Patent Description:
X-ray diagnostics and computed tomography (CT) are the most commonly used medical imaging modalities. Currently, there are two ways to detect X-rays for medical imaging purposes: either using direct solid-state detectors (<NUM>, <NUM>) (semiconductors based) or using indirect detectors based on the scintillators (<NUM>). However, the radiation dose associated CT scans is extremally high and, therefore, increase of detector sensitivity and performance speed is required. X-ray detectors can be characterized by a dead-time - the time that needs to be spent for collection of the electric charge which is proportional to the intensity of the detected radiation. Modern detectors are operated with dead-time of <NUM>-<NUM>-<NUM>-<NUM>s (corresponding to the processing frequency of MHz) (<NUM>).

<NPL>) describes the magnetoelectric properties of a Fe2B/BiFeO3 heterostructure focusing on the ferromagnetic resonance properties.

<NPL>) describes a large area photodetector array to measure intensity of electromagnetic irradiation.

<NPL>) describes nonlinear magnetoelectric properties of hexaferrite systems.

<NPL>) describes the photostrictive effect of magnetoelectric ferrites.

<NPL>) describes an photodetector array made of BiFeO3 detection units.

According to a first aspect of the invention, there is provided a method for determining intensity of electromagnetic radiation as specified in claim <NUM>.

In some embodiments of the invention, the crystal is a hexaferrite crystal, as discussed herein.

According to another aspect of the invention, there is provided a detector as specified in claim <NUM>.

Outside the scope of the invention of radiation detection, the described photoinduced nonlinear magnetoelectric effect can be utilized to create optically coupled ferromagnetic resonators with radiation source such as, but not limited to, light-emitting diode (LED) that emits ultraviolet, visible, or infrared light. These coupled resonators can then be used in various microwave signal processing devices including, but not limited to, filters, phase shifters, and delay lines. For example, by alternating the voltage applied to the LED, the spectral characteristics and phase characteristics of the resonators can be tuned. As will be understood by one of skill in the art, this will increase the dynamic range of the microwave devices incorporating these resonators and will increase the quality factor of the microwave devices. As will be understood by one of skill in the art, these LED-coupled ferromagnetic resonators could be used as adaptive microwave antennas as well.

Outside the scope of the invention, the combination of at least one suitable ferrimagnetic crystal, or suitable ferromagnetic crystal and/or a suitable ferro(ferri)magnetic-photostrictive material heterostructure with an LED provides a coupled resonator which can then be used in various microwave signal processing devices including, but not limited to, filters, phase shifters, and delay lines, as discussed herein.

As used herein, the expression "suitable ferrimagnetic crystal or suitable ferromagnetic crystal" refers to any ferri(ferro)magnetic crystals in which the photoinduced magnetoelectric effect can be observed. While not wishing to be bound or limited to a particular theory or hypothesis, the inventors believe that such a suitable ferri(ferro)magnetic crystal should possess at least one but preferably both of the following properties: (<NUM>) semiconductor behavior; and (<NUM>) while the crystal is absorbing electromagnetic radiation, the generated photocurrent should be distinguished from the dark current that naturally flows through the crystal. As will be appreciated by one of skill in the art, these two properties are necessary for the magnetic to demonstrate a photoinduced magnetoelectric effect.

In addition, as discussed above, the photoinduced magnetoelectric effect can be observed in ferro(ferri)magnetic-photostrictive material heterostructures. As will be understood by one of skill in the art, the photostrictive material will create mechanical stress due to irradiation that will be transmitted to the ferri(ferro)magnetic material in the heterostructure. This mechanical stress will alter the internal magnetic properties of the magnetic materials resulting in the resonance frequency shift, creating the photoinduced magnetoelectric effect in photostrictive-ferro(ferri)magnetic heterostructure.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described.

