1. A medical image processing apparatus comprising: an acquiring unit configured to acquire volume data of a heart; a coordinate system detecting unit configured to detect a three-dimensional left ventricle coordinate system composed of three axes including at least a left ventricle long axis of the heart from the volume data; a boundary detecting unit configured to use a boundary model expressed in the left ventricle coordinate system to detect a left ventricle boundary from the volume data by deforming the boundary model so that an error between a boundary pattern obtained by applying the boundary model to the volume data and a predetermined boundary pattern model becomes small; and a display monitor configured to display the detected left ventricle boundary on a cross-sectional image of the heart, the cross-sectional image being orthogonal to at least one axis of the three axes of the left ventricle coordinate system, wherein the boundary pattern model is obtained by modeling a pattern of brightness values in a myocardium and a surround of the myocardial boundary in an image enhanced by a contrast agent by learning, the apparatus further comprises: a calculating unit configured to calculate a degree of staining indicating a concentration of the contrast agent of a predetermined regional area in the volume data on the basis of the left ventricle coordinate system, and a correcting unit configured to use a degree of staining of the regional area to perform a correction to cause brightness values of the volume data and brightness values of the boundary pattern model to go close to each other in the corresponding regional area, and the boundary detecting unit detects a myocardial boundary in the volume data by using the data after the correction by the correcting unit.