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EP0107253A1
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Method of and device for determining the distribution of radiation attenuation in a body section
Assignee: Koninklijke Philips NV
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Filing: 1983-10-19
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Granted: —
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Expires: 2003-10-19
Abstract
Abstract Using the method and device for computer tomography in accordance with the invention, always the same filtering operation can be performed for image reconstruction, regardless of the method used to measure the measurement data: parallel (whether equidistant or not) or divergent (whether equiangular or not), provided that before and after the filtering of the measurement data there is performed a multiplication by a pre- conditioning factor and a post-conditioning factor, respectively. The pre-conditioning and post-conditioning factors are dependent on the type of reconstruction and on the position of a measurement data value in the group of measurement data filtered or to be filtered. The fact that the same filter can be used is important notably for computer tomography apparatus in which the (convolution) filter is embodied in an electronic circuit (by means of hardware). The back-projection device of a device in accordance with the invention is suitable for the back-projection of measurement data along either divergent measurement paths or parallel measurement paths for which it requires only the modification of a few constants applied to the back-projection device.
Inventors
- 👤 Willem Harry Marie Deckers
- 👤 Johannes Catharina Antonius Op De Beek
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-10-25 13:26
CPC Classifications
G; G06; G06T; G06T12/00; G06T12/20; G06T2211/00; G06T2211/40; G06T2211/421; Y; Y02; Y02A; Y02A90/00; Y02A90/10; Y10; Y10S; Y10S378/00; Y10S378/901
Claims Analysis
Total Claims: 13
Claim 1
Independent Claim
266 words
A method of determining the distribution of radiation attenuation in a body section which is irradiated from a plurality of directions by a flat, fan-shaped beam of penetrating radiation which diverges from a source point, all directions extending through a central point and being situated in the body section, a group of measurement data being determined, for each measurement direction, which is a measure of the radiation attenuation along an associated group of divergent measurement paths, each group of measurement data being either convolved with a group of convolution numbers, or Fourier transformed, filtered and back-transformed, after which the measurement data thus treated is back-projected into a matrix of pixels, characterized in that prior to convolution or Fourier transformation, each measurement data value of a group is multiplied by a pre-conditioning factor which is determined by the cosine of the angle included between a central line connecting the point source to the central point, and the measurement path associated with the measurement data value to be pre-conditioned, after which the pre-conditioned measurement data is either convolved or Fourier transformed, filtered and inverse-Fourier transformed as though the pre-conditioned measurement data had been measured along parallel measurement paths, the convolved or filtered measurement data being multiplied by a post-conditioning factor which is inversely proportional to the variation and directly proportional to the square of the cosine of the angle between the central ray and the measurement path associated with the convolved or filtered measurement data, after which the measurement data values thus post-conditioned are back-projected along the same measurement paths along which those measurement data values were determined.
Claim 2
Independent Claim
266 words
A method of determining the distribution of radiation attenuation in a body section which is irradiated from a plurality of directions by a flat, fan-shaped beam of penetrating radiation which diverges from a source point, all directions extending through a central point and being situated in the body section, a group of measurement data being determined, for each measurement direction, which is a measure of the radiation attenuation along an associated group of divergent measurement paths, each group of measurement data being either convolved with a group of convolution numbers, or Fourier transformed, filtered and back-transformed, after which the measurement data thus treated is back-projected into a matrix of pixels, characterized in that prior to convolution or Fourier transformation, each measurement data value of a group is multiplied by a pre-conditioning factor which is determined by the cosine of the angle included between a central line connecting the point source to the central point, and the measurement path associated with the measurement data value to be pre-conditioned, after which the pre-conditioned measurement data is either convolved or Fourier transformed, filtered and inverse-Fourier transformed as though the pre-conditioned measurement data had been measured along parallel measurement paths, the convolved or filtered measurement data being multiplied by a post-conditioning factor which is inversely proportional to the variation and directly proportional to the square of the cosine of the angle between the central ray and the measurement path associated with the convolved or filtered measurement data, after which the measurement data values thus post-conditioned are back-projected along the same measurement paths along which those measurement data values were determined.
