The invention relates to a method for the read-out of a latent image recorded in a stimulable phosphor sheet by X-ray radia¬ tion or the like, in which method a separate phosphor sheet of a flexible material, which carries a latent image, is posi¬ tioned in a curved shape along the circumference of a cylinder; this curved phosphor sheet is rotated around the axis of the cylinder circumference; a stimulating beam of light obtained from a source of light is aimed at the phosphor surface of the sheet carrying the latent image, the light emitted from the phosphor surface under the effect of this beam being directed to a photodetector; and the stimulating beam of light and the phosphor sheet are moved relative to each other in a direction parallel to the said axis for the read-out of the phosphor sheet. The invention also relates to an apparatus for the read¬ out of such a phosphor sheet while it is positioned in a curved shape.

Phosphor sheets in which a latent image is recorded by X- radiation are commonly used in radiography. After the exposure of the radiograph, this latent image is read out by aiming at the sheet, point by point, a stimulating beam of light under the effect of which there is produced an emission light ac¬ tivated by the latent image, and this emission in turn is de¬ tected by a detector and is converted into an electronic form for further processing. Numerous different read-out methods and read-out apparatuses have been proposed for the read-out of the latent image in such a phosphor sheet. For introducing the emitted light to a detector or detectors there is used, for example, a light collector, which is made up of an inwardly reflective tube extending over the entire width of the phosphor sheet, an inwardly reflective conical piece extending over the entire width of the phosphor sheet or an optical fiber bundle

extending over the entire width of the phosphor sheet, which collector is moved in a direction parallel to the length of the phosphor sheet, this direction being perpendicular to the said width of the light collector. At the same time the stimulating beam of light is scanned along the whole width of the light collector, either through a slot in the tubular light collector or via a mirror surface positioned in a massive light collector based on fiber optics or in a similar manner on total reflec¬ tion, the stimulating beam scanning the light collector from one end to the other. In this manner the entire phosphor plate is covered by the movement, the scanning traces forming a zig¬ zag pattern over the surface area of the phosphor sheet. In all of these described embodiments, the phosphor sheet is main¬ tained flat during the read-out. Such methods and apparatuses suitable for the read-out of a phosphor sheet are described in publications US-4 629 890, US-4 742 225, US-4 743 759 and US- 4 829 180. The apparatuses and methods of the type described above have a number of disadvantages. First, the manufacture of light collectors which extend over the entire width of the image is very complicated and expensive. The manufacture of the stimulating-beam scanning device inevitably associated with them requires top precision because it is at a distance from the phosphor plate, and also because it is necessary to know the mutual locations of each position of the turning light beam and the corresponding image point in the phosphor sheet, in order to avoid errors in the final image pattern. This part is thus also expensive. Since in such apparatuses the angle of incidence of the stimulating beam against the phosphor surface of the phosphor sheet varies, in which case the beam at the edge of the phosphor sheet is at a considerable angle as com¬ pared to being perpendicular to the phosphor sheet, this causes imprecision in the final read-out image, especially in the area of the edges of the image. In addition, in such large-sized light collectors, which are truly large as compared with the cross section of the stimulating beam and to the photosensitive area of the detector, the losses of light are very large, in

which case there tends to be noise and other interference in the obtained signal.

Publication US-4 922 102 describes a phosphor sheet read-out method in which the phosphor sheet is rotated in its plane while it is being read by means of a point-like reading head which is, between rotations, moved in a direction parallel to the rotational radius of the phosphor sheet, which remains flat. In this case the phosphor sheet is read one circular trace at a time, one inside another. Non-public patent appli¬ cation FI-950048 describes a phosphor sheet read-out method in which the phosphor sheet is moved in its plane in one direction while a read-out apparatus having a point-like reading head is rotated around an axis perpendicular to the plane of the phos¬ phor sheet. In this case the phosphor sheet is read in succes¬ sive curved traces. The rotating of the phosphor sheet in a plane and the use of a rotating reading head are relatively poor in the efficiency of their use of the read-out time, since for a considerable proportion of the total read-out time the reading beam is outside the rectangular surface area of the phosphor sheet. These structures presuppose of the rotating apparatus a precision which is difficult to implement in order that the distance between the reading head made up of a stimul¬ ating beam and a detector on the one hand and the phosphor sheet on the other should remain precisely the same at each point of the phosphor sheet.

Publication US-5 416 336 describes an apparatus in which a stimulating beam is scanned in one direction on a mirror inside an emitted radiation collector, while the phosphor plate is being moved in another direction. By this arrangement, indeed a high read-out precision and low losses of light are achieved, but the apparatus is best suited for use for the read-out of relatively small-sized images.

