U.S. patent application number 09/770043 was filed with the patent office on 2002-03-07 for cradle for x-ray ct system and x-ray ct system.
Invention is credited to Maida, Masashi.
Application Number | 20020027969 09/770043 |
Document ID | / |
Family ID | 18573439 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020027969 |
Kind Code |
A1 |
Maida, Masashi |
March 7, 2002 |
Cradle for X-ray CT system and X-ray CT system
Abstract
In order to reduce artifacts entering an X-ray tomographic image
of a subject in performing a scan at an end portion of a cradle,
and offer a reliable diagnosis environment, the cradle has a top
surface forming a concave curve as viewed from the front, and has a
generally bowl-like shape in a cross section as viewed from the
carrying direction. The cradle further has a shape at its end
portion which, when points at the same position with respect to the
carrying direction on two peripheries along which the top surface
and two lateral sides abut are represented as Points A and B, and a
point at the center position on a minimal curve connecting Points A
and B is represented as Point C, is cut in a plane passing through
Points A and C, and through a Point F for descending the plane in a
direction opposite to the carrying direction, and which is cut in a
plane passing through Points B and C, and through a Point G for
descending the plane in a direction opposite to the carrying
direction.
Inventors: |
Maida, Masashi; (Tokyo,
JP) |
Correspondence
Address: |
MOONRAY KOJIMA
BOX 627
WILLIAMSTOWN
MA
01267
US
|
Family ID: |
18573439 |
Appl. No.: |
09/770043 |
Filed: |
January 25, 2001 |
Current U.S.
Class: |
378/20 ;
378/209 |
Current CPC
Class: |
A61B 6/0442
20130101 |
Class at
Publication: |
378/20 ;
378/209 |
International
Class: |
A61B 006/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 28, 2000 |
JP |
2000-051837 |
Claims
What is claimed is:
1. A cradle for placing thereon a subject in an X-ray CT system and
for carrying the subject toward a scan position: which has a top
surface for placing the subject thereon that forms a concave curve
as viewed from the carrying direction; which has a generally
bowl-like shape in a cross section as viewed from the carrying
direction; and representing points at the same position with
respect to said carrying direction on two peripheries along which
said top surface and two lateral surfaces abut as A and B, and n
points on a minimal curve connecting Points A and B on said top
surface as C1, . . . , Cn, which has at an end portion a shape that
is cut in a plane passing through Points A and C1 and descending
toward a direction opposite to the carrying direction, is cut in a
plane passing through Points Ci and Ci+1 and descending toward a
direction opposite to said carrying direction, and is cut in a
plane passing through Points B and Cn and descending toward a
direction opposite to said carrying direction.
2. The cradle as recited in claim 1, wherein the number of said
points on the minimal curve connecting Points A and B is one.
3. The cradle as recited in claim 1, wherein the number of said
points on the minimal curve connecting Points A and B is two.
4. An X-ray CT system having a cradle as recited in claim 1, 2 or
3.
5. The cradle as recited in claim 1, 2 or 3, comprising a head rest
portion attached to the surface having a generally bowl-like shape
on a side of said cradle in the carrying direction.
6. The cradle as recited in claim 5, wherein an attachment surface
of said head rest portion against said cradle has a shape fitting
the surface having a generally bowl-like shape of said cradle.
7. The cradle as recited in claim 5 or 6, wherein said head rest
portion has a top surface forming a concave curve as viewed from
the carrying direction for placing the subject thereon.
8. The cradle as recited in claim 5, 6 or 7, wherein the attachment
surface of said head rest portion against said cradle is provided
with a protruding portion, and an attachment surface of said cradle
facing said head rest portion is provided with a slot portion for
receiving said protruding portion.
9. The cradle as recited in claim 8, wherein said slot portion
provided in the attachment surface of said cradle facing said head
rest portion has a cross-sectional shape homothetic to the
cross-sectional shape of said cradle.
10. The cradle as recited in claim 1, 2 or 3, wherein said cradle
is formed from a foam material enclosed by carbon fiber reinforced
plastic.
11. The cradle as recited in claim 10, wherein said foam material
is acrylic resin.
12. An X-ray CT system comprising a cradle as recited in claim 5 or
6.
13. An X-ray CT system comprising a cradle as recited in claim
7.
14. An X-ray CT system comprising a cradle as recited in claim
8.
15. An X-ray CT system comprising a cradle as recited in claim
9.
