Role of the activator in the performance of scintillators used in X-ray imaging.

The aim of this study was to investigate how the activator type affects the performance of X-ray scintillators. To this aim the behavior of scintillator materials was modeled under X-ray excitation conditions, similar to those used in imaging techniques. The model describes the light emission efficiency, the spectral compatibility with optical detectors (films, photodiodes, and photocathodes), and the imaging capabilities of a scintillating layer. Using the model equations the role of the activator type in scintillator performance was examined. Activators affect some important properties of materials, like the intrinsic X-ray to light conversion efficiency, the spectrum of the emitted light, and the light attenuation coefficients. The performances of a high-efficiency material (Gd2O2S) combined with either Tb3+ or Eu3+ activators were compared. Results showed that the terbium-activated material exhibited high emission efficiency (number of emitted photons per incident X-ray) and modulation transfer function (spatial resolution and image contrast) while the europium activated material showed slightly better signal-to-noise ratio properties at low spatial frequencies. Both materials were found to exhibit high spectral compatibility with currently used modern optical detectors. In conclusion, the choice of activator may improve spectral compatibility, but care must be taken because it may also alter emission efficiency and image quality.     

Evaluating scintillators used in radiation detectors of medical imaging systems by the effective fidelity index method.

OBJECTIVE: The performance of medical X-ray image receptors depends: (1) on the scintillator light emission efficiency; and (2) on the compatibility of the scintillator light spectrum with the spectral sensitivity of the light detector (film, photocathode, or photodiode), employed in conjunction with the scintillator. In this study, a scintillator performance measure, the effective fidelity index (EFI), is defined as function of both the scintillator light emission efficiency and spectral compatibility. MATERIALS AND METHOD: CsI:Na, Gd2O2S:Tb and La2O2S:Tb scintillators were employed in the form of phosphor screens prepared in our laboratory with various coating thicknesses. The screens were irradiated with X-rays employing tube voltages ranging between 50-120 kVp. RESULTS: The EFI performance of CsI:Na was found to increase with screen coating thickness and it was best when combined with the orthochromatic film or the ES/20 photocathode. Gd2O2S:Tb showed peak EFI performance at 70 mg/cm2 coating thickness and it was well combined with the light detectors considered. CONCLUSION: In accordance with our results, CsI:Na may be employed in radiography when adequately protected against humidity. Gd2O2S:Tb suitability for conventional imaging was verified and it was found that it may be useful in all types of digital imaging. La2O2S:Tb could also be used in digital detectors of imaging applications demanding medium X-ray tube voltages.     

Measurement of depth dose distribution using plastic scintillator

We propose a new method to measure the depth dose distribution in matter. The method is to use organic plastic scintillator as a phantom, which has the effective atomic number close to that of human soft tissue, and to measure the position distribution of the scintillation light by a wavelength analyzer through a thread of plastic optical fiber. The purpose of this study is to examine possibility to measure the depth dose distribution with the proposed method. We carried out the following three measurements. First, we measured the effective field of view (FOV); the plastic optical fiber accepts the light from the FOV. Then, we measured the dose dependence of the amount of emission light. Finally, the light distribution in the depth direction by the present method was compared to the depth dose distribution by the ionization chamber. The results are as follows. The angular range of FOV is 7.03 degrees +/- 0.21 degrees The dose dependence shows a good linearity. In other words, the amount of emission light is proportional to the absorbed dose. Both of the light and the dose distributions are proportional each other, and they show the same attenuation in the region deeper than 15 mm. From these results, we conclude that the present method can be used for the measurement of the depth dose distribution in matter.     

Experimental determination of elemental compositions and densities of several common liquid scintillators

New accurate data for the density and the elemental composition of several common liquid scintillators have been determined in this work. These data can be used to correctly determine or calculate the counting efficiency of radio-nuclides as well as to evaluate more accurately the uncertainties in LSC measurements due to variability on the composition of scintillators. The discrepancy between nominal densities at 20°C and the real densities at 20°C or 16°C can reach up to 4% among different batches of commercial scintillators all having the same nominal composition. Also significant differences in the elemental composition of commercial cocktails have been found compared to the nominal values. These differences range from 2% up to 260% depending on the element and the scintillator being measured.     

