BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and are not limitative of the present invention, and wherein:
FIG. 1 is a perspective view of a test phantom according to an exemplary embodiment of the present invention;
FIG. 2 illustrates an absolute coordinate system calibration configuration for detectors at 0 and 180 degree positions;
FIGS. 3
a and 3b are images from respective detectors of FIG. 2;
FIG. 4 is a three-dimensional depiction of a 2D Gaussian surface fitting for the center point source P of the absolute coordinate system;
FIG. 5 illustrates an absolute coordinate system calibration configuration for detectors at 90 and 270 degree positions;
FIGS. 6
a and 6b are images from respective detectors of FIG. 5; and
FIG. 7 is a data flow diagram illustrating the entire calibration process to establish an absolute coordinate system according to an exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
Referring to FIG. 1, a test phantom 10 is provided with a number of radioisotope point sources 12, with the center point source being denoted as point source P. In an exemplary embodiment, the test phantom comprises five point sources. Each point source comprises a capsule containing an appropriate amount of radioactive material, such as 99Tc or 57Co. The point source capsules 12 typically are mounted on a plate in a manner such that attenuation artifacts caused by interaction between the plate material and the point sources are minimized. As shown, the test phantom structure is such that a plurality of point source isotopes are located such that lines connecting any two pairs of said point source isotopes will be skewed with respect to each other.
The phantom with loaded point sources is then subjected to SPECT imaging over four projection view angles (ie., 0, 90, 180 and 270 degrees). First, as shown in FIG. 2, panel detectors 20 and 21 are placed in 0 and 180 degree rotation positions, respectively, about the test phantom 10 (Step 701, FIG. 7). Coordinate system 23 is fixed to the image reconstruction space.
A cross-hair mark 24 is projected on the surfaces of the panels 20 and 21 at the location of an image center denoted by the position vector
{right arrow over (v)}
c=(n−1,n−1)/2 (1)
where n is the image size.
The patient bed on which the test phantom is mounted is then adjusted along the Z-axis until the projection of point source P on the detector panel surface is substantially within the cross-hair mark 24.
Radiation distribution data then is acquired by the detectors 20 and 21, resulting in images as shown in FIGS. 3(a) and 3(b), respectively. A 2D Gaussian surface, which can be written as
is then fitted to the peak of point source P for each image. The elliptical function U in Equation (2) above is represented as
where the lengths of the axes of ellipse U are 2σx and 2σy and the center of ellipse U is located at (x0, y0). Ellipse U is then rotated τ degrees from the X axis in the clockwise direction as shown by arrow 25 in FIG. 2, such that the rotated coordinate system is defined as
FIG. 4 shows an acquired single pinhole image and its 2D Gaussian fit. In this Gaussian model, there are seven parameters: c0, λ, σx, σy, x0, y0, and τ, of which x0 and y0 are used directly for patient bed position displacement calculation.
The position vector of point source P in the image when the detector is rotated i degrees is denoted as
{right arrow over (v)}
0
i=(x0i, y0i), (i=0, 180 deg.) (5)
Then, the origin of the absolute coordinate system can be determined as the position vector
The patient table then is adjusted by the amount {right arrow over (v)}-{right arrow over (v)}c along each of the X and Y axes (Steps 702 and 703, FIG. 7), such that the projection of the point source P on the X-Z plane of the absolute coordinate system is now aligned with the image reconstruction space coordinate system.
As shown in FIG. 5, the detectors are then placed in 90 and 270 degree rotation positions about the test phantom (Step 704, FIG. 7), the point source P is moved to the cross-hair, and radiation projection data is acquired by each detector as shown in FIGS. 6(a) and 6(b). The 2D Gaussian fit process is then repeated to obtain the origin vector position in the Y-Z plane. The height of the patient table then is adjusted by the vector difference amount obtained in the 90-270 detector placement (Step 705, FIG. 7), such that the origin of the absolute coordinate system can be aligned with the center of the image reconstruction space coordinate system in the Y-Z plane.
With the patient bed so adjusted, there is achieved a full calibration of the image reconstruction space coordinate system with the absolute coordinate system, such that accurate correlation of clinical SPECT image data acquired from the two detectors can be performed (Step 706, FIG. 7).
The invention having been thus described, it will be obvious to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention.
For example, while a test phantom using five point sources has been described, the number of point sources may be varied to more or less than five, and their spatial relationships also may be varied, in order to obtain an optimal configuration for detector head calibration purposes as taught by the present invention. Any and all such modifications are intended to be included within the scope of the following claims.
Patent Application
20070290125