16. An X-ray CT system comprising a cradle as recited in claim 10
or 11.
17. A cradle for holding and carrying a subject toward a scanning
position in an X-ray CT system, said cradle comprising: a top
surface for holding said subject, said top surface forming a
concave curve as viewed from a carrying direction; a generally
bowl-like shape in a cross section as viewed from said carrying
direction; and representing points at a same position with respect
to said carrying direction on two peripheries along which said top
surface and two lateral surfaces abut as A and B, and n points on a
minimum curve connecting points A and B on said top surface as C1,
. . . , Cn; wherein said cradle further comprises: an end portion
thereof having a shape that (a) is cut in a plane passing through
Points A and C1 and descending toward a direction opposite to said
carrying direction, (b) is cut in a plane passing through Points Ci
and Ci+1 and descending toward a direction opposite to said
carrying direction, and (c) is cut in a plane passing through
Points B and Cn and descending toward a direction opposite to said
carrying direction.
18. The cradle of claim 17, wherein number of said points on said
minimum curve connecting Points A and B is one.
19. The cradle of claim 17, wherein number of said points on said
minimum curve connecting Points A and B is two.
20. The cradle of claim 17, further comprising a head rest portion
attached to a surface of a side of said cradle having a generally
bowl-like shape in said carrying direction.
21. The cradle of claim 20, wherein said head rest portion
comprises an attachment surface having a shape which fits said
surface having a generally bowl-like shape.
22. The cradle of claim 20, wherein said head rest portion has a
top surface formed of a concave curve as viewed from said carrying
direction.
23. The cradle of claim 21, wherein said attachment surface of said
head rest portion is provided with a protruding portion; and
wherein an attaching surface of said cradle facing said head rest
portion is provided with a slot for receiving said protruding
portion.
24. The cradle of claim 23, wherein said slot has a cross-sectional
shape homothetic to a cross sectional shape of said cradle.
25. The cradle of claim 17, wherein said cradle is formed of a foam
material enclosed by carbon fiber reinforced plastic.
26. The cradle of claim 25, wherein said foam material is acrylic
resin.
27. An X-ray CT system comprising a cradle for holding and carrying
a subject toward a scanning position, said cradle comprising: a top
surface for holding said subject, said top surface forming a
concave curve as viewed from a carrying direction; a generally
bowl-like shape in a cross section as viewed from said carrying
direction; and representing points at a same position with respect
to said carrying direction on two peripheries along which said top
surface and two lateral surfaces abut as A and B, and n points on a
minimum curve connecting points A and B on said top surface as C1,
. . . , Cn; wherein said cradle further comprises: an end portion
thereof having a shape that (a) is cut in a plane passing through
Points A and C1 and descending toward a direction opposite to said
carrying direction, (b) is cut in a plane passing through Points Ci
and Ci+1 and descending toward a direction opposite to said
carrying direction, and (c) is cut in a plane passing through
Points B and Cn and descending toward a direction opposite to said
carrying direction.
28. The system of claim 27, whrein number of said points on said
minimum curve connecting Points A and B is one.
29. The system of claim 27, wherein number of said points on said
minimum curve connecting Points And an B is two.
30. The system of claim 27, further comprising a head rest portion
attached to a surface of a side of said cradle having a generally
bowl-like shape in said carrying direction.
31. The system of claim 30, wherein said head rest portion
comprises an attachment surface having a shape which fits said
surface having a generally bowl-like shape.
32. The system of claim 30, wherein said head rest portion has a
top surface formed of a concave curve as viewed from said carrying
direction.
33. The system of claim 31, wherein said attachment surface of said
head rest portion is provided with a protruding portion; and
wherein an attaching surface of said cradle facing said head rest
portion is provided with a slot for receiving said protruding
portion.
34. The system of claim 33, wherein said slot has a cross-sectional
shape similar to the cross sectional shape of said cradle.
35. The system of claim 27, wherein said cradle is formed of a foam
material enclosed by carbon fiber reinforced plastic.
36. The system of claim 35, wherein said foam material is acrylic
resin.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a cradle for supporting a
subject (human subject) in an X-ray CT system that obtains an X-ray
tomographic image of the subject by means of X-ray exposure.
[0002] An X-ray CT (computerized tomography) system and apparatus
comprises an apparatus (generally referred to as a gantry
apparatus) having a toroidal cavity portion therein, and an
operating console for supplying several types of control signals to
the gantry apparatus and reconstructing an X-ray tomographic image
for display based on signals (data) acquired by the gantry
apparatus.
