PAT schematic
A schematic diagram showing photoacoustic tomography. A laser pulse is fired at a group of nanometer-sized particles, which vibrate at a specific frequency, which in turn can be detected ultrasonically. The vibrational pattern can be processed into an image. Wikipedia

Various imaging techniques can see inside the body, but most lack detail necessary for looking at anything smaller than a few millimeters across. The only way to get a really good look is with a biopsy.

At Washington University in St. Louis, Dipanjan Pan thinks he and his collaborators have found an answer to that problem, which could reduce the need for biopsies and detect diseases such as cancer much earlier than before, using a combination of light from a laser, sonic waves and tiny pieces of gold.

The technique is called photoacoustic tomography, or PAT. Computed tomography (CT) scans work by taking X-rays that are layered using software to form an image. Magnetic resonance imaging uses a powerful magnetic field and a radio frequency signal to make the water molecules in the body give off small amounts of energy, detected as photons.

PAT uses particles, small enough to pass through the bloodstream. Each particle is about 150 nanometers in diameter, about one-twentieth the size of a typical red blood cell. The particles are made of material derived from a mixture of almond oil and lipids. Inside each one is a one to two-nanometer sized bit of gold.

When a near-infrared laser is fired at the particles, they vibrate. An ultrasound detector can pick that vibration up, and offer a high-resolution picture of what is happening inside the body. More importantly, the image is of higher resolution than traditional scanning techniques.

PAT uses infrared lasers because human tissue absorbs visible light strongly. In addition he has thus far tested the technique on mice, not people. In Pan's mouse experiments, he implanted the animals with a material that stimulates blood vessel growth, similar to what happens in tumors.

The PAT images showed early formation of blood vessels, so small traditional scanning techniques wouldn't have seen them. Blood vessel growth, called angiogenesis, is often a sign that a tumor is starting to grow. PAT images could thus detect cancer much earlier, Pan says. This could replace the biopsy for things like breast cancer, Pan said.

Detecting blood vessel growth could also apply to areas where new vessels are desirable, such as in skin or tissue grafts.

Pan says his group is also testing it for larger structures in the body, such as imaging the carotid artery. CT scans and MRIs can show the carotids, which are the major vessels supplying the brain, but they are located deep inside the neck and detail is often hard to see.

Besides imaging, the particles could be used to deliver drugs, Pan says. In those cases one could see that the drugs were getting to the right place.