Magnetic resonance imaging (MRI) is a powerful non-invasive, non-ionizing imaging modality that is primarily used for medical imaging but also plays an important role in basic science research. It can provide anatomical, functional, metabolic, cellular and molecular information of tissues in vivo
with high resolutions in three dimensions, routinely down to 1 mm at clinical field strengths and even down to about 50 μm in research settings (Strijkers et al., 2007
). Soft tissue contrast in MR images is generated by highlighting differences in the longitudinal (T1
) and transverse (T2
) relaxation times of protons in different tissues. Tissue differences can be made greater by shortening T1
; shortening of T1
is often preferred as this leads to gain of signal versus loss of signal via shortening of T2
In order to shorten either T1
, contrast agents (CAs), which are diagnostic pharmaceutical compounds containing paramagnetic or superparamagnetic metal ions (Bellin, 2006
), are administered. The influence of a paramagnetic ion on the relaxation time depends directly upon the number of unpaired electrons generating the electronic spin that interferes with the nuclear spin of hydrogen (Idee et al., 2006
). Gd (III) ions, which are paramagnetic, contain 7 unpaired electrons and yield very strong T1
relaxation properties (Brasch, 1992
); making Gd compounds the most widely used CAs in MRI. For Gd to be safely administered in vivo
, a chelator such as diethylene triamine pentaacetic acid (DTPA) that prevents Gd toxicity, must be used.
In order to verify the in vivo imaging results obtained using Gd-chelates in MRI, a complementary technique is required to map Gd spatially and simultaneously quantify Gd down to the parts per million level and normalized to the density of the tissue. No technique has so far exhibited this capability. Such a technique would also allow one to look at the tissue distribution of the agent in relation to tissue composition and explore whether the agent may highlight specific disease components. The aim of this study was to examine the capability of nuclear microscopy to map and quantify Gd in tissue sections.
There were three main objectives of this study. First, to determine whether or not nuclear microscopy is sufficiently sensitive to detect Gd in atherosclerotic aortic sections from cholesterol-fed rabbits after intravenous injection of bis-5-hydroxytryptamide-diethylenetriamine-pentaacetate gadolinium (bis-5HT-DTPA(Gd)). Bis-5HT-DTPA(Gd) is an enzyme-activatable agent targeting myeloperoxidase (MPO) (Querol et al., 2005
, Querol et al., 2006
and Chen et al., 2006
). MPO is an abundant heme enzyme released by activated leukocytes and catalyzes the formation of a number of reactive species that, among many harmful biological effects, can modify low-density lipoprotein (LDL) to a form that converts macrophages into lipid-laden or ‘foam’ cells, the hallmark of atherosclerotic lesions (Wada et al., 2000
, Carr et al., 2000
). Validation of this novel MPO-sensing imaging agent is an extremely important step toward the ultimate goal of producing MR images that reflect atherosclerotic plaque vulnerability. For control, we sacrificed a cholesterol-fed rabbit one week after injection of the clinical standard agent diethylenetriamine-pentaacetate gadolinium (DTPA(Gd)). This agent is the parent compound on which bis-5HT-DTPA(Gd) is based though it possesses no molecular specificity.
Second, we explored the use of this technique to detect changes in Gd levels over time by examining aortic sections obtained at 2 hours and 4 hours after bis-5-HT-DTPA(Gd) injection. Thirdly, high-resolution studies were also performed to evaluate if this particular agent had any preferential distribution within the aorta.