Measurements of relative depth doses and Cerenkov light using a scintillating fiber-optic dosimeter with Co-60 radiotherapy source

In this study, we fabricated a scintillating fiber-optic dosimeter, which consists of an organic scintillator and a plastic optical fiber, for radiotherapy dosimetry. To select an adequate kind and length of scintillator for γ-rays generated from a Co-60 source, scintillating light from various kinds and lengths of organic scintillators is measured. Using a scintillating fiber-optic dosimeter, the γ-rays generated from a Co-60 therapy unit are measured and relative doses are obtained according to the field size of the γ-ray beam and the depth in a water phantom. Also, Cerenkov light generated by the interactions of primary or secondary electrons and the plastic optical fiber is measured with different field sizes and depths of a water phantom using a background optical fiber.     

Light transport feature for SCINFUL.

An extended version of the scintillator response function prediction code SCINFUL has been developed by incorporating PHOTRACK, a Monte Carlo light transport code. Comparisons of calculated and experimental results for organic scintillators exposed to neutrons show that the extended code improves the predictive capability of SCINFUL.     

New emulsive scintillator for liquid scintillation measurements

A new emulsive scintillator for liquid scintillation measurements has been proposed, which removes the inherent disadvantages of conventional emulsive scintillators. The new scintillator never forms gels and two phases, both of which are very inconvenient for user, and maintains a state of soft sol within a water content from 0 to 50%. It also possesses excellent properties with respect to stability, solubility and chemiluminescence as compared with conventional scintillators.     

An efficient analytical calculation of probability matrix in 2D SPECT.

Expectation Maximization iterative reconstruction algorithms are being widely used in PET and SPECT imaging. The system probability matrix is usually calculated by using Monte Carlo simulations, since the analytical calculation is a rather complicated problem, especially in 3D reconstruction; however, realistic Monte Carlo simulations in 3D are time consuming and simplifications are necessary. In this paper, the probability matrix in the case of 2D SPECT is analytically calculated, using only two basic parameters: (i) the total number of image pixels and (ii) the number of projection angles. In the more general phase three more parameters will be taken into account: (i) the distance between the detectors and the object to be imaged; (ii) the collimator parameters; (iii) scintillator cells parameters (in the case of pixilated scintillators) and relative position between scintillator cell and collimator hole. It is shown that the accuracy of the probability matrix affects the quality of reconstructed images, especially in the case of pixilated scintillators, which are used in many dedicated SPECT systems. The methods presented here can be extended to 3D SPECT and also 2D and 3D PET. In addition, this analytically calculated matrix can be a reference matrix in order to be compared with Monte Carlo generated matrices.     

Statistics of the LS-detector in the case of low counting efficiency.

In the case of high-quenched 3H and 63Ni sources a distinct incompatibility of theoretical and experimental detection efficiency in the triple liquid scintillation (LS) detector was observed (Appl. Radiat. Isot. 52 (2000) 643). The authors concluded, that the Poisson distribution does not properly describe the detection process, when less than one photoelectron is expected in one of the photomultipliers. Application of other distributions of photoelectrons, e.g. binomial, does not solve the observable problem of incompatibility. Measurements of a set of 55Fe sources have been performed with phototubes defocusing and grey filters for decreasing the counting efficiency of the TDCR detector. Differences between counting results of the 55Fe source and the light emitting diode (LED), simulating the scintillation source, excited by a pulse generator have been observed. Various distributions (Poisson, binomial and Polya) were used for the determination of the theoretical counting efficiency in both cases. The Poisson law gave a good result in the case of the LED but the Polya law had to be applied in the case of 55Fe. The results were independent of the scintillator volume. It seems that the validity of the Polya law in the case of LS-sources is related to the scintillator itself. Measurements of the 3H solution confirmed that conclusion.     

Scintillation crystals for PET.

In PET, inorganic scintillator crystals are used to record gamma-rays produced by the annihilation of positrons emitted by injected tracers. The ultimate performance of the camera is strongly tied to both the physical and scintillation properties of the crystals. For this reason, researchers have investigated virtually all known scintillator crystals for possible use in PET. Despite this massive research effort, only a few different scintillators have been found that have a suitable combination of characteristics, and only 2 (thallium-doped sodium iodide and bismuth germanate) have found widespread use. A recently developed scintillator crystal, cerium-doped lutetium oxyorthosilicate, appears to surpass all previously used materials in most respects and promises to be the basis for the next generation of PET cameras.     

Background components of Ge detectors in Ogoya underground laboratory.

