X-ray devices have been used for many years in medical, security and other applications. Conventional, analog X-ray devices employ an X-ray source and photographic film, which are placed on either side of an object to be examined. The X-rays are absorbed, scattered or unaffected as they pass through the object, depending on the density of features within the object. The film captures the X-rays, thereby creating an attenuation image of the features.
While analog X-ray devices are effective for some applications, the need to use chemical-based film causes them to be disfavored in applications where multiple images need to be produced rapidly or even continuously, as is the case with security X-ray machines. In addition, analog film images cannot be easily manipulated by a computer, thereby often requiring an additional step before digital storage, analysis and/or manipulation of the X-ray data can occur.
Thus, digital X-ray technology replaces the film that is used in analog X-ray devices with a digital detector. The digital detector detects the X-rays after they have passed through the object and generates electrical signals that can be interpreted by a computer to produce a corresponding image. Thus, the image may be generated very quickly (e.g., in real-time), can be digitally stored and can even be used to generate continuous images. In addition, digital X-ray technology provides higher detective quantum efficiency (“DQE”) and larger dynamic range, as is known to those skilled in the art. Therefore, digital X-ray technology enables better quality images when compared to analog X-ray technology.
Both analog and digital X-ray technology, however, collapse 3-D features into 2-D plane images, which causes the overlap of features within the object. While 2-D images may be acceptable in some applications, this overlapping of features, commonly referred to as “structured noise,” can become problematic in some medical lesion diagnosis and other applications. Lesions tend to provide a lower level of visual contrast in 2-D X-ray images because their density is similar to that of surrounding tissue. Thus, a low contrast lesion may be hidden behind dense, and therefore higher-contrast, tissue such as bone. In the case of mammography, false negative and false positive test results are quite common because of the poor contrast provided by conventional 2-D X-ray techniques, whether analog or digital.
A conventional solution to the problem of poor image contrast is to employ a technique that enables material decomposition, where an object is subjected to X-rays of varying X-ray photon energy spectra. As a result, object features of different compositions will interact with the X-rays differently, depending on the photon energy, thereby creating images with differently-emphasized features. Typically, material decomposition (or an atomic number Z and corresponding density calculation) separates an image into two images corresponding to two base materials, where the two base materials have distinct X-ray attenuation characteristics. To perform such a material separation, two distinctive incident X-ray spectra are needed to measure the same object. Conventionally, such distinctive spectra are produced with two different tube high voltage settings, or at the same tube high voltage setting but using two different beam filtration materials. A shortcoming of such material decomposition methods is that such methods mainly work on tissues that have different X-ray attenuation coefficients, such as bone and soft tissue, and are not very sensitive to slight material composition changes.
Because the image data generated by a digital detector is digital in nature, a computer may be used to manipulate the data to create additional images that may be able to isolate desired features. For example, in conventional tomosynthesis, multiple views of an object are taken at several projection angles with a large area digital detector panel. A “shift-and-add” algorithm may then be applied on the digital data to focus on a slice depth, where out-of-plane features are de-emphasized and in-plane features are enhanced. Thus, while each slice image is still 2-D, the ability to de-emphasize out-of-plane features reduces the effects of such features so as to enable images of higher quality. A typical system implementation usually has more than 10 projection angles to further enhance the focusing accuracy.
In Computerized Axial Tomography (CAT), for example, a series of radially-oriented view projection images are taken of a patient at various angles and input into a computer. The computer applies mathematical algorithms to the image data to create additional representations of the object. As a result, the digital image data may be electronically manipulated to generate the best view for the intended application (e.g., cross-sectional images generated from projection image data). Thus, visual contrast between features within an object may be increased.
Digital X-ray technology typically requires a detector area of approximately 45 cm by 45 cm for chest radiography and 25 cm by 30 cm for mammography for properly-sized images. Often, smaller detectors cannot be used because the inevitable gaps between the detectors results in lost data and therefore poor quality images. Unfortunately, fabrication of large field digital X-ray detectors is often difficult and expensive. In addition, digital detectors having the sizes discussed above (referred to as “large field” detectors) typically have a pixel size ranging from 50 μm to 200 μm. While this level of resolution may be sufficient for some applications, other applications such as mammography require even higher resolutions for effective diagnosis. Because they are easier to manufacture, small-field digital detectors can achieve smaller pixel sizes, which therefore yields greater image resolution.
An additional shortcoming of a typical tomosynthesis device is that the device's X-ray source must be rotated, which adds to the complexity, size and cost of such a device. For example, a CAT scan machine has a gantry within which an X-ray source and large digital detector are placed on opposite sides of an object to be analyzed. The gantry rotates the X-ray source and digital detector to enable numerous images of the object that is positioned within the gantry at different view angles. The rotating gantry assembly is a complex mechanism that is very expensive to construct and maintain. Also, vibrations caused by the rotating gantry may adversely affect the alignment of the X-ray source and the digital detector, thereby necessitating a robust, vibration-reducing design that needs to be maintained very precisely. In addition, conventional CAT scan machines are very large and therefore require a specialized and substantially permanent site, as well as a high-voltage power supply. The large size of a CAT scan machine further renders it unwieldy or unusable when attempting to obtain images of smaller body parts, as is the case in mammography.