Title: X-ray CT Apparatus and exposure dose calculating method
Abstract: For the purpose of displaying a more accurate exposure dose value without imposing additional work on an imaging operator, an exposure dose calculating method in an X-ray CT apparatus comprises the steps of: inputting the age and a region to be examined of a subject; calculating the head or chest diameter of the subject based on the age and region to be examined of the subject input at the inputting step, and on statistics on the head or chest circumference of a human body; calculating an exposure dose of a phantom having a diameter equivalent to the calculated diameter of the head or chest, based on prior information on an exposure dose to a phantom having a predetermined diameter when a predetermined X-ray dose is applied to the phantom; and displaying the exposure dose value.
Patent Number: 6,954,513 Issued on 10/11/2005 to Horiuchi
| Inventors:
|
Horiuchi; Tetsuya (Tokyo, JP)
|
| Assignee:
|
GE Medical Systems Global Technology Company, LLC (Waukesha, WI)
|
| Appl. No.:
|
738754 |
| Filed:
|
December 17, 2003 |
Foreign Application Priority Data
| Dec 20, 2002[JP] | 2002-370591 |
| Current U.S. Class: |
378/4; 378/98; 378/165 |
| Intern'l Class: |
G21K 001/12 |
| Field of Search: |
378/4,16,18,97,98,982,108,165,207
|
References Cited [Referenced By]
U.S. Patent Documents
| 5379333 | Jan., 1995 | Toth.
| |
| 6141398 | Oct., 2000 | He et al.
| |
| 6141402 | Oct., 2000 | Toth.
| |
| 6404844 | Jun., 2002 | Horiuchi et al.
| |
| Foreign Patent Documents |
| 2001/-178713 | Jul., 2001 | JP.
| |
Primary Examiner: Glick; Edward J.
Assistant Examiner: Thomas; Courtney
Attorney, Agent or Firm: Horton, Esq.; Carl B., Armstrong Teasdale LLP
Claims
1. An X-ray CT apparatus comprising:
a gantry including an X-ray source that generates a prespecified amount of X-rays
and rotates around a subject and a detector for detecting X-rays passing through
said subject at different emission angles;
an operation console for reconstructing an X-ray tomographic image of said subject
based on data received from said gantry;
an input device for inputting information on said subject's age and information
on an X-ray irradiated region in said subject;
a first storing device for storing statistics on the outer periphery lengths
of regions in a human body binned with respect to information on age;
a second storing device for storing information on an exposure dose to a phantom
having a predetermined diameter measured by irradiating said phantom with a predetermined
X-ray dose in a predetermined time period;
a diameter calculating device for calculating a diameter of the X-ray irradiated
region in said subject based on said information on the age of said subject and
said information on the X-ray irradiated region in said subject input by said input
device, and on said statistics on the outer periphery lengths stored in said first
storing device;
an exposure dose calculating device for calculating an exposure dose to a phantom
having a diameter equivalent to said calculated diameter of the X-ray irradiated
region based on said information on the exposure dose stored in said second storing
device; and
a display device for displaying said calculated exposure dose.
2. The X-ray CT apparatus of claim 1, wherein said second storing device stores
said information on the exposure dose to said phantom for each tube voltage of
the X-ray source in measuring said information.
3. The X-ray CT apparatus of claim 2, wherein said second storing device stores
said information on the exposure dose to said phantom along with a tube current,
slice thickness and gantry rotation speed of the X-ray source in measuring said information.
4. The X-ray CT apparatus of claim 3, wherein
said apparatus further comprises setting device for setting a tube current, slice
thickness and gantry rotation speed of the X-ray source in irradiating said subject
with X-rays; and
said exposure dose calculating device calculates said exposure dose based on
respective ratios between the tube current, slice thickness and gantry rotation
speed set by said setting device and the tube current, slice thickness and gantry
rotation speed stored in said second storing device.
