1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus for imaging organs within a living body and so on by transmitting and receiving ultrasonic waves to generate ultrasonic images to be used for diagnoses.
2. Description of a Related Art
In medical fields, various imaging technologies have been developed for observing inside of an object to be inspected so as to make diagnoses. Especially, ultrasonic imaging for acquiring interior information of the object by transmitting and receiving ultrasonic waves enables image observation in real time and provides no exposure to radiation unlike other medical image technologies such as X-ray photography or RI (radio isotope) scintillation camera. Accordingly, ultrasonic imaging is utilized as an imaging technology at a high level of safety in a wide range of departments including not only the fetal diagnosis in obstetrics, but gynecology, circulatory system, digestive system, and so on.
The principle of ultrasonic imaging is as below. Ultrasonic waves are reflected at a boundary between regions with different acoustic impedances like a boundary between structures within the object. Therefore, by transmitting ultrasonic beams into the object such as a human body, receiving ultrasonic echoes generated within the object, and obtaining reflection points where the ultrasonic echoes are generated and reflection intensity, outlines of structures (e.g., internal organs, diseased tissues, and so on) existing within the object can be extracted.
The acoustic impedance is a constant intrinsic to a material as expressed by equation (1), and the unit of MRayl (mega Rayl) is generally used therefor and 1 MRayl=1×106 kg·m−2·s−1.Z=ρ·C  (1)where ρ represents density of an acoustic medium and C represents acoustic velocity within the acoustic medium.
Further, given that the acoustic impedance of the first medium is Z1 and the acoustic impedance of the second medium adjacent to the first medium is Z2, the vertical reflectance R of ultrasonic waves at the interface between the first medium and the second medium is given by the following equation (2).R=(Z2−Z1)/(Z2+Z1)  (2)
Generally, an ultrasonic image is generated based on the intensity of ultrasonic waves reflected at the respective sample points within the object. Since the acoustic velocities are different depending on tissues within the object, there are problems that defocusing occurs in reception focusing processing and/or transmission focusing processing, the resolving power becomes lower, and image blurring occurs.
As related technologies, Japanese Patent Application Publication JP-A-5-329159 discloses an ultrasonic diagnostic apparatus in which images in the optimum focused state can be obtained consistently regardless of variations in conditions of acoustic velocity distribution within the object and so on. The ultrasonic diagnostic apparatus includes transmission focusing means for performing transmission focusing by sequentially selecting plural transmission focusing patterns and determining driving timings of plural ultrasonic vibrators according to the selected transmission focusing patterns, reception focusing means for performing reception focusing on echo signals outputted from the plural ultrasonic vibrators by providing delays according to plural reception focusing patterns in time sequence or at the same time, signal processing means for generating image data based on the echo signals reception-focused by the reception focusing means, image data storage means for storing plural image data using plural combinations of transmission focusing patterns and reception focusing patterns for the same part of the object, focusing evaluation means for selecting image data in the optimum focused state by comparing predetermined feature quantities of the respective image data of the plural image data stored in the image data storage means, and image forming means for forming tomographic images based on the image data selected by the focusing evaluation means.
Japanese Patent Application Publication JP-A-59-2127 91 discloses an ultrasonic imaging system of aperture type for automatically focusing on a part desired to be observed. According to the ultrasonic imaging system, all acoustic velocity setting values are determined such that the sharpness of the image within a small region of interest is the maximum, and then, an acoustic correction value for a partial aperture is determined such that the image within the small region due to the partial aperture has the same correlation value with the first obtained image within the small region, and the acoustic correction values corresponding to the respective parts of the entire aperture are determined while the partial aperture of interest is moved and varied in size to cover the entire aperture.
Japanese Patent Application Publication JP-P2007-7045A discloses realization of estimation of the acoustic velocity of a living body in an ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus includes an ultrasonic probe having arranged plural vibrators, a transmission circuit for transmitting ultrasonic waves to the object via the ultrasonic probe, a reception circuit for receiving echo signals from the object via the ultrasonic probe, an intensity distribution generating unit for generating plural ultrasonic intensity distributions at different acoustic velocities set for delay control.
As disclosed in these documents, it is possible to obtain the optimum acoustic velocities in the respective regions within the object. However, if the optimum acoustic velocities are obtained in the small regions, there are problems that the number of data for obtaining the acoustic velocities is reduced and the calculation accuracy becomes lower, and the number of regions is increased and the time for calculation is increased. Further, if an image is formed by using acoustic velocities different with respect to each region, artifacts would occur at boundaries between regions.