Apparatus and method for eliminating residual charges in an image capture panel

An image capture panel includes a plurality of sensors arrayed adjacent to a substrate layer of dielectric material, and a radiation sensitive layer disposed over the sensors. The radiation sensitive layer is exposed to a first substantially uniform pattern of light radiation for partially neutralizing residual electrical charges trapped within the image capture panel. The radiation sensitive layer is then exposed to a second substantially uniform pattern of light radiation sufficient to neutralize substantially all residual electrical charges trapped within the image capture panel.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to an image capture panel for capturing 
direct radiographic images. More particularly, the present invention 
pertains to a method and apparatus for eliminating residual electrical 
charges residing in the image capture panel prior to capture of a 
subsequent radiographic image. 
2. Description of the Related Art 
Traditional medical diagnostic processes record X-ray image patterns on 
silver halide films. These systems direct an initially uniform pattern of 
interrogating X-ray radiation through a patient to be studied, intercept 
the consequently imagewise modulated pattern of X-ray radiation with an 
X-ray radiation intensifying screen, record the intensified pattern in a 
silver halide film, and chemically transform this latent radiation pattern 
into a permanent and visible image, called a radiogram. 
Radiograms have also been produced by using layers of radiation sensitive 
materials to directly capture radiographic images as imagewise modulated 
patterns of electrical charges. Depending on the intensity of the incident 
X-ray radiation, electrical charges generated either electrically or 
optically by the X-ray radiation within a pixelized area are quantized 
using a regularly arranged array of discrete solid state radiation 
sensors. U.S. Pat. No. 5,319,206, issued to Lee et al. on Jun. 7, 1994 and 
assigned to E. I. du Pont de Nemours and Company, describes a system 
employing a layer of photoconductive material to create an imagewise 
modulated areal distribution of electron-hole pairs which are subsequently 
converted to corresponding analog pixel (picture element) values by 
electrosensitive devices, such as thin-film transistors. U.S. Pat. No. 
5,262,649 (Antonuk et al.) describes a system employing a layer of 
phosphor or scintillation material to create an imagewise modulated 
distribution of photons which are subsequently converted to a 
corresponding image-wise modulated distribution of electrical charges by 
photosensitive devices, such as amorphous silicon photodiodes. U.S. Pat. 
No. 5,254,480 (Tran) describes a system which combines a luminescent 
layer, to create a distribution of photons, with an adjacent layer of 
photoconductive material to create a corresponding image-wise modulated 
distribution of electrical charges which are subsequently converted to 
corresponding analog pixel values for the image by electrosensitive 
devices. These solid state systems have the advantage of being useful for 
repeated exposures to X-ray radiation without consumption and chemical 
processing of silver halide films. 
In systems utilizing a photoconductive layer, before exposure to imagewise 
modulated X-ray radiation, the top areal surface of the photoconductive 
layer is uniformly biased relative to electrical charge read-out means by 
application of an appropriate electric field. During exposure to X-ray 
radiation, electron-hole pairs are generated in the photoconductive layer 
in response to the intensity of the imagewise modulated pattern of X-ray 
radiation, and these electron-hole pairs are separated by the applied 
biasing electric field. The electron-hole pairs move in opposite 
directions along the electric field lines toward opposing surfaces of the 
photoconductive layer. After the X-ray radiation exposure, a latent image 
in the form of an imagewise distribution of electrical charges of varying 
magnitude is captured within the photoconductive layer, representing a 
latent electrostatic radiogram. A plurality of charge capture elements and 
switching devices proximate the photoconductive layer are adapted to 
readout the imagewise distribution of electrical charges, thereby 
providing a pixelized radiogram. 
A problem with such an electrical charge capture and readout scheme is that 
after exposure to X-ray radiation is stopped and the electronic charge 
distribution within the photoconductive layer is determined by readout, 
some of the electrical charges induced within the photoconductive layer 
may continue to reside as charges trapped not only within the 
photoconductive layer but also at planar interfaces between the surfaces 
of the photoconductive layer and adjacent layers. These residual 
electrical charges must be fully eliminated prior to the next X-ray 
exposure. Otherwise, a false image pattern related to the previous 
radiation pattern may be added to subsequent radiograms. 
