Abstract:
The present invention concerns x-ray sources for mammography. A microfocused x-ray source of small size (30 μμm and smaller) with x-ray spectrum optimized for enhanced mammography is obtained with a method and system according to the invention. The proposed x-ray source is based on the use of plasmas created by the energy distribution of suprathermal electrons that are produced during the interaction of the laser beam with a solid target. These hot electrons penetrate the surface layer of cold plasma and interact with the solid core of the target. The method and system according to the present invention allows optimizing the x-ray source size, its spectral distribution, and the conversion efficiency in the 17.3-28 5 keV range (adapted to the breast thickness).

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to x-rays production. More specifically, the present invention is concerned with a method and system for generating microfocused laser-based x-rays suitable for mammography  
         BACKGROUND OF THE INVENTION  
         [0002]    Mammographic signs of early cancer include calcifications, small masses (densities), neo-densities and architectural distortions. Screening mammography aims at revealing these signs and is highly sensitive and specific. However, still a notable fraction of mammography produces false positive or false negative results These outcomes most occur in the thick and/or dense breast. False positive results may result in an unnecessary additional imaging and biopsy. False negative results, which occur sometimes even when a cancer is palpable, have a severe adverse effect due to delayed biopsy and delayed cancer diagnosis.  
           [0003]    Presence of calcifications usually helps to detect breast pathology. Although calcifications demonstrate well-pronounced contrast in mammograms, their detection threshold size is a function of x-ray tube focal spot size, radiographic technique factors including radiation quality, signal-to-noise ratio and exposure time, as well as function of detector and display devices. Even though, in early stages of breast disease microcalcifications are present in approximately half of the cases, they are not apparent on regular screening mammographs before reaching the detection threshold size, which is typically around 100 μμm. This especifically applies to punctuate calcifications  
           [0004]    Some of the inherent limitations of the x-ray tube based screen-film mammography include:  
           [0005]    relatively large focal spot (typically nominal ˜100 μm);  
           [0006]    varying effective focal spot size across the imaged breast;  
           [0007]    suboptimal (below 13 lp/mm) spatial resolution in the upper quadrants for breast thickness above 6 cm:  
           [0008]    off-focal radiation;  
           [0009]    a relatively low output (especially with microfocus);  
           [0010]    a suboptimal spectral characteristic of x-rays for imaging dense fibroglandular tissue and/or thick breast;  
           [0011]    restricted latitude of film-screen mammography, and  
           [0012]    restricted contrast of film-screen mammography.  
           [0013]    Regarding the two last restrictions, film-screen mammography utilizes film as a recording medium to properly record and display relatively narrow dynamic range of x-ray exposures. This should be contrasted with specially designed detectors that can correctly record four orders of magnitude of x-ray exposure. This information can be subsequently displayed on specially designed monitors. Moreover, since film-screen mammography aims at obtaining very high contrast images, the slope of the optical density/x-ray exposure curve is very high. As a result, only a very limited range of x-ray exposure is acceptable If exposure is too high or too low, it will produce exceedingly high or low optical density thus rendering the image not useful clinically.  
           [0014]    While the two last restrictions can be alleviated to a great extent by digital mammography, the other limitations stem from the inherent limitations of x-ray tube and are unlikely to be overcome in the framework of this technology.  
           [0015]    New technology for generating x-rays had emerged in recent years It relies on emission of x-rays from laser-produced plasma (LPP). This phenomenon occurs when a visible or infrared laser beam is focused onto the surface of solids or liquids If the optical power density exceeds a material dependent threshold value, continuous bremsstrahlung and characteristic x-ray emission lines result. However, initially very expensive and large laser systems were required to obtain the required optical power and the LPP x-ray sources were rather large (100 μm-1 mm). The invention of chirped-pulse amplification (CPA) in the late 1980&#39;s allowed achievement of high optical power density delivered to the target by the laser beam from compact and significantly cheaper table-top terawatt ultra-fast lasers.  
