BACKGROUND OF THE INVENTION X-Ray films known in the art are characterized by sensitivity response curves, as shown, for example, in Fig. 1. This figure shows the optical density of a typical medical X- Ray screen film combination, after exposure and development, as a function of the log of the incident X-Ray exposure (log IT). Over a limited range of exposures, defined as the "latitude" of the film, the resultant optical density falls within a useful range of density values. Within this range, the response curve is generally roughly linear, with an average gradient that is identified as the "gamma" of the film. The higher the gamma of the film, the greater will be the relative image contrast, i.e., the variation in optical density, for a given variation in log IT falling within the film's latitude. Fig. 1 and other information cited here in the Background of the Invention are based on "Radiographic Imaging, A practical Approach," by D.P. Roberts published by Churchill Livingstone (1988), which is incorporated herein by reference and especially on chapters 4 and 8 thereof.

As further described in this book, for commonly-used medical X-Ray screen films and viewing devices, the useful range of density values is limited to net densities (ND, not including the film's "basic fog", which is generally 0.2-0.3) above about 0.25 and below about 2.0. Below about ND 0.25, as may be seen in Fig. 1, the image contrast is generally too low for observation of fine detail. Above ND 2.0, the image is generally too dim for details to be clearly discerned, although X-Ray films are generally capable of reaching maximum ND of 3.0to4.

Medical X-Ray films generally have gamma in the range of 2.0 to 2.5, in order to enhance the contrast of image details. Lower values of gamma are considered to give inadequately low image contrast, making the developed film appear to be lacking in detail.

The high values of gamma that must be used, together with the narrow useful range of density values, necessarily limits the latitude of the film to a small range of exposures. As a result, details in dimmer portions of the image, outside the latitude, are lost from view. Furthermore, as a result of this limited latitude, there is a greater chance that a medical X-Ray image will be over- or under-exposed if the radiation is not precisely and correctly controlled, so that the exposure must be adjusted and repeated.

Similar latitude limitations are encountered in presenting digital radiographic data on film. Digital radiographs are acquired by capturing an X-Ray image of a subject on a radiation- sensitive detector, such as a thermoluminescent panel detector, (used for example by Fuji

Corporation for digital radiography), a selenium detector or a CCD or screen/CCD line array, as are known in the art. Images captured in this manner typically have very low noise threshold and wide dynamic range. For example, many digital radiographic systems quantize the X-Ray values to 12 bit accuracy, i.e., to 1 point in 4000.

However, when such information is presented on film for viewing by physicians, the range of density values used is limited to the range described above with respect to direct X- Ray film generation, since the same problems of viewing film having a wide dynamic range exist no matter what the source of the image. Therefore, before a digital radiographic image is printed on film, the range of gray scale values in the image is generally compressed and processed, using image processing methods known in the art (for example using histogram equalization techniques), to accommodate the expected viewing conditions. In particular, digital radiographic images are generally presented with an O.D. range of about 0.4-1.7 in order to overcome the effects of film viewing conditions, such as glare, on contrast discernability. As a result, the visibility of low-contrast image features is reduced.

This limitation in range results in a number of well known diagnostic limitations for both conventional and digital radiography as presently practiced. In imaging extremities, for example, the radiation level is generally adjusted to properly visualize the bone structure (for example, to determine if a lesion is present in a bone or if it fractured or to visualize the small bones in X-Ray images of the foot). However, under similar clinical conditions trauma, pathology and/or lesions are also (or only) present and/or visible in the surrounding soft tissue.

Due to the limited range of present X-Ray images and to the limited presentation range of digital radiographic images, it is difficult or impossible to determine the effects of such trauma from an image. In some cases, more than one exposure must be acquired to visualize both soft and bone tissue.

Another example of a well known limitation in conventional X-Ray imaging and the presentation of digital radiographs is the fact that in lateral cranial images, the nasal structure is practically indiscernible. Thus, fractures of the nose are difficult or impossible to discern.

Various devices and methods are known in the art for enhancing the discernability and detectability of low contrast details during viewing of X-Ray images. Exemplary patents include U.S. patent 2,436,162, to Cadenas; U.S. patent 4,004,360, to Hammond; U.S. patent 4,373,280, to Armfield; and U.S. patent 4,510,708, to Porkinchak, which are incorporated herein by reference. Generally, these patents describe light box viewing apparatus for X-Ray film, which mask off the margins of the film during viewing. Because the margins tend to have low density values but no useful image information (since they are shielded from the X-Rays during exposure of the film), masking them reduces the level of background light reaching the eye of an observer, so that dim details may more readily be seen.

