WO2007045847A1

WO2007045847A1 - Optical head for computed radiography

Info

Publication number: WO2007045847A1
Application number: PCT/GB2006/003841
Authority: WO
Inventors: John Haig Marsh
Original assignee: Intense Limited
Priority date: 2005-10-18
Filing date: 2006-10-17
Publication date: 2007-04-26
Also published as: GB2431536A; GB0521107D0

Abstract

A linear scan head for a computed radiography imaging plate scanner uses a monolithic array of semiconductor lasers to provide a plurality of separate output beams for focusing on an imaging plate and stimulating emission of radiation therefrom. A photodetector array simultaneously detects emitted radiation from each of a plurality of pixels of the imaging plate. Each pixel corresponds to a single one of said separate output beams. It is then possible to individually stimulate and read individual pixels on the imaging plate.

Description

OPTICAL HEAD FOR COMPUTED RADIOGRAPHY

The present invention relates to computed radiography systems, and in particular to optical systems for stimulating and reading storage phosphors of imaging plates that have been exposed to x-rays during radiographic imaging.

Computed radiography (CR) systems use imaging plates coated with storage phosphors to capture x-rays after they have passed through a subject being imaged, such as a patient. The phosphor materials include small amounts of dopant impurities such as Europium that alter their crystalline form and physical properties, enabling these enhanced phosphors to absorb and store x-ray energy. This stored energy comprises a latent image of the x-radiation to which the imaging plates have been subjected. . When an imaging plate is subsequently stimulated by optical energy of an appropriate wavelength, the stored energy is released, by emission of optical energy at a different wavelength.

In modern CR systems, this stimulation is usually effected with a low-energy laser and the resulting emission of optical energy is captured by a photodetector and converted into an electrical signal. The electrical signal is converted to a digital data stream that can be stored, transmitted to remote S3'stems or locations and displayed on laser-printed films or softcopy workstations.

CR systems have been widely accepted not least because of their compatibility with existing analogue x-ray equipment including generators, x-ray tubes, examination tables and upright chest exam systems. A CR imaging plate cassette is loaded into an x-ray system instead of .a film cassette, and the x-ray is taken. The exposed image plates are placed in a reader, scanned, and processed to create the final digital radiographic image. The imaging plates, once scanned, can be reused by first electronically 'deleting' the stored. image.

CR offers a number of advantages over conventional x-ray film, including elimination of film consumables, elimination of chemical processing, faster image availability, reduction of image retakes and duplication costs and a greater number of options for displa3άng, sharing and storing images.

Prior art CR scanners generally use 'flying-spot' scanning, as shown schematically in figure 1. A laser 10 together with a beam shaping and intensity control device 11 produces a tightly focused laser beam 5. A rotating mirror 12 is used to scan the beam 5 across an imaging plate 15 in the x-direction, while the imaging plate itself is moved slowly in the y-direction. The laser beam 5 stimulates the latent image in the imaging plate 15 one point at a ^•time over the entire imaging plate surface. The resulting emission of optical energy 16 from the imaging plate 15 is captured using appropriate collection optics (not shown), filter 17. and photodetector 18, to give an output analogue electrical signal. This analogue output signal is then sampled and quantized to produce a digital output which can be used to generate a digital image corresponding to the stored x-ray image on the imaging plate 15.

A potential problem with such 'flying-spot' scanning systems is that the hardware required to produce and deflect the laser beam 5, as well as the components needed to collect the emitted light and convert it into an electrical signal, have limited susceptibility to miniaturisation and the number of discrete components adds cost and complexity. .

The throughput of a flying-spot CR scanner depends on many factors, one of them being how much of the stored latent image in the imaging plate 15 must be extracted in order to make an output image with reasonable image quality. This is a component of system gain and largely determines signal-to-noise ratio (SNR).

