Challenges for Molecular Neuroimaging with MRI

Abstract

Magnetic resonance (MRI)-based molecular imaging methods are beginning to have impact in neuroscience. A growing number of molecular imaging agents have been synthesized and tested in vitro, but so far relatively few have been validated in the brains of live animals. Here, we discuss key challenges associated with expanding the repertoire of successful molecular neuroimaging approaches. The difficulty of delivering agents past the blood-brain barrier (BBB) is a particular obstacle to molecular imaging in the central nervous system. We review established and emerging techniques for trans-BBB delivery, including intracranial infusion, BBB disruption, and transporter-related methods. Improving the sensitivity with which MRI-based molecular agents can be detected is a second major challenge. Better sensitivity would in turn reduce the requirements for delivery and alleviate potential side effects. We discuss recent efforts to enhance relaxivity of conventional longitudinal relaxation time (T(1)) and transverse relaxation time (T(2)) MRI contrast agents, as well as strategies that involve amplifying molecular signals or reducing endogenous background influences. With ongoing refinement of imaging approaches and brain delivery methods, MRI-based techniques for molecular-level neuroscientific investigation will fall increasingly within reach.

Figure 1

Delivery of T₁ and T₂ contrast agents to the brain. A: Coronal MRI scans obtained after convection enhanced delivery of 1:70 Gd-DTPA [top; Mardor et al. (2005)] or 0.2 mg/mL dextran-coated ~80-nm diameter SPIOs [bottom; Perlstein et al. (2008)] to rat brain. Infusion rates were 1–4 µL/min over 15–90 min periods. B: Delivery of gadolinium-based [top; Kroll et al. (1998)] and iron oxide [bottom; Muldoon et al. (2005)] contrast agents to rat brain following 25% mannitol-induced blood brain barrier disruption. Mannitol was infused at 0.09 mL/s for 25–30 s into the carotid artery, followed immediately by 0.2 mmol/kg gadoterol (ProHance, Bracco Diagnostics) or 10 mg Fe/kg ~100-nm diameter SPIO (Ferumoxides, Berlex Laboratories). C: Localized accumulation of Gd-DTPA [top; Treat et al. (2007)] and 60-nm diameter SPIO [bottom; Liu et al. (2009)] contrast agents to rat brains following focused ultrasound (FUS) treatment, in conjunction with intravascular contrast agent delivery. Gd-DTPA (Magnevist, Berlex Laboratories) and SPIO (Resovist, Schering-Plough) doses were 125 and 15 µmol/kg, respectively. White arrows in the horizontal scans indicate sites of FUS power deposition (2 × 0.6 W top, 3 W bottom); black arrows in lower panel indicate major blood vessels. Images shown in panels (A–C) are either T₁-weighted scans (A–C, top), and T₂^*- (A, C, bottom) or T₂-weighted scans (B, bottom). Panel (A), top, reproduced with permission from Figure 1 in Mardor et al., Cancer Res, 2005, 65, 6858–6863, © American Association for Cancer Research; panel (A), bottom, reproduced with permission from the publisher, Duke University Press; panel (B), top, reproduced from Kroll et al., Neurosurgery, 1998, 43, 879–886, © Lippincott Williams & Wilkins; panel (B), bottom, reproduced from Muldoon et al., Neurosurgery, 2005, 57, 785–796, © Lippincott Williams & Wilkins; panel (C), both images, reproduced with permission from Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.

Figure 2

Ligand buffering by responsive contrast agents. A: The dynamics of a hypothetical ligand in the brain were simulated using a single compartment kinetic model (Helmchen and Tank, 2005). In an approximation to neural signaling phenomena such as release and uptake of neurotransmitters or intracellular second messengers, a single pulse of 10 µM ligand was simulated at time 0.2 s, followed by a process that removed ligand at a rate of 20 s⁻¹ on a background of weak endogenous buffering (buffer concentration 1 µM, K_d = 1 µM). The graph shows time courses of free (unbound) ligand in the presence of 0 µM (solid line), 5 µM (dashed line), or 30 µM (dotted line) of a responsive T₁ contrast agent assumed to bind the ligand with a dissociation constant of 1 µM. The simulation shows that the peak amount of free ligand, and its rate of decay back to baseline, are diminished by increasing contrast agent concentrations. The inset shows partial saturation of the contrast agent as a function of time for 5 and 30 µM concentrations. B: Peak T₁-weighted MRI signal changes were computed from the fractional saturation curves of panel (A), assuming background T₁ = 1 s, TR = 0.2 s, and ligand-induced relaxivity change from 0 to 10 mM⁻¹ s⁻¹. A greater ligand-dependent signal change is observed in the presence of 30 µM contrast agent (CA), which also buffered the ligand more severely.

