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Optical mammography offers complementary screening

01 Mar 2007

While X-ray mammography is the method of choice for breast screening, its effectiveness depends on parameters such as the patient's age, breast density and body-mass index. Optical mammography – the use of light to probe and image tissue – could provide an important adjunct to traditional screening, and ART Advanced Research Technologies (ART) of Canada says that its SoftScan optical imaging device could be just the system to enable this.

ART's SoftScan system uses diffuse optical tomography to measure the migration of near-infrared (NIR) photons through the breast. Pulsed diode lasers irradiate the breast at four wavelengths (690, 730, 780 and 830 nm), with the pulses time-multiplexed into one optical fibre. After travelling through the breast tissue, the photons are collected via five optical fibres located in a mobile detector head.

"SoftScan then uses a time-domain data-acquisition technique, which allows for quantitative measurement of both the absorption and scattering components of the diffused NIR light," Mario Khayat, ART's vice-president, optical products, told medicalphysicsweb.

Examining four different wavelengths means that the system can assign the measured data to the different tissue components in the breast. "Multiple wavelengths are required to properly decouple the absorption contributed by each of the main breast chromophores: oxyhaemoglobin, deoxyhaemoglobin, water and lipids," Khayat explained. "Otherwise you end up with an intensity image representing the contribution of all of them combined."

SoftScan's review workstation uses the levels of these different chromophores to characterize the tissue. For example, to differentiate between malignant and benign lesions, the system examines perfusion (indicated by total haemoglobin concentration) and metabolism (the ratio of deoxyhaemoglobin to total haemoglobin). A significant increase in both perfusion and metabolism in the region of interest compared with surrounding tissue is indicative of a malignant tumour.

"Results from our prospective statistical analysis indicate that oxyhaemoglobin might be a good indicator to separate malignant from benign tumours, while the scattering index is an indicator of diseased [malignant and benign] versus normal tissue," added Khayat.

Recent results

At the Biomedical Optics Symposium (BiOS) (San Jose, CA) last month, Khayat presented results from a clinical study examining SoftScan's effectiveness for breast tissue characterization. The study took place at three medical centres: CHUM Hotel-Dieu Hospital and the Royal Victoria Hospital (both in Montreal, Quebec) and Avon Comprehensive Breast Evaluation Center, Massachusetts General Hospital (Boston, MA).

To perform a tissue characterization, the patient lies on the SoftScan's tabletop with one breast suspended in the scanning cavity. The breast is gently compressed between two immobilization plates and surrounded by optical compensation fluid (oil in water). The total scanning procedure, which takes about an hour, comprises a contour imaging scan, a reference (single-wavelength) scan and the multiwavelength scan.

The researchers analysed data from 69 patients: 15 with malignant lesions (17 lesions in total), 25 with benign disease and 29 with normal tissue. The reference scans were used to select a 100 cm2 window for the main scan – either to encompass the lesion or, for healthy volunteers, at an arbitrary location in each breast. To analyse the collected data, they compared the lesion data with data from a background region representative of normal tissue.

Overall the study showed that the SoftScan technique achieved a sensitivity of 88.2% and a specificity of 88% – values that compare favourably with other diagnostic modalities used for breast imaging, such as MRI and ultrasound. Khayat and co-workers concluded that these results "demonstrate the ability of optical mammography to non-invasively characterize breast lesions in a statistically significant manner when used adjunctively with X-ray screening".

ART has recently received CE certification and Health Canada approval for the SoftScan system, enabling it to be sold throughout Canada and the European market. In the US, meanwhile, the company is conducting a pivotal study for approval by the Food and Drug Administration (FDA) of SoftScan as an adjunct to mammography.

"We expect the study to be completed by the end of 2007," said George Desypris, ART's director, clinical operations. "Once the data have been analysed and the final clinical study report is released, we will be filing for approval with the FDA. We expect this to occur in the second half of 2008."

ART is also undertaking a pilot study at Sunnybrook Health Sciences Centre in Toronto to assess the potential of optical imaging for monitoring the effectiveness of neoadjuvant treatment in breast cancer patients.

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