X-ray or gamma ray systems or devices – Accessory – Testing or calibration
Reexamination Certificate
2000-11-07
2002-04-30
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Accessory
Testing or calibration
C378S116000, C378S165000
Reexamination Certificate
active
06379044
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for recording a radiation image on a recording medium.
The invention more particularly relates to such a method and apparatus for use in the field of mammographic radiation image recording.
BACKGROUND OF THE INVENTION
In mammography a radiation image obtained through X-ray exposure of a patient's breast is commonly recorded in a dedicated mammographic image recording apparatus on a recording material such as a light-sensitive photographic film.
Such a dedicated image recording apparatus generally consists of a X-ray generator controlling the amount and the characteristics of the X-rays emitted by an X-ray source, a support for supporting the breast that it to be irradiated and a slide wherein a recording system, for example a cassette comprising a light-sensitive recording material and an intensifying screen, is positioned so that it is exposed to X-rays transmitted through the breast. The apparatus commonly also comprises an electronic signal processing unit by means of which different irradiation programs can be selected and by means of which the settings of the different parts of the apparatus are controlled and adjusted.
Mammographic images are taken when there is an instant need for diagnosis, e.g. when the patient is referred to the radiography department e.g. on request of a gynaecologist.
Apart from these occasions, mammographic images are frequently taken as a part of breast cancer screening programs.
Breast screening programs comprise the recording of a mammographic image of a person on repeated occasions, for example once a year. Part of the screening is a comparison of the mammographies taken in past years with a newly taken mammography in order to detect any changes in the breast tissue or in order to follow the evolution of suspicious elements which have been detected in the breast.
A prerequisite for optimal comparison of mammographies taken at different points of time e.g. as a part of such screening programs is that the image quality of the mammographic image is high, i.e. that a high resolution and high contrast image is obtained, and that this high quality remains constant over time. Even very small differences in the photographic characteristics of the mammographies may have an influence on the results of a comparison of mammographic images taken at different points in time, such differences may make such a comparison an unreliable basis for diagnosis.
This fact has been recognised by several international organisations among which the American College of Radiology: “Widespread mammographic screening has the potential to significantly reduce mortality from breast cancer. However, the effectiveness and success of such screening and of all mammography depends on the consistent production of high resolution, high contrast mammographic images”.
Several organisations have recommended the performance of quality assurance tests and have set up detailed manuals on the requirements of these tests.
Several governments have made the performance of quality assurance tests and the quality monitoring mandatory for breast screening programs to be accepted.
There is thus a constant aim among manufacturers of recording media and among manufacturers of recording equipment to enhance the consistency and the reproducibility of the image recording. As these companies are often not the same, it may be possible that the characteristics of the recording media, and more especially the occasional changes of these characteristics, are not fully known to the equipment manufacturer and can thus not be taken into account to an adequate extent.
In a radiographic image recording apparatus and more specifically in a mammographic image recording apparatus of the kind described higher, a large number of factors have an influence on the ultimate quality of the recorded image and on the consistency of the recording throughout time.
Among these factors the most important are:
the transfer function of the X-ray source, i.e. the spectrum of irradiation as a function of the set value of mAs, kV, anode characteristics, filtration of the X-ray source,
the conversion of the input X-ray spectrum into an output X-ray spectrum modulated by the object that is irradiated, in casu the patient's breast,
the transfer function of the recording material,
In case the recording material is a photographic material this transfer function expresses the conversion of the X-ray image into a latent image.
the transfer function expressing the conversion of the latent image into a visible image.
In case the recording material is a photographic material or a photographic material which is accompanied by an intensifying phosphor screen, this material requires chemical development of the latent image to gain a visible image. This transfer function then expresses the characteristics of the development.
Although for the ease of explanation it is assumed hereinafter that the recording medium is a photographic material or a photographic material which is accompanied by an intensifying screen, other kinds of recording systems may be envisaged.
For example a photostimulable phosphor screen might also be a suitable recording material.
It is also possible to record the radiation image by means of a direct radiographic sensor such as a sensor comprising charge coupled devices or amorphous silicon flat panel detectors or the like.
It will be clear that in case the latter kinds of recording systems are used, chemical processing and corresponding adjustments are not required.
The transfer function of the X-ray source is known to the manufacturer of X-ray image recording equipment.
When setting the radiographic recording apparatus the expected characteristics of the X-ray modulation by the object that is irradiated is taken into account through the use of a model-description which is stored in the electronic signal processing unit controlling the setting of the apparatus or through intervention of the X-ray technologist having the knowledge to do so.
The effect of other very important factors however, namely the transfer function of the recording material and the characteristics of the conversion of a latent image into a visible image (which in the case of a photographic recording material are the characteristics of the chemical processing), is generally not completely known to the manufacturer of the radiographic image recording equipment.
These effects may vary to a certain extent due to batch-to-batch changes of the recording material, day-to-day variations of the processing conditions due to temperature variations, humidity, decay of the chemical properties of the processing bath, non-uniform characteristics of the intensifying screens used with a photographic film, X-ray absorption by a cassette conveying a radiographic film or intensifying screen-radiographic film combination, etc.
To a large extent these effects are taken into account by performing a calibration of the apparatus. During this calibration characteristics of the recording material as well as characteristics of the developing process (in case the recording material is a radiographic film) are determined and the recording equipment is adjusted taking into account these characteristics.
Nowadays these calibrations are generally performed by a service technician on a regular basis. However, since this kind of intervention by a service technician is expensive, the frequency of these interventions is kept rather low. Several causes of inconsistency such as day-to-day variations of the developer conditions, batch-to-batch film variations etc. occurring in between succesive interventions of the service technician cannot be solved.
Consequentially it is possible that in between two successive interventions the image quality of the radiation images becomes inconsistent.
EP 432 722 relates to an apparatus for writing information on X-ray films. More particulary means are provided for identifying a type of film, e.g. tagged with a mark S such as a notch that has been cut in each film.
Limbergen Jürgen Van
Vastenaeken Guy
Agfa-Gevaert
Barber Therese
Hoffman, Warnick & D'Alessandro LLC
Merecki John A.
Porta David P.
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