Hemodynamics of small aneurysm pairs at the internal carotid artery

Cerebral aneurysms carry significant risks because rupture-related subarachnoid hemorrhage leads to serious and often fatal consequences. The rupture risk increases considerably for multiple aneurysms. Multiple aneurysms can grow from the same location of an artery, and the interaction between these aneurysms raises the rupture risk even higher. Four aneurysm pair cases at the internal carotid artery are investigated for their hemodynamic behaviors using patient-specific modeling. For each case, aneurysms are separated from the parent artery and three models are reconstructed, one with two aneurysms and the other two models with only one of the two aneurysms. Results show that the relative anatomic location of one aneurysm to the other may determine the hemodynamic environment of an aneurysm. The presence of a proximal aneurysm reduces the intra-aneurysmal flow into the distal aneurysm; the proximal aneurysm and larger aneurysm have a greater area under low wall shear stress. The average intra-aneurysmal inflow ratio ranges from 16% to 41%, and reduction of the inflow ratio by an aneurysm pair varies from 6% to 15%. The maximum wall shear stress increases for serial aneurysms, but decreases for parallel aneurysms. Interaction between parallel aneurysms is not significant; however, the proximal aneurysm in serial aneurysms may be subject to a greater rupture risk.     

The Familial Intracranial Aneurysm (FIA) study protocol

Background  

Subarachnoid hemorrhage (SAH) due to ruptured intracranial aneurysms (IAs) occurs in about 20,000 people per year in the U.S. annually and nearly half of the affected persons are dead within the first 30 days. Survivors of ruptured IAs are often left with substantial disability. Thus, primary prevention of aneurysm formation and rupture is of paramount importance. Prior studies indicate that genetic factors are important in the formation and rupture of IAs. The long-term goal of the Familial Intracranial Aneurysm (FIA) Study is to identify genes that underlie the development and rupture of intracranial aneurysms (IA).     

影响颅内小型动脉瘤破裂的影像解剖特征分析

目的 探讨颅内小型动脉瘤的破裂与其影像解剖特征的关系,建立动脉瘤破裂风险评分的预测模型,为颅内小型动脉瘤破裂高危患者的早期识别干预提供参考。方法 回顾性分析2015年1月—2020年5月郑州大学附属郑州中心医院经头颈CT血管造影或全脑血管造影证实的182例颅内小型动脉瘤(最大径＜5 mm)患者的临床资料,其中男62例、女120例,年龄31~83(56.85±11.51)岁,动脉瘤破裂组95例、未破裂组87例。两组患者动脉瘤的部位、形状、有无子囊、生长方向及相关解剖学参数等临床特征和影像解剖特征的比较采用单因素分析方法;动脉瘤破裂的独立危险因素分析采用多因素logistic分析方法,建立动脉瘤破裂的影像解剖特征的预测模型。结果 两组患者基线资料比较差异均无统计学意义(P值均＞0.05)。单因素分析显示,两组间动脉瘤部位、瘤壁形状、子囊、入射夹角、瘤高与瘤颈的比值(AR)差异均有统计学意义(P值均＜0.05)。多因素logistic回归分析显示,有子囊、入射夹角＞117.75°、AR值＞1.65是动脉瘤破裂的独立危险因素(P值均＜0.01)。根据logistic预测模型,其动脉瘤破裂风险评分(R),受试者操作特征曲线下面积为0.812(95%可信区间0.750~0.874,P＜0.01),R=2为最佳截断值,灵敏度72.6%,特异度78.2%。结论 颅内小型未破裂动脉瘤患者符合以下两种及两种以上影像解剖特征者,即动脉瘤位于颈内动脉后交通段或前交通动脉、瘤壁不规则、有子囊、入射夹角＞117.75°、AR值＞1.65等,为动脉瘤破裂的高危人群。     

水解脱弹簧圈填塞治疗急性期破裂颅内动脉瘤1例

背景：以往多采用电解脱弹簧圈填塞治疗颅内动脉瘤,但电解脱弹簧圈解脱时间较长,解脱区质地硬,并发症发生率较高,不利于微小动脉瘤的栓塞。 目的：观察水解脱弹簧圈填塞治疗急性期破裂颅内动脉瘤的效果。 方法：经头颅CT检查确认蛛网膜下腔出血并有脑叶出血1例,数字减影血管造影系统证实为颅内动脉瘤,采用水解脱弹簧圈栓塞动脉瘤。 结果与结论：经过治疗,患者破裂动脉瘤致密栓塞,填塞达99%,出血得到控制。提示水解脱弹簧圈的超柔软性及对瘤体壁的顺应性,操作方便,提高了动脉瘤的致密性栓塞的可能性,减少并发症的可能,应该为目前急性期颅内动脉瘤破裂较好的栓塞方法。     

