Computed tomography of abdominal aortic aneurysm. An in vivo pathological report with a note on dynamic predictors

Computed Tomography Attenuation Values of the High-Attenuating Crescent Sign Can Discriminate Between Rupture, Impending Rupture, and Non-Rupture of Aortic Aneurysms

BACKGROUND

Although the high-attenuating crescent (HAC) sign can indicate aortic aneurysm (AA) impending rupture, the relation of its computed tomography (CT) value to the aneurysmal status remains unclear. This study compared the HAC sign CT-attenuation values among rupture, impending rupture, and non-rupture AA cases.Methods and Results:This included 76 patients (mean age: 77.0 years) diagnosed with HAC sign-associated AA between January 2005 and July 2015. The CT-attenuation values of the HAC sign (H) and aortic lumen (A) using region-of-interest methodology were measured and the H/A ratio was calculated. The study classified patients into the rupture group (R-G, n=36), impending rupture group (IR-G, n=16), and non-rupture group (NR-G, n=24); the H and the H/A ratio were compared among them. Additionally, the H and the H/A ratio cut-offs between the IR-G and NR-G groups were evaluated. The H and the H/A ratio were significantly higher in the R-G and IR-G than in the NR-G (both P<0.001); the H/A ratio was significantly higher in the R-G than in the IR-G (P=0.038). The optimal cut-off for H between the IR-G and NR-G was 50.3 Hounsfield units (area under the curve [AUC]=0.875; sensitivity=87.5%; specificity=87.5%), and that for the H/A ratio was 1.3 (AUC=0.909; sensitivity=91.7%; specificity=87.5%).

CONCLUSIONS

Among patients with AA, the H and the H/A ratio were significantly higher in cases of rupture and impending rupture than in those of non-rupture.

Intraluminal thrombus: Innocent bystander or factor in abdominal aortic aneurysm pathogenesis?

BACKGROUND

Abdominal aortic aneurysms (AAAs) represent a complex multifactorial hemodynamic, thrombotic, and inflammatory process that can ultimately result in aortic rupture and death. Despite improved screening and surgical management of AAAs, the mortality rates have remained high after rupture, and little progress has occurred in the development of nonoperative treatments. Intraluminal thrombus (ILT) is present in most AAAs and might be involved in AAA pathogenesis. The present review examined the latest clinical and experimental evidence for possible involvement of the ILT in AAA growth and rupture.

METHODS

A literature review was performed after a search of the PubMed database from 2012 to June 2020 using the terms "abdominal aortic aneurysm" and "intraluminal thrombus."

RESULTS

The structure, composition, and hemodynamics of ILT formation and propagation were reviewed in relation to the hemostatic and proteolytic factors favoring ILT deposition. The potential effects of the ILT on AAA wall degeneration and rupture, including a review of the current controversies regarding the position, thickness, and composition of ILT, are presented. Although initially potentially protective against increased wall stress, increasing evidence has shown that an increased volume and greater age of the ILT have direct detrimental effects on aortic wall integrity, which might predispose to an increased rupture risk.

CONCLUSIONS

ILT does not appear to be an innocent bystander in AAA pathophysiology. However, its exact role remains elusive and controversial. Despite computational evidence of a possible protective role of the ILT in reducing wall stress, increasing evidence has shown that the ILT promotes AAA wall degeneration in humans and in animal models. Further research, with large animal models and with more chronic ILT is crucial for a better understanding of the role of the ILT in AAAs and for the potential development of targeted therapies to slow or halt AAA progression.

Intraluminal Thrombus Deposition Is Reduced in Ruptured Compared to Diameter-matched Intact Abdominal Aortic Aneurysms

BACKGROUND

The aim of this study is to compare the pattern of intraluminal thrombus (ILT) deposition in diameter-matched ruptured and nonruptured abdominal aortic aneurysms (AAAs).

METHODS

We performed a single-center, retrospective study. Ruptured AAAs were collected during 24 months. Diameter-matched intact lesions were randomly selected in a 2:1 ratio and served as controls. ILT cross-sectional area, relative area, maximum thickness, and asymmetric distribution were recorded at the site of maximum aneurysm size and compared between groups. Moreover, additional comparisons were performed inside the group of ruptured AAAs, between the site of maximum size and the site of rupture.

RESULTS

Fifteen ruptured cases were compared with 30 nonruptured cases. ILT relative area (37.5% vs. 73.5%, P = 0.004) and maximum thickness (14.5 vs. 28 mm, P= 0.0017) were significantly reduced among ruptured compared to intact AAAs. The latter group presented mostly an anterior eccentric ILT deposition, while the former presented a more symmetrical pattern. The site of rupture was located at the site of maximum size in only 2 cases. In general, ILT was reduced at the site of rupture compared to the site of maximum aneurysm size in ruptured cases but differences did not reach statistical significance.

CONCLUSIONS

In similar sized AAAs, ILT is reduced in ruptured compared to nonruptured cases.

