# Photoacoustic Imaging for Management of Breast Cancer: A Literature Review and Future Perspectives
## 1 Introduction
Due to low soft tissue contrast and strong operator dependency, US imaging is not a very efficient tool for breast cancer detection. Both the US image and X-ray mammogram provide morphological information related to the breast [10]. As in the early stages, breast cancer does not produce significant morphological changes, and X-ray mammogram and US imaging may not be suitable for early detection of breast cancer. MRI is a noninvasive imaging modality that does not involve any ionizing radiation. It is capable of providing better sensitivity than X-ray mammogram or US imaging for breast cancer detection [15]. Imaging of the dense breast tissues and breast implants can be successfully accomplished using MRI while they cannot be performed using X-ray mammogram [14; 16; 17]. MRI plays an important role as a diagnostic tool especially in detecting multifocal and contralateral or multicentric breast cancers, in detecting occult primary breast cancer, which is common in patients with axillary lymph node metastasis, and also in monitoring neoadjuvant chemotherapy and identifying the recurrences [14; 16; 18; 19]. It provides good spatial resolution and it may also be useful in indicating whether or not the breast cancer has spread into the chest wall. The quality of the MRI depends on the patient's hormonal status [20; 21] so it requires contrast agents. It is an expensive detection method and consequent inaccessibility is its major drawback. It takes a longer time compared to mammography. Apart from high cost and longer imaging time, MRI also has some other limitations such as low specificity and high false positive rates [15; 17; 18; 22]. In the preoperative setting, this low specificity of MRI leads to diagnostic ambiguity, which may lead the surgeons to perform more extensive breast cancer operations. This causes no improvement in overall survival rates and also fails to reduce the local recurrence rates [15]. Although MRI is generally considered to be more accurate than X-ray mammograms, occasionally it may fail to detect breast cancer, particularly involving calcium depositions for a particular patient, which can be detected by X-ray mammography [23; 24]. Due to factors such as occasional low specificity, high cost, the requirement of contrast agents, unacceptability for pregnant women,and patients with metal implants, MRI is not a suitable imaging modality for early detection of breast cancer [10; 14; 18; 25; 26]. Angiogenesis, i.e., the production of new blood vessels, is a primary hallmark for detection of malignant lesions [27; 28; 29]. This hallmark can be detected by probing the breast with optical radiation at the NIR (near-infrared) region. However, pure optical imaging suffers from degrading spatial resolution with an increase of the depth in soft tissue, whereas photoacoustic (PA) imaging is capable of maintaining good spatial resolution inside the tissue [30]. PA imaging is an emerging hybrid imaging modality based on the PA effect. In the PA effect, after absorbing a short duration (nanosecond) pulse laser light, the light absorbing material (soft tissue) generates US waves [31; 32]. The absorbed light energy creates localized heating inside the light-absorbing material which, in turn, produces an increase of pressure and this pressure is released in the form of broadband US waves [33]. As it uses light to probe the tissue, PA imaging can determine functional information in terms of optical properties of the tissue which change with the change of the distribution of blood vessels, as well as other chromophores (light-absorbing tissue elements) inside the tissue. At the same time, since it detects US waves which do not scatter much inside soft tissue, PA images do not suffer from the problem of poor spatial resolution deep inside the soft tissue. The low-intensity non-ionizing laser light used in PA imaging poses no health hazard, unlike the ionizing radiation associated with X-ray screening mammography.