X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-08-01
2002-01-08
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S018000
Reexamination Certificate
active
06337895
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for determining a constant in a relational expression concerning the end-tidal air velocity constant and the arterial blood velocity constant, and a xenon CT apparatus. In particular, the present invention relates to a method for determining a constant in a relational expression concerning the end-tidal air velocity constant and the arterial blood velocity constant, and a xenon CT apparatus which make it possible to correctly measure the cerebral blood flow rate by utilizing, for example, a xenon gas supply unit and an X-ray CT unit.
2. Description of the Related Art
A method is known, for example, in which a tomographic image of a head of a patient as a specimen is obtained by using an X-ray CT apparatus, while the patient is allowed to inhale a mixed gas composed of xenon gas and oxygen gas fed from a gas inhalation unit for a certain period of time by the aid of a breathing mask, and then the patient is allowed to breathe ordinary air, wherein the tomographic image is analyzed to measure the blood flow in the head of the patient.
That is, according to this measuring method, the mixed gas is absorbed into the pulmonary vein from the lungs of the patient, it passes through the heart, and it flows as the arterial blood flow into the tissue of the head. The mixed gas passes through the head tissue, it returns to the heart via the venous blood flow, and it is returned to the pulmonary artery via the heart. During this process, the time-dependent change of the xenon gas concentration in the tissue of the head is observed by using the X-ray CT apparatus, and it is compared with the time-dependent change of the xenon gas concentration of a head in which the tissue is normal. Thus, it is possible to diagnose the head of the patient.
In order to obtain the cerebral blood flow rate by using the measuring method described above, it is necessary to obtain the xenon gas concentration in the artery together with the xenon gas concentration in the cerebral tissue. Recently, the xenon gas concentration in the end-tidal air, which can be detected by a noninvasive method, is substitutively used as the xenon gas concentration in the artery.
The inventors of this application have found out and revealed the correlation between the xenon gas concentration in the artery and the xenon gas concentration in the end-tidal air {see “The Effect of Xenon Inhalation Speed on Cerebral Blood Flow Obtained Using the End-Tidal Method in Xenon-Enhanced CT”, Shigeru Sase, Journal of Computer Assisted Tomography, 22 (5): 786-791, 1988}.
The correlation is as follows. That is, the velocity constant of the xenon gas concentration in the artery is expressed by a linear exponential function of the velocity constant of the xenon gas concentration in the end-tidal air by using a conversion constant.
SUMMARY OF THE INVENTION
This invention has been made taking the knowledge as described above into consideration, an object of which is to provide a method for determining a constant in a relational expression concerning the end-tidal air velocity constant and the arterial blood velocity constant, and a xenon CT apparatus which make it possible to correctly determine the cerebral blood flow rate by using the xenon gas concentration in the end-tidal air by utilizing the correlation between the xenon gas concentration of the blood flow in the artery and the xenon gas concentration in the end-tidal air.
According to the present invention, there is provided a method for determining a constant &ggr; in a relational expression:
Ka=&ggr;×(1−exp(−Ke/&ggr;))
wherein Ke represents an end-tidal air velocity constant and Ka represents an arterial blood velocity constant in a xenon CT examination, the method comprising a step A of setting a region of interest on a xenon CT image; and a step B of determining the constant &ggr; with which a xenon distribution coefficient &lgr; most closely approaches a predetermined target value in the preset region of interest. As described above, it is possible to obtain a correct value of the constant &ggr; by determining the constant &ggr; by using the xenon distribution coefficient &lgr; as an index.
In the step B, the distribution coefficient &lgr; is calculated by varying the constant &ggr; within a desired range from 0.24 to 7.7 to determine the constant &ggr; with which the distribution coefficient &lgr; most closely approaches the target value. As described above, the range of the constant &ggr; to be considered is limited, and thus it is possible to shorten the processing period of time required to determine the constant &ggr;.
In this case, it is also preferable that the desired range is a range from 0.3 to 2.5.
In the step B, the constant &ggr;, with which a value of the distribution coefficient &lgr; most closely approaches the target value, is determined for each of predetermined picture elements included in the region of interest, and obtained values of the constant &ggr; are averaged to estimate an objective value of the constant &ggr;. Accordingly, it is possible to determine the constant &ggr; more correctly.
Further, in the step A, the region of interest is set to a region including cerebral white matter, and in the step B, the target value is determined depending on a hematocrit value. Accordingly, it is possible to correctly determine the constant &ggr; in order to determine the cerebral blood flow rate.
According to the present invention, there is provided a xenon CT apparatus comprising a gas supply unit for supplying xenon gas to a specimen; a concentration-measuring unit for measuring a xenon gas concentration (hereinafter referred to as “expiration gas xenon gas concentration”) in end-tidal air of the specimen; a main X-ray CT apparatus body for obtaining CT image data of an examination site in order to obtain a xenon gas concentration (hereinafter referred to as “examination site xenon gas concentration”) of the examination site of the specimen; and a data processing unit for determining the examination site xenon gas concentration on the basis of the CT image data, and determining a blood flow rate of the examination site on the basis of the examination site xenon gas concentration and the expiration gas xenon gas concentration; wherein the data processing unit determines a xenon gas distribution coefficient &lgr; between the examination site and blood of the specimen on the basis of a conversion constant &ggr; for converting a velocity constant (hereinafter referred to as “expiration gas velocity constant”) of the expiration gas xenon gas concentration into a velocity constant (hereinafter referred to as “arterial blood velocity constant”) of a xenon gas concentration (hereinafter referred to as “arterial xenon gas concentration”) of blood flow in artery, and it establishes, as a true value, the conversion constant &ggr; with which the distribution coefficient &lgr; most closely approaches a predetermined target value. As described above, it is possible to obtain a correct value of the conversion constant &ggr; by determining the conversion constant &ggr; by using the distribution coefficient &lgr; as an index.
In this arrangement, the data processing unit includes a conversion constant-setting means for determining the conversion constant &ggr;; and the conversion constant-setting means has an assumed value-setting means for setting an assumed value of the conversion constant &ggr; and varying the assumed value.
The conversion constant-setting means includes a temporary velocity constant-setting means for determining a temporary calculated value of the arterial blood velocity constant from the expiration gas velocity constant on the basis of the assumed value of the conversion constant &ggr;; a temporary distribution coefficient-calculating means for determining a temporary calculated value of the distribution coefficient &lgr; from the temporary calculated value of the arterial blood velocity constant and the examination site xenon gas concent
Anzai Medical Kabushiki Kaisha
Bruce David V.
Guss Paul A.
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