In vivo mapping of vascular inflammation using multimodal imaging - PubMed (original) (raw)
In vivo mapping of vascular inflammation using multimodal imaging
Benjamin R Jarrett et al. PLoS One. 2010.
Abstract
Background: Plaque vulnerability to rupture has emerged as a critical correlate to risk of adverse coronary events but there is as yet no clinical method to assess plaque stability in vivo. In the search to identify biomarkers of vulnerable plaques an association has been found between macrophages and plaque stability--the density and pattern of macrophage localization in lesions is indicative of probability to rupture. In very unstable plaques, macrophages are found in high densities and concentrated in the plaque shoulders. Therefore, the ability to map macrophages in plaques could allow noninvasive assessment of plaque stability. We use a multimodality imaging approach to noninvasively map the distribution of macrophages in vivo. The use of multiple modalities allows us to combine the complementary strengths of each modality to better visualize features of interest. Our combined use of Positron Emission Tomography and Magnetic Resonance Imaging (PET/MRI) allows high sensitivity PET screening to identify putative lesions in a whole body view, and high resolution MRI for detailed mapping of biomarker expression in the lesions.
Methodology/principal findings: Macromolecular and nanoparticle contrast agents targeted to macrophages were developed and tested in three different mouse and rat models of atherosclerosis in which inflamed vascular plaques form spontaneously and/or are induced by injury. For multimodal detection, the probes were designed to contain gadolinium (T1 MRI) or iron oxide (T2 MRI), and Cu-64 (PET). PET imaging was utilized to identify regions of macrophage accumulation; these regions were further probed by MRI to visualize macrophage distribution at high resolution. In both PET and MR images the probes enhanced contrast at sites of vascular inflammation, but not in normal vessel walls. MRI was able to identify discrete sites of inflammation that were blurred together at the low resolution of PET. Macrophage content in the lesions was confirmed by histology.
Conclusions/significance: The multimodal imaging approach allowed high-sensitivity and high-resolution mapping of biomarker distribution and may lead to a clinical method to predict plaque probability to rupture.
Conflict of interest statement
Competing Interests: The authors have declared that no competing interests exist.
Figures
Figure 1. Multimodal nanoparticle probes are taken up by macrophages in the aortic valve of ApoE−/− mice with vascular inflammation induced by ligation.
(A) The aortic valve before administration of probes. Scale bar = 2.5 mm. (B) Position of the aortic valve is indicated by the dotted purple lines. The three leaflets of the valve are clearly evident. (C) The MR signal intensity in and around the aortic valve (dotted outline) decreases after introduction of the multimodal PET/T2-MRI probes, shown 24 h post injection. (D) Coregistration of the PET data with the MRI data illustrates that the PET signal correlates with a broad region including and around the aortic valve. In A–D to determine the grayscale intensity of each pixel in the image and the color intensity of the PET signal, we use bilinear interpolation between the four nearest voxels in the slice. This resampling provides a smoother reconstruction without excessive blurring or loss of data. (E) 3D reconstruction of MRI and PET data shows that the PET signal intensity is a diffuse cloud (orange-yellow) broadly localized around the aortic arch and carotid arteries. This 3D image was generated by combining three different rendering models; details provided in Materials and Methods S1. F) Immunohistochemistry demonstrates that the nanoparticulate probes are localized to macrophages in the aortic arch. Macrophage presence in the aortic valve leaflets is confirmed by positive HRP reaction with DAB, macrophages = brown stain (10X magnification). The region in the red box is shown at higher magnification in the inset image (60X) and demonstrates that the iron oxide particles, stained blue, overlap with regions staining for macrophages (brown).
Figure 2. Multimodal macromolecular probes accumulate in the injured vessel of the rat carotid clamp injury model.
(A) Coronal view PET image of the rat thorax shows a region of high signal intensity indicating probe accumulation in macrophages, however it is difficult to interpret the tissue of origin for the signal without anatomical information. Scale bar = 20 mm (B) “Zoomed in” MR images for the volume indicated by the boxed area in A reveal that the PET signal correlates to the carotid artery on the left. Scale bar = 5 mm. This vessel also shows elevated MR contrast and thicker vessel walls compared to the vessel on the right, (C) 3D reconstruction of MRI and PET data from the carotids is shown protruding from a plane in the MR image. This view illustrates that the probes are localized to the vessel wall of the injured carotid artery (purple), MR signal intensities elevated relative to vessel background signal are rendered in green, and PET intensities are in orange. The contralateral uninjured vessel is yellow. Images rendered by segmentation. MRI high contrast region, in green donut shape, at white arrow is 0.6×1.8 mm (volume is 0.4×2.6×0.6 mm3; volume includes regions out of view in this 2D image). MRI high contrast signal at yellow arrow, in green horseshoe shape, is 0.8×1.2 mm, each arm is ∼0.2 mm wide (volume is 1×0.22×3.8 mm3). PET volume (orange) is 4.5×8.3×4.0 mm3. Scale bar = 5 mm.
Figure 3. Multimodal macromolecular probes localize to the injured vessel in the rat copper cuff model.
(A) Overlay of PET with MRI for nearly whole body 24 h post injection of the PET/T1-MRI probes (head at top, out of field of view). Probe accumulation as monitored by PET is found in a diffuse cloud (red-orange) around the vessel that was clamped by the copper cuff, left (rendered in black and white). The PET and MR signal from vessels was rendered in color using the assignments shown in the scale bar, and MR signal from rest of the body is given in black-and-white for anatomical reference. The plane through the image indicates the position of the image slice given in (B), a view zoomed in around the carotid artery at the superior end of the copper cuff. This coregistered MRI/PET image shows diffuse cloud of PET signal in the region around the injured vessel (clavicle is the dark region indicated by the white arrow) and region of higher MR intensity on the right side of the vessel. scale bar = 2.5 mm. (C) The MR image from the same plane clearly shows that the vessel (red arrow) has elevated MR contrast in the walls of the vessel (yellow arrows). (D) 3D reconstruction of MRI and PET data in an oblique orientation demonstrates the mapping capabilities for MRI, which is able to identify discrete accumulations of macrophages on the vessel wall. The indicated MR volume (green) at the white arrow is 0.74×0.57×0.46 mm3. The volume at the yellow arrow is 0.68×0.60×0.32 mm3. Scale bar = 5 mm. This view is zoomed out from the FOV in panels B and C to include both vessels. Injured carotid artery is purple, increased MR signal intensity relative to vessel background is green, PET signal is orange, and contralateral vessel is gold.
Figure 4. Immunohistochemistry demonstrates that the multimodal probes accumulate in macrophages found in the intimal layer (I) of the injured vessel wall.
(A) Cell nuclei stained with DAPI (100X). A single cell is circled in yellow that will be referenced in the other panels. (B) The TAMRA signal from the multimodal agent is found in several locations in the intima. It can be seen in places to be in a punctate patern (arrowheads), suggesting internalization. A pronounced accumulation is shown for the circled cell. (C) Anti-CD68 monoclonal antibody staining also highlights a region in the intima (circled cell at white arrow). (D) Overlay of the three channels, A-C, demonstrates that the TAMRA signal colocalizes with the CD68 positive cell with the nucleus indicated in A, supporting macrophage labeling by the probes. Scale bar = 25 microns.
References
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- P30 CA093373/CA/NCI NIH HHS/United States
- U24 CA 110804/CA/NCI NIH HHS/United States
- R01EB000993/EB/NIBIB NIH HHS/United States
- U24 CA110804/CA/NCI NIH HHS/United States
- R01 EB000993/EB/NIBIB NIH HHS/United States