ACR Appropriateness Criteria pulsatile abdominal mass, suspected abdominal aortic aneurysm

Clinical palpation of a pulsating abdominal mass alerts the clinician to the presence of a possible abdominal aortic aneurysm (AAA). Generally an arterial aneurysm is defined as a localized arterial dilatation C50 % greater than the normal diameter. Imaging studies are important in diagnosing the cause of a pulsatile abdominal mass and, if an AAA is found, in determining its size and involvement of abdominal branches. Ultrasound (US) is the initial imaging modality of choice when a pulsatile abdominal mass is present. Noncontrast computed tomography (CT) may be substituted in patients for whom US is not suitable. When aneurysms have reached the size threshold for intervention or are clinically symptomatic, contrast-enhanced multidetector CT angiography (CTA) is the best diagnostic and preintervention planning study, accurately delineating the location, size, and extent of aneurysm and the involvement of branch vessels. Magnetic resonance angiography (MRA) may be substituted if CT cannot be performed. Catheter arteriography has some utility in patients with significant contraindications to both CTA and MRA. The American College of Radiology Appropriateness Criteria O are evidence-based guidelines for specific clinical conditions that are reviewed every 2 years by a multidisciplinary expert panel. The guidelineThe American College of Radiology seeks and encourages collaboration with other organizations on the development of the ACR Appropriateness Criteria O through society representation on expert panels. Participation by representatives from collaborating societies on the expert panel does not necessarily imply individual or society endorsement of the final document.
development and review include an extensive analysis of current medical literature from peer reviewed journals and the application of a well-established consensus methodology (modified Delphi) to rate the appropriateness of imaging and treatment procedures by the panel. In those instances where evidence is lacking or not definitive, expert opinion may be used to recommend imaging or treatment.
Keywords Appropriateness criteria A Aortic aneurysm A Ultrasonography A Computed tomography A Magnetic resonance angiography A Catheter arteriography

