Ultrasound vs CT for Abdominal Aortic Dissection

January 20, 2021

By: Karen Custodio, DO

Reviewed by: Jonathan Kaplan, MD

Ultrasonographic Measurement of Aortic Diameter by Emergency Physicians Approximates Results Obtained by Computed Tomography

Knaut AL, Kendall JL, Patten R, Ray C. Ultrasonographic measurement of aortic diameter by emergency physicians approximates results obtained by computed tomography. J Emerg Med. 2005;28(2):119-126. doi:10.1016/j.jemermed.2004.07.013

https://pubmed.ncbi.nlm.nih.gov/15707804/

Introduction and Background

  • Ruptured Abdominal Aortic Aneurysm (AAA) claims approximately 8,700 lives each year in the United States.
  • Currently, prevalence in people age >50 is 1%-5%.
  • Risk of rupture for an aneurysm increases over time as diameter exceeds 4 cm.
  • As many as 2/3rd of AAA go undiagnosed before rupture, but if diagnosed early and repaired, long term survival matches that of the general population.

Purpose/Objectives

  • The purpose of this study was to assess the agreement between emergency department (ED) physician measurements of the diameter of the abdominal aorta (AA) using ultrasound (US) versus measurements obtained by CT scan.

Why is this important?

  • Previous research has shown that Abdominal US for AAA performed by an emergency physician can be a fast and effective screening instrument in the ED.
  • Understanding how well this correlates with CT can help in clinical decision making.

Study Selection:

  • Inclusion criteria: Male or Female. Aged 50 years or older presenting to the ED with complaint of abdominal pain. Must also have a CT abdomen/pelvis with contrast ordered as part of their clinical evaluation.
  • Exclusion criteria: Patient not considered if risk of deterioration in clinical status was too great to wait for US or patient declined.
  • 104 patients enrolled. Average age 68.6. 51% male. Average BMI 27.3 (60% met criteria for overweight and 32.4% obese).

Methods:

  • Double blinded, prospective study using a convenience sample of patients.
  • Enrollment over 26 month period at an urban ED which sees approximately 55,000 visits /year. Staffed by residents and attending physicians who had prior US training.
  • Prior to CT, an abdominal US was performed by the emergency physician. Radiologist blinded to US results independently measured aortic diameter on CT scan.
  • US identified two points – take off of the superior mesenteric artery and the iliac bifurcation.
  • US measurements taken form inner wall to inner wall. CT measured outer wall to outer wall.
  • ED Physicians asked to report level of training and estimated time of study completion.

Results:

  • Time of procedure
    • Nearly 70% of scans took less than 5 minutes to complete. 27.8% required 5-10 minutes, and only 3.1% took greater than 10 minutes. No study took more than 15 minutes.
  • Accuracy of measurements
    • 5 patients diagnosed with a AAA on CT. All 5 were correctly identified by US.
    • 4 patients diagnosed with ruptured AAA by US, but were not included in the study because they went directly to the OR without a CT being obtained.
    • US had the greatest PPV and NPV at the iliac bifurcation.
    • At the SMA, difference between CT and US measurement expected to be less than 1.41 cm 95% of the time. Longitudinal measurements differed by less than 0.94 cm 95% of the time, and at the bifurcation the difference was less than 1.05 cm 95% of the time.

Discussion:

  • Direct comparison of aortic diameter measurements by US and CT is difficult due to interobserver variability, real time variation in aortic diameter in systole and diastole, and different standards or techniques for measuring.
  • This study suggests that ED physicians can measure abdominal aortic diameter rapidly (5-10 minutes) with US, and produce results similar to CT.
  • Agreement was poorest at the SMA, however it was best at the iliac bifurcation where the diagnosis of AAA is most critical (~1cm difference between CT and US).
  • It is noteworthy that all 5 of the ruptured AAA were identified on US scan by ED physicians.

Limitations:

  • Patients enrolled on a convenience basis. It is likely that patients who were more difficult to scan (body habitus/difficult anatomy) were not included.
  • US measurements done from inner wall to inner wall, while CT measured outer wall to outer wall. This is inconsistent with standard practice of the ED US measurements.
  • The amount of training among attending physicians and residents of different levels was not controlled for.
    • This may impact the length of time and accuracy of the US study performed.
    • Unable to assess the appropriate length of training is required or how this may have influenced the results.

Conclusion:

  • ED physicians can measure AAA diameter with US quickly and accurately.
  • These measurements are similar to those produced by CT, and can provide useful clinical information.
  • There is potential for ED physicians to participate in screening of elderly patients who have high likelihood of having a AAA before a more advanced disease process occurs.

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