1. Field of the Invention
The present invention relates to an acoustic wave measuring system configured to receive an acoustic wave and generate image signals thereof; the present invention also relates to a method of generating image date based on the image signals generated by the measuring system.
2. Description of the Related Art
In recent years, photoacoustic tomography (PAT) devices have received increasing interest due to the remarkable advantages of PAT. In PAT, a subject is irradiated with pulsed light emitted from a light source such as a laser. The pulsed light is propagated through the subject, and an acoustic wave (e.g., an ultrasonic wave) is generated when the pulse light propagating through the subject is absorbed by a target for measurement located within the subject. The acoustic wave is received and measured by an acoustic wave receiver (transducer) placed around the subject. Then, the distribution of initially generated pressures and/or that of absorption coefficients observed within the subject is changed into image data (imaging) based on an electrical signal output from the acoustic wave receiver.
As a method of generating image data of a light absorber provided in the subject (performing the image reconstruction), there are known methods of generating image data by back-projecting a measured signal, including a time domain method, a Fourier domain method, etc. Taking a system for making a diagnosis of human body as an example, the sound speeds of parts (mediums) of the structure of the system may be different from each other. For example, when the acoustic wave receiver is provided on one side of a human body, and the subject is fixed and held with a compression plate, the sound speed of the subject may be different from that of the compression plate. Indeed, different tissue parts including a fat layer, a muscle layer or the like that are included in the subject have different sound speeds. In addition, acoustic wave refraction occurs on each interface between the above-described compression plate and the different tissue parts having different sound speeds. The acoustic wave refraction coupled with the different sound speeds causes image degradation. Therefore, a method of correcting the effect of the acoustic wave refraction occurring on the interface between the mediums (e.g., an interface between the compression plate and the subject) by applying Snell's law, and generating image data according to the time domain method has been proposed in U.S. Pat. No. 6,607,489.
However, according to the method of correcting the acoustic wave refraction by Snell's law, which is exemplarily disclosed in U.S. Pat. No. 6,607,489, points that are provided after the acoustic wave refraction is corrected are not provided at regular intervals. More specifically, the points are provided before the acoustic wave passes through the compression plate, and a signal s0(x0, t0) (illustrated in FIG. 7B herein) is obtained at each of the points. Hereinafter, each of the above-described points is referred to as a “virtual reception point”. Accordingly, an applicable back-projection method is limited only to the time domain method. Further, when the above-described refraction correction is applied for the time domain method, the virtual reception points are not provided at regular intervals, and an effective viewing angle is reduced because the distribution of the virtual reception points is tilted.