In WO 96/17269, the disclosure of which is incorporated by reference, an LCD array is used to mask the portion of the viewbox outside the film or to provide an ROI mask which masks the area outside a region of interest. The back lighting of the region of interest or the

entire film is adjusted to provide optimal viewing conditions for the illuminated portion. While this type of device does improve the discemability and detectability of low contrast details in both conventional and digital radiographic films, the inherent limitations in these films, based on the way they are produced, still remains.

SUMMARY OF THE INVENTION It is an object of the present invention to provide methods for producing and viewing wide-latitude X-Ray film images.

In one aspect of the present invention, a viewing device enhances the visibility of details in a region of interest (ROI) in a wide-latitude X-Ray film image.

In one aspect of the invention, digital radiographic images are presented with an increased range of optical densities and improved contrast visibility.

The present invention is based on principles of human visual perception, which are summarized by Weber's Law. This law states, in general terms, that the ratio of the minimum perceptible stimulus contrast 6L is a linear function of the background luminance L, i.e., the ratio 6L/L is a constant. For this reason, fine, low-contrast details in an X-Ray image, particularly in dim regions of the image, are difficult or impossible for a user to distinguish against the overall high background luminance when the entire image is illuminated for viewing. Consequently, medical X-Ray films known in the art must have relatively narrow useful range and high gamma, resulting in narrow latitude limits, as described above.

In preferred embodiments of the present invention, fine, low-contrast details in an X- Ray film image are made more easily perceptible by masking portions of the film and/or by adjusting illumination conditions to optimize visibility. Preferably, the low-contrast details whose perceptibility is to be improved are contained within a ROI in the image, and those portions of the film outside the ROI are masked.

Preferably, the masking and adjustment functions are performed by using an adaptive X-Ray viewing device, preferably a digital film viewer, various aspects of which are described in U.S. patent 5,430,964 and in PCT patent publications WO 91/10152, WO 93/01564, WO 95/14949, WO 95/14950, WO 95/16934, WO96/35138, WO97/19371, WO 97/01126 and WO 97/01127, all of whose disclosures are incorporated herein by reference or in the aforementioned WO 96/17269. These devices allows a user to select a ROI within a larger image, whereupon the device masks off the remaining portion of the image outside the ROI and adjusts back illumination of the X-Ray film to a suitable level for viewing the ROI.

Preferably, the back illumination level is optimized to match the sensitivity response of the user's eye. More than one ROI may be selected and viewed simultaneously, optionally with back illumination in each ROI respectively optimized. Further preferably, background lighting in the area of the viewing device is simultaneously regulated to a level cooperative with the optimal back illumination level of the one or more ROI's.

Alternatively, other methods and devices, known in the art, for masking portions of the film during viewing may also be used.

Thus, in preferred embodiments of the present invention, X-Ray images are captured using a wide-latitude X-Ray film. Preferably, the film is a screen film combination and has an average sensitivity gradient, gamma, of less than 2.0 over its useful density range. More preferably, the film has gamma less than about 1.8, and most preferably, gamma approximately equal to 1.5 or even as low as 1.2 or 1.0.

Further preferably, the upper limit of the useful density range of the film is extended substantially beyond ND 2.0, so that dim ROI's, in high-density areas of the film image, may be observed. This extension of the useful range is made possible by masking brighter areas of the film, such as the margins of the film or, more preferably, areas outside a dim ROI, and optionally optimizing the back illumination for the remaining, unmasked area. More preferably, the upper limit of the useful density range is extended to at least ND 2.5, ND 3.0 or more, and most preferably, to approximately ND 2.8.

Using wide latitude film to produce X-Ray images, together with ROI-optimized viewing of the images, as described above, allows images to be produced having a substantially greater dynamic range than conventional medical X-Ray film. In this context, the term "dynamic range" refers to the overall number of perceptible gray levels within the film's useful range. Relatively large areas of a patient's body may thus be captured in a single X-Ray image, without loss of detail in either the high- or low-density portions of the image.

Consequently, physicians viewing these images may achieve greater diagnostic accuracy than was previously possible and may also be able to reduce patients' radiation exposure by reducing the number of images required to adequately capture details in high-contrast areas of the body.

In some preferred embodiments of the present invention, digital radiographic images are printed using a wider range of density values than heretofore, using either normal X-Ray film or, more preferably, wide latitude film. The use of the wide-latitude film allows the images to be printed with substantially less compression of their range of gray scale values than is normally required for printing on film of a type known in the art. In general, while laser multi-imager medical imaging film (film used for producing images from digital data) generally has a range of latitude from O.D. 0.2-4, this range is artificially restricted in the process of forming the image to comply with the expected method of viewing the images, i.e., with the conventional light box. In either case, low-contrast details in the digital radiographic images are more faithfully recorded and can be viewed clearly on the film medium.

