Liposome-Enhanced MRI of Neointimal Lesions in the ApoE-KO Mouse Willem J.M. Mulder, 1 Kim Douma, 2 Gerben A. Koning, 3 Marc A. van Zandvoort, 2 Esther Lutgens, 4 Mat J. Daemen, 4 Klaas Nicolay, 1 and Gustav J. Strijkers 1 * Conventional high-resolution MRI is capable of detecting lipid- rich atherosclerotic plaques in both human atherosclerosis and animal models of atherosclerosis. In this study we induced neointimal lesions in ApoE-KO mice by placing a constrictive collar around the right carotid artery. The model was imaged with conventional multispectral MRI, and the thickened wall could not be distinguished from surrounding tissue. We then tested paramagnetic liposomes (mean size  90 nm) for their ability to improve MRI visualization of induced thickening, using Gd-DTPA as a control. T 1 -weighted (T 1 -w), black-blood MRI of the neck area of the mice was performed before and 15 min, 45 min, and 24 hr after intravenous injection of either paramag- netic liposomes or Gd-DTPA. The collared vessel wall of mice that were injected with liposomes showed a pronounced signal enhancement of 100% immediately after injection, which was sustained largely until 24 hr postinjection. In contrast, the ves- sel wall of all controls (left carotid artery and animals injected with Gd-DTPA) did not show significant contrast enhance- ment at those time points. This study demonstrates that intimal thickening in ApoE-KO mice can be effectively detected by contrast-enhanced (CE)-MRI upon injection of paramagnetic liposomes. Magn Reson Med 55:1170 –1174, 2006. © 2006 Wiley-Liss, Inc. Key words: atherosclerosis; mouse; molecular MRI; liposome; contrast agent; ApoE-KO Atherosclerosis is the main cause of mortality in Western societies; however, it is usually not identified before a clinical event, such as myocardial infarction or stroke, occurs (1,2). A large number of imaging techniques are used to detect and identify the presence and progression of atherosclerosis. Magnetic resonance imaging (MRI) is one such technique that is becoming a key imaging modality for the detection of atherosclerosis (3). The main advan- tage of MRI as compared to conventional arteriography techniques such as x-ray angiography, ultrasound, and computed tomography is its ability to both image the ves- sel wall and characterize atherosclerotic plaque composi- tion (4). Because of its ability to noninvasively image soft tissue and generate contrast between healthy and diseased tissue on the basis of intrinsic differences in tissue prop- erties, such as T 1 and T 2 relaxation times and proton density (PD), MRI can be used to evaluate and quantify the presence and progression of atherosclerosis (5). Several studies have shown that different atherosclerotic stages can be accurately assessed by MRI in human atherosclero- sis (6) as well as animal models of atherosclerosis (7,8). Much effort has been made in studies involving multicon- trast MRI of atherosclerosis to determine plaque burden and composition (5,9). Despite the wide range of methods available to generate contrast with MRI, it is difficult to discriminate intimal thickening and early lesions from healthy vessels using intrinsic MRI contrast mechanisms. Contrast-enhanced (CE)-MRI potentially could be used to detect such lesions. For that purpose, a contrast agent should be designed that would accumulate exclusively in the diseased vessel wall and show no significant wall enhancement in nonpatho- logical vessels. Ultrasmall particles of iron oxide (USPIOs) accumulate in macrophage-rich plaques (10,11) and thus increase the contrast between the vessel wall and sur- rounding tissue or vessel lumen. USPIO contrast is based on the superparamagnetic properties of iron oxide, which result in a local shortening of T 2 and T* 2 . Accumulation of iron oxide particles consequently causes dark spots in T 2 -weighted (T 2 -w) and T * 2 -w images, which is called neg- ative contrast. This often complicates the interpretation of the images, since the decrease of signal usually has to be detected in T* 2 -w images with a poor signal-to-noise ratio (SNR) (10). Positive contrast can be generated with the use of para- magnetic contrast agents that mainly cause a decrease of T 1 . One such agent, Gd-DTPA, causes an increase in signal intensity in T 1 -w images. Gd-DTPA is a small molecule with a short circulation halflife that does not accumulate in atherosclerotic lesions for a prolonged period. A nano- particle (like USPIO) with T 1 -shortening properties (i.e., containing Gd-DTPA) would be an attractive option for T 1 -based CE plaque imaging. Gadofluorine is a gadolini- um-based micellular nanoparticle (approximately 10 nm) that has been used successfully to detect atherosclerotic plaques in heritable hyperlipidemic rabbits (12,13). For this study we designed paramagnetic liposomes of approximately 90 nm with a high payload of Gd-DTPA- based lipid and a coating of poly(ethylene glycol) (PEG) to ensure long circulation halflives (14). Liposomes are col- loidal particles that are composed of either natural or 1 Biomedical NMR, Department of Biomedical Engineering, Eindhoven Univer- sity of Technology, Eindhoven, The Netherlands. 2 Department of Biophysics, Cardiovascular Research Institute Maastricht, University of Maastricht, Maastricht, The Netherlands. 3 Department of Radiation, Radioisotopes and Reactors, Section of Radiation and Isotopes for Health, Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands. 4 Department of Pathology, Cardiovascular Research Institute Maastricht, Uni- versity of Maastricht, Maastricht, The Netherlands. Grant sponsor: Molecular Imaging of Ischemic Heart Disease, BSIK; Grant number: BSIK03033; Grant sponsor: European Union Network of Excellence Diagnostic Molecular Imaging; Grant number: 512146 (LSHB-CT-2005- 512146). *Correspondence to: Gustav J. Strijkers, Biomedical NMR, Eindhoven Uni- versity of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: g.j.strijkers@tue.nl Received 17 November 2005; revised 2 January 2006; accepted 27 January 2006. DOI 10.1002/mrm.20883 Published online 5 April 2006 in Wiley InterScience (www.interscience.wiley. com). Magnetic Resonance in Medicine 55:1170 –1174 (2006) © 2006 Wiley-Liss, Inc. 1170