JP2008216089A - Instrument for measuring three-dimensional position and direction of specimen - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To continuously measure a three-dimensional position and a three-dimensional direction of a solid marker, by a device comprising only one solid marker and one continuous imaging device, even in a slender narrow space such as a gantry inside. <P>SOLUTION: This measuring instrument photographs the solid marker 40 arranged three-dimensionally with four or more of marks, using the imaging device (camera 24), measures an apparent two-dimensional position in a photographed image, and calculates the three-dimensional position and the three-dimensional direction of the solid marker 40, based on a known geometry of the solid marker 40. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a three-dimensional position and orientation measuring apparatus for a subject, and particularly in the field of radiological image diagnosis and radiotherapy where it is necessary to measure the movement of a subject such as a PET (positron CT) inspection or a scintigraphy inspection. The three-dimensional position of the subject for detecting the position, orientation and angle of the subject from an image of the subject, which is suitable for measuring the amount of movement of the patient's part (head or body) And a direction measuring method and apparatus, and a stereoscopic marker and a stereoscopic marker imaging apparatus for use therewith.

  Nuclear medicine examinations such as PET examinations and scintigraphic examinations usually require a long imaging time of several minutes to several tens of minutes. If the subject (patient) moves even a little, the image quality deteriorates and the diagnostic accuracy deteriorates remarkably. During this time, it is common to suppress the subject's movement by using a special instrument. Such physical restraint is a great mental and physical burden for the elderly, the physically and mentally ill, infants and children. In addition, even if efforts are made to suppress movement, small movements are inevitable. In recent high-resolution devices, even a slight movement may affect the image quality. Therefore, there is a need for a nuclear medicine imaging technique that does not cause final image quality degradation even if the movement of the subject is allowed to some extent. This problem can be solved if the movement of the subject can be measured by some device other than the inspection device and the influence of the movement can be corrected.

  For this purpose, as shown in FIG. 1, in the prior art, a plurality of (two in the figure) optical continuous imaging devices 21 and 22 such as digital video cameras installed at a predetermined distance from the subject 10 are used for inspection. When measuring the movement of the head of the subject 10 in the binocular stereo method, the point-shaped markers 31, 32, 33, 34 attached to at least three places (four places in the figure) that can be seen from the imaging device are separated. Two or more continuous imaging devices 21 and 22 placed at different positions are continuously imaged from different directions, and each point marker image is extracted from the entire obtained image by image processing. The three-dimensional coordinates of the point marker are calculated by triangulation to obtain a plurality of marker coordinate values, and the position and orientation of the subject 10 are calculated by combining the obtained coordinate values.

  In Patent Document 1, a patient equipped with a headset including a reference unit that generates a magnetic field is attached to a patient during surgery, and the position of the surgical instrument is detected by detecting the magnetic field of the reference unit with a sensor. Are listed.

  Further, Patent Document 2 extracts characteristic recognition objects (feature points) such as eyebrows, eyes, nose, moles, and mouths of a subject's face from an image without using a specific marker. It describes that a three-dimensional position is measured by a binocular stereo method, and a head movement is measured from a relative position change of the feature point.

  Patent Document 3 previously proposed by the inventors describes a method of measuring the position and orientation of an object using only a single camera using a three-dimensional marker.

  On the other hand, looking beyond the medical field, Patent Documents 4 and 5 describe planar targets and markers for extracting the position of an object from an image captured by optical means.

  Further, Patent Document 6 proposes a three-dimensional target having two parallel circular surfaces as shown in FIG. 2 for measuring the direction (the inclination of the object) with high accuracy.

JP 11-318937 A Japanese Patent Laid-Open No. 11-63927 JP-A-2005-111115 JP-A-9-178447 JP 7-98208 A JP-A-11-63952

  However, in the method using the binocular stereo method shown in FIG. 1, the distance between at least two imaging devices and the distance between them and the measurement object must be separated to some extent, and the narrow space such as the inside of the gantry is used. There was a problem that was not applicable.

  In addition, in the method using a magnetic sensor as in Patent Document 1, a magnetic field transmission device is required, and the measurement situation is limited so that a substance that affects the magnetic field, such as a medical device, does not affect the measurement. There was a problem.