           [0016]    The feasibility of CPA lasers for mammography and angiography has been demonstrated in both “High Magnification Imaging With a Laser-Based Hard X-ray Source”, IEEE Journal of selected topics in Quantum Electron, Special issue on laser in medicine, 5, 911-915 (1999), by J. Yu, Z. Jiang, J. C. Kieffer, A. Krol. and “Laser-Based microfocused X-ray Source for Mammography faisability Study”, Journal of Medical Physics, 24, 725-732 (1997), by A. Krol, A. Ikhlef, J C Kieffer, D. Bassano, C. C. Chamberlain, Z Jiang, H. Pepin, S. C. Parsad. Data in these publications shows the ability to obtain focal spot size of the order of 10 μm necessary to perform high spatial resolution mammographic imaging and confirm system ability to produce x-ray spectra from a number of different target materials, including Mo, Rh, Ag, In and, Sn with characteristic emission energies spanning 17 3 keV to 28.5 keV.  
           [0017]    Even though LLP x-ray source created by CPA lasers can be very small (10 μm or less) and bright, with peak power many order of magnitude higher than conventional x-ray tubes, a drawback of CPA laser sources from the prior art has been their low average power However, in recent years, significant progress has been made in this respect 20 W average power CPA lasers are presently available, and a 50 W CPA laser is in the design stage  
           [0018]    Finally, there have been studies on sub-picosecond laser-solid-matter interaction devoted to investigation of x-rays generation, from the soft x-rays up to the very hard x-ray emission in the MeV range. However, nobody attempted to simultaneously control the emitted x-ray spectrum, the x-ray source size and conversion efficiency from laser to x-rays.  
           [0019]    Method and system for generating microfocused laser-based x-rays allowing to simultaneously optimize the x-ray source size, its spectral distribution, and the conversion efficiency in the 17 3-28.5 keV range is thus desirable.  
         SUMMARY OF THE INVENTION  
         [0020]    An object of the present invention is therefore to provide an improved system and a method for generating microfocused laser-based x-rays.  
           [0021]    Specifically an object of the present invention is to provide such method and system allowing optimizing the x-ray source size, its spectral distribution, and the conversion efficiency in the 17.3-28 5 keV range (adapted to the breast thickness and/or density) for improved mammography.  
           [0022]    More specifically, in accordance with an aspect of the present invention, there is provided a system for generating microfocused laser-based x-rays comprising an ultra-fast laser; and a solid target with elemental composition positioned in the focal plane of said ultra-fast laser; wherein said ultra-fast laser and solid target are configured so that interaction with said solid target of a laser beam produced by said ultra-fast laser produces suprathermal electrons that penetrate said solid target in a spot whose size is comparable to the ultra-fast laser focal spot size, yielding hard x-rays More specifically, in accordance with another aspect of the present invention, there is provided  
           [0023]    A method for generating microfocused laser-based x-rays using the above system, the method comprising: without The solid target, generating and amplifying from the ultra-fast laser, a first laser beam pulse train so as to control and adjust the laser energy, positioning the target in the focal plane of the ultra-fast laser so as to offer a fresh surface to the ultra-fast laser; generating and amplifying from the ultra-fast laser, a second laser beam pulse train, and releasing and focusing the second laser beam pulse train on the solid target; whereby the interaction of the second laser beam pulse train on the target generates hard x-rays.  
           [0024]    A system and method according to the present Invention allows producing a very small x-ray source with an x-ray spectrum tailored to a specific patient&#39;s breast density and thickness suitable to detect pathology in examined breast, for example microcalcifications, with increased sensitivity and specificity, as compared to method and system from the prior-art.  
           [0025]    Compared to systems and methods from the prior art, a system and method according to the present invention allows to lower calcification detection threshold size down to approximately 30 μm, improves visualization of morphology of microcalcifications and marginal characteristics of masses, improves detection of densities (masses), especially in a dense breast tissue, lowers mean grandular dose, especially in a dense and thick breast tissue, and enhances both sensitivity and specificity of mammography.  
           [0026]    A method and system according to the present invention yields very small (below 30 μm) x-ray focal spot size that will results in:  
           [0027]    significantly improved and uniform within field of view spatial resolution of the order of 20 lp/mm. In conventional mammography resolution is non-uniform and do not exceed 13 lp/mm. The upper quadrants of thicker (d&gt;6 cm) breasts are imaged with resolution below 10 lp/mm and there is strong gradient of spatial resolution across field of view (in the cathode-anode direction) in the detector plane,  
           [0028]    better visualization of pathologies, including demonstration of morphology of very small microcalcifications that cannot be imaged with conventional mammography, that will allow earlier detection of breast cancer with significant benefit for women&#39;s health; and  
           [0029]    magnification imaging without necessity of moving breast closer to the source This is necessary in conventional mammography and results in decreased field of view  
           [0030]    Moreover a method and system according to the present invention allows x-ray spectrum to be tailored to specific breast thickness/density that will result in:  
           [0031]    lower dose to thicker/denser breast with better contrast, as compared to conventional mammography. Dose saving can be as large as factor of three for very thick/dense breast (d&gt;&gt;4.5 cm); and  
           [0032]    better imaging of thicker/denser breast due to shorter imaging time. It might completely eliminate “white mammograms” presently obtained with radiopaque breast.  
