Focal Plane Tracking for Optical Microtomography

• US 20100322494A1
• Filed: 09/18/2006
• Published: 12/23/2010
• Est. Priority Date: 03/28/2001
• Status: Active Grant

• Current Assignee: University of Washington, VisionGate, Inc.
• Original Assignee: University of Washington, VisionGate, Inc.
• Inventors: Nelson, Alan C., Fauver, Mark E., Seibel, Eric J., Meyer, Michael G., Johnson, Roger H., Rahn, J. Richard, Neumann, Thomas
• Granted Patent: US 7,907,765 B2
• Time in Patent Office: Days
• US Class Current: 382/131
• CPC Class Codes: G01N 15/1433 using image recognition
• CPC Class Codes: G01N 15/147 the analysis being performed on a sample stream
• CPC Class Codes: G01N 15/1475 using image analysis for extracting features of the particle
• CPC Class Codes: G01N 21/4795 spatially resolved investigating of object in scattering medium in vivo A61B

Abstract

An optical tomography system for imaging an object of interest including a light source for illuminating the object of interest with a plurality of radiation beams. The object of interest is held within an object containing tube such that it is illuminated by the plurality of radiation beams to produce emerging radiation from the object containing tube, a detector array is located to receive the emerging radiation and produce imaging data used by a mechanism for tracking the object of interest.

Claims

1. An optical tomography system comprising:

a light source for illuminating an object of interest with a plurality of radiation beams;

an object containing tube, wherein when the object of interest is held within the object containing tube it is illuminated by the plurality of radiation beams to produce emerging radiation from the object containing tube;

an objective lens, having an optical axis, for scanning the object at a set of viewing angles to generate a set of pseudoprojection images from the emerging radiation, where each pseudoprojection image is produced by integrating a series of images from a series of focal planes integrated along the optical axis for each angle;

a detector array located to receive the set of pseudoprojection images; and

means for tracking the object of interest responsively to the imaging data, wherein the tracking means comprises means for tracking a pseudoprojection image center.

6. The system of claim 1 wherein the tracking means further comprises means for tracking a focal plane.

12. The optical tomography system of claim 1 wherein the plurality of radiation beams comprise a plurality of parallel radiation beams.

2-5. -5. (canceled)

7. A method for tracking a focal plane during rotation of an object of interest in a tube undergoing optical tomography comprising the steps of:

collecting a set of k pseudoprojection images pp₁-pp_k of the object of interest with an initial estimate for a radius from the tube center to the object center (R);

finding a set of center of mass values X_m1, X_m2, X_m3 . . . X_mk for the pseudoprojection images pp₁-pp_k;

recording a time of collection t₁, t₂, t₃, . . . , t_k for each of the set of pseudoprojection images pp₁-pp_k;

computing R and the value of Θ

at time k by calculating a minimum RMS error over the set of pseudoprojection images pp₁-pp_k;

estimating a real time value of Θ

based the set of center of mass values, the time of collection t₁, t₂, t₃, . . . , t_k and the clock for PP collection, and testing the real time value of Θ

for proximity to the value 0; and

when Θ

is anticipated to be 0 on the next clock cycle the trigger for capture of the set of pseudoprojection images is enabled.

8. The method of claim 7 wherein where k represents a number greater than 1.

9. The method of claim 7 wherein the step of calculating a minimum RMS error over the set of pseudoprojection images pp₁-pp_k comprises calculating the minimum RMS error according to the relationship:

Error=√

Σ

(X_m−

X′

_m)²/(number of pseudoprojections),

where, Xm represents an ensemble of the set of center of mass values X_m1, X_m2, X_m3 . . . X_mk over the set of pseudoprojection images pp₁-pp_k and X′

_m represents a trend in Xm.

10. The method of claim 9 wherein the trend in the center of mass X′

_m is modeled as;

X′

_m=R*cos(π

PP(1+ζ

)/NP+π

+Θ

)+PF+A

where R is defined as a distance between a tube center and object center, Θ

is defined as angular error, ζ

is defined as a controller error, NP is constant that is one less than the total number of pseudoprojections, PP is defined as a pseudoprojection number, PF is a value proportional to the pseudoprojection frame height, and A is an average offset of the tube around the tube center.

11. The method of claim 7 wherein the step of finding a set of center of mass values X_m1, X_m2, X_m3 . . . X_mk comprises the steps of:

segmenting each of a set of k pseudoprojection images pp₁-pp_k of the object of interest and computing the center of mass for a set of grey scale pixels associated with the object of interest;

determining a threshold for each pseudoprojection by finding the average light level;

using a connected components algorithm on to the thresholded image in order to segment objects where all non-zero pixels are connected to yield a labeled image;

selecting a component corresponding to the object of interest by identifying a pixel in the object of interest to yield a mask;

applying the mask to the original grey value image; and

computing the center of mass based on inverted grey values.

13. A method for tracking an object in a system for optical tomography, where the object is contained in a tube having a center of rotation, the object has a centroid, and the object is offset from the center of rotation, the method comprising the steps of:

acquiring image data by scanning the object through an extended depth of field while the object is being rotated;

calculating a distance value of the object centroid from the center of rotation, where the distance value is calculated from the acquired image data;

calculating a rotation angle value of the object in a selected position, where the rotation angle value is calculated from the acquired image data;

determining an extent of the object; and

limiting the extended depth of field being scanned to less than or equal to the extent of the object so as to increase image resolution in a resultant pseudoprojection image.

