X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-09-12
2003-07-08
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C310S211000, C310S254100, C310S266000
Reexamination Certificate
active
06590953
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT scanner and more particularly to an X-ray CT scanner with a scanner rotation mechanism suited for shortening a scan time by rotating a scanner at high speeds.
The X-ray CT scanner produces a cross-sectional image or tomogram of a subject by radiating a fan-shaped X-ray beam from an X-ray tube onto a subject, detecting X-rays that have penetrated the subject with an X-ray detector arranged at a position opposite the X-ray tube, and image-processing data on the detected X-rays.
The X-ray detector has a group of as many as several hundred detection elements arranged in arc, and is placed opposite the X-ray tube with the subject therebetween to form radially distributed X-ray paths in a number corresponding to that of the detection elements. The X-ray tube and the detector are rotated together around the subject through at least 180 degrees to detect X-rays that have penetrated the subject at intervals of a predetermined angle.
Thanks to advantages this X-ray CT scanner has achieved in recent years, such as capabilities of “scanning a wide range in a short period of time” and of “producing continuous data in a body axis direction and thereby generating a three-dimensional image,” a spiral CT performing a helical or spiral scan has found a rapidly growing range of applications.
The spiral CT has enabled a substantial reduction in the time required to perform a three-dimensional CT imaging by continuously rotating the X-ray tube and X-ray detector around a subject while moving a table on which the subject is placed, collecting cross-sectional image data in multiple layers over a wide range and reconstructing the data into an image.
The X-ray CT scanner normally includes a plurality of units: a scanner for rotating the X-ray tube and X-ray detector around a subject to take data on the X-rays that have penetrated the subject; a subject table having a table on which the subject is placed; an image processor for processing the X-ray data collected by the scanner to generate a reconstructed image; a display device for displaying the image reconstructed by the image processor; a keyboard with which to enter various commands; and a system controller for controlling a whole system.
The scanner includes an X-ray tube for radiating X-rays against a subject; a collimator for collimating the X-rays radiated from the X-ray tube into a fan beam; a cooler for cooling the X-ray tube; a high-voltage generator for applying a high voltage to the X-ray tube; a multichannel X-ray detector for detecting X-rays that have penetrated the subject; an amplifier for amplifying a weak electric output of the X-ray detector; a rotary member supporting these devices and having a circular hole in which to position the subject at the center thereof; a frame for rotatably supporting the rotary member; a reduction gear and a motor secured to the frame to rotate the rotary member; and a belt (normally a toothed belt) for coupling the rotary member and an output shaft of the reduction gear.
In a scanner of such a construction, when the motor is started, the rotary power of the motor output shaft is reduced in speed by the reduction gear and conveyed through the belt to the rotary member, which then rotates the X-ray tube and the X-ray detector around a subject to produce X-ray projection data (this is also referred to as imaging or scanning) at intervals of a predetermined angle. The rotary member carrying the X-ray tube and the high-voltage generator, because it is capable of counter-weight mounting, can easily establish a mass balance around a rotating axis. Further, since it does not need to be accelerated to high speeds, the rotary member needs only to be rotated at an almost constant speed. Hence, the motor often employs an induction motor based on an open-loop control.
The conventional X-ray CT scanner usually uses a motor for an actuator that rotates the rotary member by reducing the rotation speed of the motor by the reduction gear and transmitting the rotation through a power transmission means such as a belt to the rotary member.
In addition to the X-ray radiation unit and the X-ray detection unit, the rotary member has a high-voltage generation unit for applying a high voltage to the X-ray radiation unit, a cooling unit for cooling the X-ray radiation unit, and an amplifier unit for amplifying a weak electric output from the X-ray detection unit. These units are rigidly fixed to the center of the rotary member from the outer circumferential side by fixing means such as screws.
In the X-ray CT scanner, the widespread use of the spiral CT has led to a significant improvement on a diagnostic technique as described above. There are also growing demands for imaging dynamically moving internal organs such as heart.
To meet these demands, the rotation speed of the X-ray tube and X-ray detector needs to be increased to shorten the scan time. That is, the rotation speed of the rotary member of the scanner must be raised. While the scan time of 1 second/rotation poses no problem for organs other than heart, the imaging of such moving organs such as heart cannot be realized with the rotation speed of 1 second/rotation but requires a higher scan speed of 0.7 to 0.5 second/rotation or even 0.3 second/rotation.
Driving the rotary member at a high rotating speed less than 0.7 second/rotation by using a conventional scanner rotary drive mechanism described above, however, causes the toothed belt to produce a whizzing sound in excess of 70 dB. Since the X-ray CT scanner is used in an inspection room in a hospital where quietness is required, noise of such a level is offensive to the ear of a subject and an operator. To solve this noise problem and still realize a high speed rotation requires the rotary member to be rotated in a direct drive (DD) mode where the rotary member itself is constructed as a rotor of the motor, rather than being rotated through the reduction gear and belt.
Two DD methods are conceivable.
As with general industrial motors, one method uses a permanent magnet to generate a rotary force in the rotary member, and the other induces a rotating magnetic field around the rotary member and uses an electromotive force induced in the rotary member. In the method using the permanent magnet, however, since the rotary member has at its central part a circular hole about 1,000 mm in diameter through which to pass a subject (a subject insertion opening), if a hollow rotor with a hole about 1,000 mm across is to be made from a permanent magnet, the rotary member increases in size and cost and becomes more difficult to manufacture.
On the other hand, in the method using an induced electromotive force, because rotating fluxes generated around the rotor pass through the hole of the rotary member, if a subject is attached with a pacemaker or an electrocardiograph, these devices are likely to be operated undesirably by the rotating fluxes threading through the hole of the rotary member, which must be avoided.
Further, in either method using a permanent magnet or an induced electromagnetic force, a large amount of electromagnetic noise may leak out and interfere with a signal of the amplifier, which amplifies the weak electric output, resulting in a possible degradation of quality of a finally obtained image. To solve such a problem of electromagnetic noise, a measure should be taken to shield the DD motor including the rotary member, which in turn makes the scanner large, hindering the fast rotation of the rotary member.
On the other hand, shortening the scan time poses another problem.
As the scan time decreases, the rotary member must be rotated at an increased speed. The substantial improvements on the diagnostic technique made possible by the widespread use of the spiral CT scanner require an increase in the number of scanning operations performed, which in turn requires the X-ray radiation unit to have a large capacity.
The large-capacity X-ray radiation unit has an increased size and mass, which naturally increases the size and mass of the cool
Kobiki Takaaki
Kurome Akira
Makino Eiichi
Suzuki Tsutomu
Takano Hiroshi
Antonelli Terry Stout & Kraus LLP
Dunn Drew A.
Hitachi Medical Corporation
Ho Allen C.
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