Real-time multi-contrast magnetic particle imaging for the detection of gastrointestinal bleeding

Abstract

Gastrointestinal bleeding, as a potentially life-threatening condition, is typically diagnosed by radiation-based imaging modalities like computed tomography or more invasively catheter-based angiography. Endoscopy enables examination of the upper gastrointestinal tract and the colon but not of the entire small bowel. Magnetic Particle Imaging (MPI) enables non-invasive, volumetric imaging without ionizing radiation. The aim of this study was to evaluate the feasibility of detecting gastrointestinal bleeding by single- and multi-contrast MPI using human-sized organs. A 3D-printed small bowel phantom and porcine small bowel specimens were prepared with a defect within the bowel wall as the source of a bleeding. For multi-contrast MPI, the bowel lumen was filled with an intestinal tracer representing an orally administered tracer. MPI was performed to evaluate the fluid exchange between the vascular compartment of the bowel wall and the lumen while a blood pool tracer was applied. Leakage of the blood pool tracer was observed to the bowel lumen. Multi-contrast MPI enabled co-registration of both tracers at the same location within the bowel lumen indicating gastrointestinal bleeding. Single- and multi-contrast MPI are feasible to visualize gastrointestinal bleeding. Therefore, MPI might emerge as a useful tool for radiation-free detection of bleeding within the entire gastrointestinal tract.

Figure 1

Phantoms of a small bowel segment. (a) Sectional view of the phantom illustrates the design of the small bowel phantom in a sagittal cross section consisting of an inner bowel lumen and an outer vascular compartment. The vascular compartment was accessible via an inlet and an outlet to enable the connection to a circulatory system. (b) The transversal cross section of the control phantom shows a separation of the vascular and the luminal compartment. (c) Another phantom was created with a perforation (arrow) between both compartments representing the source of a bleeding. (d) Photograph of a phantom with separated compartments, dyed for illustration (blue = bowel lumen, red = vascular compartment). (e) The phantoms were placed within the MPI scanner and the vascular compartment was connected to a circulatory system.

Figure 2

Photograph of porcine ex vivo small bowel specimen. (a) Porcine small bowel was used for preparation of small bowel specimen of 3–4 cm (rectangle). (b) An adjacent mesenteric vessel was cannulated to enable injection of the blood pool tracer (arrow).

Figure 3

Multi-contrast MPI-derived signal intensities of different volume ratios of Perimag/LS-008 in equidistant 10% steps-contrast. MPI-derived signal intensity mean and variance of different volume ratios of Perimag/LS-008 in equidistant 10% steps obtained from 20 measurements (frames). Both curves are normalized with their maximum signal intensity and dashed lines indicate the expectation based on the linear MPI model.

Figure 4

MPI of small bowel phantoms. Images on the left show MPI reconstructions at 60 s. MPI enabled visualization of the enhancing bowel wall after injection of the blood pool tracer (red). In case of multi-contrast MPI, the luminal compartment is visualized by the intestinal tracer (blue). A representative region of interest (ROI) in the bowel wall (ROI 1) and the lumen (ROI 2) was identified manually. A signal increase of the blood pool tracer within the bowel lumen was only observed in case of a perforation between the vascular and the luminal compartment and indicated GI bleeding. For multi-contrast MPI, leakage of the blood pool tracer led to co-registration of both tracers at the same location within the lumen and indicated GI bleeding.

Figure 5

Post-processed single- and multi-contrast MPI of small bowel phantoms. Left: Digital subtraction of the reconstructed images at t₂ = 60 s and immediately after bowel wall enhancement at t₁ = 15 s. A GI bleeding was not detected for the intact bowel wall. The perforated bowel wall led to a residual signal after digital subtraction indicating GI bleeding. Right: Overlay of multi-contrast MPI-derived signal intensities above a defined threshold. The thresholds were visually determined to obtain the best delineation of the tracer mixture with the used setup and reconstruction parameters (threshold blood pool tracer: 28% of the maximum signal intensity; threshold intestinal tracer: 38% of the maximum signal intensity). Co-registration of both tracers at the same location was only observed in case of a perforated bowel wall and indicated GI bleeding.

Figure 6

MPI of ex vivo porcine small bowel specimen. These experiments were conducted in analogy to phantom experiments. Single contrast MPI enabled visualization of the enhancing bowel wall after bolus injection. Note the enhancement of the feeding vessel for this bowel segment (arrow). A signal increase of the blood pool tracer within the bowel lumen was only detected after incision in the mucous membrane. In multi-contrast MPI, co-registration of both tracers at the same location in the lumen represented the mixture of tracers and indicated GI bleeding.