Radiology: Optical + x-ray imaging may best characterize breast tumors

Reconstructed image sections for the right breast in 45-year-old woman. DBT image and HbT (micromoles per liter), So2, and Us (cm-1) images at 830 nm. The breast contains a 2.5-cm invasive ductal carcinoma. Image Source: Radiology

Coregistered optical and structural x-ray imaging was able to noninvasively differentiate malignant from benign breast lesions, offering the potential to reduce the number of unnecessary biopsies resulting from mammography, according to a study published online in Radiology.

"The well-established image interpretation, general availability, and good image resolution render conventional mammography the obvious choice for radiologists for breast screening," noted Qianquian Fang, PhD, of the Martinos Center for Biomedical Imaging at Massachusetts General Hospital in Boston, and co-authors. "However, the high false-positive rate and the marginal contrast between tumor and dense breast tissue remain major challenges" for mammography cancer screening in women.

Citing the promising improvements of adding functional imaging modalities to structural x-ray imaging for the detection and improved differentiation of malignant and benign tumors, the researchers constructed a combined optical and x-ray imaging system for noninvasive acquisition of coregistered functional optical and structural x-ray images of the targeted breast.

"Our purpose was to explore the optical and physiologic properties of normal and lesion-bearing breasts by using a combined optical [diffuse optical tomography, DOT] and DBT [digital breast tomosynthesis] imaging system," the authors wrote. Optical source and detector probes were attached to a DBT system to acquire the optical images, after which researchers obtained a DBT scan of the patient's breasts without repositioning.

The researchers used several steps to process the DOT measurements with the DBT measurements, including: reconstruction of the 3D DBT images; spatial registration between the DBT images and the optical probes; raw optical data calibration; and 3D breast mesh generation from the DBT images.

A spectrally constrained reconstruction approach was implemented to recover the hemoglobin (Hb_T) concentrations and tissue scattering images directly from multiwavelength measurement to attempt to differentiate the dissimilarities in molecular concentrations in malignant and benign lesions as well as other breast tissue.

A total of 189 breasts were included in the sample of images, including 26 histologically confirmed malignant lesions, 25 benign lesions and 138 normal findings. Average bulk Hb_T concentration for the 138 normal breasts was estimated from the 3D maps at 19.2 mol/L. Mean oxygen saturation (So₂) was measured at 0.73. Both of these values were within the normal range established within the literature, according to the authors.

Normalized Hb_T concentrations were significantly higher in cancerous tumors than benign lesions and cysts within the same breast, measuring at 1.37 in malignant tumors, compared with 1.19 (P=0.025) in benign lesions and 1.06 in cysts (P=0.0033).

The scattering coefficient contrast of adipose tissue for malignant tumors was also significantly higher than the contrast of fibroglandular tissue in normal breasts at P = 0.0083. In addition, cysts had significantly lower So₂ values than malignant and solid benign lesions, with normalized values of 0.95, 1.01 and 1.01 at P<0.0005 and P=0.026, respectively.

"We were able to recover diagnostic, relevant physiologic images, interpret the results with coregistered functional and structural image overlays, and demonstrate a significant difference in Hb_T among malignant tumors, solid benign lesions, and cysts and significant differences in So₂ between solid benign lesions and cysts," the authors reported.

The researchers outlined several limitations to the study, including the technique's tradeoff: increased temporal resolution of the optical system for reduced spatial resolution. The authors also cautioned that, "[g]iven the limited number of lesions involved in this study, the results reported here are rather preliminary and merely serve as an early indication of the diagnostic potential of the proposed method. Cases with false positive results can occur if optical images are used by the readers for lesion detection, and cases with false-negative results can occur if a malignant lesion is small and yields Hb_T values in the normal fibroglandular tissue ranges."

Still, Fang and co-authors claimed that their findings of differentiable Hb_T and So₂ values in cancerous and noncancerous breast tissue "can potentially be exploited to reduce the false-positive rate of conventional mammography and unnecessary biopsies."