1. Technical Field
The present invention relates to an ultrasonic diagnostic apparatus and in particular to an ultrasonic diagnostic apparatus which is used for diagnosis of hard tissue such as bone.
2. Related Art
Easy quantitative measurement of mechanical characteristics such as bone strength is desired for diagnosing bone metabolic diseases such as osteoporosis, judging fracture risk, and quantitatively diagnosing bone union after treatment of bone fracture.
The evaluation of bone formation and bone union depends largely on X-ray photography, but quantitatively diagnosing bone strength by means of X-ray photography is very difficult. As a conventional method of measuring bone strength there is known a strength test of a sample bone of a measurement target. However, this method requires an extraction operation for obtaining a sample bone, and thus, the method is invasive. A method of measuring an amount of bone and a bone density has employed devices such as X-ray CT and DXA (dual-energy x-ray absorptiometry). However, these devices are merely means for measuring the amount of bone and cannot provide an evaluation of bone strength. Moreover, in view that tissue is irradiated with X-rays in these methods, these methods cannot be considered non-invasive.
Other attempts to quantitatively evaluate bone strength include a strain gauge method in which a strain gauge is mounted on an external fixtator and the strain of the external fixtator is measured; a vibration wave method in which a vibration is applied to a bone from the outside and a characteristic frequency is evaluated; and an acoustic emission method in which acoustic waves generated by a bone which has reached yield stress are detected. These methods, however, have various problems in that a limitation is imposed on the treatment to which these methods can be applied, that the bone is subjected to invasion, and that evaluation precision is insufficient.
In view of the above circumstances, the inventors of the present application have proposed an ultrasonic diagnostic apparatus for noninvasively and quantitatively evaluating the mechanical characteristics of bone (Japanese Patent Laid-Open Publication No. 2004-298205).
The ultrasonic diagnostic apparatus described in Japanese Patent Laid-Open Publication No. 2004-298205 forms a plurality of ultrasonic beams on a bone, obtains a plurality of echo signals corresponding to the individual ultrasonic beams to specify a surface point corresponding to the bone surface for each echo signal, and generates shape data of the bone surface on the basis of the plurality of surface points obtained from the plurality of echo signals. Moreover, a mechanical characteristic of the bone is evaluated on the basis of a change in the shape data when an external load is applied to the bone. Thus, the apparatus is an epoch-making technology capable of noninvasively and quantitatively evaluating the mechanical characteristics of a bone in a living organism from the shape data of the bone surface on the basis of the echo signals.
The inventors of the present application have further improved the epoch-making technology described in Japanese Patent Laid-Open Publication No. 2004-298205 and have studied a method of more appropriately evaluating the mechanical characteristics and the like of hard tissue such as bone.