Another aspect of some embodiments of the present invention relates to modifying an exposure control circuit of an X-Ray imager to take advantage of the increased film latitude.

Many types of exposure control systems and exposure determination protocols are known in the art. However, most if not all, exposure systems and protocols are geared towards a particular desired utilization of the film latitude. In accordance with a preferred embodiment of

the invention, the exposure control is adjusted so that a greater latitude of the film is used. In X-Ray imagers and protocols where a manual exposure control is used, the technician will preferably take into account the increased available film latitude.

Another aspect of some embodiments of the present invention relates to the operating voltage of the X-Ray tube itself. Generally, as the voltage increases, so does the frequency and penetration ability of the generated X-Rays. In many cases, a higher voltage is used in order to maximally utilize the available film latitude. In a preferred embodiment of the invention, the operating voltage of the X-Ray tube is decreased, preferably in conjunction with changes in the exposure control, in order to generate lower frequency X-Rays and to better utilize the available film latitude.

There is therefore provided in accordance with a preferred embodiment of the invention, a method for producing X-ray images, comprising forming an anatomical X-ray image using film having gamma less than 2.0.

Preferably, forming the X-ray image using film having gamma less than 2.0 comprises using film having gamma less than 1.8. Preferably, using film having gamma less than 1.8 comprises using film having gamma approximately equal to 1.2.

Alternatively or additionally, forming the X-ray image comprises forming the X-ray image using film the upper limit of whose useful density range is at least 2.5 ND, when viewed using controlled illumination of a region of interest within an image and an area surrounding it.

Preferably, using film the upper limit of whose useful density range is at least 2.5 ND comprises using film the upper limit of whose useful density range is at least 2.8 ND.

Preferably, using film the upper limit of whose useful density range is at least 2.8 ND comprises using film the upper limit of whose useful density range is approximately 3.0 ND, or even higher.

In a preferred embodiment of the invention, forming the anatomical X-ray image comprises acquiring digital radiographic image data and presenting the data on the film.

Alternatively, forming the anatomical X-ray image using comprises: producing the image on the film using an X-Ray imager having a controllable X-Ray output frequency range; and adjusting the output frequency range to utilize, in image areas of the film, the maximum usable density of the film.

Alternatively or additionally, forming the anatomical X-ray image using comprises:

producing the image on the film using an X-Ray imager having an automatic exposure control; and adjusting the automatic exposure control to utilize, in image areas of the film, the maximum usable density of the film.

In a preferred embodiment of the invention, the method comprises controlling illumination so as to improve conditions for viewing the image.

Preferably, controlling illumination comprises selecting a region of interest within the image and controlling illumination of the region of interest and an area surrounding it so as to improve conditions for viewing the region of interest.

Alternatively or additionally, controlling illumination comprises masking off portions of the film.

Alternatively or additionally, controlling illumination comprises adjusting an illumination intensity.

Preferably, adjusting the illumination intensity comprises adjusting a background lighting level.

There is also provided in accordance with a preferred embodiment of the invention, a method of producing a digital X-Ray image, comprising: providing a presentation film; producing the image on the film, wherein the maximum density of image areas is greater than ND 2.0.

There is also provided in accordance with a preferred embodiment of the invention, a method of producing a digital X-Ray image on a presentation film for presentation on a viewbox having adjustable masking, comprising: providing a presentation film; and producing the image on the film, wherein the maximum density of image areas on the film is chosen responsive to the maximum usable density of film on the viewbox.

There is also provided in accordance with a preferred embodiment of the invention a method of producing a X-Ray image on a film for presentation on a viewbox having adjustable masking, comprising: providing a film;

producing the image on the film using an X-Ray imager having an automatic exposure control; and adjusting the automatic exposure control to utilize, in image areas of the film, the maximum usable density of the film on the viewbox.

There is also provided in accordance with a preferred embodiment of the invention a method of producing a X-Ray image on a film for presentation on a viewbox having adjustable masking, comprising: providing a film; producing the image on the film using an X-Ray imager having a controllable X-Ray output frequency range; and adjusting the output frequency range to utilize, in image areas of the film, the maximum usable density of the film on the viewbox.

Preferably, adjusting comprises reducing said output frequency range. Preferably, adjusting comprises varying a line input voltage to said X-Ray imager.

