NL8103799A - NUMERIC DEVICE FOR SUBTRACTING HIGH AND LOW ENERGY VENTILATION IMAGES. - Google Patents

Description

P, p- 'E 2348-1141 P & c

Numerical device for subtracting high and low energy X-ray images from each other.

The invention relates to subtraction fluoroscopy and in particular to the facilitation of angiographic examinations in continuously moving organs such as the heart.

In gart angiography, a liquid X-ray opaque medium such as an iodine compound is administered intravenously as close to the heart as possible while observing the heart with a fluoroscopic system.

When the iodine compound reaches the blood vessels of interest, a series of radiograms or a series of video images are recorded in order to be able to show later how the opaque medium advances through the blood vessels of the heart or how the flow is obstructed.

The peak value of the voltage (kVp) applied to the X-ray tube is chosen to obtain a spectral distribution of the X-ray photon energies with the maximum intensity occurring at the correct wavelength or energy level to obtain an optimal contrast between blood vessels in which the opaque medium is present and adjacent tissue where it is not. Furthermore, bone and tissues of different densities are located in the field of the X-ray beam, which also contains the heart. Bone shows a strong absorption of X-rays, as does the iodine compound, so that there is little contrast between the iodine-containing blood vessels coinciding with a bone. As is known, it is not possible to choose a single or narrow band of X-ray photon energies in which the X-ray image contains all shades of gray in order to distinguish between everything in the overall image.

It is known that the mass attenuation coefficient of bone and soft tissue is much lower at an X-ray photon energy corresponding to a peak value of the X-ray tube voltage of 70 kV, compared to iodine at the same energy. It is also known that as the energy is depleted to, for example, 140 kV, the mass attenuation coefficient of soft tissue changes relatively little, but that of iodine changes significantly. This has made it possible to minimize some contrast problems by resorting to subtractive radiography or fluoroscopy. In subtractive fluoroscopy, an X-ray image is recorded at a low peak value of the voltage and then a spatially near-summarizing image is recorded at a higher peak value of the voltage. If the radiographic or fluoroscopic organ has not moved between shots, the images may be subtracted and the remaining image will not be blurred. The 8103799 * Λ ». · "· - 2 - heart moves continuously, however, so images must be shot very quickly in order to avoid blurring.

It would be desirable to have an X-ray imaging system that allows visualization of rapidly moving organs such as the heart without the need to inject large amounts of contrast medium. As mentioned, a first approach is to record an image at a peak value of the voltage of about 70 kV and a second image at a peak value of the voltage of about 140 kV, after which the images are subtracted to show the presence of small amounts of 10 to emphasize present-day contrast medium in the blood vessels. It is immediately clear that an X-ray image intensifier can be used to obtain the images. A video camera can perceive the optical output image of the image intensifier in the usual manner. The analog video signals for the consecutive low energy and high energy signals can be converted into numerical signals corresponding in value to the intensity or gray value of the respective picture element (pixel) from the images. The pairs of images can then be stored in a very large memory and pairs of images at two different energy levels can be subtracted from each other to obtain difference signals which can be fed in the usual manner to a display device which converts them again into analog video signals and displays it on the screen of a video monitor.

When examining the blood vessels of the heart in which an opaque medium is located. moving forward, it is desirable to have images just one second apart. Usually, the radiologist wants to monitor the progress of the opaque medium in the heart blood vessels over a period of at least 20 seconds, which means that a very large memory would be required to store all the image data of the successive images. One way in which this requirement of enormous memory capacity could apparently be circumvented is to store the successive pairs of high and low value exposures of the peak voltage on a magnetic disk. Today, relatively inexpensive numerical calculators or processors on a magnetic disk can store a digitized video image of about 512 x 512 pixels 35 'in about half a second to 1 second. This would be fast enough to visualize the flow of a contrast agent through the blood vessels. but it is not fast enough to prevent heart movements from confusing the X-ray attenuation data subtractions.

The invention provides a system that can transmit numerically made video-40 signals that successive pairs of X-ray images with high and low peak value of the voltage in a low but nevertheless sufficiently high rate 8103799 4 ~ ♦ * 4 - 3 - to a magnetic disk, while simultaneously time, almost instantaneous subtraction of the high and low peak value images of the voltage can be obtained in pairs within a time interval corresponding to the frame scan time or frame scan time of the video camera used to record the pictures.