           [0033]    Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    In the appended drawings:  
         [0035]    [0035]FIG. 1 is a schematic view illustrating an x-ray mammography system including a system for generating microfocused laser-based x-rays according to an embodiment of the present invention;  
         [0036]    [0036]FIG. 2 is a graph illustrating The optimum x-ray energy for imaging various pathologies according to the breast thickness:  
         [0037]    [0037]FIG. 3 is a graph illustrating the energy conversion efficiency from hot electrons into characteristic line emission,  
         [0038]    [0038]FIG. 4 is a graph illustrating the optimum laser intensity as a function of the target material atomic number Z,  
         [0039]    [0039]FIG. 5 is a schematic view illustrating the laser-solid interaction as provided by the x-ray mammography system from FIG. 1 using a flat target, and  
         [0040]    [0040]FIG. 6 is a schematic view illustrating the laser-solid interaction as provided by the x-ray mammography system from FIG. 1 using a shaped target 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0041]    Turning to FIG. 1, an x-ray mammography system  10  incorporating a system for generating microfocused laser-based x-rays  12  according to embodiments of the present invention are illustrated.  
         [0042]    The x-ray generator  12  is based on both a stationary ultra-fast CPA (chirped-pulse amplified) laser  14  and a solid target  16  with elemental composition so positioned as to be in the focal plane of The laser  14 .  
         [0043]    As it will be explained hereinbelow in more detail, the ultra-fast laser  14  and solid target  16  are chosen and configured so that interaction of a laser beam produced by the ultra-fast laser  14  with the solid target  16  produces suprathermal electrons that penetrates the solid target  16  in a spot whose size is comparable to the ultra-fast laser  14  focal spot size. This allows simultaneously optimizing the source size, the x-ray yield and the photon energy  
         [0044]    The target  16  is mounted in a movable target chamber  18  provided with rapid target/filter changer (not shown), a precision target positioning system  20 , a target position monitor and feedback system  21 , a target ablation products removal system (not shown), and a vacuum pump  22  for providing vacuum condition in the chamber and more specifically on the target surface. The target chamber  18  allows to precisely positioning a selected area of the target  16  relatively to an x-ray window  24 . The x-ray window is located so as to allow hard x-rays produced by the laser-target interaction out of the target chamber in the direction an image receptor  40  as it will be explained hereinbelow in more detail. The movable target chamber allows obtaining all views required in mammography including cranio-caudal view (CC), the medio lateral oblique view (MLO), the latero media) (LM), the medio lateral (ML) and supplemental views  
         [0045]    The target/filter allows to properly shape the x-ray spectrum A precision target positioning/feedback allows to keep the x-ray focal spot small.  
         [0046]    The precision target positioning system  20  allows mounting and moving, including translation and rotation, the target  16  for reason explained hereinbelow. The target position monitor and feedback system  21  helps precisely position the target relative to the laser focal point and x-ray window  24  As will be explained hereinbelow, the target is to be moved between laser shots so as to expose for each new shot a fresh surface thereof. Since precision positioning systems, and target position monitor and feedback systems are believed to be well known in the art, and for concision purposes, they will not be described herein in more detail  
         [0047]    The x-ray window  24  is preferably made of beryllium. Of course other type of x-ray window may alternatively be used such as Kevlar thin foil  
         [0048]    Of course, the target chamber  18  includes other structures, mechanisms and electronics required for the operation and interactions of the above-mentioned component as it is commonly known in the art. Other features of the movable vacuum target chamber  18  will become more apparent upon reading the following description.  
         [0049]    The x-ray generator  12  also comprises an optical beam transfer system  26 , coupling the stationary ultra-fast CPA laser  14  and the movable target chamber  18 .  
