Background

Oxygen transport in the body is primarily done via red blood cells and is an important aspect of perfusion. Red blood cells (RBCs) carry oxygen by binding dissolved oxygen in the blood to hemoglobin, the oxygen carrier protein. This mechanism greatly increases the blood's oxygen carrying capacity and enhances oxygen delivery to the tissues. Reduced blood flow and oxygen transport can lead to significant tissue damage because cells are unable to undergo cellular respiration. This state is known as ischemia. Many cell and tissue types are sensitive to ischemic conditions and display symptoms immediately; however, certain tissues and organ systems can tolerate ischemic conditions and lack prominent symptoms. While ischemia resistance can help with the survival of the tissue and the body as a whole, it can also lead to underdiagnosis of the issue. When ischemia is left untreated, it can lead to the affected tissues’ death and cause widespread issues throughout the body. 

The digestive tract is an organ system that can tolerate hypoxic environments and impaired bowel perfusion. Because of this, intestinal ischemia is often unnoticed until major damage has occurred. There are a multitude of causes for this condition, the most common being blood clots, hypovolemic shock, and improper cauterization or clamping of blood vessels in the abdomen. Some conditions that often occur with ischemia in this region are acute mesenteric ischemia (AMI), small bowel obstruction (SBO), gastrointestinal perforation (GP), and anastomotic leaks (AL). AMI, for instance, results from reduced blood flow in the mesenteric vessels, with a mortality rate ranging from 60-80% due to the difficulty of diagnosing it early [1].  SBO and GP lead to reduced blood flow due to physical obstructions or failures in the intestine. SBO is caused by twisting or obstruction of the bowel, while GP occurs when perforations in the bowel wall. Both SBO and GP can lead to ischemia and intestinal necrosis [2-3]. AL is a common complication following bowel anastomosis surgeries, and research has shown that improved monitoring of bowel perfusion during surgery significantly reduces the incidence of leaks [4].  

Computed Tomography (CT) Scan

CT scans, especially CT angiography, are commonly used in emergency settings to detect impaired bowel perfusion. They provide cross-sectional images of the bowel, mesentery, and blood vessels [5]. The scan can detect blockages, blood vessel narrowing, pneumatosis intestinalis (gas in the bowel wall), and free abdominal fluid. CT scans are fast and highly detailed, making it suitable for emergency diagnosis. However, it does utilize X-rays, increasing risk of cancer with repeated exposure. Additionally, the contrast agents used often limit accessibility for patients with allergies to them. These patients are often accommodated by performing the scan without contrast agents, which leads to reduced scan detail.

Magnetic Resonance Imaging (MRI)

MRIs, primarily MR angiography (MRA) commonly used for assessment of perfusion, provides high-resolution images [6]. While the images are produced without ionizing radiation, MRIs often require contrast agents similar to CT scans. MRI provides high soft tissue contrast, helping in the evaluation of bowel wall changes and detecting inflammation, necrosis, or edema. However, it cannot be used on patients with metal implants (like pacemakers), take much longer to do than CT scans, are more expensive to use, and are less available than CT scans. 

Indocyanine Green (ICG) Fluorescence Imaging

ICG fluorescence imaging is an advanced technique used intraoperatively to evaluate real-time tissue perfusion. ICG dye non-covalently binds to human serum albumin (HSA) and emits fluorescence when exposed to near-infrared light [7]. This allows surgeons to visualize blood flow and determine areas of impaired perfusion in real-time. ICG fluorescence imaging can only be used in areas where the tissues are superficial or directly viewed, meaning it cannot be done preoperatively or non-invasively [8].

Hyperspectral Imaging (HSI) 

A promising technique to assess tissue perfusion is hyperspectral imaging (HSI). HSI utilizes a specialized camera that captures a wide array of light wavelengths at every point in the field of view. These wavelengths are then filtered out by a computer program to view desired light and subjects. 

HSI analysis can monitor perfusion in tissues with blood vessels near the surface [9]. The HSI camera captures infrared light reflected from hemoglobin in the blood vessels, and then narrows down onto areas with high levels of this wavelength. These areas are then determined to be blood vessels, and are then continuously monitored for their oxygen saturation. This technology can be adapted to systems with blood vessels close to the surface.  HSI analysis is already being utilized to monitor perfusion in surgical settings. One study used HSI as a tool to monitor the circulation of upper abdomen organs during  pancreatoduodenectomies [10]. In two of the surgeries, the HSI device discovered reduced blood flow and oxygen content to the liver 15 minutes after arterial reconstruction was done, allowing the surgeons to alter their procedure and restore oxygenation to the liver. These surgeries provide evidence that HSI analysis can be a crucial tool to prevent adverse post-operative conditions in patients and can be used on organ systems outside of skin. 

1. Monita MM, Gonzalez L. Acute Mesenteric Ischemia [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2021. Available from: https://www.ncbi.nlm.nih.gov/books/NBK431068/

2. Hafner J, Tuma F, Marar O. Intestinal Perforation [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2020. Available from:

https://www.ncbi.nlm.nih.gov/books/NBK538191/ 

3. Schick MA, Meseeha M, Kashyap S. Small bowel obstruction [Internet]. National Library of Medicine. StatPearls Publishing; 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK448079/ 

4. Clancy NT, Soares AS, Bano S, Lovat LB, Chand M, Stoyanov D. Intraoperative colon perfusion assessment using multispectral imaging. Biomedical Optics Express. 2021 Nov 12;12(12):7556–6. 

5. Computed Tomography Angiography (CTA) [Internet]. Johns Hopkins Medicine. 2019. Available from: https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/computed-tomography-angiography-cta 

6. Gordon Y, Partovi S, Müller-Eschner M, Amarteifio E, Bäuerle T, Weber MA, et al. Dynamic contrast-enhanced magnetic resonance imaging: fundamentals and application to the evaluation of the peripheral perfusion. Cardiovascular Diagnosis and Therapy [Internet]. 2014 Apr 1;4(2):147–64. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3996240/ 

7. Jang HJ, Song MG, Park CR, Youn H, Lee YS, Cheon GJ, et al. Imaging of Indocyanine Green-Human Serum Albumin (ICG-HSA) Complex in Secreted Protein Acidic and Rich in Cysteine (SPARC)-Expressing Glioblastoma. International Journal of Molecular Sciences [Internet]. 2023 Jan 3;24(1):850. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC9821702/ 

8. Diana M, Noll E, Diemunsch P, Dallemagne B, Benahmed MA, Agnus V, et al. Enhanced-reality video fluorescence: a real-time assessment of intestinal viability. Annals of Surgery [Internet]. 2014 Apr 1;259(4):700–7. Available from: https://pubmed.ncbi.nlm.nih.gov/23532109/ 

9. Yusef Moulla, Dorina Christin Buchloh, Hannes Köhler, Rademacher S, Denecke T, Meyer HJ, et al. Hyperspectral Imaging (HSI)—A New Tool to Estimate the Perfusion of Upper Abdominal Organs during Pancreatoduodenectomy. Cancers. 2021 Jun 7;13(11):2846–6.

10. Alexander A. Stratonnikov, Victor B. Loschenov, "Evaluation of blood oxygen saturation in vivo from diffuse reflectance spectra," J. Biomed. Opt. 6(4) (1 October 2001) https://doi.org/10.1117/1.1411979 

Credit: Vaughan Altmann