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'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] 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>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.