Writer: Venkat Karra, Ph.D.
This study was reported today in the Optical Society’s (OSA) open-access journal Optics Express (Optics Express, Vol. 20, Issue 11, pp. 11582-11597 (2012)), and provide proof-of-concept support that the technology can distinguish malignant tissue by providing high-contrast images of tumors.
In breast cancer screening, x-ray mammography and ultrasonography are primarily used to understand any morphological changes of breast tissue. However, these conventional techniques have their own drawbacks because of for example ionizing radiation that could cause leukemia after prolonged/ repeated exposure where as ultrasonography is strongly operator dependent.
Tumor vascularization is a crucial feature in breast imaging. One commonly used method that focuses on tumor vascularization is Dynamic Contrast Enhanced MRI (DCE-MRI). The high sensitivity of this technique for detecting breast cancer proves that vascularity can indeed provide additional information about the nature of tissue. However, DCE-MRI suffers from a limited specificity, requires the injection of contrast agents and is relatively expensive.
Far-red and near-infrared (NIR): It is gaining attention in (non-invasively) visualizing cancer and its associated vasculature due to its ability to provide functional and molecular information without the use of ionizing radiation. In recent studies, it has been shown that optical imaging in the form of diffuse optical tomography (DOT) can indeed visualize breast malignancies, primarily because of the high absorption of hemoglobin in the NIR regime. However, DOT suffers from low spatial resolution.
Several groups have studied the feasibility of photoacoustic image (PAI) in breast imaging due to their superior resolution capabilities to that of pure optical techniques. Photoacoustic imaging exploits the high NIR light absorption contrast between benign and malignant tissue, but provides superior resolution arising from ultrasound detection.
Scientists from Center for Breast Care, Medisch Spectrum Twente hospital, University of Twente and University of Amsterdam have developed the Twente Photoacoustic Mammoscope (PAM), to image the breast in transmission mode. The authors say that, in a first pilot study with this system in 2007, it was possible to get technically acceptable measurements on five patients with radiographically proven breast malignancies. Of those, four cases revealed a high photoacoustic contrast with respect to the background associated with tumor related vasculature. Now the authors have recently started an extended clinical study using PAM, as a continuation of the study performed in 2007.
In this new study, they have investigated the clinical feasibility of photoacoustic mammography in a larger group of patients with different types of breast lesions to obtain more information about the clinical feasibility and limitations of photoacoustic mammography and the results were compared with conventional imaging and histopathology.
Ten technically acceptable measurements on patients with malignancies (BI-RADS 5) and two measurements on patients with cysts (BI-RADS 2) were performed. In the reconstructed volumes of all ten malignant lesions, a confined region with high contrast with respect to the background was seen. In all malignant cases, the PA contrast of the abnormality was higher than the contrast on x-ray mammography. The PA contrast appeared to be independent of the mammographically estimated breast density and was absent in the case of cysts.
Authors say that technological improvements to the instrument and further studies on less suspicious lesions are planned to further investigate the potential of PAM. The authors from University of Twente hope that these early results will one day lead to the development of a safe, comfortable, and accurate alternative or adjunct to conventional techniques for detecting breast tumors.
Twente Photoacoustic Mammoscope (PAM):
This techniques combines the light-based system’s to distinguish between benign and malignant tissue with ultrasound to achieve superior targeting ability. The device is built into a hospital bed, where the patient lies prone and positions her breast for imaging. Laser light at a wavelength of 1,064 nm scans the breast. Because there is increased absorption of the light in malignant tissue the temperature slightly increases. With the rise in temperature, thermal expansion creates a pressure wave, which is detected by an ultrasound detector placed on one side of the breast. The resulting photoacoustic signals are then processed by the PAM system and reconstructed into images. These images reveal abnormal areas of high intensity (tumor tissue) as compared to areas of low intensity (benign tissue). This is one of the first times that the technique has been tested on breast cancer patients.
Note: Breast cancer is one of the most common forms of cancer among females and each year more than 450,000 women are diagnosed worldwide with the disease.
Reporter: Venkat Karra