Evaluation of a full-scale gas microstrip detector for low-dose X-ray imaging

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Nuclear Instruments and Methods in Physics Research A 536 (2005) 52–60 www.elsevier.com/locate/nima

Evaluation of a full-scale gas microstrip detector for low-dose X-ray imaging Philippe Despre´sa,, Gilles Beaudoina, Pierre Gravelb, Jacques A. de Guiseb a

Hoˆpital Notre-Dame du Centre Hospitalier de l’Universite´ deMontre´al, De´partement de Radiologie, 1560 rue Sherbrooke Est, Montre´al, Que´bec, Canada H2L 4M1 b Laboratoire de recherche en imagerie et orthope´die, E´cole de Technologie Supe´rieure, 1100 rue Notre-Dame ouest, Montre´al, Que´bec, Canada H3C 1K3 Received 6 June 2004; accepted 12 July 2004 Available online 17 August 2004

Abstract A new scanning slit digital radiography system based on a gas microstrip detector is presented. The EOS device, manufactured by Biospace Instruments, is equipped with two orthogonal xenon detectors and can produce dual-view full body scans at speeds up to 15 cm=s: The detectors are operated at 6 atm and contain 1764 channels at a pitch of 254 mm; with an avalanche signal amplification stage. The system can produce images with minimal dose requirements because of excellent scatter rejection and high quantum detection efficiency. Modulation transfer function (MTF) results for three beam qualities as well as comparisons with Monte Carlo simulations are reported. The resolution was found to be dependent on beam quality and on direction. In the horizontal direction, the measured MTF drops to half its full value at 1.0, 0.90 and 0:76 mm
1 for beams having half value layers of 1.8, 3.4 and 5:1 mm of Al. In the vertical direction, the corresponding values are 0.97, 0.95 and 0:92 mm
1 : r 2004 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 87.57.Ce; 87.59.Bh; 87.59.Hp Keywords: Digital radiography; Gas detector; Low-dose imaging

1. Introduction

Corresponding author. Tel.: +514-890-8000x25877; fax: +5145299206. E-mail address: [email protected] (P. Despre´s).

Gas detectors have been used in computed tomography (CT) since the advent of thirdgeneration scanners. Effective use of this technology for projection imaging is also reported in many studies [1,2], some of which being directly related to diagnostic radiography [3–5]. However,

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most of these systems never achieved a state of completeness sufficient for clinical imaging. Known full-scale realizations include work by Babichev et al. [6]. The principal flaws of gas detectors in radiology are low spatial resolution, long acquisition times and gas vessels’ cumbersomeness. Nevertheless, some applications, such as skeletal exams, may not suffer too much of these disadvantages. In such cases, it is possible to exploit the scanning slit gas detector qualities: high quantum detection efficiency, very good scattered photon rejection and large dynamic range. Because of these features, gas detectors can be effective even in low-radiation conditions. Dose may represent a major concern for pathologies requiring intensive radiological follow-up. A recent study has shown that diagnostic radiation is a causative factor of breast cancer in scoliotic patients [7]. This field, among others, may thus benefit from the development of gas detectors. Dose reduction factors of up to two orders of magnitude have been reported when using similar devices for typical radiological exams [4,8–10]. This work focuses on resolution measurements of EOS, an operational full-scale gas detector system for radiology. Possible applications and further improvements to the device will also be addressed.

2. The EOS imaging system The detectors of the EOS system are based on the MICROMEGAS technology [11], a descendant of the pioneering work by Charpak [12] and Oed [13]. It consists essentially of a parallel plates gas chamber with printed microstrips as readout elements, crafted with photolithographic processes. A fine mesh placed above the strips acts as a delimiter between two functional zones. In the first one, photons are converted, mainly by photoelectric effect into photoelectrons that are directed toward an amplification gap where the electric field is strong enough to induce electronic avalanches. The electric drift lines near the mesh are funnel-shaped and electronic transparencies close to 100% are obtained with proper field ratio between the conversion and amplification zones