Described herein is the use of a biased hexaferrite ferrimagnetic semiconductive crystal for detection of the radiation on GHz frequencies. Recently, the presence of current induced magnetoelectric effect has been demonstrated in hexaferrites (<NUM>-<NUM>). It was shown that the frequency of either ferromagnetic or multidomain resonance of the hexaferrite semiconductor crystal can be changed due to electric current flow. Furthermore, the value of current can be calculated based on the frequency shift measurement.

This principle can be used for radiation detection. We have successfully detected the internal photoelectric effect in Zn<NUM>Y hexaferrite crystal (<FIG>).

As will be appreciated by one of skill in the art, while this has been demonstrated for Zn<NUM>Y, any hexaferrite can be used until the hexaferrite crystal begins to exhibit semi-conductor behavior, as discussed herein.

Accordingly, the hexaferrite may be an M-type ferrite, a Z-type ferrite, a Y-type ferrite, a X-type ferrite or a U-type ferrite.

In some embodiments, the hexaferrite is a Y-type ferrite, having the general structure of (Ba/Sr)<NUM>Me<NUM>Fe<NUM>O<NUM> where Me is a small <NUM>+ ion such as for example cobalt, nickel or zinc and where Ba can be substituted by Sr.

Furthermore, any suitable ferrimagnetic crystal or suitable ferromagnetic crystal, for example, any ferri(ferro)magnetic crystals in which the photoinduced magnetoelectric effect can be observed, may be used within the invention, as discussed herein.

As will be appreciated by one of skill in the art, in some embodiments, the heterostructure comprises photostrictive material placed in mechanical contact with ferri(ferro)magnetic materials. The type of photostrictive material will depend on the type of radiation desired to be detected. For example, lead lanthanum zirconate titanate (PLZT) materials with a general structure Pb(<NUM>-x)Lax(ZryTi<NUM>-y)<NUM>-x/<NUM>O<NUM> can be used as photostrictive material for UV light detection.

Since the photostrictive material will interact with the radiation initially, there are no restrictions on the type or composition of the magnetic materials used.

Accordingly, any ferro(ferri)magnetic material can be used in conjunction with photostrictive material to create a heterostructure. For example, the yttrium iron garnet (Y<NUM>Fe<NUM>O<NUM>) can be used as a ferrimagnetic material. Alternatively, hexaferrites mentioned above can be also utilized for engineering of ferrimagnetic-photostrictive heterostructures.

In some embodiments, the ferrite garnets with a general formula of Me<NUM>Fe<NUM>O<NUM> (where Me is a metal cations) can be used in the discussed heterostructures.

As can be appreciated by one of skill in the art, any substituted materials could also be possible to use for the ferri(ferro)magnetic-photostrictive heterostructure.

While not wishing to be bound or limited to a particular theory or hypothesis, the inventors believe that such a suitable ferri(ferro)magnetic crystal or photostrictiveferro(ferri)magnetic heterostructure should possess at least one but preferably both of the following properties: (<NUM>) semiconductor behavior; and (<NUM>) while the crystal is absorbing electromagnetic radiation, the generated photocurrent should be distinguished from the dark current that naturally flows through the crystal. As will be appreciated by one of skill in the art, these two properties are necessary for the magnetic to demonstrate a photoinduced magnetoelectric effect.

According to a first aspect of the invention defined by the claims, there is provided a method for determining intensity of electromagnetic radiation.

In some embodiments of the invention, the suitable crystal is a hexaferrite crystal, as discussed herein.

According to another aspect of the invention, there is provided a detector as defined by the claims.

Outside the scope of the invention of radiation detection, the described photoinduced nonlinear magnetoelectric effect can be utilized to create optically coupled ferromagnetic resonators with radiation source such as, but not limited to, light-emitting diode (LED) that emits ultraviolet, visible, or infrared light. These coupled resonators can then be used in various microwave signal processing devices including, but not limited to, filters, phase shifters, and delay lines. For example, by alternating voltage applied to LED, the spectral characteristics and phase characteristics of the resonators can be tuned. As can be understood by one of skill in the art, this will increase the dynamic range of the described microwave devices and will increase the quality factor of the microwave devices. As will be understood by one skilled in the art, these LED-coupled ferromagnetic resonators could be used as adaptive microwave antennas as well.

