Non-invasive Imaging of GI Inflammation Using Microbubble Contrast Enhanced Ultrasonography

GI inflammation may indicate a number of complications or diseases, one of which is inflammatory bowel disease (IBD), now the second most prevalent inflammatory disorder in the world. Symptoms of the disease include severe abdominal pain, bloody diarrhea, persistent fever, weight loss, and significant malnutrition. There is also an increased risk of colon cancer. If left untreated, the disease is debilitating. Prompt intervention may reduce the amount of immunosuppressive therapy that is required to control the disease, but by the time the patient becomes symptomatic, the inflammatory response is difficult to suppress and much of the damage has already been done. It is therefore important to closely monitor patients with IBD. The location, extent, and severity of the inflammation are of primary consideration for correct diagnosis and treatment. However, no inexpensive and non-invasive procedure exists in protocol for the assessment of these factors of IBD. A non-invasive diagnostic that can detect the onset of inflammation and measure the extent of inflammatory involvement would be a valuable tool for the evaluation of patients with IBD.

Endoscopy, barium contrast X-ray studies, computed tomography (CT), magnetic resonance imaging (MRI), and transabdominal ultrasound (US) are currently the most common procedures used by gastroenterologists. The preferred manner of investigating GI inflammation includes endoscopy with biopsy, as only endoscopy can confirm the presence of inflammation. However, this procedure is highly invasive and limited to areas accessible to the endoscope. There are also limited but real risks associated with endoscopy. In addition, the cost of such a procedure may be prohibitive, or a qualified professional inaccessible, for some patients. Barium contrast X-ray studies remain the best way to visualize stricture and fistulae in the small intestine, but do not provide insight into the degree and extent of active inflammation. Repeated X-rays in chronic and younger patients also contribute to risk of irradiation. CT and MRI are the gold-standard for imaging extra-intestinal inflammatory disease, but fail in their ability to identify active inflammation. There have recently been many studies attempting to improve these means of assessing GI inflammation.

Transabdominal US presents a non-invasive means of imaging internal organs that imposes no significant health risks or undue discomfort upon the patient. The use of abdominal US for the evaluation of IBD was implemented as early as 1979, where wall thickening of the terminal ileum and cecum, with accompanying inflammatory changes in the mesentery, yielded recognizable patterns in both longitudinal and transverse images.11 These initial ultrasonographic images lacked sufficient resolution to provide a sensitive measure of disease activity, but technological advances in high frequency US have greatly improved resolution over the past twenty years. Still, the location and chronicity of certain conditions may decrease the efficacy of this imaging technique, making endoscopy the preferred method of investigation of GI problems. At present, there are several research groups actively investigating the application of US for the management of IBD. The combined results of these studies, in addition to the relatively wide availability, low cost, and easy use of US equipment, support the rationale for developing US into a useful tool for the evaluation of IBD.

Contrast-enhanced ultrasonography (CEU) is the main strategy for improving US quality. One contrast agent that has been studied in the imaging of inflammation, but which has not yet been human-tested for improvement of US quality in inflammation due to IBD, is microbubbles (MB). MB contrast agents are FDA-approved, and are becoming a common clinical tool for the enhancement of US imaging of cardiovascular hemodynamics around the world. Unlike tissue signal, which is produced by US reflection, the strong signal generated by MB is produced by radial oscillation of the MB in the acoustic field. Current MB used for perfusion imaging have lipid or albumin shells and contain high-molecular weight gases (perfluorocarbons, sulfur hexafluoride), which contribute to their high intravascular stability by preventing outward diffusion of gas. MB are generally 2-4µm in size - smaller than average capillary dimension - and passes unimpeded through the microcirculation. They are also hemodynamically inert, and behave similar to red blood cells in vivo. In animal models the acoustic properties of activated Definity® (Perflutren Lipid Microsphere) injectable suspension, were established at or below a mechanical index of 0.7 (1.8 MHz frequency). In clinical trials, the majority of the patients were imaged at or below a mechanical index of .08.

There are two ways that microbubbles might contribute to a strong signal in areas of inflammation in the small intestine or colon. The first is directly through neoangiogenesis and the increase in blood flow to the site. Defined as the growth of new blood vessels, neoangiogenesis is important to the pathogenesis of both Crohn's disease and ulcerative colitis. An expanded microvascular bed in the mucosa and submucosa of IBD patients with active inflammation has been confirmed, and is consistent with the high levels of integrins characteristic for proliferating endothelium (e.g. IL-8, bFGF, and VEGF) found in the microvessels of tissue affected by IBD. The hope is that the increased blood flow in actively inflamed IBD will be correlated with a stronger US signal from the increased concentration of MB flowing through the site.

The second way microbubble CEU may be effective at identifying active inflammation is an indirect effect of new microvasculature. Neoangiogenesis is thought to contribute to pathogenesis by fostering the recruitment and activation of an increased number of leukocyte into the inflamed mucosa. It has been observed that both albumin and lipid shell MB used for echocardiographic studies are phagocytosed intact by activated leukocytes, some of which are adherent to the inflamed endothelium of small intestine or colon. These phagocytosed MB retain a percentage of their acoustic properties, enabling US to image inflammation non-invasively in an in vivo setting. Incorporation of specific lipid moieties into the microbubble shell increases retention and phagocytosis by activated leukocytes.

An investigation of the efficacy of microbubble contrast agents in imaging GI inflammation is the first step towards such targeted imaging and tissue-targeted therapy.