Gamma camera with two sequential correction maps

Details Gamma camera with two sequential correction maps Gamma camera with two sequential correction maps

1999-04-26

2003-05-06

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

C250S363090

Reexamination Certificate

active

06559450

ABSTRACT:

RELATED APPLICATION
The present application is a US national phase application of PCT/IL97/00217, filed Jun. 29, 1997.
FIELD OF THE INVENTION
This application is related to precise distortion and error correction in Gamma Camera systems and in particular to incremental calibration thereof.
BACKGROUND OF THE INVENTION
FIG. 9
is a schematic illustration of an Anger camera. A subject
110
ingests, or is injected with, a radio-pharmaceutical, which tends to concentrate in certain body tissues, such as tissue
112
. The local concentration depends on the particular radio-pharmaceutical, the tissue type and on its metabolic processes. Periodically, the radio-pharmaceutical generates a gamma photon. Although gamma photons are emitted in all directions, some of them travel along a straight path
114
and through a collimator
115
so that they might interact with a scintillator crystal
116
. The interaction between the gamma photons and crystal
116
generates a shower of photons
118
in the visual light range. The number of secondary photons is directly dependent on the energy of the original gamma photon.
The number of photons
118
can be estimated by detecting these photons with a plurality of photo-multiplier tubes (PMT), such as tubes P
1
to P
4
. The energy of the gamma event is then calculated by a position and energy calculator
126
which sums the contributions of all the individual PMTs. If the photon is scattered along its path from tissue
112
or if it is a cosmic ray photon, its energy will not be the same as the energy of a gamma photon as emitted by the radio-pharmaceutical. Thus, the number of photons
118
will also be different from those in “normal” interactions. By windowing the detected events, so that only events with an energy within a desired range are taken into account, events which do not form a portion of the image may be rejected.
A collimator
115
, which is usually a fan collimator or a parallel-hole collimator is used to project the distribution of the radio-pharmaceutical in subject
110
onto detector crystal
116
. The position of the interaction of the gamma photon on (or in) crystal
116
indicates the travel path
114
, since collimator
115
limits the possible paths of a gamma photon from subject
110
. This position is calculated by calculator
126
. An accumulator
128
accumulates calculated interaction locations and builds an image therefrom, which is displayed on a display
130
.
There are several known methods of calculating the interaction position, the most commonly used having been invented by Anger. In the Anger method, the determined interaction position is a weighted average of the positions of the PMTs which detect the interaction. the weighting being the number of photons detected by each PMT. There are several problems with this method. First, the sensitivity of the PMTs are not the same. Thus, a calculated position will tend to be displaced towards the position of the most sensitive PMT. Second, the sensitivity of PMTs changes with time, especially when an old PMT is replaced with a new one. Third, PMTs tend to be more sensitive at some angles than at others. Fourth, at the edges of crystal
116
, some photons are lost, either by escaping the crystal or by there not being sufficient PMTs surrounding the interaction position from all directions. Fifth, different regions of crystal
116
differ in their sensitivity to gamma radiation and produce different amounts of light from an interaction of the same energy. Sixth, some portions of the crystal interact more strongly with the gamma radiation and thus generate a higher number of events for a fixed amount of radiation. Moreover, not only are position calculations inaccurate; as a second result of these problems, so are energy calculations. Seventh, the amount of light reaching the PMT is not linearly related to the distance of between the event and the PMT.
The results of these problems are generally classified as linearity errors—position determination is not exact; energy errors—energy determination is not exact; and sensitivity errors—the count of interactions at crystal
116
is not in a fixed proportion to the number of impinging photons.
One widely used methodology for correction of gamma cameras is the so-called “triple correction,” versions of which are described in U.S. Pat. Nos. 4,424,446 and 4,588,897, the disclosures of which are incorporated herein by reference. These patents describe a correction system which corrects for geometric (dislocation) distortions, energy response variations and non-uniform sensitivity of the camera. Preferably, the sensitivity correction is performed after the other two corrections, which can be performed in any order. However, such calibration maps take a long time to prepare and must be individually created from scratch for each camera, camera-collimator combination and event energy.
In general, as disclosed in the two abovementioned patents and in other patents and publications, the camera is flooded by a source of radiation. For the determination of the energy correction, the spectra of a total signal associated with events at particular positions on the surface of the camera are acquired. An energy window corresponding to valid events is adjusted to account for the variations with position of the signal spectrum acquired. Alternatively, the signal is adjusted as a function of its position and the window remains constant. For geometric distortion correction, an image of a plate having holes at regularly spaced intervals is acquired. The measured hole positions are compared to the known spacings of the holes and a correction map, later applied to actual events during imaging, is determined. To correct for sensitivity, a flood field is applied to the gamma camera. A flood field image, which is acquired, is corrected for both distortion and energy. Remaining variations in the resulting image, which are the result of incomplete correction of the energy and dislocations errors, as well as intrinsic sensitivity variations of the camera and collimator, are used to form a normalization map which is applied to events or images after energy and dislocation corrections to correct for the sensitivity variations.
While many cameras perform triple correction on the imaging data, some cameras perform only one or two corrections.
In general, the determination of the correction maps is a fairly long and tedious process. This is caused by the fact that the data acquired in determining the corrections is based on individual events of relatively low frequency. The data generally has a substantial standard deviation of energy, position and sensitivity. Thus, in order to achieve the statistical accuracy necessary to correct the camera, a large number of events must be acquired at closely spaced positions on the camera This is especially true when large corrections must be made, in which case the number of events and the amount of time necessary to acquire them is especially large. Since, in general, camera correction maps must be periodically field updated, the camera is designed such that the amount of correction required is limited, by compromising the design of the camera. This limitation in the required correction reduces the amount of correction required and hence the time it takes to determine the correction.
A typical calibration time is about three days, For this reason, calibration is typically performed only about once a year.
The problem of calibration times is especially limiting for distortion and energy correction. It is well known that for best spatial resolution there is an optimal spacing between a scintillator plate and photodetectors used in a gamma camera. It is also well known that, at this optimal spacing, the amount of geometric distortion and the amount of energy correction required is very large. This would require determining correction at a large number of individual holes in order to assure accuracy over the entire face of the camera. Since the resolution of gamma cameras is limited, this would require acquiring a substant

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