# Introduction/background
Clinical palpation of a pulsating abdominal mass alerts the clinician to the presence of a possible abdominal aortic aneurysm (AAA), a common vascular disorder seen in older individuals, more commonly in male patients with a history of hypertension and smoking [1-3]. However, the finding of a pulsatile abdominal mass can also be caused by a tortuous abdominal aorta or transmitted pulsations from the aorta to a nonvascular mass .
Generally an arterial aneurysm is defined as a localized arterial dilatation C50 % greater than the normal diameter. The term ectasia is applied to arterial dilatations \50 % of expected normal diameter. However, the normal dimension of the infrarenal abdominal aorta is up to 2 cm in anteroposterior (AP) diameter. Thus, the infrarenal abdominal aorta is considered aneurysmal if it is C3 cm in diameter or ectatic between 2 and 3 cm in diameter . The absolute threshold for aneurysm decreases along the length of the aorta and is about 10 % smaller in women than in men .
Imaging studies are important in diagnosing the cause of a pulsatile abdominal mass and, if an AAA is found, in determining its size, involvement of abdominal branches, both visceral and parietal, and any associated significant stenosis or aneurysm involving abdominal visceral and extremity arteries . Imaging studies should also categorize the extent of aneurysm (i.e., infrarenal aorta; infrarenal aorta and iliac; isolated iliac; or juxtarenal, suprarenal, or thoracoabdominal aorta) . Imaging can also be used for routine surveillance of AAAs [9, 10].
Currently, elective repair is considered for AAAs C5.5 cm in diameter . For smaller AAAs, periodic surveillance is recommended at intervals based on their maximum size [12]: every 6 months for those 4.5-5.4 cm in diameter, every 12 months for those 3.5-4.4 cm in diameter, every 3 years for those 3.0-3.4 cm in diameter, and every 5 years for those 2.6-2.9 cm in diameter.
Population-based ultrasound (US) screening studies have been recommended for male patients [65 years of age . Risk of AAA increases with a history of hypertension and smoking. For AAAs between 3 and 5.5 cm in diameter, periodic US or computed tomography (CT) imaging at 6-12-month intervals depending on rate of aneurysm enlargement on prior studies is recommended. When aneurysms have reached the size threshold for intervention (5.5 cm) or are considered clinically symptomatic, additional preintervention imaging studies should be performed to help define the optimal surgical or endovascular approach. For preintervention studies, either multidetector CT (MDCT) or CT angiography (CTA) is the optimal choice. Magnetic resonance angiography (MRA) may be substituted if CT cannot be performed (for example, because the patient is allergic to iodinated contrast). However, MRA is usually performed with gadolinium contrast, which is not suitable for patients with severe renal insufficiency. In such patients, the center where it is being performed must be able to perform MRA of AAA without the use of gadolinium contrast [14, 15] (see [fig_ref] Table 1: Clinical condition [/fig_ref].
Other types of imaging studies that have been used in the past to delineate AAAs-including abdominal radiographs, intravenous urography, and blood pool radionuclide imaging-are not recommended for diagnosis, surveillance, or preintervention imaging.
Catheter arteriography has very limited utility in the preintervention evaluation of patients with AAAs, its sole utility being in patients with significant contraindications to both CTA (significant renal dysfunction) and MRA (significant renal dysfunction, cardiac pacemakers, claustrophobia). In patients with significant renal dysfunction, the combination of noncontrast CT and the lower load of iodinated contrast material that can be used with intraarterial injection can decrease the risk of contrast-induced nephropathy.
Many imaging studies for assessing AAA can also identify other disease that could affect preoperative management of AAA, such as coronary artery disease Ultrasound US examination of the abdominal aorta should be a dedicated examination and not a component of a generalized abdominal US study. If possible, complete longitudinal evaluation of the full extent of the aneurysm and involvement of common iliac arteries should be performed. These studies should include a measurement of the leading-edgeto-leading-edge AP diameter in the proximal, mid, and distal infrarenal aorta and of the common iliac arteries.
Lining mural thrombus should be delineated. Right and left kidneys should be imaged to determine size, parenchymal thickness, and presence or absence of hydronephrosis. In order to permit US to be used instead of CT for AAA follow-up, interindividual reproducibility of diameter measurements should be within B4 mm . US tend to underestimate the size of aneurysms by 4 mm compared to CTA . Color Doppler imaging is not a necessary component of sonographic screening or surveillance examination. New, 3-D volumetric US techniques offer similar measurements but speed up imaging significantly .
Approximately 5 % of AAAs will be juxtarenal or juxta/ suprarenal , and it may not be possible to accurately delineate the upper margin of such aneurysms or the precise involvement of abdominal visceral branches by sonographic study. That is why a more definitive study, such as CTA, should be performed prior to intervention.

