The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus of tube current modulation for radiographic imaging, e.g. computed tomography (CT).
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray- detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
As described above, CT imaging is an imaging modality predicated upon the projection of radiographic imaging, e.g. x-rays, and reconstructing an image of the subject based on the subject's attenuation of the projected x-rays. Generally, driving an x-ray source at higher current levels produces images with less noise. On the other hand, extremely low x-ray tube current levels can cause serve artifacts in the reconstructed image. X-ray tube current may be characterized as being directly related to the amount of radiographic energy received by the subject, i.e. patient dose. As such, as x-ray tube current increases, so does the radiation dose received by the subject. While higher x-ray tube current levels result in less noisy images, higher tube current levels expose the subject to increased x-ray dose. Therefore, in establishing an imaging protocol for a given subject, a trade-off must be made between tube current and subject dose. Ideally, it is preferred to use the minimum radiation dose necessary to generate a diagnostically valuable image.
A number of techniques have been developed to determine a tube current modulation profile that achieves the two desire objectives: (1) diagnostically valuable images; and (2) minimum radiation exposure to the subject. A number of these techniques are predicted upon the acquisition and analysis of scout scan data to shape a tube current modulation profile that satisfies the above objectives. Notwithstanding the advancements achieved by these known imaging techniques, it has been shown that over-exposure as well as under-exposure of radiation can still be problematic and therefore expose the subject to unnecessary radiation or result in a noisy image that therefore requires re-scanning of the subject.
A number of tube current modulation techniques have been developed to enhance waveform shaping and x-ray generation using projection data from one or more scout scans. These techniques assume that the tube current modulation profile is symmetrical throughout a single gantry rotation cycle. It has been shown, however, that the ideal tube current modulation waveform may not be symmetrical. That is, these known techniques to determine tube current modulation fail to account for the asymmetry of the ideal modulating waveform. This asymmetry results in the subject being over-exposed or under-exposed to radiation depending upon the diagnostic objectives of the scan.
It would therefore be desirable to design a method and apparatus for tube current modulation that accounts for the asymmetry of an ideal tube current modulating profile.
The present invention relates directly to a method and apparatus for tube current modulation that overcomes the aforementioned drawbacks.
Scout scan data is acquired of a subject and analyzed to determine a peak-to-peak modulation amplitude of a normalized waveform indicative of subject size and shape. The scout scan data provides a representation of patient size and shape such that an ideal tube current modulation waveform or profile may be developed. The ideal tube current modulation profile may then be sampled or approximated at various points to determine a tube current modulation profile for implementation. The differences between the tube current modulation profile for implementation and the ideal tube current modulation profile are reduced as the number of sampling points is increased.
Therefore, in accordance with one aspect of the present invention, a method of tube current modulation for radiographic data acquisition is provided. The method includes identifying a plurality of modulation points on a waveform indicative of at least one of subject size and subject shape, and determining a modulation factor at the plurality of the modulation points. The method further includes generating a modulation tube current waveform that substantially approximates the waveform indicative of at least one of subject size and subject shape based on a modulation factor at the plurality of modulation points.
According to another aspect of the present invention, a CT system is disclosed. The CT system includes a rotatable gantry having an opening to receive a subject to be scanned and a high frequency electromagnetic energy projection source configured to project a high frequency electromagnetic energy beam towards the subject. A scintillator array having a plurality of scintillator cells is also provided such that each cell is configured to detect high frequency electromagnetic energy passing through the subject. The CT system further includes a photodiode array optically coupled to the scintillator array and comprising a plurality of photodiodes to detect light output from a corresponding scintillator. A data acquisition system (DAS) is provided and connected to the photodiode array and configured to receive photodiode outputs. The CT system further includes an image reconstructor connected to the DAS and configured to reconstruct an image of the subject from the photodiode outputs received by the DAS. The CT system further includes a computer programmed to determine an ideal tube current modulation waveform to control projection of high frequency electromagnetic energy by the high frequency electromagnetic projection source for CT data acquisition of the subject. The computer is further programmed to evaluate the ideal tube current modulation waveform at a plurality of magnitudes and determine an approximate tube current modulation waveform from the plurality of magnitudes.
In accordance of with another aspect of the present invention, a computer readable storage medium having a computer program stored thereon is provided. The computer program represents a set of instructions that while executed by a computer causes the computer to command a radiographic data acquisition system to carry out a scout scan to acquire pre-scan data indicative of subject size and subject shape. The computer is also caused to determine a first tube current modulation waveform ideal for the subject size and subject shape from the pre-scan data. The set of instructions further causes the computer to evaluate a portion of the first tube current modulation waveform corresponding to 90 degrees of gantry rotation and determine a second tube current modulation waveform that approximates the first tube current modulation waveform from the portion of the first modulation waveform.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.