Deficiencies in estrogen-mediated regulation of cerebrovascular homeostasis may contribute to an increased risk of cerebral aneurysm pathogenesis and rupture in menopausal and postmenopausal women

Despite the catastrophic consequence of ruptured intracranial aneurysms, very little is understood regarding their pathogenesis, and there are no reliable predictive markers for identifying at-risk individuals. Few studies have addressed the molecular pathological basis and mechanisms of intracranial aneurysm formation, growth, and rupture. The pathogenesis and rupture of cerebral aneurysms have been associated with inflammatory processes, and these have been implicated in the digestion and breakdown of vascular wall matrix. Epidemiological data indicate that the risk of cerebral aneurysm pathogenesis and rupture in women rises during and after menopause as compared to premenopausal women, and has been attributed to hormonal factors. Moreover, experimental evidence supports a role for estrogen in the modulation of each phase of the inflammatory response implicated in cerebral aneurysm pathogenesis and rupture. While the risk of aneurysm rupture in men also increases with age, this increased risk has been attributed to other recognized risk factors including cigarette smoking, use of alcohol, and history of hypertension, all of which are more common in men than women. We hypothesize, therefore, that decreases in both circulating estrogen levels and cerebrovascular estrogen receptor density may contribute to an increased risk of cerebral aneurysm pathogenesis and rupture in women during and after menopause. To test our hypothesis, experiments are needed to identify genes regulated by estrogen and to evaluate gene expression and intracellular mechanisms in cells/tissues exposed to varying concentrations and duration of treatment with estrogen, metabolites of estrogen, and selective estrogen receptor modulators (SERMs). Furthermore, it is not likely that the regulation of cerebrovascular homeostasis is due to the actions of estrogen alone, but rather the interplay of estrogen and other hormones and their associated receptor expression. The potential interactions of these hormones in the maintenance of normal cerebrovascular tone need to be elucidated. Additional studies are needed to define the role that estrogen and other sex hormones may play in the cerebrovascular circulation and the pathogenesis and rupture of cerebral aneurysms. Efforts directed at understanding the basic pathophysiological mechanisms of aneurysm pathogenesis and rupture promise to yield dividends that may have important therapeutic and clinical implications. The development of non-invasive tools such as molecular MRI for the detection of specific cells, molecular markers, and tissues may facilitate early diagnosis of initial pathophysiological changes that are undetectable by clinical examination or other diagnostic tools, and can also be used to evaluate the state of activity of cerebral aneurysm pathogenesis before, during, and after treatment.     

Three-Dimensional Geometrical Characterization of Cerebral Aneurysms

The risk of rupture of cerebral aneurysms has been correlated with the size of the aneurysm sac. It is conceivable that geometrical shape, not just size may also be related to aneurysm rupture potential. Further, aneurysm shape may also be a factor in deciding on treatment modalities, i.e., to clip or coil. However, our ability to make use of available information on aneurysm shape remains poor. In this study, methods were developed to quantify the seemingly arbitrary three-dimensional geometry of the aneurysm sac, using differential and computational geometry techniques. From computed tomography angiography (CTA) data, the three-dimensional geometry of five unruptured human cerebral aneurysms was reconstructed. Various indices (maximum diameter, neck diameter, height, aspect ratio, bottleneck factor, bulge location, volume, surface area, Gaussian and mean curvatures, isoperimetric ratio, and convexity ratio) were utilized to characterize the geometry of these aneurysm surfaces and four size-matched hypothetical control aneurysms. The physical meanings of various indices and their possible role as prognosticators for rupture risk and presurgical planning were discussed.     