## Computed tomography
Noncontrast CT is diagnostically equivalent to US for AAA detection and is recommended in patients for whom US is not suitable (for example, those with obese body habitus). CT may be used as a diagnostic and preintervention study, suitable for patients presenting with pulsatile abdominal mass with or without clinical suspicion of contained aortic rupture, and in planning endovascular or surgical intervention in patients with AAAs [5.5 cm in external AP diameter . In tortuous aneurysms, where a single dimension may be artifactually accentuated by the curvature of the aorta, the short-axis diameter of the aorta may be substituted for the AP diameter.
Contrast-enhanced multidetector CTA is the best diagnostic and preintervention planning study, accurately delineating the location, size, and extent of aneurysm and the involvement of branch vessels, allowing for accurate quantitative 3-D measurements . CTA can also assess thrombus in aneurysm. Larger thrombus and eccentric thrombus seem associated with rapid enlargement of the aneurysm and increased incidence of cardiovascular events . There are several research protocols that use modern CT technologies. Multiphase MDCT can assess compressibility of thrombus that can act as a biomechanical buffer . Using delayed imaging, aortic wall enhancement is associated with AAA diameter, biochemical markers of inflammation, and thrombus size [36]. Short-term follow up by CTA does not decrease the suitability of aneurysms for endovascular intervention [bib_ref] Surveillance of small aortic aneurysms does not alter anatomic suitability for endovascular..., Yau [/bib_ref].
In patients with suspected thoraco AAA, CTA may be tailored for an angiographic examination of the chest, abdomen, and pelvis [bib_ref] Suspected aortic dissection and other aortic disorders: multidetector row CT in 373..., Hayter [/bib_ref] [bib_ref] Multidetectorrow computed tomographic angiography of thoracic and abdominal aortic aneurysms: comparison of..., Kubo [/bib_ref] [bib_ref] Iodixanol in multidetector-row computed tomography angiography (MDCTA): diagnostic accuracy for abdominal aorta..., Nonent [/bib_ref]. In patients with suspected coexistent lower-extremity arterial disease, the arterial system from the diaphragm to the feet can be studied with MDCT or CTA [bib_ref] Infrarenal aortic and lower-extremity arterial disease: diagnostic performance of multidetector row CT..., Catalano [/bib_ref].
Volume rendering, subvolume maximum-intensity projection (MIP), and curved planar reformations are integral components of the 3-D analysis. Semiautomated measurements of vessel diameter and length in relation to the proximal and distal aneurysm margins and branch vessels can be readily obtained with software supplied by multiple vendors. Additional research methods include ECG-gated MDCT that can assess decreased distensibility of aortic aneurysms [bib_ref] Quantification of aortic distensibility in abdominal aortic aneurysm using ECG-gated multi-detector computed..., Ganten [/bib_ref]. Advanced postprocessing of CT data can assess wall stress. Rapidly expanding AAAs has higher shoulder and wall stress [bib_ref] Association between aneurysm shoulder stress and abdominal aortic aneurysm expansion: a longitudinal..., Li [/bib_ref] [bib_ref] The influence of wall stress on AAA growth and biomarkers, Speelman [/bib_ref]. Calcification of the aneurysm increases wall stress and decreases the biomechanical stability of AAA [bib_ref] Impact of calcification and intraluminal thrombus on the computed wall stresses of..., Li [/bib_ref]. AAA peak wall stress at maximal blood pressure is higher in symptomatic or ruptured aneurysms compared to asymptomatic aneurysms [bib_ref] Peak wall stress measurement in elective and acute abdominal aortic aneurysms, Heng [/bib_ref] [bib_ref] Wall stress analysis in small asymptomatic, symptomatic and ruptured abdominal aortic aneurysms, Truijers [/bib_ref]. In patients with suspected contained rupture, nonintravenous contrast-enhanced CT is performed to better diagnose dissecting hematoma in the lining of the intra-aortic thrombus (the crescent sign) and other signs consistent with imminent or contained rupture [bib_ref] CT of acute abdominal aortic disorders, Bhalla [/bib_ref] [bib_ref] Spectrum of CT findings in rupture and impending rupture of abdominal aortic..., Rakita [/bib_ref] [bib_ref] Bleeding into the intraluminal thrombus in abdominal aortic aneurysms is associated with..., Roy [/bib_ref] , including a draped aorta and adjacent vertebral erosion [bib_ref] Sealed rupture of abdominal aortic aneurysms: CT features in 6 patients and..., Apter [/bib_ref]. In patients who have contained rupture, a rapid CT angiographic study provides a template for decision making about endovascular aneurysm repair or surgical aneurysmectomy [bib_ref] Spiral-CT angiography to assess feasibility of endovascular aneurysm repair in patients with..., Willmann [/bib_ref].

