Patent Application
20080035867
According to the present invention a method is provided of reading a radiation image, stored in a CsBr:Eu type binderless needle-shaped photostimulable or storage phosphor screen after X-ray exposure of said screen, said method comprising the steps of:
According to the method of the present invention the step of erasing thermally stimulable energy is thus performed by exposing said screen to infrared radiation in the wavelength range from 1000 nm to 1550 nm.
In a more particular embodiment according to the method of the present invention the step of erasing thermally stimulable energy is performed by exposing said screen to infrared radiation in the wavelength range from 1030 nm to 1130 nm.
With respect to particular radiation emission sources suitable for use in the method of the present invention, the step of thermally stimulable energy stored in shallow traps of the phosphor crystals is performed by exposing said screen to infrared radiation by means of a Nd:YAG laser as a source of infrared radiation.
In another embodiment according to the method of the present invention, the step of thermally erasing stimulable energy by exposing said screen to infrared radiation is performed by means of a Nd:YLF laser as a source of infrared radiation.
In still another embodiment according to the method of the present invention, the step of thermally erasing stimulable energy is performed by exposing said screen to infrared radiation by means of a tungsten lamp with an optical filter as a source of infrared radiation.
In a still further embodiment according to the method of the present invention, the step of thermally erasing stimulable energy is performed by exposing said screen to infrared radiation by means of an infrared LED as a source of infrared radiation.
In another embodiment according to the method of the present invention, the step of thermally erasing stimulable energy is performed by exposing said screen to infrared radiation by means of a diode laser as a source of infrared radiation.
In a particular embodiment according to the method of the present invention a combination of consecutively erasing shallow traps, directly followed by read-out of deep traps by scanning with one and the same laser is made available. Such a particularly suitable laser therefore e.g. is a Nd:YAG laser. So a first scanning with the said laser, while blocking stimulating blue light by a filter and allowing transmittance of radation of long wavelengths in the infrared wavelength range is advantageously followed by a direct scanning with the same laser, without a filter blocking the stimulating blue light now in order to allow stimulation of stored energy and to provide emission of energy, stored in deep energy traps, in order to provide representation of the radiation image of an X-ray exposed subject with less noise and a better image quality.
So in a device for reading information stored in a phosphor layer, as in U.S. Pat. No. 6,369,402; a transparent carrier material including the CsBr:Eu phosphor layer is provided further with a radiation source for emitting excitation or stimulating radiation; a receiver for receiving emission radiation emitted by the phosphor layer, wherein the radiation source is arranged on one side of the carrier material and the receiver is arranged on the other side of the carrier material, so that an optical path is defined between the radiation source and the receiver and at least one thin reflective layer disposed in the optical path between the radiation source and the receiver for reflecting at least a portion of the stimulating excitation radiation away from said receiver. In such a device the reflective layer is arranged between the radiation source and the phosphor layer and designed to reflect a wavelength range of the excitation radiation which is not used to excite the phosphor layer. More particularly when, as in the present invention, it is advantageous to have two reflective layers, in that the first reflective layer is arranged between the phosphor layer and the receiver and in that the second reflective layer is arranged between the radiation source and the phosphor layer and designed to reflect a wavelength range of the stimulating radiation not used to excite the phosphor layer. The device advantageously is a construction wherein the carrier material and the phosphor layer have a fixed location in the device, wherein the radiation source is arranged on a side of the carrier material facing away from the phosphor layer and the receiver is arranged on a side of the carrier material facing towards the phosphor layer and where there is a straight optical path between the radiation source and receiver; and between the phosphor layer and receiver, wherein the receiver is provided with an optical imaging means capable of capturing the emission radiation emitted by the phosphor layer and imaging the emission radiation onto the receiver. The device is further provided with imaging means comprising optical waveguides. In such a device the radiation source is a line light source for exciting an individual row of the phosphor layer and the receiver, therefor comprising a plurality of pixels for point-by-point reception of the emission radiation and wherein the emission radiation emitted by the excited row of the phosphor layer can be simultaneously received by the pixels, so that the phosphor layer can be read row by row. In the present invention it is advantageous to first excite, row by row, the shallow traps in the phosphor, i.e. that in a first scanning, line per line, the blue laser light of the NdYAG laser is blocked and/or reflected while the long infrared wavelengths are erasing the shallow traps, whereas in a second scanning, whether or not almost immediately following the first scanning, the blue laser light is transmitted and is stimulating the deep traps generated by X-ray exposure and energy storage of the latent image, which should be read-out in order to represent the image-wise X-ray exposed subject.
