The transducer module consists of four components fabricated in the following order: a US transducer, the first optical prism, a PA detector, and the second optical prism. The laser light passes through the optical prisms mounted on both sides of the PA detector. This transducer module is scanned in the plane to measure a wide area of the breast. The main scan is a continuous reciprocating movement in the horizontal direction, synchronized with laser pulse irradiation. The typical scan speed used in our study was in the horizontal direction. The position detected by one element is the same as that of the neighboring element at the next laser pulse irradiation because both the interval between laser pulses and the time required to move the component to the next element are 100 ms. Therefore, as the PA detector is equipped with 20 elements in the horizontal direction, an average of 20 data points can be acquired in the same position by one-way scanning. The wavelengths of the laser irradiation in the first and second halves of a main reciprocating scan are 756 and 797 nm, respectively. The transducer module moves 10 mm in the vertical direction after each main reciprocating scan, which corresponds to 10 elements along the vertical axis. The main reciprocating scans overlap three times because the PA detector is equipped with 30 vertical elements; thus, an average of 60 data points can be obtained. This scan pattern contributes to lower levels of system noise because of the averaging of multiple data points and decreases the streak artifacts because the image is reconstructed using data from the entire scanned area. The subjects’ measurement area can be selected on the operation window of the personal computer that controls the PAM machine. The available area is 100 or 150 mm along the horizontal axis, and 30, 40, 50, 60, or 90 mm along the vertical axis, as shown in Fig. 5. The machine operator can select the position and size of the scan area according to breast size. More time is required for wider measurement areas. For example, the measurement time is for an area of and for an area of , each acquiring an average of 60 data points. In the clinical study, an appropriate scan area was set by a medical doctor based on the patient’s anatomy and general condition, as well as the extent of the tumor.