# US 4,394,738 A · Method of and device for determining the distribution of radiation absorption in a slice of a body

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- Number: US 4,394,738 A
- Document ID: US4394738A
- Status: Granted
- Kind: A
- Application number: 06/216,478
- Filed: 1980-12-15
- Granted: 1983-07-19
- Earliest claimed: 1979-06-16
- Assignees: U.S. Philips Corporation
- Inventors: Wolfgang Wagner
- Web page: https://patentgrep.com/patent/US4394738A

## CPC

- A61B 6/032: {Transmission computed tomography [CT]}
- A61B 6/4035: {the source being combined with a filter or grating}
- A61B 6/482: {involving multiple energy imaging}
- A61B 6/405: {Source units specially adapted to modify characteristics of the beam during the data acquisition process (A61B6/4021, A61B6/4035 take precedence)}
- A61B: DIAGNOSIS; SURGERY; IDENTIFICATION (usefulness limited to only animals A61D)

## Abstract

If only the absorption distribution of a part, for example, a separate organ, of a body slice to be examined is desirable, the radiation dose administered to the body can be limited. The part is irradiated with the full intensity only during a first measuring cycle. In order to avoid reconstruction errors, the entire body slice is irradiated with a second intensity during a second measuring cycle which is substantially lower than the first intensity. Thus, along each measuring path through the part of the slice there are available two absorption values wherefrom suitable correction factors can be determined for correction of all second absorption values which subsequently no longer exhibit any deviations, or only small deviations, with respect to the first absorption values, so that they do not introduce artefacts during the reconstruction.

## Claims

### Claim 1

A method for determining the distribution of radiation absoption properties in a body by computed tomography which includes the steps of:
radiating a planar examination zone in the body, from a plurality of directions, with radiation having a first intensity so that the radiation passes through the examination zone along a large number of measurement paths in the plane of the examination zone, collimating said radiation so that the radiation having a first intensity does not pass through the body along paths which do not pass through the examination zone, and measuring the intensity of the radiation which has passed through the examination zone along the paths to obtain as a first set of measurement values;
radiating a positioning zone, which lies in the plane of and completely encloses the examination zone, from a plurality of directions with radiation having a second intensity which is smaller than the first intensity, so that the radiation passes through the positioning zone along a large number of measurement paths in the plane of the zones, and measuring the intensity of the radiation which has passed through the positioning zone to obtain a second set of measurement values;
calculating first absorption values from the first set of measurement values;
calculating second absorption values from the second set of measurement values; and
reconstructing the distribution of radiation absorption properties in the examination zone from using the first absorption values and the second absorption values.

### Claim 2 (depends on 1)

A method for determining the distribution of radiation absorption properties in a body by computed tomography comprising the steps of:
utilizing the method of claim 1 to reconstruct the distribution of radiation absorption properties in a first planar slice of the body;
displacing the body in a direction transverse to the slice plane to radiate further body slices which are situated parallel to and adjacent to the first body slice;
radiating further parallel, planar examination zones in one or more of the further slices of the body, from a plurality of directions with radiation having the first intensity so that the radiation passes through the further examination zones along a number of measurement paths; measuring the intensity of the radiation which has passed through the further examination zones along the measuring paths to obtain further first sets of measurement values;
calculating further first sets of absorption values from the further first sets of measurement values;
approximating a second set of absorption values for the further slices from the second set of measuring values which were obtained while radiating the first slice along measuring paths which extend outside of the examination zone; and
utilizing the further first sets of absorption values and the further second sets of absorption values to reconstruct radiation absorption distributions in the further slices.

### Claim 3 (depends on 1, 2)

A method as claimed in claim 1 or 2 further comprising the steps of producing the radiation with a X-ray tube and changing the intensity of the radiation between the first intensity and the second intensity by changing a current in the X-ray tube.

### Claim 4 (depends on 1, 2)

The method of claim 1 or 2 further comprising the steps of producing the radiation with an X-ray tube and changing the intensity of the radiation between the first intensity and the second intensity by changing a voltage applied to the X-ray tube.

### Claim 5 (depends on 1, 2)

A method as claimed in claim 1 or 2 further comprising the step of producing the radiation from a radiation source, wherein the step of radiating the examination zone with radiation of a first intensity comprises situating the radiation source at a first distance from the examination zone and wherein the step of radiating the positioning zone with radiation of the second intensity comprises situating the radiation source at a second distance, which is larger than the first distance, from the examination zone.

### Claim 6

Computed tomography apparatus for determining the distribution of radiation absorption properties in a planar slice of a body comprising:
radiation source means which generate a planar beam of penetrating radiation and radiate a plane in the body therewith from a plurality of directions;
detector means which measure radiation from the radiation source means which has passed through the body and which determine therefrom absorption values which are a measure of the radiation attenuation along measuring paths in the plane which pass through the body from the source;
a positioning zone for the body situated between the source means and the detector means;
central processing means which determine absorption coefficients of the radiation absorption distribution from the absorption values;
a memory for storing the absorption values and the absorption coefficients; and
display means which display the radiation absorption distribution;
wherein an examination zone in the plane lies within and is wholly surrounded by the positioning zone and, as an improvement, the apparatus further comprises:
means which operate the radiation source means to produce two different intensities of radiation so that a first, higher intensity of radiation is used to radiate the body along paths which pass through the examination zone and a second, lower intensity of radiation is used to radiate the body along all paths which pass through the entire positioning zone, the higher intensity radiation being used to determine a first set of absorption values and the lower intensity of radiation being used to determine a second set of absorption values; and
arithmetic means which determine the number k of measuring paths which extend through the examination zone in each direction, which determine and sum quotients of the first absorption values Q.sub.1 (P.sub.1, .theta.) and second absorption values Q.sub.2 (P.sub.2, .theta.) associated with the measuring paths, which divide the sum thus obtained by the number of measuring paths, thereby determining a correction factor C(.theta.) in accordance with the formula
C(.theta.)=((.upsilon.(Q.sub.1 (P.sub.1,.theta.)/Q.sub.2 (P.sub.1,.theta.)k
and which multiply those second absorption values which were measured along measuring paths in that direction which pass through the positioning zone and do not pass through the examination zone by the correction factor.

### Claim 7 (depends on 6)

A device as claimed in claim 6 wherein the detector means comprises:
two long, parallel rows of detector elements disposed opposite the radiation source means to receive radiation which has passed through the positioning zone, the detector elements at the ends of the long rows being disposed to receive radiation from the source means along paths which are tangent to the perimeter of the positioning zone;
one or more short rows of detectors disposed between the long rows and parallel thereto to receive radiation from the source which has passed through the examination zone, and
wherein the apparatus further comprises interpolation means which calculate interpolated values from the absorption values supplied by the long detector rows.