Ultrasound Clin 2 (2007) 437–453 Sonographic Evaluation of the Abdominal Aorta ... It...

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  • U L T R A S O U N DC L I N I C S

    Ultrasound Clin 2 (2007) 437453

    437

    Sonographic Evaluationof the Abdominal AortaShweta Bhatt, MDa, Hamad Ghazale, MS, RDMSb,Vikram S. Dogra, MDa,*

    - Sonographic anatomy and technique- Abdominal aortic aneurysm- Screening

    Screening testsSelective screeningEffectiveness of screening

    - Making the diagnosisClassification of abdominal aortic

    aneurysm- Aneurysm rupture

    Predictors of aneurysm ruptureSonographic findings

    Inflammatory abdominal aortic aneurysmMycotic abdominal aortic aneurysmPseudoaneurysm

    - Endograft evaluationEndoleak detection

    - Aortic dissection- B-flow imaging- Limitations of sonography for evaluation

    of abdominal aortic aneurysm andrupture

    - Summary- References

    Abdominal aortic aneurysm (AAA) is a disease ofaging, and the prevalence is expected to increase asthe population of elderly patients grows. AAAs areassociated with high mortality, as rupture of anAAA is the tenth leading cause of death in theUnited States. AAA can be diagnosed with CT, mag-netic resonance imaging (MR imaging), and ultra-sonography (US). US has nearly 100% sensitivityin detecting AAA and is readily available for routineand emergency evaluation [1]. In addition, thereare no risk factors associated with US examinationof the aorta as there are with CT and MR imaging,including exposure to ionizing radiation, risks asso-ciated with intravenous contrast, expense, and per-haps most important, time delay in performing thestudy. This review describes the sonographic

    technique for evaluation of the abdominal aorta(AA) and associated pathologies of the AA, includ-ing AAA.

    Sonographic anatomy and technique

    The AA is a continuation of the thoracic aorta, be-ginning at the aortic hiatus in the diaphragm (tho-racic vertebra level 12) and ending at the fourthlumbar vertebra, where it divides into the commoniliac arteries. It descends in the midline, anterior tothe vertebral column and to the left of the inferiorvena cava. The normal luminal diameter of the in-frarenal AA varies with age and gender. In youngpatients who do not have vascular disease, the in-frarenal AA measures 2.3 cm in males and 1.9 cm

    a Department of Imaging Sciences, University of Rochester Medical Center, University of Rochester School ofMedicine, 601 Elmwood Avenue, Box 648, Rochester, NY 14642, USAb Diagnostic Medical Sonography Program, Rochester Institute of Technology, 153 Lomb Memorial Drive,Rochester, NY 14623, USA* Corresponding author.E-mail address: vikram_dogra@urmc.rochester.edu (V.S. Dogra).

    1556-858X/07/$ see front matter 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.cult.2007.06.001ultrasound.theclinics.com

    mailto:vikram_dogra@urmc.rochester.edu

  • Bhatt et al438

    in females [2]. It increases in diameter with age. Inone study, men who had a mean age of 70.4 yearswho did not have AAA had an average luminaldiameter of 2.8 cm [3].

    Abdominal aortic ultrasound is preferably per-formed after 8 to12 hours of fasting. Fasting reducesbowel gas, which helps provide a better view of theAA. The standard protocol for scanning the AA con-sists of obtaining longitudinal and transverse im-ages from the level of the diaphragm to the levelof bifurcation of the AA, where the common iliacarteries are visualized like binoculars in the trans-verse view (Fig. 1). Abdominal aortic diameter is re-corded at the proximal, mid, and distal aorta, alongwith measurement of the common iliac arteries justdistal to the bifurcation. The inferior vena cava isalso evaluated to document normal flow.

    Sonographic evaluation of the AA is performed ina supine position or right and left lateral decubitusor right and left posterior oblique positions. Imagesare obtained in coronal and transverse planes. Plac-ing the patient in the left lateral position and imag-ing in a coronal scan plane allows for better

    visualization of both iliac arteries in one image(Fig. 2). Application of gentle pressure or compres-sion and changing the transducer angle may alsohelp displace bowel gas and improve visualizationof the aorta. The patients body habitus plays an im-portant role in determining the optimal type oftransducer and frequency to be used. Usually 2.5to 5 MHz sector, curvilinear array transducers pro-vide optimal visualization of the aorta.

    Sonographically the normal aorta has an an-echoic echo-free lumen with echogenic walls. Arti-factual intraluminal echoes may result fromincreased gain and slice-thickness or reverberationartifacts (Fig. 3). These types of echoes can be con-fused with thrombus or intraluminal tumor. Thesonographer must change the patients position orangulation of the transducer and correct the gainto see if these echoes can be eliminated. If these ech-oes disappear, they most likely represent an artifact.