The fraction of the stored signal released from the imaging plate . 15 during

• scanning, called the read-out depth, depends on the amount of stimulation energy deposited per unit imaging plate area, which is proportional to the incident laser intensity and exposure time. Increasing the laser intensity and/or exposure time increases the total emitted signal until the stored, latent image is essentially depleted (i.e. all stored energy has been extracted). The read-out depth per pixel depends on the amount of time the laser beam spends illuminating the pixel area, also known as the dwell time. Dwell time is inversely proportional to beam velocity.

The fundamental throughput trade-off in a flying-spot scanner is, therefore, ^• scan time versus signal strength. Higher scan speeds reduce scan time and increase throughput but also reduce dwell time, which lowers read-out depth and decreases image quality. Increasing the laser intensity can compensate for this exposure reduction at higher velocities. However, increasing the laser intensity also broadens the incident beam within the phosphor layer (due to light scattering) which produces a degradation in the modulation transfer function (MTF) and loss of image quality, e.g. loss of image resolution.

Such limitations can be partially overcome by using a technique in which the imaging plate is scanned line by line, rather than one point at a time, using a linear scan head. In particular, the size limitations imposed by the bulk}^" flying-spot components are no longer relevant. Furthermore, the trade-offs between throughput and image quality can be largely circumvented. For example, dwell times in the linear scan head technique are measured in milliseconds rather than microseconds, yet scan times are still significantly shorter than those of flying-spot scanners.

Figure 2, 3 and 4 show schematic diagrams of such a linear scan head. The scan head 20 includes a linear array of laser sources 31 , beam-shaping optics in the form of cylindrical lenses 32, electronic intensity control system (not shown), light collector optics 33, filter 34, and a linear array of photodetectors, e.g a CCD

^■ (charge coupled device) array 35 or photodiode array. These are all contained in one compact linear scan head 20 which passes over a stationary. imaging plate 21 in a scan direction 22 orthogonal to the axis of the scan head. The scan head is capable of covering an imaging plate area of approximately 43 x 43 cm² in approximately 5 seconds. The control electronics to .pre-process the captured signal are also provided on the linear scan head 20 so that raw, 14-bit digital image data can be transferred directly from the lineal" scan head to a control computer for further processing and distribution. Recent CR systems tend to use laser diodes rather than gas lasers for stimulating the storage phosphor during reading of the imaging plate 21. Laser diodes are available at wavelengths (red) and output powers (tens _. of mW) that are well matched to the stimulation spectra and sensitivities of contemporary storage phosphor materials. In addition, they are very compact light sources.

Existing linear scan heads 20 use a linear array of discrete laser diodes 31 (figure 3). Although the sampling resolution of the CCD photodetector array 35 is typically approximately 50 μm₅ providing one laser diode 31 for each detector element would he cost-prohibitive using discrete lasers. Therefore, the prior art linear scan head uses the divergent, strongly asymmetric output beam profile of laser diodes 31 to reduce the number of emitters required to several dozen.

As shown schematically in figure 5, the laser diodes 31 have an asymmetrical beam profile 41 (figure 5a). In existing scan heads, the laser diodes 31 are placed so that, along their strongly diverging axes (in the plane of figure Sc)₅ the laser beams 41 overlap with one another along the axis of the linear scan head 20, producing a continuous line 42 of stimulating light with sufficient power to reach an acceptable read-out depth in the imaging plate 21. By inserting two planoconvex cylindrical lenses 32 (see also figure 5b) into the beam path, it is possible to focus the less divergent beam axis (as seen in figure 5b) down to about 80 μm transverse to the line 42 to maintain, a reasonable MTF in that direction. The actual line width scanned ^' within the imaging plate is largely determined by light scattering. Each individual discrete laser diode 31 is provided with a drive current 43 to try to maintain as uniform intensity of light output along the axis of the line 42 as possible.