Acute non-traumatic disease of the abdominal aorta

Acute pathology in the abdominal aorta is associated with significant morbidity and mortality. The most feared complication of abdominal aortic disease is acute rupture in the setting of atherosclerotic abdominal aortic aneurysm. Although frank rupture often is easily diagnosed on CT, other findings such as a hyperattenuating crescent, discontinuous intimal calcium, and draping of the aorta are subtle signs of aneurysm instability. A true aneurysm should be distinguished from a rapidly growing, saccular pseudoaneurysm in the setting of infectious aortitis, as treatment strategy differs. Acute aortic syndrome involving the abdominal aorta, such as dissection and intramural hematoma, often is an extension of thoracic aortic disease, whereas penetrating atherosclerotic ulcers occasionally involve only the abdominal aorta. The goal of treating acute aortic pathology is to repair and prevent rupture, as well as restore and maintain perfusion of the lower extremities, kidneys, and mesentery. However, both open and endovascular repair of the abdominal aorta may become acutely complicated, resulting in compromise of these goals. Examples include aortoenteric fistula, endoleak, anastomotic pseudoaneurysm, graft infection, and thrombosis or kinking of a stent graft resulting in ischemia of the limbs and mesentery.

Predicting the Risk of Rupture of Abdominal Aortic Aneurysms by Utilizing Various Geometrical Parameters: Revisiting the Diameter Criterion

The authors estimated noninvasively the wall stress distribution for actual abdominal aortic aneurysms (AAAs) in vivo on a patient-to-patient basis and correlated the peak wall stress (PWS) with various geometrical parameters. They studied 39 patients (37 men, mean age 73.7 ± 8.2 years) with an intact AAA (mean diameter 6.3 ± 1.7 cm) undergoing preoperative evaluation with spiral computed tomography (CT). Real 3-dimensional AAA geometry was obtained from image processing. Wall stress was determined by using a finite-element analysis. The aorta was considered isotropic with linear material properties and was loaded with a static pressure of 120.0 mm Hg. Various geometrical parameters were used to characterize the AAAs. PWS and each of the geometrical characteristics were correlated by use of Pearson's rank correlation coefficients. PWS varied from 10.2 to 65.8 N/cm2 (mean value 37.1 ± 9.9 N/cm2). Among the geometrical parameters, the PWS was well correlated with the mean centerline curvature, the maximum centerline curvature, and the maximum centerline torsion of the AAAs. The correlation of PWS with maximum diameter was nonsignificant. Multiple regression analysis revealed that the mean centerline curvature of the AAA was the only significant predictor of PWS and subsequent rupture risk. This noninvasive computational approach showed that geometrical parameters other than the maximum diameter are better indicators of AAA rupture.

Unstable abdominal aortic aneurysms: a review of MDCT imaging features

Abdominal aortic aneurysms are commonly encountered during abdominal CT imaging, and size-based parameters for surgical or endovascular repair are well established. Aneurysms greater than 5 cm in diameter are recognized as representing an increased rupture risk and meriting intervention. Increasingly, additional interest has been generated in recognizing imaging features which may herald instability and portend a higher chance for potentially catastrophic rupture. This article will review and illustrate such signs, including hyperattenuation of mural thrombus, rapid expansion, low thrombus to lumen ratio, intimal calcification disruption, posterior mural draping, and saccular outpouching. Other features of complicated abdominal aneurysms including perianeurysmal inflammation, aortocaval and aortoenteric fistula formation will also be addressed. Heightened awareness of these features and their prognostic implications, as well as timely communication with the clinical service, is critical for the interpreting radiologist.

[Ultrasound of the large abdominal vessels]

Ultrasound has recently become an indispensable tool for the family physician, whether exercised in primary care and emergency department; and likewise it has spread to many other specialties: internal medicine, critical care, neurology, pneumology, digestive, etc. and that ultrasound has proven to be a safe diagnostic tool and have great capacity. We firmly believe that ultrasound done to «bedside» the patient by the family doctor, can greatly complement the physical examination and greatly improve clinical effectiveness, allowing the browser an immediate view of the anatomy and physiology of certain structures. It is within this context is particularly relevant ultrasonography of the Aorta and large abdominal vessels, made by the family doctor or the emergency itself, which will develop along this chapter.

Rupture signs on computed tomography, treatment, and outcome of abdominal aortic aneurysms

OBJECTIVES

Abdominal aortic aneurysm (AAA) rupture has a high mortality rate. Although the diagnosis of a ruptured AAA is usually straightforward, detection of impending rupture signs can be more challenging. Early diagnosis of impending AAA rupture can be lifesaving. Furthermore, differentiating between impending and complete rupture has important repercussions on patient management and prognosis. The purpose of this article is to classify and illustrate the entire spectrum of AAA rupture signs and to review current treatment options for ruptured AAAs.

METHODS

Using medical illustrations supplemented with computed tomography (CT), this essay showcases the various signs of impending rupture and ruptured AAAs. Endovascular aneurysm repair (EVAR) and open surgical repair are also discussed as treatment options for ruptured AAAs.

RESULTS

CT imaging findings of ruptured AAAs can be categorised according to location: intramural, luminal, and extraluminal. Intramural signs generally indicate impending AAA rupture, whereas luminal and extraluminal signs imply complete rupture. EVAR has emerged as an alternative and possibly less morbid method to treat ruptured AAAs.

CONCLUSIONS

AAA rupture occurs at the end of a continuum of growth and wall weakening. This review describes the CT imaging findings that may help identify impending rupture prior to complete rupture.