    The anteroposterior (AP) diameter of the aortashould be measured from a longitudinal image, be-cause this allows correct placement of the calipersperpendicular to the long axis of the vessel. The

    Fig. 1. Normal aorta. (A) Longitudinal and (B) transverse gray scale sonogram demonstrates the normal appear-ance of the proximal abdominal aorta (A, abdominal aorta; IVC, inferior vena cava; P, portal vein; L, liver). (C, D)Transverse gray scale and corresponding power Doppler sonograms of aortic bifurcation into common iliacarteries (arrows) are seen as binocular in appearance.

  • Sonographic Evaluation 439

    Fig. 2. Aortic bifurcation. (A) Longitudinal gray scale and (B) color flow Doppler images demonstrate the normalaortic bifurcation.

    measurement should be taken from outer wall toouter wall and should not exceed 3 cm in diameter[4]. The diameter of the aorta decreases as it coursesinferiorly. Consequently the AP measurementvaries from one segment of the aorta to anotherand is also depends on age and the presence or ab-sence of disease. When an aneurysm is observed,the operator must obtain the maximum true length,width, and transverse dimensions of the aneurysmand must measure the true lumen. The shape andlocation of the aneurysm should be determined.The exact relationship of the AAA to the origin ofthe renal arteries and the bifurcation should be

    noted. Involvement (ie, dilatation) of the commoniliac arteries should also be documented.

    Color flow Doppler is helpful in determining thepatency and direction of blood flow in the aorta.The color box should be kept small, which im-proves the frame rate and enhances the color reso-lution. The color Doppler gain should be set atless than noise level and a low pulse repetition fre-quency (PRF) should be avoided to prevent alias-ing. Methods of optimization of color flowDoppler are given in Table 1 [5].

    Normal blood flow in the aorta is laminar.The flow pattern in the aorta is considered a

    Fig. 3. (A) Longitudinal gray scale sonogram of the proximal aorta demonstrates a hypoechoic area of low levelechoes along the anterior wall (arrow), mimicking a thrombus. This is an artifact secondary to partial volumeaveraging. (B) Longitudinal gray scale sonogram of the proximal aorta demonstrates a luminal echogenic focus(arrow), mimicking an intraluminal thrombus, arising secondary to a reverberation artifact.

  • Bhatt et al440

    high-resistance pattern. The proximal aorta normallydemonstrates a biphasic waveform with reversalof flow in early diastole. The distal aorta demon-strates a triphasic waveform with a small componentof forward flow in late diastole (Fig. 4) [6].

    Table 1: Optimization of color flow Doppler

    Color box Keeping the color boxsmall results inimproved frame rateand better colorresolution

    Doppler gain Just below the noiselevel

    Color scale (PRF) Low PRF is moresensitive to low volumeand low velocity flowbut may lead toaliasing

    Beam steering Adjust to obtainsatisfactory vesselangle

    Gate size (samplevolume)

    Set the sample volumeto a correct size, usuallytwo thirds of the vessellumen

    Wall filter A higher filter cuts outthe noise and but alsothe slower velocityflow; keep the filter at50100 Hz

    Focal zone Color flow image isoptimized at the levelof the focal zone

    (From Bhatt S, Dogra V. Doppler imaging of the uterusand adnexae. Ultrasound Clinics 2006;1:20121; withpermission.)

    Abdominal aortic aneurysm

    An aortic aneurysm is defined as a focal dilation ofthe aorta with a diameter of at least 1.5 times that ofthe expected normal diameter of that given aorticsegment; in the AA, enlargement of the aortic diam-eter of more than 3 cm is usually considered aneu-rysmal. Alternatively an AAA can be defined asa ratio of infrarenal to suprarenal aortic diameterof 1.2, or a history of AAA repair [7]. Prevalenceof AAA has been estimated at 1.2% to 12.6% formen in the sixth to ninth decades, with almosttwo thirds of AAAs involving only the AA [8]. Over-all, up to 13% of all patients in whom an aortic an-eurysm is diagnosed have multiple aneurysms, with25% to 28% of patients who have thoracic aorticaneurysms having concomitant AAAs [9]. Pub-lished data from the National Vital Statistics Reporton deaths from the year 2000 showed that AAAsand aortic dissection were the tenth leading causeof death in white men of 65 to 74 years of ageand accounted for nearly 16,000 deaths overall[10]. The high mortality associated with AAAs hasled to an increase in the need to identify risk factorsso that appropriate screening procedures can beundertaken for early diagnosis. AAA is most com-monly a sequelae of atherosclerosis; therefore, pre-disposing risk factors for atherosclerosis, such asolder age, smoking, and hypertension, are stronglyassociated with the development of AAA [11,12]. Al-though moderate alcohol consumption has beenfound to have a beneficial effect on coronary arterydisease because of its positive effect on