         [0050]    The optical beam transfer system  26  includes adjustable mirrors  28  and an optical beam-focusing device  30 . The optical beam-focusing device  30  advantageously includes optical feed back system (not shown) for stable laser beam aiming at the target  16 , and an adjustable parabolic mirror  32 . The vacuum target chamber  18  includes an optical window  34  allowing a laser beam focused by the parabolic mirror  32  to enter the chamber  18  and hit the target  16 . The optical window  34  is chosen so as to be transparent to the wavelength of the laser  14  and is advantageously made of MgF. Other suitable transmitting material can also be used. The flat and parabolic mirrors  28 ,  34  are obviously provided with appropriate mounts (not shown)  
         [0051]    Of course, the optical beam transfer system  26  may alternatively have other configurations than the one depicted on FIG. 1 For example, the CPA laser  14  may be positioned so that its beam directly aims at the target  16 , with the target positioned at the focal point of the laser  14 .  
         [0052]    The target chamber  18  further includes a set of filters  36  that are mounted to the x-ray window  24 , and an x-ray collimator  37 . They allow to properly shape the x-ray spectrum and the x-ray field of view to maximize the image contrast within constrains imposed by the maximum allowable glandular radiation dose and the maximum exposure time.  
         [0053]    The x-ray mammography system  10  further includes conventional compression paddles  38  for breast, an image receptor  40  optionally provided with removable mammographic grid  42  allowing for contact magnification imaging, and an automatic exposure and target selection control  44 . The automatic exposure control is well known in conventional mammography and is used to avoid over- or under-exposure of the imaged breast. The automatic target selection allows perfect match of the x-ray spectrum and a specific patient&#39;s breast thickness/density, resulting in the best contrast at the lowest dose Conventional mammographic x-ray units typically use Mo target Some units have an additional Rh target but the maximum load (mAs) for Rh is significantly constrained, as compared to Mo, and Rh is used infrequently. The image receptor  40  is a mammographic film/screen combination or a mammographic digital detector  
         [0054]    The compression paddles  38  are located between the image receptor  40  and the target chamber  18 , and are movable (represented by double-arrow  46  on FIG. 1) so as to advantageously yield an adjustable air-gap between the paddles and the image receptor  40 . Very small x-ray focal spot size allows the possibility to perform mammography in a full field of view magnification mode with the air-gap acting as anti-scatter device and without necessity to move imaged breast closer to the x-ray source Thus, it is possible to simply move away the image receptor from the breast while keeping breast at the same distance from the x-ray source as in the contact mammography (e.g. 65 cm), and obtain good quality magnified images. In conventional mammography only spot magnification (i.e. with very small field of view) images might be obtained and the imaged breast has to be positioned much closer to the source (e.g. at 40 cm), as compared to contact technique (e.g. 65 cm).  
         [0055]    The image receptor  40  may take many forms such as a mammographic film/screen combination or a digital detector. Since digital detectors and mammographic film/screen combination are believed to be well known in the art, and for concision purposes, they will not be explained herein in more detail  
         [0056]    Of course, the x-ray mammography system  10  also includes other well-known non-illustrated components such as optical and x-ray shielding for operator and patient protection, and a system diagnostics  
         [0057]    The operation of the x-ray generator  12  will now be described in more detail.  
         [0058]    Without target  16 , a first sequence of high voltages is triggered for the generation and amplification of a first laser beam pulse train in order to control and adjust the laser energy  
         [0059]    The target  16  is then positioned in the focal plane of the optical beam.  
         [0060]    It is to be noted that the motion of the target (rotation and or translation in a fixed plane) is triggered in order to offer a fresh surface at every laser shot.  
         [0061]    A second sequence of high voltages is then triggered for the generation and amplification of the interacting laser beam pulse train with appropriate energy and pulse number. The pulse number is selected using an adjustable gate  
         [0062]    The pulse is compressed in an optical compressor composed of a set of gratings and mirrors. The shutter is opened at the compressor exit to release the pulse train on the target  16 . The laser pulse  50  (see FIG. 5) is of course focused onto the target  16  by means of the parabola  32 .  
         [0063]    Hard x-rays  54  are produced during the interaction laser beam-target interaction.  
         [0064]    Turning now to FIG. 5, the laser-target interaction is schematically illustrated.  
         [0065]    [0065]FIG. 5 illustrates the laser-solid interaction for the flat target case. In this example, the focal spot size is about 3 μm in diameter and the incidence angle of the laser beam  50  hitting the surface  16  is 45°. The hatched area represents the 2D slab  52  of thermal plasma generated via radiation pressure confinement.  