TEACHING POINTS

• AAA rupture occurs at the end of a continuum of growth and wall weakening. • Intramural imaging findings indicate impending AAA rupture. • Luminal and extraluminal imaging findings imply complete AAA rupture. • Some imaging findings are not specific to AAA ruptures and can be seen in other pathologies. • EVAR has emerged as an alternative and possibly less morbid method of treating ruptured AAAs.

Abdominal vascular emergencies: US and CT assessment

Acute vascular emergencies can arise from direct traumatic injury to the vessel or be spontaneous (non-traumatic).The vascular injuries can also be divided into two categories: arteial injury and venous injury.Most of them are life-treatening emergencies, sice they may cause an important ipovolemic shock or severe ischemia in their end organ and require prompt diagnosis and treatment.In the different clinical scenarios, the correct diagnostic approach to vascular injuries isn't firmly established and advantages of one imaging technique over the other are not obvious.Ultrasound (US) is an easy accessible, safe and non-invasive diagnostic modality but Computed Tomography (CT) with multiphasic imaging study is an accurate modality to evaluate the abdominal vascular injuries therefore can be considered the primary imaging modality in vascular emergencies.The aim of this review article is to illustrate the different imaging options for the diagnosis of abdominal vascular emergencies, including traumatic and non traumatic vessel injuries, focusing of US and CT modalities.

Investigation on regional variation of intraluminal thrombus: A mechanical and histological study

The abdominal aortic aneurysm is an irreversible dilatation of the artery and when this pathology is localized in the aortic district, the intraluminal thrombus (ILT) is present in approximately 75% of cases. The rupture risk of AAA is thought to be associated with increased levels of wall stress. In recent decades, finite element analysis (FEA) have been used to predict wall stresses in a patient-specific, noninvasive manner. However, FE simulations are mainly based on homogeneous behavior for ILT material. The purpose of this work was to investigate the biomechanical behavior of ILT and to derive an appropriate constitutive relation for ventral and posterior regions. Uniaxial tensile tests were carried out and histological investigations were performed on four fresh thrombi to correlate with the change in the mechanical properties. A second-order polynomial, large-strain, hyperelastic constitutive model was developed and used to fit the uniaxial tensile testing data for determination of the material parameters. Our results indicate that the microstructure of ILT differs between the ventral and dorsal region, with the area region characterized by a nonlinear behavior and the lateral posterior area stiffer than the budge region. The peak stretch values were (mean±SEM) 1·24±0·04 and 1·1±0·09 for the ventral and lateral posterior area of the ILT, respectively. The material models for the thrombi are in excellent agreement with the experimental data.

Morphological aspects of mural thrombi deposition residual lumen route in infrarenal abdominal aorta aneurisms

PURPOSE

To assess the most frequent deposition site of mural thrombi in infrarenal abdominal aorta aneurisms, as well as the route of the residual lumen.

METHODS

Assessment of CT scan images from 100 patients presenting asymptomatic abdominal aorta aneurism, and followed at HC-FMRP-USP.

RESULTS

In 53% of the cases the mural thrombus was deposited on the anterior wall; from these, in 22%, the residual lumen described a predominantly right sided route; in 22%, a left sided route; on the mid line in 5%; and crossing over the mid line in 1%. In 23%, the deposition of thrombi was concentric. In 11% it occurred on the posterior wall; from these, in 5%, the route of the residual anterior lumen was predominantly right sided; in 5%, left sided; and crossed over the mid line in 1%. In 13% complex morphological deposition patterns were found.

CONCLUSION

Mural thrombi formation was predominantly found on the anterior wall of the aneurismatic mass, with the route of the residual lumen projecting towards the posterior wall.

Spectrum of CT Findings in Rupture and Impending Rupture of Abdominal Aortic Aneurysms

Prompt diagnosis of rupture and impending rupture of abdominal aortic aneurysms is imperative. The computed tomographic (CT) findings of ruptured abdominal aortic aneurysms are often straightforward. Most ruptures are manifested as a retroperitoneal hematoma accompanied by an abdominal aortic aneurysm. Periaortic blood may extend into the perirenal space, the pararenal space, or both. Intraperitoneal extravasation may be an immediate or a delayed finding. Discontinuity of the aortic wall or a focal gap in otherwise continuous circumferential wall calcifications may point to the location of a rupture. There usually is a delay of several hours between the initial intramural hemorrhage and frank extravasation into the periaortic soft tissues. Contained or impending ruptures are more difficult to identify. A small amount of periaortic blood may be confused with the duodenum, perianeurysmal fibrosis, or adenopathy. Imaging features suggestive of instability or impending rupture include increased aneurysm size, a low thrombus-to-lumen ratio, and hemorrhage into a mural thrombus. A peripheral crescent-shaped area of hyperattenuation within an abdominal aortic aneurysm represents an acute intramural hemorrhage and is another CT sign of impending rupture. Draping of the posterior aspect of an aneurysmal aorta over the vertebrae is associated with a contained rupture.