Vascular aneurysms in Ehlers-Danlos syndrome subtypes: A systematic review

Aneurysmal lesions are commonly seen in Ehlers-Danlos Syndrome (EDS). To better identify the regional and vessel-specific spectrum of aneurysms in different subtypes of EDS, we performed a systematic review. We searched Medline for relevant studies from 1963 to April 2022. Studies providing a report of any EDS subtype by genetic diagnosis, histologic analysis, or clinical criteria were included. A total of 448 patients from 220 studies were included. 720 vessel-specific aneurysms were reported: 386 in the abdominopelvic area, 165 in the intracranial region, 98 in the thorax, 2 in the extremities, and 6 in the venous system. In 27 out of the 65 patients with ruptured aneurysms, the ruptured aneurysm was the initial presentation. Multiple aneurysms were present in 163 out of 249 patients who had been systematically evaluated for other locations of aneurysms. The head and neck and abdominopelvic regions are two potential foci for aneurysm formation in patients with EDS. The aneurysm development in EDS is not confined to arteries; the venous system and cardiac septa may also be affected. Many patients develop multiple aneurysms, either at the time of the initial presentation or throughout their lifetime and aneurysm formation or rupture may be the first presentation of EDS.     

Physical Factors Effecting Cerebral Aneurysm Pathophysiology

Many factors that are either blood-, wall-, or hemodynamics-borne have been associated with the initiation, growth, and rupture of intracranial aneurysms. The distribution of cerebral aneurysms around the bifurcations of the circle of Willis has provided the impetus for numerous studies trying to link hemodynamic factors (flow impingement, pressure, and/or wall shear stress) to aneurysm pathophysiology. The focus of this review is to provide a broad overview of such hemodynamic associations as well as the subsumed aspects of vascular anatomy and wall structure. Hemodynamic factors seem to be correlated to the distribution of aneurysms on the intracranial arterial tree and complex, slow flow patterns seem to be associated with aneurysm growth and rupture. However, both the prevalence of aneurysms in the general population and the incidence of ruptures in the aneurysm population are extremely low. This suggests that hemodynamic factors and purely mechanical explanations by themselves may serve as necessary, but never as necessary and sufficient conditions of this disease’s causation. The ultimate cause is not yet known, but it is likely an additive or multiplicative effect of a handful of biochemical and biomechanical factors.     

颅内前循环动脉瘤夹闭术的显微外科技术

目的 总结我院自2004年以采采用显微神经外科手术治疗的104例颅内前循环动脉瘤的经验，探讨颅内前循环动脉瘤夹闭术的显微手术技巧，以期进一步提高该病的临床治疗效果。方法 104例颅内前循环动脉瘤病人中，1例胼周动脉瘤采用前纵裂入路，1例后交通合并胼周动脉瘤采用翼点和前纵裂的联合入路，其余部位的动脉瘤均采用改盘翼点入路。手术采用显微外科技术解剖脑池，锐性分离瘤颈并夹闭之。术中采用控制性降压、栽瘤动脉临时阻断和动脉瘤体翻转等技术以防止破裂出血和误夹正常血管。结果 104例手术病人中，102例成功夹闭瘤颈，2例采用了动脉瘤包裹术。术后病人恢复盘好78例，轻残17倒，重残6例，植物生存2例。死亡1例。结论 采用显微外科技术夹闭动脉瘤颈是治疗颅内前循环动脉瘤的根本方法；术中采用控制性降压、载瘤动脉临时阻断和动脉瘤体翻转等技术以防止破裂出血和误夹正常血管。可以提高颅内前循环动脉瘤的手术治疗效果。     

Long-lasting narrowing of the parent artery after bilateral clipping of mirror-image aneurysms of distal anterior cerebral arteries: a case report

Because multiple intracranial aneurysms are not rare, accurate preoperative detection of asymptomatic aneurysms is important. In this paper, we report a ruptured distal anterior cerebral artery (DACA) aneurysm associated with an unruptured mirror-image aneurysm in a 62-year-old man presenting with headache. Although delayed vasospasm after subarachnoid hemorrhage has been reported to persist for 2 to 3 weeks, angiographic parent artery narrowing was far more prolonged in our case. Computed tomography revealed a subarachnoid hemorrhage in the interhemispheric and right sylvian fissures and a right frontal lobe hematoma. Digital subtraction angiography demonstrated bilateral symmetric saccular aneurysms of DACAs. On the day of admission, both aneurysms were clipped using an interhemispheric approach in a one-stage procedure, and the hematoma was aspirated. Angiography performed 8 days after the surgery demonstrated a residual aneurysm neck on the left side. Follow-up digital subtraction angiography performed on day 42 from onset showed resolution of the residual aneurysm neck along with narrowing of the left A2. However, at 7 months, the A2 narrowing had lessened. The location of the bilateral aneurysms near the midline facilitated a single approach but necessitated the application of juxtaposed clips. Regarding the pathogenesis of the bilateral aneurysms, previous reports have suggested symmetry of congenital anatomic defects and hemodynamic stress as potential causes. The persistent narrowing that was observed could have resulted from proliferative vasculopathy or from fibrosis possibly induced by the clips.     