Once stimulated, the imaging plate 21 emits blue light 36 (figure 4) in proportion to the stored x-ray exposure. This light emission 36 is approximately Lambertian, i.e. it is emitted equally in all directions. The purpose of the light-collection optics 33 is to collect and direct as much of this emitted light 36 as possible onto the active surface of the photodetector 35. In flying-spot scanners, this is usually done with acrylic light pipes, fibre optics, mirrors or integrating cavities. Maximum light collection efficiency is realized when the entrance surface of the collection optics 33 is as close to the imaging plate surface as possible (i.e. the numerical aperture is large). For the same reason, the exit surface of the collection optics must be (at least, optically) close to the imaging plate 21.

The approach of the prior art linear scan head design is to accommodate the divergence of the laser beam outputs 41 and the relative size and spacing of discrete laser diode components 31 by overlapping the output beams 41 along the axis of the linear scan head 20 to provide an averaged, even intensity profile along the scan head axis, thereby relying entirely on the pliotodetector elements 35 and collection optics 33 to resolve individual 'pixels' along the scan head axis during simultaneous read out of all pixels along the linear scan head axis. With such a design approach, maintaining a uniform intensity of light output along the head axis is essential to obtain accurate measurement of the stimulated emission from the imaging plate 21. Maintaining uniform intensity is not easy when controlling a large number of discrete laser diodes 31.

lt is an object of the present invention to provide an improved linear scan head for a computed radiography imaging plate scanner.

According to a first aspect, the present invention provides a linear scan head for a computed radiography imaging plate scanner comprising: an array of semiconductor lasers adapted to provide a plurality of separate output beams for focusing on an imaging plate and stimulating emission of radiation therefrom; a photodetector array for simultaneously detecting emitted radiation from each of a plurality of pixels of the imaging plate, each pixel corresponding to a single one of said separate output beams. _.

According to a second aspect, the present invention provides a linear scan head for a computed radiography scanner having a plurality of laser diode elements in an array disposed for stimulating emission of radiation from an imaging plate which. elements are configured to produce an array of output beams having a linear spatial pitch parallel to the scan head axis that is at least as small as the linear spatial resolution of a corresponding arraj' of photodetector elements in the scan head for receiving stimulated emission from the imaging plate.

According to a third aspect, the present invention provides a linear scan head for a computed radiograph)' scanner having a plurality of laser diode elements configured to produce an array of output beams along the axis of the scan head for stimulating emission of radiation from an imaging plate, in which the number of separate output beams in the array is substantially equal to or exceeds a number of detection elements along the axis of the scan head.

Preferably, the array of semiconductor lasers or laser diode elements comprises a monolithic array. Preferred features associated with the first aspect of the invention in this specification, and particularly the claims, may also be used in conjunction with the second and third aspects of the invention.

Embodiments of the present invention will now be described by wa3^? of example and with reference to the accompanying drawings in which: Figure 1 shows a perspective schematic diagram of a prior art computed radiography scanning system implementing 'flying-spot' type scanning;

Figure 2 shows a perspective schematic diagram of a prior art linear scan head for a computed radiography scanning system;

Figure 3 shows a perspective view of elements of the linear scan head of figure 2;

Figure 4 shows a side view of elements of the linear scan head of figure 2; Figure 5 shows elements of the linear scan head of figure 2, in which figure 5a is a perspective view of a laser element in the scan head, figure 5b is a side view of the scan head viewed along the x-axis, figure 5c is a plan view of the x-y plane of part of the scan head, and figure 5d is a plan view of the x-y plane of the scan head;

Figure 6 shows a plan view of a laser array for a linear scan head having a laser diode element and a microlens element for each photo detection pixel; Figure 7 shows a plan view of a laser array for a linear scan head having a laser diode element and an optical waveguide element for each photodetection pixel;

Figure 8 shows. a plan view of a laser array for a linear scan head having a laser diode element and a microleiis element for each photodetection pixel and group addressing of the laser diode elements;

Figure 9 shows a plan view of a laser array for a linear scan head having plural separate output beams provided by each laser diode element in the array;

Figure 10 shows a plan view of a laser- array for a linear scan head having plural monolithic arrays of laser diode elements to form an extended array of laser diode elements;

Figure 11 shows a plan view of the disposition of plural monolithic arrays of laser diode elements, each arraj' having a corresponding drive and control circuit mounted on a common printed circuit board; Figure 12 shows a front view (along the laser beam axes) of an alternative disposition of plural monolithic arrays to form a higher density pitch laser array scan head;

Figure 13 shows a schematic diagram of a partially combined optical path for the output beams of the laser array and the input beams to a photodetector array; and

Figure 14 shows a schematic diagram of an alternative partially combined optical path for the output beams of the laser array and the input beams to a photodetector array.