Claim 3
Dependent Claim
65 words
A method as claimed in Claim 1, characterized in that the measurement data values to be back-projected are multiplied by a distance factor which is inversely proportional to the distance between the source point and a projection point on the central line which is determined by the projection on the central line of a pixel for which a contribution to the radiation attenuation is calculated.
Claim 4
Dependent Claim
225 words
A method as claimed in Claim 1, in which the distance R2 between the central point and the source point is not equal to the distance Rl.betweem tje central point and a detection position at which the radiation is measured after having passing through the body in positions of the source point which are equiangularly situated with respect to the central point, for each detection position there being obtained a group of measurement data values which have been measured along a group of measurement paths which diverge from the detection position, characterized in that the pre-conditioning factor Q(k) is determined by wherein v is the quotient of the distance R1 and the distance R2, and k. Ao{ is the acute angle enclosed by the connecting line between a detection position and the central point and by a connecting line between a source point position and the measuring path along which the measurement data value has been determined, the post-conditioning factor being determined by: wherein 1. δα is the angle enclosed by the connecting line between a detection position and the central point and by the connecting line between the central point and a source position in which the filtered measurement data value has been determined, Δα being the angle enclosed by two connecting lines which extend from the central point to two successive source positions.
Claim 5
Independent Claim
165 words
A method of determining the distribution of radiation attenuation in a body section, which is irradiated from a plurality of directions by a flat, fan-shaped beam of penetrating radiation which diverges from a source point, all the irradiation directions extending through a central point and being situated in the body section, a group of measurement data being determined for each measurement direction, which are a measure of the radiation attenuation along an associated group of divergent measurement paths, after which from the groups of measurement data there are formed sub-groups of measurement data which are associated with measurement paths which extend substantially parallel to one another, after which each sub-group of selected measurement data is convolved and subsequently back-projected into a matrix of pixels along the measurement paths associated with the sub-group, characterized in that the convolved measurement data are multiplied by a post-conditioning factor which is inversely proportional to the variation of the distance (dr(l)/dl) between a measurement path 1 and the central point.
Claim 6
Independent Claim
165 words
A method of determining the distribution of radiation attenuation in a body section, which is irradiated from a plurality of directions by a flat, fan-shaped beam of penetrating radiation which diverges from a source point, all the irradiation directions extending through a central point and being situated in the body section, a group of measurement data being determined for each measurement direction, which are a measure of the radiation attenuation along an associated group of divergent measurement paths, after which from the groups of measurement data there are formed sub-groups of measurement data which are associated with measurement paths which extend substantially parallel to one another, after which each sub-group of selected measurement data is convolved and subsequently back-projected into a matrix of pixels along the measurement paths associated with the sub-group, characterized in that the convolved measurement data are multiplied by a post-conditioning factor which is inversely proportional to the variation of the distance (dr(l)/dl) between a measurement path 1 and the central point.
Claim 7
Dependent Claim
61 words
A method as claimed in Claim 4, in which the measurement paths in the divergent beam are equiangularly distributed and exhibit an angular spacing A Of, characterized in that the post-conditioning factor is inversely proportional to the cosine of the angle enclosed by the central ray and a measurement path along which the measurement data to be post-conditioned has been determined.
Claim 8
Independent Claim
391 words
A device for determining the distribution of radiation attenuation in a body slice, comprising: at least one source for generating a flat, fan-shaped beam of penetrating radiation for irradiating the body along a plurality of measurement paths which diverge from the source and which are situated in the body section, from a plurality of source positions which are regularly distributed over at least 180 0 , a central ray in the divergent beam passing through a central point from each source position, an array of detectors for detecting radiation after passing through the body along the measurement paths in order to supply, for each source position, groups of measurement data which are a measure of the radiation attenuation occurring along the respective divergent measurement paths through the body section, a processing device which comprises a memory for the storage of measurement data, - a filter device for filtering each group of measurement data, - a back-projection device for allocating and back-projecting the filtered measurement data to matrix elements of a memory matrix for storing the sum of the back-projected, filtered measurement data, and - a display device for displaying the accumulated back-projected measurement data stored in the memory matrix, characterized in that the processing device furthermore comprises arithmetic means for multiplying each measurement data value of each group by a pre-conditioning factor which is determined by the cosine of the angle included between a line connecting the source to the detector which measured the measurement data value, and a central line connecting the source to a central point, and for multiplying each f ltered measurement data value by a post-conditioning factor which is inversely proportional to the change of the angle between the central line and the measurement path associated with the filtered measurement data value, as a function of the associated measurement path, and which is proportional to the square of the cosine of this angle, the filtering device for filtering the pre-conditioned measurement data being a filtering device for filtering parallel and equidistantly measured measurement values, the back-projection device comprising means for multiplying a filtered, post-conditioned measurement data value by a distance factor which is inversely proportional to the square of the distance between a source point and a projection point of a pixel which is situated on the central connecting line and in which a back-projection is determined.
