- Meeting abstract
- Open Access
2131 The dual contrast mechanism in inversion recovery with on-resonant water suppression magnetic resonance angiography (IRON-MRA) after administration of iron oxide nanoparticles
© Vonken et al; licensee BioMed Central Ltd. 2008
- Published: 22 October 2008
- Contrast Enhancement
- Iron Oxide Nanoparticle
- Small Bandwidth
- Paramagnetic Contrast Agent
- Magnetization Preparation
After administration of a paramagnetic contrast agent, the resonance frequency of the vessels shifts in a geometry-dependent way. This frequency change has recently been exploited for angiographic contrast generation ('ORCA') by Edelman . However, this contrast enhancement was shown to depend on the angle Θ of the vessel relative to the magnetic field. IRON provides an alternative off-resonance contrast enhancement technique . It is hypothesized that IRON-MRA is less dependent on Θ, because only a narrow band in the frequency domain is attenuated, and because concomitant T1-lowering by the contrast is exploited. To address this hypothesis, the relative contribution of off-resonance and T1-lowering to the contrast enhancement in IRON-MRA was investigated in vitro and in vivo at 3 T.
To measure the relative contribution of off-resonance to the contrast enhancement in IRON-MRA after iron-oxide nanoparticles adnministration.
To study the effect of Θ in vitro, a rod-shaped phantom with 1.6 mM MION-47 (a prototype 30 nm iron oxide nanoparticle, CMIR/MGH) in rabbit blood was placed at different angles to the field. IRON imaging (αIRON = 100°, BWIRON = 107 Hz, 70 bpm triggered segmented gradient echo, TR/TE/α = 3.5 ms/1.4 ms/15°, 19 profiles per shot) was performed while the center frequency of the pre-pulse varied from -500 Hz to 500 Hz.
To quantify the off-resonance in vivo, a range of pre-pulse bandwidths (107–1700 Hz) was applied in IRON-MRA of a rabbit aorta after MION-47 (1 mM). From the blood signals and an estimate of T1, the relative contribution of off-resonance to the total signal was quantified. For visual comparison, a regular T1-MRA (TR/TE/α = 25 ms/2.6 ms/20°) was obtained.
With an on-resonant pre-pulse, the 0 and 90 degree blood curves (Figure 1) are not at their respective minimum, demonstrating relative signal conservation due to off-resonance, while the background signal is effectively suppressed. The feasibility of the exploitation of this effect in vivo is shown in Figure 2, where suppression with relatively small bandwidths yields substantial off-resonance components to the total signal.
While ORCA utilizes off-resonance excitation, thereby imaging the positive frequency shifts only, IRON-MRA uses a narrow-bandwidth on-resonant frequency selective magnetization preparation. Not only do both ends of the frequency spectrum contribute to the signal, but simultaneously, the inherent T1 shortening of the contrast agent is used, which yields additional MRA contrast irrespective of Θ (aorta still visible in Figure 3b). The relative contribution of the off-resonant signal is also apparent by visually comparing Figure 3b and 3c.
The signal formation of IRON-MRA is partly the result of the susceptibility induced spectral shift after iron-oxide nanoparticle injection. The relative contribution of this off-resonance effect has been characterized in vitro and in vivo. While the off-resonance contrast depends on the angle between the blood-sample and the magnetic field, this angular dependent signal-attenuation was found to be relatively small, which is attributable to the shortened T1.