The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.
In the drawings:
The operating environment of the present invention is described with respect to a four-slice computed tomography (CT) system. However, it will be appreciated by those skilled in the art that the present invention is equally applicable for use with single-slice or other multi-slice configurations. Moreover, the present invention will be described with respect to the detection and conversion of x-rays. However, one skilled in the art will further appreciate that the present invention is equally applicable for the detection and conversion of other high frequency electromagnetic energy. The present invention will be described with respect to a “third generation” CT scanner, but is equally applicable with other CT systems.
Referring to
Rotation of gantry 12 and the operation of x-ray source 14 are governed by a control mechanism 26 of CT system 10. Control mechanism 26 includes an x-ray controller 28 that provides power and timing signals to an x-ray source 14 and a gantry motor controller 30 that controls the rotational speed and position of gantry 12. A data acquisition system (DAS) 32 in control mechanism 26 samples analog data from detectors 20 and converts the data to digital signals for subsequent processing. An image reconstructor 34 receives sampled and digitized x-ray data from DAS 32 and performs high speed reconstruction. The reconstructed image is applied as an input to a computer 36 which stores the image in a mass storage device 38.
Computer 36 also receives commands and scanning parameters from an operator via console 40 that has a keyboard. An associated cathode ray tube display 42 allows the operator to observe the reconstructed image and other data from computer 36. The operator supplied commands and parameters are used by computer 36 to provide control signals and information to DAS 32, x-ray controller 28 and gantry motor controller 30. In addition, computer 36 operates a table motor controller 44 which controls a motorized table 46 to position patient 22 and gantry 12. Particularly, table 46 moves portions of patient 22 through a gantry opening 48.
As shown in
In one embodiment, shown in
Switch arrays 80 and 82,
Switch arrays 80 and 82 further include a decoder (not shown) that enables, disables, or combines photodiode outputs in accordance with a desired number of slices and slice resolutions for each slice. Decoder, in one embodiment, is a decoder chip or a FET controller as known in the art. Decoder includes a plurality of output and control lines coupled to switch arrays 80 and 82 and DAS 32. In one embodiment defined as a 16 slice mode, decoder enables switch arrays 80 and 82 so that all rows of the photodiode array 52 are activated, resulting in 16 simultaneous slices of data for processing by DAS 32. Of course, many other slice combinations are possible. For example, decoder may also select from other slice modes, including one, two, and four-slice modes.
As shown in
The present invention is directed to a method and apparatus of determining a tube current modulation waveform profile that is tailored to a specific subject to reduce subject exposure to radiation without sacrificing image quality. By generating an ideal tube current modulation profile for a range of subjects to be encountered and analyzing points along that profile, the present invention is able to develop and cause generation of x-rays consistent with a radiation dose profile that substantially approximates the ideal profile for the given subject. The present invention analyzes data acquired during a CT scan or computer modeling to represent a model of the subject as an ellipse. Through experimentation and computer modeling using fundamental x-ray physics equations it can be shown that the ideal tube current modulation waveform has a shape or profile that is a function of an elliptical subject model. Specifically, the ideal tube current modulation waveform is a function of a small axis diameter and oval ratio of the subject. The oval ratio and the small axis diameter are preferably determined from a pair of scouts. It contemplated however that additional techniques in addition to using scout projections may be use to determine a model of the subject.
Referring now to
For example, the ideal tube current modulation waveform 84 may be sampled at a number of points and evaluated thereat to determine the approximate tube current modulation waveform 86. In the example illustrated in
As shown in
Referring now to
P10=ao+a1O+a2D+a3OD+a4O2+a5D2+a6O2D+a7OD2+a8D3 Eqn. 1.
Further, the modulation factor at 90 which corresponds to 100 percent of the peak-to-peak modulation amplitude may be defined by:
P100=b0+b1O+b2D+b3OD+b4O2+b5D2+b6O2D+b7OD2+b8D3 Eqn. 2;
In addition, to avoid over-exposure or under-exposure, it is contemplated that an error margin may be used to offset the values determined at the evaluated modulation points. For example, in one embodiment, the calculated or determined values are temporally (in x) offset by a negative amount or in magnitude (in y) by a positive amount. A typically value of adjustment is 0.02 and is applied in (y) where the waveform slope at the particular modulation point is less than one and is applied in (x) when the waveform slope at a particular modulation point is greater than one. The high frequency electromagnetic energy projection source will be commanded to generate x-rays or other radiographic energy in a matter as defined by the tube current modulation waveform 86 that is a linear approximation of the ideal tube current modulation waveform.
