Abstract
A new imaging contrast agent is reported that provides an increased fluorescent signal upon application of ultrasound (US). Liposomes containing lipids labelled with pyrene were optically excited and the excimer fluorescence emission intensity was detected in the absence and presence of an ultrasound field using an acousto-fluorescence setup. The acousto-fluorescence dynamics of liposomes containing lipids with pyrene labelled on the fatty acid tail group (PyPC) and the head group (PyPE) were compared. An increase in excimer emission intensity following exposure to US was observed for both cases studied. The increased intensity and time constants were found to be different for the PyPC and PyPE systems, and dependent on the applied US pressure and exposure time. The greatest change in fluorescence intensity (130%) and smallest rise time constant (0.33 s) are achieved through the use of PyPC labelled liposomes. The mechanism underlying the observed increase of the excimer emission intensity in PyPC labelled liposomes is proposed to arise from the “wagging” of acyl chains which involves fast response and requires lower US pressure. This is accompanied by increased lipid lateral diffusivity at higher ultrasound pressures, a mechanism that is also active in the PyPE labelled liposomes.
Highlights
Fluorescence imaging has become a cornerstone technology which enables direct visualization of the physiological processes in a living cell or tissue as well as extraction of unique functional information about biological systems and their microenvironment [1,2]
Liposomes containing different molar concentrations of lipids with pyrene labelled on the phospholipid fatty acid tail (1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine (β-PyC10-HPC); abbreviated to PyPC) and lipids with pyrene labelled on the phospholipid head group (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(1-pyrenesulfonyl) (18:1 pyrene PE); abbreviated to PyPE) were studied
An increase in excimer emission intensity following exposure to US was observed for all cases studied with the increase in intensity and time constants being different for the PyPC and PyPE labelled liposomes and dependent on the applied US pressure and exposure time
Summary
Fluorescence imaging has become a cornerstone technology which enables direct visualization of the physiological processes in a living cell or tissue as well as extraction of unique functional information about biological systems and their microenvironment [1,2]. To improve the imaging capabilities of fluorescence techniques, methods based on models of photon transport in tissue combined with image reconstruction have been investigated including continuous-wave (CW) fluorescence diffuse tomography, time-domain fluorescence diffuse tomography and frequency-domain fluorescence diffuse tomography [7]. Whilst these techniques provide some improvement to image quality they require solution of an inverse problem, which has inherent assumptions about the nature of light propagation and can be computationally expensive.
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