## Magnetic resonance angiography
Contrast-enhanced MRA is an alternative and effective diagnostic and preintervention study [bib_ref] MR angiography for abdominal and thoracic aortic aneurysms: assessment before endovascular repair..., Atar [/bib_ref]. The acquisition speed and spatial resolution of contrast-enhanced MRA has improved with the introduction of parallel imaging techniques, narrowing the gap with CTA in relation to image quality [bib_ref] High-resolution renal MRA: comparison of image quality and vessel depiction with different..., Michaely [/bib_ref] [bib_ref] Parallel imaging in MR angiography, Wilson [/bib_ref]. The introduction of blood pool contrast agents now enables longer image acquisition to improve image resolution [bib_ref] Prospective evaluation of high-resolution MRI using gadofosveset for stent-graft planning: comparison with..., Wolf [/bib_ref]. Caution should be used in patients with severe renal dysfunction, generally considered as estimated glomerular filtration rate (GFR)\30 ml/kg/min, who may be at risk for nephrogenic systemic fibrosis [bib_ref] Gadoliniumenhanced MR imaging and nephrogenic systemic fibrosis: retrospective study of a renal..., Collidge [/bib_ref]. In these patients, a non-contrast-enhanced study may be substituted. Sequences and imaging expertise required for a full evaluation of AAA without contrast are becoming more mainstream.
Three-dimensional display techniques, including multiplanar reformation, MIP display, and volume rendering, are integral to the display and analysis of 3-D MRA. Cine techniques can also assess distensibility and, with suitable measurements of central venous pressure, can assess aortic compliance [bib_ref] Biomechanical properties of abdominal aortic aneurysms assessed by simultaneously measured pressure and..., Van&apos;t Veer [/bib_ref]. Vessel wall shear stress can also be measured using newer 4-D flow-sensitive MRI techniques [bib_ref] In vivo assessment of wall shear stress in the atherosclerotic aorta using..., Harloff [/bib_ref].

## Catheter arteriography
Patients with significant contraindications to both CTA and MRA may have diagnostic catheter arteriography performed with a relatively low-contrast material load following US documentation of AAA and/or noncontrast CT findings [bib_ref] The Amsterdam acute aneurysm trial: suitability and application rate for endovascular repair..., Hoornweg [/bib_ref].
Catheter arteriography may not demonstrate the aneurysm diameter accurately, as only the contrast column of an aneurysm containing lining mural thrombus may be displayed. In patients with marginal renal function, rapid intra-arterial injection of a relatively low volume of dilute contrast material from a catheter located in the mid descending thoracic aorta can be used for a diagnostic CTA study.

## Positron emission tomography
Although primarily a research tool, positron emission tomography using fluorine-18-2-fluoro-2-deoxy-D-glucose (FDG-PET) imaging has promise in the evaluation of patients with AAA. Increased metabolic activity and FDG uptake (SUV max [ 2.5) is noted in aneurysms [bib_ref] Increased metabolic activity in abdominal aortic aneurysm detected by 18F-fluorodeoxyglucose (18F-FDG) positron..., Kotze [/bib_ref] [bib_ref] In vivo imaging of abdominal aortic aneurysms: increased FDG uptake suggests inflammation..., Truijers [/bib_ref] and even higher in inflammatory aneurysms and symptomatic aneurysms and correlates well with histologic and metabolic evidence of inflammation [bib_ref] Increased 18F-fluorodeoxyglucose uptake in abdominal aortic aneurysms in positron emission/computed tomography is..., Reeps [/bib_ref]. Increased FDG uptake is also seen in areas of high wall stress and rupture [bib_ref] High levels of 18F-FDG uptake in aortic aneurysm wall are associated with..., Xu [/bib_ref]. Aneurysm calcification is unrelated to FDG uptake [bib_ref] Increased metabolic activity in abdominal aortic aneurysm detected by 18F-fluorodeoxyglucose (18F-FDG) positron..., Kotze [/bib_ref].

## Summary
- The consensus of the literature supports aortic US as the initial imaging modality of choice when a pulsatile abdominal mass is present. Noncontrast CT may be substituted in patients for whom US is not suitable (for example, those with obese body habitus).

## - us is recommended as a screening technique in the
Medicare-eligible male population at highest risk. - For definitive diagnosis and preintervention imaging, CTA and MRA are recommended. - Currently, CTA is regarded as the superior test, as it is readily available, is robust, and provides high spatial resolution 3-D displays suitable for interventional planning as well as delineation of pathology in abdominal visceral arterial branches and extremity outflow vessels. - Contrast-enhanced MRA has improved significantly in terms of speed and spatial resolution with the advent of parallel processing techniques and blood pool contrast agents. It may replace CTA for interventional planning in patients for whom iodinated contrast is contraindicated. - Noncontrast MRA sequences for full evaluation of AAA are becoming more mainstream and should only be performed in centers with expertise in this technique. - Appropriate preintervention measurements of the aortoiliac arterial system can be obtained with either technique. - Both CTA and MRA can be used for thoracoabdominal aortic and extremity studies, all in the same imaging session. - FDG-PET remains primarily a research tool but shows promise for assessing the metabolic activity of aneurysms.

