# Photoacoustic Imaging for Management of Breast Cancer: A Literature Review and Future Perspectives
## 3 Results
### Description of Studies
#### 3.1.1 Phantom Studies  
It was found from the literature that during 1997-2016, several research groups have performed phantom studies to understand the potential of PA imaging for breast cancer detection. In 1997, the research group headed by [PERSON] [54] performed the first phantom study to evaluate the capability of PA imaging for breast cancer detection. In 2005, research groups led by [PERSON] [59] and [PERSON] [60] performed phantom studies using PA imaging for breast cancer detection. Later on, other research groups performed phantom studies using PA imaging for breast cancer detection. Details of these studies were given below. [PERSON] et al. [54] reported the usefulness of photoacoustic imaging for breast cancer detection using a phantom study. They successfully visualized small spherical tumor phantoms of 2 mm diameter which are located within the bulk collagen gels at a depth of up to 60 mm within breast phantoms of 100 mm thick. [PERSON] et al. [61] built the first PA imaging system--laser optoacoustic imaging system (LOIS)--which could be used for breast cancer detection in a clinical environment. This imaging system was operated at a single wavelength 1064 nm, contained a 12-element linear array US transducer. Using this system, gel spheres of 8 mm diameters were detected at a depth of 50 mm inside breast tissue-mimicking phantom. Using LOIS-02, in which the linear transducer array of 12 elements of the LOIS system was replaced by a 32 element arc-shaped transducer array, [PERSON] et al. [62] successfully detected tumors of size 2 mm into the 6 cm deep breast tissue-mimicking phantoms. The Bandwidth of the US transducer was in the range from 20 kHz to 2 MHz. The in-depth resolution and lateral resolution of this system were 0.4 mm and 1-2 mm depending on the position of the tumor relative to the transducer array. In another modification of the LOIS system, [PERSON] et al. [63] used a 32-element arc-shaped polyvinylidene fluoride (PVDF) transducer array for PA wave detection. This system successfully detected blood vessels of 2 mm (diameter) at a depth of 7.5 cm inside the breast tissue-mimicking phantoms. [PERSON] et al. [64] performed another phantom study with LOIS system in which PA images were acquired at two different wavelengths, 757 nm and 1064 nm, and the acquired PA data were used to calculate oxygen saturation (SO2) of blood. [PERSON] et al. [65] developed a new version of LOIS system, named LOIS-128 in which a linear US transducer array of 128 elements with a system cutoff frequency of 4 MHz was used to detect PA waves. They claimed that LOIS-128 was capable of continuous planar imaging of small tumors (less than 1 cm) at depths of more than 6 cm inside the tissue. [PERSON] et al. [66] developed and used the LOIS-64 system which contained US transducers in the form of an arc-shaped array of 64 elements with bandwidth up to 2.5 MHz to acquire PA images of breast phantoms and achieved a spatial resolution of 0.5 mm. [PERSON] et al. [59] developed a PA mammography system, 'Twente PAM', which contained a laser light delivery system at 1064 nm and a flat US detector matrix of 590 elements. The central frequency of US detector was 1 MHz with a bandwidth in the range from 0.45 MHz to 1.78 MHz. This system was capable of imaging inhomogeneities of 2 mm diameter at depths up to 32 mm with an axial resolution of 3.5 mm. Using Twente PAM, [PERSON] et al. [67] imaged breast tumor vasculature inside Polyvinyl Alcohol (PVA) breast phantom. The lateral resolution of the US detector was in the range 2.5 mm to 3.9 mm and an axial resolution was in the range from 2.3 mm to 3.9 mm. [PERSON] et al. [68] developed a 3D photoacoustic full breast tomography system and evaluated its performance using tissue-mimicking phantoms. US transducer was a linear array of 8 elements with 0.6 MHz center frequency and 75% bandwidth.