In another embodiment a tunable laser, providing ability to change its emitted wavelength as desired, is used in order to provide read-out by energy having an optimally chosen wavelength in the stimulation spectrum in order to get a stimulated emission signal as high as possible.
Whereas use of one and the same laser requires transport of the plate twice, thereby doubling the read-out time, an alternative is offered by providing a read-out system having two lasers, positioned adjacent to each other, so that the steps of erasure and read-out immediately follow subsequently.
The described scan-head type differs from the conventional flying spot type in that in the scan-head type the image read-out is line-wise whereas in the conventional flying spot type read-out unit the reading is performed in a point-by-point fashion.
In such an arrangement, the first reflective layer is arranged between the imaging means and the receiver. In a particular embodiment the radiation source and the receiver are connected to each other and the device further comprises a driver for providing a relative motion in a transport direction between the radiation source, the receiver and the phosphor layer. Further the device has a first reflective layer which is arranged between the imaging means and the receiver.
The device wherein the radiation source and the receiver are connected to each other further comprises a driver for providing a relative motion in a transport direction between the radiation source, the receiver and the phosphor layer.
In one embodiment the read-out unit comprises a linear light source for emitting stimulating light onto the photostimulable phosphor screen. This linear light source comprises 4096 individual laser diodes arranged in a row. This light source provides simultaneous illumination of all pixels of a single line of the photostimulable phosphor screen.
The read-out unit further comprises a fiber optic plate for directing light emitted by the phosphor screen upon stimulation onto a linear array of sensor elements, i.e., more particulary charge coupled devices. The fiber optic plate comprises a number of mounted light guiding fibers arranged in parallel, in order to guide the light emitted by each individual element of an illuminated line onto a sensor element.
Alternatively the fiber optic plate can be replaced by an arrangement of selfoc lenses or microlenses. A light guide member might even be avoided.
In still another embodiment the array of stimulating light sources, the fiber optic plate and the sensor array are arranged at the same side of the photostimulable phosphor screen. After read-out the photostimulable phosphor screen is erased so that the energy remaining in the screen after read-out is released, so that the screen is in a condition for reuse.
In the type of read-out apparatus wherein stimulation is performed by means of light emitted by a linear light source extending parallel to a scan line on the stimulable phosphor screen, the erasure unit preferably forms part of the read-out unit.
An additional reflective layer for reflecting emission radiation emitted by the phosphor layer is arranged between the radiation source and the phosphor layer in order to reflect emission radiation back to the phosphor layer.
In the device the reflective layer advantageously has a thickness equal to one quarter of the wavelength of the excitation radiation which should be reflected by that reflective layer.
In one aspect according to the method of the present invention, erasing is performed with at least one laser.
In another aspect according to the method of the present invention, erasing is performed with one and the same laser for all of the erasing steps.
In a particular embodiment according to the method of the present invention, said laser is a tunable laser.
In a further particular embodiment according to the method of the present invention, the main wavelength of the said laser is mixed with one or more harmonics thereof, obtained by frequency doubling.
In the method according to the present invention, performing erasure with said one and the same laser proceeds by a longer erasing wavelength in a first erasing step and a shorter erasing wavelength in a last erasing step.
Moreover according to the method of the present invention, performing erasure with said longer erasing wavelength in a first erasing step proceeds in the presence of a filter in order to prevent transmission of said shorter erasing wavelength.
Further according to the method of the present invention, performing erasure with said shorter erasing wavelength in a last erasing step proceeds without filter.
In a further embodiment according to the method of the present invention, the step of stimulating is performed with a linear array of laser diodes as a light source.
In another aspect according to the method of the present invention, the step of detecting is performed with a linear array of charge coupled device elements as an array of transducer elements converting the said detected light emitted upon stimulation into an electrical signal representation.
In the method according to the present invention, said CsBr:Eu phosphor is advantageously prepared by mixing CsBr as an alkali metal halide salt and wherein as a lanthanide dopant salt use is made of EUX2, EuX3, EUOX or EuXz, wherein 2<z<3 and wherein X is one of Br, Cl or a combination thereof.
In another embodiment thereof, according to the present invention, said CsBr:Eu phosphor is advantageously prepared by mixing CsBr as an alkali metal halide salt and wherein between 10 and 5 mol % of a Europium compound selected from the group consisting of EUX2, EUX3, EuOX, or EuXz, wherein 2<z<3 and wherein X is one of Br, Cl or a combination thereof, firing the mixture at a temperature above 450° C., cooling said mixture, and recovering the CsBr:Eu phosphor.
In still another embodiment thereof, according to the present invention, said CsBr:Eu phosphor is advantageously prepared by mixing CsBr as an alkali metal halide salt and a combination of an alkali metal halide salt and a lanthanide dopant salt according to the formula CsxEuyX′x+αy, wherein x/y>0.25, wherein α≧2 and wherein X′ is a halide selected from the group consisting of Cl, Br and I and combinations thereof.
