X-ray spectroscopy and dosimetry with a portable CdTe device

X-ray spectroscopy and dosimetry with a portable CdTe device

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 571 (2007) 373–377 www.elsevier.com/locate/nima X-ray spectroscopy and dosime...

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ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 571 (2007) 373–377 www.elsevier.com/locate/nima

X-ray spectroscopy and dosimetry with a portable CdTe device Leonardo Abbenea,b,, Angelo La Mannaa,b, Francesco Faucia,b, Gaetano Gerardia,b, Simone Stumboc,d, Giuseppe Rasoa,b a

Dipartimento di Fisica e Tecnologie Relative,Universita` di Palermo,Viale delle Scienze, Edificio 18, Palermo 90128, Italy b INFN, Sezione di Catania, Catania, Italy c Struttura Dipartimentale di Matematica e Fisica dell’Universita` degli studi di Sassari, Sassari, Italy d INFN Sezione di Cagliari, Cagliari, Italy Available online 13 November 2006

Abstract X-ray spectra and dosimetry information are very important for quality assurance (QA) and quality control (QC) in medical diagnostic X-ray systems. An accurate knowledge of the diagnostic X-ray spectra would improve the patient dose optimization, without compromising image information. In this work, we performed direct diagnostic X-ray spectra measurements with a portable device, based on a CdTe solid-state detector. The portable device is able to directly measure X-ray spectra at high photon fluence rates, as typical of clinical radiography. We investigated on the spectral performances of the system in the mammographic energy range (up to 40 keV). Good system response to monoenergetic photons was measured (energy resolution of 5% FWHM at 22.1 keV). We measured the molybdenum X-ray spectra produced by a mammographic X-ray unit (GE Senographe DMR) at 28 kV and 30 kV under clinical conditions. The results showed the good reproducibility of the system and low pile-up distortions. Preliminary dosimetric measurements have been regarded as exposure and half value layer (HVL) values obtained from direct measurements and from measured X-ray spectral data, and a good agreement between exposure attenuation curves and the HVL values was obtained. The results indicated that the portable device is suitable for mammographic X-ray spectroscopy under clinical conditions. r 2006 Elsevier B.V. All rights reserved. PACS: 29.30.Kv; 87.66.Pm; 87.64.Gb Keywords: X-ray spectroscopy; CdTe detectors; Exposure; HVL

1. Introduction Quality assurance (QA) and quality control (QC) of radiographic systems require accurate investigations on the incident X-ray spectra. Diagnostic X-ray tube spectra measurements are essential for many procedures, such as radiation protection calculation, patient dosimetry and measurement of radiographic imaging properties (Detective Quantum Efficiency). Several semiconductor detectors (Ge and Si) have been proposed for X-ray spectra measurements under clinical conditions [1–3]. Despite the excellent energy resolution of silicon and germanium detectors, several distortions due to their low detection efficiency and secondary X-ray escape are visible in Corresponding author. Tel.: +39 091 6615053; fax: +39 091 6615063.

E-mail address: [email protected] (L. Abbene). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.10.112

measured X-ray spectra [1–3]; moreover, the use of large cryogenics systems in Si and Ge detectors (necessary to reduce the thermal noise) is a critical issue for X-ray measurements under clinical conditions. Recently, several room-temperature compound semiconductors (CdTe, CdZnTe) are promising X-ray detectors and are suitable for portable systems [4–6]. The high atomic number (Zmax ¼ 52) and the large band gap (CdTe: EG1.5 eV; CdZnTe: EG1.6 eV) of these materials, in comparison with Si and Ge, give high quantum efficiency and good room-temperature performances. In this work a portable device for X-ray spectroscopy, based on a CdTe detector, is described. The system is able to directly measure X-ray spectra and X-ray fluence at high photon count rate, as typical of diagnostic X-ray systems. In order to evaluate the spectroscopic performances and calibrate the system, preliminary measurements were carried out by using

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laboratory radioactive sources. We investigate on X-ray spectra measurements in the mammographic energy range (up to 40 keV) to minimize the typical spectral distortions (tailing, X-ray escape) in CdTe measured spectra [4]. We measured the X-ray spectra from a GE Senographe DMR mammographic unit that is currently used in the Radiology Unit of Maurizio Ascoli Oncological Hospital (Palermo, Italy). Exposure and half value layer (HVL) were calculated from the measured X-ray spectra and compared with exposure and HVL measured using a PMX-III meter with a R100 solid-state dose detector (RTI electronics). 2. Materials and methods 2.1. Portable device The portable device consists of four blocks: a detector case, a shaping amplifier, an ADC card and a notebook computer. The CdTe detector (2  2  1 mm3 crystal) and the preamplifier were both within the detector case (Amptek XR-100T-CdTe). Schottky contacts and a thermoelectric cooling (20 1C) of both the CdTe crystal and the preamplifier input FET ensure low noise and good stability. The shaping amplifier (Amptek PX2T), with a shaping time (FWHM) of about 620 ns, processed the preamplifier output signal. The shaped pulses were recorded by a 12-bit ADC card (ADLink Technology NuDAQ PCI 9812), with a 10 MHz sampling rate. A dedicated software [6] calculated the incident photon count and the energy spectrum by analyzing the sampled signals. 2.2. Detector response and calibration The detector was calibrated for energy scales, linearity checks and energy resolution by using X-ray and g-ray calibration sources (109Cd: 22.1 keV and 24.9 keV; 241Am: 59.5 keV) and the fluorescent lines of Rb (Ka ¼ 13.37 keV and Kb ¼ 14.97 keV), Mo (Ka ¼ 17.44 keV and Kb ¼ 19.63 keV), Ba (Ka ¼ 32.06 keV and Kb ¼ 36.55 keV) and Tb (Ka ¼ 44.23 keV and Kb ¼ 50.65 keV). 2.3. X-ray spectroscopy in mammographic energy range X-ray spectroscopy was performed in the mammographic energy range under clinical conditions. We directly measured the molybdenum spectra transmitted through 0.03 mm Mo filter and produced by a mammographic X-ray unit (GE Senographe DMR) at 28 kV and 30 kV. The detector was placed on the cassette holder with a 57 cm focus–detector distance, and the compression paddle was removed during the measurements. In order to minimize pile-up distortions, we used a dedicated collimation system (W collimator, 50 mm f and 1 mm thick); further details about the collimation system and the alignment procedure have been already described in a previous work [7].