5. An exposure dose calculating method for an X-ray CT apparatus comprising a
gantry including an X-ray source that generates a prespecified amount of X-rays
and rotates around a subject and a detector for detecting X-rays passing through
said subject at different emission angles, and an operation console for reconstructing
an X-ray tomographic image of said subject based on data received from said gantry,
comprising the steps of:
an inputting step for inputting information on said subject's age and information
on an X-ray irradiated region in said subject;
a first storing step for storing statistics on the outer periphery lengths of
regions in a human body binned with respect to information on age;
a second storing step for storing information on an exposure dose to a phantom
having a predetermined diameter measured by irradiating said phantom with a predetermined
X-ray dose in a predetermined time period;
a diameter calculating step for calculating a diameter of the X-ray irradiated
region in said subject based on said information on the age of said subject and
said information on the X-ray irradiated region in said subject input at said inputting
step, and on said statistics on the outer periphery lengths stored at said first
storing step;
an exposure dose calculating step for calculating an exposure dose to a phantom
having a diameter equivalent to said calculated diameter of the X-ray irradiated
region based on said information on the exposure dose stored at said second storing
step; and
a display step for displaying said calculated exposure dose.
6. The exposure dose calculating method of claim 5, wherein said second storing
step stores said information on the exposure dose to said phantom for each tube
voltage of the X-ray source in measuring said information.
7. The exposure dose calculating method of claim 6, wherein said second storing
step stores said information on the exposure dose to said phantom along with a
tube current, slice thickness and gantry rotation speed of the X-ray source in
measuring said information.
8. The exposure dose calculating method of claim 7, wherein said method further
comprises a setting step for setting a tube current, slice thickness and gantry
rotation speed of the X-ray source in irradiating said subject with X-rays; and
said exposure dose calculating step calculates said exposure dose based on respective
ratios between the tube current, slice thickness and gantry rotation speed set
at said setting step and the tube current, slice thickness and gantry rotation
speed stored at said second storing step.
9. A control program embodied on a computer readable medium for calculating an
exposure dose of a patient, said program configured to:
receive information on a subject's age and information on an X-ray irradiated
region in the subject;
receive statistics on the outer periphery lengths of regions in a human body;
bin the information of the subject's age and the statistics on the outer periphery
lengths in a common bin;
receive information on an exposure dose to a phantom having a predetermined diameter
measured by irradiating the phantom with a predetermined X-ray dose in a predetermined
time period;
calculate a diameter of the X-ray irradiated region in the subject based on the
information on the age of the subject and the information on the X-ray irradiated
region in the subject input, and on the statistics on the outer periphery lengths;
calculate an exposure dose for a phantom having a diameter equivalent to the
calculated diameter of the X-ray irradiated region based on the information on
the exposure dose; and
display the calculated exposure dose.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Application No. 2002-370591
filed Dec. 20, 2002.
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT (computerized tomography) apparatus
for acquiring a tomographic image of a subject by X-ray irradiation.
An X-ray CT apparatus acquires (reconstructs) an image (X-ray tomographic image)
of a cross-sectional plane (a plane at a slice position, i.e., slice plane) in
an X-ray irradiated region in a subject (patient) by rotating an X-ray source for
generating X-rays around the patient, detecting X-rays passing through the patient
at different emission angles by an X-ray detector, and computer-processing the
X-rays at an operation console.
When capturing the X-ray tomographic image of the patient using the X-ray CT
apparatus, an exposure dose value (CTDI) that indicates the degree of exposure
to the patient by the image capture is displayed on an operation console beforehand.
IEC and FDA recommend use of a value measured using a specified phantom as the
exposure dose value displayed at that time. Specifically, when the head of the
patient is imaged, there is employed a 16-cm acrylic phantom (see FIG. 5) provided
with holes for inserting a measuring tool, one hole in the center and four near
the outer periphery, and a value obtained by multiplying the dose (unit: [mGy])
measured with the measuring tool inserted into the holes by a prespecified weight
is used as the displayed exposure dose. Similarly, when the body of the patient
is imaged, a 32-cm acrylic phantom (not shown) is employed.
One technique for reducing the exposure dose to the subject to the minimum required
amount is described in Patent Document 1.
Patent Document 1
Japanese Patent Application Laid Open No. 2001-178713.