It is known to intentionally flash expose a photoconductive layer of an 
image capture element to a large dose of actinic radiation to eliminate 
residual electrical charges stored in the photoconductive layer by 
momentarily rendering the photoconductive layer substantially conductive, 
for example, as described in U.S. Pat. No. 5,166,524, issued to Lee et al. 
on Nov. 24, 1992 and assigned to E. I. du Pont de Nemours and Company. 
However, such an image capture element must be partially disassembled by 
physical separation of a conductive contacting layer, such as conductive 
foam or rubber, from an array of charge capturing microplates before the 
flash exposure is made. Also, a large neutralizing current may be locally 
created by the flash exposure and exceed the current capacities of nearby 
readout components. Residual charges have also been minimized by 
application of a reversed and decreasing electric field, for example, as 
described in U.S. Pat. No. 5,319,206. However, this process involves 
multiple applications of a decreasing and reversed electric field to fully 
neutralize residual electrical charges remaining in the photoconductive 
layer. 
SUMMARY OF THE INVENTION 
The present invention provides an apparatus and method for eliminating 
residual charges in an image capture panel that includes a plurality of 
sensors arrayed adjacent to a substrate layer of dielectric material. A 
radiation sensitive layer is disposed over the sensors. The radiation 
sensitive layer is exposed to a first substantially uniform pattern of 
light radiation for partially neutralizing residual electrical charges 
trapped within the image capture panel. The radiation sensitive layer is 
then exposed to a second substantially uniform pattern of light radiation 
sufficient to neutralize substantially all residual electrical charges 
trapped within the image capture panel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
FIG. 1 shows an X-ray image capture panel 10 for capturing an imagewise 
modulated pattern of incident radiation. Radiation is herein meant to 
include high energy electromagnetic waves, especially X-rays with energy 
levels between about 20 Key and about 150 Key. The image capture panel 10 
comprises a dielectric substrate layer 7 having a top and bottom surface. 
A plurality of radiation sensitive sensors 12n (i.e., 12a, 12b, 12c, . . . 
, 12n) are arrayed adjacent the top surface of the layer 7 in a matrix of 
rows and columns. The sensors 12n are read out by suitable electrical 
readout means 5n. The readout means 5n typically comprises means for 
electronically accessing each of the sensors 12n individually, and means 
for individually determining electric charge captured in the sensors 12n. 
The sensors 12n comprise a plurality of switching devices 14n (i.e., 14a, 
14b, 14c, . . . , 14n) and a plurality of sensing elements 16n (i.e., 16a, 
16b, 16c, . . . , 16n). The sensing elements 16n comprise a first 
plurality of conductive microplates 24n (i.e., 24a, 24b, 24c, . . . , 24n) 
deposited on the substrate layer 7 and covered with a capacitive 
dielectric material 15, for example, silicon dioxide. A second plurality 
of conductive microplates 26n (i.e., 26a, 26b, 26c, . . . , 26n) are 
deposited over the substrate layer 7. The substrate layer 7 preferably 
comprises a material substantially transparent to light, for example, 
glass. The conductive microplates 24n and 26n have a low optical density, 
for example, a thin layer of indium-tin-oxide (ITO) or a thin layer 
between 50 and 100 .ANG. of metal, such as gold or aluminum. 
FIG. 1 shows the sensing elements 16n which function as charge storage 
capacitors formed by the microplates 26n, the microplates 24n and the 
capacitive dielectric material 15. Each microplate 26n is connected to the 
adjacent switching device 14n. The switching devices 14n are preferably 
thin-film field effect transistors (FETs) comprising a layer 13 of 
hydrogenated amorphous-silicon, crystalline silicon, polycrystalline 
silicon or cadmium selenide, an insulating layer 99, a conductive gate 21 
and two conductive electrodes 23 and 25. Each electrode 23 is connected to 
one of a plurality of readout lines 20n (shown in FIG. 2) and acts as a 
drain electrode. Each electrode 25 is connected to one of the microplates 
26n and acts as a source electrode. Each switching device 14n has its gate 
21 connected to a switching line 18n (shown in FIG. 2), and serves as a 
bi-directional switch allowing current flow between the readout line 20n 
and the sensing element 16n depending on whether a bias voltage is applied 
to the gate 21 through the switching line 18n. Each switching device 14n 
is covered with a passivation layer 98. Preferably, the passivation layer 
98 has a thickness greater than that of the insulating layer 99. The 
technology for creating the switching devices 14n is well known in the 
art. See, for instance, "Modular Series on Solid State Devices," Volume 5 
of Introduction to Microelectronics Fabrication by R. C. Jaeger, published 
by Addison-Wesley in 1988. 
