Medical Imaging Detection Research Articles

A theoretical model describing the light emission efficiency of single-crystal scintillators in the diagnostic energy range

The aim of this study was to develop a theoretical model to examine emission features of single-crystal scintillators, used in medical imaging detectors, under X-ray excitation. For this purpose, the number of optical photons that were produced inside the crystal and escaped to the output was modeled for variant X-ray tube voltages in the energy range of Computed Tomography and for different thicknesses of the crystalline material. The theoretical model that was used to estimate the optimum dimensions and the radiation conditions of the crystal, was validated against experimental data obtained by a single-crystal scintillator irradiated by X-rays. For implementation a Gd2SiO5:Ce crystal was used. Theoretical and experimental results will be useful in designing Hybrid Tomographic imaging systems based on a common gamma-ray and X-ray detector (PET/CT or SPECT/CT).

Quantifying the limitations of small animal positron emission tomography

The application of position sensitive semiconductor detectors in medical imaging is a field of global research interest. The Monte-Carlo simulation toolkit GEANT4 [ http://geant4.web.cern.ch/geant4/] was employed to improve the understanding of detailed γ -ray interactions within the small animal Positron Emission Tomography (PET), high-purity germanium (HPGe) imaging system, SmartPET [A.J. Boston, et al., Oral contribution, ANL, Chicago, USA, 2006]. This system has shown promising results in the field of PET [R.J. Cooper, et al., Nucl. Instr. and Meth. A (2009), accepted for publication] and Compton camera imaging [J.E. Gillam, et al., Nucl. Instr. and Meth. A 579 (2007) 76]. Images for a selection of single and multiple point, line and phantom sources were successfully reconstructed using both a filtered-back-projection (FBP) [A.R. Mather, Ph.D. Thesis, University of Liverpool, 2007] and an iterative reconstruction algorithm [A.R. Mather, Ph.D. Thesis, University of Liverpool, 2007]. Simulated data were exploited as an alternative route to a reconstructed image allowing full quantification of the image distortions introduced in each phase of the data processing. Quantifying the contribution of uncertainty in all system components from detector to reconstruction algorithm allows the areas in need of most attention on the SmartPET project and semiconductor PET to be addressed.

Flat-panel Detectors For Medical Imaging

Flat-panel Detectors For Medical Imaging

The growth and scintillation properties of CsCe 2Cl 7 crystal

We report on a new scintillation crystal, CsCe 2Cl 7, for γ-ray spectroscopy. The crystal was grown using the Czocralski pulling method. X-ray excited luminescence measurements of the CsCe 2Cl 7 showed a broad emission band in the wavelength range from 370 to 470 nm with a peak centre at 410 nm. An energy resolution (FWHM of peak position) for the 662 keV full energy peak of 5.5% was observed at room temperature. The results showed a good proportionality for light output versus γ-ray energy. The light output deviation from the linear response is about 10% between the energy range of 31 and 1333 keV. We measured a light yield of 28,000 photons/MeV of absorbed γ-ray energy. The scintillation decay curve of CsCe 2Cl 7 can be described by a single exponential decay function with a decay time of 50 ns. Overall, these measurements clearly indicate that CsCe 2Cl 7 can exhibit attracting scintillation properties, and we believe that the CsCe 2Cl 7 crystal is a promising material for medical imaging and radiation detection.

Luminescence Emission Properties of $({\rm Lu},{\rm Y})_{2}{\rm SiO}_{5}$:Ce (LYSO:Ce) and $({\rm Lu},{\rm Y}){\rm AlO}_{3}$:Ce (LuYAP:Ce) Single Crystal Scintillators Under Medical Imaging Conditions

LYSO:Ce and LuYAP:Ce are single crystal non-hygroscopic scintillators of high density, high light yield and short decay time, which have been successfully used in small animal PET imagers. In the present study, the luminescence emission properties of (Lu0.9, Y0.1)2SiO5:Ce (LYSO:Ce) and (Lu0.7, Y0.3)AIO3:Ce (LuYAP:Ce) crystals were investigated for use in X-ray medical imaging. Both crystals had dimensions of 2 times 2 times 8 mm3, with all surfaces polished. Evaluation was performed by determining the X-ray luminescence efficiency (XLE) (emitted light energy flux over incident X-ray energy flux) and the detector optical gain (DOG) (emitted light photons per incident x-ray photon) in a wide range of X-ray energies employed in mammography (22-49 kVp) and in general X-ray imaging (50-140 kVp). Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. The emission spectrum under X-ray excitation was measured using an optical grating monochromator to determine the spectral compatibility to various optical photon detectors incorporated in medical imaging detectors. Optical characteristics such as transmission and absorption spectra were investigated in addition to the scintillation properties. The light emission performance of the two scintillation materials studied was found adequately high for X-ray imaging.