  In addition, the method of extracting a characteristic recognition target from an image as in Patent Document 2 has a problem that it is easily affected by the influence of hair, clothes, etc., and complicated deformation motion, and a plurality of imaging operations. Since an apparatus is necessary, there is a problem similar to the method of FIG.

  In addition, since the method of Patent Document 3 uses only one camera, it can be installed in an elongated space such as the inside of the gantry, but can accurately measure the coordinates representing the distance between the camera and the object. First, in order to measure a three-dimensional position, there is a problem that some auxiliary measuring means must be used together.

  In addition, the targets and markers used in Patent Documents 4 and 5 are both planar, and there is a limit to the measurement accuracy of the direction and the rotation angle.

  Further, in the three-dimensional target used in Patent Document 6, since the target is not marked, the rotation angle around the target central axis cannot be detected, and two parallel targets are further formed on the target. Since a figure needs to exist, there is a problem that it is difficult to attach to a subject's head or the like when a relatively large target is used.

  The present invention has been made in order to solve the above-described conventional problems. Even in a narrow space such as the inside of a gantry, the present invention is an apparatus comprising only one stereoscopic marker and one continuous imaging device. It is an object to enable continuous measurement of the three-dimensional position and the three-dimensional orientation.

  In the present invention, a three-dimensional marker in which four or more marks are three-dimensionally arranged is photographed using an imaging device, an apparent two-dimensional position in the photographed image is measured, and a known geometric shape of the three-dimensional marker is measured. Based on the above, the above-mentioned problem is solved by calculating the three-dimensional position and orientation of the three-dimensional marker.

  It is desirable that the distance between the three-dimensional marker and the imaging device be sufficiently small so that the apparent magnification can be measured with sufficient accuracy.

  One of the three-dimensional markers can be attached to the surface near the examination site of the subject at the time of examination, and the movement of the subject measurement site can be measured.

  In the present invention, three or more marks are arranged in a plane, and one or more marks are arranged at a predetermined height from the plane toward the image pickup apparatus, whereby four or more marks are arranged in three dimensions. The present invention provides a three-dimensional marker for use in the method for measuring the three-dimensional position and orientation of the subject.

  It is desirable that the predetermined height be sufficiently large so that three types of angular coordinates representing a three-dimensional orientation can be measured with high accuracy.

  The present invention is also characterized by comprising a camera for capturing a moving image of the marker of the three-dimensional marker and measuring a two-dimensional position of the marker in the image, and a moving image processing apparatus. A stereoscopic marker imaging device for use in a method for measuring a three-dimensional position and orientation of a body is provided.

  The present invention also measures the apparent two-dimensional position in the image picked up using the three-dimensional marker, the three-dimensional marker imaging device, and the three-dimensional marker imaging device. And a means for calculating the three-dimensional position and orientation of a three-dimensional marker based on the geometric shape.

  The three-dimensional marker imaging device can be disposed in a gantry.

  According to the present invention, even in a narrow and narrow space such as the inside of a gantry, it is an apparatus composed of only one stereoscopic marker and one continuous imaging device, and the three-dimensional marker 3D can be used without using triangulation. Position and 3D orientation can be measured and their temporal changes can be tracked.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  As shown in FIG. 3, the three-dimensional marker 40 according to the present invention has three or more marks 41, 42, 43 on a predetermined plane, and at least one mark 44 at a predetermined height from the plane. It is used by attaching to the body surface of a subject at the time of nuclear medicine inspection such as PET inspection or SPECT inspection.

  As shown in FIG. 3, a continuous imaging device (simply referred to as a camera) 24 that images the stereoscopic marker 40 continuously includes marks 41 to 44 of the stereoscopic marker 40 from a predetermined position, such as a digital video camera, as in the prior art. Can be captured.

  As shown in FIG. 3, the computer unit 50 that processes the image acquired by the camera 24 includes an image processing unit 52 that processes the captured image, a subsequent arithmetic processing unit 54, and a mark 41 of the three-dimensional marker 40 in advance. The storage unit 56 that stores the arrangement, shape, relative position, and the like of .about.44 can be configured.