The sensing elements 16n further comprise a plurality of conductive 
collecting elements 6n (i.e., 6a, 6b, 6c, . . . , 6n) deposited, 
respectively, over the microplates 26n and over the switching devices 14n. 
Over the top surface of the collecting elements 6n is disposed a charge 
blocking layer 28 having a thickness selected to prevent charge leakage. 
Both the conductive collecting elements 6n and the charge blocking layer 
28 also have a low optical density. The charge blocking layer 28 is 
preferably provided by an aluminum oxide layer formed on the collecting 
elements 6n although other blocking interfaces may also be used. 
FIG. 2 shows a plurality of conductive switching lines 18n and conductive 
readout lines 20n disposed in the spaces between the sensors 12n. The 
switching devices 14n are disposed in the spaces between the sensing 
elements 16n, the switching lines 18n, and the readout lines 20n. The 
switching lines 18n are individually accessible through a plurality of 
leads connected to a multiplexing device 32. As mentioned before, each 
switching device 14n serves as a bi-directional switch allowing current 
flow between the readout line 20n and the sensing element 16n depending on 
whether a bias voltage is applied to the gate 21 through the switching 
line 18n. 
FIG. 1 further shows a radiation sensitive layer 8, preferably a layer of 
photoconductive material, disposed over the charge blocking layer 28, the 
switching lines 18n and the readout lines 20n. The combination of layers 
6n, 28 and 8 behaves as a blocking diode, inhibiting one type of charge 
flow in one direction. The photoconductive layer 8 preferably exhibits 
very high dark resistivity and may comprise amorphous selenium, lead 
oxide, thallium bromide, cadmium telluride, cadmium sulfide, mercuric 
iodide or any other such material. The photoconductive layer 8 may also 
comprise organic materials such as photoconductive polymers loaded with 
X-ray absorbing compounds, which exhibit photoconductivity. In a preferred 
embodiment, the photoconductive layer 8 comprises 300 to 500 micrometers 
of amorphous selenium in order to provide high efficiency in X-ray 
radiation detection, and the charge blocking layer 28 has a thickness 
greater than 100 .ANG.. 
A top dielectric layer 11 having a thickness greater than one micrometer is 
disposed over the top surface of the photoconductive layer 8. Mylar.RTM. 
(i.e., polyethylene terephthalate) film with a thickness of 25 micrometers 
may be laminated for the dielectric layer 11 or, preferably, a dielectric 
material such as Parylene.RTM. (i.e., poly-xylylene) may be vacuum 
deposited to form the dielectric layer 11. A final top conductive layer 9, 
having a low optical density, for example, a thin layer of 
indium-tin-oxide (ITO) or a thin layer between 50 and 100 .ANG. of metal 
like chromium, being essentially transparent to light, is formed uniformly 
over the top dielectric layer 11. The entire image capture panel 10 can be 
made by depositing sensors 12n, blocking layer 28, photoconductive layer 
8, top dielectric layer 11, and top conductive layer 9 upon the dielectric 
substrate layer 7. Fabrication may be accomplished, for example, by 
plasma-enhanced chemical vapor deposition, vacuum deposition, lamination, 
or sputtering. 
FIG. 1 shows light exposure means L juxtaposed the image capture panel 10. 