         [0066]    The angle of incidence of the laser pulse  50  on the target  16 , the laser polarization and the target  16  are so chosen as to match the density gradient scale length set by the radiation pressure confinement. Under these conditions, the resonance absorption mechanism that is at the origin of the hot electrons  48 , and thus the energy transfer from the laser  14  to the hot electron population  48  is optimized As the resulting plasma is a thin two-dimensional slab (as illustrated in FIG. 5) or three-dimensional droplet (not illustrated) (gradient scale length smaller than the laser beam focal spot size), the trajectories of the hot electrons  48  are limited to the laser focal spot, which can be as small as a few micrometers The electrons  48  penetrate the target in a spot whose size is comparable to the laser spot size. The generated hot electron distribution f(e), is Maximilian More specifically:  
           f ( e )= AE   1/2   exp (− E/kT   H ,  
         [0067]    where T H  is the hot electron temperature and k is Planck constant.  
         [0068]    A judicious choice of target geometry and composition, of the laser pulse duration, polarization and/or intensity, allows shaping the x-ray spectrum and to maintain and control the hot electron temperature between 10 and 50 keV. The conversion efficiency is drastically reduced for hot electron temperatures lower than 10 keV It has also been found that hot electron temperatures higher than 50 keV induce a degradation of imaging contrast due to the high-energy tail in the x-ray spectrum The emitted x-ray spectrum is composed of continuum bremsstrahlung and discrete emission lines. The K α  lines radiation is dominant over bremsstrahlung and can be used, with appropriate filtering for imaging  
         [0069]    As it will be explained hereinbelow in more detail, the laser  14  is configured for providing sufficiently high-contrast sub-picosecond pulses with peak and average power optimized for the imaging task, i.e. breast thickness, composition and imaging time.  
         [0070]    According to the present invention, the radiation pressure of the pulse produced by the laser  14  is used to balance the thermal plasma pressure occurring on the target  16  during the laser pulse. This allows the thermal and hydrodynamic mechanisms tending to spread the plasma to be overcome and thus the plasma size (perpendicularly to the target surface) to remain very small. The trajectories of hot electrons  48  above the target  16  surface are then limited and controlled, thus minimizing the x-ray source size  
         [0071]    The laser parameters are controlled as follows. A very high contrast laser pulse (10 10 ·1), indicative of the intensity peak/background noise ratio, is required to help prevent significant plasma expansion before the arrival of the main pulse The pulse duration is set larger than 300 fs and the laser intensity is selected so as to be in the range 10 18  W/cm 2 -10 19  W/cm  2 . At lower intensity the thermal pressure is dominant and the plasma freely expands during the laser pulse, while at higher intensities the radiation pressure can be so high that plasma profile modification becomes a problem In the optimum intensity range, a still two-dimensional plasma slab  52  having characteristic dimensions of optical laser diameter (typically 3 μm) by thermal penetration depth (typically 0.3 μm) is produced during the laser pulse, resulting in constrained hot electron  48  trajectories above the target surface  55   
         [0072]    The control of the x-ray source size imposes utilization of a well-defined narrow intensity range (10 18  W/cm 2 -10 19  W/cm 2 ). It has been found that this intensity range is also the most appropriate one for the x-ray photon energy adjustment to optimize imaging for a given breast thickness/density It has also been found that the energy conversion efficiency (η k ) from hot electrons into characteristic line emission reaches maximum when the hot electron temperature (kT n ) is about 3 times the photon energy (E k ) of the characteristic line emission (see FIG. 2) Therefore, the photon energy E k  is adapted to a specific breast thickness, by adjusting the hot electron temperature so as to yield kT n /E k =3.  
         [0073]    The hot electron temperature can be adjusted by a careful laser intensity control indeed, it has been found through experiments that  
           kT   n =α( lλ   2 ) 1/3 ,  
         [0074]    where l is the laser intensity and λ is the laser wavelength. Radiation Confined Plasmas yields α=5×10 −5  when kT n  is in keV, l in W/cm2 and λ in μm as deduced from experiments with a 400 fs laser pulse at 0.53 μm wavelength. This yields the following expression for the optimum intensity (corresponding to kT n /E k =3)  
           l   opt   =bE   K   3 /λ 2    
         [0075]    This optimum intensity as a function of the target material atomic number Z is shown in FIG. 4 for a 400 fs laser pulse at 0 53 μm wavelength (l opt (W/cm 2 )=7.7×10 14  E K   3  (with E K  in keV)). Consequently, energy and yield-optimized x-ray spectra for particular breast thickness and density can be generated by careful selection of both target material and laser intensity.  
