In Vivo quantitative imaging of angiogenesis-targeted PFOB nanoparticles in a hypercholesterol rabbit model using 19F-MRI with ultra-short echo time balanced SSFP
Journal of Cardiovascular Magnetic Resonance volume 14, Article number: M8 (2012)
Herein, initial results are presented as obtained in a hypercholesterol rabbit model with the simultaneous 19F/1H balanced UTE-SSFP technique and using ανβ3-targeted PFOB nanoparticles to establish the feasibility of high sensitivity MR molecular imaging of Gd-free, fluorine-based, clinically-relevant contrast agents.
ανβ3-integrin targeted nanoparticle (NP) emulsions have been shown to detect and quantify angiogenesis and anti-angiogenic therapy in small animal models of atherosclerosis. While these NP were visualized in high resolution pre- and post-injection 1H-MRI via a Gadolinium (Gd) chelate, we seek to image the perfluoro-octyl bromide (PFOB) core directly via 19F MR. Early in vivo successes of 19F MR molecular imaging exploited the single resonance peak of perfluoro-crown-ether. However, PFOB, which is the more clinically-relevant NP with a better-understood human safety profile, has a more complex spectrum with seven 19F resonance peaks and multiple relaxation conditions, leading to chemical shift artifact and intra-voxel destructive interference. We hypothesize that a new technique—simultaneous dual-frequency 19F/1H ultra-short echo time (UTE) balanced steady state free precession (b-SSFP) sequence with 3D radial readout—will allow efficient, sensitive imaging of the complex PFOB signal without the need for Gd and in sufficient resolution to discern the anatomy even in the presence of cardiac and respiratory motion.
The study was performed using a dual-tuned transmit/receive surface coil (7×12cm) on a 3T clinical whole-body scanner (Achieva, Philips Healthcare) modified for truly-simultaneous 19F/1H operation. Male New Zealand White rabbits were fed high cholesterol chow for 20 weeks. Imaging was performed 2h post-injection of 1.0ml/kg of the ανβ3-targeted PFOB-NP. A UTE b-SSFP sequence with simultaneous 19F/1H excitation and 3D radial readout was acquired at six time points post-injection with the following parameters: FOV=140mm, matrix 1123, isotropic voxel Δx=1.25mm, α=30°, excitation bandwidth exBW=9kHz, pixel bandwidth pBW=900Hz, TR=2.0ms, TE=100μs (FID sampling), total scan time 28 min. The radial k-space data was reconstructed at full resolution for the 1H component, and at lower resolutions with higher signal to noise for the 19F component (Nyquist radius 7%). 19F-data from subsequent time points were combined to provide an image of the spatial NP distribution. The 19F-signal was calibrated for 19F concentrations using an agar phantom containing PFOB-NP at 150mM19F.
In vivo imaging of angiogenesis-targeted PFOB nanoparticles was successful using the 19F/1H UTE b-SSFP sequence. Figure 1a shows an example of the proton image quality in a selected slice at the aorta, which is robust against motion due to the simultaneous 3D radial acquisition. The isotropic voxel allows multi-planar reformatting for visualizing anatomy and prescribing ROIs for analyzing the directly-corresponding 19F NP signal. In this example, ανβ3-targeted PFOB-NP were detected in the aorta ROI (Fig.1b) in concentrations ranging from 10 to 16mM.
Dual frequency 19F/1H radial 3D balanced ultra-short TE is a versatile pulse sequence that allows high-sensitivity, high-resolution in vivo detection of angiogenesis-targeted PFOB-NP despite the possible complex resonant peak interaction.
NIH R01 HL073646.
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Goette, M.J., Schmieder, A.H., Williams, T.A. et al. In Vivo quantitative imaging of angiogenesis-targeted PFOB nanoparticles in a hypercholesterol rabbit model using 19F-MRI with ultra-short echo time balanced SSFP. J Cardiovasc Magn Reson 14 (Suppl 1), M8 (2012). https://doi.org/10.1186/1532-429X-14-S1-M8