DALLA LUCE VISIBILE AI RAGGI X: NUOVI RIVELATORI DI IMMAGINI PER RAGGI X A DISCRIMINAZIONE IN ENERGIA ED APPLICAZIONI

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1 DALLA LUCE VISIBILE AI RAGGI X: NUOVI RIVELATORI DI IMMAGINI PER RAGGI X A DISCRIMINAZIONE IN ENERGIA ED APPLICAZIONI D. Pacella ENEA - Frascati LIMS, Frascati ottobre 2015

2 Come per la fotografia: dal bianco e nero ai colori, dalla pinhole camera alle lenti. Cosi per i raggi X Vi condurro per mano..

3 Outline Overview of the X-ray imaging detectors New C-MOS and Gas detector in photon counting X-ray color imaging Laser produced plasma: very short X-ray sources X-rays like light: X-ray polycapillary lenses Future improvements of the system source-object-detector

4 X-ray tubes and detectors X-ray tubes They produce continuos spectra up to the value KV p Detectors In the table are the different weight of the absorption processes (Photoelectric and Compton) of X-ray into human body: Compton effect then is not a favorable process, because it degrades the spatial and energy resolution

5 Imaging Quality Imaging quality in radiography is due to three major contributions: Detectors (efficiency, noise, spatial resolution, dynamic range, contrast..) Geometry (source-subject-detector distances, focal spot size, plane misalignment..) Object (nature, atomic numbers of elements, shape ) The most relevant imaging quality parameters for an X-ray detector are: Spatial resolution Detection efficiency Sensitivity and dynamic range Contrast Noise Spatial resolution is fully described by means of the Modulation Transfer Function (MTF) and it is often quantified as lines/mm that can be discriminated. SNR is the signal to noise ratio DQE Detection efficiency and SNR are combined in the Detection QuantumEfficiency

6 FILMS For almost a century X-ray radiographies were made with photographic films, based on gel with AgBr or AgI grains. Response (OD) is purely analogic Detection efficiency is about 1% X-ray Imaging detector IMAGING PLATES Nowadays photographic films have been widely replaced by the Imaging Plates, called also storage phosphors where X-rays absorbed in the material produce metastable states and the plate is read-out by recording the luminescence induced by a laser. They have Detection Efficiency similar to film but the response with exposure is linear and the dynamic range is almost four orders of magnitude

7 Flat Panel Detectors In the last two decades the so called Flat Panel Detectors (FPD) have been developed to combine the advantages of having an electronic conversion of the radiation, like for CCDs, high detection efficiency and large active area at the same time. Two approaches have been studied: indirect (having a scintillator as X-ray converter in visible light) and direct conversion.

8 New Imaging Detectors in Photon Counting: C-MOS Imagers and pixellated gas detectors Different semiconductors: SI, CdTe, CZT, GaAs C-MOS imager Wide developments in the 2010 s years Photon counting Advantages: Noise free (only statistic) higher efficiency, SNR, sensitivity and dynamic range Discrimination in energy Post process analysis (true digital) Major limitation is now the active area (a few cm 2 ). Mosaic of ASICs are now under realization, at least for scanning radiography

9 Comparison between detectors : Efficiency and Sensitivity (2 out of many ) Attenuation coefficient Detection threshold Detector Minimum fluence (ph/mm 2 ) Film 10 6 Imaging Plate 10 4 Flat Panel 10 4 C-MOS Imager 1

10 X-RAY COLOR IMAGING In analogy with visible light, where the spectrum is divided in colors, X-ray radiography has been done so far in black and white (grey scale) based on intensity and not their energy ( colors ) In the X-ray imaging we are now at the transition between black/white and colors (not yet for applications)

11 Dual Energy Imaging Dual-energy imaging is based on the acquisition of two distinct images taken in different spectral ranges The Transmission I/I 0 of a X-ray beam of energy E and intensity I 0 entering in an homogeneous target of thickness t (cm) with a given composition, results: I 0 I I/I 0 = exp (-a(e) rt) r Density a mass absorp. coeff Dual energy imaging is useful when two different materials are present in the target (for example bones and soft tissues like in bone densitometry), or in functional imaging, where a contrast agent has to be imaged (Iodine, Gadoliium, Barium). Dual energy X-ray imaging is now a reality, in more than one application.