CT Findings of Rupture, Impending Rupture, and Contained Rupture of Abdominal Aortic Aneurysms

OBJECTIVE

With the increasing use of cross-sectional imaging for a variety of medical and surgical conditions affecting the abdomen and pelvis, familiarity with the imaging features of aneurysm rupture--and the findings suspicious for impending or contained aneurysm rupture--is crucial for all radiologists. This pictorial essay will review the imaging findings of rupture of abdominal aortic aneurysms and of complicated aneurysms.

CONCLUSION

Prompt detection of abdominal aortic aneurysm rupture or impending rupture is critical because emergent surgery may be required and patient survival may be at stake.

Elemental composition, morphology and mechanical properties of calcified deposits obtained from abdominal aortic aneurysms

Calcified deposits exist in almost all abdominal aortic aneurysms (AAAs). The significant difference in stiffness between these hard deposits and the compliant arterial wall may result in local stress concentrations and increase the risk of aneurysm rupture. Calcium deposits may also complicate AAA repair by hindering the attachment of a graft or stent-graft to the arterial wall or cause vessel wall injury at the site of balloon dilation or vascular clamp placement. Knowledge of the composition and properties of calcified deposits helps in understanding the risks associated with their presence. This work presents results of elemental composition, microscopic morphology, and mechanical property measurements of human calcified deposits obtained from within AAAs. The elemental analyses indicate the deposits are composed primarily of calcium phosphate with other assorted constituents. Microscopy investigations show a variety of microstructures within the deposits. The mechanical property measurements indicate an average elastic modulus in the range of cortical bone and an average hardness similar to nickel and iron.

CT of nontraumatic thoracic aortic emergencies

Computed tomography (CT), especially multidetector row CT (MDCT), is often the preferred imaging test used for evaluation of nontraumatic thoracic aortic abnormalities. Unenhanced images, usually followed by contrast-enhanced arterial imaging, allow for rapid detailed aortic assessment. Understanding the spectrum of acute thoracic aortic conditions which may present similarly (aortic dissection, aneurysm rupture, penetrating atherosclerotic ulcer, intramural hematoma) will ensure that patients are diagnosed and treated appropriately. Familiarity with imaging protocols and potential mimics will prevent confusion of normal anatomy and variants with aortic disease.

A Review of Biological Factors Implicated in Abdominal Aortic Aneurysm Rupture

Abdominal aortic aneurysm (AAA) rupture is the 13th commonest cause of death in the Western World. Although considerable research has been applied to the aetiology and mechanism of aneurysm expansion, little is known about the mechanism of rupture. Aneurysm rupture was historically considered to be a simple physical process that occurred when the aortic wall could no longer contain the haemodynamic stress of the circulation. However, AAAs do not conform to the law of Laplace and there is growing evidence that aneurysm rupture involves a complex series of biological changes in the aortic wall. This paper reviews the available data on patient variables associated with aneurysm rupture and presents the evidence implicating biological factors in AAA rupture.

Size and location of thrombus in intact and ruptured abdominal aortic aneurysms

PURPOSE

This study compared the volume and morphology of intraluminal thrombus (ILT) in intact and ruptured abdominal aortic aneurysms (AAAs).

METHODS

ILT volume in 67 intact AAAs and in 31 ruptured AAAs was assessed by using computed tomography (CT) angiography to measure the major and minor diameter of the outer wall and lumen of AAA as outlined by contrast at multiple sites. ILT thrombus morphology was recorded by AutoCAD 2000 software. Four equidistant images traced from the CT scan were recorded along the length of AAA. Thrombus volume was categorized as anterior-eccentric if the calculated area of thrombus was greater anteriorly, posterior-eccentric if greater posteriorly, eccentric-equal if the difference between the anterior and posterior thrombus was <or=10%, and no thrombus.

RESULTS

Patients were well matched for age, gender, and other demographic variables except hypertension, which was significantly higher in ruptured AAA group ( P = .018). Ruptured AAAs were larger in diameter compared with intact AAAs (7.3 +/- 1.7 cm vs 6.0 +/- 1.2 cm, P = .0002). ILT volume was greater in ruptured AAAs (148.9 +/- 90.4 cm 3 ) compared with intact AAAs (92.1 +/- 75.6 cm 3 , P = .0031). However, the ILT volume/aneurysm volume ratio was similar in the two groups (0.49 +/- 0.19 in intact AAAs, 0.47 +/- 0.18 in ruptured AAAs; P = .8). Two patients in intact AAA group and three patients in the ruptured AAA group did not have ILT. Eccentric thrombus was present in 65 of 67 in the intact group and in 28 of 31 in the ruptured AAA group. Eccentric-anterior thrombus was predominant in both groups (46 of 67 in intact AAAs; 14 of 31 in ruptured AAAs). Thrombus location was statistically similar in both groups ( P = .101).

CONCLUSION

Ruptured AAAs are larger in diameter and have a greater volume of thrombus compared with intact AAAs. However, there was no difference in the thrombus volume/aneurysm volume ratio in the two groups. In both intact and ruptured AAA groups, the thrombus was usually anterior and eccentric.

A comparative study of aortic wall stress using finite element analysis for ruptured and non-ruptured abdominal aortic aneurysms

BACKGROUND

The decision to repair an asymptomatic abdominal aortic aneurysm (AAA) is currently based on diameter (> or =5.5 cm) alone. However, aneurysms less than 5.5 cm do rupture while some reach greater than 5.5 cm without rupturing. Hence the need to predict the risk of rupture on an individual patient basis is important. This study aims to calculate and compare wall stress in ruptured and non-ruptured AAA.

