Radiation imaging using simultaneous emission and transmission

Details Radiation imaging using simultaneous emission and transmission Radiation imaging using simultaneous emission and transmission

1999-10-21

2002-04-30

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

C250S363020

Reexamination Certificate

active

06380540

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to systems and methods of emission tomography, and specifically to gamma cameras for simultaneously producing tomographic maps based on emission and transmission of radiation.
BACKGROUND OF THE INVENTION
Gamma cameras known in the art produce tomographic images that are indicative of physiological activity. Such cameras receive radiation that is emitted by radioisotope markers or tracers, which are introduced into the body of a subject and are taken up by an organ of the body in proportion to the physiological activity of interest. The radiation emitted is generally received by scintillator/detector systems, which produce signals responsive to photons of the radiation incident thereon. The signals are processed and back-projected, using computerized tomography methods known in the art, to produce a three-dimensional image indicative of localized activity within the organ.
In the context of the present patent application, the terms “tomographic” and “tomographic images” will be taken to include any three-dimensional images produced using a gamma camera, whether based on emission signals or on transmission signals, as will be described below. Gamma cameras known in the art for producing such three-dimensional tomographic images include Single Photon Emission Computerized Tomography (SPECT) and Positron Emission Tomography (PET) systems.
In some SPECT systems, known in the art, a transmission image of the body, preferably a tomographic image, including the region of the organ of interest, is produced concurrently with the emission image. This transmission image is useful in relating the emission image to the actual position and dimensions of the organ. It also provides information regarding attenuation of the radiation as it passes through the body along different paths, which is used to correct the emission signals for variations in attenuation thereof.
FIGS. 1-3
schematically illustrate, in axial, sectional views, systems known in the art for simultaneous emission and transmission imaging.
FIG. 1
shows a SPECT camera
20
for emission and transmission tomography as known in the art. Due to the limited view of the source a 360° rotation is generally necessary for producing an emission tomographic image. Camera
20
includes a planar emission detector
22
and a planar transmission detector
24
, positioned on opposite sides of a body
26
of a subject. A radiation line source
28
, having its long axis perpendicular to the plane of the figure, emits a fan beam
30
of radiation, which is collimated so as to be limited to rays substantially parallel to the plane of the figure. These rays are received by transmission detector
24
after passing through body
26
and, preferably, through a collimator
23
, adjacent to detector
24
. Collimator
23
is intended to block a considerable fraction of photons emitted and scattered from body
26
from reaching detector
24
.
Detectors
22
and
24
and source
28
are mounted to revolve about body
26
, as indicated by arrows
32
. As the detectors revolve, they generate signals responsive to radiation photons incident thereon. A computer
34
receives these signals over 180° or, preferably, 360° of revolution of the detectors and source, and then processes and back-projects them to produce emission and transmission tomographic images.
Although detector
24
is referred to here as a transmission detector, some emission from body
26
also reaches this detector, to the extent that this emission is not blocked by collimator
23
. Therefore, line source
28
is generally made to emit photons of substantially different energy from those characteristic of the radioisotope marker injected into body
26
, and the emission- and transmission-responsive signals from detector
24
are separated by energy measurement, as is known in the art. It will be appreciated, however, that some cross-talk between the emission and transmission signals is generally unavoidable. Because camera
20
has only a single emission detector
22
, the scan time needed to form the emission image is also relatively long, and the camera cannot be used in PET imaging. Moreover, the geometry of the transmission rays is cumbersome and computationally inconvenient.
FIG. 2
schematically illustrates another SPECT camera
40
, known in the art, for example, as described in “NON-UNIFORM ATTENUATION CORRECTION USING SIMULTANEOUS TRANSMISSION AND EMISSION TOMOGRAPHY” by C-H Tung IEEE TRANS. Nuc. Sci. V. 3, No. 4, P1134-1143, which is also incorporated herein by reference. Camera
40
includes two planar emission detectors
42
and one planar transmission detector
44
in a triangular arrangement, with line source
28
at the apex of the triangle opposite detector
44
. A collimator
46
may be placed adjacent to detector
44
, so that the rays reaching the detector are substantially limited to those from the direction of source
28
that pass within one of a plurality of axial slices through body
26
, the slices being generally parallel to the plane of the figure. The elements of camera
40
shown in
FIG. 2
revolve around body
26
, and the detector signals are received and processed by computer, as described above with reference to
FIG. 1
(although for the sake of simplicity, the computer is not shown here or in certain others of the figures). Camera
40
will generally be capable of reconstructing SPECT emission and transmission images with greater speed than camera
20
, for example, but at the expense of an additional planar detector. Camera
40
is not suitable for PET imaging.
FIG. 3A
schematically illustrates still another existing SPECT camera
50
, based generally on the camera described in “A SCANNING LINE SOURCE FOR SIMULTANEOUS EMISSION AND TRANSMISSION MEASUREMENTS IN SPECT” by P. Tan, et al. J. Nuc. Med. 1993; 34:1752-1760, which is also incorporated herein by reference. Camera
50
includes two planar detectors
52
arranged at right angles, wherein both detectors serve as both emission and transmission detectors. Two line sources
28
are mounted respectively opposite the two detectors
52
so that each of the line sources is translatable in a direction parallel to the face of the respective detector, as indicated by arrows
54
in the figure. A collimator
29
is associated with each of sources
28
, in order to limit the radiation impinging on body
26
from the respective source to rays that are within an axial slice through the body and are generally perpendicular to the respective detector
52
opposite the source. At each angular position, as the elements of camera
50
revolve as shown by arrow
32
, sources
28
are swept laterally across body
26
along arrows
54
, and the transmission signals from detectors
28
are used to reconstruct tomographic images. Because of the right-angle geometry of camera
50
, only a 90° revolution of the elements shown of the camera is needed to reconstruct emission and transmission images.
In camera
50
, detectors
52
serve as both emission and transmission detectors, and the transmission- and emission-responsive signals produced by the detectors must be distinguished from one another in order to produce the tomographic images. The signals may be separated by energy, as described above in reference to camera
20
, shown in FIG.
1
. Alternatively or additionally, an area of each of detectors
52
opposite moving source
28
may be temporarily “blanked” with respect to emission signals. Camera
50
is not suitable for PET imaging.
FIG. 3B
shows another version of this camera, marketed by Picker under the name PRISM 2000 XP STEP OPTION, in which two parallel planar detectors
72
a
and
72
b
are used. Two collimators are associated with detectors
72
a
and
72
b
. A line source
96
collimated by a collimator
96
is situated in front of one of the collimators and scans across the collimator as shown by arrow
120
.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide gamma cameras that produce tomographic emission and transmiss

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