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 Sobject 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    ⁢          xe2x80x83        ⁢          σ      pixel        ≈                              S          object                          (                      default_mAs            xc3x97            Th                    )                      .  
The standard deviation SD"sgr"pixel is regarded as an image noise value.
The projected area Sobject 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"xe2x80x2pixel 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    xc3x97                            (                                    SD              ⁢                              xe2x80x83                            ⁢                              σ                pixel                xe2x80x2                                                    SD              ⁢                              xe2x80x83                            ⁢                              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.
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 reconstruction, a radiation beam width, and a movement speed of an imaging table for placing a subject.
In the tomographic imaging scan condition determining method of the fourth aspect, when a weighting function for image reconstruction is altered, the image thickness can be adjusted by a calculational process only, without mechanical control. When the radiation beam width is altered, the image thickness can be adjusted by, for example, controlling the opening width of a collimator aperture. When the movement speed of an imaging table is altered, the image thickness can be adjusted by controlling a drive system of the imaging table.
In accordance with a fifth aspect of the invention, there is provided a tomographic imaging scan condition determining method as described regarding any one of the first through fourth aspects, comprising the step of correcting the radiation dose with a dose correction factor based on a reference value, the reference value being a radiation dose in performing an axial scan with a slice thickness equal to the image thickness.
In the tomographic imaging scan condition determining method of the fifth aspect, the radiation dose is corrected with a dose correction factor based on a radiation dose in an axial scan. Therefore, a radiation dose proper for scan conditions can be calculated by a simple calculation.
In accordance with a sixth aspect of the invention, there is provided a tomographic imaging method, comprising the step of producing a tomographic image by performing a helical scan at the tomographic imaging scan conditions determined by the tomographic imaging scan condition determining method as described regarding any one of the first through fifth aspects.
In the tomographic imaging method of the sixth aspect, a tomographic image is produced using tomographic imaging scan conditions determined by the aforementioned tomographic imaging scan condition determining method. Therefore, a tomographic image with required image quality can be obtained by a minimum exposure.
In accordance with a seventh aspect of the invention, there is provided a tomographic imaging scan condition determining method comprising the steps of: when an X-ray tomographic image having an image thickness is to be produced by a helical scan by an X-ray CT apparatus comprising a multi-detector having a plurality of detector rows arranged in parallel, provisionally determining an X-ray dose in obtaining said X-ray tomographic image by single-slice CT using a single-slice CT X-ray dose determining algorithm; selecting scan conditions for the helical scan to be performed; correcting said X-ray dose to an X-ray dose such that an image noise value of the X-ray 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 X-ray dose.
In the tomographic imaging scan condition determining method of the seventh aspect, a disadvantage that image quality of an X-ray tomographic image is extremely degraded due to an insufficient X-ray dose can be avoided, and a subject can be prevented from unnecessary exposure due to an excessive X-ray dose.
In accordance with an eighth aspect of the invention, there is provided the tomographic imaging scan condition determining method as described regarding the seventh aspect, comprising the step of preferentially specifying either an X-ray tube current or an emission time in order to emit X-rays with the corrected X-ray dose.
In the tomographic imaging scan condition determining method of the eighth aspect, when the X-ray tube current is preferentially specified, the X-ray tube is assuredly prevented from being overloaded and decreased in lifetime. 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 ninth aspect of the invention, there is provided a tomographic imaging scan condition determining method as described regarding the seventh or eighth aspect, comprising the step of correcting said X-ray dose with a dose correction factor based on a reference value, said reference value being an X-ray dose in performing an axial scan with a slice thickness equal to said image thickness.
In the tomographic imaging scan condition determining method of the ninth aspect, the X-ray dose is corrected based on an X-ray dose in an axial scan. Therefore, an X-ray dose proper for scan conditions can be calculated by a simple calculation.
In accordance with a tenth aspect of the invention, there is provided a tomographic imaging scan condition determining method as described regarding the ninth aspect, comprising the steps of: obtaining a dose correction factor that expresses an X-ray dose by a multiplying factor based on a reference value, said reference value being an X-ray dose in performing an axial scan by single-slice CT on a phantom imitating a CT value distribution of a standard subject with a slice thickness equal to a desired image thickness, said X-ray dose being an X-ray dose for obtaining an X-ray tomographic image by a helical scan employing a multi-detector with an image noise value equivalent to that in said axial scan; and correcting said X-ray dose by multiplying said provisionally determined X-ray dose by said dose correction factor.
In the tomographic imaging scan condition determining method of the tenth aspect, the X-ray dose is corrected with a dose correction factor based on an X-ray dose in performing an axial scan on a phantom. Therefore, an X-ray dose proper for imaging of a subject can be calculated with good accuracy.
In accordance with a eleventh aspect of the invention, there is provided a tomographic imaging method, comprising the step of producing an X-ray tomographic image by performing a helical scan at the tomographic imaging scan conditions determined by the tomographic imaging scan condition determining method as described regarding any one of the seventh through tenth aspects.
In the tomographic imaging method of the eleventh aspect, an X-ray tomographic image is produced using tomographic imaging scan conditions determined by the aforementioned tomographic imaging scan condition determining method. Therefore, an X-ray tomographic image with required image quality can be obtained by a minimum exposure.