Figure 6 illustrates a plan view of a laser arra}⁷ 60 of a linear scan head in which the individual laser diode elements 61 form part of a monolithic array of lasers, i.e. formed in a single semiconductor substrate. Recent developments in process technology for manufacturing laser diode monolithic arays by the present applicant have enabled the production of high power laser arrays having a pitch less than 200 microns and also down to around 80 microns, while still maintaining output power in the range of several hundred mW. The expression 'laser pitch' is used herein to denote the distance between corresponding points on adjacent lasers, i.e. the total of the width of laser output facet plus the spacing between adjacent laser output facets. Each laser diode element 61 is optically coupled to a respective lens element 63 in a microlens array 64 which focuses the optical output of each laser element 61 to provide a respective output beam 65.

In the embodiment of figure 6, each separate output beam 65 is provided by a corresponding single laser element 61 in the array 62 and the output beam pitch is substantially the same as the laser pitch, although as discussed later, this need not be the case. The expression 'beam pitch' is used herein to denote the distance between corresponding points on adjacent beam profiles, e.g. intensity peak to peak distance. The use of the arrangement of figure 6 allows the generation of output beams of pitch down to the pitch of the lasers, e.g. around 80 microns using present technologies for monolithic semiconductor laser array fabrication and wire bonding.

Preferably, as shown in figure 6, each laser diode element 61 is separately controlled by a control circuit (not shown) coupled to the laser diode by current injection control lines 66. In one preferred arrangement, all laser diode elements 61 are driven simultaneously to provide multiple simultaneous output beams 65. The control lines 66 may be used to adjust the level of drive current for each laser individually to ensure identical optical output of each laser element 61 thereby ensuring that each output beam 65 is identical.

Individual laser control may be used to compensate for normal variabilities in laser output, e.g. as a function of manufacturing process, operating temperature and other conditions. In particular, lasers having optical output in the red part of the optical spectrum commonly required for CR S3'stems are often very sensitive to operating temperature. By providing individually addressable ^' laser elements, thermal variations across the laser array 62 can be readify compensated for.

Each output beam 65 is directed to a separate region of the imaging plate 21 to stimulate emission of optical radiation therefrom, corresponding to prior x-ray exposure of that region of the imaging plate. A photodetector array similar to that described in connection with figure 4 is used to detect stimulated emission from a plurality of pixels of the imaging plate 21. The expression 'pixel' is used herein to mean an area of the imaging plate 21 over which the photodetector arraj⁷ 35 is capable of spatially resolving emitted radiation.

hi the preferred embodiment of figure 6, each one of the separate output beams 65 corresponds to a single pixel. In other words, each pixel resolvable by the photodetector array is stimulated by a dedicated output beam 65.

Thus, it will be recognised that individual pixels may be separately irradiated by dedicated output beams 65. This enables an 'interleaved' scanning process in which groups of non-adjacent output beams may be actuated together and at different times from other groups of output beams. For example, odd numbered laser elements in the linear array could be fired alternately with even numbered laser elements in the array. Each output beam corresponds to a pixel resolvable by the photodetector array. Thus, it will be recognised that an odd numbered laser element output beam and an adjacent even numbered laser element output beam can illuminate separate parts of the same pixel. The resolution of system then becomes twice that suggested bj' the pixel size resolvable b}^r the photodetector array, because different parts of the pixel can be read at different times by the same element of the photodetector array. The laser elements may be grouped and fired in two, three or more interleaved groups. Alternatively, the imaging plate 21 could be scanned several tunes with the scan head translated in the x-direction relative to the plate by a small distance corresponding to a fraction of a pixel.

