available, and it should be noted whether the measurement technique is consistent. AIUM 2025 does not turn the ultrasound protocol into a table of treatment thresholds: observation intervals and indications for intervention are determined by specialized vascular recommendations and clinical context.

Practical Conclusion Formulation

For negative screening: the abdominal aorta is visualized, the maximum outer diameter is less than 3.0 cm, no ultrasound signs of AAA detected. For a positive result: aneurysmal enlargement of the abdominal aorta with a maximum outer diameter of at least 3.0 cm, followed by localization, extent, thrombus, involvement of bifurcation, and technical limitations. Such a structure makes the conclusion comparable in repeat studies and corresponds to the logic of AIUM 2025.