In accordance with a twelfth aspect of the invention, there is provided an X-ray CT apparatus comprising: an X-ray tube for emitting X-rays; a multi-detector having a plurality of detector rows arranged in parallel; X-ray dose provisionally determining means for, when an X-ray tomographic image having an image thickness is to be produced by a helical scan employing said multi-detector, provisionally determining an X-ray dose in obtaining said X-ray tomographic image by single-slice CT using a single-slice CT X-ray dose determining algorithm; X-ray dose correcting means for correcting said provisionally determined X-ray dose to an X-ray dose such that an image noise value of the X-ray tomographic image obtained by performing the helical scan is not excessive or insufficient with respect to an allowed value; and scan control means for performing control of the helical scan conforming to said corrected X-ray dose.
In the X-ray CT apparatus of the twelfth aspect, an X-ray dose provisionally determined to obtain an X-ray tomographic image having a certain image thickness is corrected so that an image noise value of the X-ray tomographic image is not excessive or insufficient with respect to an allowed value. Therefore, a minimum X-ray dose satisfying an image noise value requirement can be accurately calculated, and a helical scan conforming to the condition can be performed.
Thus, a disadvantage that required image quality cannot be achieved due to an insufficient X-ray dose can be avoided, and a subject can be prevented from unnecessary X-ray exposure due to an excessive X-ray dose.
In accordance with a thirteenth aspect of the invention, there is provided an X-ray CT apparatus comprising: an X-ray tube for emitting X-rays; a multi-detector having a plurality of detector rows arranged in parallel; image thickness selecting means for selecting an image thickness of an X-ray tomographic image to be produced by a helical scan employing said multi-detector; X-ray dose provisionally determining means for provisionally determining an X-ray dose in obtaining the X-ray tomographic image having said image thickness by single-slice CT using a single-slice CT X-ray dose determining algorithm; scan condition selecting means for selecting scan conditions for the helical scan to be performed; X-ray dose correcting means for correcting said provisionally determined X-ray dose to an X-ray dose such that an image noise value of the X-ray tomographic image obtained by performing the helical scan at said scan conditions is not excessive or insufficient with respect to an allowed value; and scan control means for performing control of the helical scan conforming to said corrected X-ray dose.
The X-ray CT apparatus of the thirteenth aspect achieves the same effects as in the X-ray CT apparatus of the twelfth aspect. Moreover, the image thickness of an X-ray tomographic image to be produced can be selected using image thickness selecting means. Furthermore, scan conditions for a helical scan can be selected using scan condition selecting means.
In accordance with a fourteenth aspect of the invention, there is provided an X-ray CT apparatus comprising: an X-ray tube for emitting X-rays; a multi-detector having a plurality of detector rows arranged in parallel; image thickness selecting means for selecting an image thickness of an X-ray tomographic image to be produced by a helical scan employing said multi-detector; X-ray dose provisionally determining means for provisionally determining an X-ray dose in obtaining the X-ray tomographic image having said image thickness by single-slice CT using a single-slice CT X-ray dose determining algorithm; scan condition selecting means for selecting scan conditions for the helical scan to be performed; a dose correction factor table for storing a dose correction factor for each image thickness and each scan condition; dose correcting means for correcting said provisionally determined X-ray dose by reading a dose correction factor corresponding to said selected image thickness and scan conditions out of said correction factor table and multiplying said provisionally determined X-ray dose by said dose correction factor; and scan control means for performing control of the helical scan conforming to said corrected X-ray dose.
The X-ray CT apparatus of the fourteenth aspect achieves the same effects as in the X-ray CT apparatus of the thirteenth aspect. Moreover, the X-ray dose is corrected by using a dose correction factor read out of a dose correction factor table corresponding to a slice thickness and scan conditions to be achieved. Therefore, a radiation dose proper for scan conditions can be calculated by a simple calculation.
In accordance with a fifteenth aspect of the invention, there is provided an X-ray CT apparatus as described regarding the fourteenth aspect, wherein said dose correction factor table is created corresponding to a plurality of image thicknesses which may be selected, and stores a dose correction factor that expresses an X-ray dose by a multiplying factor based on a reference value, said reference value being an X-ray dose in performing an axial scan by single-slice CT on a phantom imitating a CT value distribution of a standard subject with a slice thickness equal to a desired image thickness, said X-ray dose being an X-ray dose for obtaining an X-ray tomographic image by a helical scan employing said multi-detector with an image noise value equivalent to that in said axial scan.
In the X-ray CT apparatus of the fifteenth aspect, the X-ray dose is corrected with a dose correction factor based on an X-ray dose in performing an axial scan on a phantom. Therefore, an X-ray dose proper for imaging of a subject can be calculated with good accuracy.
In accordance with a sixteenth aspect of the invention, there is provided an X-ray CT apparatus as described regarding the fourteenth or fifteemth aspect, wherein said dose correction factor table stores a dose correction factor for each moving speed of an imaging table and X-ray beam width at a scan center.
In the X-ray CT apparatus of the sixteenth aspect, a dose correction factor is stored in a dose correction factor table for each movement speed of an imaging table and X-ray beam width frequently used as scan conditions. Therefore, the clinical ease-of-use and imaging efficiency can be further improved.
According to the tomographic imaging scan condition determining method and tomographic imaging method of the present invention, the radiation dose to a subject is reduced to a minimum that satisfies an image noise value requirement. Therefore, a disadvantage that the subject is exposed more than necessary can be prevented.
Moreover, according to the X-ray CT apparatus of the present invention, an X-ray tomographic image that satisfies an image noise value requirement can be obtained with a minimum X-ray dose. Therefore, safety can be further improved.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.