High‐resolution magnetic resonance angiography in the mouse using a nanoparticle blood‐pool contrast agent

Abnormal vascular morphology is associated with numerous cerebral diseases. These diseases span a wide range from vasculopathies such as atherosclerosis to neurodegenerative conditions such as Alzheimer’s disease to neoplastic diseases such as glioblastoma multiforme. In the clinical domain, magnetic resonance angiography (MRA) has become one of the most valuable tools for evaluating both normal and pathologic vascular anatomy. MRA is well suited to studying vascular architecture because it provides excellent vessel contrast while being noninvasive and inherently three-dimensional (3D). In the preclinical domain, MRA is also being adopted to study corresponding animal models of human diseases. In mouse models of neurologic and neoplastic diseases, subtle changes in vascular morphology have been found to be relevant to the disease process (1,2). MRA of mouse models has been used in the study of middle cerebral artery occlusion (3), vascular changes associated with Alzheimer’s disease (2,4), collateral flow in peripheral vascular occlusion (5), and vascularity in choroid plexus carcinoma (6). To date, the highest-resolution mouse MRA studies have used 3D time-of-flight (TOF) contrast acquired with 3D gradient echo sequences (3,6). In this approach, the vessel signal comes primarily from unsaturated blood flowing into the field of view (FOV) (inflow enhancement), so it provides superb visualization of the large and proximal arteries in a particular FOV. However, because the inflowing spins are rapidly saturated by the rapid pulse repetition, no signal remains for visualizing the distal arteries and/or venous circulation. This loss of distal vessel signal becomes problematic in regions with turbulent or slow flow or with tortuous vessels, such as in the brain and tumors (4). This problem is exacerbated when the large arteries supplying a region are included in the FOV. Contrast agents can be used in MRA to increase small-vessel contrast. In contrast-enhanced MRA (CE-MRA), the vessel signal comes not only from the TOF effect but also from magnetization that recovers in the blood due to T1-shortening by the contrast agent. However, conventional low-molecular-weight contrast agents, such as Gd-DTPA, rapidly extravasate from the vascular space. As a result, the time window for imaging is considerably reduced, thus limiting acquisition of high-resolution images (7–9). Several macromolecular and nanoparticle-based contrast agents have been developed (10) to overcome these limitations with conventional contrast agents. Recently, long-circulating gadolinium contrast agents have been developed using a liposomal platform (8,11). The nanoscale size of the liposomes (50–200 nm) limits their extravasation from the vasculature, making them a “blood-pool” contrast agent. In addition, the liposomes are coated with polyethylene glycol, which slows the clearance rate and gives these agents a long in vivo residence time. These blood-pool and long-circulating features permit an extended time window for imaging, which allows acquisition of high-spatial-resolution images (12–14). The first generation of these liposomal contrast agents carried gadolinium within the liposomal core. We have now created a second generation of agents, in which the gadolinium is conjugated to the surface of the liposome (surface-conjugated or SC-Gd liposomes). Compared to core-encapsulated liposomes, SC-Gd liposomes have approximately twice the T1 relaxivity (at 7 T), because the lanthanide center is presented on the surface of the liposomes and is thus allowed more interactions with the blood protons. In this study, the utility of SC-Gd liposomes for CE-MRA was evaluated for neurovascular imaging in the mouse. Because the best mouse MRA results to date have utilized TOF-MRA (3,6), TOF-MRA was used as the reference standard for quantitative assessment. First, a TOF-MRA protocol was optimized to establish benchmarks of spatial resolution, small-vessel contrast, FOV, and scan time. Then, a corresponding CE-MRA protocol using SC-Gd liposomes was developed and compared to determine if the contrast agent could improve neurovascular mouse MRA in terms of small-vessel contrast, FOV, spatial resolution, and scan time.