         [0076]    The laser intensity window, for example, for a laser having a 400 fs pulse and 0 53 μm wavelength, is in the 2×10 18  W/cm 2 -10 19  W/cm 2  range for production of x-ray spectra suitable for optimized imaging of breast thickness between 2 cm and 8 cm with target atomic number Z between 38 and 50. The use of this intensity range simultaneously allows  
         [0077]    using RCP and thus controlling the hard x-ray source size;  
         [0078]    utilizing targets with the appropriate elemental composition to match the x-ray energy to a patient specific breast thickness/density with an optimized conversion efficiency (η K ).  
         [0079]    It has been found that a pulse duration between 200 fs and 600 fs allows to optimize the plasma confinement effect Through radiation pressure and thus to optimize the x-ray source parameters. Through experiments and calculations, it has also been found that shorter (150 fs) and longer (1 ps) pulse resulted in lower conversion efficiency and/or larger x-ray spot size. According to a preferred embodiment of the present invention, a typical value used for the pulse duration is 400 fs (with no pre-pulse and/or pedestal)  
         [0080]    A theoretical investigation of the optimum monochromatic x-ray energy for imaging various simulated pathologies in the breast for a large range of breast thickness spanning the 2-cm-8-cm range was performed. The following figure of merit (FOM) was evaluated  
           FOM =(contrast) 2 /dose  
         [0081]    Simulated pathologies included 200-μm calcifications and 5-mm masses.  
         [0082]    It has been confirmed that the shape and the exact location of maximum depend on the imaging object. However in each case the optimum energy shifts towards higher energy for thicker/denser breast for all the investigated pathologies. It has been found that the optimum photon energy increases from ˜15-18 keV for 2 cm Lucite equivalent breast to ˜25-28 keV for 8 cm Lucite equivalent breast. FIG. 2 shows the calculated optimum photon energy corresponding to FOM maximum as a function of the Lucite equivalent breast thickness  
         [0083]    For larger breast thickness/density (d&gt;4.5 cm) imaged with ultra-fast-based laser x-ray source, we expect significant improvement in the dose utilization without loss of contrast, as compared to imaging with a mammographic x-ray tube operating at increased kilovoltage (28-31 kVp)  
         [0084]    The target  16  can be made in many forms including a continuous thin tape with a plastic substrate, rotating cylindrical or planar solid targets. In any case, the target is rapidly moved between each laser shot so that a fresh surface, with possibly different elemental composition (when required), is exposed to every shot. Possible target elemental compositions include Mo, Rh, Ag, In, Sn, with the K α  lines in the 17 4-27.4 keV range. Via selection of a suitable laser target elemental composition and matching filter  36  material, x-ray spectrum from the LPP x-ray source can be tailored to a specific breast thickness and composition, as well as to a detector used.  
         [0085]    Preferably, an x-ray generator  12  according to the present invention includes a shaped target  16 ′ as illustrated in FIG. 6.  
         [0086]    In both cases, hot electron  48  trajectories (illustrated in both FIGS. 5 and 6 by full-arrows returning from the slab  52  to into the target  16 ) are constrained but with flat targets  16  only a fraction of the produced hot electrons  48  is coming back inside the target into a very small spot, producing hard x-rays  54   
         [0087]    As illustrated in FIG. 6, shaped targets produced for example by machine indentation, allows increasing the coupling of hot electrons  48 ′ to the target  16 ′ without increasing The source size. It has been found that the x-ray yield increases when the laser beam  50  interacts with preformed craters  56  (with characteristic dimensions: optical laser diameter (3 μm) by a laser ablation depth (1 μm)). The walls  58  of the indentation are capturing electrons that normally escape with a flat target  16  However the shaped target  16 ′ adds a level of complexity since the target  16 ′ positioning has to be controlled with a very high precision level using the precision target positioning system  20 .  
         [0088]    The operation of the overall x-ray mammography system  10  will not be described herein in more detail since it is believed to be well known in the art  
         [0089]    Although the present invention has been described hereinabove by way of preferred embodiments thereof, it can be modified, without departing from the spirit and nature of the subject invention as defined in the appended claims.