Image Pickup Device

The invention relates to an image pickup device comprising a matrix having horizontal rows and vertical columns of image pickup elements, at least one vertical shift register associated with each column of image pickup elements, means for supplying clock signals to the matrix of image pickup elements and to the shift registers for transferring image information in the column direction in the time-delay-and-integration mode, and at least one horizontal shift register near the end of the vertical shift registers for reading out and transferring the image information.

In radiography the directing of a flat fan-shaped beam of x-ray radiation via a body under examination to an image pickup device consisting of a large number of image pickup elements situated in a line is known from the U.S. Pat. No. 4,179,100. In this case each image pickup element receives, optionally after further processing, the radiation which has been transmitted through a corresponding part of the body under examination. As further processing it is possible for the radiation to be converted into visible light in an x-ray detector, for example, before it is picked up by the image pickup device. In the case of an image pickup device consisting of a large number of image pickup element, situated in a line and having, for example, dimensions of 0.1.times.0.1 mm, the exposure time for each image pickup element is extremely short for the usual scanning speeds of a body under examination. As a result, the signal/noise ratio of the image obtained is very poor or the source loading has to be raised to undesirably high levels to achieve a better signal/noise ratio. In order, nevertheless, to obtain an acceptable signal/noise ratio for a relatively low source loading, it is necessary to use the x-ray source more efficiently than is possible with a fan-shaped beam which has only a thickness of the size of one image pickup element.

Making use of an image pickup device comprising a matrix having rows and columns of image pickup elements, the charge of which is formed during a certain period in a certain pickup element as a result of the radiation received by said element during said period, being transferred during a reading period to an adjacent associated vertical shift register, is known from the U.S. Pat. No. 4,383,327. This takes place during the reading period for all the image pickup elements in each row. A relative movement between the body and the image pickup device is also brought about between the consecutive periods so that the same part of the body under examination is imaged on the next row of image pickup elements during the subsequent period. The charge in the shift register tracks so that in this manner, during consecutive periods, a charge is accumulated by consecutive image pickup elements in a column in the associated-shift register. The accumulated charge is the sum of the charge which is transmitted through a specific part (an image point) of the body under examination during the consecutive periods.

This method of scanning is termed TDI (time delay and integration) and appears to be particularly well suited for use in the examination of bodies by means of x-ray radiation, it being possible for a usable image to be formed in spite of the fact that each image pickup element per se generates only a very small quantity of charge in response to the radiation received. For a comprehensive discussion of the TDI principle, reference is made to the U.S. Pat. No. 4,383,327.

Generating so-called dual-energy x-ray images by alternately applying two different high-voltage values, for example 70 kV.sub.p and 130 kV.sub.p to an x-ray radiation source, the respective high-voltage values generating x-ray spectra with different energy centre points, often termed a different hardness, is known from the article entitled "Computerized dual-energy imaging: a technical description" by J. Coumans et al. in Medicamundi, Vol. 27, no. 3, 1982. By irradiating a body under examination successively with x-ray radiation having a first energy centre point and then with x-ray radiation having a second energy centre point, it is possible to process the x-ray images obtained, for example, by means of a computer, so that bones in the human body are not imaged and tissue is, as a result of which it is possible to image tissue parts situated behind, for example, ribs, considerably more clearly. All this is based on the fact that different materials in, for example, the human body, have a different absorption for x-ray radiation having a different hardness.

In addition to obtaining different x-ray radiation energies by means of switching the supply voltage of the x-ray radiation source, it is also possible to obtain from an x-ray radiation source driven with one single supply voltage source different radiation energies by

filtering the x-ray radiation beam radiated by the source before it reaches the body to be irradiated. By optionally causing this filtering to take place alternately, a beam of x-ray radiation can be obtained having alternately a first and a second x-ray radiation energy.

The object of the invention is to make an image pickup device which operates according to the above described TDI principle suitable for processing x-ray images which are simultaneously obtained by means of two x-ray radiation energies.

For this purpose the invention provides an image pickup device of the abovementioned type in which two vertical shift registers are associated with each column of image pickup elements and in which means are provided for always transferring the information from the image pickup elements to the first vertical shift register associated with each column during a first period and to the second vertical shift register associated with each column during a second period.

According to a preferred embodiment two horizontal shift registers are provided for reading out and transferring the image information, of which the first always receives the information from the first shift register associated with each column of image pickup elements and the second always receives the information from the second shift register associated with each column of image pickup elements.

Although the invention is suitable, in particular, for simultaneously processing two x-ray images obtained with different x-ray radiation energies, attention is emphatically drawn to the fact that the invention is certainly not restricted to use in radiography. The invention is capable of being used in all those cases in which images are obtained alternately with different spectral regions, which images are scanned in the TDI mode. In this connection, said different spectral regions may be situated in the region of the x-ray radiation but also in the region of visible or infra-red light. The separation into different spectral regions may in these cases also take place in the region of visible or infrared light, for example, by means of filtering.

