In the medical field, a doctor displays the medical images obtained by imaging a patient on a monitor, interprets the displayed medical images, and observes the state of a morbid portion and temporal changes in it. Apparatuses which generate this type of medical images include, for example, an X-ray imaging apparatus, X-ray computed tomography apparatus (X-ray CT), magnetic resonance imaging apparatus (MRI), nuclear medicine diagnostic apparatus (SPECT or PET), and ultrasonic image diagnostic apparatus (US). In image diagnosis in which a doctor diagnoses a patient by using the images obtained from such apparatuses which generate medical images, the doctor selects proper apparatuses depending on the region or disease to be diagnosed in consideration of the differences in characteristic between the apparatuses. In some case, a doctor performs diagnosis by using a plurality of images obtained from a plurality of apparatuses in order to improve the accuracy of the diagnosis. For example, it is possible to obtain information more effective for diagnosis by imaging the same object using both an ultrasonic image diagnostic apparatus and an MRI and using a combination of the captured images.
When performing diagnosis using a combination of ultrasonic images and MRI images, it is effective to perform diagnosis upon associating corresponding regions between two images. For this purpose, it is necessary to perform alignment between the two images. In order to implement this, there are several problems to be solved. The first problem is that since a doctor or technician images an object to be examined while holding an imaging probe (to be referred to as a “probe” hereinafter) and freely moving it, the correspondence between a position in a space with reference to the object and a captured image is not clear. The second problem is that an image captured as an ultrasonic image is generally a two-dimensional tomogram of the inside of an object to be examined, and hence differs in the spatial dimension of information which can be obtained from an MRI image obtained by capturing three-dimensional information.
One of the approaches to these problems is a method of measuring the position and orientation of a probe by using an external sensor. Japanese Patent No. 03871747 discloses a technique of obtaining a slice image corresponding to a region imaged by an ultrasonic image diagnostic apparatus from a three-dimensional medical image, captured in advance, in accordance with the measurement value from a device for measuring a position and orientation which is mounted on the probe, and displaying the images side by side. This technique allows to observe an ultrasonic image of a two-dimensional slice in association with three-dimensional information obtained by another modality.
Studies have been made to align these two modalities by using the image information of an ultrasonic image and three-dimensional medical image. The technique disclosed in W. Wein, B. Roper, and N. Navab, “Automatic registration and fusion of ultrasound with CT to radiotherapy”, Proc. MICCAI 2005, vol. 2, pp. 303-311, 2005 generates an ultrasonic simulation image based on a three-dimensional medical image obtained in advance. This technique associates an ultrasonic image actually captured by an ultrasonic image diagnostic apparatus with a simulation image based on image information.
Japanese Patent Laid-Open No. 2010-131269 discloses a technique of improving the processing efficiency by obtaining a surface shape of an object from a three-dimensional medical image, and limiting a search range for association by associating images under conditions that make the positions of the surface shape and the imaging surface of a probe match, based on the positions of the surface shape and imaging surface of the probe.
According to the technique disclosed in Japanese Patent No. 03871747, it is difficult to accurately align an ultrasonic image with a three-dimensional medical image when the accuracy of a sensor is insufficient. In addition, even with the use of a high-accuracy position and orientation sensor, if an imaging region deforms due to the body movement of an object or the pressing force of the probe, it is also difficult to accurately perform alignment. On the other hand, the technique disclosed in W. Wein, B. Roper, and N. Navab, “Automatic registration and fusion of ultrasound with CT to radiotherapy”, Proc. MICCAI 2005, vol. 2, pp. 303-311, 2005 can perform accurate alignment including the correction of the above deformation by performing alignment processing between an ultrasonic image and an MRI image based on image information. However, since this technique requires a very large amount of calculation, it is difficult to perform alignment at high speed. This impairs the synchronization between imaging and observation. In addition, since the technique disclosed in Japanese Patent Laid-Open No. 2010-131269 performs processing on the assumption that the overall imaging surface of the probe is in contact with an object, when the probe is in contact with only part of the object, the accuracy of an alignment result deteriorates.