Large Focal Spot Research Articles

Recent experiments on the hydrodynamics of laser-produced plasmas conducted at the PALS laboratory

We present a series of experimental results, and their interpretation, connected to various aspects of the hydrodynamics of laser produced plasmas. Experiments were performed using the Prague PALS iodine laser working at 0.44 μm wavelength and irradiances up to a few 1014W/cm2. By adopting large focal spots and smoothed laser beams, the lateral energy transport and lateral expansion have been avoided. Therefore we could reach a quasi one-dimensional regime for which experimental results can be more easily and properly compared to available analytical models.

High energy heavy ion jets emerging from laser plasma generated by long pulse laser beams from the NHELIX laser system at GSI

High energy heavy ions were generated in laser produced plasma at moderate laser energy, with a large focal spot size of 0.5 mm diameter. The laser beam was provided by the 10 GW GSI-NHELIX laser systems, and the ions were observed spectroscopically in status nascendi with high spatial and spectral resolution. Due to the focal geometry, plasma jet was formed, containing high energy heavy ions. The velocity distribution was measured via an observation of Doppler shifted characteristic transition lines. The observed energy of up to 3 MeV of F-ions deviates by an order of magnitude from the well-known Gitomer (Gitomer et al., 1986) scaling, and agrees with the higher energies of relativistic self focusing.

Laser-triggered ion acceleration at moderate intensity and pulse duration

Two-dimensional particle-in-cell (PIC) simulations of laser-triggered ion acceleration in overdense plasma at moderate intensity (≃1.4×1018 W/cm2) and pulse duration (≃0.5 ps) are presented. We focus on the comparison of the efficiency of ion acceleration for normal and oblique incidence of the laser light, for backward and forward directions of ion emission, and for large and small focal spots. We discuss the correlation between the properties of accelerated ions and hot electrons, and identify the tendency of the ion spectra in the forward direction to those typical for the isothermal and adiabatic regimes of plasma expansion.

Micro‐CT with respiratory and cardiac gating

Cardiopulmonary imaging in rodents using micro-computed tomography (CT) is a challenging task due to both cardiac and pulmonary motion and the limited fluence rate available from micro-focus x-ray tubes of most commercial systems. Successful imaging in the mouse requires recognition of both the spatial and temporal scales and their impact on the required fluence rate. Smaller voxels require an increase in the total number of photons (integrated fluence) used in the reconstructed image for constant signal-to-noise ratio. The faster heart rates require shorter exposures to minimize cardiac motion blur imposing even higher demands on the fluence rate. We describe a system with fixed tube/detector and with a rotating specimen. A large focal spot x-ray tube capable of producing high fluence rates with short exposure times was used. The geometry is optimized to match focal spot blur with detector pitch and the resolution limits imposed by the reproducibility of gating. Thus, it is possible to achieve isotropic spatial resolution of 100 microm with a fluence rate at the detector 250 times that of a conventional cone beam micro-CT system with rotating detector and microfocal x-ray tube. Motion is minimized for any single projection with 10 ms exposures that are synchronized to both cardiac and breathing motion. System performance was validated in vivo by studies of the cardiopulmonary structures in C57BL/6 mice, demonstrating the value of motion integration with a bright x-ray source.

Laser driven shock experiments at PALS

We report on recent laser driven shock experiments performed at the PALS laboratory in Prague. In the first series of experiments, the ablation pressure at 0.44 μm has been measured at irradiance up to 2×1014 W/cm2. The diagnostics consisted in the detection of shock breakout from stepped Al targets. By adopting large focal spots and smoothed laser beams, the lateral energy transport and “drilling effects” were avoided. The measured scaling shows a fair agreement with analytical models. Later, such shocks have been applied to the experimental study of carbon equation of state (EOS) at Megabar pressures. The rear side emissivity of “two materials — two steps” targets (Al−C) was recorded and, by applying the impedance mismatch method, allowed a direct determination of relative EOS points. Previously unreached pressures were obtained. Results are compared with previous experiments and with available theoretical models and seem to show high compressibility of carbon at Megabar pressures.