According to the method of the present invention said CsBr:Eu phosphor screen is obtained by applying said phosphor on a substrate by a method selected from the group consisting of physical vapor deposition, thermal vapor deposition, chemical vapor deposition, radio frequency deposition and pulsed laser deposition.
An image-forming system making use of the methods of the present invention as described above is thus recommended in order to provide a better signal to noise representation of the desired image.
While the present invention will hereinafter in the examples be described in connection with preferred embodiments thereof, it will be understood that it is not intended to limit the invention to those embodiments. It is further clear that all of the references cited in the detailed description hereinbefore are incorporated herein by reference.
CsBr:Eu photostimulable phosphor screens were prepared by a vapor deposition process, on flexible chromium sealed anodized aluminum plates, in a vacuum chamber by means of a resistive heating of crucibles, having as starting materials a mixture of CsBr and EuOBr as raw materials. Said deposition process onto said flexible anodized aluminum supports was performed in such a way that said support was rotating over the vapor stream.
An electrically heated oven with two refractory trays or boats—one placed on the left side, the other on the right side, were used, in which 330 g of a mixture of CsBr and EuOBr as raw materials in a 99.5%/0.5% CsBr/EuOBr percentage ratio by weight were present as raw materials in each of said crucibles in order to become vaporized. As crucibles an elongated boat having a length of 100 mm was used, having a width of 35 mm and a side wall height of 50 mm composed of “tantalum” having a thickness of 0.5 mm, composed of 3 integrated parts: a crucible container, a “second” plate with slits and small openings and a cover with slit outlet. The longitudinal parts were fold from one continuous tantalum base plate in order to overcome leakage and the head parts are welded. Said second plate was mounted internally in the crucible at a distance from the outermost cover plate which was less than ⅔ of said side wall height of 45 mm. Under vacuum pressure (a pressure of 2×10−1 Pa equivalent with 2×10−3 mbar) maintained by a continuous inlet of argon gas into the vacuum chamber, and at a sufficiently high temperature of the vapor source (760° C.) the obtained vapor was directed towards the moving sheet support and was deposited thereupon successively while said support was rotating over the vapor stream. Said temperature of the vapor source was measured by means of thermocouples present outside and pressed under the bottom of said crucible and by tantalum protected thermocouples present in the crucible. Before starting evaporation in the vapor deposition apparatus, while heating the raw mixture in the boat or crucible and to make them ready for evaporation, shutters are covering the boats, trays or crucibles.
The chromium sealed anodized aluminum support having a thickness of 800 μm, a width of 18 cm and a length of 24 cm, was covered with a parylene C precoat at the side whereupon the phosphor should be deposited, positioned at a distance—measured perpendicularly—of 22 cm between substrate and crucible vapor outlet slit.
Plates were taken out of the vapor deposition apparatus after having run same vapor deposition times, leading to phosphor plates having phosphor layers of about equal thicknesses.
In the
A test showed that a Nd:YAG laser (1064 nm) and a diode laser (1300 nm and 1550 nm) were providing the best result with respect to erasure, preference to be given to the Nd:YAG laser.
It has thereby clearly been shown that the long wavelengths cited above in fact provide the best results. However as is well known a monochromator creates harmonics, positioned at wavelengths twice as long or only half as long. In order to further prove this, an optical filter allowing transmittance of radiation having half the wavelength as set forth and blocking the longer wavelength as set forth above made clear that the desired effect indeed disappeared.
As an advantageous effect of the present invention, exposure of CsBr:Eu-type phosphors having been exposed to X-rays, before starting read-out by stimulating light in the range from 550 nm to 850 nm, with sources emitting infrared radiation in the range from 1000 nm to 1550 nm, and more particularly in the range from 1030 nm to 1130 nm, results in a remarkable decrease of thermally stimulated radiation emitted in form of undesired afterglow, and further advantageously results in a decrease of noise level of the detected and reproduced X-ray image and in a higher dynamic range of the resulting reproduced X-ray image.
Having described in detail preferred embodiments of the current invention, it will now be apparent to those skilled in the art that numerous modifications can be made therein without departing from the scope of the invention as defined in the appending claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06118704.3 | Aug 2006 | EP | regional |
This application claims the benefit of U.S. Provisional Application No. 60/839,379 filed Aug. 22, 2006, which is incorporated by reference. In addition, this application claims the benefit of European Application No. 06118704.3 filed Aug. 10, 2006, which is also incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60839379 | Aug 2006 | US |