Miliary aneurysms and “Fibrinoid” degeneration of cerebral blood vessels

Three cases of intracerebral hemorrhage are described in which there was fibrinoid degeneration of cerebral arteries and arterioles or miliary aneurysms or both. Fibrous balls are shown to be sclerosed true aneurysms. These changes occurred in the absence of malignant hypertension and perhaps in the absence of any hypertension. A further point of interest was the finding of fibr9noid at the site of apparent aneurysm formation in a small artery on the cerebral surface, a location at which miliary aneurysms are not generally thought to form. The presence of intracerebral hemorrhage in all three cases, and the ready demonstration of similar changes in other cases of intracerebral hemorrhage, suggest but do not prove that the fibrinoid degeneration or aneurysm leads to vessel rupture and to hemorrhage itself. Also unsettled is the question of whether miliary aneurysms form only at sites already displaying fibrinoid change. Our data suggest that pre-existing fibrinoid may not be a prerequisite for miliary aneurysm formation.     

A clinical analysis of twelve cases of ruptured cerebral de novo aneurysms

Formation of cerebral de novo aneurysms (CDNA) is rare, and the pathogenesis remains obscure. In this study, we investigated the factors that contribute to the formation of CDNA and suggest guidelines for following patients treated for cerebral aneurysms. We retrospectively reviewed 2,887 patients treated for intracranial aneurysm at our institute from January of 1976 to December of 2005. Of those patients, 12 were readmitted due to recurrent rupture of CDNA, which was demonstrated by cerebral angiography. We assessed clinical characteristics, such as gender, size and site of rupture, past history, and the time to CDNA rupture. Of the 12 patients, 11 were female and 1 was male, with a mean age at rupture of the first aneurysm of 44.7 years (range: 30-69 years). The mean time between the first episode of subarachnoid hemorrhage (SAH) and the second was 8.9 years (range: 1.0-16.7 years). The most common site of ruptured CDNA was the internal carotid artery (5 patients, 41.7%), followed by basilar artery bifurcation (3 patients, 25.0%). In the remaining 4 patients, rupture occurred in the anterior communicating, middle cerebral, anterior cerebral (A1), or posterior cerebral (P1) arteries. In 5 cases (41.7%), the CDNA occurred contralateral to the initial aneurysm. Eleven patients (91.7%) had a past history of arterial hypertension. There was no history of habitual smoking or alcohol abuse in any of the patients. Eight patients underwent clipping for CDNA and three patients were treated with coiling. One patient who had multiple aneurysms was treated with clipping following intra-aneurysmal coiling. Assessment according to the Glasgow Outcome Scale (GOS) of the patients after the treatment was good in 10 cases (83.3%) and fair in 2 cases (16.7%). Although formation of CDNA after successful treatment of initial aneurysm is rare, several factors may contribute to recurrence. In our study, female patients with a history of arterial hypertension were at higher risk for ruptured CDNA. We recommend follow-up imaging studies every five years after treatment of the initial aneurysm, especially in women and those with a history of arterial hypertension.     

Ultra-Early Surgery for Poor-Grade Intracranial Aneurysmal Subarachnoid Hemorrhage: A Preliminary Study

Purpose

To describe the therapeutic effect and possibility of the ultra-early surgery for poor-grade aneurysmal subarachnoid hemorrhage (Hunt-Hess grades IV - V).

Materials and Methods

Nine cases with intracranial aneurysms, demonstrated by computed tomographic angiography (CTA), were treated by ultra-early surgery under general anesthesia within 24 hours from subarachnoid hemorrhage (SAH), 5 cases were treated within 6 hours and 4 cases in 6 - 24 hours. Preoperative Hunt-Hess grade: 6 cases were IV and 3 cases were V. The clinical outcome was evaluated by Glasgow Outcome Scores (GOS).

Results

In operation, difficult dissection occurred in 5 cases (55.6%), and rupture of aneurysm occurred and temporary obstructions were performed in 4 cases (44.4%). After clipping of aneurysm, 2 cases underwent V-P shunt because of hydrocephalus, pulmonary infection occurred in 3 cases, hypothalamus reaction accompanied with upper gastrointestinal hemorrhage in 2 cases. The clinical outcome were favorable (GOS 4 - 5) in 4 cases (44.4%), dissatisfied (GOS 2 - 3) in 3 cases (33.3%), and dead (GOS 1) in 2 cases (22.2%) when patients departed from our hospital.

