The present invention relates to a method of constructing a sensor system which is sensitive to electromagnetic radiation and is suitable for use in a digital imaging camera head, said sensor system delivering an electrical signal containing the image information formed by electromagnetic radiation incident on the sensor system.
The invention also concerns a solid-state sensor system of a CCD camera head intended for digital imaging, said sensor system comprising a radiation-responsive detector element array with control electronics connected thereto and an interface unit by which the sensor system is connected to an image recording apparatus, and said sensor system further comprising a frame unit, on which the sensor system is adapted.
The present invention relates broadly to object imaging by means of electromagnetic radiation. In particular, the invention is related to digital imaging utilizing CCD sensor systems.
The method and apparatus according to the invention are intended broadly for digital imaging and particularly for medical x-ray imaging such as panoramic dental radiography and mammography.
Conventionally, digital imaging is also used in medical x-ray technology, wherein it has certain indisputable benefits over the use of radiation-sensitive film requiring a development process. Such benefits are those related to viewing, handling, storage and remote transfer of digitally imaged and stored picture information which in the future will be further accentuated when the medical services and hospital systems will increasingly move into digital techniques in general and particularly in the processing of x-ray images and similar information. Hence, it is one of the principal objects of the present invention to bring an essential advancement to this on-going trend.
As to their basic structure, solid-state sensors intended for digital imaging comprise small radiation-sensitive detector elements called pixels arranged into larger planar arrays, or in a simplified form, into a single-row line sensor. Electromagnetic radiation such as light, infra-red radiation or x-rays absorbed at the area of the detector elements generates an electrical charge in the elements with a magnitude dependent on the intensity of absorbed radiation (that is, flux density and energy of absorbed quanta) of impinging radiation. Here, the magnitude of the electrical charges grows as a function of time, which means that during the exposure time, the detector element integrates the charge generated by the radiation impinging over the sensor area, thus in principle giving a possibility of controlling the magnitude of the detector element output signal through varying the integration time.
In the prior art, CCD (Charge-Coupled Device) image sensors and their control electronics were designed and optimized always separately for a certain application and specific use, whereby such conventional image sensor systems are ill fit for other applications no matter how similar. The development of new types of CCD sensors is extremely expensive and time-consuming. Typically, a new type of CCD sensor takes at least a year or more involving an R&D budget of at least USD 1 million. Correspondingly, the development of control electronics for such a new CCD sensor takes about the same time, raising the system R&D costs by at least about USD 250,000. Given these facts, it is obvious that since the development of a new CCD sensor type presumes a practical application with a commercial potential of development cost payback in a reasonable time, applications using small quantities of CCD sensors cannot be implemented in practice within the constraints of a reasonable cost budget. On the other hand, while new CCD sensors are continually developed for applications of reasonably high volume, even in such projects the proportion of costs attributed to the CCD sensors rises relatively high when conventional techniques are used.
A single solid-state sensor chip, particularly a CCD sensor, made by concurrent technology has such a small active area that any larger image area cannot be recorded by means of a single sensor, particularly when reasonably or extremely high requirements are set for the sensor sensitivity as is the case in x-ray imaging. Resultingly, a number of CCD sensor chips with control circuit blocks are necessary in the construction of a complete digital camera head.
As known in the art, digital camera heads with their image sensors and control circuitry are generally designed for a certain application, which makes them unsuitable for modification or adaptation to any other application whether similar or not. For example, with the help of time-delayed integration (TDI) technique, applications can be found in, e.g., mammography or panoramic x-ray imaging of dental or skull areas in which a single basic construction of the imaging apparatus is produced in modified versions for these applications that differ from each other chiefly by the active area of the imaging sensor only. In mammography equipment, the length of the scanning CCD line sensor typically is either 18 cm or 24 cm, depending on whether image areas of 18.times.24 cm or 24.times.30 cm are being recorded. In panoramic imaging equipment, the length of the scanning CCD line sensor typically is approx 14 cm for panoramic imaging alone and 18 cm or 24 cm for different modes of skull imaging.
Today, the purchase decision of a digital CCD camera head must be based on knowledge covering all possible future applications, thus assuring the suitability of the purchased equipment version for all of them, because retrofitting of a camera head implemented by means of conventional technology is not either possible, or if possible, requires a complete replacement of the CCD camera head with another type, which obviously is an uneconomical operation. On the other hand, the purchase of a CCD camera head with an imaging area covering the maximum size required by any conceivable application is impractical, because the price of this type of imaging system is chiefly determined by the CCD sensor assembly itself and its control electronics, whereby the equipment price can easily double from the inclusion of a provision covering all possible future imaging needs that may well remain unrealized.
Another problem related to conventional digital CCD camera head systems is servicing of the equipment. When even a slight malfunction occurs the CCD sensor of the equipment or its control electronics, the sensor must be replaced as a whole unit, which by its servicing cost rises close to the purchase price of new camera head. If fabricated by conventional techniques, an integrally bonded CCD sensor package must always be replaced as a whole unit even if only a small component actually fails therein. Similarly, the control electronics of the imaging system have conventionally been designed and traditionally built for the entire sensor package, whereby its malfunction has also required the replacement of the entire electronics package.