According to the invention, the voltage of the X-ray tube is adjusted to generate a low-energy X-ray pulse, such as with a peak value of the voltage of 70 kV. The resulting image is made numerically and stored in a first high speed image memory. A short time later, for example 1/60 or 1/30 second later, the voltage of the X-ray tube is changed to a voltage to generate a short X-ray pulse of higher energy, such as a peak value of the voltage of 140 kV. This image is also kept in a fast memory. During the next full second, time is available for the calculator or other processing device to read the information to a magnetic disk. A relatively slow and inexpensive processing device can be used for this. The images are taken every second as required by the dynamic flow of the opaque iodine-containing agent through the blood vessels, but the images to be subtracted are only 1/60 or 1/30 second apart. This prevents movements of a patient or an organ of the patient, such as the heart, from adversely affecting the subtraction process. Since the X-ray pulses further occur on average every half a second, the X-ray flux can be much greater than is usually applied, so that the signal-to-noise ratio of the video signal is considerably better than in a system using a constant X-ray flux. The time interval between the low and high peak value images of the voltage is related to the number of lines one wishes to input into the memory and the sampling rate of the video system being used, as will be seen later.

The invention is explained in more detail below with reference to the drawing, which relates to an exemplary embodiment of a device according to the invention.

Fig. 1 is a block diagram of a system according to the invention.

Fig. 2 is a timing chart of the X-ray pulses and the write functions and read functions of the memory.

Fig. 3 is a diagram of the X-ray energy in the form of the mass attenuation coefficient for iodine, soft tissue and bone as a function of the peak value of the voltage.

In fig. 1 the patient 10 is lying with the blood vessels of the heart 11 8.1 0 3 7 9 9 - 4 - • r *.

1. v must be examined on an X-ray transmitting support 12.

In this example, the X-ray tube 13 is located under the carrier. The X-ray tube includes an anode 14, a filament or cathode 15, and a control grid 16. The power lines 17-20 of these components lead to an X-ray power supply and pulse circuit 21. The bias voltage control of the grid 16 sets the X-ray tube. allows short pulses of X-ray photons to have a peak value of energy corresponding to the peak value of the voltage between the cathode and anode of the tube at the time when the negative bias of the control grid 16 is canceled.

"10 In FIG. 1, the X-ray pulses delivered by the anode 14 are passed through a beam-defining collimator 22 and the X-ray beam. which is differentially attenuated when passing through the body 10 is received by an electronic image intensifier tube 23. As is known, a photocathode 24 in the image intensifier tube converts the X-ray image into an electron image falling on a screen 24 'with luminescent material, where it is converted into a clear and reduced optical image. An objective lens 25 is used to couple the optical image to the lens of a video camera 26 which records the optical image and provides analog video signals representative of the image and outputted via a cable 27. Preferably, a video camera is used with a imaging plate that responds quickly to changes in light level, such as a vidicon with a lead oxide imaging plate. The manner in which video signals are processed to obtain numerical data for each X-ray image to be stored on the magnetic disk of a video disk recorder (VDR) or other magnetic medium for subtraction is described later.

As mentioned earlier, the X-ray pulses follow each other rapidly and occur in pairs, with a significant time elapsing between the pairs, during which time the pixel data can be transferred to the disk memory. The timing of the X-ray pulses can be seen from part A of FIG. 2. In this view, the first pulse 60 of a pair may be a low energy pulse, preferably at a peak voltage value of about 70 kV. This pulse can have a duration of about 1 to 3 ms. Within a further video frame or 1/30 second 35 in the discussed example, a second pulse 61 of the same duration but a higher voltage such as about 140 kV is delivered to the X-ray tube. Then, a long interval of, for example, 1 second elapses before the next pair of similar pulses 62 and 63 with low and high peak values of the voltage are delivered to the tube.

40 Of course, if desired, the high energy pulse can precede the 8103799 - 5 - low energy pulse.

The video signals of each of the consecutive low-energy and high-energy X-ray images are sequentially fed to an analog-to-numeric converter 28. The output of converter 28 in line 29 is a series of pixel signals line after line which together form the image, the value of which corresponds to the intensity or gray scale of the image element of the X-ray image. Typically, an X-ray image video frame comprises 512 pixels per horizontal line and 512 horizontal lines.

The pixel data for the first image of a pair, which in this example is an image with a low peak value of the voltage, is transferred to a first memory 30. The pixel data for the second image, which in this example is a higher energy image are supplied to a second memory 31. As indicated, the pixel data 15 for each X-ray image is generated during the occurrence of X-ray pulses of typically a duration of up to three ms and the two pulses occur within a video frame or 1/30 second in this example, so that both memories 30 and 31 can be easily loaded during that time.