Publication US-4 829 180 also describes an arrangement in which

the phosphor sheet is positioned on a cylinder with the phos¬ phor surface outwards, and the phosphor sheet is read by means of a point-like reader which comprises a stimulating beam and a detector for emitted radiation. During the read-out the cylin¬ der is rotated around its axis, whereby one scanning direction of the phosphor sheet is accomplished, whereas another scanning direction, parallel to the axis of the cylinder, is accom¬ plished by moving either the reading head or the cylinder in a direction parallel to the rotation axis. This publication does not at all describe how the phosphor sheet is attached to the outer surface of the cylinder. Article Hildebolt, Vannier: "PSP-Photostimulable Phosphor Dental Radiography" - Washingto University School of Medicine, St. Louis, Missouri (Internet => HTTP: //IMACX.WUSTL.EDU/PSP/psp.HTML, Jan 09 1996) describes a corresponding phosphor sheet read-out method, but it describes additionally that the backing of the phosphor sheet is made up of a thin metal sheet which adheres magnetically to the outer surface of the cylinder. The reliability of such magnetic ad¬ hesion is not very good, at least if the cylinder is rotated at even a moderate speed. In addition, the metal backing of the phosphor sheet renders it very rigid, in which case its shaping around the cylinder is uncertain and the risk of its becoming detached during the rotation of the cylinder further increases. Thus these described arrangements have the disadvantage of a high risk that the phosphor sheet becomes detached, and for this reason it is most likely necessary to maintain a low rota¬ tion speed of the cylinder, which in turn leads to the read-out of the phosphor sheet being slow.

An object of the present invention is therefore to provide a method and apparatus wherein the phosphor sheet is, in a manner known per se, moved during its read-out, but wherein the phos¬ phor sheet, in spite of this movement, can be maintained pre¬ cisely in the required shape. This means that the distance between the read-out apparatus, which includes a source of stimulating light and a detector for emitted radiation, and the

phosphor sheet will not change but will remain as precisely as possible at what it is intended to be in each given case. Thus an object of the invention is a method and apparatus in which the phosphor sheet can be read out at a high speed without there being a risk of any change in the mutual positioning of the phosphor sheet and the read-out apparatus. Another object of the invention is a method and apparatus of this type, where¬ in the risk of the phosphor sheet being dislodged when it is being moved during the read-out is minimal. A third object of the invention is an apparatus and method of this type, wherein the idle time during the read-out, i.e. the time which is not used for effective read-out of the phosphor sheet, is as short as possible, or alternatively the read-out speed is so high that the idle time has no practical significance. A fourth object of the invention is a method and apparatus of this type, enabling phosphor sheets of considerably different sizes to be simply positioned and read out. A fifth object of the invention is a method and apparatus of this type, enabling conventional and separate phosphor sheets, i.e. phosphor sheets without special devices, to be used. A sixth object of the invention is a method and apparatus of this type, in which small-sized and efficient light collectors can be used. A further object of the invention is a method and apparatus of this type, by means of which a maximally high read-out precision can be achieved, in which case the pixel size is as small as possible and the appa¬ ratus is capable of reading out as high a number of line pairs per one millimeter as possible. One further object of the in¬ vention is a method and apparatus of this type, making an easy- to-use and economical apparatus and its use possible.

The disadvantages described above can be eliminated and the objects defined above can be achieved by using the method ac¬ cording to the invention, which is characterized in what is stated in the characterizing clause of Claim 1, and an appa¬ ratus according to the invention, which is characterized in what is stated in the characterizing clause of Claim 10.

The most important advantage of the invention is that, through the use of the method and apparatus according to it, a conven¬ tional, relatively flexible phosphor sheet can be caused to remain in precisely the desired shape and in particular in the shape of a circular cylinder, without any specific fastening devices. Another advantage of the invention is that the phos¬ phor sheet may be of any conventional type and it can be caused to remain with high precision in a cylindrical shape and to remain very firmly in place, in which case the distance between the read-out means and the phosphor surface can be set at as low a value as possible and their mutual positioning can be controlled with precision. Thereby a high efficiency in the collection of emitted light into the collector and a small size of the stimulating beam and its precise aiming at the phosphor sheet are accomplished. Furthermore, a high read-out speed is accomplished. Thus, by using the method and apparatus according to the invention, it is possible to arrive normally and with relative ease at a pixel size of 100*100 μm even when the radiograph size is large (e.g. 24 cm * 30 cm). In practice, with moderate care, a pixel size of 70*70 μm or even a pixel size of 50*50 μm can be accomplished in this radiograph size. Theoretically the achievable pixel size is in the order of 20 μm * 20 μm, which is better than the value achieved by using any other read-out method or read-out apparatus. In fact, the only factor limiting the read-out precision in the arrangement according to the invention is the precision at which the phos¬ phor sheet is capable of recording a latent image. One further advantage of the invention is the high read-out speed; a large image of the size mentioned above can be read out at least in a time less than 2 minutes, or usually in about 1 minute, or even in about 1/2 minute, whereas the time taken by read-out in known methods and apparatuses is many times this. In fact, the only factor limiting the read-out speed in the arrangement according to the invention is the frequency at which the de¬ tector is capable of detecting changes in radiation and/or the

speed at which the phosphor sheet is capable of starting and ending its stimulated emission. One more advantage of the in¬ vention is that it is possible to use small-sized and efficient light collectors, in which case the emitted light can be di¬ rected to the detector at best almost entirely, with very small losses.