METHODS

The 3D geometries of AAA were derived from CT scans of 27 patients (12 ruptured and 15 non-ruptured). AAA geometry, systolic blood pressure and literature derived material properties, were utilised to calculate wall stress for individual AAA using finite element analysis.

RESULTS

Peak wall stress was significantly higher in the ruptured AAA (mean 1.02 MPa) than the non-ruptured AAA (mean 0.62 MPa). In patients with an identifiable site of rupture on CT scan, the area of peak wall stress correlated with rupture site.

CONCLUSIONS

Peak wall stress can be calculated from routinely performed CT scans and may be a better predictor of risk of rupture than AAA diameter on an individual patient basis.

Aneurysmal hypertension and its relationship to sac thrombus: a semi-qualitative analysis by experimental fluid mechanics

OBJECTIVES

To ascertain the effect of aneurysm thrombus and luminal diameter on arterial blood pressure within the abdominal aortic aneurysm lumen and at the sac wall.

METHODS

A life-like abdominal aortic aneurysm was incorporated in a pulsatile flow unit, using systemic blood pressure settings of 140/100 mmHg and 130/90 mmHg (denoted the high and low settings, respectively). Aneurysm sac pressure was measured in the absence of thrombus within the sac. This was repeated after a thrombus analogue (gelatine) was introduced into the aneurysm model in an asymmetric fashion. Luminal and sac wall pressures were compared to the systemic pressure, and to each other, in both blood pressure settings. Statistical analysis was performed using ANOVA in Minitab 13.

RESULTS

In the empty sac, the luminal and sac wall pressures were identical to the systemic pressures at the high and low settings. After introduction of thrombus, pressure was transmitted in a monophasic pulsatile fashion, measuring 166/142/151 mmHg (SP/DP/MP) at the sac wall, while the corresponding intraluminal pressure was 164/136/145 mmHg (p<0.001, high setting). By contrast, in the low setting, these readings were 157/133/141 (sac wall) and 160/128/138 mmHg (lumen; p<0.001). The sac wall pressures were significantly higher than the luminal pressures for both high and low settings (p<0.001).

CONCLUSIONS

Thrombus has a significant effect on the intraaneurysmal lumen itself and causes localised hypertension with high intraluminal pressures. The differences between the sac wall/luminal pressures may affect regional aneurysm wall biomechanics, but needs further study.

CT of acute abdominal aortic disorders

Aortic aneurysm rupture, aortic dissection, PAU, acute aortic occlusion, traumatic aortic injury, and aortic fistula represent acute abdominal aortic conditions. Because of its speed and proximity to the emergency department, helical CT is the imaging test of choice for these conditions. MR imaging also plays an important role in the imaging of aortic dissection and PAU, particularly when the patient is unable to receive intravenous contrast material. In this era of MDCT, conventional angiography is used as a secondary diagnostic tool to clarify equivocal findings on cross-sectional imaging. Ultrasound is helpful when CT is not readily available and the patient is unable or too unstable to undergo MR imaging.

Relationship of residual intraluminal to intrathrombotic pressure in a closed aneurysmal sac

PURPOSE

This study was undertaken to determine the relationship of residual intraluminal aneurysmal sac pressure (ILASP) to intrathrombic aneurysm sac pressure (ITASP) and to define the relationship between abdominal aorta aneurysm (AAA) size (anteroposterior or transverse diameter), volume of intraluminal thrombus, and residual ITASP.

METHODS

We measured ILASP and ITASP after proximal aortic neck and distal iliac clamping by placing angiocatheters into the lumen and thrombus of an excluded aneurysm sac in 41 consecutive patients. Simultaneously, mean blood pressure was recorded and aneurysm sac pressure ratio was calculated. Changes in ILASP and ITASP after clamping of the inferior mesenteric artery were recorded. In addition, correlation between AAA size, volume of intraluminal thrombus in AAA, and residual ITASP was determined.

RESULTS

Mean ILASP/blood pressure ratio was 0.40 (SD, 0.20). Mean ITASP/blood pressure ratio was 0.37 (SD, 0.23). There was a significant positive correlation of 0.47 between ITASP and ILASP (P =.002). Clamping of the inferior mesenteric artery resulted in markedly decreased ITASP in 2 patients (n = 40) and ILASP in 4 patients (n = 41). Each centimeter increase in AAA size resulted in a 47 mL increase in thrombus volume.

CONCLUSION

Increased ILASP results in corresponding increase in ITASP, and increased AAA size is associated with increased thrombus volume. However, neither thrombus volume nor AAA size has any relationship to ITASP.