Optimization of Digital Mammography Resolution Using Magnification Technique in Computed Radiography

Purpose: To determine whether magnified digital mammography using a computed radiography system can produce better spatial resolution by reducing the focus-object distance, and to define the optimal magnification factor when a large x-ray tube focal spot is used for digital mammography using a CR system. Materials and Methods: Digital images obtained using computed radiography of a breast phantom were obtained using various magnification factors. Up to twelve acrylic blocks each measuring one centimeter in height were used to increase the distance between the breast phantom and the base plate (screen holder), in order to create the magnification images. The large (0.3 mm) focal spot of the x-ray tube was used for the entire series of images. Three radiologists participated in the evaluation of the images, in order to determine which had the best resolution. The resolving ability of the line pair structures and image clarity of the detectable artificial microcalcifications (specs) were the two factors used to determine the resolution of the images. The images were not compressed aFnd the viewing conditions, including the magnification factors, brightness and contrast, were fixed. The images were displayed on four high resolution PACS dedicated monitors (5 mega pixel LCD, BARCO Belgium). Results: A focus-object distance of 590 mm and a source-to-image receptor distance of 650 mm (set by the manufacturer) resulted in the best resolution, when combined with a magnification factor of 1.1. All three radiologists agreed on this result. Two of the radiologists believed that at least two more line pairs were better separated on the magnified image having the best resolution than on the unmagnified image, while one radiologist believed that three more line pairs were better separated on this magnified image. Using images with still larger magnification factors did not improve the resolution due to edge blurring. It was easier to determine the resolving power by means of the line-pair structures than by assessing the clarity of the artificial microcalcifications (specs). A 10% decrease in focal spot-object distance resulted in a 21% increase in radiation to the breasts. Conclusion: Magnified digital breast images taken with a computed radiography system using a large focal spot produced better quality images, because of their utilizing more pixels per volume of the breast phantom with a minimal increase in radiation dosage.

Ablation pressure scaling at short laser wavelength.

The ablation pressure at a 0.44-microm laser wavelength has been measured at irradiance up to 2 x 10(14) W/cm(2). The diagnostics consisted in the detection of shock breakout from stepped Al targets. By adopting large focal spots and smoothed laser beams, the lateral energy transport and "drilling effects" have been avoided. The measured scaling shows a fair agreement with analytical models.

Multi-detector row CT: radiation dose characteristics.

To determine the dose characteristics of multi-detector row computed tomography (CT) and to provide tabulated dose values and rules of thumb that assist in minimizing the radiation dose at multi-detector row CT. Weighted CT dose index (CTDI100w) values were obtained from three multi-detector row CT scanners (LightSpeed; GE Medical Systems, Milwaukee, Wis) for both head and body CT modes by using standard CT-dose phantoms. The CTDI100w was determined as a function of x-ray tube voltage (80, 100, 120, 140 kVp), tube current (range, 50-380 mA), tube rotation time (0.5-4.0 seconds), radiation profile width (RPW) (5, 10, 15, 20 mm), and acquisition mode (helical high-quality and high-speed modes and axial one-, two-, and four-section modes). Statistical regression was performed to characterize the relationships between CTDI100w and various technique factors. The CTDI100w (milligray) increased linearly with tube current: in head mode, CTDI100w = (0.391 mGy/mA +/- 0.004) x tube current (milliampere) (r2 = 0.999); in body mode, CTDI100w = (0.162 mGy/mA +/- 0.002) x tube current (milliampere) (r2 = 0.999). The CTDI100w increased linearly with rotation time: in head mode, CTDI100w = (34.7 mGy/sec +/- 0.2) x rotation time (seconds) (r2 = 1.0); in body mode, CTDI100w = (13.957 mGy/sec +/- 0.005) x rotation time (seconds) (r2 = 1.0). The relationship of normalized CTDI100w (milligrays per 100 mAs) with tube voltage followed a power law: in head mode, CTDI100w = (0.00016 mGy/100 mAs. kVp +/- 0.00007) x (tube voltage)(2.5+/-0.1) (r2 = 0.997); in body mode, CTDI100w = (0.000012 mGy/100 mAs. kVp +/- 0.000007) x (tube voltage)(2.8+/-0.1) (r2 = 0.996). In all scanning modes, CTDI100w decreased when RPW increased. CTDI100w was 10% higher in head mode and 13% lower in body mode compared with the value suggested by the manufacturer, which is displayed at the scanner console. When deposited power exceeded 24 kW, CTDI100w increased by 10% as a result of use of the large focal spot. The authors provide a set of tables of radiation dose as a function of imaging protocol to facilitate implementation of radiation dose-efficient studies.

The impact of increased Al filtration on x-ray tube loading and image quality in diagnostic radiology.