The invention is described below in detail, with reference to the accompanying figures.

Figure IA depicts a top view of the principal features of a first embodiment of the apparatus according to the invention, partly as a cutaway in such a way that the left side of the figure depicts a top view of the apparatus from direction I in Figure IB and the left side of the figure depicts a section through II-II in Figure IB.

Figure IB depicts a cross-section of the apparatus of Figure IA, through III-III, which is perpendicular to the rotation axis, with a phosphor sheet partly pushed inside the drum member.

Figure 2A depicts a top view of the principal features of a second embodiment of the apparatus according to the invention, as seen from direction IV in Figure 2B and partly as a cutaway, the section being through IX-IX in Figure 2B, with a phosphor sheet partly pushed into the drum member.

Figure 2B depicts a cross-section of the apparatus of Figure 2A, through V-V, which is perpendicular to the rotation axis, with a phosphor sheet shown in its place.

Figure 3A depicts a top view of the principal features of a third embodiment of the apparatus according to the invention, as seen from direction VI in Figure 3B and partly as a cutaway, the section being through VII-VII in Figure 3B, with a phosphor

sheet partly pushed from the outside to the inside or partly pulled out of the drum member.

Figure 3B depicts a cross-section of the apparatus according to Figure 3A, through VIII-VIII, which is perpendicular to the rotation axis, with a phosphor sheet partly pushed into or partly pulled out of the drum member.

Figure 4 depicts schematically a phosphor sheet bent into a cylindrical shape and, inside the cylindrical shape, the direc¬ tions of the stimulating beam and the sensitivity directions of the detector for emitted light, as well as the scanning trace formed in the read-out of the phosphor sheet, with a phosphor sheet in its place in a cylindrical shape inside the drum member.

The present invention for the read-out of a phosphor sheet does not relate to the exposure of the phosphor sheet with X-rays in order to record a latent image therein but to the read-out of a latent image recorded in a phosphor sheet in any suitable man¬ ner, after its exposure. However, it should be pointed out that in general a phosphor sheet is exposed while it is positioned in some suitable radiography cassette, in which the phosphor sheet in general remains as precisely as possible flat. For the read-out, the phosphor sheet is taken out of the cassette. It is also possible to expose the phosphor sheet with X-rays while it is in some curved shape, but this curved shape has no rela¬ tion to the curved shape during the phosphor sheet read-out described below. The point of departure in the invention is thus that the phosphor sheet is separate (i.e. not attached to a rigid or corresponding backing) , in which case it can be handled and positioned into the required shape, and it also already carries the latent image information.

The figures show a drum member 8 of a solid and rigid material, having on its inside a cylindrical circumferential surface 5

formed by a cylindrical cavity 15. The drum member 8 is at¬ tached, for example, with bearings at one end, at point 26, so that the drum 8 can be rotated around the axis 3 of the cylin¬ der circumference 5. Preferably this inner cylindrical surface 5 is a circular cylinder, and usually the opposite outer sur¬ face 25 of the drum member 8 is also cylindrical, in which case the entire drum member 8 is a tubular piece. With this design, the drum member 8 can be easily balanced so that its rotation around the axis 3 of the circumferential surface 5 is as steady as possible. When necessary, the impacts of the apertures of the drum member and of the weight of the phosphor sheet can be compensated for by using fixed or detachable or relocatable balancing means. This drum member 8 is rotated by a motor 22, the axle of which is attached at the axis 3 of the circum¬ ferential surface 5 of the drum member. However, there is no obstacle to rotating the drum member 8 in some other way, for example, on top of rollers, but the embodiment shown in the figures is most likely the most preferable, since by using it the circumferential surface 5 can be well centered and the rotation of the drum will be steady. The drum member 8 may be made from a metal, such as aluminum, or any strong and rigid plastic, or any suitable strong and rigid composite material.

The phosphor sheet 1, into which a radiation image has been recorded, usually by X-ray radiation, by any known or new method, which method is thus not shown in the figures and is not even otherwise described here, is placed according to the invention against the concave inner circumferential surface 5 of the cylindrical cavity 15 of this drum member 8 in such a manner that the stimulable phosphor surface 2 carrying a latent image will be the concave side and will thus face the axis 3. The phosphor sheet 1 may be introduced against the concave shape of the cylindrical circumference 5 in a number of ways, which are described in greater detail below.