The invention will be explained below in more detail on the basis of an exemplary embodiment with reference to the drawing, in which the FIG. shows a diagrammatic plan view of an image pickup device according to the invention which is designed to work on the TDI principle.

The device comprises m rows, 2a-2m respectively, of image pickup elements and n columns, 1a-1n respectively of image pickup elements. The image pickup device therefore consists of m x n image pickup elements. For the sake of the clarity of the perception underlying the invention to be described below, it will be assumed that the height of an image pickup element augmented by the distance between two adjacent image pickup elements in one column is s and that the total height of a column of image pickup elements is h.

It is therefore now the case for m image pickup elements in a column that h=ms. If the scanning speed, i.e. the difference in speed between the image pickup device and the body to be irradiated, is v, the imaging of said body according to the TDI principle is correct if the frequency of the clock signals f.sub.TDI = v/s=mv/h. In this case the fact is left out of consideration that the known use of a multiphase clock implies per se a higher frequency, but this is not relevant for the understanding of the present invention.

The body is now irradiated alternately with hard and soft x-ray radiation in order to obtain an x-ray image having two radiation energies. This takes place with a repetition frequency f.sub.hz =kv/h, where k=1,2,3 . . . If the hard and the soft radiation are each present for an equally long time in a period of h/kv, then every image point of the body to be irradiated is. imaged on half (m/2) the number of image pickup elements in a column with hard x-ray irradiation and on the other half of the image pickup elements in the same column with soft x-ray radiation. If the hard and the soft x-ray radiation are each present for a time which is not equally long in a period h/kv, then each image point of the body to be irradiated is imaged on a larger or smaller number respectively than half of the number of image pickup elements in a column with hard x-ray radiation and on a smaller or larger number respectively than half of the image pickup elements in the same column with soft x-ray radiation. This applies, of course, for all the corresponding image pickup elements in the n columns. There is a fixed relationship f.sub.hz/fTDI =k/m between the clock frequency and the repetition frequency of the x-ray radiation hardness. If, for example, m=64, s=0.1 mm and v=400 mm/sec, then it holds true that f.sub.TDI =4 kHz and for k=1, f.sub.hz =62.5 Hz.

According to the invention, adjacent to each column of image pickup elements there is provided in each case a first vertical shift register 3a-3n respectively and in each case a second vertical shift register 4a-4n respectively. Clock signals are fed via a bus 5 to the shift registers 3a-3n and 4a-4n in a manner known per se for transferring the image information in the vertical direction which is received by each shift register from the associated column of image pickup elements.

Each of the m image pickup elements in a particular column, for example column 1a, can be coupled, under the control of two readout port electrodes (not shown) to either the first or the second associated shift register 3a or 4a. Said gate electrodes receive clock signals via the bus 5 for optionally transferring charge from an image pickup element to a shift register. In addition, however, a clock signal is provided which runs synchronously with the alternating frequency for switching between the two x-ray radiation energies, which clock signal also controls the readout gate electrodes of all the image pickup elements.

In this manner, the image information from all the image pickup elements can be transferred during the period of the hard x-ray radiation to, for example, the respective shift registers 3a-3n, and the image information from all the image pickup elements can be transferred during the period of soft x-ray. radiation to the respective shift registers 4a-4n.

Consequently, the situation indicated in the FIGURE by arrows between the columns of image pickup elements 1a-1n and the associated vertical shift registers 3a-3n and 4a-4n arises during a single TDI cycle for a row of image points of a body to be irradiated. During the time the effected row of image points of the body receives hard x-ray radiation, the image information is transferred to the shift registers 3a-3n and during the time that the same row of image elements receives soft x-ray radiation, it is transferred to the shift registers 4a-4n.

At the end of a TDI cycle, the image information of a hard and a soft x-ray image of one row of image points of the body is therefore available at the end of, respectively, the shift registers 3a-3n and 4a-4n respectively. From this information it is possible to determine directly on the chip of the image pickup device a difference, possibly weighted, for each pair of vertical shift registers 3 and 4 by means of an analog circuit suitable for the purpose, which different signal from each column can then be read out by means of a single horizontal shift register and can be transferred to an output terminal for further processing of the signal.

Preferably, however, as shown in the FIGURE, two horizontal shift registers 6 and 7 are provided, to which clock pulse signals are fed via a bus 8. The shift register 7 is always coupled to the vertical shift registers 3a-n and the shift register 6 to the vertical shift registers 4a-n. In this manner the information of a soft and a hard x-ray radiation image of the irradiated body is always available line by line at the output terminals 9 and 10 of, respectively, the shift registers 6 and 7, which information can be processed further either on the chip of the image pickup device or outside thereof, optionally prior to the conversion thereof into digital form.