The gastrointestinal tract essentially fulfills two major functions: digestion and absorption of food, and physical and immunological barrier against environmental influences. These basic functions are critically dependent on splanchnic blood flow at both the macrovascular and microvascular levels. In particular, advances in vascular biology have revealed a central and intricate role of blood circulation in inflammatory bowel disease (IBD).
Until now, changes in blood flow have been used as surrogate markers for altered inflammatory activity in the intestine, e.g., by Doppler sonographic detection. Multispectral Optoacoustic Tomograph (MSOT) allows for non-invasive, quantitative imaging of the molecular composition of target tissues. In MSOT, similar to conventional sonography, a transducer is placed on the skin but energy is delivered to the tissue by means of laser light in the near infrared spectrum instead of ultrasound waves. This leads to a constant alternation of minimal expansions and contractions (thermoelastic expansion) of individual tissue components or molecules. The resulting ultrasound waves can subsequently be detected by the same examination unit. Previous studies have shown that quantitative determination of hemoglobin can provide information on blood flow and inflammatory activity in the intestine of adult patients with Crohn's disease. In particular, the distinction between the activity levels of the disease (remission/low/moderate/high) is promising for saving invasive measures in the future when evaluating the progression of the disease.
In addition to inflammatory processes, food intake also causes fluctuations in regional blood flow in the gastrointestinal tract. This manifests as postprandial hyperemia, which occurs sequentially in the different sections of the gastrointestinal tract from oral to aboral.
The time course of postprandial hyperemia in the different sections of the gastrointestinal tract has been scientifically investigated in many studies. While an increase in blood flow in the stomach and duodenum can be detected after 30-60 minutes, it takes much longer for postprandial hyperemia to be detected in the areas used to measure inflammatory activity with MSOT in IBD such as the terminal ileum and sigmoid colon. An increase in blood flow in the ileum can be measured after 120 minutes at the earliest, and the arrival of chyme in the colon and the accompanying local increase in blood flow occur after approximately 240-300 minutes.
It is unclear whether this postprandial hyperemia can lead to a change and potential increase in the optoacoustic hemoglobin signal of the intestinal wall, resulting in falsely high MSOT signals in the determination of inflammatory activity. This study investigates influences of a standardized dietary on the MSOT signal of the intestinal wall using a longitudinal design. Optoacoustic signals will be compared between subjects in fasting and postprandial states. Because the postprandial increase in intestinal blood flow is predominantly a result of the local presence of chyme in the intestine, a simultaneous determination of intestinal transit of chyme during MSOT measurement would be helpful to validate whether postprandial changes in MSOT signals are attributable to hyperemia in the corresponding bowel segment. Besides imaging of hemoglobin, MSOT enables the detection of exogenous chromophores (i.e. dyes). In this study, the oral administration of the nonabsorbable dye indocyanine green (ICG) will be used for noninvasive identification of the chyme. The combination of exogenous and endogenous chromophores thus allows accurate co-localization and registration of intestinal wall blood flow patterns and chyme transit. This information enables accurate anatomical mapping of interfering influences on the determination of hemoglobin using MSOT.
