# US 5,347,563 A · Method for determining the function representing the effect of non-reciprocity of a radiographic film

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- Number: US 5,347,563 A
- Document ID: US5347563A
- Status: Granted
- Kind: A
- Application number: 07/726,175
- Filed: 1991-07-05
- Granted: 1994-09-13
- Earliest claimed: 1990-07-06
- Assignees: General Electric CGR S.A.
- Inventors: Robert Heidsieck
- Web page: https://patentgrep.com/patent/US5347563A

## CPC

- A61B 6/583: {using calibration phantoms}
- H05G 1/46: Combined control of different quantities, e.g. exposure time as well as voltage or current
- A61B: DIAGNOSIS; SURGERY; IDENTIFICATION (usefulness limited to only animals A61D)
- H05G: X-RAY TECHNIQUE (investigating or analysing materials by the use of X-rays G01N23/00; apparatus for X-ray photography G03B42/02; X-ray tubes H01J35/00; TV systems having X-ray input H04N5/321)

## Abstract

The invention relates to radiology systems and, more particularly in such systems, to a method that enables the effect of non-reciprocity of the radiographic film to be determined. This effect of non-reciprocity is expressed by coefficients CNRD(d.sub.i) which are a function of the photon dose rate (d.sub.i) on the film, the coefficients CNRD(d.sub.i) being obtained from the coefficients CNRT(t.sub.i) and the determination of the reference lumination L.sub.ref received by the film.

## Claims

### Claim 1

A method for determining the function representing the effect of non-reciprocity of a radiographic film in a system of radiology designed to examine an object that includes an X-ray tube, the supply voltage V of which may assume various values V.sub.m, with continuous or discrete variation, said X-ray tube emitting an X-ray beam in the form of pulses of variable duration t.sub.i towards the object to be examined, a receiver of the X-radiation that has crossed the object to form an image of said object, a cell for the detection of the X-rays that have crossed the object to be examined, that enables the conversion of a physical variable, characterizing the X-ray beam, into a measurement signal L, an integrator circuit that integrates the measurement signal L for the duration t.sub.i and gives a signal M, and a device to compute the yield D given by the ratio of M to the product I.times.t.sub.i (or mA.s) of the anode current I of the tube by the duration ti of the exposure, wherein said method includes the following steps:
(a) determining the coefficients of non-reciprocity CNRT (t.sub.i) of the film expressed in exposure time t.sub.i for an optical density DO.sub.refo of said film;
(b) determining the reference lumination L.sub.ref received by said film;
(c) calculating the photon dose rates d.sub.i on said film by the formula: ##EQU13## (d) determining the coefficients of non-reciprocity CNRD (d.sub.i) expressed in photon dose rate d.sub.i by the application of the formula:
CNRD (d.sub.i)=CNRT (t.sub.i)
wherein the step (a) comprises the following steps:
(a1) modifying the tube heating current so as to obtain different values of said current,
(a2) reading the values M (t.sub.i) given by the integrator circuit for different exposure times (t.sub.i) so as to obtain an optical density DO.sub.1 of the film,
(a3) calculating the ratio ##EQU14## which gives the coefficient CNRT (t.sub.i) with M (t.sub.ref) the value M (t.sub.i) for t.sub.i =t.sub.ref.

### Claim 2

A method for determining the function representing the effect of non-reciprocity of a radiographic film in a system of radiology designed to examine an object that includes an X-ray tube, the supply voltage V of which may assume various values V.sub.m, with continuous or discrete variation, said X-ray tube emitting an X-ray beam in the form of pulses of variable duration t.sub.i towards the object to be examined, a receiver of the X-radiation that has crossed the object form an image of said object, a cell for the detection of the X-rays that have crossed the object to be examined, that enables the conversion of a physical variable, characterizing the X-ray beam, into a measurement signal L, an integrator circuit that integrates the measurement signal L for the duration t.sub.i and gives a signal M, and a device to compute the yield D given by the ratio of M to the product I.times.t.sub.i (or mA.s) of the anode current I of the tube by the duration t.sub.i of the exposure, wherein said method includes the following steps:
(a) determining the coefficients of non-reciprocity CNRT (t.sub.i) of the film expression in exposure time t.sub.i for an optical density DO.sub.refo of said film;
(b) determining the reference lumination L.sub.ref received by said film;
(c) calculating the photon dose rates d.sub.i on said film by the formula: ##EQU15## (d) determining the coefficients of non-reciprocity CNRD (d.sub.i) expressed in photon dose rate d.sub.i by the application of the formula:
CNRD (d.sub.i)=CNRT (t.sub.i)
wherein the step (a) includes the following operations:
(a1) making, by means of a variable time sensitograph, a first sensitogram S.sub.refo when the exposure time is set for a reference time t.sub.refo ;
(a2) making, by means of the same variable time sensitograph, q sensitograms S.sub.1 to S.sub.q for q different exposure times t.sub.i ;
(a3) choosing a reference optical density DO.sub.refo ;
(a4) measuring, on each sensitogram, the illumination step Ech.sub.refo ; Ech.sub.1 . . . Ech.sub.i . . . Ech.sub.q corresponding to the optical density DO.sub.refo ;
(a5) calculating the coefficient CNRT (t.sub.i) by the equation: ##EQU16##