  In the image processing unit 52, the marks 41 to 44 on the three-dimensional marker 40 are first extracted from the two-dimensional image by predetermined image processing. However, it is assumed that the camera is installed in advance at a position where changes in the positions and orientations of all the marks 41 to 44 can be seen.

  Due to the change in the three-dimensional position and the three-dimensional direction of the three-dimensional marker 40 accompanying the movement of the subject, the positions of the apparent marks 41 to 44 on the extracted two-dimensional image change. Conversely, from the two-dimensional positions of the marks 41 to 44 in the two-dimensional image, the three-dimensional position and the three-dimensional direction can be measured by the calculation shown in the procedure of FIG.

First, initialization is performed in step 100 of FIG. Specifically, the number of the marks 41 to 44 attached to the three-dimensional marker 40 is n (natural number) (n = 4 in this example). As shown in FIG. 5, the distance between a predetermined mark i and the mark j (i and j are natural numbers less than n and i <j) is l _ij , and the height of the mark j from the plane S formed by the mark i is h _j . In this example, since only one mark is given to the predetermined height, the height is denoted as h.

Next, as shown in FIG. 6, the three-dimensional marker 40 is installed in a coordinate system in which the central optical axis of the camera 24 is the horizontal axis Z-axis and the Y-axis is vertically upward, and the camera virtual viewpoint placed at the origin. Let's observe from. The distance from the origin to the mark i is d _i , the polar angle of the mark i in the polar coordinate system is θ _i , and the phase angle is φ _i .

Next, as shown in FIG. 7, in a two-dimensional image taken at a certain time taken by the camera 24, the pixel coordinates of the mark i are (px _i , py _i ), the distance from the origin is pr _i , and the phase angle is ψ _i. And

Next, assuming a spherical surface with a radius d ₀ centered on the camera virtual viewpoint, this is considered as a reference spherical surface, and the polar angle θ and phase angle φ on the reference spherical surface correspond to coordinates pr, ψ, and theta = theta as a function of _{d 0 (pr, ψ, d} 0), φ = φ (pr, ψ, d 0) is calibrated as expressed as. By the way, if the linearity of the camera characteristics is excellent, approximately,
θ≈c × pr (c is a constant),
φ ≒ ψ
It can be.

Thus, the three-dimensional coordinates (x _i , y _i , z _i ) of the mark i in the xyz coordinate system can be expressed as follows (step 110).

x _i = d _i × sin (θ _i ) × cos (φ _i ) (1)
y _i = d _i × sin (θ _i ) × sin (φ _i ) (2)
z _i = d _i × cos (θ _i ) (3)
θ _i = θ _i (pr _i , ψ _i , d _i ) ≈c × pr _i (4)
φ _i = φ _i (pr _i , ψ _i , d _i ) ≈ψ _i (5)

  Next, since the distance between the landmarks is known, the following constraint condition is satisfied.

lij ² = | (x _i , y _i , z _i ) − (x _j , y _j , z _j ) | ² (6)
(i and j are natural numbers less than n, i <j)

The number of constraints n (n-1) / 2 pieces, since the variables that must be determined by calculation of the n by d _i (i is a natural number not exceeding n), a sufficiently d _i If n is 4 or more It is possible to determine.

In actual calculation processing, a function to be minimized is appropriately defined, and d _i is determined by a non-linear minimization calculation method (step 120). For example, the minimization function δ is defined below.

δ = Σ | l _ij ² − | (x _i , y _i , z _i ) − (x _j , y _j , z _j ) | ² | (7)

When the variables d _i are obtained for all the landmarks, the coordinates (x _i , y _i , z _i ) of the landmarks are obtained (step 130), and as a result, the three-dimensional position and the three-dimensional orientation of the three-dimensional marker 40 are measured. it can.

Next, it will be described that the polar marker measurement accuracy in the change in orientation is improved by using a three-dimensional marker. As shown in FIG. 8, it is assumed that the three-dimensional marker 40 is slightly changed by an angle γ (polar angle) with respect to the central optical axis. Since there is a mark at the position of the predetermined height h, the change in the apparent position of the mark in the two-dimensional image is proportional to h × sinγ to hγ. If the marker is a planar marker instead of a three-dimensional marker, the change in the apparent position of the mark in the two-dimensional image is as follows.
r × (1-cosγ) ∝γ ²
Therefore, it is difficult to measure a minute change in γ.

