X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-06-03
2001-10-02
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S114000, C378S016000
Reexamination Certificate
active
06298111
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray computed tomography apparatus for reconstructing the image, i.e., a tomogram, of a slice of an inner portion of a subject on the basis of projection data on the subject in many directions.
Many recent X-ray computed tomography apparatuses have an electrocardiographic synchronization function. As shown in
FIG. 1
, with an electrocardiogram, characteristic waves such as P, Q, R, S, and T waves can be captured. One cardiac cycle is divided into a systole A, a diastole B, and equivalent diastoles C, D, and E. The volume of the heart varies greatly in the systole A and the diastole B, but varies slightly in the equivalent diastoles C, D, and E.
According to the electrocardiographic synchronization function, for example, projection data is acquired a predetermined period of time after the R wave. This data acquisition operation is repeated to obtain, for example, 360° projection data in different projecting directions during a period during which the volume of the heart remains almost the same. A tomogram having no artifact caused by variations in the volume of the heart can be reconstructed on the basis of this obtained projection data.
This electrocardiographic synchronization function, however, demands a very long scanning time. Assume that a tomogram in the systole A is obtained by an apparatus with 750 msec/rotation. Since the systole A is generally 200 msec, a period of time four times a cardiac cycle is required to obtain 360° projection data in the systole A. Since the cardiac cycle is generally one sec, the scanning time is as long as four sec.
For this reason, the subject must be kept exposed to X-rays for a period as long as four sec. In addition, if the X-ray intensity is decreased to reduce the dose of X-rays on the subject, a deterioration in image quality occurs.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to meet both the demands for a reduction in the dose of X-rays and suppression of deterioration in image quality in electrocardiographic synchronization scanning operation of an X-ray computed tomography apparatus.
In the present invention, irradiation of X-rays on a subject is stopped in a specific period in a cardiac cycle, and the subject is irradiated with X-rays in a period other than the specific period in the cardiac cycle. The dose of X-rays can therefore be reduced by the amount corresponding to the specific period in each cardiac cycle as compared with the prior art in which the subject is continuously irradiated with X-rays. In addition, since the X-ray intensity in a period other than the specific period can be set to be higher than that in the prior art, a deterioration in image quality can be suppressed.
Switching operation of irradiation
on-irradiation of X-rays on the subject can be implemented by either X-ray generation/X-ray stop operation of the X-ray tube or opening/closing operation of the shutter.
Furthermore, the dose of X-rays can be reduced even by decreasing the intensity of X-rays applied onto the subject in specific periods without stopping irradiation of X-rays on the subject in the specific periods.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
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Kabushiki Kaisha Toshiba
Kiknadze Irakli
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Porta David P.
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