Image analysis – Applications – Biomedical applications
Reexamination Certificate
1999-07-15
2003-10-14
Mehta, Bhavesh M. (Department: 2625)
Image analysis
Applications
Biomedical applications
C358S519000, C358S520000, C382S162000, C382S274000, C600S443000, C600S458000
Reexamination Certificate
active
06633657
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention generally relates to medical diagnostic imaging and in particular to a method and apparatus for adjusting the dynamic range of a digital medical image to be presented.
X-ray imaging has long been an accepted medical diagnostic tool. X-ray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial and abdominal images that often include the information necessary for a doctor to make an accurate diagnosis. When having a thoracic x-ray image taken, for example, a patient stands with his or her chest against an x-ray sensor as an x-ray technologist positions the x-ray sensor and an x-ray source at an appropriate height. The x-ray sensor then detects the x-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system scans out the detected x-ray energy and prepares a corresponding diagnostic image on a display. Optionally, the x-ray sensor may be a solid state digital image detector. If the x-ray sensor is a conventional screen/film configuration, the screen converts the x-rays to light, to which the film is exposed.
In conventional radiographic imaging systems, the x-ray technique is chosen by the operator. An operator or an automatic exposure control system selects or determines a desired exposure for the selected screen/film configuration in order to obtain a desired optical density of the exposed film. The optical density represents the “lightness” or “darkness” of the resulting film once exposed to x-rays. By controlling the manner (e.g., time, orientation, etc.) of exposure of the detector or screen/film to x-rays, the film lightness or darkness is varied. It is preferable to achieve a consistent optical density from one exposure to the next in order to facilitate diagnosis and examination by physicians when analyzing radiographic images. A screen/film combination system does not have a linear response to x-ray exposure, but instead is non-linear and defined by an x-ray sensitometry curve of the particular screen/film combination. The sensitometry curve relates input exposure to a resulting optical density. The non-linear response of a screen/film combination affords a desirable image contrast at exposure levels falling in the middle of the exposure range. The non-linear response compresses image contrast outside of the middle range. The dynamic range of a screen/film combination system is “fixed”. The operator (or via automatic exposure control system) attempts to regulate the transmitted x-ray exposure of the diagnostic regions to fall within the limited middle dynamic range of the screen/film. Different exposures arise from one patient to the next, from one film type to the next, from one medical imaging system to the next, from one orientation to the next and the like.
In the past, it has been quite difficult to maintain a consistent optical density from one exposure to the next (e.g., patient to patient, film to film, system to system, patient angle to patient angle) due to inherent differences. For instance, each patient has a slightly different size and anatomy which causes the internal organs of the patient to have different x-ray attenuation and may be located at different positions relative to the detector or screen/film. For example, when attempting to obtain an x-ray of a chest image, every patient's lungs and rib cage are of a different size. The resulting variance in x-ray attenuation creates a large variance between exposures. Variation in patient position and orientation further create variance between exposures. Variance between exposures may result in the diagnostic exposure range for a particular exposure to fall outside of the detector's “desired” exposure range and compress the diagnostic dynamic range. The resultant optical density may become variable due to particular patient pathology, to foreign objects within a patient (e.g., pacemakers and the like), to differences in patient thickness, as well as to differences in x-ray acquisition parameters (e.g., x-ray energy, dose, exposure time and the like).
automatic exposure control is commonly used with radiographic systems in an attempt to control the exposure range and the resultant optical density of the exposed film. Automatic exposure control systems typically use an x-ray sensitive ion chamber located proximate to the detector or screen/film combination and proximate to the patient. For instance, the operator positions the patient so that his/her lungs are proximate the selected ion chamber for a chest exam. The automatic exposure control terminates the x-ray exposure when a preset dose is measured.
However, automatic exposure control systems have experienced difficulties. In particular, the exact position of an individual patient's lung is unknown while the ion chamber position is fixed. Hence, different patients continue to create a large variance in the resulting exposure to the detector. For instance, the ion chambers may not actually be located proximate certain patient's lungs. When an ion chamber is located proximate an anatomy other than the lung, the automatic exposure control terminates exposure based on inaccurate measurements. A certain percentage of chest films result in creation of either too dark or too light of an image. When the image is too dark or too light, it may be necessary to repeat the x-ray examination to retake the medical image. It is quite time consuming to retake medical images. Film development may require a relatively long period of time, such as five to fifteen minutes, during which the patient may leave the image acquisition area.
Further, a resulting presentation of a medical image is determined by the selection of the type of detector or film/screen configuration in combination with the desired x-ray technique. Different types of detectors and screens/film configurations experience different amounts of image contrast, different signal to noise ratios (SNR) and different dynamic ranges. In the past, SNR has been modified by varying the input exposure. However, to maintain a constant optical density as one varies the exposure level, the detector, film or screen types must be changed to match the expected dynamic range. It is quite cumbersome to change detectors, screens or films, and thus it is rarely done.
More recently, digital detectors have been proposed for use with radiographic imaging. Digital detectors afford a significantly greater dynamic range than convention screen/film configurations, typically as much as two to three times greater. Heretofore, the automatic exposure control and/or operator must still be relied upon to limit the exposure to the digital detector to account for the detectors greater, yet finite, dynamic range. The digitally detected image may be image processed to attain the expected film-like sensitometry curve. That is, the dynamic range is mapped via a contrast look-up table to achieve the desired contrast and optical densities when printed or viewed.
Moreover, with the advent of electronics and digital technology, it has become desirable to offer “soft copies” of medical images to physicians for examination. Soft copies refer to the display of medical images on a television, computer screen and the like after the medical image has been image processed. The soft copy medical images, in many instances, replace a hard copy of the medical image, such as previously provided on exposed x-ray film. Hard copy x-ray films are held up to a back-lit light for examination by the physician. Electronic medical images may be routed more quickly and provide grater viewing flexibility than hard copies for an examining physician.
However, soft copy medical images have experienced certain drawbacks. For instance, the contrast or optical density of medical images when displayed electronically may significantly differ from the contrast or optical dens
Kump Kenneth Scott
Xue Ping
Dellapenna Michael A.
Desire Gregory
McAndrews Held & Malloy Ltd.
Mehta Bhavesh M.
Vogel Peter J.
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