Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
1999-02-03
2001-06-26
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S155000
Reexamination Certificate
active
06252531
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a data acquisition system (DAS), and more particularly, to a data acquisition system using oversampled, delta-sigma analog-to-digital (A/D) converters specifically adapted for use in computed tomography (CT) scanners.
BACKGROUND OF THE INVENTION
Certain signal processing techniques involve the simultaneous detection of a plurality of analog information signals for the purpose of acquiring data represented by the signals. For example, certain commercially available medical imaging systems such as CT scanners are used to image internal features of an object under view by exposing the object to a preselected amount and type of radiation. Detectors sense radiation from the object and generate analog signals representative of internal features of the object.
In the example of CT scanners, those of the third generation type include an X-ray source and X-ray detector system secured respectively on diametrically opposite sides of an annular-shaped disk. The latter is rotatably mounted within a gantry support so that during a scan the disk continuously rotates about a rotation axis while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.
The detector system typically includes an array of detectors disposed as a single row in the shape of an arc of a circle having a center of curvature at the point, referred to as the “focal spot,” where the radiation emanates from the X-ray source. The X-ray source and array of detectors are all positioned so that the X-ray paths between the source and each detector all lie in the same plane, referred to as the “slice plane” or “scanning plane”, normal to the rotation axis of the disk. The X-rays that are detected by a single detector at a measuring instant during a scan is considered a “ray. ” Because the ray paths originate from substantially a point source and extend at different angles to the detectors, the ray paths resemble a fan, and thus the term “fan” beam is frequently used to describe all of the ray paths at any one instant of time. The ray is partially attenuated by all the mass in its path so as to generate a single intensity measurement as a function of the attenuation, and thus the density of the mass in that path. Projection views, i.e., the X-ray intensity measurements, are typically done at each of a plurality of angular positions of the disk.
In fourth generation CT scanners the detection system comprises a circular array of detectors secured on and at equiangular positions around the gantry support, equidistant from the rotation center of the disk so that the source rotates relative to the detectors. A fan beam is defined as the ray paths from the rotating source to each detector where the point of convergence of each fan beam is the corresponding detector.
The detectors used in CT scanners are usually either of the solid state type, such as cadmium tungstate detectors each having a scintillation crystal or layer of ceramic material and a photodiode, or of the gas type, such as Xenon detectors. The X-ray source can provide a continuous wave or a pulsed X-ray beam.
An image reconstructed from data acquired at all of the projection angles during a scan of both types of machines will be a slice along the scanning plane through the object being scanned. In order to “reconstruct” or “back project” a density image of the section or “slice” of the object in the defined scanning plane, the image is typically reconstructed in a pixel array, wherein each pixel in the array is attributed a value representative of the attenuation of all of the rays that pass through its corresponding position in the scanning plane during a scan. As the source and detectors rotate around the object, rays penetrate the object from different directions, or projection angles, passing through different combinations of pixel locations. The density distribution of the object in the slice plane is mathematically generated from these measurements, and the brightness value of each pixel is set to represent that distribution. The result is an array of pixels of differing values which represents a density image of the slice plane.
While the signals generated by the detectors through the series of readings provide the required data to generate the 2-dimensional image, acquiring and processing the data can pose various design problems. For example, a large number of detectors must be used for each set of readings taken for each projection view, and a large number of projection views must be taken during a scan in order to create a detailed image with sufficient resolution (a typical third generation CT scanner contains on the order of 350 to 1000 detectors, with, for example, 600 to 3000 projection views being taken within a period of 2 seconds resulting in data values, i.e., detector readings, on the order of one million, although these numbers can clearly vary). The resolution of the image created can be improved by increasing the number of detectors used and/or sets of readings, i.e., projection views, utilized. This increases the amount of data acquired and, therefore, the amount of signal information that must be processed. Accordingly, with over approximately one million data values acquired during a typical CT scan the analog signals acquired in each set of readings or views must be quickly and efficiently digitized so that computer processing can be utilized to provide relatively fast results.
Thus, in order to process the data received from the array of detectors, a data acquisition system (DAS) is used to process the data through multiple channels substantially all at the same time. The DAS includes means for converting the plurality of sets of data received from the detectors as analog signals during each projection view into corresponding digital signals so that the latter can be processed by a digital signal processor (DSP). However, various problems exist with respect to current DAS designs. For example, many DASs required for CT scanning require high digitization resolution on the order of one million (10
6
) to one or better, i.e., 20 bits or more. While many A/D converter techniques are known, some, such as successive approximation A/D conversion, provide inadequate signal resolution and therefore are incapable of achieving a digital signal of 20 bits or more. In this regard A/D converters using integrators have been designed to provide the required high resolution.
Where DASs are used with a continuous wave X-ray source, any modulation in the X-ray source during a scan over time will create errors. Problems are also encountered when the DAS is used with a pulse X-ray source. For example, artifacts due to variable afterglow readings of X-ray pulses are not necessarily treated identically for all of the channels. These interpulse values have an overall effect on the values of the detected analog signals corresponding to the detected X-rays in response to the pulses of X-rays from the source, and the interpulse values should be taken into consideration to provide accurate readings. In addition current leakage of certain storage devices, disposed in each channel, for storing temporarily stored information can create errors in the signal conversion.
While some of these problems can be overcome by using a separate A/D converter for each channel, until recently such an approach has been impractical because of its prohibitive cost. With the dynamic range of the analog signals provided in each channel on the order of 10
6
to 1, a linear ramp A/D converter is also impractical. One DAS which overcomes or at least minimizes many of the above problems is described in U.S. Pat. No. 5,138,552, patented in the name of Hans J. Weedon and Enrico Dolazza, issued Aug. 11, 1992 and assigned to the present assignee (the “Weedon et al. Patent). The latter patent describes a DAS using non-linear digitization intervals by employing a non-linear ramp A/D converter.
In addition, CT scanners use detectors providing low level output currents. In general solid
Gordon Bernard M.
Miller Mark H.
Poulo Louis R.
Weedon Hans
Analogic Corporation
Jean-Pierre Peguy
McDermott & Will & Emery
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