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

A method according to claims 1 or 2, wherein the step (a) further includes the following step of:
determining the function of modelization of the coefficients CNRT (t.sub.i) such that:
CNRT (t.sub.i)=A.sub.0 +A.sub.1 log t+A.sub.2 [log t].sup.2

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

A method according to any of the claims 1 or 2, wherein the step (b) consists in measuring said lumination L.sub.ref by a detector cell.

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

A method according to any of the claims 1 or 2, wherein the reference lumination L.sub.ref is determined by a calibration method that includes the following steps should the image receiver be formed by at least one intensifier screen and a film sensitive to the light from this screen:
taking a shot under determined radiological conditions for a reference optical density DO.sub.ref, a thickness standard E.sub.0, a supply voltage V.sub.0, an exposure time t.sub.0 and a value of the product I.sub.0 .times.t.sub.0 ;
measuring the yield D.sub.0 ;
calculating the equivalent thickness E.sub.p0 by the formula:
E.sub.p0 =g" (V.sub.0, D.sub.0)
calculating the yield D.sub.f0 on the film by the formula:
D.sub.f0 =f' (V.sub.0, E.sub.0)-f" (V.sub.0, E.sub.p0)
calculating the luminance L.sub.film, on the film by the formula:
L.sub.film =D.sub.f0 .times.I.sub.0 .times.t.sub.0
calculating the illumination step Ech.sub.ref corresponding to the reference optical density DO.sub.refo by means of the sensitometric curve;
measuring the optical density DO.sub.m of the shot obtained and the computation of the illumination step Ech.sub.m by means of the sensitometric curve;
calculating the reference lumination L.sub.ref by the formula: ##EQU17##
with K=2/log.sub.10 (2)

### Claim 6 (depends on 5)

A method according to claim 5 wherein, for calculating the yield D.sub.f0 on the film by the equation D.sub.f0, the equivalent thickness E.sub.p0 is replaced by (E.sub.p0 -sup.filter), (sup.filter) being the equivalent thickness due to the additional filtration resulting from the attentuation between the intensifier screen and the detection cell.

### Claim 7 (depends on 5)

A method according to claim 5, when the image receiver is formed by a film sensitive to X-radiation and when the detection cell is placed before or behind said film, wherein the yield D.sub.f0 on the film is computed by the formula:
D'.sub.f0 =f"' (V.sub.m, E.sub.p)

### Claim 8 (depends on 7)

A method according to claim 7 wherein, in the equation D'.sub.f0, the thickness E.sub.p is replaced by (E.sub.p -sup.filter), (sup.filter) being the equivalent thickness due to the additional filtration resulting from the attenuation between the receiver film and the detection cell.

### Claim 9 (depends on 3)

A method according to claim 3, wherein, the luminance on the film is calculated by the formula: ##EQU18##

### Claim 10

A method for determining the function representing the effect of non-reciprocity of a radiographic film in a system of radiology designed to examine an object that includes an X-ray tube, comprising the steps of:
supplying voltage to the tube at various values, V.sub.m ;
exposing an object to X-ray beams in the form of pulses of variable duration, t.sub.i ;
exposing film to X-ray beams after passage through the object;
subjecting an X-ray radiation detector cell to tile beams for converting beam energy, passed through the object, into a measurement signal L;
integrating the signal L for tile duration ti and generating a resulting signal M;
measuring the current I supplied to an anode of the X-ray tube;
calculating the yield D from the ratio of tile signal M to the product of I.times.t.sub.i during an interval of exposure;
determining the coefficient of non-reciprocity CNRT (t.sub.i) of the film as a function of exposure time t.sub.i for an optical density DO.sub.refo of the film;
calculating the photon dose rates d.sub.i, as a function of a reference illumination on the film (L.sub.ref) by the formula: ##EQU19## and determining the coefficients of non-reciprocity CNRD (d.sub.i) as a function photon dose rate d.sub.i by the formula:
CNRD (d.sub.i)=CNRT (t.sub.i).

### Claim 11 (depends on 10)

A method according to claim 10 further comprising the steps of:
determining the function of modelization of the coefficients CNRD (d) such that--
CNRD (d)=A'.sub.0 +A'.sub.1 log 1/d+A'.sub.2.