JPH05244507A - Image quantity reinforcing method - Google Patents

Abstract

PURPOSE: To improve image quality for a diagnostic purpose by imparting specified correction signals to original image signals. CONSTITUTION: The radiation images of a patient are recorded by exposing 2 a light stimulative phosphor screen to X-rays transmitted through an object. The screen is carried by a cassette 3 and various data are recorded to an EEPROM in an identification terminal 4. Latent images stored in the screen in a digitizer 1 are read by scanning a phosphor sheet by a stimulation ray radiated by a laser. Stimulated radiation is passed through a photoelectron amplification tube, converted to a logarithm amount and then, quantized to 12 bits. The 12-bit digital image signals are transmitted to an image processor 7 and image data are limited to 10 bits, expresses the images proportional to a logarithm E for which E is exposure and are transmitted to a prior inspection display device 5. Then, the value of the logarithm E is mapped to a desirable density value through a table for reference and converted to analog signals.

Description

Detailed Description of the Invention

The present invention is in the field of digital image processing and more specifically relates to a method for enhancing the quality of radiographic images for medical purposes.

Presently, most radiographic images are recorded on radiographic film. Intensifying screen-film combinations are preferably used. This screen consists of a support and a fluorescent layer consisting of a phosphor dispersed in a binder resin, which emits light upon stimulation and a protective layer. In the radiographic operation, the image conversion screen
The fluorescent layer is placed closely on top of the film so that it faces the film. It is technical practice to have two image conversion screens housed in a housing, eg a cassette, with the radiographic film. When the object is exposed, X
The lines are transmitted by the object onto the screen-film combination. The phosphor screen emits light and enhances the exposure of the film.

The exposure contrast of the part of the human body targeted for diagnostic purposes is usually low. To overcome this problem, that is to say there is a detectable density difference, photographic films with a sharp gradation are always used, i.e. films with a gradation of about 3 are routinely used. Because of the narrow exposure area of these films, the image quality is strictly determined by the exact exposure level. In most cases, only image details that fall within the 10: 1 exposure range at best give the best image quality. As a result, details in the image are only detected if they are superimposed on a correspondingly uniform area covering the entire diagnostically relevant image. A correspondingly uniform area means that the dynamic range of the input exposure (dynamic range = ratio of maximum exposure to minimum exposure) is within the limits of which the recording system is fully functional. For films this means a range of about 10: 1.

However, the anatomy of the human body itself is the cause of image non-uniformity in many situations. For example, when imaging a hand or foot, the average exposure level in the detector plane changes somewhat linearly with increasing bone thickness.
Other parts of the human body, such as the spines of the lateral lumbar spine, produce more complex changes in average exposure level. Therefore, in many cases, the image detail present in the primary exposure is lost due to the small exposure latitude of the film and disappears in areas of under or over exposure.

This problem has been described in many ways. For example, Journal of Thoracic Imaging, Vol. 5,
1, 90, page 29, shows that the emergency department evaluation of the cervical spine in the transverse projection of the cross table was performed with sufficient exposure to allow visualization of the C-7 vertebral bodies. It is often unsatisfactory because it results in an overall overexposure of the anterior soft tissue and upper vertebral spines.

In the prior art, this problem has been associated with stepwise intensifying screens in screen-film combinations.
(Also referred to as a tonal screen). Since conventional conversion screens have comparable velocities over the entire area, the fluorescence intensity of the light emitted by the phosphor layer is constant over the entire area, while grayscale phosphor screens have a screen-film combination. Fig. 6 is a screen that provides locally varying speeds of the body. This is commonly obtained by varying the phosphor thickness of the screen.

If the screen is manufactured so that the speed of the screen-film combination is inversely related to the expected average exposure level, more uniform recording will be possible, thus allowing more detail to be identified. ..

Such a graduated screen is adapted to the anatomy of the patient being examined, including the variability in the extent and morphology of its velocity change, which may be brought by the patient, such as lean or fat, tall or short. It is only advantageous when you do. Optimally, therefore, various graded screens with different properties in their normal dimensions would be required.

