1. Field of the Invention
The present invention relates to an ultrasound imaging apparatus capable of acquiring an ultrasound image representing a subject with ultrasound waves and capable of evaluating a motional state of the subject by using the ultrasound image, and also relates to a method for processing an ultrasound image.
2. Description of the Related Art
It is very important to objectively and quantitatively evaluate the function of body tissue such as the myocardium of the heart when diagnosing the body tissue. For example, a quantitative evaluation method based on image data representing the heart that is acquired by an ultrasound imaging apparatus is proposed.
As an example, a technique of tracking using local pattern matching on a two-dimensional ultrasound image or a three-dimensional ultrasound image to calculate local wall-motion information such as displacement and strain of the myocardium (referred to as Speckle Tracking (ST) hereinafter) is practically used (e.g., Japanese Unexamined Patent Application Publication No. 2003-175041, and Japanese Unexamined Patent Application Publication No. 2003-250804).
In the ST method, the contours of the endocardium and epicardium of the myocardium are given as initial tracking positions in the end diastole (a time phase in which an initial R wave is detected) or the end systole. In the remaining time phases, the initial tracking positions are automatically tracked by using movement-vector information obtained by local pattern matching, whereby the contours of the endocardium and epicardium in all the time phases are tracked.
However, in the method according to the related art, there is a problem of occurrence of a tracking miss within one heartbeat (Problem 1).
Moreover, there is a problem of degradation of the tracking accuracy in the case of tracking over a plurality of heartbeats (Problem 2).
As for Problem 1, the contour tracking tends to deviate in a time phase T1 after a time phase T0 in which wall-motion velocity is the fastest in one heartbeat (an early diastolic phase e′ in a normal case, or an atrial contraction phase a′ in a diastolic dysfunction case). In this case, even if correction of the contour is made in the time phase T1 and the tracking is restarted in this time phase, the tracking will eventually deviate at the time of tracking the time phase T0 in the opposite direction.
This problem I will be described with reference to FIG. 1. FIG. 1 is a graph illustrating wall-motion velocity and strain (displacement). In FIG. 1, the horizontal axis takes a time t. A waveform pattern 500 represents the wall-motion velocity in the normal case. A waveform pattern 540 represents the wall-motion velocity in the diastolic dysfunction case. When the tracking does not deviate in the time phase e′ of the normal case, the strain (displacement) is accurately evaluated as a waveform pattern 510 of the normal case. On the other hand, the wall-motion velocity in the time phase e′ is the fastest within one heartbeat in the normal case. Therefore, if the tracking in the forward direction deviates, the tracking position returns to the original position in accordance with movement in the subsequent time phase a′, and the strain (displacement) forms a waveform pattern 520. In this case, it is difficult to distinguish from a waveform pattern 550 in which diastolic dysfunction results from an ischemic heart disease, etc.
Thus, a case of resetting the initial tracking position in the time phase T1 and tracking in both temporal directions in this case will be considered.
However, since an error occurs in an estimated movement vector obtained by tracking in the forward direction and the tracking in the forward direction deviates, an error similarly occurs in an estimated movement vector at the time of tracking through the time phase T0 in the opposite direction.
Because the tracking in the opposite direction deviates, the peak position cannot reach the peak position at the time of tracking in the forward direction, and the peak value of the waveform decreases as shown in a waveform pattern 530. In this case, it is difficult to distinguish from a waveform pattern in which systolic failure occurs due to an ischemic heart disease or the like. In any event, it becomes difficult to accurately evaluate the normal pattern.
In order to solve this problem, when manually correcting the tracking position deviated in the time phase T1, it is necessary to manually correct the tracking position throughout the entire interval between the time phase T1 and the end of atrial systole (a time phase in which the next R wave is detected). Therefore, such correction requires time and is not easy.
As for Problem 2, in general, tracking for long time results in accumulation of errors and easy deviation of the tracking. Moreover, if movement of a subject due to breathing, etc. or movement of an ultrasound probe occurs during data acquisition, drift components are also accumulated.
As a result, the assumption of processing using periodicity (the assumption that the position returns to the original position after one heartbeat) gradually deviates, whereby the tracking accuracy is lowered.