Wall stress studies of abdominal aortic aneurysm in a clinical model

To estimate when an abdominal aortic aneurysm (AAA) may rupture, it is necessary to understand the forces responsible for this event. We investigated the wall stresses in an AAA in a clinical model. Using CT scans of the AAA, the diameter and wall thickness were measured and the model of the aneurysm was created. The wall stresses were determined using a finite element analysis in which the aorta was considered isotropic with linear material properties and was loaded with a pressure of 120 mmHg. The AAA was eccentric with a length of 10.5 cm, a diameter of 2.5 to 5.9 cm, and a wall thickness of 1.0 to 2.0 mm. The aneurysm had specific areas of high stress. On the inner surface the highest stress was 0.4 N/mm2 and occurred along two circumferentially oriented belts--one at the bulb and the other just below. The stress was longitudinal at the anterior region of the bulb and circumferential elsewhere, suggesting that a rupture caused by this stress will result in a circumferential tear at the anterior portion of the bulb and a longitudinal tear elsewhere. In the mid-surface the highest stress was 0.37 N/mm2 and occurred at two locations: the posterior region of the bulb and anteriorly just below. The stress was circumferential, suggesting that the rupture caused by this stress will produce a longitudinal tear. The location and orientation of the maximum stress were influenced more by the tethering force than by the wall thickness, luminal pressure, or wall stiffness. In conclusion, the rupture of an AAA is most likely to occur on the inner surface at the bulb. Such analytical approaches could lead to a better understanding of the aneurysm rupture and may be instrumental in planning surgical interventions.

Clinical and patho-anatomical factors affecting expansion of thoracic aortic aneurysms

OBJECTIVE

To examine the expansion of aneurysmal aortic segments (> or = 35 mm) and to assess the impact of clinical and patho-anatomical factors on aneurysm expansion.

DESIGN

87 consecutive patients (mean age 63.6 years, range 22-84 years) were studied using serial (six month intervals) computed tomographic or magnetic resonance imaging to monitor progression of thoracic aortic aneurysms. Aortic diameter was measured at seven predetermined segments and at the site of maximum aortic dilatation (MAX).

RESULTS

780 segment intervals were identified. The median overall aneurysm expansion rate was 1.43 mm/year. This increased exponentially with incremental aortic diameter (p < 0.01) and varied by anatomical segment (p < 0.05). The presence of intraluminal thrombus (p < 0.01) but not dissection or calcification was associated with accelerated growth. Univariate analysis identified thrombus (p < 0.001), previous stroke (p < 0.002), smoking (p < 0. 01), and peripheral vascular disease (p < 0.05) as factors associated with accelerated growth in MAX. Dissection, wall calcification, and history of hypertension did not affect expansion. beta Blocker treatment was not associated with protection. Multivariate analysis confirmed the positive effect of intraluminal thrombus and previous cerebral ischaemia, and the negative effect of previous aortic surgery on aneurysm growth. These findings translated into a mathematical equation describing exponential aneurysm expansion.

CONCLUSIONS

Aneurysmal thoracic aortic segments expand exponentially according to their initial size and their anatomical position within the aorta. The presence of intraluminal thrombus, atherosclerosis, and smoking history is associated with accelerated growth and may identify a high risk patient group for close surveillance.

Spared flow tract within the mural thrombus of an aortic aneurysm: its pathogenesis and clinical importance

The purpose of this case report is to determine the unique pathogenesis of a "spared flow tract" through a thick mural thrombus of an aortic aneurysm mimicking the penetrating or dissecting tract of an impending or acute rupture of an abdominal aortic aneurysm (AAA) and to discuss its clinical importance. Three blood flow tracts (i.e., spared flow tracts) penetrating to aortic major branches (inferior mesenteric arteries in two and left renal artery in one) through thick mural thrombi of three aortic aneurysms were found on thin section spiral CT scans. Histopathological examination revealed that the tracts were formed by thrombi and partially covered with endothelial cells. In conclusion, spared flow tracts may be pathways continuing to the aortic major branches through thick mural thrombi of aortic aneurysms and are spared from thrombogenesis because of relatively high blood flows. Their pathogenesis is definitely different from penetrating or dissecting tracts within mural thrombi of ruptured AAAs. Spared flow tracts should not be misinterpreted as penetrating or dissecting tracts of impending or acute rupture.

Diagnosis of Thrombus and Blood Flow in Aortic Aneurysm Using Tagging Cine Magnetic Resonance Imaging

In order to diagnose thrombus and to evaluate blood flow in aortic diseases, we investigated the usefulness of tagging cine magnetic resonance imaging (MRI) in patients with aortic aneurysm and aortic dissection. Six abdominal aortic aneurysms (AAA), two aortic arch aneurysms, and three aortic dissections were examined using tagging cine MRI. Linear tagging bands were 3 or 5 mm wide and established to the long and short axial images of the aortic lesions. Tagging bands were confirmed as the low-intensity signal bands in all patients. The parts of tagging bands of the intraaortic thrombus did not move, whereas the parts of tagging bands of the intraaortic blood flow moved and faded as time passed. Intraaortic thrombi were diagnosed in eight patients (six AAA, one aortic arch aneurysm, and one aortic dissection) in accordance with the stain of the parts of tagging bands. In patients with aortic dissection, the parts of tagging bands in the false lumen moved slowly, whereas the parts of tagging bands in the true lumen moved quickly. Tagging cine MRI has been shown to be useful in the diagnosis of thrombus and blood flow in aortic diseases.