This situation is exacerbated by the limitations of manufacturing technology. That is, the maximum exposure range that can be generated is about 4: 1. Therefore, the adaptation of the graded screen to individual patients cannot be achieved.

Furthermore, the use of various screens increases the likelihood of making an error, resulting in a time consuming procedure for both the radiologist and the patient. Errors may result in additional exposure that is harmful to the patient and wastes photographic recording material.

A method has recently been developed in which the X-rays transmitted by an exposed object are stored on a reusable photostimulable phosphor screen. Examples of these photostimulable phosphors are extensively described in European Patent Application No. 91200511.3, filed August 3, 1991.

Since the photostimulable phosphor screen is read by stimulating the screen with a laser beam, it releases its latent image by emitting imagewise modulated light. The emitted light is then detected and converted into an electrical representation (more specifically an analog signal that is digitized). This electrical indication is subsequently used to control the recording head of the image reproduction system. In a laser recorder, for example, the image signal is used to control the modulation of the output of the recording head so as to expose the photographic film with a scanning laser beam having an intensity corresponding to the numerical value of the digital image. Redevelopment for diagnostic purposes
It is calibrated and analyzed on a light box.

A very important property of these photostimulable phosphor screens is the fact that in practice they are capable of recording exposure ratios above 500: 1. However, the final visual reproduction of the information contained in the photostimulable phosphor screen produces a visually pleasing density difference, whether on film or on a CRT (or other) display. Forms of contrast enhancement to be included. Due to the contrast dynamic range limits present in any form of display, whether film, CRT or otherwise, only a portion of the original exposure range can be inspected satisfactorily. This problem can be solved by making a series of soft or hard copies, each representing a different part of the overall original exposure range. For example, in computerized tomography, it is a technical practice to display different windows next to each other (all grayscale parts). Clearly, this solution is costly and further reduces efficiency.

It is an object of the present invention to provide a method of enhancing image quality for diagnostic purposes in reproducing an image of an object or patient that has been exposed to radiation.

Another object is that the X-rays transmitted through the patient or object are recorded on a photostimulable phosphor screen, and when the laser beam stimulates the emission lines of the phosphor screen to be detected and converted into an electrical signal. It is to provide such a method in a system that is transformed and used to control reproduction.

Still another object is to be able to optimally map signal values onto a recording material in a useful density range without introducing disturbing artifacts and without losing diagnostically relevant data. A method of reducing the dynamic range of the signal value of an image representation signal obtained by scanning a photostimulable phosphor screen.

Yet another object is to reduce the total number of soft or hard copies required to display the total amount of diagnostic data available.

Other objects will become apparent from the description below.

The present invention provides a method for enhancing the image quality of an image or a portion thereof in an image recording system, wherein a photostimulable phosphor screen exposed to a radiation image in said image recording system is scanned by a stimulating beam. And the light emitted during stimulation is detected and converted into an electrical image signal, the image signal is processed and a visible image is recorded on the recording material modulated in response to the processed image signal. By scanning the recording material with a beam,
In what is to be recorded, the image is processed by applying to the original image signal a correction signal which is a function of at least one positioning coordinate of each pixel in the image area and which is related to the local average exposure level. It is characterized by

This method can be used in medical applications, but also in non-destructive testing if the image of the object to be tested is stored on a photostimulable phosphor screen.

This method is particularly advantageous in that the range occupied by processed signal values is reduced with respect to the range occupied by unprocessed signal values. When the image processed according to the invention is reproduced on the recording material, the correction signal values are more optimally mapped to the density range on the film, so that an optimum detailed contrast in a large exposure range is obtained and for diagnostic purposes. The image quality for use is optimal. The method of the present invention overcomes the drawbacks of the above prior art methods of creating a series of displays, each representing a different portion of the overall original exposure range.

The term "image" is used when the image signal is processed in such a way that only the areas that are diagnostically relevant to the radiation image that is stored as a whole from the exposure of the object or patient and that are diagnostically relevant are retained. Means the resulting image. This diagnostically relevant area can be determined manually or through automatic segmentation.