The exposure dose value displayed when dividing only between the head and body
and irrespective of the size of the patient body as in the conventional technique
may, however, be different from the actual exposure dose value. Especially, when
imaging the head or body of a pediatric patient that is smaller than the phantom
for use in the measurement (for the head: 16 cm, and for the body: 32 cm), the
exposure dose value displayed is susceptible to the risk of underestimation relative
to the actual exposure dose value. Thus, it is desirable to display a measurement
result from a phantom of a size that corresponds as close as possible to the outer
periphery (head or chest circumference) of the X-ray irradiated region of the patient.
On the other hand, the work of measuring the outer periphery (head or chest circumference)
of the imaged region for every patient before imaging, and inputting the result
to the operation console puts an additional burden on the imaging operator (radiologist,
physician, nurse or the like).
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an X-ray CT
apparatus that is capable of displaying a more accurate exposure dose value without
imposing additional work to the imaging operator.
An X-ray CT apparatus in accordance with the present invention for solving the
aforementioned problems comprises: a gantry comprising an X-ray source that generates
a prespecified amount of X-rays and rotates around a subject and a detector for
detecting X-rays passing through said subject at different emission angles, and
an operation console for reconstructing an X-ray tomographic image of said subject
based on data received from said gantry, said X-ray CT apparatus being characterized
in comprising: input means for inputting information on said subject's age and
information on an X-ray irradiated region in said subject; first storing means
for storing statistics on the outer periphery lengths of regions in a human body
binned with respect to information on age; second storing means for storing information
on an exposure dose to a phantom having a predetermined diameter measured by irradiating
said phantom with a predetermined X-ray dose in a predetermined time period; diameter
calculating means for calculating a diameter of the X-ray irradiated region in
said subject based on said information on the age of said subject and said information
on the X-ray irradiated region in said subject input by said input means, and on
said statistics on the outer periphery lengths stored in said first storing means;
exposure dose calculating means for calculating an exposure dose to a phantom having
a diameter equivalent to said calculated diameter of the X-ray irradiated region
based on said information on the exposure dose stored in said second storing means;
and display means for displaying said calculated exposure dose.
According to the present invention, a more accurate exposure dose value
can be displayed without imposing additional work on the imaging operator.
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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing the configuration of an X-ray CT apparatus in accordance
with one embodiment of the present invention.
FIG. 2 is a flow chart showing processing for exposure dose value display in
the X-ray CT apparatus in accordance with one embodiment of the present invention.
FIG. 3 is a diagram showing the age—head circumference/chest circumference
statistical data stored in the X-ray CT apparatus in accordance with the embodiment
of the present invention.
FIG. 4 is a diagram showing a relationship between an acrylic phantom diameter
and an absorption dose.
FIG. 5 is an exterior view of the acrylic phantom.
DETAILED DESCRIPTION OF THE INVENTION
Several preferred embodiments of the present invention will now be described
in detail with reference to the accompanying drawings. Similar reference symbols
refer to identical or similar portions throughout the drawings.
<System Overall Configuration>
FIG. 1 is a system configuration diagram of an X-ray CT apparatus in accordance
with one embodiment of the present invention.
As shown in FIG. 1, the X-ray CT apparatus comprises a gantry
120 for
irradiating
a subject (patient) with X-rays and detecting X-rays passing through the borne
subject, an operation console
100 for transmitting instruction signals to
the gantry
120 to activate several kinds of settings, and reconstructing
an X-ray tomographic image based on projection data output from the gantry
120
for display, and a carrier apparatus
140 for bearing thereon the subject
and carrying the subject into the gantry.
The gantry designated by reference numeral
120 comprises a main controller
122 for conducting overall control along with the following components.
Reference numeral
121 designates an interface for communication
with the operation console
100, reference numeral
132 designates
a gantry rotating section provided therein with an X-ray tube
124 for generating
X-rays (that is drive-controlled by an X-ray tube controller
123), a collimator
127 for defining the X-ray irradiation range, and a collimator motor
126
for adjusting the slit width of the collimator
127 to define the X-ray irradiation
range and adjusting the position of the collimator
127 in the Z-axis direction
(the direction perpendicular to the drawing plane i.e., the direction in which
a top plate
142 described later is carried toward a cavity portion
133).
Such driving by the collimator motor
126 is controlled by a collimator controller
125.