It also contemplated that the approximate tube current modulation waveform may be determined by characterizing the low tube current flat region of the ideal waveform with a single reference point, e.g. ten percent of peak-to-peak modulation and the remainder of the waveform with a polynomial expression of appropriate degree e.g. a four point function. The polynomial expression may be fitted as a function of minimum diameter and oval ratio in a manner similar to that described alone with respect to piecewise approximation. Additionally, other functions may be used such as an appropriate sinusoidal, elliptical, circular, parabolic or other appropriate analytic continuous function for which the parameters are fitted as a function of minimum diameter and/or oval ratio.
The technique described above is designed to reduce overall tube use and radiation exposure to a subject without sacrificing image quality. The above technique is also applied with individualized scan protocols such that the physical variations of a scan population are taken into account with each scan. It is contemplated however that various waveforms may be developed to account for variations in scan population and stored in an accessible database. During scanner operation, the oval ratio and diameter of the patient are estimated from scout scans and functions or relationships such as those defined above by equations 1 and 2 are used to generate the appropriate tube current modulation profile for controlling x-ray generation for CT data acquisition for the patient. One skilled in the art will appreciate that other functions or equations other than those specifically identified above may be used to generate the appropriate tube current modulation profile.
Referring now to
The present invention has been described with respect to an imaging technique for optimizing radiation dose during CT data acquisition that determines ideal modulation waveforms as a function of projection angle for a set of phantoms. It is understood that the set of phantoms may be scanned and the ideal modulation waveforms may be determined from axial attenuation data. Coefficients at modulation points along the ideal modulation waveforms are determined and capture or represent the relationship between the size and shape of a phantom and an appropriate modulation waveform shape. As such, when scanning a subject, a scout scan, taken at a fixed angle of zero and/or ninety degrees, is used to determine a projection area and projection measure. From the determined coefficients as well as the projection area and projection measure, an appropriate tube current modulation waveform to use for a specific subject to optimize radiation dose may be determined as a function of z-axis subject position and gantry angle during scanning.
Therefore, in accordance with one embodiment of the present invention, a method of tube current modulation for radiographic data acquisition is provided. The method includes identifying a plurality of modulation points on a waveform indicative of at least one of subject size and subject shape, and determining a modulation factor at the plurality of the modulation points. The method further includes generating a modulation tube current waveform that substantially approximates the waveform indicative of at least one of subject size and subject shape based on a modulation factor at the plurality of modulation points.
According to another embodiment of the present invention, a CT system is disclosed. The CT system includes a rotatable gantry having an opening to receive a subject to be scanned and a high frequency electromagnetic energy projection source configured to project a high frequency electromagnetic energy beam towards the subject. A scintillator array having a plurality of scintillator cells is also provided such that each cell is configured to detect high frequency electromagnetic energy passing through the subject. The CT system further includes a photodiode array optically coupled to the scintillator array and comprising a plurality of photodiodes to detect light output from a corresponding scintillator. A data acquisition system (DAS) is provided and connected to the photodiode array and configured to receive photodiode outputs. The CT system further includes an image reconstructor connected to the DAS and configured to reconstruct an image of the subject from the photodiode outputs received by the DAS. The CT system further includes a computer programmed to determine an ideal tube current modulation waveform to control projection of high frequency electromagnetic energy by the high frequency electromagnetic projection source for CT data acquisition of the subject. The computer is further programmed to evaluate the ideal tube current modulation waveform at a plurality of magnitudes and determine an approximate tube current modulation waveform from the plurality of magnitudes.
In accordance of with another embodiment of the present invention, a computer readable storage medium having a computer program stored thereon is provided. The computer program represents a set of instructions that while executed by a computer causes the computer to command a radiographic data acquisition system to carry out a scout scan to acquire pre-scan data indicative of subject size and subject shape. The computer is also caused to determine a first tube current modulation waveform ideal for the subject size and subject shape from the pre-scan data. The set of instructions further causes the computer to evaluate a portion of the first tube current modulation waveform corresponding to 90 degrees of gantry rotation and determine a second tube current modulation waveform that approximates the first tube current modulation waveform from the portion of the first modulation waveform.
The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.