# Photoacoustic Imaging for Management of Breast Cancer: A Literature Review and Future Perspectives
## 4 Discussion
### Comparison of PA Imaging with Other Imaging Modalities
#### 4.1.1 PA Imaging Versus X-ray Mammography and US Imaging  
The most commonly used imaging modalities for breast cancer screening or detection are X-ray mammography and US imaging. As mentioned earlier, the research groups of [PERSON] et. al. [35; 117], [PERSON] et al. [111], [PERSON] et al. [37], [PERSON] et al. [106], and [PERSON] et al. [108] compared the results of PA imaging with that of X-ray as well as US imaging. [PERSON] et al. [117] showed that the PA contrast generated by malignant pathology exceeded the corresponding X-ray contrast. They identified [35] 3 cases among a total of 32 cases, for which the lesions could not be detected using X-ray mammogram while the same lesions appeared with high contrast in the corresponding PA images. They observed that in the case of X-ray mammogram, the contrast of lesions in high-density breasts was lower than those in the low-density breasts while they could not find any significant difference in the average contrast of lesion PA images between the high-density and low-density breasts. According to their results, the deviation of the average size of the lesion detected in PA mammography with respect to the corresponding histopathology was 28% whereas, in X-ray mammography, it was 34%. In a pilot study involving eight patients, with a 3D PA imaging system, [PERSON] et al. [106] detected tumors which were not clearly distinguishable with X-ray mammograms for two patients. From these studies, it was observed that PA imaging could be effectively used to visualize the tumors in radiographically dense breasts and occult lesions, which were not clearly identified with X-ray mammograms. For breast cancer detection using PA imaging, the contrast of the tumor is due to the difference in concentration of different forms of hemoglobin (primarily deoxygenated hemoglobin (Hb) and oxygenated hemoglobin (HbO\({}_{2}\))) between tumor and background tissue, which is independent of the mammographically-estimated breast density [117]. The malignant tumors are having higher total hemoglobin concentration and lower oxygen saturation as compared to background breast tissue. PA imaging provides tumor vasculature and oxygenation, which cannot be obtained by X-ray mammogram [108]. As far as US imaging is concerned, [PERSON] et al. [35] found that out of the 31 patients, for one patient the malignant lesion could not be detected using US image whereas it appeared with high contrast in the corresponding PA image. The average size deviation of the lesion detected using PA imaging with respect to the corresponding histopathology was 28%, whereas, in the case of US imaging, it was 18%. [PERSON] et al. [108] acquired in vivo images of 27 breast cancer lesions and concluded that information regarding the tumor vasculature and oxygenation could be obtained from PA images while US or X-ray imaging could not provide similar information. [PERSON] et al. [111] found that their ImagioTM breast imaging system provides better specificity and equal sensitivity as compared to conventional US imaging. [PERSON] et al. [37] performed a multi-center pivotal study with their dual mode PA-US imaging system, and from the results of their study, they claimed that the diagnostic specificity of the fused PA-US imaging system was improved 14.9% over the internal US imaging system alone. US is one of the most common imaging modalities which has been widely used by radiologists for decades, while PA imaging is an emerging imaging modality. Due to the high cost of the nanosecond duration pulse laser, the cost of PA imaging system is higher than the US imaging system. However, as both PA imaging and US imaging share the common detector technology, almost all the PA imaging systems can be used as dual mode PA US imaging systems to exploit the advantages of both PA and US imaging.