Photoacoustic imaging aims to better detect, diagnose ovarian cancer

The photoacoustic imaging method could help to improve the current standard of care for patients with ovarian cancer.

A multidisciplinary team at Washington University in St. Louis (WUSTL; St. Louis, MO) has found that a photoacoustic imaging method could lead to a promising new diagnostic imaging technique to improve the current standard of care for patients with ovarian cancer.

Quing Zhu, professor of biomedical engineering in the School of Engineering & Applied Science and of radiology, and a team of physicians and researchers at WUSTL conducted a pilot study using co-registered photoacoustic tomography with ultrasound to evaluate ovarian tumors on 16 patients at the School of Medicine and Barnes-Jewish Hospital.

"When ovarian cancer is detected at an early, localized stage—stage 1 or 2—the five-year survival rate after surgery and chemotherapy is 70–90%, compared with 20% or less when it is diagnosed at later stages, 3 or 4," says Zhu, a pioneer of combining ultrasound and near-infrared imaging modalities for cancer diagnosis and treatment assessment. "Clearly, early detection is critical, yet due to the lack of effective screening tools, only 20–25% of ovarian cancers are diagnosed early. If detected in later stages, the survival rate is very low."

In their approach, researchers use transvaginal ultrasound to obtain information about ovarian tumors, but ultrasound lacks accuracy in diagnosis of ovarian masses, Zhu says. Photoacoustic tomography, however, gives researchers a very detailed look at the tumor's vasculature, or tumor angiogenesis, and blood oxygen saturation (sO2) by lighting up the tumor's vasculature bed and allowing for more accurate diagnoses of ovarian masses seen by ultrasound.

Both tumor angiogenesis and tumor sO2 are related to tumor growth, metabolism, and therapeutic response. The WUSTL team is the only team using co-registered photoacoustic imaging and ultrasound to diagnose ovarian cancer.

In the pilot study, Zhu and her team created a sheath with optical fibers that wrap around a standard transvaginal ultrasound probe. The optical fibers are connected to a laser. Once the probe is inside the patient, Zhu turns the laser on, which shines through the vaginal muscle wall. With photoacoustic tomography, the light from the laser propagates, gets absorbed by the tumor and generates sound waves, revealing information about the tumor angiogenesis and sO2 inside the ultrasound-visible ovaries. A normal ovary contains a lot of collagen, Zhu says, but an ovary with invasive cancers has extensive blood vessels and lower sO2.

The team used two biomarkers to characterize the ovaries: relative total hemoglobin concentration (rHbT), which is directly related to tumor angiogenesis, and mean sO2. In this pilot study, the team found that the rHbT was 1.9 times higher for invasive epithelial cancerous ovaries, which make up 90% of ovarian cancers, than for normal ovaries. The mean oxygen saturation of invasive epithelial cancers was 9.1% lower than normal and benign ovaries. All five invasive epithelial cancerous ovaries, including two stage 1 and 2 cancers, showed extensive rHbT distribution and lower sO2.

These initial results will need to be validated with more patients, Zhu says, and the team is applying for funding to conduct a large clinical trial.

Full details of the work appear in the journal Radiology.

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