12 Multi Energy Imaging Multi energy is an extension of the method, potentially useful for targets made of many materials, but it is extremely challenging At the present multi energy CT is still at the level of research or pre-clinical investigations Multiple energy X-ray imaging opens the way to detect the nature of the materials present into the target. This is possible only measuring the selective absorption at the different energy bands. Useful for material discrimination or for functional imaging with makers (I, Ba, Gd)

13 Energy dependance of the Mass Absorption Coefficient

14 Demonstration of color imaging potentialities with GEM Gas Detectors A pioneering work has been done in 2003 by the presenter in developing X-ray imaging detectors, based on Gas Electron Multipliers (GEM), with energy resolution capability, demonstrating that energy resolved imaging is a powerful way to distinguish materials. The energy range (2-15 kev) was divided in 25 energy bands and a scan of threshold was performed. The spectrum of the absorbed radiation in these bands, for different samples (fat, plastic tape, CaCl), was estimated and the transmission coefficients derived Detection for Multienergy is more difficult because the response at different energies are overlapped due to the low energy resolution. Solutions are under consideration, both hardware and software

15 D. Pacella Reports in Medical Imaging, 2015 :

16 New Imaging X-ray Technique (NIXT) Laboratory in Frascati (ENEA) Fully shielded up to 120 kev Equipped for gas and solid state detectors and spectrometers It allows optical configurations at large distances (up to 7 m) All the devices are remotely controlled outside All the X-ray tubes and sources, imaging detectors, spectrometers and filters are Absolutely calibrated Experimental configurations can be simulated with absolute spectral distribution

17 C-MOS Imagers 21 Pixirad at ENEA-Laboratory Spin-off National Institute Nuclear Physics (INFN)-Pisa Active area = mm 2 Matrix = pixels Pixel dimension = μm 2 X-ray sensor = thin layer of a crystalline CdTe (650 mm) Energy range: kev Count rate > 30GHz Frame rate ~ 100 frame/s Noise free Energy discrimination Energy discrimination is not sharp because of charge sharing

18 LENSES LIKE FOR the LIGHT: X-RAY POLYCAPILLARY LENSES 4.5 cm Half lens X-ray source microfocus Transmitted X-rays Detector (medipix) No image F 1 object Full lens F 2 x y z Detector (medipix) Magnified image

19 POLYCAPILLARY LENSES FOR REMOTE DETECTION Measurement with half lens at large distances in the NIXT Laboratory (ENEA-Frascati) X-ray tube (microfocus) KV 2000 ma

20 integrated counts (ToT) Triple GEM X-ray MERGING GEM AND C-MOS: GEMpix FOR LASER PRODUCED PLASMAS Mylar window It is a new detector where a GEM gas detector is readout with ASICs at 512 x 512 pixels of 50x50 microns. Active area 30 x 30 mm 2 4 medipix ASICs Gas flux AR CO 2 single photon at 6 kev Vgem = 930 V Vgem 1000 V GEMpix: Int vs HV It works in photon counting or charge integration mode (Time over Threshold, ToT) Sensitivity can be adjusted over 3 orders of magnitude 1000 Vgem 1150 V Increasing gain, the charge cloud becomes larger high voltage (V) Ca Fe Cu Espo. (Fe) Espo. (Cu)

21 ENERGY RESOLVED IMAGING 35 Energy discrimination.diving in a photon sea Detector Pixirad THR = 2 kev 30 X-rays kev 1500 THR = 2 kev 2-15 kev THR = 12 kev kev

22 ENERGY RESOLVED IMAGING FOR NON DESTRUCTIVE TESTS Radiography with Pixirad of a snail shell kev kev kev 34 Explore the energy resolved radiography or tomography Non destructive test, material science, biomedical applications..

23 Applications Microtomography and radiography to study small components or medical devices Medical applications for scanning radiography or tomography Investigation of new materials (light materials, plastics, fibers, gel, foams..) Study of multi-layered materials Monitoring of industrial fabrication and deposition Plasma physics

24 Future Developments Improvement of the X-ray radiography requires a better matching between object (patient), spectrum and detector. Optimization starts from the object, defining the most sensitive energy bands to probe it, maximizing the contrast and minimizing the dose. Since mass absorption coefficients of the different tissues and organs vary widely in the whole energy range, in principle the X-ray spectrum should be tailored in each energy band to maximize the contrast and detail capability, taking into account the detector features. Detectors could be then optimized to work in the selected energy range

25 Uno speciale ringraziamento a: L. Gabellieri, A. Romano, G. Claps, F. Causa and the FTU technical pool (ENEA) F. Murtas (INFN-Frascati)