Advances in Gastrointestinal Imaging Using Ultrasound (Part 1)

Contrast-enhanced ultrasound originated in echocardiography to delineate anatomical contours, but has seen significant advances in subsequent years. The development of dedicated contrast agents has enabled the creation of microbubbles with sizes approaching that of red blood cells and capable of stable passage through the pulmonary circulation.Microbubbles consist of an inert gas(sulfur hexafluoride) encapsulated in a phospholipid shell, which oscillate under low acoustic pressure (optimal acoustic pressure of 3-5 MHz) to produce specific contrast images. Due to their size, they reside at the capillary level. This is a truly intravascular contrast agent, with microbubbles being cleared via the lungs after rupture.

The advantages of contrast-enhanced ultrasound are primarily those of conventional ultrasound: ease of bedside use, lack of radiation, reproducibility, and low cost. Significant additional benefits include: microbubble contrast agents are nephrotoxic, there is a lower risk of adverse events compared to CT/magnetic resonance (MR) imaging, and in some applications, CEUS is considered equivalent to CT or MR assessment.Contrast-enhanced ultrasound also shares the same limitations as conventional ultrasoundIncreased body habitus, excessive bowel gas can impair visualization of structures, while surgical wounds/dressings reduce visualization.

Contrast-enhanced ultrasound is already widespread in radiology, particularly in imaging of the liver, kidneys, testes, vasculature, trauma, and pediatrics.In cardiology practice, contrast-enhanced ultrasound is used to delineate myocardial borders and identify perfusion abnormalities. Some of these applications and techniques may be usable in critical care across different specialties, while in those areas contrast-enhanced ultrasound is still in its early stages.

Contrast-enhanced ultrasound is commonly used for liver assessment based on enhancement and washout patterns to characterize focal liver lesions (Figure 1), but there are also a number of other Applications. The disruptive imaging mode of enhanced Doppler signal by microbubble disruption was initially used for post-liver transplant assessment of the portal vein, but these microbubbles can now be preserved at low acoustic pressure.Contrast-specific images produced by conventional low mechanical index imaging allow for angiographic assessment, showing the visceral arterial and venous systems for thrombi, stenosis, or pseudoaneurysms. Similarly, at the capillary level, infarcts can be observed and quantified with the use of quantitative software.

Another common Application of liver contrast-enhanced ultrasound is for post-trauma assessment.AlthoughCT remains irreplaceable for rapid panoramic assessment, contrast-enhanced ultrasound (CEUS) is able to provide accurate focused assessment of solid organs. The plane of vascular tears is clearly visible, and active bleeding or pseudoaneurysm formation can be detected. In renal trauma, it is important to note that damage to the collecting system cannot be reliably assessed, as the contrast agent is a truly intravascular agent with no urinary excretion phase, unlike iodine contrast agents.As with the biliary system, we can directly assess the renal collecting system, but this requires indwelling lines and diluted contrast agent.

Ultrasound can accurately delineate the five ultrasound layers of the bowel and assess bowel wall thickening and focal lesions (Figure 2). Because higher frequency probes are required for gastrointestinal imaging, higher doses of contrast-enhanced ultrasound are needed to capture optimal signal.Contrast-enhanced ultrasound is primarily used for vascular assessment of focal bowel abnormalities, particularly in Crohn’s disease, to differentiate active and fibrotic disease.

Contrast-enhanced ultrasound can be used to assess renal perfusion in patients in shock and sepsis, and has recently challenged the traditional understanding of hypoechoic areas in lung ultrasound as areas of infection, these are examples of further Applications of contrast-enhanced ultrasound in critical care.

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