Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1993-11-08
2001-01-30
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S363040
Reexamination Certificate
active
06180943
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to tomograph acquisition apparatus usable, in particular, in the medical field. It relates essentially to acquiring tomographs with scintigraphy detectors that are easy to use.
The principles of scintigraphy detectors is known in nuclear medicine. They are as follows. A radioactive marker, generally technetium, is injected into a patient. As a function of its nature, the marker is distributed from its point of injection into various portions of the patient's body. In the patient's body, the marker is to be found in the organ under investigation and it reveals the function thereof. The marker produces gamma photons. After passing through a collimator, the gamma photons are detected by a scintillator crystal, whence the term “scintigraphy detector”. The crystal transforms the gamma photons into light photons. The light photons are in turn detected by photomultiplier tubes placed looking at the scintillator. The currents that flow through the photomultiplier tubes are in the form of pulses and are a function of the magnitude of the scintillation produced. These currents are applied to a resistance matrix. The resistance matrix outputs “locating” pulses that specify the position at which scintillation took place facing the photomultiplier tubes.
A counter unit connected to the output from the resistance matrix serves to sum the number of such pulses occurring at each location on the scintillator. It is then possible to create an image representative of the activity of the marker in the body by attributing brightness to each image point as a function of the number of strikes that have been counted for each of said locations. Such a method is known in the state of the art under the name “Anger” method. With very fast scintigraphy cameras, e.g. capable of counting up to 200,000 strikes per second, an image constituting a projection of a portion of the human body can be generated in about 30 seconds.
The detector is mounted in a rotary assembly called a gamma camera which also serves to aim the detector. If the detector is aimed in different directions relative to the body, multiple images can be acquired under the same conditions. By acquiring a sufficient number of images for different aiming directions of the detector, the set of image signals can be subjected to processing suitable for obtaining tomographs of the body by algorithmic reconstruction. Given that the accuracy of such tomographic images increases with the number of projection images, it can be seen that such a method leads to periods of examination that are relatively long.
Proposals have already been made to remedy this problem by constructing gamma cameras provided with two, three, or even more detectors. Under such circumstances, the duration of an examination is reduced, substantially pro rata the number of detectors.
However, another problem arises. To obtain projection images, and consequently tomographs, that have very good resolution, it is necessary for the detectors to be placed as close as possible to the body. Unfortunately, patients to be examined are not all the same size, some are fat, others are thin. In addition, depending on the examination being performed, it may be necessary for the detectors to be at various different distances from the body. For example, an examination around the belly requires the detectors to be at a different distance from the patient than an examination around the head, since the diameter of the head is smaller. A known way of solving this problem is to mount the detectors on telescopic arms and to move the detectors initially as close as possible to the patient. During an examination, the detector travels around a circle whose diameter depends on said distance from the patient. A device of this kind is for example depicted in U.S. Pat. No. 4,368,389.
The drawback of such a telescopic mechanism is that it is twice as complex when there are two detectors instead of only one, and so on. The telescopic mechanism is itself mounted on a rotary assembly enabling the detectors to be pointed in different image-taking directions.
SUMMARY OF THE INVENTION
An object of the invention is to remedy these drawbacks firstly by taking account of the need to place more than one detector on the rotary assembly in order to accelerate acquisition, and secondly to simplify the handling of the various detectors. To solve these problems, the invention begins by securing the two detectors to each other and also by giving them a certain angle of inclination relative to each other.
Thus, any one detector has a substantially plane detection surface constituting its detection field. At present, such detection fields are rectangular in shape. They have a length and they have a width. In the invention, two detectors are placed against each other so that the normals to the centres of their detection fields intersect, and so that these detection fields are adjacent to each other along one side of each of them, which sides are called “lengths”. In the following explanation, the adjacent sides are called lengths, but that does not mean that the detection field is necessarily longer in that direction than it is along a direction at rightangles.
It is preferable for the normals to intersect at an angle of 90°. However, this is not essential and the angle could be acute or obtuse. The two detectors are thus secured to each other in this configuration in such a manner that the bisector plane including the point of intersection of the normals and the adjacent lengths of the two detectors also contains the axis of rotation of the tomography.
With small patients, the assembly is displaced radially towards the axis of rotation. The corner formed by the two detectors can thus be moved closer to a small patient on the axis of the machine. Alternatively the corner can be moved further away when examining a fat patient, likewise on the axis of the machine. This simplifies the displacement mechanics.
However, by acting in this way, the information acquired during projection is not properly situated since the axis of rotation of the machine does not necessarily pass through the point of intersection of the normals to the detection fields. To be able to use the same reconstruction algorithms, it is therefore necessary to transform the image signal processing parameters as a function of the distance between the pair of detectors and the axis of rotation. It is shown that exactly the same algorithms can be used to reconstitute tomography images, while also obtaining a substantial saving on the mechanical equipment which is much simpler.
The invention thus provides an apparatus for acquiring tomographs of a subject, the apparatus comprising a pair of plane scintillation detectors carried by a support rotating about an axis of rotation, and connected to an image processor, the apparatus being characterised in that:
the detection fields of these two detectors are inclined at an angle relative to each other;
the bisector plane bisecting the angle formed between these detection fields includes the axis of rotation;
and in that the apparatus includes:
means for displacing the pair of detectors together relative to the subject in a direction which is radial to the axis of rotation; and
modification means for modifying an effective detection field of these detectors as a function of said displacement.
The invention will be better understood on reading the following description and examining the accompanying drawings. The drawings are given merely by way of indication and do not limit the invention in any way. In the drawings:
REFERENCES:
patent: 4368389 (1983-01-01), Blum
patent: 4417143 (1983-11-01), Haas
patent: 4590378 (1986-05-01), Platz
patent: 0092437 (1983-10-01), None
patent: 2250670 (1992-06-01), None
Cabou Christian G.
General Electric Company
Hannaher Constantine
Price Phyllis Y.
Skarsten James O.
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