Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/01—Measuring temperature of body parts ; Diagnostic temperature sensing, e.g. for malignant or inflamed tissue
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
  • A61B6/58—Testing, adjusting or calibrating thereof
  • A61B6/582—Calibration
  • A61B6/583—Calibration using calibration phantoms

Definitions

• the present invention concerns the field of calibration of radiology apparatuses for medical or other applications.
• the intensity of the signal I constituting the image corresponds to a desired quantity of X-rays, so as to obtain satisfactory images in terms of quantum noise.
• a system operating point is generally defined by simultaneous knowledge of the intensity of the signal constituting the signal, or that of a signal representing the latter, and of the image receiver input dose, as measured by a dosimeter under specified radiological conditions.
• the ratio of the intensity of the signal to the dose under those specified radiological conditions is called system conversion factor.
• the value measured by the dosimeter can be directly used for adjustments, or else supplied to an arithmetical unit in order to determine by a calibration the appropriate conversion factor or factors.
• the dose is usually determined under flat field conditions, that is, by inserting in the X-ray beam a uniform part consisting of one or more materials, such as water, a plastic or copper, etc., of sufficient thickness to simulate the attenuation of the X-rays by a patient's body.
• the present invention proposes a method of calibration of a radiology apparaius not requiring the presence of a dosimeter.
• the method of calibration is a radiology apparatus of type comprising a radiation source providing X photons, a for deiection of the X-radiation after it has crossed an object, means for images of which are sought, image acquisition and processing means capable of supplying, on the one hand, an image signal I and. on the other, a signal B representing the intensity of that image, and a means for display of the image connected to the detection means.
• the intensity B is obtained by processing of all or part of the image signal.
• the intensity B is equal to the mean of the points ofthe image in a predefined zone.
• the means of calculation of intensity B have a different slate between the calibration phase and the use phase.
• means of determination of the intensity B of the image include an optoelectronic detector receiving a light signal shunted from the optical path of the image.
• the means of determination of image intensity B include a dedicated detector, of optoelectronic type, for example, placed after conversion of the X-ray radiant image into an optical image and receiving a light signal shunted from the optical path of the image.
• the preceding stages are repeated in order to obtain the value of the factor f for different values of N.
• the spectrum of the X-ray beam is centered on a given energy by diminishing the high voltage supply ofthe radiation source and by placing a filter significantly reducing the proportion of low-energy X-rays.
• the calibration operations are carried out for more than one X-ray energy band.
• an image subtraction is previously carried out in the course ofthe calibration phase.
• the calibration operations are carried out for different values of X-radiation intensity, with a view to identifying the contribution to measurement of the noise sources other than the quantum noise of the
• the stages are carried out automatically.
• the invention is directed to a radiology apparatus of type comprising a radiation source, providing X photons, a means for detection of the X-radiation after it has crossed an object, images of which are sought, means for image acquisition and processing capable of supplying, on the one hand, an image signal I and, on the other, a signal B representing the intensity of that image, and a means for display of the image connected to the means for detection.
• the apparatus includes means for calibration capable of calculating the mean m and the variance ⁇ of distribution of the values 1 of the points constituting the image inside a defined zone of the image.
• the apparatus includes means for determination of image intensity B comprising a dedicated detector, of optoelectronic type, for example, placed after conversion of the X-ray radiant image into an optical image and receiving a light signal shunted from the optical path of the image.
• the invention is also directed to a computer program comprising program code means for applying the stages of the above method, when the program is operating on a computer.
• the invention is also directed to a support capable of being read by a device for reading program code means which is stored there and are suitable for application ofthe stages of the above method, when the program is operating on a computer.
• Figure 1 is a schematic view of a radiology apparatus of a first type
• Figure 2 is a schematic view of a radiology apparatus of a second type;
• Figure 3 describes a calibration operation according to the prior art.
• the radiology apparatus contains an X-ray tube 1 capable of emitting an X-ray beam 2 in the direction of an object 3 which includes an interesting detail 4 that it is desired to study, and an image receiver 5 placed in the X- ray beam 2 below the object 3.
• the image receiver 5 can comprise a solid state detector, hereinafter called flat panel, and as described in "Performance Characteristics of an Amorphous Silicon Flat Panel X-ray Imaging Detector" by Paul R. Granfors, in Proceedings of SPIE Volume: 3659, Medical Imaging 1999: Physics of Medical Imaging, (May 1999).
• the image receiver may be any other device capable of converting incident X photons into a signal usable in digital form.
• the signal representing the image is sent to imaging system by a link 6.
• the image signal is sent by a link 7 to a device 8 which extracts from it a quantity representative of the intensity of the image.
• the signal representing quantity B is sent by a link 9 to a control unit 1 1 which also receives an index signal 10, and determines a control signal sent by a link 12 to the high voltage generator 13 feeding the X-ray tube 1.
• the assembly thus constitutes a regulation loop to maintain signal B at a value equal to the index 10.
• the image receiver comprises an image intensifier 124, the output image of which is sent back over an optoelectronic image detector 105 by means of a tandem of two optical devices 122 and 123, for example, lenses, separated by an iris diaphragm 121 .
• a luminance sensor 120 placed to receive a part of the light 1 18 resulting from conversion of the X photons by the image intensifier 124 sends a signal representing the intensity B of the image through a conductor 107 to a control unit 108 which also receives an index signal through a conductor 1 10.
• the control unit 1 1 1 supplies control instructions to the high voltage generator 1 13 feeding the X-ray tube 101.
• Figure 3 describes the method of calibration of the conversion factor ordinarily used. It is represented here in relation to equipment according to Figure 1. but could just as well be represented in relation to equipment according to Figure 2.
