Microwave radiation can be used to image materials that are opaque to visible light, and is therefore used in applications such as medical imaging, ground penetrating radar and de-mining. The technique used generally involves transmitting microwave radiation through air at an object to be imaged and detecting the signal reflected back from the inside of the object. In the case of medical imaging, the use of microwave radiation also has the advantage of having less harmful side effects caused by ionisation than are caused by X-ray imaging.

However, known microwave imaging techniques have the disadvantage that as the material properties of the object to be imaged differ from that of air, reflection from the boundary between the object and the air often causes unwanted interference at the receiver and can make it difficult to extract image data from the received radiation.

Several methods of reducing this reflection can be used.

Firstly, the radiation reflected at the boundary takes less time to travel from the source to the detector than radiation reflected from a region such as a tumour inside the patient being imaged. By sending radiation pulses and sampling the signal in the time domain, the radiation from inside the object is received at a different time from the radiation reflected at the surface. However, this adds to the complexity of the imaging apparatus.

Secondly, the object to be imaged can be immersed in a gel having similar physical properties to the object. For example, in medical imaging the part of the patient's body to be imaged, as well as the microwave transmitter and receiver can be immersed in a suitable gel having similar microwave transmission properties to human tissue, thereby reducing any reflection that would have occurred at an air/tissue boundary. This method has the disadvantage of being rather messy, inconvenient and uncomfortable for the patient. Also, gel can be lossy at microwave frequencies, causing loss of signal power.

Preferred embodiments of the present invention seek to overcome the above described disadvantages of the prior art.

According to the present invention, there is provided a device for reducing reflected electromagnetic radiation in a medical imaging process, the device comprising:- a sheet of material adapted to be placed against the body of a patient and to at least partially transmit electromagnetic radiation therethrough, wherein the thickness and characteristic impedance of said material are selected to reduce the intensity of the radiation reflected back towards at least one source of said radiation, compared with the intensity of radiation reflected from the surface of said patient in the absence of said device.

By providing a device which reduces the intensity of radiation reflected back towards the radiation source as a result of the characteristic impedance and thickness of the device, this provides the advantage that the intensity of reflected radiation from the air/patient boundary is reduced without having to immerse the patient and the transmitter in a medium having similar physical properties to human tissue.

Also, because there is now no need for processing of the reflected signal to electronically remove the unwanted component, the processing of signals representing reflected radiation is simplified.

In a preferred embodiment, the thickness of the sheet of dielectric material in use is approximately an odd number of multiples of one quarter of the centre wavelength of the electromagnetic radiation in use.

By providing a sheet having a thickness of approximately an odd number of multiples of one quarter of the wavelength of the radiation in use, this provides the advantage that radiation reflected from the surface of the skin of the patient will be approximately 180° out of phase with radiation reflected from the outer surface of the sheet in use, causing destructive interference and reducing the intensity of unwanted reflection.

In a preferred embodiment, the characteristic impedance Zd of the dielectric material in use is given approximately by: Zd = A/ (Zazb) where Za is the characteristic impedance of air, and Zb is the characteristic impedance of tissue to be imaged.

The sheet may comprise a powdered first material having a first mean dielectric constant, mixed with a second material having a second mean dielectric constant, lower than said first mean dielectric constant.

In a preferred embodiment, said first material comprises at least one ceramic material.

The second material may comprise polytetrafluoroethylene.

The second material may comprise polyethylene.

The device may further comprise several sheets of different characteristic impedance placed on top of one another in order to reduce reflection of electromagnetic radiation over a predetermined bandwidth.

This provides the advantage of reducing unwanted reflection over a range of wavelengths.

In a preferred embodiment, the frequency of the electromagnetic radiation in use is in the range 1 GHz to 100 GHz.

According to another aspect of the present invention, there is provided a method of reducing unwanted reflection of electromagnetic radiation from a surface in an imaging process, the method comprising:- placing a sheet of dielectric material on the surface, said sheet adapted to at least partially transmit electromagnetic radiation. therethrough ; and transmitting electromagnetic radiation having a predetermined range of wavelengths through said sheet; wherein the thickness and characteristic impedance of said material are selected to reduce the intensity of the radiation reflected back towards at least one source of said radiation compared with the intensity of radiation reflected from said surface in the absence of said device.

In a preferred embodiment, the thickness of the sheet of dielectric material in use is approximately an odd number of multiples of one quarter of the centre wavelength of the electromagnetic radiation in use.

In a preferred embodiment, the characteristic impedance Zd of the dielectric material in use is given approximately by: Zd = #(ZaZb) where Za is the characteristic impedance of air, and Zb is the characteristic impedance of tissue to be imaged.