Figures IA and IB depict one preferred method according to the

invention for implementing this. In this arrangement, there is on the circumference of the drum member 8 a straight slot 23 parallel to the axis 3, which slot may be relatively spacious or very narrow in the direction parallel to the circumference 5. In the latter case the slot is typically parallel to a tangent to the surface of the cylinder circumference 5. After radiography, the exposed phosphor sheet 1 is normally in a cassette 20, which is shown only schematically in the figures. From the cassette 20 the phosphor sheet 1 is pushed, with its leading edge la first, by tools not shown in the figures via the slot 23, in a direction ID parallel or approximately paral¬ lel to a tangent to the cylinder circumference 5, into the cyl¬ inder cavity 15 inside the drum member 8, in particular along its inner surface 5. The direction of the push may in principle deviate considerably from the said tangential direction, for example 10°, 20° or 30° therefrom, but this will probably complicate the structure. Figure IB shows a situation in which part of a phosphor sheet 1 has been pushed into the shape of the cylinder circumference 5, and part of it is still in the cassette 20. For read-out, the entire phosphor sheet 1 is pushed to the inside of the cylinder cavity 15 into the shape of the cylinder circumference 5, which is evident when the pushing is continued in direction Dl in the case of Figure IB, until also the trailing edge lb of the phosphor sheet 1 is inside the cylinder cavity 15, against the cylinder circum¬ ference 5. The leading edge la of the phosphor sheet 1 is pushed, for example by means of a transfer member 31 extending to the inside of the drum via slots 30 parallel to the circum¬ ference, the transfer member being in this case in contact with the trailing edge lb, until the leading edge meets a limiter 34 on the cylinder circumference 5. At this time the phosphor sheet 1 is thus in its entirety along the cylinder circum¬ ference 5 formed by the cylinder cavity 15, in such a manner that the stimulable phosphor surface 2 is inwards, facing the axis 3, and forms the concave side of the phosphor sheet. After the read-out of the phosphor sheet 1, the phosphor sheet can be

removed from the inside of the drum member 8 by pushing the member 31 to the inside of the drum via the slots 30 parallel to the circumference and by pushing at the leading edge la in a direction opposite to the pushing-in direction Dl, whereupon the phosphor sheet leaves via the slot 23, the trailing edge lb first. This removal is thus simply an operation reverse to the pushing of the phosphor sheet into the cylinder cavity 15.

Figures 2A and 2B depict a second procedure for bringing a phosphor sheet against the concave cylinder circumference 5 of the cylinder cavity 15. Therein the phosphor sheet is in ad¬ vance bent into a cylindrical shape, for example by means of a ring 24 having a diameter slightly smaller than that inside diameter of the cylinder cavity which forms the cylinder cir¬ cumference 5. Thereupon this phosphor sheet 1, bent into a substantially cylindrical shape, can be pushed in a direction D2 approximately parallel to the axis 3, via the open end 19 of the drum member 8, with the first side edge lc first, into the cylinder cavity 15 of the drum member, and further against its circumference 5. In this case, also, the phosphor sheet is positioned so that its stimulable phosphor surface 2 carrying a latent image will be inwards, facing the axis 3, thus forming a concave surface. In this case the phosphor sheet 1 can be re¬ moved from the cylinder cavity by using a member 33, which extends via slots 32 parallel to the axis 3 into the cylinder cavity, the member gripping the first side edge lc of the phos¬ phor sheet and pushing the phosphor sheet in the direction opposite to the pushing-in direction D2, the other side edge Id first, out of the cylinder cavity 15. In this case, also, the removal is an operation completely reverse to the pushing in of the phosphor sheet.

Figures 3A and 3B depict a third procedure for introducing a phosphor sheet 1 into the cylinder cavity 15. This procedure resembles that shown in Figures IA and IB, except that in this one the phosphor sheet is not introduced from a cassette but is

guided by means of rolls 28 via a slot 23 parallel to the axis along the shape of the cylinder periphery 5, just as in the case of Figures IA and IB. In the present case the removal of the phosphor sheet 1 is carried out, as in Figures IA and IB, by using a member 31 pushing into the cavity or chamber 15 via slots 30 parallel to the circumference. In the figures, members 31 and 33 are, for the sake of clarity, shown as separate from the drum member 8, as they will be in a situation in which they are not being used. It is evident that they can be placed in the operating position by moving them in a direction parallel to the radius of the cylinder circumference 5, towards the axis 3.

Procedures have been described above in which the phosphor sheet is, after the read-out, removed from the inside of the drum member 8 by using the same members as were used for intro¬ ducing them. It is, however, possible to remove the phosphor sheet for re-use from the cylinder cavity 15 in any other man¬ ner. For example, if the phosphor sheet has been brought onto the cylinder surface 5 by the procedure of Figures 1A-B, it can be removed by procedures shown either in Figures 2A-B or in Figures 3A-B, and respectively the removal of a phosphor sheet introduced by the procedure shown in Figures 2A-B can be car¬ ried out by the removal procedure shown in Figures 3A-B, or by any other procedure. The procedure for introducing a phosphor sheet onto the cylinder surface 5 for read-out and the pro¬ cedure for its removal after the read-out may be procedures completely independent of one another. It is clear that numerous arrangements different in their details can be de¬ signed for the introduction and removal of the phosphor sheet.