It has been discovered that the light exposure means L is effective in 
eliminating residual electrical charges that may reside within the image 
capture panel 10 and create a false image in a subsequent imagewise 
radiation operation. The light exposure means L is positioned to provide a 
uniform pattern of low energy light radiation over the photoconductive 
layer 8. In a preferred embodiment, the light exposure means L comprises a 
first light emitting panel 22F positioned above and displaced a small 
distance, for example 2 millimeters, from the top conductive layer 9. The 
first panel 22F is adapted to expose the photoconductive layer 8 to a 
first pattern of low energy light radiation. The term "light" is used 
herein to describe low energy electromagnetic radiation having wavelengths 
in the range 400 to 800 nanometers with energy in the range 20 to 1000 
erg/cm.sup.2. This first pattern of low energy light radiation is selected 
to have wavelengths and energy such that the light is capable of 
penetrating the top conductive layer 9 and top dielectric layer 11 with 
minimal absorption but being essentially totally absorbed near the top 
portion of the photoconductive layer 8. Preferably, the light emitting 
panel 22F is a panel woven from plastic strands of optic fibers, like that 
available from Lumitex, Inc., Strongsville, Ohio. The panel 22F should be 
capable of providing a substantially uniform pattern of low energy light 
radiation, have a broad range of wavelengths in the 400 to 800 nanometers 
range, and provide energy per unit area in the range from 20 to 1000 
erg/cm.sup.2, preferably about 500 erg/cm.sup.2. Since X-rays must 
penetrate the first panel 22F during patient exposure, it is necessary 
that the first panel 22F comprise material having both low X-ray 
attenuation and a uniform X-ray density. The uniform X-ray density is 
important in order to prevent any internal panel structure from appearing 
in the X-ray image. A remote source 22L of low energy light radiation, 
preferably a tungsten-halogen source, is coupled to the light emitting 
panel 22F through polished fiber ends of the light emitting panel 22F in 
order to provide low energy light radiation into the light emitting panel 
22F. Light transmitted into the optic fibers is emitted from the sides of 
the fibers, passes through the conductive layer 9 and the dielectric layer 
11, and then onto the photoconductive layer 8. The selection of multiple 
layers of woven optic fibers allows more efficient use of the low energy 
light radiation source 22L, and enhances the brightness and uniformity of 
the low energy light radiation incident on the top conductive layer 9. 
In the preferred embodiment, the light exposure means L further comprises a 
second light emitting panel 22S positioned below the substrate 7 and 
displaced a small distance, for example 2 millimeters, from the substrate 
7. The second panel 22S is adapted to provide a second pattern of low 
energy light radiation over the substrate 7 and therethrough onto the 
photoconductive layer 8. Preferably, the light emitting panel 22S is an 
electroluminescent low energy light radiation panel, such as a panel 
commercially available from BKL, Inc., King of Prussia, Pa., under the 
tradename "Aviation Green N3."This panel 22S is composed of 
electroluminescent material sandwiched between thin foils of ITO and Al 
and is capable of providing a uniform pattern of low energy light 
radiation having wavelengths in the 400 to 800 nanometer range, and 
providing energy per unit area in the range from 20 to 1000 erg/cm.sup.2, 
preferably about 400 erg/cm.sup.2. In this second panel 22S, the source of 
low energy light radiation is energy emission from the electroluminescent 
material in response to power provided to the light emitting panel 22S. 
The light emitting panel 22S is oriented so that the ITO-coated surface is 
adjacent to the image capture panel 10. This second pattern of low energy 
light radiation is selected to have wavelengths and energy such that the 
light is capable of penetrating the dielectric substrate 7, the 
microplates 24n and 26n, the collecting elements 6n and the charge 
blocking layer 28 with minimal absorption but being essentially totally 
absorbed near the bottom portion of the photoconductive layer 8. As shown 
in FIG. 2, the first light emitting panel 22F is co-extensive with the 
array of sensors 12n. The second light emitting panel 22S is shown in 
dashed lines and is also co-extensive with the array of sensors 12n. 
In operation, an imagewise modulated pattern of X-ray radiation is incident 
upon the image capture panel 10 which is positioned proximate the patient. 
