Abstract B22: Quantitative tumor imaging using magnetic nanoparticles
Authors: Codi Gharagouzloo , Qiao Ju , Timms Liam , Ven Anne van de , Sridhar Srinivas 
Published: March 2017

Introduction: We have developed a new method of Quantitative MRI named QUTE-CE MRI that yields images of the vasculature with unparalleled clarity and definition and is quantitative. QUTE-CE MRI can produce contrast enhanced magnetic resonance angiograms (CE-MRA) using super paramagnetic iron-oxide nanoparticle (SPION), including the FDA approved ferumoxytol, with high contrast in cardiovascular, cerebral, and tumor imaging.

Based upon principles of magnetic nanoparticle interactions with neighboring water molecules, the method achieves robust, reproducible results by utilizing rapid signal acquisition at ultra-short time-to-echo (UTE) to produce positive-contrast images with pure T1 weighting and little T2* decay. The spoiled gradient echo equation (SPGR) is used to transform UTE intensities directly into concentration using experimentally determined relaxivity constants and image acquisition parameters.

Methods: All animal experiments were conducted in accordance with the Northeastern University Division of Laboratory Animal Medicine and Institutional Animal Care and Use Committee. MRI images were obtained at ambient temperature (∼25°C) using a Bruker Biospec 7.0T/20-cm USR horizontal magnet (Bruker, Billerica, Massachusetts, USA) equipped with a 20-G/cm magnetic field gradient insert (ID =12 cm, Bruker) and the same quadrature 300 MHz, 30 mm Mouse MRI coil was used for all in vivo work as previously utilized for mouse experiments in Section 3.8 (Animal Imaging Research, LLC, Holden, Massachusetts, USA).

PC 3 cells were injected into the right flank of immunocompromised FoxNu1 mice (n=5, Charles River Laboratories). After tumors reached about 0.5-1.0cm3, animals underwent three separate imaging sessions: Session 1 - pre-contrast T1, T2 and QUTE-CE measurements, Session 2 - immediate post-contrast QUTE-CE measurement and Session 3 - 24h post-contrast T1, T2 and QUTE-CE measurements. For contrast, 100μl of ferumoxytol diluted to 6mg/ml was injected i.v. to render a blood concentration of ~200μg/ml Fe (2x clinical dose).

Results: Contrary to more standard MRI techniques, QUTE-CE pre-contrast images render a nearly homogenous signal with a Gaussian distribution in the tumor. The immediate post-contrast images render the vasculature clearly and skew the distribution of voxels within the whole tumor to the left, however also increases the overall mean of the signal intensity because the movement of voxels within the tumor is to the right, leaving a long bright tail with the brightest voxels represented by those containing 100% blood. 24h after the initial administration of ferumoxytol the vasculature is no longer visible, but the locations within the tumor that have passively accumulated SPIONs resulting from the EPR effect becomes apparent. While the distribution of voxels within the tumor becomes less skewed, the overall shape is still slightly skewed to the left and the mean of the distribution has moved to the right. Nanoparticle accumulation in the post-contrast image is heterogeneous and unambiguous.

Angiography and TBV in tumors Assuming a partial 2-volume model of blood and tissue, we determine the tumor blood volume (TBV) across the entire tumor volume. The resultant TBV heatmaps show a clear range of TBV values are apparent, delineating areas of the tissue with high contrast in regard to overall vascular health, including apparently necrotic tissue.

Nanoparticle accumulation Next, a unique feature of the methodology to produce high-contrast images of purely T1-weighted signal is employed to unambiguously delineate nanoparticle accumulation in a PC3 subcutaneous tumor model with ferumoxytol accumulation 24 hours after just one dose. From this, contrast efficiency was produced compared to standard techniques with the additional benefit that pre-contrast images are not necessitated. A major advantage of delineating SPION accumulation using QUTE-CE, compared to ΔT2 or ΔT1 imaging, is that the post-contrast image contains sufficient information for nanoparticle localization, eliminating the need for pre-contrast images.

Conclusion: QUTE-CE MRI exploits physical principles of magnetic relaxation modulated by SPIONs to achieve quantitative MRI yielding exceptional vascular images. This ability to longitudinally quantify blood pool CA concentration is unique to the QUTE-CE method, and makes QUTE-CE MRI competitive with nuclear imaging. Quantitative tumor blood volume distributions are obtained at short times, while nanoparticle accumulation maps are obtained at long times. QUTE-CE MRI is a new method that can be used to study tumor properties longitudinally. The technique is immediately translatable to the clinic using the FDA approved contrast agent ferumoxytol and is expected to have a major impact on clinical tumor imaging.

Work supported by NSF-DGE- 0965843.

Citation Format: Gharagouzloo Codi, Ju Qiao, Liam Timms, Anne van de Ven, Srinivas Sridhar. Quantitative tumor imaging using magnetic nanoparticles. [abstract]. In: Proceedings of the AACR Special Conference on Engineering and Physical Sciences in Oncology; 2016 Jun 25-28; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2017;77(2 Suppl):Abstract nr B22.