## Anticipated exceptions
Nephrogenic systemic fibrosis (NSF) is a disorder with a scleroderma-like presentation and a spectrum of manifestations that can range from limited clinical sequelae to fatality. It appears to be related to both underlying severe renal dysfunction and the administration of gadolinium-based contrast agents. It has occurred primarily in patients on dialysis, rarely in patients with very limited GFR (i.e., \30 mL/min/1.73 m 2 ), and almost never in other patients. There is growing literature regarding NSF. Although some controversy and lack of clarity remain, there is a consensus that it is advisable to avoid all gadolinium-based contrast agents in dialysis-dependent patients unless the possible benefits clearly outweigh the risk, and to limit the type and amount in patients with estimated GFR rates \30 mL/min/ 1.73 m 2 . For more information, please see the ACR Manual on Contrast Media.

## Relative radiation level information
Potential adverse health effects associated with radiation exposure are an important factor to consider when selecting the appropriate imaging procedure. Because there is a wide range of radiation exposures associated with different diagnostic procedures, a relative radiation level (RRL) indication has been included for each imaging examination. The RRLs are based on effective dose, which is a radiation dose quantity that is used to estimate population total radiation risk associated with an imaging procedure. Patients in the pediatric age group are at inherently higher risk from exposure, both because of organ sensitivity and longer life expectancy (relevant to the long latency that appears to accompany radiation exposure). For these reasons, the RRL dose estimate ranges for pediatric examinations are lower as compared to those specified for adults (see [fig_ref] Table 2: RRL assignments for some of the examinations cannot be made, because the... [/fig_ref]. Additional information regarding radiation dose assessment for imaging examinations can be found in the ACR Appropriateness Criteria O radiation dose assessment introduction document. For additional information on ACR Appropriateness Criteria O , refer to http://www.acr.org/ac. (6)
[table] Table 1: Clinical condition: pulsatile abdominal mass, suspected AAA Preferred for symptomatic patients. Suitable for patients in whom US is not useful Alternative to CTA. Unable to detect calcium. Site-specific expertise important. See statement regarding contrast in text under ''anticipated exceptions'' [/table]
[table] Table 2: RRL assignments for some of the examinations cannot be made, because the actual patient doses in these procedures vary as a function of a number of factors (e.g., region of the body exposed to ionizing radiation, the imaging guidance that is used). The RRLs for these examinations are designated as NS not specified18. Aboyans V, Kownator S, Lafitte M et al (2010) Screening abdominal aorta aneurysm during echocardiography: literature review and proposal for a French nationwide study. Arch Cardiovasc Dis 103(10):552-558 19. Roshanali F, Mandegar MH, Yousefnia MA, Mohammadi A, Baharvand B (2007) Abdominal aorta screening during transthoracic echocardiography. Echocardiography 24(7):685-688 20. Alund M, Mani K, Wanhainen A (2008) Selective screening for abdominal aortic aneurysm among patients referred to the vascular laboratory. Eur J Vasc Endovasc Surg 35(6):669-674 21. Dupont A, Elkalioubie A, Juthier F et al (2010) Frequency of abdominal aortic aneurysm in patients undergoing coronary artery bypass grafting. Am J Cardiol 105(11):1545-1548 22. Long A, Bui HT, Barbe C et al (2010) Prevalence of abdominal aortic aneurysm and large infrarenal aorta in patients with acute coronary syndrome and proven coronary stenosis: a prospective monocenter study. Ann Vasc Surg 24(5):602-608 23. Gratama JW, van Leeuwen RB (2010) Abdominal aortic aneurysm: high prevalence in men over 59 years of age with TIA or stroke, a perspective. Abdom Imaging 35(1):95-98 24. Singh K, Bonaa KH, Solberg S, Sorlie DG, Bjork L (1998) Intraand inter-observer variability in ultrasound measurements of abdominal aortic diameter. The Tromso Study. Eur J Vasc Endovasc Surg 15 [/table]