[0003] The gantry apparatus has an X-ray source (X-ray tube), an
X-ray detector for detecting X-rays, and a cavity for disposing a
subject therebetween. By rotating the X-ray source and X-ray
detector, signals (data) are obtained at different rotation angles
corresponding to the amount of X-rays which has passed through (and
been attenuated by) the subject. In response to the signals, the
operating console arithmetically calculates the X-ray attenuation
factor in a small portion in a cross-sectional plane through the
subject, and displays the calculated value as a pixel value to
ultimately create an image visible by a human observer. The image
is generally referred to as an X-ray tomographic image, and the
process of creating the X-ray tomographic image is referred to as
an X-ray tomographic image reconstruction process, or more simply,
as reconstruction.
[0004] In addition to the above components, the X-ray CT system
requires a carrier apparatus for supporting and immobilizing the
subject in the cavity portion in the gantry apparatus and for
carrying the subject toward the cavity portion. A table which is
provided over the carrier apparatus and with which the subject
comes in direct contact generally referred to as a cradle.
[0005] Since the cradle, along with the subject, is exposed to
X-rays, the material constituting the cradle is required to have a
high transmission factor to X-rays. In general, a material
comprising a foam material, such as acrylic resin, reinforced by a
surrounding CFRP (carbon fiber-reinforced plastic) or the like is
employed.
[0006] In the X-ray CT system, noise images, referred to as
artifacts, sometimes appear on a reconstructed image. An artifact
occurs when an object has a higher X-ray attenuation factor (a
lower X-ray transmission factor) than its surroundings. If the
noise occurs outside the reconstructed X-ray tomographic image of a
subject, it causes no problem. However, a noise occurring inside
the reconstructed image may lead to misdiagnosis.
[0007] It is also known that the artifact phenomenon is pronounced
in the tangential direction of an object having a high X-ray
attenuation factor.
[0008] The relationship between a conventional cradle configuration
and artifacts will now be described.
[0009] FIG. 1 shows three-direction projection views of a cradle
around its end portion on a side near the gantry apparatus, in
which (a) is a top plan view of the cradle (a view from upside of a
subject, if placed on the cradle), (b) is a front (end) view from
the carrying direction (i.e., from the gantry apparatus), and (c)
is a side view.
[0010] As shown, the cross section of the cradle as viewed from the
cross section or from the carrying direction has a bowl-like shape
(inverted trapezoidal shape) whose top periphery forms a concave
curve, as shown in the end view (b), so that the lying subject is
stably supported. Moreover, the cross section as viewed from the
lateral side is formed as a shape cut at an end at an angle
.theta., as shown in the side view (c). (The reason for the cut
shape will be described later.)
[0011] FIG. 2 shows an exemplary X-ray tomographic image
reconstructed by laying a subject on the illustrated cradle and
scanning the subject at a position S shown in FIG. 1. (The scan
plane is orthogonal to the drawing plane of FIG. 1.)
[0012] Although the cradle is made from a material with a high
X-ray transmission factor, the reinforcing CFRP constituting the
surface of the cradle has a lower transmission factor than the
ambient air. Accordingly, many linear artifacts are generated along
the tangential direction of the CFRP covering the surface, as shown
in FIG. 2.
[0013] Since the cross section of the cradle has a bowl-like shape,
the artifacts generated at the lateral surfaces of the cradle
appear only in a direction away from the subject. Similarly, the
artifacts generated at the curved surface on which the subject is
placed appear only in the tangential direction and do not enter the
X-ray tomographic image of the subject. In these points, it may
safely be said that these artifacts substantially do not affect a
diagnosis based on the X-ray tomographic image in such a condition
as shown in FIG. 2.
[0014] Next, the reason for the shape of the end portion cut at an
angle of .theta. as viewed from the lateral side shown in FIG. 1
will be explained, and then problems about artifacts at the end
portion will be discussed.
[0015] A principal cause of artifacts is the existence of a
material with a low X-ray transmission factor in a scan plane.
Therefore, if the end surface of the cradle is not slanted but,
unlike that shown in FIG. 1, is perpendicular, artifacts are
generated when a position that coincides with the perpendicular end
surface is scanned, because a CFRP layer having a low X-ray
transmission factor extends in a plane that coincides with the scan
plane. Hence, the surface at the end portion of the cradle is made
slanted as shown in the side view FIG. 1(c).