Previous work has shown that for nine common radiographic projections (AP abdomen, AP cervical spine, LAT cervical spine, PA chest, LAT chest, AP hip, AP lumbar spine, LAT lumber spine, and AP pelvis) increasing the total x-ray tube filtration from 2.5 mm Al equivalent (the regulatory minimum for general diagnostic radiology) to 4.0 mm Al equivalent, reduces the average effective dose and average skin entrance dose by 9% and 16%, respectively, using a 400 speed screen-film system. In this study, the effects of this filtration increase on x-ray tube loading and image quality were assessed. For the above projections and filtration increase, mean absolute and percentage increases in tube loading were 2.9 mAs and 15%, respectively, for a constant film density and fixed kVp. Tube current (mA) increases of 25% (a worst case) resulted in no statistically significant loss in focal spot resolution due to blooming for both large (1.2 mm) and small (0.6 mm) focal spot sizes, except at high mA low kVp techniques. The latter losses were below 10%, and when the image receptor blur was incorporated, the total system spatial resolution losses were on the order of one-quarter to one-half these values for typical clinical geometries. Radiographs of a contrast phantom taken with 2.5 and 4.0 mm total Al equivalent x-ray tube filtration were compared at 60, 70, 81, 90, 102, and 121 kVp. No statistically significant changes were observed with regard to (1) test object conspicuity as reported by three observers, (2) image contrast, as measured using a densitometer with a 3 mm aperture (+/-0.0017 OD, 95% confidence level), and (3) pixel value image noise, image contrast-to-noise ratios, and image signal-to-noise ratios, as measured using a scanning densitometer with a 12-bit acquisition depth and 85 micron pixel size (+/-2.5%, +/-3.1%, and +/-2.5%, 95% confidence levels, respectively). These results, combined with the linear no-threshold model for radiation risk and the ALARA principle, suggest that general radiography should be carried out using a minimum of 4.0 mm total Al equivalent filtration.

Bio-acoustic thermal lensing and nonlinear propagation in focused ultrasound surgery using large focal spots: a parametric study

It is well known that the acoustic properties of soft tissue have a dependence on tissue temperature. This is of particular interest in focused ultrasound surgery since the mechanism of action of focused ultrasound surgery is to kill targeted tissue by inducing localized heating by ultrasound absorption, and hence cautery of that tissue. However, the act of localized heating induces a change in the acoustic properties of the targeted tissue and tissue surrounding it. This phenomenon distorts the incoming acoustic wavefront, and has been termed the thermal lens effect for this reason. Furthermore, nonlinear effects in acoustic propagation become non-negligible at the ultrasound intensities required for therapeutic action.This paper examines the importance of the thermal lens effect and nonlinear tissue properties by simulating a variety of clinically applicable phased array transducer configurations that have not yet been appropriately analysed using a full three-dimensional nonlinear treatment of acoustic propagation. The significance of the thermal lens effect is characterized by comparing the simulation of coupled acoustic and thermal propagation with an uncoupled treatment; neglecting thermal lensing typically produces a movement of 1 to 2 mm in the predicted position of the focus towards the transducer.The results also show that the classical methods of acoustic propagation can produce grossly erroneous results under certain clinically relevant transducer configurations and that an acoustic field scan with a hydrophone may not accurately predict therapeutic effect.

Calculation of mass transfer in the remote cutting of metals by radiation of a high-power repetitively pulsed CO2 laser

The mechanism of remote cutting of steel plates by radiation of a high-power repetitively pulsed CO2 laser is theoretically studied. The models of melt removal by the gravity force and the recoil pressure of material vapour are proposed and the sufficient conditions for the initiation of cutting are determined. A numerical model of a thermally thin plate was employed to describe the cutting for large focal spots.

Thoracic radiology: the past 50 years.

The radiology of 50 years ago was a primitive science compared with the radiology of today. Hospital departments were small and radiologists few in number. Night call was uncommon. Examinations consisted primarily of radiographs of the chest, bones, and gastrointestinal tract, although some early neuroradiologic studies were performed. Chest fluoroscopy was common. Film processing was done manually, often with poor results. Radiographic examinations of the chest were likewise unsophisticated by today's standards. Chest radiographs were made with low-kilovoltage, calcium tungstate phosphors and relatively large focal spots. There were no image intensifiers, nuclear medicine studies, ultrasonography, computed tomography, or magnetic resonance studies. How far we have come!

Shock impedance matching experiments in foam-solid targets and implications for “foam buffered ICF”

We studied the influence of foams on laser produced shocks. Experiments were performed at LULI using a Nd laser converted to second harmonic, and at MPQ (Max Planck Institut für Quantenoptik) using the iodine Asterix laser converted to third harmonic. In both cases, sub-ns lasers with pulse energies of several tens of joules were focused on large focal spots (hundreds of microns) to reduce 2D effects. The laser beams were optically smoothed with phase zone plates (PZP) and directly focused on layered targets made of a foam layer on the laser side and a stepped Al layer on the other side. A visible streak camera was used to detect shock breakthrough at the base and at the step of the Al target, allowing shock velocity to be determined. Using the well known SESAME Al equation of state, we determined shock pressure. A stronger pressure increase was measured when foam was present, compared to what was obtained by focusing the laser beam directly on the Al target. This was due to the impedance mismatch effect at the Al-foam interface.