However, prior to exposure of a patient to a uniform pattern of X-ray 
radiation, electrical charges residual in the image capture panel 10 are 
minimized in order to eliminate false or ghost image patterns. First, the 
operating bias voltage from the power supply 19P is reduced to zero, and 
all readout switch gates 21 are activated to bring the collecting elements 
6n and the top conductive layer 9 to a common zero potential. Next, the 
first light emitting panel 22F is activated to flash expose the 
photoconductive layer 8 to a first uniform pattern of low energy light 
radiation. This first flash exposure is adjusted in energy to create 
sufficient electrical charge carriers within the photoconductive layer 8 
to partially neutralize residual electrical charges remaining within the 
bulk of the photoconductive layer 8 and remaining near the interface 
between the photoconductive layer 8 and the top dielectric layer 11. Using 
the tungsten-halogen light source 22L coupled to the low energy light 
radiation emitting panel 22F, a light radiation flash exposure providing 
energy in the range from 20 to 1000 erg/cm.sup.2 over a time interval of 1 
to 30 seconds has proved adequate. This first creation of free electrical 
charges serves to partially eliminate any localized higher level 
accumulations of electrical charges trapped within the bulk of the 
photoconductive layer 8 and remaining near the interface between the 
photoconductive layer 8 and the sensors 12n prior to activation of the 
second light emitting panel 22S. If the localized higher level 
accumulations of electrical charges are not initially reduced in 
intensity, it is possible that during a subsequent discharging operation, 
described hereinafter, certain portions of the electrical readout means 
5n, particularly the switching devices 14n, may experience a surge in 
charge current density greater than the load limit of the electrical 
readout means 5n, thereby causing failure of the readout means 5. 
The second light emitting panel 22S is next activated to flash expose the 
photoconductive layer 8 to a second uniform pattern of low energy light 
radiation. This second flash exposure is adjusted to have an energy output 
that produces an abundant supply of electron-hole pairs within the 
photoconductive layer 8 sufficient to effectively neutralize or eliminate 
all residual electrical charges trapped either within the bulk of the 
photoconductive layer 8 or near the interface between the photoconductive 
layer 8 and the sensors 12n. By eliminating the residual electric charges, 
the electric field that exists across the dielectric layer 11 is 
eliminated. When the above-mentioned "Aviation Green N3" 
electro-luminescent low energy light radiation panel is employed, a low 
energy light radiation flash exposure providing energy per unit area in 
the range from 20 to 1000 erg/cm.sup.2 over a time interval of 1 to 30 
seconds has been found adequate to fully neutralize any residual charges 
remaining within the photoconductive layer 8. Because the first uniform 
pattern of low energy light radiation partially reduced the magnitude of 
localized residual electrical charges, the overall reduced distribution in 
the amplitudes of residual electric fields across the dielectric layer 11 
is minimized, and the danger of exceeding the current capabilities of the 
readout means 5 is minimized during subsequent elimination of these 
charges by the second flash exposure. 
FIG. 3 shows the operation of the image capture panel 10 during image 
capture and readout. Each readout line 20n is connected to a readout means 
34 comprising a charge integrating amplifier device 41n, and a 
sample-and-hold circuit 45n. The output of circuit 45n may be sampled 
sequentially to obtain an X-ray radiation image output signal, and the 
technology to do this is well known in the art. Additional electronic 
manipulations not related to the present invention may be performed before 
or after image capture to enhance the overall quality of the final 
radiation image. 
A patient to be examined is exposed to a uniform pattern of X-ray radiation 
producing an imagewise modulated pattern of X-ray radiation incident on 
the image capture panel 10. This pattern generates a corresponding 
imagewise modulated pattern of electrical charges in the array of sensing 
elements 16n. After X-ray radiation exposure is completed, a 
microprocessor controller 38 causes the multiplexing device 32 to read out 
a pattern of electrical charge values corresponding to the pattern of 
X-ray radiation absorption in the patient. 
After the readout signal has been recovered, the panel 10 is cleared of any 
residual charges using the sequence of reducing the distribution in the 
amplitudes of residual electrical charges remaining near the surface 2 of 
the photoconductive layer 8 and, thereafter, fully eliminating all 
residual electric charges retained near the surface 3 of the layer 8, 
using the first and second exposure devices 22F and 22S, respectively, as 
described herein. This residual charge reduction process may be repeated 
if appropriate until all the trapped charges are removed and the image 
capture panel 10 is prepared for a subsequent image capture operation. 
In the present invention, both the first and the second patterns of light 
radiation may be provided from a single light emitting panel, e.g., the 
second light emitting panel 22F. However, in the preferred embodiment, the 
first light emitting panel 22F is cooperatively combined with the second 
light emitting panel 22S in order to eliminate all residual electrical 
charges without exceeding the current capabilities of the readout means 5. 
The first panel 22F is utilized to reduce the overall intensity of any 
remaining trapped electrical charges, while the second panel 22S serves to 
completely eliminate all remaining electrical charges. 