14. The method of claim 13 wherein the step of acquiring image data includes focusing an objective lens on a focal plane, the method further including the step of converting the distance value and a set of rotation angle values into control signals for controlling translation of the objective lens such the focal plane tracks the center of the object.

15. The method of claim 14 wherein the step of determining an extent of the object is calculated according to the relationship:

)

object_extent=√

{square root over (((extent_θ

)²+(extent<sub>θ

+90°</sub>

)²))}{square root over (((extent_θ

)²+(extent<sub>θ

+90°</sub>

)²))}

where

object_extent represents the extent of the object;

extent_θ

represents a first measurement of the object extent taken at a selected rotation angle θ

; and

extent<sub>θ

+90°</sub>

represents a second measurement of the object extent taken at a selected rotation angle θ

+90°

.

16. The method of claim 14 wherein the control signals move the objective lens sinusoidally according to a translating centroid of the object.

17. The method of claim 13 wherein the tube has a rotation cycle and the distance value and a set of angle values are used to compute a sinusoidal function for objective lens position, the sinusoidal function having a wavelength proportional to the rotational cycle.

18. The method of claim 13 further comprising a step of creating a projection image of the object being rotated includes centering the projection image within a circle of reconstruction during tomographic reconstruction.

19. The method of claim 17 wherein the sinusoidal function is modulated by an additional function to control the scanning of the objective lens to form a pseudoprojection.

20. The method of claim 13 wherein higher spatial frequencies and image contrast are preserved in resultant images of at least one structure within the object during rotation.

21. The method of claim 13 further comprising the steps of:

scanning at least one feature of the object at a set of viewing angles to produce a set of images;

from the set of images, determining a 0 degree viewing angle to exist where the at least one feature is measured at a maximum distance from tube axis; and

calibrating the pseudoprojection system relative to the determined 0 degree viewing angle.

22. The method of claim 13, wherein the tube includes a tube axis, further comprising the steps of:

scanning at least one feature of the object at a set of viewing angles to produce a set of images;

from the set of images, determining a 90 degree position where the at least one feature appears in a first image in the middle of the tube, and closest to the lens;

then selecting a second image corresponding to rotating the tube 90 degrees to return to a 0 degree position;

measuring the second image for object distance from the tube axis to determine a radius; and

calibrating the pseudoprojection system relative to the determined 0 degree position.

23. The method of claim 13 further comprising the steps of:

scanning at least one feature of the object at a set of viewing angles to produce a set of images;

from the set of images, measuring a distance of the at least one feature from tube axis for the images corresponding to the set of viewing angles; and

calculating a cosine function fit to find radius and angle for at least one of the set of viewing angles.

24. The method of claim 13 further comprising the steps of:

obtaining an image and locating at least one feature of the object as initially positioned in the tube;

measuring a distance from the at least one feature to a tube axis;

scanning an objective lens along its optical axis to find a depth for the at least one feature; and

determining a radius and angle of the at least one feature from the distance and depth measurement.

25. The method of claim 13, wherein the tube has inner tube walls, further comprising the steps of:

setting a focus to an initial focal plane (F₀) by focusing on the inner tube walls at the section where the initial focal plane is located on a plane intersecting the tube through its central axis;

rotating the tube until a structure of interest having a centroid appears in close proximity to one of the inner tube walls;

marking the centroid of the structure of interest;

measuring a distance (a) of the centroid to the center of rotation;

calculating a change of focal plane (h_n) per rotation angle (β

), according to the equation F_n=F₀+(a sin(β

_n)); and

converting h_n into a signal transmitted to an objective positioner that moves the objective in a sinusoidal function according to the translating centroid of the structure of interest.

26. The method of claim 13 wherein the tube has a rotation cycle and the distance value and a set of angle values are used to compute a sinusoidal function for objective lens position, the sinusoidal function having a wavelength proportional to the rotational cycle and wherein the sinusoidal function is pre-distorted with pre-compensation values to correct for axial shifts in best focus across the entire field.

27. The method of claim 13 wherein the tube has a rotation cycle and the distance value and a set of angle values are used to compute a sinusoidal function for objective lens position, the sinusoidal function having a wavelength proportional to the rotational cycle and wherein a pre-compensation look-up table for adjusting the sinusoidal function is calibrated using isolated microspheres located at different regions of the field.

28. The method of claim 13 wherein the tube has a rotation cycle and the distance value and a set of angle values are used to compute a sinusoidal function for objective lens position, the sinusoidal function having a wavelength proportional to the rotational cycle and wherein a pre-compensation calibration using a specific capillary tube sandwich is performed before scanning each sample.

29. The method of claim 13 wherein the tube has a rotation cycle and the distance value and a set of angle values are used to compute a sinusoidal function for objective lens position, the sinusoidal function having a wavelength proportional to the rotational cycle and wherein a pre-compensation for adjusting the sinusoidal function is performed while the tube is rotating.

30. The method of claim 13 wherein the tube has a rotation cycle and the distance value and a set of angle values are used to compute a sinusoidal function for objective lens position, the sinusoidal function having a wavelength proportional to the rotational cycle and wherein the sinusoidal function is pre-compensation for thickness variations of gel during rotation of the tube.