X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2002-08-30
2003-10-28
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S094000
Reexamination Certificate
active
06639966
ABSTRACT:
The present application hereby claims priority under 35 U.S.C. §119 on German patent publication number 10142605.4 filed Aug. 31, 2001, the entire contents of which are hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
Imaging data from an examined test object can be obtained with modern medical diagnostic techniques, for example the computed tomography (CT). As a rule, the test object to be examined is a patient.
The imaging data are generally obtained in digital form and are thus accessible for the digital data processing. Typical operations of the digital data processing are, for example, the intensification, super-imposition or filtering.
In addition to the desired imaging information from the examined test object, the imaging data obtained by use of imaging systems for medical diagnostics normally contain information that can be traced back to interfering influences, such as parasitic induction, during the testing procedure.
In general, the problems are divided into two different categories that reduce the quality of the obtained imaging data: image background noise and artifacts.
In the following, these two problems are to be explained further with the example of the computed tomography.
The image background noise in turn can be divided into several causes.
The main share of the image background noise results from the quantum noise, caused by the fact that each emission of rays is composed of a finite quantum number, so that the quantum number that is measured always fluctuates with normal distribution around an average value.
Further causes for the image background noise are the monochromatic quantum, which are for the most part not exact, of X-ray tubes that can realistically be produced, as well as the scattering of rays based on the interaction between the X-ray radiation used and the electron sheath of atoms during the transmission through the test object.
The artifacts can also be divided further.
Aliasing, partial-volume artifacts, age-hardened artifacts, as well as movement artifacts are typical types of artifacts, the appearance of which in particular depends on the geometry or a movement by the test object.
Effects corresponding to the image background noise, described in the above with the computed tomography, and the artifacts can also be found in other imaging system for medical diagnostics.
Ring artifacts are a special form of the artifacts, caused primarily by the imaging systems themselves that are used for the medical diagnostics.
Imaging systems for medical diagnostics which use (for example in modern computed tomography machines) several detectors (in particular also several detector layers) provide the option of insufficiently calibrating the individual detectors. This is understood to mean that different detectors will measure the same amount of weakening of the rays penetrating the test object differently.
With an insufficient calibration of the individual detectors, for example in a computed tomography machine, the obtained imaging data show concentric rings around the rotational center of the computed tomography machine because of the rotation of the radiation source and the detectors around the test object during the measuring operation. These rings do not have an actual relationship to the test object that is examined. Interferences of this type in the imaging data are called ring artifacts.
Since the manufacturer of an imaging system for medical diagnostics normally calibrates the detectors, the frequency and clarity of ring artifacts is an indicator for the presentation quality of such a system. A low number and low clarity of the ring artifacts indicate great care taken by the manufacturer during the testing and calibration of the imaging system and thus a high quality.
Imaging systems of this type for medical diagnostics are therefore subjected to a final testing by the manufacturer before the system is delivered. Within the framework of this final testing, at least one test image is created for each detector layer with the aid of a test body having a known absorption coefficient (such a test body is also called a “phantom”). As with many other calibration methods, the number of tests performed (test images created) also increases the accuracy. The obtained test images are then checked visually by specially trained personnel for the presence of ring artifacts and the individual detectors are calibrate accordingly.
The disadvantage of this known method, however, is its high cost. Modern CT systems, for example, may require the visual checking of several hundred test images for each system.
German Unexamined Published Patent Application No. 31 07 170 A1 discloses a device for reducing ring-shaped artifacts arranged concentrically around the center of the area to be examined by a computed tomography machine. These artifacts are superimposed on the reconstruction image of the computed tomography machine.
A smoothing device, such as a filter, is proposed for this which smoothes the image values of the reconstruction image in an azimuth direction, relative to the center of the examined area (for example through low-pass filtering). As a result, the ring artifacts are emphasized more in the reconstruction image.
The ring artifacts are subsequently detected in the reconstruction image in a ring-detection stage through filtering along straight lines through the center of the examined area. A median filter, with a filter length that is double the width of the artifacts plus an image point (pixel) is suggested for the filter. By subtracting the image values filtered with the median filter from the smoothed image values, correction image values are computed for all image values.
The computed corrected image values are subsequently compared to a minimum threshold value and a maximum threshold value. On the one hand, this serves to suppress the ring artifacts that cannot be detected visually. On the other hand, it ensures that elements not corresponding to ring artifacts, but some other type of object structure (e.g. a bone structure), are not corrected by mistake.
The disadvantage of the known method and device is that in order to compare the detected ring artifacts to a previously selected criterion, a meridian filtering must first be carried out. Meridian filtering operations of this type are involved computations since they require a sorting function and thus iterative steps. Furthermore, it is necessary to have prior knowledge of the width of the artifacts to determine the filter length.
Furthermore, the correction value used for evaluating the visual detectability of the detected ring artifacts according to German Patent 31 07 170 A1 is a poor indicator for the visual detectability of the detected ring artifacts. This is because it is only an indicator for the expansion of the image values within the filter length and is not a direct indicator of their clarity.
In addition, the meridian filter according to this prior art is used for detecting the ring artifacts as well as for evaluating them. This results in a high dependence on the selected meridian filter as well as the selected filter length. Thus, the result is only meaningful to a limited degree as it relates to the visual detection of the ring artifacts.
SUMMARY OF THE INVENTION
It is the object of an embodiment of the present invention to provide a method and a device for checking the presentation quality of an imaging system for medical diagnostics with respect to ring artifacts, for which the visual detectability of ring artifacts can be evaluated reliably and with little expenditure.
According to an embodiment of the present invention, a method for checking the presentation quality of imaging systems for medical diagnostics with respect to ring artifacts is divided into the following steps:
Creating at least one test image; Defining concentric circles in the at least one test image, wherein the definition of the concentric circles in the at least one test image occurs around a rotational center which is fixed in the test image by the imaging system for medical diagnosti
Lautenschlager Markus
Stierstorfer Karl
Bruce David V.
Harness & Dickey & Pierce P.L.C.
Siemens Aktiengesellschaft
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