Towards registration of optical and MR signal changes in subcutaneous tumor volume in vivo after optical skin clearing

The goal of this research was in testing magnetic resonance imaging (MRI) pulse sequences for monitoring local changes of proton relaxation times after the local application of skin optical clearing (OC) compositions in vivo. We used xenograft mouse models of cancer, i.e. nu/nu mice bearing subcutaneous tumors expressing endogenous TagRFP red fluorescent protein marker and tested the changes in fluorescence intensity and lifetime (FL) of the subcutaneous tumor foci after OC application (70% glycerol, 5% DMSO, 25% water) onto the skin. By using time-correlated single photon counting within 20-30 min after the OC we observed: 1) 30-40% increase in the overall photon numbers output; 2) 50 ps increase in the median FL of TagRFP. We subsequently performed tracking of MR signal intensity changes within selected regions of interest (ROI) located close to the skin surface before, during and after OC. The analysis of 1T MR T2-weighted (T2w) fast spin-echo images showed significant quantitative differences between Gaussian noise-normalized MRI signal intensities (Mann-Whitney test, p<0.05). Our results suggest that the application of OC may cause: 1) a transient change of the peripheral tumoral microenvironment and as a consequence, FL increase and shortening of mean proton relaxation times within the voxels of subcutaneous tumor (i.e. T2w hypointensity increase); 2) potential microviscosity change due to the permeability for the OC components resulting in shortening of tissue water proton relaxation times. The results suggest that T2w 1T MRI was useful for semi-quantitative monitoring of MR signal intensity longitudinal changes in the subcutaneous space during and after OC thereby enabling registration of optical and MR signal fluctuations in the same voxels of live tissue

Interaction of laser radiation and complexes of gold nanoparticles linked with proteins

The results of numerical simulation of the near-field distribution inside and in the vicinity of two types of gold nanoparticles (nanospheres and nanorods) intended for producing complexes of gold nanoparticles linked with proteins and exciting photosensitizers in the wavelength range of 532 – 770 nm are presented. Quantitative estimates of the field localisation (enhancement) are obtained depending on the type of gold nanoparticles and dimensional factors. The tendency of the red shift of the wavelength at which the maximum local field enhancement is achieved relative to the positions of the maxima of the absorption and scattering cross sections of nanoparticles and complexes is described

Magnetic resonance contrast agents in optical clearing: Prospects for multimodal tissue imaging

Skin optical clearing effect ex vivo and in vivo was achieved by topical application of low molecular weight paramagnetic magnetic resonance contrast agents. This novel feature has not been explored before. By using collimated transmittance the diffusion coefficients of three clinically used magnetic resonance contrast agents, that is Gadovist, Magnevist and Dotarem as well as X-ray contrast agent Visipaque in mouse skin were determined ex vivo as (4.29 +/- 0.39) x 10(-7) cm(2) /s, (5.00 +/- 0.72) x 10(-7) cm(2) /s, (3.72 +/- 0.67) x 10(-7) cm(2) /s and (1.64 +/- 0.18) x 10(-7) cm(2) /s, respectively. The application of gadobutrol (Gadovist) resulted in efficient optical clearing that in general, was superior to other contrast agents tested and allowed to achieve: (a) more than 12-fold increase of transmittance over 10 minutes after application ex vivo; (b) markedly improved images of skin architecture obtained with optical coherence tomography; (c) an increase of the fluorescence intensity/background ratio in TagRFP-red fluorescent marker protein expressing tumor by five times after 15 minutes application into the skin in vivo. The obtained results have immediate implications for multimodality imaging because many contrast agents are capable of simultaneously enhancing the contrast of multiple imaging modalities

Prospects for multimodal visualisation of biological tissues using fluorescence imaging

We investigate skin optical clearing in laboratory animals ex vivo and in vivo by means of low-molecular-weight paramagnetic contrast agents used in magnetic resonance imaging (MRI) and a radiopaque agent used in computed tomography (CT) to increase the sounding depth and image contrast in the methods of fluorescence laser imaging and optical coherence tomography (OCT). The diffusion coefficients of the MRI agents Gadovist®, Magnevist®, and Dotarem®, which are widely used in medicine, and the Visipaque® CT agent in ex vivo mouse skin, are determined from the collimated transmission spectra. MRI agents Gadovist® and Magnevist® provide the greatest optical clearing (optical transmission) of the skin, which allowed: 1) an almost 19-fold increase in transmission at 540 nm and a 7 – 8-fold increase in transmission in the NIR region from 750 to 900 nm; 2) a noticeable improvement in OCT images of skin architecture; and 3) a 5-fold increase in the ratio of fluorescence intensity to background using TagRFP-red fluorescent marker protein expressed in a tumour, after application to the skin of animals in vivo for 15 min. The obtained results are important for multimodal imaging of tumours, namely, when combining laser fluorescence and OCT methods with MRI and CT, since the contrast agents under study can simultaneously enhance the contrast of several imaging methods

MR and fluorescence imaging of gadobutrol-induced optical clearing of red fluorescent protein signal in an in vivo cancer model

Multimodality registration of optical and MR images in the same tissue volume in vivo may be enabled by MR contrast agents with an optical clearing (OC) effect. The goals of this study were to (a) investigate the effects of clinical MR contrast agent gadobutrol (GB) and its combinations as a potential OC agent assisting in fluorescence intensity (FI) imaging in vivo and (b) evaluate MRI as a tool for imaging of topical or systemic application of GB for the purpose of OC. Subcutaneous tumor xenografts expressing red fluorescent marker protein were used as disease models. MRI was performed at 1 T (1) H MRI using T1 -weighted 3D gradient-echo (T1w-3D GRE) sequences to measure time-dependent MR signal intensity changes by region of interest analysis after image segmentation. Topical application of 1.0 M or 0.7 M GB-containing OC mixture with water and dimethyl sulfoxide showed similar 30-40% increases of tumor FI during the initial 15 min. Afterwards, the OC effect of GB on FI and tumor/background FI ratio showed a decrease over time in the case of 1.0 M GB, unlike the 0.7 M GB mixture, which resulted in a steady increase of FI and tumor/background ratio for 15-60 min. The use of T1w-3D GRE MR pulse sequences showed that concentrated 1.0 M GB resulted in MR signal loss of the skin due to high magnetic susceptibility and that signal loss coincided with the OC effect. Intravenous injection of 0.3 mmol GB/kg resulted in a rapid but transient 40% increase of FI of the tumors. Overall, 1 T MRI enabled tracking of GB-containing OC compositions on the skin surface and tumor tissue, supporting the observation of a time-dependent FI increase in vivo