X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-04-05
2002-03-19
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S019000
Reexamination Certificate
active
06359955
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a twin asymmetric scan slice thickness setting method and apparatus and a radiation tomography method and apparatus, and more particularly to a method of setting slice thicknesses for a twin asymmetric scan in which a scan is performed with a first slice thickness in a first detector row of a multi detector and with a second slice thickness different than the first slice thickness in a second detector row, and an apparatus which can suitably implement the method.
In a conventional X-ray CT apparatus comprising a twin detector having first and second detector rows, a slice thickness setting method is known for a twin symmetric scan in which a scan is performed with the same slice thickness in both the first and second detector rows.
The slice thickness setting method for the twin symmetric scan involves moving a position of an X-ray beam and performing a scan while keeping the width of the X-ray beam fixed at twice the slice thickness to move the position of the X-ray beam to a position at which count values for the first and second detector rows match.
However, a slice thickness setting method for a twin asymmetric scan in which a scan is performed with a first slice thickness in a first detector row and with a second slice thickness different than the first slice thickness in a second detector row is not known in the prior art.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a twin asymmetric scan slice thickness setting method and apparatus for setting slice thicknesses for the aforementioned twin asymmetric scan, and a radiation tomography method and apparatus which can suitably implement the method.
In accordance with a first aspect of the present invention, there is provided a twin asymmetric scan slice thickness setting method for performing a scan with a first slice thickness in a first detector row of a multi detector and with a second slice thickness different than the first slice thickness in a second detector row, comprising the steps of moving a position of a penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at twice the first slice thickness to move the position of the penetrating radiation beam to a position at which count values for the first and second detector rows match, storing the count value for the detector row at this time, and then moving the position of the penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at the sum of the first and second slice thicknesses to move the position of the penetrating radiation beam to a position at which the count value for the first detector row matches the stored count value.
The sensitivity profile of each of the first and second detector rows in the slice thickness direction has an inverse U-like shape having the crest in the center, gradually falling toward the ends (see FIG.
1
(
b
)). Hence, a relation (first slice thickness:second slice thickness)=(count value for the first detector row:count value for the second detector row) does not stand. That is, the ratio between the count values cannot determine the position of the penetrating radiation beam when the first slice thickness the second slice thickness.
However, since the sensitivity profiles of the first and second detector rows in the slice thickness direction can be considered to be symmetric as viewed from the boundary between the first and second detector rows, a relation (count value for the first detector row count value for the second detector row) stands when the first slice thickness=the second slice thickness. That is, the position of the penetrating radiation beam can be determined by the ratio between the count values (1:1) when the first slice thickness=the second slice thickness.
The present invention utilizes the above-described principle.
More specifically, according to the twin asymmetric scan slice thickness setting method of the first aspect, first, a position of the penetrating radiation beam is moved and a scan is performed while keeping the width of the penetrating radiation beam fixed at twice the first slice thickness to move the position of the penetrating radiation beam to a position at which count values for the first and second detector rows match (i.e., to a position at which the ratio of the count values is 1:1). At this time, the slice thickness in the first detector row and the slice thickness in the second detector row are equal to the first slice thickness. Then, the count value f or the detector rows at this time is stored. Next, the position of the penetrating radiation beam is moved and a scan is performed while keeping the width of the penetrating radiation beam fixed at the sum of the first and second slice thicknesses to move the position of the penetrating radiation beam to a position at which the count value for the first detector row matches the stored count value. At this time, the slice thickness in the first detector row is equal to the first slice thickness. On the other hand, since the slice thickness in the second detector row has a value subtracting the first slice thickness from the sum of the first and second slice thicknesses, the slice thickness in the second detector row is equal to the second slice thickness. The slice thicknesses can thus be set for a twin asymmetric scan.
In accordance with a second aspect of the present invention, there is provided a twin asymmetric scan slice thickness setting method for performing a scan with a first slice thickness in a first detector row of a multi detector and with a second slice thickness different than the first slice thickness in a second detector row, comprising the steps of: moving a position of a penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at twice the first slice thickness to move the position of the penetrating radiation beam to a position at which count values for the first and second detector rows match, storing the count value for the detector row at this time, and then moving the position of the penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at the sum of the first and second slice thicknesses to move the position of the penetrating radiation beam to a position at which the count value for the first detector row matches the stored count value, storing a ratio a between the count values for the first and second detector rows at this time; prior to performing an actual scan of a subject, acquiring a count value for a first control detector disposed corresponding to the first detector row and a count value for a second control detector disposed corresponding to the second detector row, and moving a position of the penetrating radiation beam to a position at which a ratio &bgr; between these count values matches the stored ratio &agr;.
According to the twin asymmetric scan slice thickness setting method of the second aspect, the slice thicknesses are set for a twin asymmetric scan based on the twin asymmetric scan slice thickness setting method as described regarding the first aspect. Then, a ratio a between the count values for the first and second detector rows at this time is stored. Thereafter, and prior to performing an actual scan of a subject, a count value for a first control detector disposed corresponding to the first detector row and a count value for a second control detector disposed corresponding to the second detector row are acquired, and the position of the penetrating radiation beam is moved to a position at which a ratio &bgr; between these count values matches the stored ratio &agr;. Thus, if the reference ratio &agr; is obtained by performing the twin asymmetric scan slice thickness setting method of the first aspect only once, the ratio can be used as many times as desired in the subsequent actual scans, thereby simplifying the operations for setting the slice thicknes
Bruce David V.
GE Yokogawa Medical Systems Limited
Hobden Pamela R.
Kojima Moonray
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