According to a preferred embodiment of the invention, a phos¬ phor sheet 1, positioned in the cylinder cavity, will be caused to settle precisely in the shape of a circle, i.e. the shape of the cylinder circumference 5, by rotating the drum member 8 around the axis 3 at a sufficiently high circumferential speed

Rx, which will produce a sufficient centrifugal force which will press the phosphor sheet 1 outwards in the radial direc¬ tion of the cylinder circumference 5, and thus precisely against the said concave inner surface 5 of the cylinder. This required circular velocity will, of course, depend on the rigidity and mass of the phosphor sheet 1, and according to the invention it is indeed most preferable to use a phosphor sheet 1 which is as flexible as possible. If the diameter of the con¬ cave cylinder circumference 5 is in the order of 15 cm - 20 cm, and if conventional commercially available phosphor sheets are used, the rotation speed required for producing a sufficient centrifugal force is at its minimum in the order of 500 rpm, but most phosphor sheets can be caused to settle against the surface 5 by using a rotation speed of 1000 rpm; it is most preferable to use rotation speeds of 1500-2000 rpm, whereby any commercial phosphor sheets which have not specifically been stiffened can be caused to settle and to remain very precisely in the shape of the cylinder circumference 5. In this case the magnitude of the centrifugal force corresponds to an accelera¬ tion of 300 g„ for the mass of the phosphor sheet. When espe¬ cially flexible phosphor sheets are used, a sufficient pre¬ cision of shape can be accomplished at an acceleration of 100 g _D , or possibly even at an acceleration of 30 g ₀ , for the mass of the phosphor sheet (g _n stands for an acceleration of the magnitude of the earth's gravitational pull). As is evi¬ dent, it is also preferable to use not only a phosphor sheet as flexible as possible, but at the same time as heavy as pos¬ sible, because in such a case a precise contact between the phosphor sheet and the inner surface of the cavity of the drum member can be accomplished using even a low rotation speed. Aiming at a low rotation speed, however, is not significant, since the scanning of the drum member, and thus of the cylin¬ drical phosphor sheet rotating with it, takes place during this very rotation, and for a rapid read-out of the phosphor sheet it is expedient to use as high a rotation speed as possible. Thus the rotation speed and the circular velocity Rx are

limited in the arrangement according to the invention only by the speed of the phosphor sheet and the detector, as well as of the electronics associated therewith, and the strength of the apparatus. A maximally high circumferential speed also best ensures that the phosphor sheet will remain precisely against the circumferential surface 5, i.e. in the correct shape. The flexible material of which the phosphor sheet is made is, of course, a solid material, and typically also a resilient mate¬ rial, in order that the phosphor sheet should recover its original straight shape after the read-out.

By using the above-described inner cylinder circumference 5 and method for introducing a phosphor sheet 1 against its inner surface and a sufficient rotation speed of the drum member, i.e. circumferential speed Rx, one and the same apparatus can be used for the read-out of phosphor sheets of considerably different sizes without there being the need to adjust the fas¬ tening of the phosphor sheets in any way according to the phos¬ phor sheet size. Thus such an apparatus can be used for the read-out of phosphor sheets as large as 24 cm * 30 cm, or phos¬ phor sheets as small as 30 mm * 40 mm, since the centrifugal force will hold the phosphor sheet in the correct shape. The purpose of the limiter 34 is only to ensure that the phosphor sheet will not move in the circumferential direction of the surface 5 during the read-out. The drum member according to the invention may have a plurality of longitudinal slots 23 for the introduction of phosphor sheets against the cylinder circum¬ ference 5. The drum member may also comprise more than one limiter on its circumferential surface 5, which limiters may be fixed or detachable. By these arrangements it is possible, when necessary, to arrange on the circumferential surface 5 simul¬ taneously several phosphor sheets smaller than the maximum size for being read out in one scanning. When a maximum-sized phos¬ phor sheet is placed inside the drum member, it will thus cover nearly 360° of the circumference of the circumferential sur¬ face, whereas smaller phosphor sheets will cover a smaller

proportion of this circumference. Drum members of different sizes may be arranged in the apparatus according to the inven¬ tion, according to the size of the phosphor sheets to be scanned.

Another option for pressing a phosphor sheet 1 against the con¬ cave circumferential surface is compression F-F parallel to the circumference; this force is indicated in principle in Figure 4. The control of the compressive force F is, however, dif¬ ficult. A third option is to generate a vacuum between the phosphor sheet 1 and the inner circumferential surface 5 of the drum member, this vacuum sucking the phosphor sheet fast against this surface. This option, for its part, requires ex¬ pensive and complicated additional devices. Therefore the most preferred option of the invention is to use a sufficiently high circumferential speed Rx of the rotation of the drum member 8, as described above.