Figure 3

MRI contrast agents for molecular and cellular imaging with improved sensitivity. A: Gd-hydroxypyridonate (Gd-HOPO)-based complexes have been designed to have characteristics optimized for high T₁ relaxivity (r₁). Key properties include the number of inner sphere water molecules (q), the exchange time for bound water (τ_M), and the rotational correlation time of the complex (τ_R). The HOPO compounds schematized at left have q = 2 (water ligands shown in bold) and τ_M values ≪ 100 ns (Cohen et al., 2000; Pierre et al., 2005); most Gd-polyaminocarboxylate complexes have less favorable q = 1 and τ_M 0.1–1 µs (Caravan et al., 1999). Complexes with R = CH₃ (Gd-1) or a dendrimeric substituent (Gd-2) have τ_R values of 125 and 238 ps, respectively. The longer τ_R of the larger Gd-2 complex contributes to its significantly higher r₁ across a range of magnetic field strengths, as indicated by the nuclear magnetic relaxation dispersion (NMRD) plot for Gd-2 (closed circles) and Gd-1 (open circles) shown at right. NMRD data from a more conventional macrocyclic complex, Gd-DO3A (Aime et al., 1998), is included for comparison (triangles). Theoretical curves fit to the experimental data all show a typical decline in relaxivity at high-resonance frequencies, above 100 MHz (B₀ > 2.35 T). B: SPIO contrast agents shown to generate particularly strong MRI effects include 1.6 µm diameter encapsulated microparticles of iron oxide (MPIOs), similar to those depicted at top. MPIOs were used to label proliferating cells in the subventricular zone (SVZ) of live rats, and visualized in sagittal gradient echo images (bottom) (Shapiro et al., 2006). The particles were infused into the ventricles near the SVZ, producing the large area of MRI signal dropout shown in the image. A narrow band of punctate T₂^* contrast is observed along the rostral migratory stream (RMS), extending from the SVZ to the olfactory bulb (white arrows). This band developed over several weeks following the injection, and is attributed to migrating cells, each harboring small numbers of MPIOs (scale bar = 5 mm). C: High sensitivity molecular neuroimaging can be achieved using MRI techniques with minimal or zero endogenous background. In this example, a fluorine-labeled analog (top, R = F) of a compound previously shown to label amyloid plaques (R = Br) (Skovronsky et al., 2000), was applied in a mouse model of Alzheimer’s disease (Higuchi et al., 2005). A coronal ¹⁹F MRI scan with 156 × 156 × 2000 µm resolution indicates distribution of the tracer, with no background from endogenous species (bottom left). Higher resolution anatomical data for comparison was obtained using conventional ¹H T₂-weighted imaging (bottom right). Arrows denote apparent amyloid deposits visible in both modalities. Several foci of ¹⁹F tracer accumulation do not correspond to T₂ lesions (e.g., arrowhead, left), and may represent plaques detectable more sensitively by ¹⁹F imaging. Graph in panel (A) adapted with permission from Pierre et al., J Am Chem Soc, 2005, 127, 504–505, © American Chemical Society; panel (B), top, courtesy Bangs Laboratories (Fishers, IN); panel (B), bottom, reproduced with permission from Shapiro et al., Magn Reson Med, 2006, 55, 242–249, © Wiley-Liss, Inc., a division of John Wiley & Sons, Inc.; images in panel (C) adapted with permission of the authors from Higuchi et al., Nat Neurosci, 2005, 8, 527–533, © Nature Publishing Group.