Dynamics of Mandel’shtam-Brillouin induced scattering during self-focusing of a laser beam

It is shown that the size of the focal spot has a substantial influence on the dynamics of Mandel’shtam-Brillouin induced scattering (MBIS) for the laser beam power near critical for striction self-focusing. For small focal spots MBIS suppresses self-focusing. An increase in the size of the focal spot leads to growth of the MBIS pulsations and the steady-state setup time. For large enough focal spots MBIS arises in the form of regular intense spikes. Physical processes shaping the dynamics of MBIS are discussed.

焦点寸法によるX線照射野の違い

X-ray tube assemblies with a 12-degree target angle have increased. We examined the dimensions of the x-ray field and the x-ray intensity distribution according to differences in the focal spot size of the same x-ray source assembly. In the dosemeter method and x-ray film method, the optimal object field of the x-ray source assembly by which a small focal spot touches a large spot on the anode side was almost the same. With the x-ray source assembly that was examined at this time, the range of a uniform x-ray intensity distribution to the displacement point of a small focal spot was wider than that of a large focal spot. The dimension of the x-ray filed on the anode side on the X-axis were determined by the target angle, regardless of the position of the focal spot. Differences in the dimensions of the x-ray field as affected by differences in the focal spot size differences in the x-ray intensity distribution occurred on the cathode side. These differences existed because of the size and position of the focal spot. Therefore, it it necessary to understand the x-ray field of each x-ray source assembly in order to radiography.

Effects of focal spot size on caries diagnosis with D and E speed images

Annual measurement of the x-ray unit focal spot size has been recommended by the American Academy of Oral and Maxillofacial radiology as part of the dental radiographic quality control program. This study compares the effects of focal spot size on caries diagnosis. Three x-ray units with small, medium, and large focal spot sizes were used to produce bite-wing images on extracted teeth mounted in acrylic bases. Randomized films were scored for lesion presence and depth by two general dentists. Weighted kappa statistics were used to evaluate the agreement of reviewer caries diagnosis by film speed and focal spot size. Comparisons of caries cells with small versus medium and small versus large focal spot size produced weighted kappa statistics = 0.72 and 0.70, respectively. Differences in caries calls were greater because of film speed rather than focal spot size. The results of this study suggest that the clinical significance of varied focal spot size is negligible. The value of annual measurement of focal spot size is questionable, and its recommendation should be revisited.

High intensity above-threshold ionization of He.

This is the first high resolution measurement of the electron energy spectrum from high intensity above-threshold ionization (pondermotive energy \ensuremath{\gg} ionization potential \ensuremath{\gg}h\ensuremath{\nu}) at intensities up to 7\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{cm}}^{2}$. In contrast to previous work, we have used a short pulse and a large focal spot along with low gas densities to minimize the effect of ponderomotive forces and collisions. Existing models do not agree well with electron energy spectra for either linearly or circularly polarized light. The electron temperature is too hot for the implementation of 820 nm multiphoton ionized recombination type XUV laser schemes.

Mammography focal spot measurement with a star pattern: techniques to avoid inaccuracies.

The increased emphasis on mammography acceptance testing and quality control make the star pattern test an essential component of the procedure for diagnosing poor resolution due to large focal spots. In this paper procedures for calculating magnification and the correct angle of the star pattern to use are described. The preferred placement of the star pattern in the field and proper exposure technique is also discussed.

An effect of the target position relative to the laser focus on X-ray emission from the laser plasma

The first harmonics beam generated by an iodine laser system was focused by an f/2 optics on an Al foil target. The X-ray output from the laser plasma both in the line and broad-band spectra was registered over an interval around the “ideal” focus. It was found that the maximum X-ray power is not obtained in the focus itself but for a somewhat larger focal spot outside the focus. To explain this phenomena, temperature and density measurements were in addition made. The plasma temperature evaluated from both the line (He-like Al XII resonant line and j, k, l satellites) and broad-band spectra (two foil method) was also measured and found to be largely constant in the vicinity of the focus. The line and broad-band temperatures differ, the broad-band temperature being about 25% higher. The electron density was equally determined using an intercombination line.

Radiographic detail and variation of the nominal focal spot size: the "focal effect".

It is not generally appreciated that there is a gradation of the focal spot size from the anode to cathode end of an x-ray field that can dramatically affect radiographic detail from one end of a radiograph to the other. The authors name this gradation the "focal effect." Gradations in radiation intensity and focal spot size were measured from anode end to cathode end of a 14 x 17-inch field and were visually demonstrated with lymphangiograms and line-pair patterns. The degradation in spatial resolution along the anode-cathode axis was found to be as much as 75%, depending on the orientation of the patient with respect to the x-ray tube. Radiographic detail is, therefore, significantly improved (even when the large focal spot is used) by exploiting the focal effect and placing the body part requiring the best radiographic detail at the anode end of the table.