1. Field of the Invention
The present invention is directed to an X-ray computer tomography apparatus (i.e., X-ray CT apparatus) configured to scan using multiple row detection in which a plurality of detector elements are arrayed along a central axis direction of subject identified as the slices thickness direction.
2. Description of the Related Art
Conventional x-ray CT devices are single slice devices. A single slice x-ray CT apparatus has an x-ray source and a detector arranged at both sides of an object (for example, a patient). The detector includes about 1000 channels arranged in an arc shape along a direction perpendicular to a body axis (the channel direction).
In a single slice x-ray CT apparatus the x-ray source emits x-ray beams in a fan shape corresponding to a slice of the object to be examined. The detector detects the x-ray beams passes through the slice plane of the object. The acquired X-ray transmission data is sent to a data acquisition system (DAS) having a plurality of data acquisition elements corresponding to x-ray detector elements. The X-ray transmission data is amplified by each of the acquisition elements and projection data is acquired. Data at a predetermined rotation angle is called one view.
The conventional, single-slice x-ray CT apparatus repeats the data acquisition process about 1,000 times per rotation, emitting x-rays while the x-ray source and the detector rotate together around the object to be examined. Thus, the conventional x-ray CT apparatus acquires multidirectional projection data of the object to be examined and reconstructs an image of the slice plane based on the acquired multidirectional projection data.
The conventional x-ray CT apparatus is limited in its ability to scan a wide area in a short time because it only acquires a single image of a plane.
To address this limitation, multi-slice x-ray CT devices have been developed in recent years. The detector of the multi-slice x-ray CT apparatus is a multiple row detector (two-dimensional detector). The multiple row detector comprises detector elements arranged in M channels of N segments. The multi-slice x-ray CT has an x-ray source that emits fan shaped x-ray beams. In addition, the multi-slice CT acquires projection data of a plurality of slice planes simultaneously by emitting cone shaped x-ray beams and then detecting the x-rays passed through the object with the two-dimensional detector. Thus, the multi-slice x-ray CT can scan the wider area than the single slice x-ray CT.
The typical multi-slice x-ray CT apparatus includes a 4 slice type multi-slice CT. Recently, demand has risen for x-ray CT devices with a larger number of detector rows (e.g., 8 slice or 16 slice) that can produce wider area images. However, technical challenges are associated with building devices with larger numbers of detector rows.
The multi-slice x-ray CT apparatus emits fan shaped x-ray beams having a width spread in the direction of the body axis (in other words, a cone shaped x-ray beam). The multi-slice x-ray CT apparatus reconstructs a slice image using a fan-beam reconstruction method in which x-ray beams are assumed to intersect perpendicularly with the rotation center axis of the x-ray source.
However, if the number of the detector rows increases beyond 4, the x-ray beam emitted to the two-dimensional detector can not be assumed to intersect perpendicularly with the rotation center axis (especially, at the end of detector rows). This is because the x-ray beams are assumed to intersect perpendicularly with the rotation axis and a plurality of images as the multiple slices are then reconstructed, the images would generate artifacts which could lead to faulty images which may prevent their use in diagnosis.
In response to these limitations industry has sought an effective and practical method for cone-beam x-ray CT. A cone-beam x-ray CT apparatus can acquire a wider range of data more quickly than the multi-slice x-ray CT apparatus. Thus, it can shorten the scanning time. But, because the cone-beam x-ray CT apparatus needs to reconstruct an image by taking the cone angle into consideration, when many images of the thick slice thickness are reconstructed simultaneously the reconstruction time could become so long as to be limited by processing time.
Also, in order to acquire the data using a plurality of rows detector (for example, 32 rows and 64 rows), the number of data acquisition systems (DAS) must correspond to the number of rows of the detection element rows. However there is a limitation in the number of DAS elements that can be used due to limited mounting space in a system (gantry), cost, performance, and other considerations.
Japanese Patent Publication (KOKAI) NO. 9-192126, the entire contents of which are hereby incorporated by reference, discloses an x-ray CT apparatus which selects fan-beam reconstruction or cone-beam reconstruction according to the slice position to be reconstructed. However, the image quality (noise level) of each slice is not constant when reconstructing a plurality of slices in a scanning range. This is because the quality of the fan-beam reconstructed image is different from the quality of the cone-beam reconstructed image and the CT apparatus reconstructs a plurality of slice image in the scanning range using the cone-beam reconstruction and the fan-beam reconstruction. For example, when 6 images are reconstructed from projection data obtained from one scan, the prior art generates 6 images whose quality levels (noise levels) are a little different, because the 4 central images are reconstructed using the fan-beam reconstruction while the 2 peripheral images (one on each side of the 4 central images) are reconstructed using the cone-beam reconstruction. The difference in the image quality of the images produced from the common scan may cause confusion or extra efforts to a doctor to equalize the image quality. The difference of image quality is caused by the difference of the reconstruction method. In the fan-beam reconstruction, a tomographic image is backprojected along “single ray” of x-ray beams, because x-ray beams are assumed to intersect perpendicularly with the rotation center axis of the x-ray source. But, in the cone-beam reconstruction a tomographic image must be backprojected along “a plurality of” rays of x-ray beams. In the cone-beam reconstruction, it is assumed that the image comprises a plurality of voxels, and respective voxel is backprojected along a ray of x-ray beam. By not properly taking into account these differences in backprojection, image distortion can occur.