The term "according to" means that the modulation of the output of the recording head in the recording device is obtained by the processed signal itself or by converting the processed signal, for example by a sensitometric conversion. It means to be controlled by a signal. With such a conversion, the signal value of the processed signal is mapped to the specific density value obtained in the reproduction, so that the image screen exposure represented by the electrical signal is converted to the output density and reproduced. A predetermined sensitometry is obtained at.

The recording can be carried out so as to obtain a visible image on a hardcopy material such as photographic film. It will be apparent to be within the scope of the invention to scan the phosphor in a CRT display device (monitor) with an electron beam that is modulated by the processed image signal so that a soft copy is obtained. ..

It is generally known that monitors have a more limited dynamic range than photographic film. Therefore, the advantages of the method of the present invention are often more pronounced when displaying radiographic images on a monitor.

In a particular embodiment, the original image signal is represented in logarithmic form. In this case, the correction signal is subtracted from the original image signal. Other forms of original image signal and other arithmetic operations (division, multiplication ---)
Are within the scope of the invention. The subtraction of the following terms can be replaced by other arithmetic operations according to the form of the original image signal.

The modified signal in the method of the present invention is preferably normalized to prevent loss of detail. In a particular embodiment where the original image signal is represented in logarithmic form, the correction signal is determined such that at least one pixel has a zero value and the other pixels are non-negative.

The correction signal can be a predetermined signal corresponding to the type of examination. The correction signal is a function of at least one placement determination coordinate of each pixel in the image area and is related to the expected local average exposure level. Expected local average exposure level means the local average exposure level determined through experience gained from examining multiple exposures or through calculations that take into account the data for multiple exposures.

The more important said correction signal is the original image signal itself so that it is a function of at least one placement determination coordinate of each pixel and it is related to the calculated local average exposure value of said image. Can be calculated from

Each pixel in an image area depends on its location within the image area, in other words its coordinates (x, y),
And its pixel value P (x, y) corresponding to the signal value obtained by stimulating the phosphor at that location with coordinates (x, y) and sensing the stimulated emission line.

The inventor uses, for example, delta P (x, y) = ax + b, delta P (x, y) = a + by, and delta P (x, y) = a + b as a linear function of coordinates in each pixel.
It has been found to be advantageous to apply to the original image signal a correction signal with the signal value obtained as a function of x + cy or the like. With this kind of function, the image quality can be improved appropriately without requiring a long calculation time.

The functions delta P (x, y) = ax + b and delta P (x, y) = a + by are functions of one variable that provides gray levels in one direction. These functions are convenient for enhancing the quality of the reproduction of the X-ray image of the hand or foot when the patient is in a fixed position with respect to the exposure table.

The correction signal preferably represents the average exposure level, but approximates the original image signal so that it is superimposed on this level and does not represent diagnostically important details. As a result of the application of the correction signal, the processed image signal value occupies a narrower range than the original image signal. When reproducing the image corresponding to the processed image signal, the processed image values can be more optimally mapped onto the density range on the film, resulting in an optimal detailed contrast within a wide dynamic range and diagnostic The image quality for the purpose is optimal.

This method provides the advantage that the correction function can be adapted to the anatomy of the patient being exposed, in addition to the advantage of applying a fixed and preset function.

In this method, a linear function of coordinates in each pixel, for example, delta P (x, y) = ax + b, delta P (x, y) = a + by, delta P (x, y) = a
It is advantageous to subtract a signal equal to a function such as + bx + cy from the original image signal. With this kind of function, the image quality can be improved appropriately without requiring a long calculation time.

In some applications it will be useful to have a more complex modification, ie to use higher order polynomials.

For the purpose of limiting the calculation time of the modification, the following types of functions are convenient. That is, delta P (x,
y) = A. F (x) + B. G (y) + C. Here, F (x) and G (y) are arbitrary functions. For example, in the case of quadratic correction, F (x) is equal to ax ² + bx, and G (g)
(Y) is equal to cy ² + dy.

In the case of the knee joint, good results have been obtained with a primary correction in the direction of the bone and a parabolic correction in the direction perpendicular to the direction of the bone.

The coefficient of the function is determined in consideration of the original image information by means of the first least squares approximation method.

