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The Essential Physics Of Medical Imaging

Scintillation materials and detectors that are used in many applications, such as medical imaging, security, oil-logging, high energy physics and non-destructive inspection, are reviewed. The fundamental physics understood today is explained, and common scintillators and scintillation detectors are introduced. The properties explained here are light yield, energy non-proportionality, emission wavelength, energy resolution, decay time, effective atomic number and timing resolution. For further understanding, the emission mechanisms of scintillator materials are also introduced. Furthermore, unresolved problems in scintillation phenomenon are considered, and my recent interpretations are discussed. These topics include positive hysteresis, the co-doping of non-luminescent ions, the introduction of an aimed impurity phase, the excitation density effect and the complementary relationship between scintillators and storage phosphors.

https://www.jstage.jst.go.jp/article/pjab/94/2/94_PJA9402B-02/_pdf

Inorganic scintillating materials and scintillation detectors.

The quest for the ideal inorganic scintillator

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Rare-Earth Doping in Nanostructured Inorganic Materials.

The aim of the current work was to investigate the modulation transfer function (MTF) of a non-destructive testing (NDT)/ industrial inspection CMOS sensor in conjunction with a calcium tungstate (CaWO4) thin screen, following both the IEC 62220-1:2003 and IEC 62220-1-1:2015 methods. Thin screen samples, with dimensions of 2.7x3.6 cm2 and mean coating thickness of 36.26 mg/cm2 (actual thickness: 118.9 μm estimated from scanning electron microscopy-SEM images) were extracted from an Agfa Curix universal screen and were manually coupled to the active area of a high resolution, active pixel (APS), complementary metal oxide semiconductor (CMOS) sensor. Experiments were performed using the RQA-5 beam quality, as described in the IEC series. Modulation transfer function was assessed using the slanted-edge method. The final MTF, following IEC 62220-1-1:2015 was obtained through averaging the oversampled edge spread function (ESF), using a custom-made software developed in our laboratory. MTF values were found with close agreement in the low and medium spatial frequency ranges, for both methods. Thereafter, MTFs calculated following the 62220-1:2003 protocol, were found overestimated for spatial frequencies higher than 5.7 cycles/mm. The combination of the thin calcium tungstate screen and the CMOS sensor provided very promising image resolution properties and thus could be also considered for use in CMOS based X-ray imaging devices, for various applications.

Towards the enhancement of medical imaging with non-destructive testing (NDT) CMOS sensors. Evaluation following IEC 62220-1-1:2015 international standard

Light Emission Efficiency of (GOS:Eu) Powder Screens Under X-Ray Mammography Conditions

The aim of this work is to provide a comparative evaluation of luminescence efficiency of Lu 2 O 3 :Eu phosphors screens, in transmission mode, prepared with different grain shape and size, and manufactured by the sedimentation method. More specific, three screens were prepared with spherical grains of size 50 nm, 200 nm and 5 μm. Furthermore, two screens with rod-like shape grains and size of 500 nm and 1–8 μm. The behavior of Absolute Luminescence Efficiency (AE) at low energy X-rays (50 kVp) appeared to differ with the grains size and shape. Furthermore the rod-like grain screens appeared with reduced luminescence efficiency values.

Grains size and shape dependence of luminescence efficiency of Lu 2 O 3 :Eu thin screens

Breast cancer may manifest as microcalcifications (μCs) in X-ray mammography. However, the detection and visualization of μCs are often obscured by the overlapping tissue structures. Dual-Energy imaging technique offers an alternative approach for imaging and visualizing μCs. With this technique, a high- and a low-energy image is acquired and their differences are used to “cancel” out the background tissue structures. In this report, various combinations of maximum tube voltages, 23-30kVp for low energy and 50- 60kVp for high energy, and filter thicknesses were examined in order to obtain quasi-monochromatic spectra. The filters applied to Tungsten (W) spectra were selected according to their K-edge (K-edge filtering). The impact of these imaging parameters on the calcification signal-to-noise ratio (SNR) for a fixed entrance exposure was studied. The optimization of the study was accomplished by the maximization of the calcification SNR (SNRtc ≥ 3) and the minimization of the coefficient of variation of the incident photons, for the minimum μC size that can be detected. The best results were obtained from filtered spectra at 30kVp and 60kVp for low and high energy respectively. These spectra were filtered with 300 μm Cd and 800 μm Sm with mean energy values 23.9 and 42.2 keV for low and high energy respectively.

Calcification Detection Optimization in Dual Energy Mammography: Influence of the X-Ray Spectra

Single crystal scintilators are used in various radiation detectors applications. The efficiency of the crystal can be determined by the Detector Optical Gain (DOG) defined as the ratio of the emitted optical photon flux over the incident radiation photons flux. A parameter affecting DOG is the intrinsic conversion efficiency (nC) giving the percentage of the X-ray photon power converted to optical photon power. nC is considered a constant value for X-ray energies in the order of keV although a non-proportional behavior has been reported. In this work an analytical model, has been utilized to single crystals scintillators GSO:Ce, LSO:Ce and LYSO:Ce to examine whether the intrinsic conversion efficiency shows non proportional behavior under X-ray excitation. DOG was theoretically calculated as a function of the incident X-ray spectrum, the X-ray absorption efficiency, the energy of the produced optical photons and the light transmission efficiency. The theoretical DOG values were compared with experimental data obtained by irradiating the crystals with X-rays at tube voltages from 50 to 140 kV and by measuring the light energy flux emitted from the irradiated screen. An initial value for nC (calculated from literature data) was assumed for the X-ray tube voltage of 50 kV. For higher X-ray tube voltages the optical photon propagation phenomena was assumed constant and any deviations between experimental and theoretical data were associated with changes in the intrinsic conversion efficiency. The experimental errors were below 7% for each experimental setup. The behavior of nC values for LSO:Ce and LYSO:Ce were found very similar, i.e., ranging with values from 0.089 at 50 kV to 0.015 at 140 kV, while for GSO:Ce, nC demonstrated a peak at 80 kV.

George Fountos

Papers

Towards the enhancement of medical imaging with non-destructive testing (NDT) CMOS sensors. Evaluation following IEC 62220-1-1:2015 international standard

Light Emission Efficiency of (GOS:Eu) Powder Screens Under X-Ray Mammography Conditions

Grains size and shape dependence of luminescence efficiency of Lu 2 O 3 :Eu thin screens

Calcification Detection Optimization in Dual Energy Mammography: Influence of the X-Ray Spectra

Studying the energy dependence of intrinsic conversion efficiency of single crystal scintillators under X-ray excitation