Date Published: October 5, 2017
Publisher: Springer US
Author(s): Vasileios Rafailidis, Cheng Fang, Gibran T. Yusuf, Dean Y. Huang, Paul S. Sidhu.
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.
CEUS is performed with the intravenous administration of a bolus dose of the UCA, and essentially always performed after a complete unenhanced ultrasonographic examination. This allows the examiner to identify the area of interest, establish an initial opinion, ascertain the viability of a subsequent CEUS examination and plan the procedure to maximize the diagnostic outcome. Once the unenhanced ultrasonographic protocol is complete, having appreciated the gray-scale, color, power Doppler, and spectral analysis findings, an intravenous catheter can be placed in the antecubital fossa. It is best to insert the intravenous catheter following the baseline US to avoid unnecessary cannulation if the CEUS examination is not deemed useful. In general, the amount of UCA administered varies depending on the ultrasound machine’s sensitivity and the product used. SonoVue™ (Bracco SpA, Milan, Italy) is the most widely used contrast agent in Europe and consists of microbubbles containing an inert gas (sulfur hexafluoride) encapsulated by a phospholipid shell, marketed as Lumason™ (Bracco SpA, Milan, Italy) in the United States. A dose of 2.4 mL of Lumason™/SonoVue™ per injection is considered adequate for the liver and other abdominal vascular procedures. A second dose of 2.4 mL can be administered if needed. UCA are strict intravascular agents, large enough (10 μm) to preclude passage through the vascular endothelium, but small enough to circulate through small capillaries. Crucially, the metabolism of UCA renders them independent of renal excretion, the phospholipid shell is metabolized by the liver and the contained inert gas is exhaled by the lungs. As a result, CEUS can be safely performed in patients with renal impairment. In order to achieve optimal visualization of the UCA, a contrast-specific ultrasonographic technique should be applied. Pulse inversion and amplitude-modulation techniques which in general suppress echogenic signals originating from static tissues while visualizing echogenic signals produced by oscillating microbubbles are used. This results in the optimal echogenicity distinction between UCA and static tissues and offers the best spatial and temporal resolution. Two valuable techniques in vascular CEUS include the replenishment mode after a high-Mechanical Index pulse and the Temporal Maximum Intensity Projection (MIP) mode. In the first technique, a high-MI ultrasound pulse is used to disrupt all the microbubbles lying within the imaging field with replenishment allowing observation of the enhancement pattern of structures. In the second technique, the ultrasound device aggregates bright echoes of the UCA and creates cumulative images which illustrate the vascular pattern or architecture of structures under investigation [1, 3, 4, 6].
The introduction of UCA has significantly expanded the role of US in the investigation of abdominal vascular diseases. CEUS is superior to conventional US techniques in term of tissue contrast, spatial, and temporal resolution and its dynamic and real-time nature in assessment of tissue perfusion and vascular lumen opacification. Experience has shown that CEUS plays a key role in certain clinical scenarios such as evaluation of abdominal trauma, diagnosis of organ ischemia, imaging surveillance of post-EVAR aorta or the differential diagnosis of malignant vs. benign portal vein thrombosis in patients with liver cirrhosis and hepatocellular carcinoma. CEUS is also useful in assisting ultrasonographic evaluation of other blood vessels, although the widespread availability of CTA and MRA has limited its role in the renal arteries and mesenteric arteries.