Claim 9
Independent Claim
391 words
A device for determining the distribution of radiation attenuation in a body slice, comprising: at least one source for generating a flat, fan-shaped beam of penetrating radiation for irradiating the body along a plurality of measurement paths which diverge from the source and which are situated in the body section, from a plurality of source positions which are regularly distributed over at least 180 0 , a central ray in the divergent beam passing through a central point from each source position, an array of detectors for detecting radiation after passing through the body along the measurement paths in order to supply, for each source position, groups of measurement data which are a measure of the radiation attenuation occurring along the respective divergent measurement paths through the body section, a processing device which comprises a memory for the storage of measurement data, - a filter device for filtering each group of measurement data, - a back-projection device for allocating and back-projecting the filtered measurement data to matrix elements of a memory matrix for storing the sum of the back-projected, filtered measurement data, and - a display device for displaying the accumulated back-projected measurement data stored in the memory matrix, characterized in that the processing device furthermore comprises arithmetic means for multiplying each measurement data value of each group by a pre-conditioning factor which is determined by the cosine of the angle included between a line connecting the source to the detector which measured the measurement data value, and a central line connecting the source to a central point, and for multiplying each f ltered measurement data value by a post-conditioning factor which is inversely proportional to the change of the angle between the central line and the measurement path associated with the filtered measurement data value, as a function of the associated measurement path, and which is proportional to the square of the cosine of this angle, the filtering device for filtering the pre-conditioned measurement data being a filtering device for filtering parallel and equidistantly measured measurement values, the back-projection device comprising means for multiplying a filtered, post-conditioned measurement data value by a distance factor which is inversely proportional to the square of the distance between a source point and a projection point of a pixel which is situated on the central connecting line and in which a back-projection is determined.
Claim 10
Dependent Claim
64 words
A device as claimed in Claim 6, characterized in that the arithmetic means of the processing device comprises a first multiplier and a pre-conditioning factor memory whose output is coupled to the input of the first multiplier and a further input of which forms an input for measurement data, an output thereof being used to apply the pre-conditioned measurement data to the filtering device.
Claim 11
Dependent Claim
67 words
A device as claimed in Claim 6 or 7, characterized in that the arithmetic means of the processing device comprise a second multiplier and a post-conditioning factor memory whose output is connected to an input of the second multiplier and a further input of which forms an input for filtered measurement daca, an output thereof being used to apply the post-conditioned measurement data to the back-projection device.
Claim 12
Dependent Claim
88 words
A device as claimed in Claim 7 or 8, characterized in that the means of the back-projection device comprises several back-projection segment processors and an adder, the back-projection segment processors being connected in parallel to one another, the output of each segment processor being connected to an input of the adder for adding the filtered measurement data determined in each back-projection segment processor and back-projected into the same matrix element, the filtering device supplying each back-projection segment processor with a different group of post-conditioned, filtered measurement data values.
Claim 13
Dependent Claim
93 words
A device as claimed in Claim 9, characterized in that each back-projection segment processor comprises a profile memory for storing post-conditioned, filtered measurement data, a distance factor generator, a multiplier and addressing means, the addresses generated by the addressing means being capable of fetching a distance factor and a post-conditioned, filtered measurement data value via the distance factor generator and from the profile memory, respectively, which can be applied from outputs of the distance factor generator and the profile memory to the multiplier whose output is connected to an input of the adder.
References Cited (Backward Citations)
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