With reference to figure 7, other forms of waveguiding or focusing element (other than the microlens array of figure 6) may be used. In figure 7, each output beam 75 is guided and/or focused by a respective waveguide fibre 73 or other waveguide element arranged in an array. Other optical systems that may be used include holographic optics or bulk optics, or a combination thereof. The optical elements are preferably integrated into a unitary structure, e.g. microlens array, or may be discrete structures (e.g. fibre optics) separated by free space or some other medium. Also shown in figure 7, the individual laser diode elements 71 of the monolithic array 72 may be controlled in groups 77, each of four elements as shown in this example, to reduce control line 76 complexity. The stimulating power incident on individual pixels can still be carefully controlled (given that immediately adjacent devices on a monolithic array often exhibit closely similar operating characteristics) whilst reducing the number of electrical interconnects and electronic control circuits.

Figure 8 illustrates an alternative array configuration 80 similar to figures 6 and 7 in which common control lines 86 are shared between groups 87 of laser elements 81 on a monolithic substrate 82 while each laser element 81 has a respective rnicrolens element 83 of a microlens array 84. .

In some circumstances, it may be advantageous to use a single laser to generate a number of output beams. An example is shown in figure 9. Each laser element 91 may be used to provide a divergent optical output 97 that is coupled into a beam splitting device to produce multiple (in this exemplary case, four) output beams 95. λ^arious arrangements of beam splitter are possible. The example of figure 9 comprises holographic phase plates 98 in combination with a microlens array 94 comprising plural microlenses 93, one for each output beam 95, to generate a plurality of parallel coUimated beams. In this example, the array of semiconductor lasers 91 is adapted to provide a plurality of output beams 95 by virtue of the requisite beam splitter elements coupled thereto. The beam splitters may also be incorporated into appropriate waveguide elements. It will also be appreciated that the array of beams produced after the first holographic phase plate 98 could be used to illuminate the imaging plate directly.

The approach of figure 9 allows the individual laser elements to be wider spaced and therefore higher power.

With reference to figure 10, multiple monolithic arrays 102 of semiconductor laser elements 101 may be mounted on a common substrate 103 to create an extended array 100 of laser elements 101. Separate monolithic arrays 102 can be positioned adjacent to one another in a linear array sufficiently close to enable the pitch of the lasers to be maintained across the entire extended array, for example using techniques described in the present applicant's co-pending patent specification no PCT/GB2005/001964.

The extended array shown in figure 10 may be used in conjunction with optical elements such as holographic phase plates 98, array 94 of microlenses 93 as described with reference to figure 9, or any other optical configuration for providing plural output beams as described herein, e.g. as shown in figures 6, 7 and 8.

With reference to figure 11, an extended array 100 of monolithic laser arrays 102 are mounted on a carrier 110 with printed circuit board for connecting respective ASIC drive circuits 103, 104, one for each monolithic laser array 102. In the example shown, sixteen monolithic arrays each comprising sixteen laser elements . having a laser pitch of 150 microns have been used to create an extended array of 256 laser elements over a total array width of approximate^ 38 mm. Multiple such extended arrays 100 can be incorporated into one linear scan head. It will be appreciated that a single ASIC may be used to drive one or more monolithic laser arrays or that a single large monolithic laser array might be driven by more than one ASIC.

