In Vivo quantitative imaging of angiogenesis-targeted PFOB nanoparticles in a hypercholesterol rabbit model using 19F-MRI with ultra-short echo time balanced SSFP

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.


Summary
Herein, initial results are presented as obtained in a hypercholesterol rabbit model with the simultaneous 19 F/ 1 H balanced UTE-SSFP technique and using α ν β 3targeted PFOB nanoparticles to establish the feasibility of high sensitivity MR molecular imaging of Gd-free, fluorine-based, clinically-relevant contrast agents.
Background α ν β 3 -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 1 H-MRI via a Gadolinium (Gd) chelate, we seek to image the perfluoro-octyl bromide (PFOB) core directly via 19 F MR. Early in vivo successes of 19 F 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 19 F resonance peaks and multiple relaxation conditions, leading to chemical shift artifact and intra-voxel destructive interference. We hypothesize that a new technique-simultaneous dualfrequency 19 F/ 1 H 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.

Methods
The study was performed using a dual-tuned transmit/ receive surface coil (7×12cm) on a 3T clinical wholebody scanner (Achieva, Philips Healthcare) modified for truly-simultaneous 19 F/ 1 H 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 α ν β 3 -targeted PFOB-NP. A UTE b-SSFP sequence with simultaneous 19 F/ 1 H excitation and 3D radial readout was acquired at six time points postinjection with the following parameters: FOV=140mm, matrix 112 3 , 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 1 H component, and at lower resolutions with higher signal to noise for the 19 F component (Nyquist radius 7%). 19 F-data from subsequent time points were combined to provide an image of the spatial NP distribution. The 19 F-signal was calibrated for 19 F concentrations using an agar phantom containing PFOB-NP at 150mM 19F .

Results
In vivo imaging of angiogenesis-targeted PFOB nanoparticles was successful using the 19 F/ 1 H 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 1 acquisition. The isotropic voxel allows multi-planar reformatting for visualizing anatomy and prescribing ROIs for analyzing the directly-corresponding 19 F NP signal. In this example, α ν β 3 -targeted PFOB-NP were detected in the aorta ROI (Fig.1b) in concentrations ranging from 10 to 16mM.

Conclusions
Dual frequency 19 F/ 1 H radial 3D balanced ultra-short TE is a versatile pulse sequence that allows high-sensitivity, high-resolution in vivo detection of angiogenesistargeted PFOB-NP despite the possible complex resonant peak interaction.

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Submit your manuscript at www.biomedcentral.com/submit Figure 1 Simultaneous 19 F/ 1 H molecular imaging of angiogenesis targeted perfluoro-octyl-bromide nanoparticles in a rabbit model of atherosclerosis using 3D radial balanced UTE-SSFP. Proton images (a) with 1.25mm isotropic voxels show anatomy, upon which 19 F image can be over-laid (b). The ROI in (b) is surrounding the aorta, which has a diameter of about 5mm. The 19 F overlay within the aortic region is in green, and extra-aortic 19 F signal is blue.