The apparatus of the invention includes read-out devices, de¬ scribed below in greater detail, for the read-out by scanning of a phosphor sheet positioned in the manner described above, in which the phosphor surface 2 carrying a latent image forms the concave side and preferably a circular cylinder, which is rotated around its axis 3. The read-out of the phosphor sheet is started after it has been brought, by any of the procedures described above, against the cylinder surface 5 and has been pressed, for example under the centrifugal force described above, with precision of shape against a surface having prefer¬ ably the shape of a circular cylinder. The read-out means com¬ prise a transfer member 21 for aiming stimulating light from a light source 4 at the phosphor surface 2 of the phosphor sheet in a direction SI which is away from the axis 3. The read-out means also comprise a photodetector 7 for detecting light emit¬ ted form the phosphor surface under the effect of stimulation, the main direction of the emitted light being thus from the cylinder circumference 5 towards the center. The figures depict

preferred embodiments of the invention for accomplishing this. In all of these embodiments, the transfer members 21 are me¬ chanical members which extend to the inside of the said cylin¬ der cavity 15 and carry means for transmitting a stimulating beam 6 and a detector for detecting radiation emitted from the phosphor surface. In this case also the light source 4 and the photodetector 7 are located inside the cylinder cavity, at least in part and for at least a large proportion of the read¬ out time of the phosphor sheet. However, there is no obstacle to the arranging of a transfer member structure in which either the source 4 of stimulating light and/or the photodetector 7 receiving the light emitted from the phosphor surface are/is located outside the cylinder cavity. In such a case the trans¬ fer members would possibly comprise only members for trans¬ ferring optical transmission components or for turning them in a direction M parallel to the axis 3 for the read-out of the phosphor sheet. It is necessary to move the read-out means in direction M only over the distance over which the phosphor sheet or phosphor sheets cover the cylinder circumference 5 in a direction parallel to the axis 3.

Figures 1A-3B depict structures in which there are, inside the drum member 8, guides 12, such as guide bars, parallel to the axis, and a slide 13 moving in the axial direction along these guides, the slide carrying a source 4 of stimulating light and a detector 7 for emitted light. During the rotation Rx of the phosphor sheet, the slide is moved in a direction M parallel to the said axis 3 for the read-out of the phosphor sheet, trace by trace, which traces are made up of parts of a continuous spiral. For this purpose the direction SI of the stimulating light beam 6 and the principal sensitivity direction S2 of the detector 7 for emitted light are in the slide in unchanging positions relative to each other, but their combination, i.e. the slide 13, is moved in direction M.

Figures 1A-1B depict an embodiment in which the light source 4

transmits a stimulating beam 6 directly towards the cylindrical phosphor surface 2, the stimulating beam 6 being parallel to a radius of the circumferential surface 5. The stimulating beam 6 travels via a light collector 9. The light emitted from the phosphor surface 2 is collected by the light collector 9, the first end 11 of which is as close as possible to the phosphor surface 2 and which is made up of an inwardly reflective mem¬ ber, such as a tubular part. The stimulating beam 6 meets the phosphor surface preferably within the area of this first end 11. The emitted light collected by the light collector 9 is de¬ flected, for example, by means of a prism 29, from a direction which is on average from the phosphor surface 2 towards the axis 3 to a direction substantially parallel to the axis, to meet a detector positioned in the axial direction, in which case this emitted light is detected by detector 7 and is con¬ verted by it into an electronic form for further processing. When in Figures 1A-1B the phosphor sheet 1 is rotated at a steady circumferential speed Rx and the slide together with the light source 4 and detector 7 is moved at a steady speed M in a direction parallel to the axis 3, the concave phosphor surface of the phosphor sheet will be scanned one trace K at a time, as is evident and has been depicted schematically in Figure 4.

Figures 2A-2B depict a slightly different arrangement, wherein the stimulating beam 6 transmitted by the light source 4 meets the phosphor surface 2 at a small angle, which angle is in a radial plane passing via the axis 3. The principal direction of sensitivity of the detector 7, for its part, is parallel to a radius of the circumferential surface 5. In other respects, also in this case these members have been arranged in such a manner relative to one another that the stimulating light beam 6 meets the phosphor surface 2 within the area of the first surface 11 of the light collector 9, preferably in the center of this area 11. The structure of this light collector 9 is in this embodiment, as in the embodiment described above, an in¬ wardly reflective tubular part. In this case the stimulating

beam will not impinge against any point in the side wall of the light collector 9.

Figures 3A-3B depict an embodiment which somewhat resembles the embodiment shown in Figures 2A-2B. In this, also, the principal sensitivity direction of the photodetector 7 is perpendicular to the phosphor surface 2, i.e. parallel to a radius of the circumferential surface 5. In this case, however, the light source 4 is in a position so much deviating from the perpen¬ dicular to the phosphor surface, and at a suitable distance from the end surface 11 of the light collector 9, that the stimulating beam 6 is reflected from one inwardly reflective surface 14 of the light collector 9 and impinges via it, via the first end 11, against the phosphor surface 2.

