GB1569883A - Xeroradiography - Google Patents

Description

(54) IMPROVEMENT IN XERORADIOGRAPHY (71) We, NATIONAL RESEARCH DE VELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66-74 Victoria Street, London S.W.1., do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to improvements in xeroradiography and relates particularly to a filter for use in xeroradiography.

The well known technique of xerography is used in medical diagnosis in the form of xeroradiography in which an x-ray beam is passed through a patient to a xerographic plate. However, a major disadvantage of the technique is the need for a greatly increased radiation dosage in comparison with conventional radiography using silver halide film as a recording medium.

The dose may be greater by a factor of three or four, and there may be considerable reluctance to expose a patient to such high levels of x-rays.

According to the invention, apparatus for use in xeroradiography comprising the combination of a source arranged to produce a beam of x-rays; closely adjacent the source a sheet arranged to filter the x-ray beam before the beam impinges on a specimen said sheet comprising nickel or copper or zinc between 0.10 millimetres and 0.35 millimetres thick and at least 98% pure or an alloy of at least one of said metals of such thickness as to provide equivalent x-ray absorption at a selected x-ray wavelength; and means for supporting a xeroradiographic plate so as to be irradiated by the filtered x-ray beam.

Preferably the sheet thickness is in the range 0.15 mm to 0.30 mm. The atomic numbers of the metals are nickel 28, copper 29 and zinc 30; a relatively thick nickel sheet will have equivalent absorbance to a relatively thin zinc sheet. In a preferred example, a sheet of copper 0.25 mm thick is used.

The sheet will normally be between 3 centimetres square and 10 centimetres square.

It has been found that the provision of a copper filter allows the use of xeroradiography at much lower dosages than has previously been possible. It is expected that the major use will be the study of living human bodies but there may also be other occasions when xeroradiography at low dosages is required. The technique has been used experimentally to study dead animals and dead human organs.

A filter device according to the invention is provided in addition to the conventional aluminium filter used in xeroradicJaphy.

Also according to the invention, a method of obtaining a xeroradiogram of a specimen comprises passing a beam of x-rays through a sheet of nickel or copper or zinc of thickness between 0.15 mm and 0.35 mm and at least 98% pure or an alloy of at least one of said materials of such thickness as to provide equivalent x-ray absorption at a selected x-ray wavelength, and then through the specimen on to a xerographic recording material.

The invention will now be described by way of example with reference to the drawings filed with this specification in which: Figure 1 is a diagrammatic section of xeroradiographic apparatus according to the invention used to record a living human body; Figure 2 shows the spectral distributions of x-ray beams with and without filters; Figure 3 shows the absorption by human tissue of a copper-filtered x-ray beam; and Figure 4 shows typical values of the percentage charge remaining on a xero raphic plate after exposure to x-rays which have passed through various materials.

In Figure 1, a patient indicated by reference 1 is placed at a specimen stage above a xerographic recording plate 2. An x-ray source of conventional type is indicated by reference 3, and x-rays from the target 4 are arranged to pass through a filter 5 according to the invention, consisting of a sheet of copper 0.25 mm thick, through a conventional xeroradiographic filter o consisting of a sheet of aluminium 2.0 mm thick and through a collimator indicated generally by reference 7 which provides a collimated beam 8 which passes to the patient l; after passage through the patient the residual beam is recorded by the plate 2.

In Figure 2, curve A shows the spectral distribution of the x-ray beam used in conventional xeroradiography after passage through a sheet of aluminium 2.0 mm thick; curve B shows the effect of providing in addition a filter comprising a 0.25 mm sheet of copper; curve C is the relative sensitivity of a conventional xerographic plate.

The human body completely absorbs xrays at energies lower than about 35 KeV, and x-rays of higher energy are required to provide an x-ray record, whether conventional or xerographic. The absorption cut off is in the region indicated by line D and it can easily be seen from Figure 2 that a beam of x-rays represented by curve A contains a considerable proportion of unwanted x-rays below the cut off. Curve B shows a much lower proportion of x-rays below the cut off, so that a xeroradiographic record obtained using such a beam would require exposure of a patient to a much lower dosage of unwanted radiation. In some cases the total does may be reduced by a factor of three or four, bringing the required dose as low as, or even lower than, that required for conventional radiography.

Figure 3 shows the spectral distribution of a copper and aluminium filtered beam represented by curve B in Figure 2 after transmission through 10 cm of muscle or through 7 cm muscle plus 3 cm of bone.

There is relatively little radiation at low energies.

Referring again to Figure 2, curve C shows that the sensitivity of a xerographic plate decreases rapidly below 35 KeV so use of a copper filter causes little loss in sensitivity. In practice, the quality of the image may even be improved. This effect is illustrated in Figure 2, which compares the residual charge on a xerographic plate after exposure to x-ray beams filtered by a 2 mm thick sheet of aluminium (shown by the chain-dotted line) and by the 2 mm thick aluminium plus a 0.25 mm thick copper sheet (shown by the full line). In both cases the beams themselves without attenuation cause almost complete discharge of the plate. Attenuation by 10 cm muscle leaves residual charges of almost 6% and almost 2% for beams respectively with and without a copper filter, and for 7 cm muscle plus 3 cm bone the figures are over 16% and almost 11.5%.The important feature is the increase in size of the steps between the residual charges for different types of attenuation. Use of a copper filter increases the step between muscle and muscle plus bone by about 9% and increases the step between an unattenuated beam and attenuation by 10 cm muscle by about 140%. Soft tissue is therefore more clearly recorded per se when a copper filter is used, and can also be recorded on the same plate as bone.