  Next, an improvement in the measurement accuracy of the movement in the distance direction between the stereoscopic marker and the camera, which has been a problem in Patent Document 3 of the prior art, will be described including trial results. If the distance z slightly changes by dz, the rate of change of the apparent enlargement ratio in the two-dimensional image is about dz / z. If the size of the marker in the two-dimensional image is about pr in terms of the number of pixels, the detectable change in z is about dz to z / pr. That is, if the stereoscopic marker is photographed with a size of about several hundred pixels, a distance measurement accuracy of about one hundredth of the distance z can be obtained. In the embodiment shown in FIG. 8, when the motion of the digital video camera 24 having a maximum number of pixels of 640 × 480 is measured by moving the stereoscopic marker 40 close to a distance of about 20 cm, pr is about 200 pixels, and the z direction Good measurement accuracy was obtained.

  As illustrated in FIG. 9, the camera 24 can be disposed in the gantry 60, for example, in the vicinity of the three-dimensional marker 40 attached to the subject 10 in the upper gantry 60A. In the figure, 60B is the lower part of the gantry.

  In the above embodiment, a three-dimensional marker having four marks is used, but the number of marks is not limited to four and may be five or more. Further, the continuous imaging device is not limited to a digital moving camera.

  Furthermore, it is possible to combine two or more three-dimensional marker imaging devices of the present invention, increase redundancy, and increase measurement accuracy by comparing data with each other.

The perspective view which shows an example of the prior art by a twin-lens stereo system The perspective view which shows the example of the solid marker proposed by Unexamined-Japanese-Patent No. 11-63952 The perspective view which shows the measurement condition by embodiment of this invention The flowchart which shows the arithmetic processing procedure of the said embodiment The perspective view which shows the solid marker used in the said embodiment The front view which shows the positional relationship of a camera virtual viewpoint and a mark for demonstrating the principle of this invention Similarly, a front view showing the position of the mark in the two-dimensional photographed image Similarly perspective view for explaining the measurement accuracy of polar angle and distance Sectional drawing which shows the example of arrangement | positioning to the gantry of the said embodiment

Explanation of symbols

10 ... Subject 24 ... Continuous imaging device (camera)
40 ... Solid marker 41-44 ... Mark 50 ... Computer part 52 ... Image processing part 54 ... Arithmetic processing part 56 ... Storage part 60 ... Gantry

Claims (6)

Photograph a three-dimensional marker in which four or more landmarks are arranged in three dimensions using an imaging device,
Measure the apparent two-dimensional position in the captured image,
A method for measuring a three-dimensional position and orientation of a subject, wherein the three-dimensional position and orientation of a three-dimensional marker are calculated based on a known geometric shape of the three-dimensional marker.   The method for measuring a three-dimensional position and orientation of a subject according to claim 1, wherein the three-dimensional marker is attached to a surface near the examination site of the subject at the time of examination, and the movement of the subject measurement site is measured.   Three or more marks are arranged in a plane, and one or more marks are arranged at a predetermined height from the plane toward the image pickup apparatus, whereby four or more marks are arranged in three dimensions. A three-dimensional marker for use in the method for measuring a three-dimensional position and orientation of a subject according to claim 1.   A moving image processing apparatus comprising: a camera for capturing a moving image of the mark of the three-dimensional marker according to claim 3 and measuring a two-dimensional position of the mark in the image; A three-dimensional marker imaging device for use in the method for measuring a three-dimensional position and orientation of a subject described in 1. The three-dimensional marker according to claim 3,
The three-dimensional marker imaging device according to claim 4,
The apparent two-dimensional position in the image imaged using the three-dimensional marker imaging device is measured, and the three-dimensional position and orientation of the three-dimensional marker are calculated based on the known geometric shape of the three-dimensional marker. Means,
A three-dimensional position and orientation measuring device for a subject characterized by comprising:   6. The three-dimensional position and orientation measuring device for a subject according to claim 5, wherein the three-dimensional marker imaging device is disposed in a gantry.

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