Dose evaluating method and X-ray CT apparatus

• US 20060115039A1
• Filed: 11/30/2005
• Published: 06/01/2006
• Est. Priority Date: 12/01/2004
• Status: Active Grant

• Current Assignee: GE Medical Systems Global Technology Company LLC (GE Healthcare Technologies, Inc.)
• Original Assignee: GE Medical Systems Global Technology Company LLC (GE Healthcare Technologies, Inc.)
• Inventors: Gohno, Makoto
• Granted Patent: US 7,756,243 B2
• Time in Patent Office: Days
• US Class Current: 378/4
• CPC Class Codes: A61B 6/032 Transmission computed tomography [CT]
• CPC Class Codes: A61B 6/488 involving pre-scan acquisition
• CPC Class Codes: A61B 6/542 involving control of exposure
• CPC Class Codes: G01T 1/02 Dosimeters G01T1/15 takes precedence

Abstract

With the objective of evaluating whether a dose is excessive for a subject, such a dose that image noise reaches less than or equal to a predetermined value is estimated upon an axial scan on the basis of projection data acquired by a scout scan. The dose is set as a maximum dose. An area other than air, of an image obtained by the axial scan is partitioned into a plurality of small areas i. Image noises N(i) at the respective small areas i are calculated. A warning image G1 in which all of pixel values of pixels in small areas in which the image noises N(i) are less than or equal to a predetermined value, are substituted with pixel values expressed in black level, is created and displayed.

Claims

1. A dose evaluating method comprising the steps of:

collecting projection data (called fixed view projection data) without rotating an X-ray tube and an X-ray detector about a body axis of a subject; and

evaluating, based on the fixed view projection data, a dose at the time when at least one of the X-ray tube and the X-ray detector is rotated about the body axis of the subject to collect projection data.

2. The dose evaluating method according to claim 1, further comprising the steps of determining an average penetration density and a maximum penetration length from the fixed view projection data, estimating a dose at which image noise reaches less than or equal to a predetermined value, from the average penetration density and the maximum penetration length, and setting the dose as a criterion for evaluation.

3. The dose evaluating method according to claim 2, further comprising the step of exhibiting the dose set as the criterion for evaluation as a maximum dose when an operator designates a dose at the time when at least one of the X-ray tube and the X-ray detector is rotated about the body axis of the subject to collect the projection data.

4. A dose evaluating method comprising the steps of:

collecting projection data (called fixed view projection data) at a plurality of z-coordinate positions without rotating an X-ray tube and an X-ray detector about a body axis of a subject and by moving the X-ray tube and the X-ray detector relative to the direction of the body axis thereof; and

evaluating, based on the fixed view projection data, doses at the respective z-coordinate positions at the time when at least one of the X-ray tube and the X-ray detector is rotated about the body axis of the subject to collect projection data.

5. The dose evaluating method according to claim 4, further comprising the steps of determining average penetration densities and maximum penetration lengths at the respective z-coordinate positions from the respective fixed view projection data, estimating doses at which image noises at the respective z-coordinate positions become less than or equal to a predetermined value, from the respective average penetration densities and the maximum penetration lengths, and setting the respective doses as criteria for evaluation at the respective z-coordinate positions.

6. The dose evaluating method according to claim 5, further comprising the step of exhibiting a dose set as a criterion for evaluation at a given z-coordinate position as a maximum dose when an operator designates a dose at the time when at least one of the X-ray tube and the X-ray detector is rotated about the body axis of the subject to collect projection data.

7. A dose evaluating method comprising the steps of:

partitioning an area for a target image, other than air into a plurality of small areas;

calculating image noise at each of the small areas; and

if a small area in which the image noise is less than or equal to a predetermined value exists, determining as being excessive, a dose at the time when the target image is photographed.

8. The dose evaluating method according to claim 7, further comprising the step of creating and displaying a warning image which enables a distinction from each small area in which image noise is larger than a predetermined value, by changing a display form of the small area at the time when the dose is determined as being excessive.

9. The dose evaluating method according to claim 7, further comprising the step of creating and displaying a histogram in which the value of image noise is defined as a horizontal axis and the number of small areas is defined as a vertical axis.

10. An X-ray CT apparatus comprising:

a fixed view projection data acquiring device for collecting projection data (called fixed view projection data) without rotating an X-ray tube and an X-ray detector about a body axis of a subject; and

a dose evaluating device for evaluating, based on the fixed view projection data, a dose at the time when at least one of the X-ray tube and the X-ray detector is rotated about the body axis of the subject to collect projection data.

11. The X-ray CT apparatus according to claim 10, wherein the dose evaluating device determines an average penetration density and a maximum penetration length from the fixed view projection data, estimates a dose at which image noise reaches less than or equal to a predetermined value, from the average penetration density and the maximum penetration length, and sets the dose as a criterion for evaluation.

12. The X-ray CT apparatus according to claim 11, further comprising:

a dose designating device for allowing an operator to designate a dose at the time when at least one of the X-ray tube and the X-ray detector is rotated about the body axis of the subject to collect projection data; and

a maximum dose exhibiting device for exhibiting the dose set as the criterion for evaluation as a maximum dose.

13. An X-ray CT apparatus comprising:

a fixed view projection data acquiring device for collecting projection data (called fixed view projection data) at a plurality of z-coordinate positions without rotating an X-ray tube and an X-ray detector about a body axis of a subject and by moving the X-ray tube and the X-ray detector relative to the direction of the body axis thereof; and

a dose evaluating device for evaluating, based on the fixed view projection data, doses at the respective z-coordinate positions at the time when at least one of the X-ray tube and the X-ray detector is rotated about the body axis of the subject to collect projection data.

14. The X-ray CT apparatus according to claim 13, wherein the dose evaluating device determines average penetration densities and maximum penetration lengths at the respective z-coordinate positions from the respective fixed view projection data, estimates doses at which image noises at the respective z-coordinate positions become less than or equal to a predetermined value, from the respective average penetration densities and the maximum penetration lengths, and sets the respective doses as criteria for evaluation at the respective z-coordinate positions.

15. The X-ray CT apparatus according to claim 14, further comprising:

a dose designating device for allowing an operator to designate a dose at the time when at least one of the X-ray tube and the X-ray detector is rotated about the body axis of the subject at a given z-coordinate position to collect projection data; and

a maximum dose exhibiting device for exhibiting a dose set as a criterion for evaluation at the given z-coordinate position as a maximum dose.

16. An X-ray CT apparatus comprising:

an image noise calculating device for partitioning an area for a target image, other than air into a plurality of small areas and calculating image noise at each small area; and

a dose evaluating device for, if a small area in which the image noise is less than or equal to a predetermined value exists, determining as being excessive, a dose at the time when the target image is photographed.

17. The X-ray CT apparatus according to claim 16, further comprising:

a warning image creating/displaying device for creating and displaying a warning image which enables a distinction from each small area in which image noise is larger than a predetermined value, by changing a display form of the small area at the time when the dose is determined as being excessive.

18. The X-ray CT apparatus according to claim 16, further comprising:

a histogram creating/displaying device for creating and displaying a histogram in which the value of image noise is defined as a horizontal axis and the number of small areas is defined as a vertical axis.