Moreover, the gantry rotating section designated by reference numeral
132
comprises an X-ray detecting section
131 for detecting X-rays passing through
the subject, and a data collecting section
130 for collecting projection
data acquired by the X-ray detecting section
131. The X-ray detecting section
131 comprises a plurality of detector rows arranged in the Z-axis direction,
each row having a detector element group comprised of a plurality of detector elements (channels).
The X-ray tube
124 and collimator
127 and the X-ray detecting section
131 are opposingly disposed with respect to the cavity portion
133,
and the gantry rotating section
132 is configured to rotate in a direction
indicated by an arrow
135 while maintaining their mutual relationship. The
rotation is conducted by a rotary motor
129 whose rotation speed is controlled
by driving signals from the rotary motor controller
128 at a prespecified
control cycle.
The carrier apparatus
140 has a top plate
142 on which the subject
is directly rested and a table
143 for supporting the top plate
142.
The top plate
142 is driven in the Z-axis direction by a top plate motor
141 (i.e., the direction of carrying the top plate=the Z-axis direction),
and the drive of the top plate motor
141 at a carrying speed is controlled
based on driving signals from a top plate motor controller
134 at a prespecified
control cycle.
The main controller
122 analyzes several kinds of instruction signals
received via the I/F
121, and based on the signals, outputs several kinds
of control signals to the X-ray tube controller
123, collimator controller
125, rotary motor controller
128, top plate motor controller
134,
and data collecting section
130. Moreover, the main controller
122
also executes processing for sending the projection data collected at the data
collecting section
130 to the operation console
100 via the I/F
121.
The operation console
100 is what is generally called a workstation, and
comprises a CPU
105 for conducting overall control for the entire apparatus,
a ROM
106 storing a boot program, etc., a RAM
107 serving as a main
storage device (memory), and the following components, as shown in FIG.
1.
HDD
108 is a hard disk device, and stores an OS and a diagnostic program
for controlling the entire X-ray CT apparatus. The HDD
108 also stores a
control program for causing the operation console
100 to execute the exposure
dose calculating method in accordance with the present invention. The exposure
dose calculating method in accordance with the present invention is achieved by
the CPU
105 reading and executing the control program. At that time, a program
code itself read from the HDD
108 implements the exposure dose calculating
method, and the HDD
108 that stores the program code constitutes the present invention.
Returning to FIG. 1, a VRAM
101 is a memory for developing image
data to be displayed (256×256 pixels), and the development of the image data,
etc., on the VRAM
101 enables an X-ray tomographic image and a calculated
exposure dose value (which will be described later) to be displayed on a CRT
102.
Reference numerals
103 and
104 designate a keyboard and a mouse for
several kinds of settings. Reference numeral
109 designates an interface
for communication with the gantry
120.
<Flow of Exposure Dose Calculation Processing>
FIG. 2 is a flow chart showing exposure dose calculation processing in the X-ray
CT apparatus in accordance with one embodiment of the present invention. The flow
chart of FIG. 2 will be described hereinbelow with reference to FIGS. 3-5 as necessary.
At Steps S
201 and S
202, the age and a region to be examined (=X-ray
irradiated region; particularly, the head or chest) of the patient are input using
the keyboard
103 or mouse
104. It should be noted that the age and
X-ray irradiated region of the patient are items that have been conventionally
input when imaging the patient, and they are not newly required input items to
implement the present invention (that is, the input operation does not impose additional
work on the operator, and the work load does not change as compared with the conventional technique).
Based on the age and X-ray irradiated region of the patient input at Steps
S
201 and S
202, a head or chest circumference corresponding to the
size of the patient body is accessed (Step S
203). The access to the head
or chest circumference is conducted based on a table saved beforehand in the hard
disk
108 in the operation console. FIG. 3 is an example of the table, and
it allows derivation of a more realistic value by using, for example, statistical
data of the age and the head and chest circumferences from a survey made by the
Ministry of Health, Labour and Welfare. At Step S
203, a head circumference
is accessed if the X-ray irradiated region input at Step S
202 is the head,
and a chest circumference is accessed if the region is the chest.
Next, at Step S
204, the diameter of an acrylic phantom is calculated
based on the accessed head circumference (or chest circumference). Specifically,
on the assumption that the head (or chest) is round, its diameter is calculated
from the accessed head circumference (or chest circumference) (that is, the diameter
is calculated by head circumference/π, or chest circumference/π).
