X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-11-21
2002-06-11
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S016000
Reexamination Certificate
active
06404844
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a tomographic imaging scan condition determining method, tomographic imaging method and X-ray CT (computed tomography) apparatus, and more particularly to a tomographic imaging scan condition determining method, tomographic imaging method and X-ray CT apparatus that can determine a radiation dose without excess or insufficiency with respect to an allowed value of image noise.
FIG. 1
is a flow chart showing an example of a conventional X-ray dose determining process in an X-ray CT apparatus comprising a single detector, i.e., a detector having one detector row.
In Step SU1, scout imaging is performed in two orthogonal directions to produce a sagittal plane image and a coronal plane image.
In Step SU2, a tube voltage, slice thickness and reconstruction function are selected.
In Step SU3, a scan position (slice position) in X-ray tomographic imaging is determined with reference to the scout images.
In Step SU4, a standard deviation SD&sgr;
pixel
of an image SD assuming the cross section of the subject to be circular is calculated based on a projected area S
object
in imaging a subject at default imaging conditions, a default X-ray dose default_mAs (which is the product of the tube current and the emission time) and a slice thickness Th, as follows:
SD
σ
pixel
≈
S
object
(
default_mAs
×
Th
)
.
The standard deviation SD&sgr;
pixel
is regarded as an image noise value.
The projected area S
object
is roughly evaluated from the sagittal and coronal plane images.
In Step SU5, the standard deviation SD&sgr;
pixel
is corrected according to the attenuation ratio between the sagittal plane image and coronal plane image to obtain a standard deviation SD&sgr;′
pixel
according to the actual cross-sectional shape of the subject.
In Step SU6, an allowed value SD&sgr;
target
for the standard deviation (image noise value) of the image SD is input.
In Step SU7, an X-ray dose scan_mAs satisfying the standard deviation SD&sgr;
target
is calculated for each slice as follows:
scan_mAs
=
default_mAs
×
(
SD
σ
pixel
′
SD
σ
target
)
2
.
The basic principle of an X-ray dose determining process like the above is disclosed in, for example, Japanese Patent Application Laid Open No. H11-104121.
Recently, a technique has been developed involving performing a helical scan by an X-ray CT apparatus comprising a multi-detector, i.e., a detector having a plurality of detector rows arranged in parallel, and combining weighted data of respective slices (multislice) corresponding to the rows of the multi-detectors for image reconstruction, to thereby increase the substantial slice thickness, or image thickness. That is, even when the X-ray beam width is decreased, an X-ray tomographic image with a large image thickness can be obtained by using an extended range of data in image reconstruction (i.e., by increasing the number of rotations of the X-ray tube and multi-detector to more than one). In this case, the X-ray dose required to obtain an X-ray tomographic image with the same image noise value can be reduced compared to that by one rotation.
However, since the X-ray CT apparatus comprising the multi-detector still employs the X-ray dose determining process for the conventional single-slice CT (see FIG.
1
), the actual image noise level is greater or smaller than the allowed image noise value at some scan conditions. When the actual image noise level is greater than the allowed value, required image quality cannot be achieved; and when the actual image noise level is smaller, the X-ray dose is unnecessarily large.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a tomographic imaging scan condition determining method, tomographic imaging method and X-ray CT apparatus that can determine a radiation dose without excess or insufficiency with respect to an allowed value of image noise in performing a helical scan by a CT apparatus comprising a multi-detector.
In accordance with a first aspect of the invention, there is provided a tomographic imaging scan condition determining method comprising the steps of: when a tomographic image having an image thickness is to be produced by a helical scan by a CT apparatus comprising a multi-detector having a plurality of detector rows arranged in parallel, provisionally determining a radiation dose in obtaining the tomographic image by single-slice CT using a single-slice CT radiation dose determining algorithm; correcting the radiation dose such that an image noise value of the tomographic image obtained by performing the helical scan is not excessive or insufficient with respect to an allowed value; and determining tomographic imaging scan conditions proper for the corrected radiation dose.
In the tomographic imaging scan condition determining method of the first aspect, a radiation dose provisionally determined to obtain a tomographic image having a certain image thickness is corrected so that an image noise value of the tomographic image obtained by performing a helical scan is not excessive or insufficient with respect to an allowed value. Therefore, a minimum radiation dose satisfying an image noise value requirement can be accurately calculated.
Thus, a disadvantage that required image quality cannot be achieved due to an insufficient radiation dose can be avoided, and a subject can be prevented from unnecessary exposure due to an excessive radiation dose. Moreover, since the most suitable radiation dose can be determined automatically or semi-automatically, the complexity is reduced for a human operator.
In accordance with a second aspect of the invention, there is provided a tomographic imaging scan condition determining method comprising the steps of: selecting an image thickness of a tomographic image to be produced by a helical scan by a CT apparatus comprising a multi-detector having a plurality of detector rows arranged in parallel; provisionally determining a radiation dose in obtaining the tomographic image having said image thickness by a single-slice CT using a single-slice CT radiation dose determining algorithm; selecting scan conditions of the helical scan to be performed; correcting said provisionally determined radiation dose to a radiation dose such that an image noise value of the tomographic image obtained by performing the helical scan at said scan conditions is not excessive or insufficient with respect to an allowed value; and determining tomographic imaging scan conditions proper for said corrected radiation dose.
The tomographic imaging scan condition determining method of the second aspect achieves the same effects as in the tomographic imaging scan condition determining method of the first aspect. Moreover, the image thickness of an X-ray tomographic image to be produced can be selected. Furthermore, scan conditions for a helical scan can be selected.
In accordance with a third aspect of the invention, there is provided a tomographic imaging scan condition determining method as described regarding the first or second aspect, comprising the step of preferentially specifying either an electric current to be supplied to an emitting source or an emission time, in order to emit radiation with the corrected radiation dose.
In the tomographic imaging scan condition determining method of the third aspect, when the electric current to be supplied to an emitting source is preferentially specified, the emitting source can be assuredly prevented from being overloaded. When the emission time is preferentially specified, the scan time can be adjusted in consideration of an effect of subject's body motion on the image quality etc.
In accordance with a forth aspect of the invention, there is provided a tomographic imaging scan condition determining method as described regarding the second or third aspect, comprising the step of adjusting the image thickness by altering at least one of a weighting function in combining data of the rows of said multi-detector for image
Gohno Makoto
Horiuchi Tetsuya
Barber Therese
GE Medical Systems Global Technology Company LLC
Kojima Moonray
Porta David P.
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