Alternatively or additionally, adjusting comprises adjusting the frequency range to enhance soft tissue contrast.

In a preferred embodiment of the invention, where the X-Ray imager has an automatic exposure control the method comprises: adjusting the automatic exposure control in conjunction with adjusting said output frequency range to utilize, in image areas of the film, the maximum usable density of film on the viewbox.

In a preferred embodiment of the invention where the viewbox has adjustable back lighting, the maximum density of image areas is chosen responsive to the maximum usable density of masked film under the maximum lighting conditions available.

Alternatively or additionally, image areas on the film have a density greater than ND 2.0.

In a preferred embodiment of the invention, the presentation film is a photographic film and wherein producing the image comprises exposing the film to light.

Preferably, the density is greater than ND 2.5. Alternatively, the density is greater than ND 2.7.

In a preferred embodiment of the invention, the film has a gamma of less than 2.0.

Alternatively, said film has a gamma of less than 1.8. Alternatively, said film has a gamma of less than 1.2.

There is also provided in accordance with a preferred embodiment of the invention, a transparency having a digital X-Ray image produced thereon, wherein image areas on the transparency have a density greater than ND 2.0.

Preferably, image areas on the transparency have a density greater than ND 2.5.

Alternatively, image areas on the transparency have a density greater than ND 2.9.

In a preferred embodiment of the invention, the transparency is an exposed photographic film.

There is also provided in accordance with a preferred embodiment of the invention, a medical X-Ray film having a gamma of less than 2.0. Preferably, the film has a gamma of less than 1.8. Alternatively, the film has a gamma of less than 1.5. Alternatively, the film has a gamma of less than 1.2. Alternatively, the film has a gamma of approximately 1.0.

There is also provided in accordance with a preferred embodiment of the invention a medical X-Ray screen-film combination, comprising: a screen which produces light in response to X-Rays; and a film as described above, which is exposed by light produced by the screen.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: Fig. 1 is a graph schematically illustrating a sensitivity curve of a medical X-Ray film known in the art; Fig. 2 is a graph schematically illustrating a sensitivity curve of a wide-latitude X-Ray film, in accordance with a preferred embodiment of the present invention; and Fig. 3A is a schematic illustration of an X-Ray image captured using the wide-latitude film of Fig. 2, in accordance with a preferred embodiment of the present invention; and Fig. 3B schematically illustrates a method of viewing a region of interest in the X-Ray image of Fig. 3A, in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is now made to Fig. 2, which schematically illustrates a sensitivity curve of a wide-latitude X-Ray screen-film combination, for use in producing medical X-Ray images in accordance with a preferred embodiment of the present invention. By comparison with the

narrow-latitude film whose characteristics are illustrated in Fig. 1, it will be appreciated that the wide-latitude film has: (1) Larger useful density range, from ND 0.25 (or less) to about ND 3.0; (2) Lower sensitivity gradient gamma for example about 1.5; and therefore, (3) Wider latitude, from log IT 1.36 to 3.19.

Thus, while the conventional, narrow-latitude, film allows image details to be captured over less than one order of magnitude of X-Ray exposure, from log IT 1.36 to 2.05, the wide- latitude films allows details to be captured over nearly two orders of magnitude. This wide latitude is useful, for example, in imaging thick and thin sections of bone simultaneously and in capturing small contrast differences within the wide range of exposures typically encountered in chest, bone, head, extremities and other types of X-Rays. It will further be appreciated that even without extension of the useful density range, the lower value of gamma still allows a wider latitude of exposures.

Fig. 3A very schematically illustrates an X-Ray image 20 captured using wide-latitude film 22 and having characteristics as illustrated in Fig. 2. Fig. 3A (and Fig. 3B) are not meant to represent any particular anatomy, but to present how varying density values are treated in accordance with preferred embodiments of the invention. Image 20 includes bright areas 24 and 26, having low optical densities in the range ND 0.5-1.0, as well as other areas of intermediate density, such as area 28, and a dense area 30, having optical density in the range of ND 2.5. When the entire image 20 is back illuminated, bright area 24 will have approximately 100 times the average luminance of dense area 30. Fine details of diagnostic significance within the dense area, for example, local density variations on the order of 1%, as are commonly encountered in medical X-Ray images, will be "washed out" by the brightness of area 24. Under these circumstances, a physician inspecting image 20 will probably fail to observe such details.