Effect of intraluminal thrombus thickness and bulge diameter on the oxygen diffusion in abdominal aortic aneurysm

The intraluminal thrombus (ILT) commonly found within abdominal aortic aneurysm (AAA) may serve as a barrier to oxygen diffusion from the lumen to the inner layers of the aortic wall. The purpose of this work was to address this hypothesis and to assess the effects of AAA bulge diameter (dAAA) and ILT thickness (delta) on the oxygen flow. A hypothetical, three-dimensional, axisymmetric model of AAA containing ILT was created for computational analysis. Commercial software was utilized to estimate the volume flow of O2 per cell, which resulted in zero oxygen tension at the AAA wall. Solutions were generated by holding one of the two parameters fixed while varying the other. The supply of O2 to the AAA wall increases slightly and linearly with dAAA for a fixed delta. This slight increase is due to the enlarged area through which diffusion of O2 may take place. The supply of O2 was found to decrease quickly with increasing delta for a fixed dAAA due to the increased resistance to O2 transport by the ILT layer. The presence of even a thin, 3 mm ILT layer causes a diminished O2 supply (less than 4 x 10(-10) mumol/min/cell). Normally functioning smooth muscle cells require a supply of 21 x 10(-10) mumol/min/cell. Thus, our analysis serves to support our hypothesis that the presence of ILT alters the normal pattern of O2 supply to the AAA wall. This may lead to hypoxic cell dysfunction in the AAA wall, which may further lead to wall weakening and increased potential for rupture.

Biomechanics of abdominal aortic aneurysm in the presence of endoluminal thrombus: experimental characterisation and structural static computational analysis

OBJECTIVES

To evaluate the role played by biomechanical and geometrical parameters of endoluminal thrombus and of aortic wall on abdominal aortic aneurysm (AAA) behaviour.

MATERIALS AND METHODS

Tensile tests on 21 AAA thrombus specimens from six patients undergoing AAA repair and numerical evaluation of aneurysmal aortic wall stress and strain distribution. Parameters of the analysis were lumen eccentricity, thrombus Young's Modulus and the aortic wall constitutive equation.

RESULTS

There was a linear stress/strain for all the thrombus specimens. The numerical analyses show the mechanical behaviour of AAA as a function of lumen eccentricity and biomechanical parameters.

CONCLUSIONS

Well organised thrombus reduces the effect of the pressure load on the aneurysmal aortic wall.

Effect of intraluminal thrombus on abdominal aortic aneurysm wall stress

PURPOSE

Abdominal aortic aneurysms (AAAs) rupture when the wall stress exceeds the strength of the vascular tissue. Intraluminal thrombus may absorb tension and reduce AAA wall stress. This study was performed to test the hypothesis that intraluminal thrombus can significantly reduce AAA wall stress.

METHODS

AAA wall stresses were determined by axisymmetric finite element analysis. Model AAAs had external diameters ranging from 2.0 to 4.0 cm. Model parameters included: AAA length, 6 cm; wall thickness, 1.5 mm; Poisson's ratio, 0.49; Young's modulus, 1.0 MPa; and luminal pressure, 1.6 x 10(5) dyne/cm2. Stresses were calculated for each model without thrombus, and then were recalculated with thrombus filling 10% of the AAA cavity. Calculations were repeated as thrombus size was increased in 10% increments and as thrombus elastic modulus increased from 0.01 MPa to 1.0 MPa. Maximum wall stresses were compared between models that had intraluminal thrombus and the unmodified models. Stress reduction greater than 25% was considered significant.

RESULTS

The maximum stress reduction of 51% occurred when thrombus with elastic modulus of 1.0 MPa filled the entire AAA cavity. Stresses were reduced by only 25% as modulus decreased to 0.2 MPa. Similarly, decreasing thrombus size by 70% resulted in stress reduction of only 28%. Large AAAs experienced greater stress reduction than small AAAs (48% vs 11%).

CONCLUSION

Intraluminal thrombus can significantly reduce AAA wall stress.

Cellular content and permeability of intraluminal thrombus in abdominal aortic aneurysm

PURPOSE

A pathologic feature commonly associated with abdominal aortic aneurysms is the presence of variably sized and shaped intraluminal thrombus, which may be fundamental to the disease process. However, the precise role of the intraluminal thrombus in the formation, enlargement, and rupture of abdominal aortic aneurysms is unknown. The hypothesis tested in this study was whether there were structural features of aortic thrombi to suggest that it may be involved in the pathogenesis of abdominal aortic aneurysms. We have investigated this hypothesis using a variety of structural and biochemical techniques.

METHODS

Tests performed were light, transmission, and scanning electron microscopy; fluid permeability measurements; and Western blots.

RESULTS

Intraluminal thrombus found in abdominal aortic aneurysms is structurally complex and is traversed from the luminal to abluminal surface by a continuous network of interconnected canaliculi. Quantitative microscopic analysis of the thrombus shows cellular penetration for at least 1 cm from the luminal surface of the thrombus. Macro-molecular penetration may be unrestricted throughout the entire thickness of the thrombus. Fibrin deposition occurred throughout the thrombus, whereas fibrin degradation occurred principally at the abluminal surface.

CONCLUSIONS

These principally structural studies support the hypothesis that the thrombus is a self-sustaining entity that may have significance in the pathophysiologic mechanism of abdominal aortic aneurysms.