If the tissue in the human body is assumed to be equivalent to water, then, by
correcting the calculated diameter using a ratio between acrylic and water X-ray
attenuations, an equivalent diameter of an acrylic phantom can be determined.
For example, the average head circumference of a six-month-old male infant is
43.7 cm according to FIG.
3. The diameter is about 13 cm assuming that the
head is round as mentioned above. If the attenuation ratio between acrylic and
water at a tube voltage of 120KV is about 0.9, an acrylic phantom diameter equivalent
to the head of a six-month-old male infant is determined as about 12 cm.
At Step S
205, several kinds of setting values in imaging the patient are
read. Specifically, the setting values include a tube voltage, tube current, slice
thickness, and gantry rotation speed.
Then, at Step S
206, an exposure dose value is calculated. In calculating
the exposure dose value, a graph representing a relationship between the acrylic
phantom diameter and absorption dose as shown in FIG. 4 is referred to.
In FIG. 4, the horizontal axis represents the acrylic phantom diameter (cm),
and
the vertical axis represents the absorption dose (mGy) measured for each acrylic
phantom diameter. Reference numeral
401 designates the absorption dose measured
by a measurement tool inserted into a hole in the center of the acrylic phantom,
and reference numeral
402 designates the absorption dose measured by the
measurement tool inserted into a hole near the periphery of the acrylic phantom.
These values are measured beforehand for a plurality of tube voltages under
a prespecified slice thickness, tube current and gantry rotation speed, are binned
with respect to the tube voltages in the measurement process, and are stored along
with the slice thickness, tube current and gantry rotation speed in the measurement
process, as shown in FIG.
4.
A graph (one of those shown in FIG. 4) corresponding to the tube voltage read
at
Step S
205 is referred to, and an absorption dose is calculated based on
the acrylic phantom diameter calculated at Step S
204. At that time, the
tube current, slice thickness and gantry rotation speed read at Step S
205
are used. That is, since the exposure dose value is proportional to the tube current,
slice thickness and gantry rotation speed, a resultant exposure dose value is calculated
from respective ratios of the tube current, slice thickness and gantry rotation
speed read at Step S
205 to a measurement condition (tube current, slice
thickness, gantry rotation speed) in the referenced graph (FIG.
4).
The exposure dose value calculated at Step S
206 is displayed on the operation
console at Step S
207. In the display, a value obtained by summing a weighted
absorption dose
401 measured by the measurement tool inserted into the hole
in the center of the acrylic phantom, and a weighted absorption dose
402
measured by the measurement tool inserted into the hole near the periphery of the
acrylic phantom is displayed. In the present embodiment, the sum of the measured
value in the center multiplied by ⅓ and the measured value in the peripheral
portion multiplied by ⅔ is displayed as the exposure dose value.
As can be clearly seen from the preceding description, according to the present
embodiment, by merely inputting the age and measured region of the patient as in
the conventional technique, a more realistic exposure dose value than that in the
conventional technique can be displayed.
Other Embodiments
Although an HDD is employed as a storage medium for supplying the program
code in the aforementioned embodiment, the storage medium is not limited thereto
but may be, for example, a floppy (registered trademark) disk, optical disk, magneto-optical
disk, CD-ROM, CD-R, magnetic tape, non-volatile memory card, or ROM.
Moreover, it will be easily recognized that the present invention encompasses
not only the case in which the function of the aforementioned embodiment is implemented
by a computer reading and executing a program code but also the case in which the
function of the aforementioned embodiment is implemented by an OS (operating system),
for example, running on the computer, executing part or all of the actual processing
based on instructions by the program code.
Furthermore, it will be easily recognized that the present invention
encompasses the case in which the function of the aforementioned embodiment is
implemented by writing the program code read out from a storage medium into a memory
provided in a feature expansion board inserted into the computer or in a feature
expansion unit connected to the computer, and then, executing part or all of actual
processing based on instructions by the program code by a CPU provided in the extension
board or unit.
Many widely different embodiments of the invention may be configured without
departing from the spirit and the scope of the present invention. It should be
understood that the present invention is not limited to the specific embodiments
described in the specification, except as defined in the appended claims.
*
