X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-07-05
2002-05-21
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S098800, C250S370110, C250S370090
Reexamination Certificate
active
06393092
ABSTRACT:
BACKGROUND
1. Field
The patent specification relates to an X-ray detection device which is used in an X-ray CT (computerized tomography) apparatus, and, in particular, relates to a multi element solid state X-ray detection device with a high spatial resolution and a high S/N ratio which is suitable for concurrently detecting X-ray transmission data of a plurality of slices of an object body being inspected
2. Related Art
Currently, the X-ray detection device used for an X-ray CT apparatus tends to be a solid state detection device using a scintillator that has improved X-ray detection accuracy in comparison with conventional X-ray detection using a xenon ionization detector, Such a solid state detection device comprises a multiplicity of channels of X-ray detection elements arranged in an arcuate shape around an X-ray source, each element being formed by a scintillator for converting incident X-ray into light and a light detection element such as a silicon photo diode which detects the light converted by the scintillator and outputs the same as an electrical signal.
In an X-ray CT apparatus, in order to improve throughput of the device, it is desired to shorten the time required to obtain CT images. The following two methods are generally enumerated;
(1) Shorten the time required for a rotation of a scanner.
(2) Increase the number of tomographic images taken for every rotation of a scanner.
With regard to (1) above, an improvement in rotating speed of the scanner can be achieved by reducing the weight of an X-ray tube serving as an X-ray generating device. On the other hand, the above (2) can be achieved by arranging a row of X-ray detection elements for an X-ray detection device conventionally arranged in one dimension in the channel direction in a plurality of rows, in that two rows or more than two rows are arranged along a slice direction (which is perpendicular to the channel direction).
Such an X-ray detection device is called a multi slice type X-ray detection device (the conventional X-ray detection device in which the X-ray detection elements are simply arranged in one dimension is generally called a single slice type X-ray detection device).
FIG. 8
shows a schematic diagram of an example of such a multi slice type X-ray detection device applied to a CT apparatus. In
FIG. 8
, a relationship between a multi slice type X-ray detection device
13
, a body
11
to be inspected and an X-ray tube
10
is illustrated. The multi slice type X-ray detection device
13
has four rows of X-ray detection elements
12
, from element row
1
to element row
4
arranged in slice direction, and can measure image data for a region covering four slices from slice
1
to slice
4
of the body
11
by concurrently receiving X-ray beams
14
irradiated from an X-ray tube
10
. As a result, the utilization efficiency of the X-ray beams
14
from the X-ray tube
10
is improved four times in comparison with the conventional single slice type X-ray detection device. Further improvements in efficiency can be achieved as the number of X-ray detection element rows
12
increases.
With the background of conventional single slice type X-ray detection device, many of such multi slice type X-ray detection devices are constituted by simply arranging several single slice type X-ray detection device in the slice direction. However, in such a multi slice type X-ray detection device, the respective X-ray detection elements must match in performance. If they do not match sufficiently well, ring artifacts can appear in the reconstructed CT images and thereby deteriorate image quality.
Further, when the performance of X-ray detection elements varies in the slice direction, the measured data can differ depending on which X-ray detection element row in the slice direction obtained the measurements; therefore, it is possible, even when the measurements of image data are performed with regard to the same slice plane of the body
11
, that the image quality of the CT images and medical information obtained from the CT images would vary. The obtained CT images should not differ because of differences in performance of X-ray detection elements used for the measurement, when image data are measured with regard to a same slice plane of a same body
11
. For this reason, it is required that the X-ray detection elements are sufficiently matched in performance in bot the channel an the slice directions.
For the reasons explained above, it can be difficult to manufacture a multi slice type X-ray detection device that performs well.
Further, in order to match the performance characteristic values of the respective X-ray detection elements, it is important to reduce electrical cross talk as well as optical cross talk between respective neighboring elements in the detection device in addition to matching the performance characteristics of the scintillators and silicon photo diodes included in the respective elements.
FIG. 4
is a perspective view showing a basic structure of a conventional single slice type X-ray detection device.
In
FIG. 4
, numeral
1
is a scintillator which converts incident X-ray
5
into light, numeral
2
a
is an isolation wall between neighboring X-ray detection elements and numeral
3
is a silicon photo diode array which converts the light converted by the scintillators
1
into electrical signals. Each of the X-ray detection element is constituted by adhering a scintillator
1
on the upper surface of a respective photo receiving portion provided on the surface of the silicon photo diode array
3
, and an X- ray detection element array is constituted by arranging the thus constituted X-ray detection elements in parallel with a predetermined pitch on a circuit substrate
6
while interleaving the isolation walls
2
a
therebetween. Further, numeral
7
is an upper face reflection plate which efficiently reflects light from the scintillators
1
and introduces the same toward the respective photo receiving portions on the silicon photo diode array
3
.
In the above structured X-ray detection device, the incident X-ray on and into the detection device is converted by the scintillators
1
into visible light having a local intensity proportional to the local intensity of the incident X-ray
5
. The converted light is transmitted through the scintillators
1
in part through reflection such as at the surface of the upper face reflection plate
7
, the surfaces of the isolation walls
2
a
and boundaries and surfaces of the scintillators
1
, and is introduced onto the photo receiving portions provided on the surface of the silicon photo diode array
3
in which a photo-electric conversion is performed and electrical signals (photo currents) having intensities proportional to the intensity of light, namely proportional to the intensity of X-ray is detected.
The performance of an X-ray detection device is evaluated primarily depending on S/N ratio and spatial resolution thereof. The S/N ratio is determined by the contribution rate of the incident X-ray
5
on the output signal, namely by the X-ray utilization efficiency and electrical signals (noise signals) induced in an electrical circuit system including the silicon photo diode array
3
when no X-ray is incident into the X-ray detection device. Then, the X-ray utilization efficiency is determined by a luminous efficiency (light conversion efficiency) of the scintillators
1
, a light conversion efficiency of the silicon photo diode array
3
, a spatial utilization efficiency which represents a spatial X-ray utilization efficiency by the X-ray detection device and a light transmission efficiency in the X-ray detection device.
Noise signals are primarily caused by shot noise and dark currents due to recombination currents in depletion layers in the silicon photo diode array
3
and noise currents in the electric current system such as a preamplifier. Among the causes which affect the above X-ray utilization efficiency, the luminous efficiency (light conversion efficiency) of the scintillators
1
and the light conversion efficiency of the silico
Hitachi Medical Corporation
Kim Robert H.
Yun Jurie
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