• a dose detector 31 is placed in the input plane of the image receiver 5, connected to a measuring device 32. which supplies measurements of the dose on that plane.
• the usual calibration phase consists of the following stages:
• a difficulty with that method is the need to use a dosimeter, an apparatus which in turn requires periodic calibrations, and especially the fact that it is highly sensitive to variations of position of the measuring probe 31 by reason, on the one hand, of the variation of intensity of X-radiation with the square of the distance to the source and, on the other, the sensitivity to backscattered radiation by the image receiver 5 itself.
• the disclosed method makes it possible to remedy those two problems simultaneously.
• the image intensity is proportional to the quantity of N photons having contributed to the formation of that image.
• the dose at the inlet of the image receiver It is therefore possible to replace calibration of the operating point establishing the relation between the image intensity and the dose on input of the image receiver with a relation of the same type between the image intensity and the number of photons on .input of the image receiver, knowing that simultaneous application of the usual method and of the method disclosed under specified conditions makes possible a passage from one to the other.
• a relation of type B fxN is established, in which N is the number of X photons and f is a conversion factor. It is therefore sought to determine the value of the number of N photons. It is known that the distribution of values in a homogeneous zone of the image is affected by a random variation obeying a Poisson distribution.
• the means 8 making it possible to calculate the value of B from the values of the points ofthe image can furnish a simple arithmetical mean in a predefined zone of the image, adapted in size and shape to the particular needs of the application, or else have a more improved behavior, for example, with threshold effects on the highest or lowest levels, or nonlinear transfer functions.
• An example of those improved treatments can be found in EP 1 ,004,875.
• the reliability of the estimate of quantum noise and. therefore, of number N can. if necessary, be improved by replacing simple measurement of the mean and of the variance of the points of the ROI by an estimate of the parameters of the Poisson distribution (or in practice of the normal law), which is best adjusted to distribution of the points measured; the influence of possible aberrant points can thus be minimized.
• the relation between number of photons and dose can be established as follows. Let us take, as an example, a beam generated under 80 kV and crossing 20 cm of tissues. According to the tables of HPA Scientific Report 30, the mean energy of the photons on output of the object is 54.4 keV and the quantity of photons produced at 0J5 m from the source is 2.27* 10 4 photons/(mAs.mm 2 ). Furthermore, the same tables indicate for that high voltage value an exposure of 0J ⁇ Gy.mAs. A number of photons of 3.24* 10 4 photons ( ⁇ Gy.mm ) is derived therefrom. The same result can be arrived at by other means. In Physics of Radiology (Johns &
• these calculations establishing the relation between dose and number of photons are based on the mean energy of the photons present in the beam. Results closer to the conditions of the calculation will be obtained by restricting the energy range of the spectrum.
• a material of atomic number high enough to reduce the quantity of low-energy X phoions will advantageously be chosen for the phantom, while reducing the high- voltage value, in order to reduce the quantity of high-energy photons X contributing to the image.
• a copper plate and a voltage of less than approximately 75 kV will be used.
• Such a beam can be generated, for example, by using 60 to 70 kV and 2-mm-thick copper.
• the doses customarily indicated in adjustment procedures or the object of regulation are by necessity measured in a plane different from that of the image receiver (not attainable), which generally necessitates the application of corrective factors and before the mechanical protection elements ofthe image receiver, which attenuate the beam.
• only photons actually reaching the plane of the image receiver are measured, which is preferable from the standpoint of image quality, but can raise problems from the standpoint of radiation protection. If the point of operation obtained in terms compatible with traditional procedures has to be specified, it will then be necessary to resort either to calculation of a correction factor or to perform a calibration proper to the equipment, but not needing to be remeasured on all the installations.
• the noise term measured can present increases due to other effects (Swank effect, Lubberts effect), or inversely be reduced by the fact that the transmission of spatial modulations in the image is not 100% for all frequencies. That behavior is described by the modulation transfer function (MTF).
• the apparent noise reduction factor associated with the MTF can be calculated independent of the latter or again determined by an initial calibration.
• the MTF can be measured by using the known methods, such as measurement of amplitude on a sinusoidal target, or calculation from measurement of the effective value of modulation in a rectangular periodic target image (Droege's method), or derived from the response to the image of an edge.
• those same operations can be provided for after a subtraction of images making it possible to retain only the terms which vary from one image to another and corresponding to the fluctuation from one measurement to another due to the amplification noise and to the quantum noise.
• the amplification noise can be eliminated by calculation, using, for example, two measurements made at appreciably different image levels, obtained by adjusting the supply current of the X-ray tube or the duration of exposure.
• That method makes possible a simplified calibration, reducing the labor necessary to put the radiology apparatus into service, resulting in a cost saving.
• the method of calibration is based on an absolute measurement and does not require the presence of external measuring instruments, which eliminates the need for external calibration of those measuring instruments.
• the method of calibration can be applied in any type of digital radiology apparatus.
• the digital X-ray detector is used as a relative photon counter during the operating phase in order to determine the appropriate calibration factor to be used during the operational use phases of the apparatus.
• the quantum nature of the X-rays makes the noise of an image, as measured by the variance of levels of the points constituting that image, proportional to the X- ray signal, when the random variations of level in the image (also known and described as noise) are exclusively due to the quantum nature of the X photons
• image intensifier on the one hand, and optoelectronic image detector, on the other). for example, by varying the numerical aperture o the optics ensuring formation ofthe image gathered by the radiological image intensifier on the CCD detector.
• the disclosed method is advantageous to the development of flat detectors, which render it much sturdier.

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