As is well known, an X-ray beam emanating from an X-ray tube may be passed through a selected portion of a patient to produce a shadow image of the internal structure of the patient on X-ray film. For quality assurance testing of X-ray imaging equipment and for teaching aids in training technicians, nurses, and the like in how to use X-ray imaging equipment, X-ray phantoms are well known. Prior art X-ray phantoms are available in a number of variations. Selected phantoms comprise plastic replicas of the human body or specific body portions, whereas other phantoms comprise actual human bones cast in plastic.
More particularly, an X-ray phantom is typically composed of a material that mimics human tissue in its ability to produce absorption and scattering of radiation, i.e., mimics radiopacity/radiolucency characteristics of human anatomy. The ability to absorb and scatter radiation is expressed by the attenuation coefficient, which is a function of chemical elements of which the material is composed and the spectrum of energies used in the X-ray examination. Variations of attenuation coefficients and thicknesses among materials produce contrast in an X-ray image. Two substances with the same attenuation coefficient and thickness will similarly absorb and scatter X-rays under given imaging conditions and will produce the same contrast with respect to a third substance during an X-ray examination. Many prior art phantoms include calibration patterns or test patterns consisting of metal objects (for instance, solid circles, hollow circles, and/or parallel lines) for assisting the technician in providing adjustments in the X-ray system with respect to optical density, spatial resolution, and contrast-detail.
One example of an X-ray phantom is shown in U.S. Pat. No. 4,794,631 to Ridge. This patent describes a cardiovascular phantom including a sandwich of circuit boards and plates encapsulated within an acrylic block. Specifically, 6 circuit boards have a lead coating that is etched away to provide the arborescent appearance of major arteries, and also each circuit board contains a space therein to allow for the lung field. A copper plate is included in the phantom and has an opening therein analogous to the spaces in the circuit boards for creating the lung field, and a copper diaphragm plate is provided.
An X-ray phantom is described in U.S. Pat. No. 4,126,789 to Vogl et al. This patent describes a phantom having a radiolucent sealed case of a high impact plastic material, such as polycarbonate, and containing body part objects, such as a genuine femur, suspended in salt water. Other objects in the phantom include traditional X-ray machine evaluation and calibration devices, such as a resolution test pattern of spaced metal bars (i.e., parallel lines), a step wedge penetrometer, and a wire mesh test pattern.
Another phantom is shown in U.S. Pat. No. 4,323,782 to Riihimaki et al. which describes a human skull phantom that includes an aluminum cylinder with a wall thickness of about 3 mm so that the absorption in the cylinder corresponds to the absorption in the bones of a human skull. The cylinder has on its respective ends two end caps made of acrylic plastic. Smaller vessels, also cylindrically shaped, are contained within the cylinder. An image can be taken of the phantom with an X-ray computed tomography system in which can be seen small circles corresponding to the small vessels. Then, another image can be made by placing the phantom into another X-ray computed tomography system. The resultant images are compared with each other to see whether an equal number of the smaller vessels can be seen, and if so, then the absorption resolutions of both tomography systems are determined to be equally good.
An angiographic X-ray phantom for use with digital X-ray equipment is disclosed in U.S. Pat. No. 4,649,561 to Arnold. The phantom is representative of human tissue containing variable concentrations of iodine and serves as a test device for assessing the performance of X-ray imaging systems such as digital subtraction angiographic apparatus. The phantom contains a test pattern including disc details and elongated cylindrical details, with all details being provided in varying iodine concentrations and the elongated details also being provided in varying diameter cross-sections simulating arterial and venous configurations.
Also of interest with respect to X-ray phantoms, typically including test patterns, are U.S. Pat. No. 4,097,793 to Shapiro et al.; U.S. Pat. No. 4,550,422 to VanPelt et al.; U.S. Pat. No. 4,578,767 to Shapiro; U.S. Pat. No. 4,638,502 to Yaffe; U.S. Pat. No. 4,759,045 to Lasky; U.S. Pat. No. 4,818,943 to Chandra; U.S. Pat. No. 5,063,583 to Galkin; U.S. Pat. No. 5,236,363 to Sandrik et al.; and U.S. Pat. No. 5,276,726 to Galkin.
