X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-07-17
2004-03-02
Arana, Louis (Department: 2859)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S019000
Reexamination Certificate
active
06700947
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-217531, filed Jul. 18, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for optically transmitting data between a rotor and a stator and an X-ray CT apparatus having the apparatus incorporated therein.
2. Description of the Related Art
Known is an optical transmission apparatus for transmitting data by utilizing an optical beam transmitted in the space between a rotor and a stator. In the optical transmission apparatus, a cable is not used and, thus, the rotation of the rotor is not restricted so that the rotor can be freely rotated. In addition, a mechanical contact or contacts are not required in the optical transmission apparatus unlike the transmission apparatus using a slip ring, leading to a high reliability.
The particular optical transmission apparatus is used, for example, in an X-ray CT (computed-tomography) apparatus, which is one of medical image diagnostic apparatuses. The X-ray CT apparatus comprises a hollow rotor and a hollow stator, and a patient (human body) is put in the bore of the X-ray CT apparatus. An X-ray tube and an X-ray detector are arranged in the rotor, and the patient put in the X-ray CT apparatus is irradiated with an X-ray emitted from the X-ray tube while rotating the rotor to permit the X-ray to scan, for example, spirally the patient. The X-ray passing through the patient is detected by the X-ray detector so as to be taken out as an electric signal. This electric signal is converted into a digital signal and, then, converted again into a light beam so as to be transmitted to the side of the stator. The light beam transmitted to the side of the stator is received by a photo-detecting element and is converted into an electric signal, and the electric signal is processed to obtain a tomographic image information or data of the patient.
In the optical transmission apparatus between the rotor and the stator utilized in the conventional X-ray CT apparatus, a plurality of light-emitting elements
83
, e.g., 64 light-emitting elements
82
, are arranged at a predetermined interval on a circle having a predetermined radius on a rotor
81
, as shown in FIG.
1
. For simplifying the drawing, only two light-emitting elements
83
a
and
83
b
are shown in FIG.
1
. Also, a single photo-detecting element
84
is arranged on a circle having the radius equal to that noted above on a stator
82
. A light beam modulated in accordance with the image data is transmitted between the light-emitting element
83
and the photo-detecting element
84
. In accordance with rotation of the rotor
81
, the light-emitting elements
83
a
and
83
b
are successively faced to the photo-detecting element
84
.
FIG. 1
shows the state immediately after the light-emitting element
83
a
has being faced to the photo-detecting element
84
and the state immediately before the light-emitting element
83
b
reaches the photo-detecting element
84
.
Light beams
85
a
,
85
b
emitted from the light-emitting elements
83
a
,
83
b
is diverged, respectively. In the state shown in
FIG. 1
, the photo-detecting element
84
is positioned intermediate between the light-emitting elements
83
a
and
83
b
so as to receive the edge portions of the light beams
85
a
,
85
b
, i.e., the weak portions of the light power. However, since the photo-detecting element
84
simultaneously receives the two light beams
85
a
,
85
b
, the power of the received light beams (incident light intensity) is maintained at a relatively high level.
In order to ensure a sufficiently high optical power even in the case where the photo-detecting element
84
is positioned intermediate between the light-emitting elements
83
a
and
83
b
, it is necessary to set the distance between the light-emitting elements
83
a
and
83
b
such that the light beams
85
a
,
85
b
emitted the light-emitting elements
83
a
,
83
b
are allowed to partially overlap each other, as shown in FIG.
1
. Because of this requirement, the number of light-emitting elements is increased. For example, it is necessary to arrange 64 light-emitting elements. In this connection, the number of IC's for driving the light-emitting elements and the wiring for connecting these IC's is increased, leading to serious problems. For example, the power consumption is increased. In addition, the reliability is lowered and the manufacturing cost is increased.
In order to avoid the problems pointed out above, it may be advisable to increase the distance between the rotor
81
and the stator
82
so as to widen the distribution of the light beams emitted from the light-emitting elements and, thus, to decrease the number of photo-detecting elements. It should be noted in this connection, however, that the intensity of light is inversely proportional to the square of the distance. In other words, if the distance between the rotor
81
and the stator
82
is increased, the power of light received by the photo-detecting element is weakened so as to give rise to a new problem that a transmission error is increased.
The X-ray CT apparatus is required in recent years to transmit a large amount of data at a high speed by the shortening of the irradiating time and the employment of a multi-slice system for acquiring a large number of tomographic images simultaneously. In this connection, transmission of a large amount of data at a high speed is required in respect of the optical transmission apparatus between the rotor and the stator. However, it is difficult for the conventional construction described above to satisfy the particular requirement because of the reasons described below.
Concerning the specific parts used in the optical transmission apparatus between the rotor and the stator, it is possible to use a light-emitting diode (LED) or a laser diode (LD) as the light-emitting element. In the case of transmitting the data of a large capacity roughly exceeding hundred Mbps at a high transmitting speed, an LD is used in many cases. On the other hand, a photodiode (PD) is generally used as the photo-detecting element. The LD and the PD have characteristic parameters called the maximum light output and the minimum light receiving sensitivity, respectively. An allowable loss called optical power budget is determined by the values of these characteristics parameters.
In order to achieve an error-free data transmission by the light beam between the rotor and the stator, it is necessary to make the optical power budget as large as possible. On the other hand, in view of the increase in the data transmission, the light output of the LD tends to be decreased with increase in the data transmission speed. Also, in order to increase the data transmission speed for the PD, it is necessary to decrease the parasitic capacitance so as to diminish the light receiving area, with the result that the received optical power is decreased. It follows that the optical power budget is diminished with increase in the data transmission speed.
It should be noted that, if the transmission speed is increased with the distance between the adjacent light-emitting elements set constant, the light receiving power of the photo-detecting element is diminished in accordance with the transmission speed. Therefore, it is necessary to arrange the light-emitting elements at a higher density in order to ensure a desired light receiving power so as to make the above-noted problems more serious in respect of the increase in the power consumption accompanying the increase in the number of light-emitting elements used as well as the reduction in the reliability and the increase in the manufacturing cost.
As described above, in the conventional light transmission apparatus between a rotor and a stator for performing the data transmission directly by the light beam from a plurality of light-emitting elements arranged
Matsuyama Hiroshi
Oshima Shigeru
Arana Louis
Kabushiki Kaisha Toshiba
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