2.4. Dosimetry and HVL measurements Preliminary dosimetric measurements have been regarded as exposure and HVL measurements obtained with measured X-ray spectral data and using a PMX-III meter with a R100 solid-state dose detector (RTI electronics). We used eight aluminum filters (Type 1100, Al 99.0% purity, 0.1–0.8 mm) for HVL measurements. The exposure value X (mR) for monoenergetic photons was calculated using the following formula [8]: X ðmRÞ ¼ 1:83  106 fEðmen =rÞair ,

(1)

where f is the photon fluence (photons/mm2), E the photon energy (keV) and (men/r)air the air mass energy absorption coefficient (cm2/g). The air mass energy absorption coefficient data were obtained using a polynomial function as suggested in Assiamah et al. [9]. We calculated the exposure of mammographic spectra by summing monoenergetic exposures over all the energy range. The transmitted exposure for each combination of the aluminum filters was used to calculate the attenuation curves and the HVL values. The HVL values were obtained by interpolation of the two data points neighboring the HVL thickness [10]. 3. Results and discussion The detector response to 109Cd is shown in Fig. 1. The detector shows good spectral performances (energy resolution of 5% FWHM at 22.1 keV). Low tailing characterizes the full-energy peaks of the spectrum and no escape peaks are visible; high tailing is evident in the 241Am spectrum as reported in our previous work [7]. These results confirm that tailing and X-ray escape are not severe in the mammographic energy range. We measured the detector calibration curve showing a good linearity and a 0.14 keV/ channel sensibility. Two measured spectra of the X-ray mammographic tube (molybdenum target) are shown in Fig. 2; the tube settings were: 60 mAs, 28 kV and 30 kV. The system reproducibility was verified by measuring X-ray spectra several times; the standard deviation of the photon number was smaller than 0.5% (10 measurements). Table 1 shows the average results from 10 measured molybdenum spectra both at 28 kV and 30 kV. The results confirm the good spectral and temporal performances of the detector and the good pile-up recognition–correction capability of the system (rejected pile-up events at 28 kV and 30 kV: 1.5% and 1.9%, respectively). The comparison of exposure attenuation curves obtained from the CdTe spectra with curves from direct exposure measurements (PMX III) is shown in Fig. 3 (28 kV) and Fig. 4 (30 kV). Statistical errors were taken into account for CdTe exposure measurements; we considered direct exposure measurements accurate to within 75%. Table 2 reports the HVL measurements. We got a good agreement between HVL values obtained from direct exposure measurements with values from

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Fig. 1. Measured X-ray spectrum of

375

109

Cd (22.1 keV and 24.9 keV). The CdTe detector shows an energy resolution of 5% FWHM at 22.1 keV.

Fig. 2. Molybdenum X-ray spectra measured with the portable device under clinical conditions. The tube settings were: 60 mAs, 28 kV and 30 kV.

Table 1 Results of 10 mammographic X-ray measurements Tube voltage (kV)

Detected photons

Recognized photons

Photon count rate (photons/s)

28 30

252907120 337107170

249207120 330607170

396007190 541907300

measured X-ray spectral data. The differences between the exposure attenuation curves (Fig. 4) are probably caused by the increasing of incident scattered radiation with larger filter thickness [4].

4. Conclusions A portable device based on a CdTe detector was employed in diagnostic X-ray spectra measurements.

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Fig. 3. Relative attenuation curve calculated from the CdTe spectra and curve from direct exposure measurements (PMX III meter, RTI electronics) at 28 kV. Errors are too small to be visible in the figure.

Fig. 4. Relative attenuation curve calculated from the CdTe spectra and curve from direct exposure measurements (PMX III meter, RTI electronics) at 30 kV. Errors are too small to be visible in the figure. Table 2 Results of HVL measurements Tube voltage (kV)

HVL CdTe (mm)

HVL PMX III (mm)

28 30

424717 490730

423750 490720

The device is able to directly measure X-ray spectra at high photon fluence rates, as typical of clinical radiography. In particular, we investigated the system performances in the mammographic energy range. The response to monoener-

getic photons (22.1 keV) showed good spectral performances of the system in the mammographic energy range; we observed a good energy resolution (5% FWHM 22.1 keV), low tailing and no X-ray escape in the measured spectra. The mammographic X-ray spectra showed the good spectral and temporal performances of the detector and the good pile-up recognition–correction capability of the system (rejected pile-up events at 28 kV and 30 kV are about 1.5% and 1.9%, respectively). The agreement between HVL values obtained from direct exposure measurements with values from measured X-ray spectral data confirmed the good perfomances of the system. In

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addition, the compactness and its easy operation make the system suitable for X-ray spectroscopy under clinical conditions. References [1] T.R. Fewell, R.E. Shuping, Med. Phys. 12 (3) (1977) 187. [2] R. Birch, M. Marshall, Phys. Med. Biol. 24 (1979) 505.

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