Since ultrasonic waves do not cause exposure and are basically non-invasive, ultrasonic image diagnosis is widely used. Particularly in recent years, the delay time generated for forming an ultrasonic beam can be easily controlled at high precision because of the advancement of digitization of ultrasonic imaging apparatuses. By this, a high quality biological tomographic image can be obtained using a compact apparatus, which is used for the diagnosis of each area. In addition to an ultrasonic echo diagnostic apparatus which irradiates ultrasonic waves onto a subject, receives the echo and generates an image thereof, a photoacoustic imaging apparatus which irradiates pulsed lights and receives photoacoustic waves (ultrasonic waves) generated inside the subject, and generates an image thereof, has also been proposed.
In conventional ultrasonic imaging apparatuses, delay time control, for forming an ultrasonic beam, is often performed based on the assumption that an ultrasonic wave propagates through a medium of which sound velocity is constant. On the other hand, if a material of which sound velocity is different from a biological tissue, that is a subject, such as a stationary plate for securing an acoustic lens layer on the surface of an ultrasonic probe and the biological tissue, exists between the biological tissue and a transducer which transmits/receives ultrasonic waves, the ultrasonic waves are refracted at the boundary of the areas having different sound velocities. As a result, in the case of a conventional delay time control, a focal point of an ultrasonic beam is not formed well, and image quality deteriorates. When a thick fat layer exists on the subject surface as well, ultrasonic waves are refracted in the same manner, since the sound velocities of the fat layer and the biological tissue under the fat layer are different, and as a result, the tissue under the fat layer is not clearly imaged.
A quantity of delay when a layer having a different sound velocity exists between a subject and a transducer can be analytically determined by considering a path of refraction using Snell's Law. Some methods of correcting the delay time using Snell's Law have been proposed.
U.S. Pat. No. 6,607,489 (PTL 1) discloses an ultrasonic apparatus having a stationary plate for securing a biological tissue, where delay time, due to the difference of the sound velocities between the stationary plate and a target tissue based on an accurate calculation, is stored in the apparatus in advance for each transducer and each focal point, and delay of the transmit/receive beam is controlled using this delay time. Calculating the delay time in real-time is also disclosed, but a concrete numerical computing algorithm is not disclosed.
WO 01-026555 (PTL 2) discloses an ultrasonic imaging apparatus which corrects the influence on refraction by an acoustic lens and a fat layer on the surface of a tissue, so as to suppress deterioration on the quality of an image of the tissue area under the fat layer of the subject. Since the influence on the refraction by a fat layer, which is relatively thick and which thickness is different depending on the target subject, the delay time in each transducer is determined by computation using a delay time correction approximate expression based on Snell's Law. In the method of WO 01-026555, the delay time in the next transducer is calculated using recurrence relation based on the delay time in the adjacent transducer. Thereby the computing time is reduced.    (PTL 1) U.S. Pat. No. 6,607,489    (PTL 2) International Publication No. WO01-026555