Magnetic Particle Imaging for Highly Sensitive, Quantitative, and Safe in Vivo Gut Bleed Detection in a Murine Model

Abstract

Gastrointestinal (GI) bleeding causes more than 300 000 hospitalizations per year in the United States. Imaging plays a crucial role in accurately locating the source of the bleed for timely intervention. Magnetic particle imaging (MPI) is an emerging clinically translatable imaging modality that images superparamagnetic iron-oxide (SPIO) tracers with extraordinary contrast and sensitivity. This linearly quantitative modality has zero background tissue signal and zero signal depth attenuation. MPI is also safe: there is zero ionizing radiation exposure to the patient and clinically approved tracers can be used with MPI. In this study, we demonstrate the use of MPI along with long-circulating, PEG-stabilized SPIOs for rapid in vivo detection and quantification of GI bleed. A mouse model genetically predisposed to GI polyp development (Apc^Min/+) was used for this study, and heparin was used as an anticoagulant to induce acute GI bleeding. We then injected MPI-tailored, long-circulating SPIOs through the tail vein, and tracked the tracer biodistribution over time using our custom-built high resolution field-free line (FFL) MPI scanner. Dynamic MPI projection images captured tracer accumulation in the lower GI tract with excellent contrast. Quantitative analysis of the MPI images show that the mice experienced GI bleed rates between 1 and 5 μL/min. Although there are currently no human scale MPI systems, and MPI-tailored SPIOs need to undergo further development and evaluation, clinical translation of the technique is achievable. The robust contrast, sensitivity, safety, ability to image anywhere in the body, along with long-circulating SPIOs lends MPI outstanding promise as a clinical diagnostic tool for GI bleeding.

Keywords: blood pool contrast; gastrointestinal bleeding; magnetic particle imaging; medical imaging; superparamagnetic iron-oxide nanoparticles.

Figure 1. Custom-built vertical bore 6.3 T/m Field-Free Line scanner

(a) The FFL is rastered across the imaging field of view, and 2D projection images are acquired over time. (b) Top: MPI images of a concentration phantom (5 concentrations ranging from 0.0625 to 2 mg Fe/mL, each 10 μL in thin tubing spanning ~1 cm) at two different projection angles. The 0° projection resulted in a line image, while the 90° projection resulted in a point image. Bottom: MPI is linearly quantitative (r² = 0.99), and the total detected signal was consistent between projection angles (red, 90°, blue, 0°).

Figure 2. Long circulating MPI-tailored SPIO tracer

(a) Bright field TEM image of uncoated iron oxide cores of LS-017 and (inset) Selected Area Electron Diffraction pattern showing crystal morphology and characteristic spinel diffraction rings. (b) Histogram of particle core size from bright field TEM (mean = 28.11, σ = 0.08). (c) Magnetization curves measured by VSM at room temperature. Magnetic size was 28.7 nm (σ = 0.07). This figure was adapted from Nano Lett. 2017, 17, 1648–1654.

Figure 3. Dynamic projection MPI captures whole body tracer bio-distribution

MPI co-registered with projection X-ray for anatomical reference. (a) Representative images of an Apc^Min/+ mouse over time. MPI clearly captures dynamics of tracer extravasation into the gut. (b) Representative images of a Wild-Type mouse over time. No tracer extravasation into the gut is seen.

Figure 4. MPI Subtraction Images

Each MPI image was further processed by digital subtraction. The MPI image at the first time point was subtracted from all remaining images in the time course to capture tracer accumulation. (a) Representative subtracted images of an Apc^Min/+ mouse over time. The GI bleed is visualized with extraordinary contrast. (b) Representative subtracted images of a wild-type mouse over time. The tracer was predominantly in the blood pool throughout the study, hence not observed after subtraction. Additionally, no GI bleed is observed.

Figure 5. Gut Bleed Flow Quantitation

(a) Schematic of two compartment model used to quantify the rate of bleed into the gut. (b) Bar graph of flow rate from both groups. The flow rates were significantly different between the Apc^Min/+ and wild-type groups for both fitting methods: NLLS method (p = 0.014) and patlak LLS method (p = 0.007). Representative NLLS fit results for (c) Apc^Min/+ mice and (d) wild-type mice. Axes: iron in blood on left and iron in gut lumen on right. Representative LLS Patlak fit results for (e) Apc^Min/+ mice and (f) wild-type mice.

Figure 6. Ex Vivo MPI Scans

(a) Representative photo and corresponding MPI image of Apc^Min/+ mouse GI tract after dynamic MPI study. Tracer accumulation is seen in the cecum and small intestine. (b) Ex vivo and in vivo MPI signal from the gut were compared for all mice. Near unity slope of 0.98 was obtained for the linearity fit (r² = 0.98). H&E stained histological section of the intestinal lumen: (c) Control mouse with well-ordered mucosal and serosal layers (20× magnification, scale bar 50 μm) and (d) Apc^Min/+ mouse with a polyp along the intestinal lining and RBCs in the lumen (10× magnification, scale bar 100 μm).

Figure 7. Experimental work-flow

C57BL/6-Apc^Min/+ mice (n = 5) and C57BL/6-WT mice (n = 3) were used for this study. Polyps spontaneously developed in the Apc^Min/+ mice with age, causing them to bleed and become anemic. The hematocrit levels were monitored weekly throughout the experiment. At age 11–13 weeks, anti-coagulant (Heparin) and MPI tracer (LS-017) were injected through the tail vein in both Apc^Min/+ and WT mice, followed by dynamic 2D projection MPI scans (21 projections over 130 minutes).