Polynomials with coefficients that can be calculated by means of the first least squares approximation are preferred, as their calculation requires a reasonable calculation time.

For example, the function delta P (x, y) = ax ²
The coefficients + bx + cy ² + dy + c can be obtained by solving the following system of linear equations expressed in matrix construction:

[Equation 1]

The choice of coordinate system is important. This is because one processes a large number of pixels (more than 5 million pixels in an image), and the calculation of the above equation is very time consuming. The image area reaches -L / 2 to L / 2 and -K / 2 to K / 2 (L is the number of pixels per column and K is the number of pixels in a row of the image area) By choosing a coordinate system in this way, the number of calculations to be performed is reduced, so that the method can be implemented without the need for a high-performance computer and can be performed online.

In the present invention, the radiation transmitted from the patient or object is first stored within the graded photostimulable phosphor screen so that in the region where only a small number of photons expose the screen, the signal to noise ratio is increased. It does not exclude that it is emphasized.

The graded photostimulable screen can be made according to the method disclosed under 287464 in Research Disclosure 287 (March 1988). However, the conventional phosphors described are photostimulable phosphors, for example European patent application No. 1 filed August 3, 1991.
It replaces one of the phosphors described in 91200511.3.

The graded photostimulable phosphor screen can be selected to fit certain types of exams (eg, hand, foot, lumbar spine). It is not necessary or possible to completely match the degree and morphology of the screen speed variations to the anatomy of the patient being examined. The graded stimulable phosphor screen, once stimulated and read out, can be further processed according to the invention by applying a correction signal to the image signal. The correction signal is a function of at least one placement determination coordinate of each pixel and is related to the average exposure value.

Specific aspects of the invention and its preferred embodiments are described below with reference to the drawings.

FIG. 1 shows generally the equipment to which the method of the invention can be applied.

A radiographic image of the object or part thereof, eg the patient, is recorded on the photostimulable screen by exposing it to x-rays transmitted through the screen. The stimulable phosphor screen is carried in a cassette 3, which in this embodiment is
Programmable read-only memory (E
EPROM) is provided. At the identification terminal 4, various data, such as patient identification data (name, date of birth) and data related to exposure and / or signal processing, are stored in the EEP.
Recorded in RPM.

In the digitizer 1, the latent image stored on the photostimulable phosphor sheet is read by scanning the phosphor sheet with stimulating rays emitted by a laser. The stimulation line is deflected in the main scanning direction by means of a galvanometer deflection unit. The sub-scanning is performed by moving the phosphor sheet in the sub-scanning direction. The stimulated radiation is directed to a photoelectron amplifier tube for conversion into an electrical image display.

The output signal of the photoelectron amplifier tube is converted into a logarithmic quantity.

The signal is then quantized to 12 bits.
This quantized image is called the original image. This 12-bit digital image signal is transmitted to the image processing device (FIG. 1, numeral 7) and is stored therein in the internal buffer. Without modification, the digital image signal can also be transmitted from the image processing device to the inspection console, where it is temporarily stored on the hard disk. With this backup, signals are never lost in the event of a component failure of the equipment,
It is guaranteed that the and signals can be searched for any kind of subsequent processing, for example processing with different parameter settings.

This function is used when the results of online processing are insufficient due to poor exposure conditions or improper selection of processing parameters. The original image can be recorded and stored on an optical disc (not shown).

The latitude of a 12-bit image is too large to print on film, which is about 2.7 decades. Actual object tolerance is limited to 1.5 decades or even lower for many applications. The remaining 1.2 decades of the dynamic range of the digitizing system sets a safety margin for under or over exposure. The actual object exposure latitude and level must be determined for each acquired image in order to limit the coverage of the digital image to the diagnostically relevant areas.

The process of limiting the dynamic range to the diagnostically useful region is called requantization, so that the image data is limited to 10 bits and the image is proportional to the logarithm E, where E is the exposure. Express. The gray levels above and below the diagnostic area are fixed at 0 and 1023, respectively.

At this stage, the image is displayed on the pre-inspection display device 5.
And the display device first displays the acquired data, so that if the acquired data is defective, it can be fed back to the operator.

