NL1013865C2 - Method for determining at least one material in an object, and devices for its implementation. - Google Patents

Description

Method for determining at least one material in an object, and devices for its implementation.

The invention relates to a method for determining at least one material in an object, more in particular to the analysis of inorganic and organic materials by means of ionizing radiation, such as X-rays and γ-radiation, including such materials in humans and animals. It is thus possible to determine the material qualitatively, quantitatively or qualitatively and quantitatively. The invention also relates to devices suitable for carrying out this method.

Devices and methods have already been developed, in particular to measure the amount of bone in the human body. It is thus possible, for example, to determine the possible occurrence of osteoporosis, a phenomenon which can lead to fracture of the bone, for example of the wrist, hip or vertebrae.

For example, US patent Re.34,511 discloses an apparatus with which a patient is scanned with a so-called peneil beam, and in US patent 5,432,834 with a so-called fan beam (fan beam) ionizing radiation, usually X-rays. The part to be examined hereby is scanned essentially twice, the first time with radiation with a certain energy and the second time with radiation with a different energy. When scanning with a pencil beam, one detector element is placed behind the part to be examined, and in the case of a fan beam a row of detector elements. The detector (s) is (are) usually mechanically connected to the radiation source to track the scanning beam. 30 By measuring the absorbance at two different energies and entering it in equations 1013865 2 1) ix / Ix = e "** 11 x e-®" 2 2) i2 / I2 = e'YHi xe "4 * 12 where M1 represents the distance that the radiation 5 has to travel in material 1, M2 represents the distance that the radiation must travel in material 2, ix stands for the radiation with energy 1 exiting the object, which is applied to the relevant detector 10 i2 stands for the radiation emitting from the object with energy 2, which falls on the relevant detector, lx stands for the radiation falling on the object with energy 1, I2 stands for the radiation falling on the object, with energy 2, a, β, γ and δ represent the absorption coefficients associated with the relevant materials and the energy 1, 20 and 2 of the radiation offered, for example, the amount of bone material in the relevant part of the body can be determined.

For measuring bone strength, most devices include a bed on which the patients are in a suitable position and a scanning installation that can be moved relative to the bed and the patient. Such an apparatus is described in U.S. Patent No. 5,778,045.

When using a pencil beam, the beam-30 beam of U.S. Pat. No. 4,811,373 is usually first moved transversely of the patient's longitudinal direction from one shoulder to the other, then the beam is moved slightly in the longitudinal direction and then reversed is moved to the other shoulder 35. This continues until the entire target area is scanned. To measure bone density, measure 1013865 3 twice in each position with different energy spectra.

Since scanning with a peneil beam with only one detector takes a relatively long time, other 5 devices have been developed which scan the patient with a fan beam (U.S. Pat. No. 5,432,834), the radiation source generating a fan beam which is thin in the longitudinal direction of the patient, usually about the width of the detectors with which the beams 10 are received, and fanned out in the transverse direction, thereby covering at least part of the transverse width of the patient. The full transverse width is then fully covered either by the fan beam or by tilting the radiation source and detector row assembly about an axis passing through the ray source parallel to the patient's longitudinal axis. After the entire width has been scanned, said assembly is then moved in the longitudinal direction, after which it is scanned again over the entire width, after which it is moved in the longitudinal direction again until the intended area is fully scanned. To obtain the results of measurements from two different energy spectra, the patient can be scanned first with one energy spectrum and then with the other energy spectrum, with the risk of intermediate patient movement affecting the reliability of the data, or two measurements are taken consecutively at each position with different energy spectra.

For the fan beam, but also often for the pen beam, an X-ray source, such as an X-ray tube, is usually used to generate the radiation. To obtain two different energy spectra, it can be operated with two different high voltages or alternatively with one high voltage, whereby different energy spectra are obtained by placing filters in the beam of rays. A combination of both is also possible.

1 o 1 3865 4

In the methods previously used for scanning a particular area, it is necessary that each measuring point is irradiated with two beams with mutually different energy spectrum in order to obtain a good result.

It would be desirable to beam the object only once. This would speed up the measurement and limit the amount of radiation.

The object of the invention is now to provide a method for considerably shortening the time required for scanning and reducing the amount of radiation required.