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[14]. The mesh ensures the rapid collection of ions created during the avalanche and inhibits potential drawbacks like gain inhomogeneities and charge accumulation on the strip substrate. This design is successfully used in the field of particle physics to achieve detection with high gains and excellent space and time resolutions [15]. In such experiments, radiation is usually incident near the normal to the readout strips. X-ray imaging has also been conducted in this manner with similar detectors [3]. However, in radiological applications, it is better to adapt the geometry so that the read-out elements projections converge on the focal spot of the X-ray tube. In this radial layout, incoming photons follow a path parallel to the strip, ensuring that spatial information is preserved. Moreover, the length of the path can be extended to improve detection efficiency without compromising resolution. Fig. 1 shows the geometry of the detector, which contains 1764 channels spanning 44:8 cm at a 254 mm pitch. Such a configuration has already been used with microwires [16] and with printed strips [6,5] but never with a signal amplification stage. This feature is important as it allows a better signalto-noise ratio. The fan-shaped X-ray beam is collimated by a 0:5 mm slit and then enters the detector by a 0:5 mm thick aluminum window. This design prevents almost all of the scattered radiation from being detected. The photons eventually interact in the 10 cm long conversion zone. The gas mixture composed of xenon with 5% ethane used as a quencher provides a high photoelectric crosssection within the radiological energy range. The predominance of this interaction over scattering is important to prevent unwanted energy deposition elsewhere in the detector. The images on the EOS system are acquired one line at a time by sweeping the fan beam across the subject. Both detectors and X-ray tubes are moving together on motorized vertical rail mounts orthogonally positioned. Square pixels are obtained by adjusting the integration time per line and the scanning speed to the pitch of the microstrips. The maximum translational speed is 15 cm=s: The source-detector distance is set to 1:3 m and cannot be changed to preserve the

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~1 mm

Ec microgrid avalanche

Em

~100 µm

~10 cm amplifiers

incoming photon

Fig. 1. Detector geometry as seen by an incoming photon. The electrons released at an interaction site drift downward, cross the microgrid and are multiplied by avalanche in the amplification zone.

focused condition. The images produced are intrinsically orthographic in the scan direction while magnified in the other. Signal comes from current integration on each channel. The images are stored in 16-bit TIFF format. Because of slight gain variations between preamplifiers in each channel, a calibration procedure is executed prior to image acquisition. It consists of an offset signal acquisition followed by a full illumination that generates the reference signal upon which further measurements will be compared. Even if the tubes are oriented so as to avoid the heel effect along the detector slit, slight misalignment of the collimators may lead to slow spatial modulation of input light. This is corrected by calibration, which provides a uniform background in flat-field images. To avoid the possible drift in amplifier gains, calibration is reset whenever an image parameter is changed. Since this procedure is automatic and lasts for only 15 s; it does not constitute an obstacle to clinical use. The parameters of the EOS device are presented in Table 1. The system under investigation is equipped with two PX1557 tubes (Dunlee, Aurora IL, USA) driven by Editor MP 601 (Ro¨ntgenwerk Bochum, Germany) generators. These tubes, designed for CT scan requirements, are cooled by an oil circuit. Tube load is a major concern when using the EOS system since X-rays must be turned on for prolonged periods of times and most of the output is wasted in collimation. The beam qualities were measured with a calibrated dosimeter (model

Table 1 Characteristics of the EOS device Characteristics

Value

Pixel size (mm2 ) Number of channels Detector gas Detector depth (cm) Image width (mm) Maximum scanning speed (cm/s) Operation pressure (atm) Detection efficiency (80 kVp; 6 mm Al)

0:254  0:254 1764 Xe + 5% ethane 10 448 15 6 71%

35050A, Keithley Instruments, Cleveland, OH) and a set of 1100 aluminum attenuators. The spectra was modeled to perform Monte Carlo simulations (Section 3.2) and to compute a correction of the MTF for transmission through the jaws of the slit camera (Section 4.1). This was done with a program based on the spectra generation method of Boone et al. [17]. The resulting spectra was numerically filtered with tabulated attenuation coefficients [18] until both quality and exposure matched the measured values.

3. Experimental method 3.1. Modulation transfer function The presampled modulation transfer function (MTF) of the system was measured using a

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tantalum slit camera (model 07-624, Nuclear Associates, Hicksville, NY) with a 41 bevel. The slit was slightly angled (31–71) relative to the pixel mesh to avoid phase effects and to provide a better sampled line spread function (LSF). Since the scanning process introduces some sort of anisotropy, the LSF was obtained for both scan (vertical) and subscan (horizontal) directions. In each case, four images of the slit were taken and numerically added to provide more accurate statistics. 3.2. Monte Carlo simulations Monte Carlo studies were conducted to give an insight of the physics of gas detectors. The GEANT4 software toolbox (version 4.6.1), available from CERN [19], was used to simulate the energy deposition profile inside the conversion zone of the detector. The geometry of the detector and the slit camera were reproduced, including the angle between the slit and the pixel grid. The photons were launched according to the modeled spectra. The focal spot size of 0:7 mm was also simulated. The energy deposition on each channel volume was recorded for each scanned line to reconstruct an image of the slit camera, exactly as it would occur on the experimental setup. This synthetic image is then treated like those obtained with the device. The avalanche was not simulated since the amplification zone being relatively short, lateral dispersion does not contribute to the spatial energy spread, and the effect on the LSF is thus minimal.

