Photoacoustics is an emerging field within medical imaging. As photoacoustics relies on detection of the acoustic waves generated via optical absorption and the consequent heating/expansion process, the technology is closely tied to ultrasound. Typically, an intensity modulated light source, or short pulse source (i.e., laser), is used as the excitation source. The light is typically shined at the tissue surface, but can also be delivered from inside by means of minimally invasive delivery systems (e.g., endoscope, catheter, light-delivery needle). It penetrates the tissue predominantly via light scattering, thus illuminating a large volume. The light gets absorbed by blood/tissue chromophores, or non-targeted and targeted exogenous contrast agents such as optical dyes or nanoparticles configured for this purpose. The absorption, and consequent expansion, produces the acoustic wave, i.e., ultrasound or acoustic signal. The blood vessels (with different sizes and densities within a tumor, as well as different blood oxygenation level) and the surrounding tissue differ as to their light absorption. The resulting difference in the optically generated ultrasound produced provides contrast used in imaging. The technique's popularity is seen to be growing rapidly within the research community, focusing around some preclinical work such as whole body small animal imaging and monitoring pharmacokinetics, and clinical applications in oncology such as for breast or prostate cancer.
However, commonly-assigned International Publication Number WO 2009/057021 to Wang et al., (hereinafter “Wang”), entitled “Photoacoustic Imaging Contrast Agent and System for Converting Optical Energy to In-Band Acoustic Emission”, which is incorporated herein by reference in its entirety, notes, and illustrates therein by FIGS. 1(a), 1(b), 2(a), 2(b), that photoacoustic (PA) signals generated by irradiating, with short laser pulses, a point absorber such as a PA contrast agent particle, are broadband, and only a fraction of the PA signal energy falls within the receive frequency range of a regular medical ultrasound transducer. A largely predominant portion of the energy falls outside the range, i.e., into a higher frequency range.
To address this, Wang places microbubbles and/or nanobubbles in close proximity of the PA contrast agent.
In particular, each nanoparticle in Wang incorporates evaporating material and light-absorbing material. When the light-absorbing material is excited or “activated” by irradiation, it evaporates its evaporating material to thereby create an attached bubble.
Advantageously, the system can be tuned so that the bubbles re-radiate the energy principally within the receive frequency range of a regular medical ultrasound transducer. The energy re-radiated has been amplified, and has spread out in all directions, including in the direction of an ultrasound transducer.
The nanoparticles, before activation, are small enough to cross the boundary between the vasculature and lymphatic system. Accordingly, permeability can be measured. Also as a consequence, more anatomy can be imaged.
Material from which a bubble is formed, and the light-absorbing material that causes formation of the bubble, are combined in a particle, or droplet, in ways that differ according to the embodiment, thereby collectively offering a range of bubble size, and of bubble longevity over repeated expansions.