Computed tomography scanning findings associated with rapid expansion of abdominal aortic aneurysms

PURPOSE

Early repair of abdominal aortic aneurysms (AAA) is particularly appropriate for those that are most likely to expand. Our aim was to define features on computed tomography (CT) scanning associated with subsequent rapid aneurysm expansion.

METHODS

We reviewed CT scans of 80 patients with AAA (> 3.0 cm) who underwent CT scanning of the abdomen and pelvis two times, at least 6 months apart, between 1986 and 1992. The aneurysms initially measured 4.4 +/- 0.6 cm, and the mean interval between obtaining scans was 22 +/- 12 months. Clinical variables assessed included age, sex, medical risk factors, underlying cardiovascular and pulmonary diseases, and administration of beta blockers and lipid-lowering agents. Computer-aided measurements on each CT scan section included the maximal and minimal diameters and area of the aneurysm. Dimensions of the luminal thrombus and the arc of aneurysm wall covered by thrombus (TARC). Maximal aneurysm dimensions were related to juxtarenal aortic and second lumbar vertebral body dimensions.

RESULTS

Mean aneurysm expansion was 0.26 +/- 0.25 cm/yr. CT scanning variables that correlated significantly with rate of expansion included the mean TARC (r = 0.43, p < 0.001), thrombus volume fraction (r = 0.37, p < 0.001), TARC on the largest aneurysm cross section (r = 0.34, p < 0.01), and thrombus area fraction (r = 0.30, p < 0.01). Rapid expansion (> 0.5 cm/yr) occurred in 15 (19%) aneurysms. The two predictors for rapid expansion on logistic regression analysis were mean TARC (p < 0.005) and the presence of carotid artery disease (p < 0.05).

CONCLUSION

An increased AAA thrombus load is associated with a higher likelihood of rapid expansion and should weigh in favor of early surgical repair.

Morphology predicts rapid growth of small abdominal aortic aneurysms

This study evaluated CT scans of small abdominal aortic aneurysms (AAAs) (< 5 cm) to assess anatomic features associated with rapid expansion. Serial CT scans obtained at least 10 months apart (mean 15 months) from patients with small AAAs were reviewed. Each cross-sectional image of the AAAs was analyzed using a computer-assisted design program. The circumference of the AAA in each CT image was divided into eight equal arcs, from which the apparent radius of curvature (Rc) for each segment was calculated. Flattening of the wall curvature results in an increased segmental Rc. The CT scans of nine patients with expanding AAAs (expansion > or = 0.5 cm/yr) were compared to those of 10 patients with stable AAAs (expansion < or = 0.2 cm/yr). To adjust for differences in AAA size, the Rc for each segment was normalized by dividing each individual Rc by the average of the eight Rcs (RcAvg) calculated for that cross-sectional CT image. Analysis of variance showed that the left posterolateral segments in expanding AAAs had larger Rc/RcAvg ratios than those segments in stable AAAs (1.14 +/- 0.19 vs. 0.80 +/- 0.09, p < 0.02). Laplace's law indicates that the left posterolateral segment in AAAs that grow more rapidly is subjected to greater wall tension. Flattening in the curvature of the left posterolateral wall segment was significantly associated with an increased rate of expansion in small AAAs. This finding, readily derived from standard CT scan images, may predict which small AAAs are more prone to rapid expansion.

Biomechanical factors in abdominal aortic aneurysm rupture

Hitherto the size of abdominal aortic aneurysms (AAA) has been considered the most important factor in determining the risk of rupture. For this reason most interest has been devoted to physical, echographic and tomographic analyses of the shape of AAA. However, it is known that rupture can also occur in small AAA. Other factors must be considered to have an important role in the natural history of aneurysms. The aim of this study was to characterise the mechanical stress in the wall of an AAA due to pressure in the presence of atherosclerosis, intraluminal thrombus and anatomical restraints. The Finite Elements Method (FEM) was used to determine wall stress distribution. Due to the simplicity of the AAA structure an axisymmetric model has been built. The results of the structural analysis confirms that maximum stress increases with diameter. These effects may be reduced by the presence of intraluminal thrombus, which in the models reduces maximum stress by up to 30%; however this is not the case for dissecting thrombus. On the other hand atherosclerotic plaques cause stress concentration and a significant increase in maximum wall stress. The risk of rupture can increase by about 200%. Finally the investigation shows the FEM is a versatile tool for studying the mechanics of vascular structures. It enables the influence of various parameters on wall stress to be quantified in diagnostic settings, and so could be useful for predicting the rupture of AAA, although at present such predictions are limited by data leakage and by the approximations used in the model.

The role of computerized tomography in aorto-iliac vascular disease

Computerized tomography (CT) now has a definite place in the assessment of aortic vascular disease. In a study of 96 patients with abdominal or thoracic aorto-iliac problems, CT proved most useful in the management of haemodynamically stable patients with abdominal aortic aneurysms that were suspected of leaking. The complex anatomy associated with thoracic and abdominal aneurysms and aortic dissection was clearly defined. The interpretation of scans on postoperative aortic graft patients was difficult and less often helpful. The incidental finding of aortic disease during abdominal scans for a variety of other indications was infrequent and seldom contributed to patient management. The indications for CT have become far more selective.