It is clear that in all of these three described embodiments, the light source 4 transmitting a stimulating light beam 6 and the light collector 9 with its detector 7 can be rotated around that perpendicular to the phosphor surface 2 which passes via the first end 11 of the light collector, into any position. Thus, for example, in the embodiments of Figures 2A-2B and 3A- 3B, the light source 4 and the detector 7 are in the same plane passing through the axis 3. If the members are rotated, for example through 90°, they will be located relative to each other in other respects in the positions depicted in the fig¬ ures but in a plane perpendicular to the axis 3, i.e. in a plane perpendicular to the image plane. If thus, for example in a situation corresponding to the case of Figure 2A-2B, in which the light collector 9 and the detector 7, as well as the source 4 of stimulating light, are located in the same plane perpen¬ dicular to the axis 3, this configuration is raised somewhat above the horizontal plane passing through the axis 3, the stimulating beam will meet the phosphor surface 2 in a perpen¬ dicular direction (unlike the situation shown in Figure 2A-2B), in which case the resolution is at its best.

This very high resolution made possible by the structure ac¬ cording to the invention and the very high efficiency in the collection of the light emitted from the phosphor surface are based, among other things, on the fact that the direction SI of the stimulating light beam 6 is either perpendicular or at a very small angle α to the phosphor surface 2, and the radiation emitted from the phosphor surface 2 is collected, by a light collector 9 having a relatively small diameter 27, directly from around the meeting point P of the stimulating beam and the phosphor surface into the detector 7, the principal sensitivity direction S2 of which is also either perpendicular or at a small angle β to the phosphor surface. This resolution and this luminous efficiency are further improved by the fact that the stimulating beam 6 and the light collector 9 are constantly in a fixed position relative to each other, regardless of the rotational movement Rx of the drum member 8 and the rectilinear movement M of the slide, and advantageously the stimulating beam 6 impinges against the phosphor surface 2 in the center of the light collector end 11. The efficiency of the collection of emitted light is further improved by the fact that this end 11 of the light collector, or the light collector 9 in general, or directly the detector 7 can, by using the arrangement according to the invention, be located very close to the phosphor sur¬ face, since the phosphor sheet will be held well in shape by the procedure described above. In the invention the distance between these optical means and the phosphor surface is typi¬ cally less than 10 mm or 5 mm, but the distance can easily be made to be less than 2 mm or even less than 1 mm. Distances of 0.5 mm and down to 0.1 mm are possible. The small distance makes it possible to use a small collector, in which case losses of light can easily be minimized. The purpose of Figure 4 is to illustrate the orientations of the direction SI of incidence of the stimulating beam 6 and the principal sensi¬ tivity direction S2 of the detector used for the read-out of the light emitted from the phosphor surface 2. Figure 4 shows, first, schematically a plane T2, which is perpendicular to the

cylinder axis 3, and the direction SI of the stimulating beam 6 is at an angle α to a radius of the cylinder circumference 5, and the principal sensitivity direction of the detector 7 is at an angle β to the same radius. These directions SI and S2 are thus incident upon one point P on the phosphor surface 2. Fur¬ thermore, Figure 4 shows schematically a plane Tl, which is a plane passing through the axis 3, in which plane the direction SI of the stimulating beam is at angle α to a radius of the cylinder circumference 5 and the principal sensitivity direc¬ tion S2 of the detector 7 is at an angle β to the same radius, in which case the directions SI and S2 meet at the same point P, which is a point on the phosphor surface. It is clear that angles α and β may also form at positions other than those shown in Figure 4, i.e. they may be in a plane other than a plane parallel to the axis 3 or perpendicular to it, or they may be in different planes relative to each other, but it is preferable if the directions SI and S2 meet at the same point P on the phosphor surface 2. In order to accomplish a high reso¬ lution, both angle α and angle β are at maximum approx. 45°, and preferably they are smaller than approx. 30°. It is to be noted that when we speak about the directions SI and S2, we mean directions in the area of the phosphor surface 2 , and thus the radiation direction of the light source 4 may be any direc¬ tion whatsoever, as long as by means of mirrors, prisms or other means it can be caused effectively to impinge against the phosphor surface 2 within the limits defined above. Likewise, the sensitivity direction of the detector may be any direction whatsoever, as long as by means of mirrors, prisms or other means it can be caused effectively to impinge against the phos¬ phor surface 2 within the limits defined above. Thus, for example, in the arrangement of Figure IA the sensitivity of the detector 7 points in a direction parallel to the axis 3, but owing to the design of the light collector 9 its sensitivity direction in the area of the phosphor surface 2 is perpen¬ dicular to the phosphor sheet. The same applies to the light source 4 depicted in Figure 3A, the original direction of radi-

ation of which deviates from the targeted direction, but the beam reflected from the mirror surface 14 of the light col¬ lector fulfills the condition defined above in the area of the phosphor surface 2. There is no obstacle to the use of even larger angles of incidence, but in such a case the resolution and the light collection capacity are clearly lowered.