[0016] The angle .theta. should be larger than a certain value.
This is because a scan may possibly be performed with the gantry
apparatus tilted by a certain angle .theta.0, rather than always
being performed in a plane orthogonal to the carrying direction of
the cradle, as shown in FIG. 3. The tilt is made because, for
example, a range of vertebrae constituting the backbone of a human
subject extends in a gentle S-shaped curve and an X-ray tomographic
image sometimes needs to be reconstructed in a plane orthogonal to
a certain portion in such a curved range. Therefore, if the slope
angle .theta. at the end portion of the cradle is smaller than the
maximum tilt angle, there occurs a situation in which the tilt
angle of the gantry is equal to .theta. in an actual use, causing
those artifacts described above to be generated. Hence, the slope
angle .theta. of the end surface of the cradle is required to be
larger than the maximum tilt angle of the gantry apparatus.
However, if the gantry apparatus does not have a tilt function, a
moderate angle will do without the above limitation on the
angle.
[0017] Moreover, when a subject is laid on the cradle, the end
portion may support either the head or the feet of the subject.
Since a scan requires a stable condition of the subject, some
cradles include a mechanism at the end portion for stably securing
the head of the subject. This is achieved by inserting a head rest
into an attachment slot provided at the end surface of the cradle,
as shown in FIG. 4.
[0018] Consider a case of performing a scan at a position,
designated by reference symbol S in FIG. 5, in the carrying
direction (generally referred to as the Z-axis).
[0019] In the illustrated case, a scan plane intersects two
portions Pa and Pb at the corners of the cradle. The cross section
of the scan plane is shown at "S-cross section" in FIG. 5, and two
end cross sections Pa and Pb of the cradle are formed.
[0020] As can also be seen from the enlarged view of the cross
sections Pa and Pb of the end portion of the cradle, bottom
peripheries Pa1 and Pb1 of these cross sections (which are in a
layer of CFRP) are both horizontal, and in addition, are collinear.
Consequently, artifacts are generated, or tend to be generated, on
a line connecting the bottom peripheries Pa1 and Pb1. Therefore, if
a certain site of the subject is placed at the end portion of the
cradle, the artifact connecting the bottom peripheries Pa1 and Pb1
enters the X-ray tomographic image, as shown in FIG. 5, preventing
an accurate diagnosis.
SUMMARY OF THE INVENTION
[0021] The present invention was made in consideration of such
problems, and is directed to providing a cradle and an X-ray CT
system for reducing artifacts entering an X-ray tomographic image
of a subject in performing a scan at an end portion of the cradle,
and offering a reliable diagnosis environment.
[0022] In order to solve such problems, a cradle for an X-ray CT
system of the present invention has, in one embodiment, the
following configuration:
[0023] a cradle for placing thereon a subject in an X-ray CT system
and for carrying the subject toward a scan position:
[0024] which has a top surface for placing the subject thereon that
forms a concave curve as viewed from the carrying direction;
[0025] which has a generally bowl-like shape in a cross section as
viewed from the carrying direction; and
[0026] representing points at the same position with respect to the
carrying direction on two peripheries along which the top surface
and two lateral surfaces abut as A and B, and n points on a minimal
curve connecting Points A and B on the top surface as C1, . . . ,
Cn,
[0027] which has at an end portion a shape that is cut in a plane
passing through Points A and C1 and descending toward a direction
opposite to the carrying direction, is cut in a plane passing
through Points Ci and Ci+1 and descending toward a direction
opposite to the carrying direction, and is cut in a plane passing
through Points B and Cn and descending toward a direction opposite
to the carrying direction.
[0028] According to the present invention as described above,
artifacts entering an X-ray tomographic image of a subject can be
reduced in performing a scan at an end portion of a cradle, and a
reliable diagnosis environment can be offered.
[0029] 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
[0030] FIG. 1 illustrates a shape at an end portion of a
conventional cradle.
[0031] FIG. 2 illustrates a relationship between an X-ray
tomographic image and artifacts at a position S in FIG. 1.
[0032] FIG. 3 is a view for explaining a tilting operation of a
gantry apparatus.
[0033] FIG. 4 illustrates how a head rest is attached to a
cradle.
[0034] FIG. 5 illustrates a relationship between an X-ray
tomographic image and artifacts in performing a scan at the end
portion of the cradle shown in FIG. 1.