Ogoya underground laboratory, (OUL; 270 m water equivalent), has 5 well, 4 planar, and 1 coaxial-type ultra-low background Ge detectors with passive shield of old lead. An anticoincidence system with a plastic scintillator (PS) has been tested to reduce cosmic-ray-induced background. Energy spectra of PSs, coincidence counting rate, and angular distribution of cosmic rays were studied. Based on this study, we propose a new index of Ge surface area-normalized background counting rate (d(-1) cm(-2)) to compare shielding efficiency between different types of Ge detectors.     

An intra-operative positron probe with background rejection capability for FDG-guided surgery

For radio-guided surgery on tumors using F-18-FDG, detection of annihilation gamma photons emanating from other parts of the body produces background radiation counts and limits its use in clinical situations. To overcome this limitation, we have developed an intra-operative positron probe with background-rejection capability. The positron probe uses a phoswich detector composed of a plastic scintillator and a bismuth germinate (BGO). A positron from a positron emitter such as F-18 is detected by the plastic scintillator and emits annihilation photons. The BGO detects one of the annihilation photons while a photo-multiplier tube (PMT) detects scintillation photons from both scintillators. The decay time differences of these two scintillators are used to distinguish whether the event is a true event where a positron and a following annihilation photon are detected simultaneously, or a background event. In this configuration, only positrons can be selectively detected, even in an environment of high background gamma photon flux. Spatial resolution was 11-mm full width at half maximum (FWHM) 5 mm from the detector surface. Measured sensitivity for the F-18 point source was 2.6 cps/kBq 5 mm from the detector surface. The background count rate was less than 0.5 cps for a 20-cm diameter cylindrical phantom containing 37 MBq of F-18 solution measured on the phantom surface, while the positron count rate was almost linear over a range of approximately 6 kcps. These results indicate that our developed intra-operative positron probe is valuable for radio-guided surgery on tumors using F-18-FDG in a high flux of background annihilation gamma photons.     

Performance evaluation of X-ray CT using visible scintillation light

We proposed a new method of performance evaluation for X-ray CT using visible scintillation light and examined its usefulness in this study. When we scanned a plastic scintillator disk in a gantry opening of the X-ray CT, we could observe visible scintillation light. The rotation of the light-emitting area of the disk corresponded to that of the X-ray tube. We were able to record the scintillation light by digital video camera. By analyzing the area of visible scintillation light, the rotation speed of the X-ray tube, angular spread of the X-ray beam, uniformity of the incident X-rays, and change in X-ray energy were measured. No other method is available to obtain the above parameters of X-ray CT during a single CT scan. In the measurements of the uniformity of incident X-rays and change of X-ray energy, our method showed good accuracy in detecting the attenuation caused by the couch between the X-ray tube and the plastic scintillator disc. The proposed method is inexpensive and easy-to-use. We conclude that the method is a useful tool for performance evaluation as well as a maintenance tool for X-ray CT.     

The influence of photon depth of interaction and non-collinear spread of annihilation photons on PET image spatial resolution

Purpose The quality of PET imaging is impaired by parallax errors. These errors produce misalignment between the projected location of the true origin of the annihilation event and the line of response determined by the coincidence detection system. Parallax errors are due to the varying depths of photon interaction (DOI) within the scintillator and the non-collinear (NC) emission of the annihilation photons. The aim of this work was to address the problems associated with the DOI and the NC spread of annihilation photons and to develop a quantitative model to assess their impact on image spatial resolution losses for various commonly used scintillators and PET geometries.Methods A theoretical model based on Monte Carlo simulations was developed to assess the relative influence of DOI and the NC spread of annihilation photons on PET spatial resolution for various scintillator materials (BGO, LSO, LuAP, GSO, NaI) and PET geometries.Results The results demonstrate good agreement between simulated, experimental and published overall spatial resolution for some commercial systems, with maximum differences around 1 mm in both 2D and 3D mode. The DOI introduces an impairment of non-stationary spatial resolution along the radial direction, which can be very severe at peripheral positions. As an example, the radial spatial resolution loss due to DOI increased from 1.3 mm at the centre to 6.7 mm at 20 cm from the centre of a BGO camera with a 412-mm radius in 2D mode. Including the NC, the corresponding losses were 3.0 mm at the centre and 7.3 mm 20 cm from the centre.Conclusion Without a DOI detection technique, it seems difficult to improve PET spatial resolution and increase sensitivity by reducing the detector ring radius or by extending the detector in the axial direction. Much effort is expended on the design and configuration of smaller detector elements but more effort should be devoted to the DOI complexity.     

Statistical image analysis of high-sensitivity neutron images obtained by cooled CCD systems.

