Exploring phase contrast imaging with a laser-based K_α x-ray source up to relativistic laser intensity

Abstract

This study explores the ability of a hard K_α x-ray source (17.48 keV) produced by a 10 TW class laser system operated at high temporal contrast ratio and high repetition rate for phase contrast imaging. For demonstration, a parametric study based on a known object (PET films) shows clear evidence of feasibility of phase contrast imaging over a large range of laser intensity on target (from ~10¹⁷ W/cm² to 7.0 × 10¹⁸ W/cm²). To highlight this result, a comparison of raw phase contrast and retrieved phase images of a biological object (a wasp) is done at different laser intensities below the relativistic intensity regime and up to 1.3 × 10¹⁹ W/cm². This brings out attractive imaging strategies by selecting suitable laser intensity for optimizing either high spatial resolution and high quality of image or short acquisition time.

Introduction

The interest of develo** new x-ray sources and/or improving their performances in terms of brightness, stability, and compactness is still growing since decades. This is strongly motivated by applications of x-ray sources for imaging and related applied developments to biology, medicine and material science. In particular, the advent of synchrotron radiation sources in the seventies as well as the development of optical components for x-rays, definitively allowed to transfer the phase contrast imaging (PCI) techniques from the visible spectral range to the x-ray one. Phase contrast x-ray imaging is sensitive to phase shift induced by an object placed in the x-ray path and does not rely on its absorption. Thus, it can image weakly absorbing materials, such as carbon-based materials and biological objects. In addition, it should be noted that the sensitivity of absorption contrast decreases as the photon energy (E) increases¹ as E^–3, whereas that of phase contrast methods decreases only as E^–2. Therefore, phase contrast methods are more sensitive at high photon energies (E = 10 to 100 keV), compared to absorption methods. In that case, for comparable image quality, the absorbed x-ray dose is smaller than with conventional radiography. Challenges addressed by hard x-ray PCI are numerous such as the detection of complex damages in composite materials

Figure 4

X-ray phase images based on Paganin phase retrieval algorithm^50,51. (a–c) are retrieved from the raw images presented in Fig. 3 (a–c). (a) for I = 2.7 × 10¹⁷ W/cm², (b) for I = 1.0 × 10¹⁸ W/cm², (c) for I = 7.2 × 10¹⁸ W/cm².

Figure 5

Phase grey histogram of region of interest from images (a–c) of Fig. 4. Error bars are correlated to the non-uniform response of the detector and take into account the (L_s/${{\rm{L}}}_{\perp }$)_body ratios. They are similar for each point of the corresponding histogram.

To explore the limit of ASUR laser source, an ultimate x-ray PCI experiment has been carried out at the maximum laser intensity of I = 1.3 × 10¹⁹ W/cm². The CCD camera is kept at 90 cm from the wasp and the raw image shown in Fig. 6a is obtained with an accumulation of only 200 x-ray pulses (2 seconds with 100 Hz driver laser). The effective source size of the x-ray source is 52 µm.

Figure 6

(a) Raw x-ray phase contrast image of the wasp acquired at I = 1.3 × 10¹⁹ W/cm² and with 200 x-ray pulses. (b) Line profile of the signal perpendicular to the edge of a thorax zone. (c) Corresponding x-ray retrieved phase image.

Compared to the previous images of Fig. 3 the quality of the raw image is undeniably degraded with less visible details. Nevertheless, the signal of the edge contrast still exceeds the noise level (Fig. 6b). This signal value is sufficient to extract a phase image shown in Fig. 6c for the air/wasp interface. For the body structure of the wasp, less phase curvature signal is also visible, involving a degraded phase extraction compared to previous images in Fig. 4, with an average grey value of 30 rad and σ = 15 rad (orange ROI in Fig. 6c). Even so, these last results demonstrate good applicability of our x-ray source and 100 Hz TW-class laser driver for phase-contrast imaging at high intensity and short exposure time.

