# Photoacoustic Imaging for Management of Breast Cancer: A Literature Review and Future Perspectives
## 3 Results
### Description of Studies
#### 3.1.1 Phantom Studies
The field of view (FOV) and penetration depth of the system were \(170\times 170\times 170\) mm and 40 mm, respectively, while its spatial resolution was 2 mm in XY plane and 6 mm along the Z axis. Combining thermoacoustic (TA) imaging with PA imaging, [PERSON] et al. [60] developed a dual mode imaging system to detect lesions and objects deeply situated in biological tissues. Their PA imaging system contained four US transducers with different central frequencies of 2.25 MHz, 3.5 MHz, 5 MHz, and 10 MHz and all with bandwidths ranging from 50% to 80% of the corresponding central frequencies. Using PA images acquired at 1064 nm with a 3.5 MHz cylindrically focused US transducer, their system was able to detect blood objects embedded in chicken breast muscle up to a depth of 5 cm. [PERSON] et al. [69] developed a multimodal breast cancer imaging system containing TA and PA imaging systems. PA images of breast-tissue mimicking phantoms were acquired at 1064 nm using two US transducers of 13 mm and 6 mm diameter active area. The central frequency of these US transducers was 2.25 MHz, and their resolution was reported to be approximately 0.7 mm. [PERSON] et al. [70] developed a tri-modality imaging system by integrating PA and TA imaging in a modified commercial ultrasound scanner, and it was tested on a chicken breast tissue. PA images were acquired at 650 nm using a US transducer phased array consisting of 80 elements with a frequency band of 1 to 5 MHz. Maximum penetration depth achieved with this system was 6.6 cm at a signal to noise ratio (SNR) nearly equal to 24 dB. The axial resolution of the PA imaging system was 640 \(\upmu\)m, and the lateral resolution was in the range from 720 \(\upmu\)m to 2 mm, whereas the best elevation resolution achieved with this system was 3.5 mm. [PERSON] et al. [71] developed a 3D PA imaging system in line confocal mode which used a 64 element linear US transducer array with a central frequency of 7.5 MHz and 70% bandwidth to detect the PA signals. Using a phantom containing carbon points, they found an improved spatial resolution of the PA imaging system when it was operated in the confocal mode. [PERSON] et al. [72] combined PA imaging system with diffuse optical tomography (DOT) system to develop a hybrid imaging system for breast cancer detection. US transducer consists of a ring-shaped array of 64 elements with frequency response from 380 KHz to 1.48 MHz, and maximum frequency response was up to 2 MHz. They reported that the spatial resolution of the PA imaging system used in this study was \(\sim\)0.5 mm. This system was tested on a tumor (removed from a rat bearing a 4T1) of 3.5 mm in radius embedded in the phantom background. The absorption image of PA imaging system estimated the size of the tumor as 3 mm in radius. [PERSON] et al. [73] developed a photoacoustic imaging system for breast cancer with a spherical detector aperture along which 512 US detectors were arranged. The center frequency of the US detector was 2 MHz with a 70% bandwidth. Using suitable phantoms, they showed that the imaging system could clearly visualize a target up to a depth of 53 mm inside tissue with a minimum spatial resolution equal to 0.42 mm  
[PERSON] et al. [74] employed a photoacoustic tomography system at 800 nm wavelength using a 10 MHz single element focused US transducer to image astaxanthin-mediated gold nanoparticles (Atx-AuNPs)-treated breast cancer cells in a gelatin-based phantom mimicking biological tissue. Details of various materials used to mimic breast tissue, tumors/blood vessels/targets in phantoms of the above phantom studies were summarized in Table 3.