4. Analysis

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In the first step the background signal was subtracted from the slit image to remove any LSF distortion. The background region around the slit was located using thresholding and the resulting binary mask was eroded to take into account the extended tail of the LSF. A quadratic surface was fitted to the background intensity data and subtracted from the original image. This procedure was used to remove any lowfrequency trend that might have been present and to prepare the image for slit angle measurement. The Radon transform of the background-free slit image was computed and presented an extended peak at the slit angle which value could be determined with a sub-pixel accuracy as follows. The peak region was located by thresholding the Radon image and its surface was modeled, using the selected pixel intensities, as the product of a fourth-degree polynomial function (along the angular axis) and a Gaussian (along the spatial axis). The peak maximum, and thus the slit angle, were determined analytically using the surface parameters. In the second step, the pixel positions were projected according to the slit angle and resampled in bins having the third of the original pixel size. In the last step, the presampling MTF was obtained from the Fourier transform (F) of the LSF, MTFðf Þ ¼ jFfLSFðxÞgj;

(1)

normalized by its value at f ¼ 0: A correction factor accounting for the finite width of the slit and the transmission through the beveled jaws was applied on the MTF [21]. The transmission profile of the slit was computed from the modeled spectra and tabulated attenuation data [18]. This treatment resulted in a 2% maximum improvement in MTF up to the Nyquist frequency. No extrapolation of tail signal [20] was attempted since this procedure is somewhat arbitrary.

4.1. MTF The slit image was processed following a threestep procedure adapted from [20,21]. The first step involved measuring the slit angle. Using this information, the slit image was then processed to extract the LSF. Finally, the MTF was computed from the LSF and corrected for camera geometry.

5. Results 5.1. Images Fig. 2 shows typical dual-view head to foot images produced by the EOS system. These images

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were taken at 15 cm=s with tubes set at 60 kVp: The entrance skin exposure in each image is close to 1 mR; well below the 13 mR median value for chest exams on film, as reported in Ref. [22]. Moreover, these images are valuable for spine inspection even if the exposure used here was at least 100 times less than the one reported for the same exam in Ref. [22]. 5.2. MTF Figs. 3 and 4 show the LSF and associated MTF for the subscan and scan directions respectively, at 50, 80 and 110 kVp: The corresponding beam HVLs are 1.8, 3.4 and 5:1 mm of Al. Because of the line by line acquisition mode, the energy deposition profiles are qualitatively different depending on the direction. The photoelectrons can travel along the subscan direction and contribute to signal on adjacent channels. This explains the tail of the LSF in this direction. This cannot be the case in the scan direction since each line is temporally uncorrelated from others. An electron traveling vertically will not contribute to the previous or next lines. Spatial correlation is, a fortiori, not present in the scanning direction. This explains why the MTF is beam-quality dependent in this direction while almost unaffected in the other. The resolution in the scan direction is mainly tributary to the detector’s collimation slit width, as Section 5.3 will show. Resolution degradation with harder beams is due to the production of more energetic photoelectrons, which have a longer range in the gas. Larger primary clouds are created and induce signals on more strips. The line by line acquisition feature also contributes to the shape of the noise power spectrum (NPS), shown in Fig. 5. The NPS was obtained by averaging 2D Fourier transforms of flat-field images. The noise is white in the scan direction while colored in the other. Pixel intensities are uncorrelated along the vertical direction since no relation links a line of acquisition to another. This feature can be related to visual artifacts in images caused by a modulation of X-ray flux impinging on the detector.

Fig. 2. Human subject images taken simultaneously at 15 cm=s with tubes set at 60 kVp:

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position (mm)

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2

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0 0

0.5

1

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Fig. 3. LSF and MTF in the subscan (horizontal) direction for 50, 80 and 110 kVp: Resolution is beam quality dependent.

Fig. 4. LSF and MTF in the scan (vertical) direction for 50, 80 and 110 kVp: Resolution is independent on beam quality.