As illustrated schematically in Fig. 3B, however, this problem is overcome by masking bright and intermediate areas 24, 26 and 28 of image 20, so that only a region of interest (ROI) 32 within dense area 30 is visible, in accordance with a preferred embodiment of the present invention. Preferably, this masking is accomplished using an X-Ray viewing device (not shown in the figures) as described in the above-mentioned U.S. 5,430,964 patent. The physician selects ROI 32, whereupon the viewing device masks all other areas of image 20.

The device increases the level of illumination of ROI 32, either automatically, in response to a measurement of the average optical density within the ROI, or under the physician's control, to a suitable level for this high-density portion of the image. Preferably the illumination is adjusted to optimally match the sensitivity response of the physician's eyes. Further preferably, the room lighting in the room where the viewing device is situated is concomitantly reduced, to enhance the visibility of the dim image.

Under these conditions, as shown in Fig. 3B, the total optical density range that the physician observes within ROI 32 varies from about ND 2.3 to ND 2.7. Under these

conditions, density variations within the ROI on the order of 1%, i.e., ND +0.02 or less, now become visible.

A similar situation, and similar (if less pronounced) improvements in visibility apply when the outside of the film is masked.

Similarly, digital radiographic images may be printed on film to obtain a greater range of densities than heretofore, by using a wide latitude conversion function for translating the X- Ray intensity values to optical film density values. For example, utilizing a density range of ND 0.1 to ND 3.0 and in particular utilizing an upper density of greater than ND 2.0, and more preferably, greater than ND 2.5, 2.8 or ND 3.0. These results can be achieved using either ordinary high gamma film or low gamma film. Since the range of printed densities can be adjusted in almost any digital imager, this methodology can result in improved discemability and detectability of low contrast details of the radiographic image without using any special equipment and even without using any special film. Preferably, the image printed in this manner is viewed using methods of ROI viewing or masked viewing as described above.

Preferably the printing density is varied in anticipation of the viewing of the image on a digital film viewer and responsive to the range of gray scales in the image portions of the image.

In particular, the ability, using masking techniques and adjustable back lighting, results in a different, more optimal mapping of the X-Ray densities to optical densities to take account of the wider range of visible optical densities. Using such a wider range of densities and masking automatically results in better visualization, i.e., improved discernability and detectability by the viewer, of low contrast details.

In contrast with digital X-Ray images of the prior art which are produced with a range of densities between ND 0.1 and 1.8, the contrast discernability of images of the present invention can be increased by between one and two orders of magnitude, or more.

Another aspect of some embodiments of the present invention relates to modifying an exposure control circuit of an X-Ray imager to take advantage of the increased film latitude.

Many types of exposure control systems and exposure determination protocols are known in the art. However, most if not all, exposure systems and protocols are geared towards a particular desired utilization of the film latitude. In accordance with a preferred embodiment of the invention, the exposure control is adjusted so that a greater latitude of the film is used. In one example, the exposure is set so that the films become "over exposed". Due to the increased effective range of the film and/or the digital film viewer, such over exposing does not adversely affect the image quality. Rather, it utilizes the increased available film latitude. It should be appreciated that such over exposing is also useful using standard type film, albeit to a lesser degree, since a film viewer, such as those described in the above referenced patents and publications, will increase the effective available film density range. In X-Ray imagers and protocols in which a manual exposure control is used, the technician will preferably take into account the increased available film latitude in making the exposure setting.

Another aspect of some embodiments of the present invention relates to the operating voltage of an X-Ray tube used to generate the X-Rays for the imaging. Generally, as the voltage increases, so do the frequency and penetration ability of the generated X-Rays. In many cases, a higher voltage is used in order to maximally utilize the available film latitude. In a preferred embodiment of the invention, the operating voltage of the X-Ray tube is decreased, preferably in conjunction with changes in the exposure control, in order to generate lower frequency X-Rays and to better utilize the available film latitude. In one example, using lower frequency X-Rays increases their absorption in soft tissue. Thus, the resulting images show more details in the soft tissues, especially near the bone. Such changes in X-Ray hardness (i.e., frequency) may also be applied in conjunction with manual exposure control.

Although the preferred embodiment described above is based on masking areas of the film outside a certain ROI, the principles of the present invention may similarly be applied using methods and devices for viewing X-Ray images in which only the margins of the film are masked, as are known in the art. For digital radiographic images, for example, it may be desirable to mask a clear border which is often left on the film. When only the margins are masked, the extension of the film's useful density range may not be as great as in the above preferred embodiment. But the reduction in the amount of background light and, preferably, optimal adjustment of the illumination of the film still allow the low-gamma film to be used, so that a wider latitude of exposures may be captured and viewed.

It will be appreciated that the preferred embodiments described above are cited by way of example, and the full scope of the invention is limited only by the claims.