Conclusion

The ultra-early surgery can avoid early rebleeding of intracranial aneurysm, therefore, should be considered in the treatment of Hunt-Hess grade IV-V intracranial aneurysms. The appliance of CTA can make it possible to use of ultra-early surgery and improve the therapeutic effect.     

Endothelial cells from human cerebral aneurysm and arteriovenous malformation release ET-1 in response to vessel rupture

Cerebral aneurysms and arteriovenous malformations (AVM) are a common cause of stroke and cerebral hemorrage. Both are often discovered when they rupture, causing subarachnoid hemorrhage (SAH). SAH-induced vasospasm is mediated by enhanced vasoconstriction due to endothelin-1 (ET-1). We investigated whether endothelial cells (ECs) obtained from aneurysm and AVM express phenotypic and genotypic alterations contributing to the development of vasospasm after SAH. We isolated ECs from human AVM and aneurysm and then confirmed their EC origin by polymerase chain reaction and immunocytochemistry with endothelial markers. Experiments were also carried out with human cerebral microvascular and umbilical vein ECs (HCECs and HUVECs respectively) for comparison. We tested EC proliferation ability and microtubule formation in Matrigel at different cell passages. Five aneurysm (3 ruptured, 2 unruptured) and 3 AVM (2 ruptured, 1 unruptured) ECs were tested for ET-1 release in the culture medium. Aneurysm and AVM ECs expressed von Willebrand factor, Adrenomedullin, and exhibited a progressive reduction of proliferation and in vitro angiogenic ability after the V passage. Significantly higher levels of ET-1 have been detected in ECs from ruptured aneurysms and AVMs. We report the first successful isolation and characterization of primary EC lines from human cerebral vascular lesions. Augmented release of ET-1 is correlated with the rupture of the abnormal vessel confirming its role in vasospasm after SAH. Furthermore, ECs obtained from these vascular malformations can be used as an experimental model to study SAH-induced vasoconstriction.     

Blood flow dynamics in patient-specific cerebral aneurysm models: the relationship between wall shear stress and aneurysm area index

Hemodynamics plays an important role in the progression and rupture of cerebral aneurysms. The temporal and spatial variations in wall shear stress (WSS) within the aneurysmal sac are hypothesized to be correlated with the growth and rupture of the aneurysm. The current work describes the blood flow dynamics in 34 patient-specific models of saccular aneurysms located in the region of the anterior and posterior circulation of the circle of Willis. The models were obtained from three-dimensional rotational angiography image data and blood flow dynamics was studied under a physiologically representative waveform of inflow. The three-dimensional continuity and momentum equations for unsteady laminar flow were solved with commercial software using non-structured fine grid sizes. The vortex structure, the wall pressure, and the WSS showed large variations, depending on the morphology of the artery, size of the aneurysm, and form. A correlation existed between the mean WSS on the aneurysmal sac for lateral unruptured and ruptured aneurysms with an aneurysm surface index, which is defined as the ratio between the aneurysm area and the artery area at model inlet, respectively.     

Applications of multislice CT angiography in the surgical clipping and endovascular coiling of intracranial aneurysms

Prompt diagnosis and therapy of aneurysms are critical for patients with nontraumatic subarachnoid hemorrhage (SAH). The aim of our study was to assess the clinical usefulness of multislice computed tomography angiography (CTA) in the surgical and endovascular treatment of intracranial aneurysms. A total of 195 cases with 206 intracranial aneurysms underwent CTA. Fifty (24%) aneurysms underwent surgical clipping while 156 (76%) aneurysms underwent endovascular coiling. In the five missed aneurysms at digital substraction angiography and the nine aneurysms with mass intracerebral hematomas, surgical treatment was successfully performed based on 16-slice CTA alone, and the other 36 aneurysms were clipped on the main basis of the CTA. The intraoperative findings correlated well with the CTA findings and all aneurysms were clipped successfully. Sixteen-slice CTA image information has been shown to determine the choice of aneurysm therapy and assist the surgical and endovascular treatment of intracranial aneurysms.     