To that end, adjacent measuring areas, hereinafter referred to as strips, are irradiated mainly with radiation with mutually different energy spectrum. When using two different energy spectra for consecutive areas, these energy spectra can be used alternately. This makes it possible to obtain the intended analysis results, such as, for example, bone density, by measuring once per position. In this way, two radiation images are obtained, which are staggered at least one position relative to each other. For each radiant energy, the radiant image from the intermediate positions (which have not been measured) can be reconstructed using interpolation.

Strips, as used herein, are elongated regions corresponding to the dimensions of a detector system, which consists of more than one row of detector elements, one strip corresponding to all rows of detector elements.

The invention thus relates to a method for determining at least one material in an object, which consists of at least two materials, by means of energy subtraction, wherein the object to be examined is at least partially irradiated by a number of energy spectra that can be distinguished in practice, which is lower than necessary for the application of energy subtraction in the usual way and the data missing for a certain part of the object in the first instance for the usual application are derived from the information obtained from the measurement of the transmission through at least one other part of the object.

It is preferable that the energy-wise spatial modulation of the radiation pattern is effected by a radiation beam, which irradiates part of the object, and the object to move relative to each other and to change the energy of the radiation beam. This creates mutually separated strips of ratio numbers (ratio of striking to outgoing amount of radiation), which belong to one energy. The information for the intermediate strip or strips is reconstructed by interpolation. This interpolation does not have to be linear. Other interpolation methods may be used if desired and if necessary. It will be appreciated that the strips of distinct energy can overlap, although they may also be adjacent or even separated. In practice, however, it is now preferable to use a partial overlap because it allows more easily relatable reference points to be obtained.

It is also possible, if desired, to use more than two irradiations with mutually different energy spectra with distinct hardness, which makes it possible to distinguish as many different absorbent materials as there are different penetrating radiations. The limiting factor here is that the successive positions for one and the same radiation may not be so far apart that interpolation of the area between the two positions leads to unacceptable measurement errors.

The invention is further elucidated with reference to the annexed drawing.

In the drawing, 1013865 6 FIG. 1 shows a principle sketch of an apparatus for scanning according to the invention, FIG. 2 shows a radiation pattern with alternating spectra with the corresponding pulse duration.

FIG. 1 shows an apparatus using a fan beam. Thereby, an assembly of an X-ray source (1) with focus (2) of X-ray source (1) and also axis of rotation of the fan beam detector system, which axis is perpendicular to the plane of the drawing, becomes a collima-10 torsion system (3 ) and a slit former (4) of collimator system (3) which moves with the fan beam detector system, and a detector system (8) composed of several detector elements (9), between which the object (6) to be examined is on a bed (7) is placed, rotated about the axis. The detector system (8) is long enough to accommodate a fan beam (5) covering the entire width of the object (6). In the case shown, the width (45 cm by 1 cm) is formed by a number of rows of detector elements with a size of 100 μπι. 20 The detector system consists of 100 rows with 100 detector elements each. The dimensions of this detector system correspond to a strip. The detector-25 elements are switched transversely to the direction of the rows of detector elements, i.e. in the direction of the relative movement relative to the object, so that after a short time the signal from the first detector is shifted to the second detector element in the width direction, and the result of the second detector element is shifted back to the third detector element and so on, while the previous 30 detector elements start to measure again. This is a so-called TID detector. The detector is mechanically or electrically rigidly connected to the X-ray source, such that the detector always catches the beam after irradiation of the patient. The distance between X-ray source and detector can be varied if desired. For example, the absorption of the radiation by the patient is measured in successive strips. These strips abut against each other in the longitudinal direction of the patient. A complete X-ray image is thus built up.

Fig. 2 shows in the bottom diagram the X-ray dose in vertical direction and along the horizontal axis the dimension in cm. As shown in the diagram, alternating spectrum A (11) and spectrum B (12) are broadcast. In the illustrated embodiment, the distance between the start of the maxima of consecutive spectra is 1 cm.

The corresponding upper diagram in Fig. 2 shows the X-ray energy in the vertical direction and the time in ms along the horizontal axis. The pulse duration, i.e. the width of each "tooth" is 2.5 με.