Since an individual phosphor sheet point is read out at point P and the slide, and thus the directions SI and S2 of the arriv¬ ing and leaving beams, will shift in direction M of the axis 3 while the phosphor surface moves in direction R, it is clear that the read-out point P will draw along the circumferential surface of the cylinder 5 a spiral line K, of which Figure 4 shows two, i.e. K _n and _n+1 , i.e. two successive scanning traces. It is clear that earlier, corresponding partial spirals of a spiral part K _n extending around the circumferential sur¬ face 5 can be indicated by K _n _ _πι and any subsequent spiral line parts by K _n+m . The distance between these spiral lines, i.e. scanning traces, can be adjusted by joint adjustment of the circumferential speed Rx and the transfer speed M of the slide carrying the read-out means 4, 7. A resolution of 6-7 line pairs/mm, and also a resolution of 12 or more line pairs/mm, can easily be accomplished by means of the invention. The phos¬ phor sheet read-out is implemented as a steady, continuous action, in which case neither the radiation source - detector combination not the drum member is stopped, nor is their direc¬ tion changed; they are both maintained in as steady a motion as possible.

The light collector 9 is preferably a hollow chamber 17 in a solid material, such as metal, plastic or glass, the inner surface of the chamber being reflective of the light emitted from the phosphor surface. This light collector 9 may comprise a branch and/or any other wall portion transparent to stimulat¬ ing radiation, as in Figures 2A and 3A, through which the stim¬ ulating beam 6 has access to the area of the first end 11 of

the light collector. In addition, the light collector 9 may have mirrors or prisms, either for guiding the stimulating beam to the phosphor surface or for guiding to the detector 4 the radiation emitted from the phosphor surface. However, here the radiation source - detector combination is very simple in structure, since it does not require any parts movable relative to each other, as can be seen in the embodiments of the fig¬ ures. In order to accomplish a high efficiency, the diameter of the light collector 9, i.e. in general the diameter 27 of its first end 11, is preferably at least three times the diameter of the sensitive area of the photodetector 7, the first end 11 being as close as possible to the phosphor surface 2. The light collector is preferably a tubular member having a circular or oval cross-section, and in many cases its diameter is at maxi¬ mum approx. twice the diameter of the sensitive area of the photodetector 7, but the diameter 27 may also be approximately equal to the diameter of the sensitive area of the detector 7. It is also possible to use a collimator suitable for the pur¬ pose, in which case a light collector having a diameter 27 even greater than those mentioned above, for example a light collec¬ tor five times that of the detector, can be used.

The light source 4 used is typically a laser of a suitable type, such as a semiconductor laser or, preferably, a laser diode. These are small in size and can be used continuously over long periods. In addition, the apparatus may comprise a radiation source providing steady light, not shown in the fig¬ ures, by means of which the image is erased from the phosphor sheet after it has been read out.

The slide 13 carrying the source 4 for stimulating light and the photodetector 7 can be moved in direction M, which is parallel to the axis 3, by any mechanism known per se. One option is to shape one of the guides 12 as a toothed rack against the teeth of which a toothed gear is rotated by a motor in the slide. Another option is to provide, in the hollow

chamber 15 or outside its ends, gears around which there is tightened a toothed belt to which the slide 13 is fastened. In this case, by the rotation of the gear or gears of the toothed belt, the slide can be caused to move in direction M along the guides 12. A third option is to arrange one of the guides 12 as a threaded bar rotating around its axis, and its counterpiece in the slide as a nut or the like, in which case the rotation of the bar will move the slide 13 in direction M. These struc¬ tures are not shown in the figures in greater detail, since many different new or known structures can be applied to them in the manner described above. The guides 12 may be supported at both ends projecting out from the cavity 15 of the drum member 8, or at only one end. The drum member may also be sup¬ ported by bearings at both ends or at only one end. If the procedure depicted in Figures 1A-1B and 3A-3B is used for feed¬ ing the phosphor sheet into the cavity 15 against the cylinder circumference 5 and out of it, the possibilities for other structural options are better. If the procedure depicted in Figures 2A-2B is used for feeding in the phosphor sheet and for taking it out, the structural options are limited, since the end 19 of the cavity 15 must be free.

In principle, of course, it is possible, while remaining within the scope of the invention, to keep the light source 4 and the photodetector 7 stationary and to produce a partial scanning movement in a direction M parallel to the axis 3 by moving the drum member also in the axial direction of the cylinder. How¬ ever, this option will most likely lead to a complicated struc¬ ture in which it will be difficult to accomplish a steady move¬ ment.

The read-out of a latent image recorded in a phosphor sheet by X-ray radiation has been discussed above. It is clear that the method and apparatus according to the invention can be used for all corresponding read-out functions regardless of the type of electromagnetic radiation or particle radiation by which the

latent image has been produced. Likewise, the wavelength of the stimulating light can be any wavelength by means of which emis¬ sion corresponding to the image can be obtained from a phosphor sheet. Thus the wavelength of the stimulating light, as well as the wavelength of the emitted light, may be outside the visible range. In the present application by term light is also meant corresponding radiation the wavelength of which is invisible to the human eye.