It will be understood that the values in Figure 4 are illustrative and by way of example only.

The copper filter should be fitted as close as possible to the target in the x-ray tube, should be easily removable to allow conventional radiographic use of the x-ray source, and is preferably fitted with indicating means connected to the control panel of the apparatus to provide a clear indication when the copper filter is not in an operative position.

Use of filters of higher atomic number or greater thickness than stated above would not improve the spectral distribution of the x-ray beam and would need longer exposure times with the attendant risk of the patient moving and causing a blurring of the image.

WHAT WE CLAIM IS:- 1. Apparatus for use in xeroradiography comprising the combination of a source arranged to produce a beam of 'c-rays: closely adjacent the source a sheet arranged to filter the x-ray beam before the beam impinges on a specimen, said sheet comprising nickel or copper or zinc between 0.10 millimetres and 0.35 millimetres thick and at least 98% pure or an alloy of at least one of said metals of such thickness as to provide equivalent x-ray absorption at a selected x-ray wavelength; and means for supporting a xeroradiographic plate so as to be irradiated by the filtered x-ray beam.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. raphic plate after exposure to x-rays which have passed through various materials. In Figure 1, a patient indicated by reference 1 is placed at a specimen stage above a xerographic recording plate 2. An x-ray source of conventional type is indicated by reference 3, and x-rays from the target 4 are arranged to pass through a filter 5 according to the invention, consisting of a sheet of copper 0.25 mm thick, through a conventional xeroradiographic filter o consisting of a sheet of aluminium 2.0 mm thick and through a collimator indicated generally by reference 7 which provides a collimated beam 8 which passes to the patient l; after passage through the patient the residual beam is recorded by the plate 2. In Figure 2, curve A shows the spectral distribution of the x-ray beam used in conventional xeroradiography after passage through a sheet of aluminium 2.0 mm thick; curve B shows the effect of providing in addition a filter comprising a 0.25 mm sheet of copper; curve C is the relative sensitivity of a conventional xerographic plate. The human body completely absorbs xrays at energies lower than about 35 KeV, and x-rays of higher energy are required to provide an x-ray record, whether conventional or xerographic. The absorption cut off is in the region indicated by line D and it can easily be seen from Figure 2 that a beam of x-rays represented by curve A contains a considerable proportion of unwanted x-rays below the cut off. Curve B shows a much lower proportion of x-rays below the cut off, so that a xeroradiographic record obtained using such a beam would require exposure of a patient to a much lower dosage of unwanted radiation. In some cases the total does may be reduced by a factor of three or four, bringing the required dose as low as, or even lower than, that required for conventional radiography. Figure 3 shows the spectral distribution of a copper and aluminium filtered beam represented by curve B in Figure 2 after transmission through 10 cm of muscle or through 7 cm muscle plus 3 cm of bone. There is relatively little radiation at low energies. Referring again to Figure 2, curve C shows that the sensitivity of a xerographic plate decreases rapidly below 35 KeV so use of a copper filter causes little loss in sensitivity. In practice, the quality of the image may even be improved. This effect is illustrated in Figure 2, which compares the residual charge on a xerographic plate after exposure to x-ray beams filtered by a 2 mm thick sheet of aluminium (shown by the chain-dotted line) and by the 2 mm thick aluminium plus a 0.25 mm thick copper sheet (shown by the full line). In both cases the beams themselves without attenuation cause almost complete discharge of the plate. Attenuation by 10 cm muscle leaves residual charges of almost 6% and almost 2% for beams respectively with and without a copper filter, and for 7 cm muscle plus 3 cm bone the figures are over 16% and almost 11.5%.The important feature is the increase in size of the steps between the residual charges for different types of attenuation. Use of a copper filter increases the step between muscle and muscle plus bone by about 9% and increases the step between an unattenuated beam and attenuation by 10 cm muscle by about 140%. Soft tissue is therefore more clearly recorded per se when a copper filter is used, and can also be recorded on the same plate as bone. It will be understood that the values in Figure 4 are illustrative and by way of example only. The copper filter should be fitted as close as possible to the target in the x-ray tube, should be easily removable to allow conventional radiographic use of the x-ray source, and is preferably fitted with indicating means connected to the control panel of the apparatus to provide a clear indication when the copper filter is not in an operative position. Use of filters of higher atomic number or greater thickness than stated above would not improve the spectral distribution of the x-ray beam and would need longer exposure times with the attendant risk of the patient moving and causing a blurring of the image. WHAT WE CLAIM IS:-

1. Apparatus for use in xeroradiography comprising the combination of a source arranged to produce a beam of 'c-rays: closely adjacent the source a sheet arranged to filter the x-ray beam before the beam impinges on a specimen, said sheet comprising nickel or copper or zinc between 0.10 millimetres and 0.35 millimetres thick and at least 98% pure or an alloy of at least one of said metals of such thickness as to provide equivalent x-ray absorption at a selected x-ray wavelength; and means for supporting a xeroradiographic plate so as to be irradiated by the filtered x-ray beam.

2. Apparatus according to Claim 1 in

which the sheet is between 0.15 millimetres and 0.30 millimetres thick.

3. Apparatus according to Claim 1 or Claim 2 in which the sheet is a copper sheet 0.25 millimetres thick.

4. A method of obtaining a xeroradiogram of a specimen comprising passing a beam of x-rays through a sheet of nickel or copper or zinc of thickness between 0.10 millimetres and 0.35 millimetres thick and at least 98% pure or an alloy of at least one of said metals of such thickness as to provide equivalent x-ray absorption at a selected x-ray wavelength, and then through the specimen on to a xerographic recording material.

5. Apparatus for use in xeroradiography substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.