The sheet may comprise a powdered first material having a first mean dielectric constant, mixed with a second material having a second mean dielectric constant, lower than said first mean dielectric constant.

The first material may comprise at least one ceramic material.

The second material may comprise polytetrafluoroethylene.

The second material may comprise polyethylene.

The sheet may further comprise several sheets of different characteristic impedance placed on top of one another in order to reduce reflection of electromagnetic radiation over a predetermined bandwidth.

In a preferred embodiment, the frequency of the electromagnetic radiation in use is in the range 1 GHz to 100 GHz.

A preferred embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings in which:- Figure 1 is a schematic diagram representing the principle of operation of the present invention; Figure 2 is a schematic representation of a medical imaging process using the device embodying the present invention.

Referring to Figure 1, a region of a medium 2 to be imaged, such as tissue of a patient, is covered by a layer 4 of dielectric material having thickness t placed in direct contact with the skin of the patient above the area to be imaged. The sheet 4 may be in the form of a sheet to be placed over the region to be imaged, or may be in the form of a tightly fitting garment similar to a wetsuit. A microwave source at the detector (not shown) irradiates the patient 2 and layer 4 through the air.

The characteristic impedance Z of a material is a measure of its resistance to transmission of electromagnetic radiation such as optical or microwave radiation, and therefore its refractive index. For an electromagnetic wave travelling from air 6 of characteristic impedance Za into tissue 2 having characteristic impedance Zb, it is found that the total intensity of radiation reflected at the air/sheet interface and sheet/patient interface received at the receiver (not shown) to a minimum when the radiation reflected at one interface is in antiphase with the radiation reflected at the other interface. This condition occurs when the characteristic impedance Zd of the dielectric sheet 4 is given by: Zd = V/(Zazb) A beam 10 of microwave radiation having predominant wavelength X is transmitted through the air 2 and is incident on dielectric sheet 4. A part 12 of beam 10 is reflected from the air/dielectric boundary 20 and a part 14 of beam 10 is transmitted through sheet 4. Similarly, at the dielectric/tissue boundary 22, a part 18 of beam 14 is reflected and a part 16 of beam 14 is transmitted into the tissue 6. Reflected beam 18 is transmitted as beam 19 back into the air 6.

In order to reduce the intensity of reflected radiation beam 19, the thickness t of dielectric layer 4 having characteristic impedance Zd is chosen to cause destructive interference at the air/dielectric boundary 20. If beams 12 and 19 are approximately 180° (one half wavelength) out of phase then they will substantially cancel each other out, thus substantially eliminating the reflected beam 19. In order to accomplish this, the path difference between beams 12 and 19 must be equal to an odd number of multiples of one half of the wavelength A. Accordingly, the thickness t of layer 4 must be half of this i. e. t = (2n + 1) X/4 where n is a whole number. In the simplest case, t is a quarter wavelength X/4 such that at boundary 20 beams 12 and 19 are 180° out of phase.

Referring to Figure 2, the layer 4 of dielectric material is placed onto the skin of a patient 24. The sheet 24 is formed from a mixture of a high dielectric powdered ceramic material and a low dielectric plastic material such as polytetrafluoroethylene or polyethylene. The relative proportions of the ingredients are chosen to obtain the correct value of characteristic impedance Zd as defined above. It should be understood that any material having the correct dielectric properties and low intensity loss could be utilised.

For the sheet 4 to work efficiently, it is required that. it makes as good a contact with the skin as possible.

Accordingly, the region of the skin of the patient 24 on which the sheet 4 is to be placed should be shaved before application of the sheet.

A microwave transmitter 26 transmits microwave radiation 28 having a frequency in the range of 1 GHz to 100 GHz through sheet 4 and into the patient 24. The wavelength of this radiation is in the order of millimetres to centimetres, and the thickness of the sheet is chosen to be approximately one quarter of the wavelength of the radiation 28 in use. As a consequence, waves reflected from the air/sheet boundary and waves reflected from the sheet/skin boundary will be approximately 180° out of phase, and will destructively interfere causing a significant reduction in radiation reflected from the skin being received at receiver 32.

Radiation passing into the patient's body is reflected from an area of interest, which may be a bone fracture, tumour or other medical condition. The reflected radiation 30 emerges from the patient and is collected by receiver 32. A control system 34 then analyses the reflected signal, for interpretation by a user. Persons skilled in the art will understand that the dielectric sheet will reduce unwanted reflection from a number of different medical imaging techniques, and also ground penetrating radar systems.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.