If scanning 40 cm in the longitudinal direction of the patient takes about one second and the X-ray source 15 gives a relatively short pulse every 25 ms, the spectrum of which is changed alternately, the resulting X-ray image consists of a series in the longitudinal direction of the approximately adjacent 1 cm wide image strips alternately taken with a different spectrum. It is technically difficult to make the X-ray pulses so short that the movement of the detector during those pulses is small relative to the size of a detector element. Therefore, a TID mode detector is provided whose charge propagation speed to the adjacent detector elements is exactly synchronized with the detector's moving speed relative to the patient. This means that the transmitted radiation from one object element (ie a part of the object, the dimensions of which correspond to the size of one detector element) is received by all detector elements passing successively, but the charge corresponding to that transmitted radiation always is passed to the subsequent detector, so that the charge, which is finally measured during reading, corresponds to the transmitted radiation of the object element during the entire time that the detector system passes the object element. During the 2.5 race in which the X-ray pulse is given, 10 detector elements will shift their charge during the X-ray pulses against the direction of movement of the detector system to the device, which reads out the charge. This means that the charge read first after the appearance of the X-ray pulse corresponds to only about one-tenth of the dose, compared to the dose associated with the charge last read before the next X-ray pulse. occurs. The charge, which is read second, corresponds to approximately two-tenths of that dose. With each subsequent reading, the dose increases to the eleventh time the charge is read after the X-ray pulse appears. From that reading, the dose remains approximately constant until the last charge, which is read before the next X-ray pulse occurs.

After the X-ray pulse has ended, the charges are shifted and read out more quickly, so that the detector is fully read out and reset for the next X-ray pulse. For example, an X-ray image of the patient can be constructed consisting of strips one cm wide if the intensity of the image of the first ten rows of pixels associated with each X-ray pulse is corrected for the lower initial dose with which the image was taken. Two complete images of the patient can then be reconstructed by always interpolating the space between two image bars associated with the same X-ray energy. Thus, two complete X-ray images are obtained at two different energies, from which one can determine, for example, the amount of bone in the body by applying the formulas 1 and 2 and 30 after the necessary calibration.

Virtually all elements in the above explanation can be varied. For example, the detector elements can be made larger or smaller. The number of detector elements in the direction of movement can be made larger or smaller. The speed of movement of the detector can be varied so that the 1013865 9 strips partially overlap or are spaced apart. It is also possible to think of varying the speed of movement during one measurement.

The pulse duration can be made relatively and absolutely smaller and larger. And the X-ray output of the X-ray tube can also be varied during the measuring process in order to optimize the signal-to-noise ratio during the measuring procedure. The total measuring procedure can be made longer or shorter, for example depending on the available output of the X-ray tube. In addition, an embodiment can be devised in which the X-ray tube does not have to be pulsed, but emits a continuous, albeit varying energy and / or output.

It is thus possible to make a detector system of only one row of detector elements, for example larger than 0.1 mm, that scan the image and alternately image a strip with high and one with lower X-ray energy, from which, after interpolation, the quantity of bone in a patient can be identified.

20 One does not have to limit yourself to two energies.

In principle, one can work with more energy levels with which more materials can be analyzed in the object to be investigated. For example, a detector consisting of one row of detector elements, the readout speed of which is relatively high, can be moved in a linear uniform motion over the object to be examined, while the X-ray energy varies periodically. The specific size of this periodicity must be large relative to the size of a detector element but small enough to allow interpolation with the required accuracy.

The invention therefore more particularly relates to a method in which the radiation is intermittently presented to the object in the form of pulses. It is also possible that the radiation is continuous and changes in energy in time.

1013865 10

This principle can again be extended to the use of a TID detector, composed of several rows of detector magnets, which moves unanimously over the object to be scanned, while the X-ray energy preferably varies periodically over time. The specific size of the periodic X-ray pattern should preferably be more than twice the size of the detector in the direction of the scanning direction, but not so large that interpolation required to analyze the different materials is not sufficient can be done accurately.

The reading of the charge, which corresponds to radiation transmitted by one object element, is a superposition of charges which is generated in successive detector elements by the transmitted radiation during the relative movement. That transmitted radiation is again the superposition of energy-varying radiation, which is sent by the radiation source through said object element during the passage of the detector system. This superposition of different spectra, belonging to 20 different energies, can in itself be effectively considered a spectrum. The charges so read in succession are associated with different effective spectra, because the specific size of the periodic radiation pattern in the direction of motion is preferably more than twice the size of the detector system in the same direction. In this way, the transmission of many rows of object elements (transverse to the relative direction of movement) corresponding to an effective spectrum that varies from row to row is measured.

