Contrast enhanced duplex ultrasound imaging of the mesenteric arteries

The radiologist's guide to duplex ultrasound assessment of chronic mesenteric ischemia

OBJECTIVE

This article reviews the relevant anatomy and physiology of the mesenteric vasculature, familiarizes the radiologist with the accepted diagnostic criteria for mesenteric artery stenosis and its role in the diagnosis of chronic mesenteric ischemia, describes Doppler imaging techniques, and provides protocols for the assessment and surveillance of the mesenteric vasculature before and after revascularization. It also discusses expected changes following revascularization and reviews common post-procedural complications.

RESULTS

Duplex sonography plays an important role in the diagnosis and management of chronic mesenteric ischemia (CMI). Establishing a successful diagnosis is dependent upon knowledge of mesenteric arterial anatomy and physiology as well as sufficient expertise in image optimization and scanning techniques. Although there has been a trend toward utilization of other noninvasive [computed tomographic angiography (CTA), magnetic resonance angiography (MRA), and invasive (digital subtraction angiography (DSA)] imaging modalities for assessment of the mesenteric vasculature, a new era of "imaging wisely" raises legitimate concerns about the effects of ionizing radiation as well as potential effects of CT and MR contrast agents. These concerns are obviated by the use of ultrasound, and recently developed techniques, such as contrast-enhanced ultrasound and vascular applications focused on the evaluation of slow flow, have revealed the vast potential of vascular ultrasound in the evaluation of chronic mesenteric ischemia.

CONCLUSION

Duplex sonography is a cost-effective and powerful tool that can be utilized for the accurate assessment of mesenteric vascular pathology, specifically mesenteric arterial stenosis, and for the evaluation of mesenteric arterial system post revascularization.

Visceral Artery Aneurysms: Diagnosis, Surveillance, and Treatment

Visceral artery aneurysms (VAAs) are a rare, but serious clinical entity as rupture confers a high rate of mortality. Data regarding the prevalence, treatment, and prognosis of VAAs is largely from case series, as true randomized trials are lacking. The incidence of VAAs has risen over the decades with advances in imaging technology, availability, and utilization. Even in the presence of symptoms, the prompt diagnosis of VAAs may be challenging as symptoms are often nonspecific and varied. Although there are no definitive treatment guidelines, asymptomatic VAAs require treatment in the following scenarios: when the aneurysm diameter is greater than 2 cm, when identified during pregnancy, when multiple aneurysms are present, and in the case of hepatic transplant. Similar to therapeutic trends in other vascular beds, advances in endovascular devices and techniques have driven an "endovascular first" approach for the treatment of VAAs. However, although the success rates of endovascular repair are impressive, surgical intervention is still necessary in treating centers. This paper reviews the pathophysiology, clinical features, anatomic characteristics, and management strategies required for the effective diagnosis and treatment of VAAs.

Contrast-enhanced ultrasound (CEUS) of the abdominal vasculature

Vascular diseases account for a significant proportion of abdominal pathology and represent a common referral source for abdominal ultrasonographic examinations. B-mode, color Doppler, and spectral Doppler analyses are well-established in the evaluation of abdominal blood vessels although they may occasionally be limited by lower sensitivity for slow flow visualization or the deeper location of abdominal vascular structures. The introduction of microbubbles as ultrasonographic contrast agents has rendered contrast-enhanced ultrasound (CEUS), a valuable complementary ultrasonographic technique, which is capable of addressing clinically significant problems and guiding patient management. The purpose of this pictorial review is to analyze the use of CEUS in the evaluation of abdominal vascular pathology and illustrate such applications by presenting representative images. Pathology discussed includes abdominal aortic aneurysm, post-endovascular treatment aorta, portal vein thrombosis, abdominal vascular trauma, and organ transplantation along with its complications.

Evolving clinical applications of contrast-enhanced ultrasound (CEUS) in the abdominal aorta