5.3. Monte Carlo simulations

the conversion zone when photoelectrons deposit energy far from the interaction site. Monte Carlo simulations were conducted to predict the system’s behavior in different conditions. For example, Fig. 9 shows a way to reduce the primary spread by increasing the pressure inside the chamber. The mean free path of photoelectrons is then reduced and the LSF is sharper. Higher pressure also means better quantum detection efficiency. Within diagnostic radiology energies, it is possible to convert almost all X photons at reasonable pressures. Interestingly enough, this improvement do not affect the resolution as it is the case with phosphor detection technologies when the plates are thickened. In fact, resolution also benefits from an increased pressure. However, a potential drawback of using higher

Fig. 6 shows typical experimental and simulated slits. Comparisons between simulated and measured LSFs are presented in Figs. 7 and 8. Despite a relatively good agreement, the simulated LSFs are slightly narrower than the measured ones for both directions. This could be explained by possible photon scattering on detector material not taken into account by the simulation. The possible misalignment of the focal spot with the microstrip projections may also be responsible for the observed discrepancy. Simulations have shown that this issue can have a significant effect on the LSF. However, this is difficult to verify experimentally. Globally, Monte Carlo studies indicate that most of the resolution degradation occurs in

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experiment simulation

1

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-1.016

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Fig. 7. Simulated and experimental LSF for the subscan direction at 80 kVp:

1. 2

Fig. 5. Typical 2D NPS for the EOS system. Anisotropy is caused by the line by line acquisition mode.

experiment simulation

1

LSF

0. 8

0. 6

0. 4

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0 -2.032

-1.016

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1.016

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position (mm)

Fig. 8. Simulated and experimental LSF for the scan direction at 80 kVp:

Fig. 6. Typical (a) experimental and (b) simulated slit images.

pressures is that it requires a sturdier gas vessel and can possibly lead to leakage problems. The maximum proportional gain achievable may also suffer from an increase in pressure. Simulations have also confirmed that resolution in the scan direction could be greatly improved by using a tighter collimation. Fig. 10 demonstrates this effect. However, a smaller entrance slit can also complicate the alignment of the beam and increase the system’s sensitivity to vibrations, which could lead to intensity modulation artefacts

in the images. A smaller entrance slit also requires a greater X-ray exposure to preserve the photons statistics. The ability to reduce the slit width may therefore be limited by the tube load capacity.

6. Applications Given their geometry, scanning slit devices are intrinsically good candidates for low-dose imaging [23,24]. Digital flat panels have certainly contributed to dose reduction in radiology but these devices still need an anti-scatter grid to perform well. This leads to a significant loss of useful

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will also be installed shortly in two Montre´al QC hospitals.

6atm 12atm 24atm

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MT F

7. Conclusion 0.6 0.5 0.4 0.3 0.2 0.1 0

0.5

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Fig. 9. Effect of pressure on the subscan direction MTF as computed by simulations at 80 kVp:

1

500 µm 250 µm 100 µm

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0.6 0.5 0.4 0.3 0.2

The general functioning principles of the EOS X-ray imaging device were presented together with resolution measurements and simulations. Good agreement between the model and the actual machine was found, which will certainly ease future development of the system. The data acquisition is currently conducted in chargeintegration mode because large fluxes prohibit the use of a counting mode. However, it is reasonable to believe that it could be implemented in the near future [3]. Many benefits will arise from using the counting mode: improved resolution from event centroid calculations, better signal-tonoise ratios and possible online multi-energy imaging with a pulse-height circuitry. The EOS system certainly constitutes a low-dose imaging alternative to other modalities that will be beneficial for a sensitive clientele, namely pregnant women and scoliotic patients. Future work will enlarge the spectrum of applications, especially in the field of dual-energy imaging.

0.1 0 0

0.5

1

1.5

2

frequency (mm-1)

Fig. 10. Effect of collimation on the scan direction MTF as computed by simulations at 80 kVp:

signal. Several good quality images of full human spines were taken with the EOS system at an entrance skin dose of 10 mR: This is well below the median value of 145 mR reported for film in a large scale survey [22]. Dose-sensitive patients like pregnant women would certainly benefit from the use of EOS-like devices. Skeletal pathologies requiring many radiological images for surgery planning and follow-up are also good targets for this technology. For instance, scoliosis-related imaging is under intensive development with the EOS system. This new imaging device is currently used for such purposes at the E´cole Nationale Supe´rieure d’Arts et Me´tiers in Paris, France. It

Acknowledgements This work was conducted with the help of Fonds de la Recherche en Sante´ du Que´bec, Valorisation Recherche Que´bec, Fondation Canadienne pour l’Innovation, Minisite`re de l’E´ducation du Que´bec and Socie´te´ Biospace. The authors wish to thank Ce´dric Fedelich, Wafa Skalli and David Mitton from ENSAM Paris for their support. References [1] T. Tanimori, Y. Nishi, A. Ochi, Y. Nishi, Nucl. Instr. and Meth. A 436 (1999) 188. [2] G.C. Giakos, S. Chowdhury, S. Vedantham, A. Dasgupta, B. Pillai, P. Ghotra, S. Suryanarayanan, J. Odogba, Feasibility study of a gas microstrip detector for medical applications, in: Medical Imaging 1997: Physics of Medical

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