With reference to figure 12, multiple extended arrays 100 can be combined within a scan head 120 such that the effective output beam pitch is reduced over that available from the laser beam pitch of each array. This is achieved by disposing each one of the monolithic arrays 62, and each one of the extended arra3's 100, at an angle oblique to the axis of the scan head 120, and consequently also oblique to the scan direction 121. The figure 12 shows the scan head as viewed along the beam axes. The angle of 48.19° to the scan direction as shown is purely illustrative and is adapted to reduce the effective pitch of a laser array at 150 μm to a beam pitch along the scan head axis of 100 μm: Corresponding oblique mounting of the photodetector arra)' (not shown) may be used to maintain relationship between each pixel and a respective output beam. It will be understood that individual pixels of the imaging plate can then be stimulated and read by appropriate timing of the firing of laser elements as a function of scan position of the linear scan head 120 along the scan direction 121. The relative position of the scan head and imaging plate can be controlled in conjunction with appropriate timing of the firing of laser diode elements.

Oblique mounting of arrays 100 as shown in figure 12 also allows the ends of each array 100 to overlap, to compensate for possible larger margins between the last laser element in an array and the edge of the substrate than half the beam pitch.

Oblique mounting of laser diode arrays as in figure 12 also allows improved forced air cooling of the arrays as air can be directed at the arra3^rs in the same direction as the scan direction marked, so that the arrays act as baffles or vanes to the cooling air flow. Further details of oblique mounting of laser arrays are found in the present applicant's co-pending patent application PCT/GB2005/001971.

In preferred embodiments of the invention, ever}' output laser beam from the monolithic array or arrays as described in connection with figures 6 to 12 is associated with a corresponding photodetector array element for resolving stimulated emission from a respective pixel of the imaging plate such that there is one-to-one relationship between output beams 65, 75, 85, 95 and a corresponding photodetector element. In a preferred embodiment as shown in figure 13, each output beam 130 of the semiconductor lasers in laser array 131 and the corresponding emitted radiation 132 from each of the plurality of pixels 133 of the imaging plate 21 shares a common optical path 134 between the imaging plate 21 and a beam splitter 135.

This enables at least some of the imaging optics to be common for the stimulating beam and for the light collection. This allows the photodetector elements to operate with a very large effective numerical aperture, e.g. with a large lens placed close to the imaging plate, thus avoiding a compromise associated with the current state of the art in which the light collection lens has to be displaced back from the imaging plate to allow optical access for the stimulating beam as shown in figure 4. In the arrangement of figure 13, a dichroic filter is used as a beam splitter to separate, the stimulating output beams 130 (typically red) from the emitted light 132 (typically blue) with high efficiency. It will be appreciated that the optical imaging system shown in figure 13 can be augmented by adding extra elements to • give independent control of the laser spot size on the imaging plate 21 and the numerical aperture of the light collection s)^?stem.

In practice, any wavelength selective element could be used as a beam splitter, for example a wavelength division multiplexer (WDM) coupler 145. In the arrangement of figure 14 a first waveguide 140 directs the output beam 141 from a laser diode element (not shown) into ^"Vv⁷DM coupler 145, the output beam emerging therefrom on common optical path 144 to the imaging plate 21. Stimulated emission from the imaging plate 21 travels on the common optical path 21 to ^"Vv⁷DM coupler 145 which diverts the emitted light 142 to the respective photodetector element via optical fibre 146.

Other beam splitting devices ma}' be used, e.g. bulk optical components such as etalons or partially reflecting mirror assemblies.

As discussed in connection with figure 9, it is possible to split the optical outputs of each laser diode element 91 so that one laser generates several beams, e.g. a 100 mW laser can generate 4 x 25 mW beams, and illuminate four pixels of the imaging plate. This technique can be used to create a scan head with an optical beam pitch much smaller than that which is achievable using individual laser diode elements.

It will be appreciated that scattering and other effects in the imaging plate cause a . stimulating output beam to excite a pixel somewhat larger than the beam spot size. The exciting beam spot is therefore preferably focused with appropriate waveguiding or focusing optics (e.g. microlens arrays 64, 84, 94, waveguides 73, coupler 145 or other lenses 138) to result in an output beam spot size smaller than the pixel area sampled by the detector. The output beam spots are preferably focused to be centred on each pixel area of the imaging plate, The beam spots may have a circular shape on the imaging plate or may be arranged to be elliptical or rectangular with the spot short axis orthogonal to the axis of the scan head, i.e. orthogonal to the line being scanned.