Accordingly, in some embodiments outside the scope of the invention, there is provided use of at least one suitable ferrimagnetic crystal, or suitable ferromagnetic crystal and/or a suitable ferro(ferri)magnetic-photostrictive material heterostructure with a light-emitting diode (LED).

In some embodiments, the LED emits ultraviolet, visible, or infrared light.

As will be appreciated by one of skill in the art, the combination of at least one suitable ferrimagnetic crystal, or suitable ferromagnetic crystal and/or a suitable ferro(ferri)magnetic-photostrictive material heterostructure with an LED provides a coupled resonator which can then be used in various microwave signal processing devices including, but not limited to, filters, phase shifters, and delay lines, as discussed herein.

Accordingly, in some embodiments outside the scope of the invention, there is provided a coupled resonator comprising at least one suitable ferrimagnetic crystal, suitable ferromagnetic crystal and/or suitable ferro(ferri)magnetic-photostrictive material heterostructure and a light-emitting diode (LED).

In some embodiments, the LED emits ultraviolet, visible, or infrared light. <FIG> shows the measured DC current-voltage characteristics (CVC) of the sample measured without (black curve), and with, UV irradiation (red curve). The dark CVC became nonlinear for the applied voltages higher than <NUM> V, indicating semiconductor behavior. Once irradiated with UV light, nonlinear behavior of CVC was detected at <NUM> V and the photocurrent increased non-linearity of the CVC. The detectable difference between the dark current and photocurrent indicates the possibility of utilizing Zn<NUM>Y for radiation detection since this difference causes an additional change in the ferromagnetic resonance of the crystal.

Following the detection of photoelectric effect in Zn<NUM>Y crystal, we have successfully detected a photoinduced nonlinear magnetoelectric effect in the monocrystal of Zn<NUM>Y (<FIG>). The ferromagnetic resonance of the hexaferrite crystal was observed at <NUM> (<FIG> black curve). Once 17V bias was applied, the resonance frequency shifted down to the <NUM> (<FIG> red curve). When the crystal was irradiated by ultraviolet light, an <NUM>% larger shift in the ferromagnetic resonance frequency was observed (<FIG> blue curve). The ferromagnetic resonance frequency under ultraviolet radiation was equal to <NUM>. The dependence of the ferromagnetic resonance frequency shift on bias voltage is shown in <FIG>.

These results demonstrate how radiation can be detected by exploiting this photoinduced nonlinear magnetoelectric effect. Specifically, once the crystal is irradiated, the photocurrent induces change in resonance frequency (<FIG>) of the sample by affecting its internal magnetic field distribution. This resonance frequency change is proportional to the induced photocurrent and, therefore, the radiation intensity. By measuring the resonance frequency change caused by exposing a hexaferrite crystal to photoinduced nonlinear magnetoelectric effect it is possible to obtain the intensity of the incidental radiation.

This phenomenon can be extended for the detection of any type of electromagnetic radiation. If hexaferrite crystal is biased and irradiated by any electromagnetic waves, and/or experiences an internal photoelectric effect, the frequency of ferromagnetic resonance will be changed proportionally to the intensity of absorbed radiation. Therefore, by conducting frequency measurement of the ferromagnetic resonance similar to the approach demonstrated above, it is possible to measure the intensity of the incoming radiation.

As will be appreciated by one of skill in the art, using a matrix of hexaferrite resonators placed for example on copper transmission lines produces a radiation detector capable of detecting radiation locally.

In some embodiments, to make the resonance frequency of every resonator unique, a matrix or array of crystals can be placed in a 2D magnetic field gradient. In these embodiments, the reference signal can be generated by, for example, a sweep generator with a spectrum broad enough to cover all resonance frequencies of the crystals in the hexaferrite matrix. In these embodiments, the resonance frequency measurements are made using for example a microwave spectrometer. Since the resonance frequency of each crystal is unique, it is easy to quantify the intensity of electromagnetic radiation which was absorbed by every pixel on the matrix. As will be apparent to one of skill in the art, detectors built using this approach can be used for detection of any radiation for example but by no means limited to visible light, ultraviolet light, X-rays and the like. Thus, these detectors can be implemented in a variety of spectrometers and medical imaging modalities.