Industrial silicon detectors, advancements in planar technology

Thirty years ago planar technology was first successfully applied to the manufacture of silicon detectors and this technique led to detectors with better performance than traditional surface barrier and diffused junction types. Today, these detectors are widely used in nuclear and high-energy physics research and are widely deployed in the industrial market as well. These silicon detectors are used in health physics, X-ray fluorescence and diffraction, medical imaging, space applications, and photon (light) detection. As an industrial company, Canberra was challenged to provide detectors with high reliability along with low leakage current, thin entrance windows, a wide range of thicknesses, low noise, and position sensitivity in both single devices and arrays. A new challenge is the readout of pixilated devices. Electronics integrated into or onto the detector chip is a topic of current interest.

Characterization of a-Se(As:Cl) Film for Phosphor Coupled X-Ray Light Modulators

Amorphous selenium (a-Se) film has the potential to fulfill the requirements of a novel x-ray image detector because of its good photo-to-dark impedance ratio, large area coverage, and low temperature deposition. In this work were studied the structural, optical and electrical properties of thermally- evaporated a-Se film for the phosphor-light modulator (PLM). From the x-ray diffraction and electron microscopy experimental results, the deposited film had an amorphous phase without any re-crystallization or defects. Also, the light absorption in widely visible range of 400 ~ 630 nm was over 95 %. From the electrical measurements, the low dark current density of 2.8 nA/cm2 was obtained at 10 V/㎛. The x-ray sensitivity of the 270㎛-Gd2O3:Eu phosphor coupled 20㎛-Se film was 7.31 nC/cm2-mR. From such experimental results, the novel x-ray detector which incorporates phosphor coupled x-ray light modulator, makes an operation at low x-ray exposure possible, therefore, the applications as a medical imaging detector are shown below.

Monte Carlo analysis of the degradation of the spectrum produced by an X-ray tube in conventional radiography

Medical imaging detectors used in diagnostic radiology are very sensitive to the spectra of the X-ray beams used. In this work, we use Monte Carlo simulation to analyse the degradation of various X-ray spectra at different stages of their travel from the X-tube focus to the detector. Special attention is paid to the antiscatter grid that is normally used in diagnostic radiology. For the present study, we have considered various measured spectra corresponding to accelerating voltages of 60, 80, 90 and 125 kV. We have assumed that the X-ray beams are parallel beams going successively through an aluminium filter, the air gap between the focus and the patient, a patient-equivalent water phantom and a parallel cross antiscatter grid. The Monte Carlo simulation code PENELOPE has been used to find out the X-ray spectrum at three planes: the patient, the grid, and the detector entrance. Additional simulations with monoenergetic beams of 30, 40, 55 and 100 keV have been carried out for comparison. We have investigated in detail the dependence of the attenuation of the primary beam with the initial energy, the percentage of the initial X-rays reaching the grid and the ratio of scatter to primary photons reaching the grid. The role played by the grid has been studied in depth. In particular, we have analysed the modification of the incident spectrum that it produces and its efficiency in removing the scattered photons.

Radiation detector based on liquid crystal light valve for large-area imaging applications

Currently most large-area radiation detectors rely on the use of a phosphor screen and an optical sensor. In such conversion process, there will be signal loss due to inefficiencies in coupling between phosphor screen and later light-detection components. This paper presents a radiation detector based on X-ray induced light transmittance of liquid crystal (LC) as a function of radiation. Our detector incorporates a europium-doped gadolinium oxide (Gd 2O 3:Eu) phosphor and an amorphous selenium (a-Se)-coupled LC light valve. The Gd 2O 3:Eu is synthesized using a low-temperature solution–combustion method, and the optical properties of the Gd 2O 3:Eu are investigated as a function of a sintering temperature. The transmission–voltage ( T– V) curve and the optical response of the detector are also investigated. Our experimental results show good T– V characteristics of the LC, switching between 12% and 86% transmission. The radiation dose is also presented in this work. The results show that our detector can operate at low X-ray dose and, therefore, a promising application is the medical-imaging detector.