[0035] FIG. 6 illustrates an end portion of a cradle blank from
which a cradle that is a first embodiment of the present invention
is made.
[0036] FIG. 7 is an exterior perspective view of the end portion of
the blank shown in FIG. 6.
[0037] FIG. 8 illustrates the shape of the end portion of the
cradle in the first embodiment.
[0038] FIG. 9 is a cross-sectional view in performing a scan at the
end portion of the cradle in the first embodiment.
[0039] FIG. 10 illustrates artifacts in performing a scan at the
end portion of the cradle in the first embodiment.
[0040] FIG. 11 is an exterior perspective view of a cradle blank
for defining the shape of an end portion of a cradle that is a
second embodiment.
[0041] FIG. 12 illustrates the shape of the end portion of the
cradle in the second embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0042] Embodiments in accordance with the present invention will
now be described in detail with reference to the accompanying
drawings.
[0043] For convenience in appreciating the shape of a cradle
according to a first embodiment, consider a cradle blank from which
the cradle is made, having the shape shown in FIG. 6. The
illustrated blank has a width W and a height H, and surfaces of the
blank will be represented by the designations in FIG. 6
hereinbelow.
[0044] In FIG. 6, the shape of the blank as viewed from the
carrying direction is the same as that shown in the front view of
FIG. 1(b). However, the end surface at this stage is orthogonal to
the bottom surface (has not yet been angled). FIG. 7 is a
perspective view of the blank.
[0045] Points A-G shown in FIG. 7 are defined as follows:
[0046] Points A, B: Two corners on the top surface of the blank.
These points are at the same position with respect to the carrying
direction (generally referred to as the Z-axis);
[0047] Point C: A center position on a curved periphery connecting
the top and end surfaces of the blank (a center on a minimal curve
connecting Points A and B on the top surface);
[0048] Points D, E: Two corners on the bottom surface of the
blank;
[0049] Point F: A point on a periphery connecting the bottom
surface and a lateral surface 1 of the blank at a distance L from
Point D; and
[0050] Point G: A point on a periphery connecting the bottom
surface and a lateral surface 2 (not visible in FIG. 7) of the
blank at a distance L from Point E;
[0051] The so-defined Points A, C and F define a plane (which is a
plane descending toward a direction opposite to the carrying
direction). Points B, C and G define another similar plane. The
shape of the end portion of the cradle in the present embodiment is
formed by cutting the blank in these two planes.
[0052] FIG. 8 shows three-direction projection views of the shape
of the end portion of the cradle in the present embodiment. In FIG.
8, reference numerals A-C and F and G are the same as those shown
in FIG. 7.
[0053] It should be noted that the distance L may be set such that
an angle .theta. of a ridgeline R between the two cut surfaces with
respect to the vertical surface is .theta. shown in FIG. 1. The
significance of the angle .theta. was explained earlier and will
not be repeated here.
[0054] An X-ray tomographic image reconstructed by performing a
scan at a position S0 with respect to the carrying direction offers
the same result as in the image at the position S in FIG. 1
described before, and artifacts which are generated or tend to be
generated are the same as those shown in FIG. 2. Therefore,
explanation thereof will not be repeated.
[0055] An explanation will be made for the case of performing a
scan at a position S1 in FIG. 8. The scan plane (a plane
perpendicular to the drawing plane) at the scan position S1
intersects three portions Pa, Pb and Pc of the cradle, as shown in
FIG. 8.
[0056] FIG. 9 shows the respective tomographic images of the
portions Pa, Pb and Pc when a scan is performed at the position S0.
Each of the portions Pa, Pb and Pc has a generally triangular shape
as shown (though strictly speaking, the top periphery of each
triangle is curved), and the fact that the bottom periphery Pa1 of
the portion Pa and the bottom periphery Pc1 of the portion Pc are
collinear should be noted. The reason for the collinearity is that
the peripheries Pa1 and Pc1 are both on an intersection line
between the plane defined by Points A, C and F described with
reference to FIG. 7 and the scan plane at S1 in FIG. 8, i.e., on an
intersection line between two planes. Similarly, the bottom
periphery Pb1 of the portion Pb and the bottom periphery Pc2 of the
portion Pc are collinear.
[0057] Accordingly, an X-ray tomographic image reconstructed, and
artifacts which are generated or tend to be generated, by actually
laying a subject on the cradle configured as above and performing a
scan at the position S1 will be those shown in FIG. 10.