For example, to perform X-ray imaging using photoinduced non-linear magnetoelectric effect as described herein, an array of MxN hexaferrite resonators as described herein is arranged. As will be apparent to one of skill in the art, the matrix size can vary depending on the end application. For, example, a CT detector matrix will be larger than a digital mammography detector or a fluoroscopy detector. The shape of the detector may also vary depending on the potential application - for example, CT detectors would likely be curved but in other applications, the detector may be flat or substantially flat. As discussed above, the MxN hexaferrite matrix is placed in a gradient magnetic field in order to magnetize ferrimagnetic resonators and to create a specific resonance frequency for each respective one resonator. The array size will also vary depending on the application. For instance, for digital mammography, the array might be on the order of 25x25 cm<NUM> so as to be similar in size to previously developed direct digital detectors. An example of a suitable detector construction can be seen in <FIG>.

Specifically, as can be seen from <FIG>, it is placed in the external magnetic field B0, and the gradients of the external magnetic field are created along X- and Y-axes (Gx and Gy respectively). In use, similar to prior art devices, the patient would be placed between the radiation source and the detector. The radiation source and the detector could either rotate around the patient to obtain images at different angles (for CT or fluoroscopy imaging purposes) or remain stationary (for conventional 2D X-ray imaging). The radiation attenuated by the patient will be detected by the crystalline array, as discussed above.

Unlike radiation detectors that rely on photocurrent measurement and quantification, an array of hexaferrite resonators does not require any additional electronic circuits designed for charge collection and/or current measurement. Therefore, manufacturing of the hexaferrite radiation detector described herein will avoid the difficulties associated with solid-state radiation detectors. In addition, unlike conventional radiation detectors that frequently utilize amorphous and polycrystalline materials, the hexaferrite radiation detector utilizes hexaferrite monocrystals, which significantly improves the charge carrier mobilities. Finally, the hexaferrite radiation detector has a working frequency of <NUM>-<NUM> which is up to <NUM> orders of magnitude higher than currently available radiation detectors. Utilization of such operating frequencies produces increases in signal detection speed and will result in significant reduction of radiation dose required for a single scan. In addition, faster performance of the hexaferrite radiation detector will allow for a substantial increase in the resulting image quality.

The primary advantage of this radiation detection device is the high operating frequencies, as discussed above, which result in a significantly shorter dead time for the detector. For example, the known hexaferrite crystal Zn<NUM>Y experiences ferromagnetic resonance around <NUM>. Barium hexaferrite (BaFe<NUM>O<NUM>) has a resonance frequency - <NUM>. These operating frequencies are several orders of magnitude higher compared to the frequencies commonly used with for example X-ray detection. Therefore, hexaferrite detectors based on photoinduced magnetoelectric effect potentially will have significantly shorter dead-time which means that a lower radiation dose is needed to obtain the same signal-to-noise ratio of an X-ray image.

Claim 1:
A method for determining intensity of electromagnetic radiation, comprising:
providing a plurality of ferrimagnetic and/or ferromagnetic crystals suitable for producing a photoinduced magnetoelectric effect arranged in an array, each respective ferrimagnetic or ferromagnetic crystal having a different control resonance frequency;
exposing the ferrimagnetic and/or ferromagnetic crystals
to electromagnetic radiation of unknown intensity or variable intensity such that the ferrimagnetic and/or ferromagnetic crystals undergo an internal photoelectric effect,
measuring an irradiated resonance frequency of the irradiated ferrimagnetic and/or ferromagnetic crystals; and
determining the intensity of the electromagnetic radiation irradiating the ferrimagnetic and/or ferromagnetic crystals from the respective control resonance frequency and the respective irradiated resonance frequency.