Detector characterization for an inline PET scanner in hadrontherapy

Our group at the “Institut de Physique Nucléaire de Lyon” (IPNL) is working on physics and detectors for medical imaging. We are presently developing a small animal Positron Emission Tomograph (PET) scanner prototype with an innovative slow control and data acquisition features, for a demonstration purpose and within the crystal clear international collaboration. We also investigate a feasibility study of an online PET dedicated for inline and in situ dose deposition control in hadrontherapy. Here, we present the characterization setup and method we used to calibrate the detector heads of our PET prototype. Each of these heads consists of a single block continuous scintillating LySO crystal coupled to a multi-anode photomultiplier equipped with its proper acquisition readout chain.

Microelectronics technologies for new detectors in medical imaging

The use of silicon chips for instrumentation developments in elementary particle physics serves as an example for other applications and digital imaging detectors could find use in medical and molecular imaging. Attractive features are direct quantum conversion in a semiconductor matrix, innovative three-dimensional modular detector construction, multilayer devices, very fast signal processing, on-line data pre-processing and massive parallelism at the system level. Cost aspects of such semiconductor imager options have to be taken into account in the R&D phase. With the integrated electronics and high density interconnects in the Medipix development as an example, the ultimate aim of single photon imaging comes within reach.

State-of-the-art radiation detectors for medical imaging: Demands and trends

Over the last half-century a variety of significant technical advances in several scientific fields has been pointing to an exploding growth in the field of medical imaging leading to a better interpretation of more specific anatomical, biochemical and molecular pathways. In particular, the development of novel imaging detectors and readout electronics has been critical to the advancement of medical imaging allowing the invention of breakthrough platforms for simultaneous acquisition of multi-modality images at molecular level. The present paper presents a review of the challenges, demands and constraints on radiation imaging detectors imposed by the nature of the modality and the physics of the imaging source. This is followed by a concise review and perspective on various types of state-of-the-art detector technologies that have been developed to meet these requirements. Trends, prospects and new concepts for future imaging detectors are also highlighted.

A large-area monolithic array of silicon drift detectors for medical imaging

Monolithic arrays of silicon drift detectors (SDDs) have been recently proposed to be used with scintillators for high-position-resolution γ-ray imaging applications. Thanks to the low electronics noise due to the small value of the output capacitance, the SDD offers better noise performances with respect to conventional photodiodes of the same geometry. Small monolithic arrays of SDDs have been used as photodetector of the scintillation light in a first prototype of Anger Camera for γ-ray imaging characterized by an intrinsic resolution better than 0.3 mm. In this work, we present a new large-area monolithic array of SDDs. It consists of a single chip composed of 77 single hexagonal units, each one with an active area of 8.7 mm 2, for a total active area of the device of 6.7 cm 2. It represents the largest monolithic array of SDDs with on-chip JFETs produced up to now for X-ray and γ-ray detection. The results achieved in the experimental characterization of a first prototype of the detector array are presented, both with X and visible photons. The energy resolution measured at 6 keV with the single unit of the array is of 142 eV at −10 °C, while a QE>90% was measured at λ=550 nm.

Ionizing Radiation Detectors for Medical Imaging [Book Review

Ionizing Radiation Detectors for Medical Imaging [Book Review

Growth of mercuric iodide crystals

Mercuric Iodide (HgI2) is a semiconductor candidate for the construction of X- and gamma-ray detectors for digital medical imaging due to its high atomic number (Z Hg = 80, Z I = 53). Also, HgI2 has a wide optical band-gap (2.13 eV) and high photon absorption coefficient for high-energy radiation. Different structures can lead to varying electrical and optical properties of the final material. In this work, HgI2 crystals were produced by the solvent evaporation technique. The solvents used were ethanol (solubility around 20 mg/ml at 25ºC), ether (solubility around 3.5 mg/ml at 25ºC) and acetone (solubility around 24mg/ml at 25ºC). The evaporation conditions were varied in order to produce different final crystals. The Bérnard cells are responsible for crystallites formation due to the Bérnard-Maragoni convection in the liquid. Millimeter-sized crystals can be obtained as seen by Scanning Electron Microscopy.