Neutron sensitivity and noise characteristics of cooled CCD NR systems were investigated. Eighteen species of neutron sensitive scintillators were tested by the use of two types of cooled CCD devices. The statistical analysis of the S/N ratio of the neutron images showed that the ZnS+LiF type was the highest sensitivity scintillator. The generation rate of the CCD noises increased with increasing exposure time, temperature of the device and dose rate of the environmental radiation. The generation rate and the pulse height distribution of the noise were quite different between the two CCD devices. Thus, the origin of the noise is considered to be strongly related to the internal structure of the CCD devices.     

On the stochastic dependence between photomultipliers in the TDCR method

The TDCR method (Triple to Double Coincidence Ratio) is widely implemented in National Metrology Institutes for activity primary measurements based on liquid scintillation counting. The detection efficiency and thereby the activity are determined using a statistical and physical model. In this article, we propose to revisit the application of the classical TDCR model and its validity by introducing a prerequisite of stochastic independence between photomultiplier counting. In order to support the need for this condition, the demonstration is carried out by considering the simple case of a monoenergetic deposition in the scintillation cocktail. Simulations of triple and double coincidence counting are presented in order to point out the existence of stochastic dependence between photomultipliers that can be significant in the case of low-energy deposition in the scintillator. It is demonstrated that a problem of time dependence arises when the coincidence resolving time is shorter than the time distribution of scintillation photons; in addition, it is shown that this effect is at the origin of a bias in the detection efficiency calculation encountered for the standardization of ³H. This investigation is extended to the study of geometric dependence between photomultipliers related to the position of light emission inside the scintillation vial (the volume of the vial is not considered in the classical TDCR model). In that case, triple and double coincidences are calculated using a stochastic TDCR model based on the Monte-Carlo simulation code Geant4. This stochastic approach is also applied to the standardization of ⁵¹Cr by liquid scintillation; the difference observed in detection efficiencies calculated using the standard and stochastic models can be explained by such an effect of geometric dependence between photomultiplier channels.     

塑料闪烁体探测器在光子束和电子束中的性能研究

目的:探讨W2塑料闪烁体探测器在兆伏级光子束和电子束辐射中的性能。方法:采用直线加速器提供的光子束和电子束能量对W2闪烁体进行数据采集。研究内容包括静电计读数稳定性、W2剂量和剂量率线性以及角度响应,同时研究W2校准系数给剂量测量带来的不确定度。结果:静电计读数稳定性平均值的标准偏差在0.03～0.47之间;W2剂量的...     

The light output and the detection efficiency of the liquid scintillator EJ-309

The light output response and the neutron and gamma-ray detection efficiency are determined for liquid scintillator EJ-309. The light output function is compared to those of previous studies. Experimental efficiency results are compared to predictions from GEANT4, MCNPX and PENELOPE Monte Carlo simulations. The differences associated with the use of different light output functions are discussed.     

Sector-shaped fast organic liquid scintillation detectors based neutron coincidence counter

A simulation based on the Monte Carlo method is described which has been performed using MCNPX 2.6.0, to model the geometry of a sector-shaped liquid scintillation detector in response to coincident neutron events. The detection of neutrons from a mixed-oxide (MOX) fuel pellet has been simulated for different thicknesses of scintillators. A layer of lead has been used to reduce the gamma-ray fluence reaching the scintillator and, the effect of lead for neutron detection has also been estimated by considering different thicknesses of lead layers.     

Standardization of tritiated water and 204Tl by TDCR liquid scintillation counting.

The triple to double coincidence ratio method was used with a combined uncertainty of 1-2% to standardize tritiated water and a solution of 204Tl. The vial with liquid scintillator, in which the sample to be measured was dissolved, is optically coupled to three photomultipliers. The electronic module MAC-3 assures the selection of double and triple coincidences count rates, D and T, from the three counting channels. It contains the gating circuits, necessary to obtain the livetime value and the extended deadtime circuit. Specific computer programs were used to calculate the free parameter value, the efficiency of D and so, the value of the activity. The optimal value of the Birks ionisation-quenching parameter, kB, was evaluated by changing the detection efficiency with grey filters. Three types of liquid scintillators, namely InstaGel, PPO+POPOP+Triton in toluene and Ultima Gold, were employed. 204Tl was measured in the frame of an international comparison organized by BIPM. For tritiated water a comparison was made with LNHB-Saclay; the relative difference between the obtained values for the massic activity was only 0.2%.