Scan heads as described above offer a number of advantages over prior art scan heads. Simultaneous stimulation along the axis of the scan head for smaller pixel areas results in higher power density on the imaging plate thereby increasing scan speed {reducing dwell time required). Individual addressing of laser elements allows multiple pass scans (e.g. interleaved scans) and temperature compensation across the array. Output beam pitch down to at least 30 microns is possible, with consequent improvements in resolution. Providing a common optical path provides optimum light collection efficiency.

A particular advantage of the new linear CR scan head is improvement in the sensitivity of CR systems for mammography where there is a need to image small microcalcifications of the order of 100 μm in size which also have low contrast. In order to characterize margins a better resolution is required. The linear CR scan heads described herein are capable of imaging at pixel sizes of down to 25 to 50 microns.

Other embodiments are intentionally within the scope of the accompanying claims.

Claims

1. A lineai- scan head for a computed radiography imaging plate scanner comprising: an array of semiconductor lasers adapted to provide a plurality of separate output beams for focusing on an imaging plate and stimulating emission of radiation therefrom; a photodetector array for simultaneously detecting emitted radiation from each of a plurality of pixels of the imaging plate, each pixel corresponding to a single one of said separate output beams.

2. The linear scan head of claim 1 in which each separate output beam is provided by a corresponding single laser element in the array.

3. The linear scan head of claim 1 in which each separate output beam is provided by a laser element in conjunction with a single waveguiding element in an array of waveguiding elements.

4. The linear scan head of claim 3 in which the array of waveguiding elements comprises a microlens array.

5. The linear scan head of claim 3 in which the array of waveguiding elements comprises an array of optical fibres.

6. The linear scan head of claim 2 in which each laser element is optically coupled to a beam splitter to provide plural ones of said output beams from each single laser element in the array.

7. The linear scan head of claim 1 in which there is provided a separate photodetection element corresponding to each one of the separate output beams.

8. ' The linear scan head of claim 1 in which array of semiconductor lasers comprises a monolithic array.

9. The linear scan head of claim 8 in which each laser element in the monolithic arrajr of semiconductor lasers is separately addressable,

10. The linear scan head of claim 8 further comprising a plurality of said monolithic arra)^rs of semiconductor lasers adjacent one another forming an extended array of laser elements.

11. The linear scan head of claim 10 in which each of said plurality of monolithic arrays are disposed at an angle oblique to the axis of the scan head.

12. The linear scan head of claim 1 in which each output beam of the semiconductor lasers and the corresponding emitted radiation from each of the plurality of pixels of the imaging plate shares a common optical path between the imaging plate and a beam splitter.

13. The linear scan head of claim 12 in which the beam splitter is a wavelength division multiplexer.

14. The linear scan head of claim 1 in which the dimension of each output beam focused on the imaging plate is smaller than each pixel resolvable b}^? elements of the photodetector array.

15. The linear scan head of claim 1 in which the number of separate output beams is substantially equal to or greater than the number of pixels resolvable by elements of the photodetector array.

16. The linear scan head of claim 1 in which laser elements in the array are adapted to be driven to provide adjacent output beams at different times.

17. A linear scan head for a computed radiography scanner having a plurality of laser diode elements in an array disposed for stimulating emission of radiation from an imaging plate which elements are configured to. produce an array of output beams having a linear spatial pitch parallel to the scan head axis that is at least as small as the linear spatial resolution of a corresponding array of photodetector elements in the scan head for receiving stimulated emission from the imaging plate.

18, A linear scan head for a computed radiography scanner having a plurality of laser diode elements configured to produce an array of output beams along the axis of the scan head for stimulating emission of radiation from an imaging plate, hi which the number of separate output beams in the array is substantially equal to or exceeds a number of detection elements along the axis of the scan head.

19. A linear scan head for a computed radiography scanner substantially as described herein with reference to the accompanying drawings.