[0058] As shown, although several causes for generating artifacts
are present, those which are generated nearest to the X-ray
tomographic image of the subject are artifacts 40 and 41 shown in
FIG. 10. It will be easily recognized that the artifacts 40 and 41
are a line connecting the peripheries Pa1 and Pc1 and a line
connecting the peripheries Pb1 and Pc2, respectively, shown in FIG.
9.
[0059] Comparing FIGS. 10 and 5, it will be appreciated that
artifacts entering the X-ray tomographic image of the subject are
successfully and markedly reduced when a scan is performed near the
end portion of the cradle of the present embodiment.
[0060] Therefore, when an X-ray tomographic image is reconstructed
at an end portion of a cradle, the shape of the end portion of the
cradle of the present embodiment can reduce artifacts entering the
X-ray tomographic image of the subject, and a highly reliable X-ray
tomographic image can be reconstructed throughout the length of the
cradle.
[0061] It should be noted that the cradle described according to
this embodiment can be provided with an attachment slot for
connecting the head rest as shown in FIG. 4. It is desirable that
the shape of the slot be one for suppressing artifacts entering the
X-ray tomographic image of the subject. The slot may simply have a
cross-sectional shape homothetic to that of the cradle. In this
case, the surface of the head rest abutting against the cradle may
be formed in a shape that closely contacts the end surface of the
cradle so as to reduce artifacts entering the X-ray tomographic
image of the subject, even if artifacts are generated by the end
surface of the head rest.
[0062] <<Second Embodiment>>
[0063] The above embodiment has a shape obtained by cutting the
cradle blank as shown in FIG. 6 in two planes. However, two
artifacts lines 40 and 41 nearest to the X-ray tomographic image of
the subject shown in FIG. 10 lie above the curved top surface of
the cradle at a very small distance, i.e., enter the X-ray
tomographic image of the subject.
[0064] Reduction of the amount of entry can be achieved by a shape
obtained by cutting the blank shown in FIG. 6 in three planes or
more.
[0065] A case of cutting the member in three planes will therefore
be described as a second embodiment hereinbelow.
[0066] To define the cutting positions, FIG. 11 illustrates these
positions for the blank shown in FIG. 6.
[0067] In FIG. 11, Points A, B and D-G are the same as those shown
in FIG. 7, and explanation thereof will be omitted. Points C1, C2
and H are defined as follows:
[0068] Points C1 and C2: Equally trisected positions on a curve
connecting the top and end surfaces (i.e., positions equally
trisecting a minimal curve connecting Points A and B on the top
surface); and
[0069] Point H: A midpoint of a line connecting Points F and G on
the bottom surface.
[0070] According to the second embodiment, the blank is cut in a
surface defined by Points A, C1, and F, a plane defined by Points
C1, C2 and H and a plane defined by Points B, C2 and G. The shape
after cutting forms the end portion of the cradle in the second
embodiment.
[0071] FIG. 12 shows three-direction projection views of the cradle
in accordance with the second embodiment. It should be noted that
Point H is not necessarily the midpoint on a line connecting Points
F and G, and it may lie nearer to the periphery DE. The essential
thing is to assuredly maintain the angle .theta. of the end surface
with respect to the vertical plane.
[0072] When a scan is performed at a position S2 in FIG. 12, four
end portions of the cradle Pa, Pc1, Pc2 and Pb are intersected, as
shown in FIG. 12. As can be easily seen from the front elevation in
FIG. 12, what is meant by the four portions is exactly that the
number of divisions on the curve along the top surface of the
cradle is increased by one relative to that shown in FIG. 9. It
will be easily recognized by those skilled in the art that a line
connecting the four bottom peripheries of those portions serve to
approximate a curved surface more accurately. The reason is the
same as why a regular polygon having N+1 sides approximates a
circle more accurately than one with N sides. Thus, artifacts
generated also come closer to the curved top surface, extending in
a direction away from the X-ray tomographic image of the
subject.
[0073] According to the second embodiment as described above, the
number of artifacts entering an X-ray tomographic image of a
subject can be reduced more than in the cradle of the first
embodiment.
[0074] Moreover, although Points C, C1 and C2 on a minimal curve
connecting Points A and B on the top surface are described as
dividing the curve into equal portions with reference to FIGS. 7
and 11, these points are not limited thereto, and the number of the
points is not limited to one or two but may be more.
[0075] 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.
* * * * *