X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2002-01-16
2002-10-08
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000
Reexamination Certificate
active
06463116
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to radiographic apparatus of the non-CT (Computed Tomography) type used in medical, industrial and other fields for producing sectional images of patients or objects under examination. More particularly, the invention relates to a technique for shortening a processing time needed for image reconstruction and reducing artifacts appearing in the sectional images.
(2) Description of the Related Art
Conventional radiographic apparatus include an X-ray radiographic apparatus, for example. The X-ray radiographic apparatus has an X-ray tube and an image intensifier opposed to each other across an object under examination. The X-ray tube is linearly moved in a first direction, and in synchronism therewith the image intensifier is moved in a second direction counter to the first direction. With this movement, the apparatus intermittently performs radiography while varying an angle of X-ray emission from the X-ray tube to the object, such that a given point in a particular sectional plane of the object always corresponds to the same location on the detecting plane of the image intensifier. Then, a process is carried out simply to add detection signals so as to overlap a plurality of projected images acquired by radiography done from varied angles. In this way, the apparatus derives image information on a particular section of the object and image information on adjacent sections at opposite sides of the particular section.
Thus, the above X-ray radiographic apparatus is based the non-CT type radiographic technique distinct from the X-ray CT type radiographic technique which has made remarkable progress in recent years. That is, the X-ray CT type radiographic technique acquires transmitted images by driving an X-ray tube and an image intensifier opposed to each other across an object under examination to make one revolution (at least a half revolution) about the body axis of the object. An image reconstruction is carried out based on transmitted images acquired from one revolution (at least a half revolution) about the body axis of the object, to produce a sectional image seen in a direction along the body axis of the object. The non-CT type radiographic technique, as does the foregoing X-ray radiographic apparatus, produces a sectional image seen in a direction along the body axis of the object, without causing the X-ray tube and image intensifier to make a half or more revolution about the body axis of the object.
Nowadays, further improvement is desired for the non-CT type radiographic technique, and in this context, a possibility of employing what is known as a back projection method is being explored. This method does not perform an image reconstruction to obtain two-dimensional slice image data by adding detection signals so as to overlap a plurality of projected images acquired by radiography done from varied angles. Instead, an image reconstruction is carried out to produce three-dimensional volume data of a region of interest by projecting a plurality of projection images obtained by radiographing the region of interest from varied angles, back to predetermined lattice points on a three-dimensional lattice virtually set to the region of interest of the radiographed object. The back projection method of the non-CT type radiographic technique can acquire three-dimensional volume data of a region of interest of an object in one radiographic operation. This provides an advantage of enabling a particular sectional image to be selected and displayed promptly after the radiographic operation.
However, the conventional technique noted above has the following drawback. The back projection method of the non-CT type radiographic technique carries out an image reconstruction to produce three-dimensional volume data of a region of interest by projecting a plurality of projection images obtained by radiographing the region of interest from varied angles, back to predetermined lattice points on a three-dimensional lattice virtually set to the region of interest of the radiographed object. The number of lattice points on the three-dimensional lattice, for example, corresponds to the tube of the number (100 to 1,000) of points equally arranged along each of the three axes. A projection image obtained from a particular angle, i.e. data detected by pixels on the detecting plane of the image intensifier, is projected back to predetermined lattice points on the three-dimensional lattice. This is done for a plurality of projection images obtained from varied angles. Thus, an enormous volume of data is back-projected, resulting in an extended processing time for the image reconstruction to generate three-dimensional volume data of the region of interest.
SUMMARY OF THE INVENTION
This invention has been made having regard to the state of the art noted above, and its object is to provide a radiographic apparatus which requires a reduced processing time for image reconstruction.
To fulfill the above object, Inventor has made intensive research and attained the following findings. The non-CT type radiographic technique is characterized in that a radiation source such as an X-ray tube and an area detector such as an image intensifier are not driven to make more than a half revolution about the body axis of an object under examination. To illustrate a radiographed region of interest of the object with a three-dimensional coordinate system, only a small amount of information is collected for the direction of an axis extending substantially through the center of the region of interest and perpendicular to a sectional plane. Resolution is thus lower in this direction than in the directions along the two remaining axes (which are within the sectional plane). Consequently, lattice spacing along the axis extending through the slice plane may be made larger than the lattice spacing along the other axes of a three-dimensional lattice virtually set to the region of interest of the object, without wasting image information for the direction of this axis. Rather, the processing time may be shortened.
Based on the above findings, this invention provides a radiographic apparatus for generating three-dimensional volume data of a region of interest of an object under examination by an image reconstruction of projection data acquired by radiographing the object from varied scan positions, and obtaining sectional images from the three-dimensional volume data, the apparatus comprising:
a radiation source for irradiating the object with penetrating electromagnetic waves;
an area detector for detecting electromagnetic waves transmitted through the object;
the radiation source and the area detector being arranged across sectional planes of the object and synchronously operable for scanning action; and
a back projection unit for performing the image reconstruction to generate three-dimensional volume data of the region of interest by projecting projection data detected in the varied scan positions back to predetermined lattice points of a three-dimensional lattice virtually set to the region of interest of the object radiographed;
the back projection unit generating the three-dimensional volume data, with lattice spacing along a sectional axis extending substantially through the center of the region of interest and perpendicular to the sectional planes, among three orthogonal axes of the three-dimensional lattice, made larger than lattice spacing in the two other directions.
With the apparatus according to this invention, a three-dimensional lattice is virtually set to the region of interest of the object radiographed. The back projection unit increases the lattice spacing along a sectional axis extending substantially through the center of the region of interest and perpendicular to the sectional planes, among three orthogonal axes of the three dimensional lattice, to be larger than the lattice spacing in the two other directions. Then, the back projection unit performs an image reconstruction to generate three-dimensional volume data of the region o
Bruce David V.
Rader & Fishman & Grauer, PLLC
Shimadzu Corporation
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