Early detection has been shown to increase breast cancer survival rates, but many women avoid having their mammograms taken as often as they should because of the discomfort involved. Researchers say they have developed something better: a laser-sonic scanner that can find tumors in as little as 15 seconds by shining pulses of light into the breast. The scanning system, known as photoacoustic computed tomography, or PACT, was developed in the lab of Lihong Wang, Caltech’s Bren Professor of Medical Engineering and Electrical Engineering.
PACT works by shining a near-infrared laser pulse into the breast tissue. The laser light diffuses through the breast and is absorbed by oxygen-carrying hemoglobin molecules in the patient's red blood cells, causing the molecules to vibrate ultrasonically. Those vibrations travel through the tissue and are picked up by an array of 512 tiny ultrasonic sensors around the skin of the breast. The data from those sensors are used to assemble an image of the breast's internal structures in a process that is similar to ultrasound imaging, though much more precise. PACT can provide a clear view of structures as small as a quarter of a millimeter at a depth of 4 centimeters. Mammograms cannot provide soft-tissue contrast with the level of detail in PACT images, Wang says.
Because the laser light at the currently used wavelength is so strongly absorbed by hemoglobin, PACT can construct images that primarily show the blood vessels present in the tissue being scanned. The speed with which a PACT scan can be performed gives it advantages over other imaging techniques. For example, magnetic resonance imaging (MRI) scans can take 45 minutes. MRI scans are also expensive and sometimes requires contrast agents to be injected into the patient's blood.