Ultrasound (US) represents the initial modality in the workup of abdominal aortic pathology based on the plethora of advantages including widespread availability, low cost, safety profile and repeatability. However, US has inherent limitations including limited spatial information of pathologic processes to neighboring structures, lower sensitivity to slow blood flow and aortic luminal irregularities. For evaluation of aortic pathology angiography has long been considered the gold standard. Non-invasive cross-sectional imaging techniques like computed tomography angiography (CTA) and magnetic resonance angiography (MRA) have gradually replaced interventional angiography for the evaluation of aorta, currently being regarded as the diagnostic imaging modalities of choice for diagnosis of virtually every aortic disease. Interventional angiography is currently primarily performed for treatment purposes of aortic pathology. The introduction of microbubbles as ultrasonographic contrast agents has rendered contrast-enhanced ultrasound (CEUS) an evolving valuable complementary technique with markedly increased diagnostic accuracy for certain aortic applications. CEUS is characterized by the potential to be performed in patients with impaired renal function. Due to its superior spatial and temporal resolution, ability for prolonged scanning and dynamic and real-time imaging, it provides clinically significant additional information compared to the standard Duplex US. The purpose of this paper is to discuss the currently available literature regarding abdominal aortic applications of CEUS, briefly elaborate on CEUS technique and safety and present cases in order to illustrate the added value in aortic pathologies. Conditions discussed include abdominal aortic aneurysm (AAA), aneurysm rupture, aneurysm surveillance after endovascular repair, dissection and aortitis.

Pharmacological modulation of gut mucosal and large vessel blood flow

BACKGROUND

Constipation, diminished gut blood flow, ischaemic colitis and drug therapy may be associated.

AIM

To study the effect of constipating medication on, and the regulation of, gut blood flow.

METHODS

24 healthy females (mean age 30) received, in a double-blind, three-way crossover study: (i) placebo, (ii) ipratropium 40 microg by inhalation (positive control known to reduce rectal mucosal blood flow) and (iii) oral loperamide 4 mg. Mucosal blood flow was measured at the splenic flexure and rectum using laser Doppler flowmetry. Blood flow in the superior and inferior mesenteric arteries was measured by trans-abdominal Doppler ultrasound.

RESULTS

Ipratropium decreased rectal mucosal blood flow by 16% (P=0.009) and splenic flexure mucosal blood flow by 8% (P=0.075). Loperamide caused no change in rectal (P=0.40) or splenic flexure mucosal blood flow (P=0.73). Neither treatment changed superior or inferior mesenteric artery blood flow. Splenic flexure mucosal blood flow showed a positive correlation with rectal mucosal blood flow (r=0.69; P<0.0001).

CONCLUSIONS

Vasoactive agents may reduce gut mucosal blood flow in the absence of reduced large vessel flow. Constipating drugs do not necessarily reduce gut blood flow. Rectal mucosal blood flow correlates with splenic flexure mucosal flow, and potentially may be used as a more convenient surrogate for studying splenic flexure blood flow.

Duplex imaging of the renal arteries with contrast enhancement

Duplex ultrasound evaluation of the renal arteries is a technically challenging procedure. Its accuracy is significantly influenced by operator expertise and patient factors, such as overlying bowel gas and obesity. Intravenous microbubble contrast agents enhance vascular reflective acoustic signals and may improve ultrasound diagnostic accuracy. The clinical usefulness of such a contrast agent in the renal vasculature was examined prospectively. A total of 22 patients (16 males and 6 females) with mean age of 63 +/-3 years with suspected abdominal vascular disease were studied prospectively. A complete color flow duplex imaging study of the renal vasculature was performed. This was then followed by an identical examination during which an ultrasound contrast agent (Definity, DuPont Pharmaceutical) was infused intravenously at a rate of 2 to 4 mL/min. In addition to imaging of the vessels, the peak systolic velocity and Doppler waveforms of the aorta and renal arteries were examined. These results were independently compared to results with contrast angiography. A mean of 67 mL of contrast was used per patient. Of the total of 43 renal arteries examined, the accuracy for the detection of occlusions was 75% (3 of 4) for both standard and contrast-enhanced duplex ultrasound. The accuracy for the detection of hemodynamically significant stenosis was 50% (6 of 12) for standard and 75% (9 of 12) for contrast-enhanced duplex ultrasound. Visualization of normal or minimally diseased arteries was 94% (30 of 32) for standard and 97% (31 of 32) for contrast-enhanced ultrasound. Although overall accuracy was not enhanced by the infusion of ultrasound contrast, 5 of 7 arteries not visualized by color flow duplex were detected following the infusion of contrast agent, resulting in an additional 10% (5 of 48) of vessels visualized. Peak systolic velocities were increased by an average of 10% in normal or minimally diseased vessels and 12% in stenotic vessels following contrast administration but these differences were not statistically significant. Contrast-enhanced duplex imaging of the renal arteries is safe but not routinely required when performed by an experienced sonographer. However, it may increase visualization and accuracy in patients with stenoses or when the vessels are not initially visualized. Although increased velocities are seen when contrast agent is used, this does not appear to necessitate different Doppler criteria at this time.