It should be noted that all of the above-described phantoms except Arnold are designed for use with analogue X-ray systems. Even though the X-ray phantom described in the above-mentioned U.S. Pat. No. 4,649,561 to Arnold is intended for use with a digital X-ray system, the parts of that phantom do not radiographically resemble human anatomy parts. Thus, none of the prior art X-ray phantoms addresses the increasing need, which has resulted from the increasing use of digital imaging X-ray systems with automated, examination-specific, and content-sensitive image processing techniques, for a class of phantoms that radiographically resemble human anatomy and provide X-ray images with features useful for objective quality assurance evaluation of the digital X-ray imaging system.
To be more specific, conventional analogue projection radiographic imaging systems use a screen-film sandwich to detect X-ray photons and to create a radiographic image on the film. For a given incident X-ray energy, these conventional image receptors possess fixed characteristics including X-ray sensitivity, latitude (i.e., dynamic range, which is the range of X-ray exposures that can be accurately recorded in an image), contrast, spatial resolution, and noise. Conventional receptors, therefore, can be thought of as static systems in which the properties of the imaging system are not dependent on image content. Additionally, for a given X-ray energy (kilovoltage peak, abbreviated herein as kVp), only a narrow range of acceptable exposure settings exist that will produce an acceptable film image. An under-exposed conventional film is too light (transparent) whereas an over-exposed film is too dark. Both conditions make the resultant films undesirable for diagnostic use.
Because the properties of conventional analogue radiography systems do not depend on image content, various test objects that do not produce a radiographic image shape that resembles the corresponding human anatomy part, as can be seen from a review of the above-described patents involving X-ray phantoms, often have been used. If the phantom contains a test pattern, typically used are step wedges (such as stair-step-like objects of increasing thickness which, when radiographed, produce a series of nearly-uniform film areas with decreasing optical densities), line resolution phantoms that can be visually evaluated to estimate the smallest visible structure on film, and contrast detail phantoms consisting of objects of decreasing size and contrast (circles and/or parallel lines) which can be visually evaluated to assess barely detectable features in the image on the film.
Unlike analogue X-ray imaging systems, in which the film serves as both detector and display medium, digital X-ray imaging systems employ a multi-stage process that separates X-ray detection from image display. In digital systems, X-rays are first detected by a receptor (typically a wide-latitude receptor), and the image is converted to a matrix of digital values (known in the art as pixels) which correspond to X-ray intensity at each position in the image. Later, the digital image data are computer processed and then transformed back into analogue form for display, typically on laser-printed film or on a video monitor such as at a diagnostic workstation.
The separation of digital image detection and display makes possible a wide variety of capabilities that are not possible with analogue screen-film systems, including, but not limited to:
(1) The capability to use a wide range of X-ray exposure levels in the acquisition of an image, without causing the final film image to be too dark or too light because digital systems typically employ a wide-latitude image receptor; PA1 (2) The capability to analyze automatically the digital image data in an effort to identify the useful exposure range so as to optimize contrast resolution that affects final image quality; and PA1 (3) The capability to digitally process the image in different ways, for instance, enhancing contrast and/or spatial resolution, reducing noise, and the like, for optimum visualization of key features as needed for specific diagnostic tests.
Each of the three above-mentioned capabilities of digital X-ray imaging systems can change the appearance of the radiographic image produced from the phantom. For example, variations in X-ray exposure level produce changes in the signal-to-noise ratio in the image, which can ultimately affect object detectability. Also, image processing can change the image appearance in any number of subtle ways, depending on the operations performed.
Commercial digital radiographic imaging systems are programmed to perform these operations conditionally, in ways that depend on the nature of the image itself, i.e., the image content. Typically, the behavior of the X-ray imaging system is determined by the histogram of the digital data within the full image or a prescribed image subregion. Since the histogram of human anatomy is quite different from that of most non-anatomic radiographic test pattern objects, images of anatomy and non-anatomic test pattern objects can be expected to be processed quite differently. Accordingly, it is not possible from examining the properties of the resultant film of a radiograph of a step wedge to infer anything about how a subsequent chest image produced by the same X-ray imaging system would appear.
Therefore, a need exists for the ability to evaluate image quality, particularly digital image quality, and to evaluate the ability of an X-ray imaging system, particularly a digital imaging system, to produce a clinical image. The X-ray phantom should resemble human anatomy in both average radiographic density and appearances (possessing both a shape regionally similar to that of the corresponding real human anatomy portion and a histogram similar to that of the corresponding real human anatomy portion), and also should contain designated test pattern region(s) for optical density measurements, spatial resolution, and contrast-detail measurements, all without unduly changing the "human-like" qualities of the resultant overall image. Applicants have developed a phantom to meet the long-felt need for such a phantom.