FIG. 4a shows a simplified electrical description of this combination of 
intensity reduction of localized trapped electrical charges prior to 
elimination of all trapped electrical charges. The preferred process for 
neutralizing any residual charges remaining within the photoconductive 
layer 8 utilizes two sources of low energy light radiation 22F and 22S. 
Although the readout means 5 and the top conductive layer 9 are shorted to 
ground, and the operating bias voltage from the power supply 19P is 
reduced to zero, an uneven distribution of electrical charges may remain 
"trapped" within the photoconductive layer 8 because of the limited charge 
transport range of the "minority carrier" of the radiation induced 
electron-hole pair. These charges are trapped in the bulk of the 
photoconductive layer 8 as well as near the interface with the dielectric 
layer 11, and attract electric charges of the opposite polarity in the 
nearby grounded layers. The dielectric layer 11 and the radiation 
sensitive photoconductive layer 8 thus form two capacitors in series, 
designated as Cd and Crs, respectively, for each of the sensors 12n. In 
FIG. 4a, the lower surface of the dielectric layer 11 is indicated by the 
character 1, and the juxtaposed surface of the photoconductive layer 8 is 
indicated by the character 2. When the photoconductive layer 8 has 
positively charged holes as the "majority carrier", such as P type 
selenium, the minority charge carriers created in the layer 8 by an 
incident imagewise modulated pattern of radiation will have a limited 
charge transport range and a corresponding shorter range within the bulk 
of the layer 8. Consequently, following a first radiation exposure and 
readout operation similar to that described above, trapped charges 
residual in the photoconductive layer 8, shown as "-" charges, will 
generally be non-uniformly distributed throughout the image capture panel 
10. 
FIG. 4b shows a simplified electrical equivalent circuit illustrative of 
the present invention wherein the first flash exposure, indicated by FR in 
FIG. 4b, is incident through the top dielectric layer 11 and into the 
photoconductive layer 8. As described above, this first flash exposure is 
largely absorbed near the surface of the photoconductive layer 8 but is 
adjusted in energy to create sufficient electron-hole pairs within the 
photoconductive layer 8 to combine with residual trapped electrical 
charges and partially eliminate any electrical charges retained primarily 
near the surface 2 of the photoconductive layer 8. Because of the limited 
charge transport range of the minority charge carriers, only a limited 
number of the residual trapped electrical charges will be eliminated. 
However, by partially reducing the number of residual trapped electrical 
charges, the first flash exposure beneficially produces an overall reduced 
maximum amplitude of residual electric fields in Cd throughout the sensors 
12n within the panel 10. 
FIG. 4C shows a simplified electrical equivalent circuit illustrative of 
the present invention wherein the second flash exposure, indicated by SR 
in FIG. 4c, is incident through the dielectric substrate 7 and through the 
plurality of sensors 12n. As described above, this second flash exposure 
is largely absorbed near the surface of the photoconductive layer 8 but is 
adjusted in energy to create sufficient electron-hole pairs within the 
photoconductive layer 8 to combine with the residual trapped electrical 
charges, thereby fully eliminating all residual electric charges retained 
within the panel 10. The second flash exposure SR may be performed by 
applying the second pattern at a rate in the range from 20 to 800 
erg/cm.sup.2 per second. Preferably, the second flash exposure rate is 
time-wise adjusted so that the incident radiation is applied at an 
increasing rate from 20 to 800 erg/cm.sup.2 per second, thereby generating 
a slowly increasing number of electron-hole pairs while eliminating the 
residual electrical charges. This process of first reducing the 
distribution in the amplitudes of residual electrical charges remaining 
near the surface 2 of the photoconductive layer 8 and thereafter fully 
eliminating all residual electrical charges retained near the top 
conductive layer 9 is effective in protecting the readout means 5n within 
the panel 10 because the amplitudes of electric fields attributable to 
residual charges being discharged from the panel 10 through the readout 
means 5n are lowered. 
In an alternative embodiment, an N type radiation sensitive material may be 
used, such as cadmium selenide which has negatively charged electrons as 
the "majority carrier". In this instance, the minority charge carriers, 
which are created in the photoconductive layer 8 and have a limited charge 
transport range, are positively charged. The process described above is 
again effective in protecting the readout means 5n within the panel 10 
although the operating bias voltage applied to the top conductive layer 9 
is reversed.