In order to improve the signal / noise ratio, the output can also be varied when the energy of the radiation source varies.

It is also possible to use a non-periodically varying X-ray energy.

The above methods can also partly be applied to devices that use only one detector element and a so-called pencil beam, by using radiation with different energy spectra at the different positions at which the detector measures.

The scanning movement is preferably obtained by rotating the array of the radiation source and the detector about an axis passing through the focus of the radiation source. However, it is also possible to move the object to be examined along the system of radiation source and detector system. This movement should preferably be done in a rocking motion about the focus of the beam source, so that each part of the object to be examined is irradiated by the conically shaped beam. In practice, however, in many cases a linear movement along the system of radiation sources detector system suffices. The latter also makes the described methods suitable for scanning, for instance, luggage, for example at airports, or for scanning, for example, animals in a slaughtering system in order to determine, for example, the fat-meat-bone ratio of the animals.

Another application, which can make use of the above-mentioned principle of interpolation, is that in which the spatial modulation of the radiation pattern is obtained by irradiating larger parts of the object to be examined with a radiation source provided with a radiation filter, which is approximately perpendicular the direction of the radiation beam is modulated, so that different parts of the object to be examined are irradiated with radiation with a different energy spectrum.

When using the method in which a radiation source with associated detector system is rotated around the object to be examined, a CT device is particularly suitable in which successive slices with X-rays of different energy are scanned.

In a variant of this, in a CT device using spiral scanning, the energy of the X-rays used is changed during scanning.

1013865 12

More specifically, a CT device can be used in which successively recorded profiles with different energy are generated. The beam source is preferably controlled in such a manner that the modulation of the energy of the beam source created during the rotation contains at least one periodic component.

An apparatus suitable for making rotational-angio recordings can also be used for carrying out the method.

In particular, an apparatus is suitable for taking pictures with the general method, wherein the object and radiation source to be examined move relative to each other with the associated detector system and the detector system consists of more than one row of detector element and, 15 with the detector elements in TID mode switched in the direction of the relative movement of the object.

1013865

Claims (15)

Method for determining at least one material in an object, which consists of at least two materials 5, by means of energy subtraction, characterized in that the object to be examined is at least partially irradiated by distinguishing a number in practice energy spectra, which is lower than necessary for the usual application of energy subtraction 10 and the data missing for a certain part of the object initially for the usual application are derived from the information obtained from the measurement of the transmission by at least one other part of the object. Method according to claim 1, characterized in that the energy-wise spatial modulation of the radiation pattern is effected by a radiation beam, which irradiates part of the object, and having the object displaced with respect to each other and the energy of 20 to change the radiation beam. Method according to claim 2, characterized in that the radiation is presented to the object intermittently in the form of pulses. 4. Method according to claim 3, characterized in that the energy of the different radiation pulses is different. Method according to claim 2, characterized in that the radiation is continuous and changes in energy over time. Method according to claim 5, characterized in that the modulation of the generated radiation pattern contains at least one periodic component when viewed spatially. Method according to claim 1, characterized in that the detector is stationary relative to the object and is large enough to receive and measure the transmission of the radiation pattern of the entire part of the object to be examined. 1013865 Method according to claim 1, characterized in that a radiation source and an associated detector system revolve around the object to be examined and the energy of the radiation source is changed during rotation. CT device for use in the method according to claim 8, characterized in that successive slices with X-rays of different energy are scanned. CT device using spiral scanning for use in the method according to claim 8, 10, characterized in that the energy of the X-rays used is changed during scanning. CT device for use in the method according to claim 8, characterized in that successively recorded profiles with a different radiation spectrum are generated. CT device according to claim 11, characterized in that the modulation of the energy of the beam source created during rotation contains at least one periodic component. Apparatus for use in the method according to any one of claims 1 to 7, characterized in that it is suitable for simultaneously measuring bone density and taking a picture of the thorax or other parts of the body. Device for use in the method according to claim 8, characterized in that it is suitable for making a rotational-angio recording. 15. Device for use in the method according to any one of claims 1-6, characterized in that the object to be examined and the radiation source move relative to each other with the associated detector system and in that the detector system consists of more than one row of detector elements, the detector elements being switched in TID mode in the direction of relative movement relative to the object. 1013865