Prognosis of abdominal aortic aneurysms. A population-based study

Information is incomplete about the rate of expansion of abdominal aortic aneurysms and the risk of rupture in relation to their size. To address these questions, we conducted a population-based study. Of the 370 residents of Rochester, Minn., with an aneurysm initially diagnosed from 1951 through 1984, 181 had the aneurysm documented by ultrasound examination. Among the 103 patients who underwent more than one ultrasound study, the diameter of the aneurysm increased by a median of 0.21 cm per year. Only 24 percent had a rate of expansion of 0.4 cm or more per year. Among the 176 patients who had an unruptured aneurysm at the time of the initial ultrasound study, the cumulative incidence of rupture was 6 percent after 5 years and 8 percent after 10 years. However, the risk of rupture over five years was 0 percent for the 130 patients with an aneurysm less than 5 cm in diameter and 25 percent for the 46 patients with an aneurysm 5 cm or more in diameter. All 16 patients who had ruptures had aneurysms that were 5 cm or more in diameter at the time of the rupture. These population-based data challenge the clinical perception that aneurysms typically expand at a rate of 0.4 to 0.5 cm per year. Our data also suggest that for aneurysms less than 5 cm in diameter the risk of rupture is considerably lower than has been reported previously. However, the risk of rupture is substantial for aneurysms 5 cm or more in diameter.     

3D Shape Analysis of Intracranial Aneurysms Using the Writhe Number as a Discriminant for Rupture

Intracranial aneurysms are polymorphic focal arterial dilations, which harbor a variable risk of rupture leading to high morbidity and mortality. Increased detection of incidental aneurysms by non-invasive imaging has created a need for rupture risk stratification tools, in addition to simple aneurysm size, to guide optimal treatment strategy. To this end, shape analysis has emerged as a possible differentiator of rupture likelihood. A novel set of morphological parameters based on the writhe number are introduced here to describe aneurysms and discriminate rupture status. Classification in 117 saccular aneurysms (52 ruptured and 65 unruptured) is based on statistical analysis of writhe number distribution on the aneurysm surface. Aneurysms are analyzed both in isolation and including a portion of their parent vessel. Sidewall and bifurcation aneurysm subtypes were found to be best described by disjoint sets of shape parameters, yielding a morphological dichotomy between the two aneurysm classes. Writhe number analysis results in 86.7% accuracy on sidewall (SW) aneurysms and 71.2% accuracy on bifurcation (BF) aneurysms. This represents a 12% accuracy increase for both subtypes compared to the performance of seven established 2D and 3D indexes. The results support the utility of writhe number aneurysm shape analysis, with potential clinical value in rupture risk stratification.     

Multiple intracranial aneurysms associated with branchio-oto-dysplasia

Branchio-oto-dysplasia is characterized by abnormalities of embryonic branchial arch system and deafness inherited as autosomal dominant with variable gene expression. We present a rare case of multiple intracranial aneurysms associated with branchio-oto-dysplasia. A 40-yr-old man with severe headache presented as spontaneous subarachnoid hemorrhage on brain computed tomographic scan. The patient also manifested clinical features of branchio-oto-dysplasia and right hemifacial hypoplasia. Carotid angiogram confirmed an aneurysm in the anterior communicating artery. Intraoperative findings demonstrated multiple aneurysms in the anterior communicating artery and in the left posterior communicating artery, which were clipped successfully. Postoperative course was uneventful. This condition has not been reported previously. We also reviewed literatures to discuss whether the intracranial aneurysm was as a coincidental finding or a part of this malformation.     

Experimental investigation of steady flow in rigid models of abdominal aortic aneurysms

Abdominal aortic aneurysms occur in as much as 2–3% of the population, and their rupture produces a mortality rate of 78–94% (1), causing 15,000 deaths per year in the U.S. alone. As an investigation into the mechanical factors that lead to aneurysm rupture, flow field measurements are presented for steady flow through a range of aneurysm sizes and Reynolds numbers. Seven rigid symmetric models of aneurysms were constructed with uniform lengths of 4d and diameters that ranged from 1.4 to 3.3d, whered is the inner diameter of the undilated entrance tube. Color Doppler flow imaging was used to visualize the flow fields, while laser Doppler velocimetry was used to quantify the flow field velocities and to determine critical Reynolds numbers for the onset of, and complete transition to, turbulent flow. Estimates of mean and peak wall shear stresses were derived from velocity measurements. Flow in these models varied from fully laminar to fully turbulent over the range of Reynolds numbers corresponding toin vivo flows. There was a large range over which the flow was intermittently turbulent. High wall shear occurred in the models when the flow was turbulent, suggesting that turbulence inin vivo aneurysms may contribute significantly to their risk of rupture.