Stoichiometry, surface and structural characterization of lead iodide thin films

In this work we present the structural properties and stoichiometry analysis of thin films of lead iodide (PbI 2). This material is a very promising semiconductor material for the development of X-ray detectors in digital medical imaging. An alternative deposition method called Spray Pyrolysis was used. We discuss the main advantages and limitations of the deposition process comparing three different starting material powders. Extra iodine atmosphere during deposition and the effect of post-deposition thermal treatment is also discussed. The structural properties were studied by X-ray diffraction, Atomic Force Microscopy (AFM), Scanning Electronic Microscopy (SEM) and stoichiometry analysis were performed using Energy Dispersive Spectroscopy (EDS).

Application of epitaxial GaAs to medical imaging

We show that GaAs is a promising material for X-ray medical imaging detectors. Indeed, fluorescence degrades the spatial resolution, and GaAs optimises the compromise between absorption and spatial resolution. We describe the performances (spatial resolution, linearity, energy resolution and temporal response) reached by detectors made of the thick epitaxial GaAs layers we produce, in order to illustrate that such detectors qualify for medical imaging. Finally the main limitation, related to the level of residual doping in an epitaxial layer, is discussed.

A theoretical model evaluating the angular distribution of luminescence emission in X-ray scintillating screens

The aim of this study was to examine the angular distribution of the light emitted from radiation-excited scintillators in medical imaging detectors. This distribution diverges from Lambert's cosine law and affects the light emission efficiency of scintillators, hence it also affects the dose burden to the patient. In the present study, the angular distribution was theoretically modeled and was used to fit experimental data on various scintillator materials. Results of calculations revealed that the angular distribution is more directional than that predicted by Lambert's law. Divergence from this law is more pronounced for high values of light attenuation coefficient and thick scintillator layers (screens). This type of divergence reduces light emission efficiency and hence it increases the incident X-ray flux required for a given level of image brightness.

On the relationship between the GLRT and UMPI tests for the detection of signals with unknown parameters

The generalized likelihood ratio test (GLRT) is widely used in signal processing applications such as image processing, wireless communications, medical imaging, classification, and signal detection. However, the GLRT does not have many known properties, other than that it is invariant, uniformly most powerful invariant (UMPI) for problems that fit the linear model, and asymptotically (N/spl rarr//spl infin/) UMPI in general. Since it is invariant, it belongs to the class of tests for which the UMPI test is optimal. In this paper, we consider a general class of detection problems in which unknown signal parameters imply a problem invariance that can be described analytically by orthogonal subgroups. This invariance is natural for problems with unknown signal parameters and, for example, include those of the matched subspace detectors of Scharf and Friedlander. We derive the GLRT and UMPI detectors for this general signal class for the case of Gaussian noise. An expression is found that relates the two test statistics showing the UMPI statistic to be the sum of two terms, one of which is the GLRT. Using this, we find that the GLRT and UMPI tests are asymptotically equivalent as signal-to-noise ratio (SNR) approaches infinity (or as probability of false alarm approaches zero). These results are illustrated by extending an example given by Nicolls and de Jager to show the analytic relationship between the GLRT and UMPI tests. The results indicate that the performance between the tests becomes close at signal-to-noise ratios (SNRs) associated with operating points of the receiver operating curve that are typically of interest in signal detection applications.

HgI/sub 2/ detector with a virtual Frisch ring

We investigated the performance of the modification of a Frisch-ring device design applied to the bar shaped HgI/sub 2/ detector. In this device, the ring electrode, separated from the detector surface by thin insulating layer, is extended to cove the entire area of side surfaces of the crystal, and connected to the cathode. To fabricate this device the side surfaces of 3 /spl times/ 3 /spl times/ 6 mm/sup 3/ HgI/sub 2/ crystals were wrapped in a Cu foil separated from the crystal by a thin layer of a Teflon tape. These bar-shaped HgI/sub 2/ detectors can be used in large-area high-efficiency detector arrays that are in demand for medical imaging and nuclear materials detection. We obtained an energy resolution of /spl sim/5% FWHM at 511 keV with our tested detectors. This is a very good result for the HgI/sub 2/ detector of such thickness. However, this resolution is far away from the theoretical limit predicted for HgI/sub 2/ material. This paper analyzes several factors affecting the energy resolution of this type of the virtual Frisch-grid devices, of which the fluctuation of charge loss due to electron trapping by local defects with high concentration of traps seems is a dominant factor.