The logarithmic E value is then mapped through a look-up table to a density value corresponding to the desired sensitometry and applied to a digital / analog converter, the output of which then controls the modulator of the laser recorder 6. To produce a hard copy on photographic film.

In FIG. 3, the method of the present invention is illustrated by means of a fictitious example. In FIG. 3a, the abscissa is the X coordinate of the pixel, and the ordinate is the detected signal value at the pixel. For simplicity, this example shows one dimension of the image (
For example, it is limited to the image line).

In FIG. 3b, the correction signal is depicted. FIG. 3C shows a modified signal (from the original image signal
FIG. 3B shows the corrected image signal obtained by subtracting (see FIG. 3B). This obtained signal is then applied for output modulation of the recording head in the recording device or output control of the display unit.

From these figures, the dynamic range of pixel values that need to be recorded to obtain image redevelopment is uncorrected (see FIG. 3a) and modified (see FIG. 3b). ), And by modulating the recording head in response to the modified signal value, the dynamic range of the reproduction material can be used in an optimal way, thus improving the quality of the reproduction. I understand.

In FIG. 4a, a column diagram of the image signal corresponding to the image of the knee joint is shown. FIG. 4b shows a histogram of the same knee joint image signal subtracted with the correction signal according to the invention. In FIG. 4b, the primary correction is performed in the direction of the leg and the parabolic correction is performed in the direction perpendicular to the leg. In FIG. 4c, the same knee joint signal is parabolic corrected in the direction of the leg and in the direction perpendicular to the leg.

[Brief description of drawings]

FIG. 1 generally shows an instrument in which the method of the invention can be used.

FIG. 2 shows a data path.

FIG. 3 shows the effect of the present invention applied to an aerial image signal.

4A is a columnar view of an image of a knee joint, and FIG.
4b is a columnar view corresponding to an image of the same knee joint where the primary correction is in the direction of the leg and the parabolic correction is in the direction perpendicular to the leg, and FIG. 4c is in the direction of the leg and It is a column diagram of the image signal of the same knee joint which was parabolic-corrected in the direction perpendicular to the leg.

[Explanation of symbols]

 1 Digitizer 2 Radiation Source 3 Cassette 4 Identification Unit 5 Display Unit 6 Laser Recording Device 7 Processing Device

Front Page Continuation (72) Inventor Emile Paul Skotel, Septestraat, Mortzel, Belgium 27 Agfa Gewert Namrose Rose Bennault Chap

Claims (9)

[Claims] 1. A method for enhancing the image quality of an image or a portion thereof in an image recording system, wherein a photostimulable phosphor screen exposed to a radiation image in said image recording system is scanned by a stimulating beam and is emitted during stimulation. The detected light is detected and converted into an electrical image signal, the image signal is processed to form a visible image on the recording material, and the recording material is a recording beam modulated according to the processed image signal. By applying a correction signal to the original image signal that is a function of at least one placement determination coordinate of each pixel in the image area and is related to the local average exposure level An image quality enhancement method, wherein the image is processed. 2. The method of claim 1, wherein the correction signal is a predetermined signal that is a function of at least one placement determination coordinate of each pixel in the image area and is associated with an expected average exposure level. .. 3. The method of claim 1, wherein the correction signal is a function of at least one placement determination coordinate of each pixel in the image area and is related to a calculated average exposure level. 4. The method according to claim 1, wherein the correction signal is a linear function of at least one coordinate of each pixel in the image area or a part thereof. 5. The method according to claim 1, wherein the correction signal is a quadratic function of at least one coordinate of each pixel in the image area or a part thereof. 6. The method according to claim 1, wherein the correction signal is a function of a single coordinate of each pixel in the image area. 7. The method of claim 1, wherein the correction signal corresponds to a quadratic function of one coordinate of each pixel in the image area and a quadratic function of another coordinate of each pixel in the image area. 8. The method according to claim 1, wherein the coefficient of the equation defining the function is determined by approximation of the image signal by the first least square method. 9. The method of claim 1, wherein the screen is a graded photostimulable phosphor screen.