High-contrast photoacoustics tracks tumor cells

10 Mar 2015

Novel approach uses cell engineering and optical detection of ultrasound to image deep tumor.

Scientists at University College London (UCL) have developed a new approach to photoacoustic imaging that promises to reveal previously unknown secrets of organ development and cancer growth in vivo.

Using cells genetically engineered to produce a pigment allowing high-contrast imaging without the need for a fluorescent dye, and a novel etalon-based ultrasound sensor, the UCL team was able to image the development of a human tumor implanted into a live mouse.

And in a paper published in the journal Nature Photonics, they say that the state-of-the-art technique will ultimately allow them to study the growth and behavior of entire organs in living animals.

Martin Pule, one of the lead researchers on the study, said: “Anything you could possibly think of in terms of imaging complex activity within an organ, this technique would you allow you to do.”

“Whether it's watching immune cells attack a tumor or an infection, or watching an organ develop embryonically, function, or react to damage or stress, all of these things you could observe at an organ level, which is something it would have been very difficult to do before.

“This technique lets you genetically label particular parts of an organ and then study it, over time, in a non-invasive way, without having to administer a contrast agent.”

The approach is said to produce photoacoustic images of unprecedented clarity at depths of up to 10 mm in a live animal – raising the prospect of studying cellular and genetic processes in mammals in real time.

UCL video: 3D view of the photoacoustic imagery of a tumor:

Genetically encodable contrast
While photoacoustic imaging is not new, and early developers like Lihong Wang and Alexander Oraevsky have already gained much recognition from within the photonics community, it remains in its infancy as far as clinical applications are concerned.

Wang and others see early-stage detection of cancer as the ultimate application for the technique, and it is currently under evaluation in clinical trials for distinguishing between aggressive and less virulent forms of breast cancer. Here, it promises to reduce the rate of false-positive diagnoses and also the number of unnecessary surgical interventions.

But whereas most photoacoustic scanners can only detect only blood vessels, the UCL scientists have genetically engineered tumor cells so that they express tyrosinase - an enzyme that produces the pigment melanin in skin. Aside from red blood cells, most cells in the body have no natural pigment, and cannot provide the contrast need for photoacoustic imaging. The UCL team has shown it is possible to introduce genetically encodable contrast that can be differentiated from hemoglobin.

This alters the cells so that they absorb light from the excitation laser much more effectively, and deliver high-contrast images at depth – even though the in vivo experiments were conducted without signal averaging and using laser powers below the levels determined as safe for human skin.

All-optical ultrasound sensor
On top of that, the UCL team used an all-optical ultrasound detector based around a high-finesse Fabry-Pérot polymer film etalon. In their paper, they claim that it has a number of advantages over the more conventional piezoelectric receivers used to detect photoacoustic waves and thereby generate images.

The etalon itself comprises a 40 µm-thick Parylene C polymer spacer, sandwiched between a pair of dichroic dielectric mirrors. Saying that this offers better acoustic performance, resulting in improved image quality, they write:

“The transparent nature of the sensor head is clearly advantageous in terms of irradiating the tissue, in that it avoids the challenges associated with delivering the excitation light around an opaque piezoelectric planar detector array.

“However, perhaps the most important advantage of this type of sensor derives from its exceptional acoustic performance in terms of bandwidth, element size and sensitivity.”

Individual cell imaging “possible”
Having shown that the new approach works, the UCL researchers are now developing other pigments, to increase the palette of colors with which they could label different parts of an organ. That should allow them to study complex behaviors of several different types of cells.

Ultimately, they add, detection of individual cells may even be possible, by optimizing the excitation wavelength, light delivery and detector sensitivity.

And because eumelanin, the pigment that is produced by tyrosinase, absorbs light through to near-infrared wavelengths, they believe that it should be possible to detect labelled cells at depths approaching 10 mm by using a longer wavelength.

Future applications in cancer research could also include imaging the evolution of labelled tumor growth, as well as metastasis and response to different therapies.

The UCL team developed the technique with funding from the UK’s Biotechnology & Biological Sciences Research Council (BBSRC). Also providing support were NIHR University College London Hospital Biomedical Research Centre, King's College London, the UCL Comprehensive Cancer Imaging Centre, Cancer Research UK, the Engineering and Physical Sciences Research Council (EPSRC), the Medical Research Council, the Department of Health UK, and the European Union.