Conclusion

In summary, this study demonstrates the excellent ability for phase contrast imaging of a K_α x-ray source produced by a high repetition rate multi-TW class Ti:Sa laser system up to a very high range of laser intensity never experimentally explored. By a rigorous characterization of the x-ray source in terms of flux and effective source size done simultaneously for each PCI acquisitions, we clearly obtain raw phase contrast images and phase extraction with detectable phase information over the entire range of laser driver intensity explored from ~10¹⁷ W/cm² to the maximum intensity tested of 1.3 × 10¹⁹ W/cm². Thanks to the high temporal contrast of the laser system, the increase of the effective x-ray source size is restrained from 11 µm at 2.7 × 10¹⁷ W/cm² to 52 µm at 1.3 × 10¹⁹ W/cm². For PET samples as well as for the wasp a decrease of the edge contrast signal is observed when raising the laser intensity in agreement with a lowering of the x-ray source spatial coherence. The quality of the raw phase contrast image of the wasp is almost unchanged between 2.7 × 10¹⁷ W/cm² to 1.0 × 10¹⁸ W/cm² and with only ~10% loss of the C_edge factor and a stable dynamic and phase average value. Meanwhile, the number of laser pulses is reduced by a factor 2.5. At higher laser intensity, up to 1.3 × 10¹⁹ W/cm², where a more pronounced increase of the x-ray source size happens, a degradation of the image quality is observed in comparison with PCI images obtained in a lower laser intensity regime. Nevertheless retrieved phase images of the wasp are still shown with a drastic shortening of the acquisition time (a factor 200 between the wasp image done at 2.7 × 10¹⁷ W/cm² and the one done at 1.3 10¹⁹ W/cm²) thanks to the very high number of K_α photons delivered by laser-produced plasma at such high laser driving intensity. Thus, the present study shows a compromise between achieving good image quality and short exposure time depending on the targeted application. This compromise can be balanced by adjusting only one parameter, the laser intensity on target. At low laser intensity, the best x-ray source spatial coherence and the lowest L_s/${{\rm{L}}}_{\perp }$ ratios are achieved which offer the highest edge contrast signal enhancement, suitable for applications such as the control of material fabrication processes²². On the opposite, at very high laser intensity, faster imaging acquisition can be done thanks to high available x-ray photon flux with a L_s/${{\rm{L}}}_{\perp }$ ratio still below unity.

Methods

Data extraction and treatment

The following procedure to collect the measurements and evaluate the C_edge parameter is done for each laser intensity. The K_α x-ray flux (Fig. 1b) and the x-ray spectrum (Fig. 1c) are acquired²⁹ with a 16 bits direct detection PIXIS-XB: 1024BR camera from Princeton Instruments, cooled down to −60 °C, with 1024 × 1024 pixels arrays and a pixel size of 13 × 13 µm². The sample is then inserted. For imaging applications requiring detector with a large surface, a more suitable detector is used. It is a 16 bits indirect detection CCD camera Quad-RO: 4320 from Princeton Instruments cooled down to −25 °C with 2084 × 2084 pixels and a pixel size of 24 × 24 µm². A raw image of the PET sample and the knife edge is presented in Fig. 7a. In a single image the characterization of the x-ray source size (Fig. 7b) and the extraction of C_edge for the three thicknesses of PET films (Fig. 7c) are done. The x-ray source size is extracted thanks to the well-known knife edge technique, using a sharp tungsten knife²⁹. The corresponding edge spread function (ESF) is plotted from the acquired image (red line profile of the signal in Fig. 7b). We further determine the x-ray source size by applying a Fermi function fit (black dashed line in Fig. 7b) on the resulted ESF. Moreover, the signal for each PET thickness is extracted in order to calculate a value of C_edge. The limit of a detectable phase contrast signal is calculated for each image and is equal to C_noise = $\frac{{{\rm{N}}}_{{\rm{\max }}}-{{\rm{N}}}_{{\rm{\min }}}}{{{\rm{N}}}_{{\rm{\max }}}+{{\rm{N}}}_{{\rm{\min }}}}\times 100 \%$ (see Fig. 7c). The same procedure is applied for the wasp images.

Figure 7

(a) Raw image of tungsten knife (black shadow in the left top) and of the sample. Image is acquired at I = 1.0 × 10¹⁸ W/cm², with 6000 laser shots; the corresponding x-ray source size is 18 µm and the K_α x-ray flux 3 × 10⁹ ph/sr/s. The sample is made of PET films of different thicknesses. (b) Line intensity profile corresponding to the vertically integrated red area of the ESF. (c) Vertically integrated intensity line profile of PET films. Note that this line profile only serves as an example to show the three PET films with different thicknesses in a same graph. Different levels of absorption are visible as well as edge enhanced contrast corresponding to the signature of the refractive index variation between PET and air.