The time intervals corresponding to the time required by the video camera 20 to read two interlaced frames or an image after each X-ray pulse are indicated at 64 and 65 in Part A of Fig. 2. As a typical video system with 525 lines per image, applied, the reading or scanning time for an image is 1/30 second and the X-ray pulses of a pair must be at least as far apart. The timing for recording numerical information output by the analog-to-digital converter 28 in the first and second memories is shown in part B of FIG. 2. The write unblock pulses are shown at 66 and 67 and substantially coincide with the reading intervals 64 and 65 of the video camera. The write pulse 66 starts at the end of the X-ray pulse 60 and the start of the video image readout 66 opens the first memory for receiving and recording data associated with the X-ray pulse 60. Once the first memory is loaded, the X-ray pulse occurs 61 and the video image reading interval 65 begins as branched at A in FIG. 2 and the memory writing interval 67 in part B 35 begins, so that the second memory is opened to receive the numerical pixel data from the analog-numeric converter for the second image of the pair of consecutive images to be subtracted.

The intention is to transfer the pixel data from the first and second memory 8103799 of 9 t * - 6 - in that order to a magnetic disk memory in a video disc recorder (VDR) 32. The data for many pairs of high and low energy X-ray images can be recorded on the disc, so that the progress of the X-ray contrast medium through the blood vessels of, for example, the moving heart can be observed later, almost faithfully for long intervals such as 20 seconds or more.

The transfer of the data from the first and second memory to the disk memory can take place with a suitable controller which may consist of an inexpensive and relatively slow microprocessor system, which is conveniently referred to here as a calculator and is shown at 33 in Fig. 1. . As can be seen from FIG. 2, after pixel data for the consecutive low and high peak value images of the voltage from the analog-to-numeric converter is recorded in the first and second memories, a significant time is available in which the processor is able to can transfer data to the write memory. As can be seen from the portion C of Fig. 2, once the writing of the data into the second memory has been completed, the reading of the first memory as indicated at 68 begins and continues until the time 69. Once the contents of the first memory have been read and transferred to the write memory 32, the reading of the second memory begins at time 70 and continues until time 71 indicated in part D of FIG. 2. The next X-ray pulse 62 of a pair may even begin before the contents of , the second memory has been completely transferred to the disk memory.

As is known, the signals for synchronizing writing into and reading from the analog-numeric conversion memories and generation of the video frames can be derived from the video system and are therefore not discussed here in more detail. The control signals are processed in a collector 34. It is clear that this collector is connected to the video system and to the conversion terminal C and the stop terminal S of the analog-numeric converter and the write terminal W and read terminal R of the inputs of both memories.

A manifold 35 connects the calculator 33 to the disk recorder 32 and a further manifold 36 connects the disk recorder to the processor. The pixel data is recorded on the disk or other memory medium in non-subtracted form. They must therefore be taken from the disk memory by means of the manifold 36 in order to be subtracted in the computer 33. The pixel data representing the difference between the X and X images with high and low voltage 8103799 * - 7 - is then sequentially fed for each image to a conventional video display 37 through a collector 38. The controller receives the pixel data line by line and converts this line by line into analog video signals and outputs them via a cable 39 to a video monitor 40 on the screen 41 whose subtracted image is displayed.

Thus, it is clear that while the X-ray pulses occur almost simultaneously and last very short, the movements of the heart are essentially frozen, so that no blur can occur in the image based on data obtained by subtracting rapidly successive images. Furthermore, the disclosed system makes it possible to transfer the data to the disk memory relatively slowly, which can easily take place within an interval of 1 second, so that the transfer speed limitations inherent in inexpensive processing devices or calculators are not involved .

Described above is a system for instrumenting the basic facets of the invention, consisting of taking two X-rays with different voltages from the X-ray tube to facilitate subtraction of the images for highlighting the X-ray contrast medium blood vessels moving heart, taking these pictures in such a rapid sequence that the influence of heart movements disappears, transferring the data for each image of a pair to be subtracted to separate memories, simultaneously obtaining the data, and then recording the data successively reading the data to a magnetic memory medium such as a video disk recorder during the relatively long interval between successive pairs of X-ray pulses, so that a relatively slow and therefore inexpensive processor or calculator can be used to transfer the data from the memory to the disk.

The video camera 26 in this example had an image rate corresponding to 1/30 second and 525 lines per image. However, video cameras with other operating parameters can be used advantageously to prevent better freezing of motion, which means better coverage of the images and less blurring of the subtracted images.

For example, a television camera or video camera with 875 or 1024 lines per 1/30 second frame and half thereof, such as 437 or 512 lines per 1/60 second frame, can be used. At a high line frequency such as 512 lines per frame, the low voltage X-ray pulse can be delivered and the frame read, converted to. .. _fe »8103799

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I t t * I '- 8 - numerical values and with good resolution are recorded in a memory within 1/60 second. Then, the high voltage pulse can be delivered to obtain a second frame which can be read, converted and stored for transfer to the disk memory. This scheme 5 allows only 1/60 second to pass between the low and high voltage X-ray pulses, so that movements can be better frozen, but it requires video signal amplifiers with a larger bandwidth.

A video camera with 525 lines per image and 262 lines per frame 10 may be used in a manner other than that described above if any loss of resolution is acceptable. Each frame of 262 lines can be preceded by an X-ray pulse and can be read as an image and data for each image can be stored as described above. This makes it possible to reduce the spacing between X-ray pulses 15 of a pair to 1/60 second. Otherwise, if low and high voltage X-ray pulses are applied at 1/30 second intervals, about 1/60 second would be available to rid the display in the camera of any residual charges caused by the low voltage image before high voltage-20 image occurs. The removal of such residual charges can be conducive to increasing the difference between the brightness levels of the two images, so that the contrast-filled blood vessels exhibit greater contrast in the subtracted image.

As it appears, various modes of operation for the video camera 25 can be employed, requiring compromises between the bandwidth, the resolution and the freezing of motion.

The theory underlying the subtraction of X-ray images obtained with high and low energy X-ray beams is known, so that it will be discussed only briefly with reference to Figure 3.

FIG. 3 is a diagram of the mass attenuation coefficient of different materials as a function of the X-ray energy indicated as the peak value of the applied voltage. Curve 75 is the iodine attenuation curve at energies well above its K absorption edge. Curve 76 applies to bone and curve 77 applies to water and corresponds approximately to that of soft tissue. Curve 75 shows that at a peak voltage value of 70 kV, for example, iodine causes strong attenuation, while bone and soft tissue are more transparent and cause less attenuation. At high energies such as a peak value of the voltage of 140 kV, the mass attenuator coefficient of 8103799 - 9 - iodine strongly determines, but the mass attenuator coefficients of bone and soft tissue decrease less and follow almost the same curves at this high energy. Therefore, if the pixel data obtained at high energy are subtracted from the pixel data obtained at low energy, the influence of tissue in the image is almost completely suppressed, while the iodine-containing blood vessels are emphasized, in the end this makes possible small blood vessels that low in iodine and also makes it possible to use less concentrated iodine compounds or smaller amounts thereof without losing details from the displayed image.

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Claims (5)

A system for subtracting each X-ray image from a pair from another X-ray image of the pair, wherein a series of such pairs of images are recorded, characterized by an X-ray tube for delivering X-ray beams through a body and a means for guiding the short duration X-ray pulse tube in the form of closely matched pairs to generate corresponding images, the energy of one pulse being greater than that of the other pulse and the interval between successive pairs of X-ray pulses being long compared to the interval within which the impulse pairs occur, a means for converting the differential attenuated X-ray images obtained with the X-ray pulses into visible images, a video camera for generating analog video signals corresponding to the intensities of the picture elements (pixels) from which the images concerned exist, a means for converting the analog signals for each image in a pair of respective sets of numerical signals corresponding to the pixel intensities of the images, a first memory means for storing the set of numerical pixel signals from the converter for the first image during the occurrence of the first X-ray pulse a pair and a second memory means for storing the set of numerical signals from the converter for the second image during the occurrence of the second pulse of a pair, a device for recording numerical signals on a magnetic medium, a means for transferring of the sets of numerical pixel signals from the memory devices in a sequence to the recording device during the intervals that pass between pairs of X-ray pulses and for subtracting the sets of signals received from the recording device to obtain a set of numerical pixel signals representative of the difference obtained by subtraction, a we display device 30 for converting the numerical signals representing the differences into analog video signals and a video monitor displaying images obtained by subtraction in response to the analog signals. System according to claim 1, characterized in that each set of signals corresponds to a matrix of pixel signals consisting of 512 lines, 35 pixels and 512 pixels per line. System according to claim 1, characterized in that the duration of each of the X-ray pulses from a pair is between 1 and 3 ms and the pulse pairs occur at intervals of about 1 s. System according to claim 1, characterized in that the duration of each of the 8103799 - 11 X-ray pulses of a pair is between about 1 and 3 axis and the pulses of a pair occur within 1/60 to 1/30 s and the pairs occurring at intervals of about 1 s. System according to one or more of claims 1 to 4, characterized in that the peak energy value of the high-energy X-ray pulse corresponds to about 150 kV and the peak value of the low-energy X-ray pulse of a pair corresponds to about 70 kV. 8103799