ERASMUS+ PROGRAM Phosphor-based X-ray detectors: Basic principles in medical imaging Panagiotis Liaparinos Dpt: Biomedical Engineering, TEI of Athens, Greece

Dpt: Biomedical Engineering, Technological Educational Institute of Athens, Greece

Radiography

Basic principle in radiology

Exponential law of X-ray attenuation : Mass attenuation coefficient : Path length (material thickness) : Incident number of X-rays

It depends on the: 1. Material compound 2. X-ray energy Attenuation coefficients Energy (keV) μ (mm -1 ) bone muscle fat

For every X-ray energy For every X-ray energy For every chemical compound For every chemical compound Determination of mass attenuation coefficients NIST XCOM: Photon Cross Sections Database Evaluation of the X-rays absorbed within a material (e.g., an organ in human body) Attenuation coefficients

X-ray attenuation: X-ray interactions with matter Photoelectric effect Compton scattering Rayleigh scattering Pair production

interaction Inelastic Scattering Photoelectric absorption Excited ionPhotoelectron K-fluorescence Auger Process Electron escape x-ray escape Energetic electrons create conduction-band electrons Elastic scattering Scattered photon Electron Photon escape Incident x-ray

1895 – Announced the discovery of x-rays 1901 – Nobel price Radiography from his wife’s hand, Wilhelm Conrad Röntgen (27 March 1845 – 10 February 1923)

What are X-rays ? X-rays are Electromagnetic waves generated from electron clouds of atoms. No charge No mass Travel at the speed of light Categorized in two groups depend on the energy Low energy Non-ionizing radiation High energy----- Ionizing radiation Can cause “ionization”

Ionization Ionization Atom Electron Ionizing Radiation Ion More Reactive !! Biological Effect

Electromagnetic spectrum

Phosphor layer protective coating Optical sensor 200 μm 20 μm X-rays Detector structure

Conversion of X-rays to light quanta - First stage: Absorption of X-rays - Second stage: Conversion of X-rays to light - Third stage: Light diffusion at the output “Indirect detection”

Detector core is the phosphor material Detector Readout Engineering Design Technology MATERIAL

Scintillators study – multidisciplinary science * Synthesis * Development * Technology Optics Chemistry Solid state Physics Engineering Electronics Light collection Crystal Growth Scintillator Powder Crystals Columnar

Material choise is based on the application Material LED Microscopy Probes Microscopy Diodes MEDICAL IMAGING

Which is the optimum phosphor?  High density  Z  40  Low decay time  Low afterglow  High luminous efficiency (at suitable wavelength)  Industrial growth (e.g., large area detectors)

year Scintillation discovery CaWO 4 ZnS:Ag NaI:Tl CdWO 4 CsI:Tl CsF CsI LiI:Eu Si glass:Ce CaF 2 :Eu ZnO:Ga CdS:In CsI:Na BaF 2 slow Bi 4 Ge 3 O 12 YAlO3:Ce BaF 2 fast (Y,Gd) 2 O 3 :Ce CeF 3 PbWO 4 Lu 2 SiO 5 :Ce LuAlO 3 :Ce RbGd 2 Br 7 :Ce LaCl 3 :Ce LaBr 3 :Ce LuI 3 :Ce Eu 2+ doped SrCsI 3 :Eu 2012 BaBrI:Eu 2011 Ba 2 CsI 5 :Eu 2009 SrI 2 :Eu 2007 Ce 3+ doped LuI 3 :Ce 2003 LaBr 3 :Ce 2001 LYSO:Ce 2001 LuYAP:Ce 2001 LaCl 3 :Ce 2000 LuAP:Ce 1994 LSO:Ce1982 Conventional scintillators NaI:Tl, CsI, CsI:Na, CsI:Tl

Physical properties of several phosphor materials

- powder phosphors (e.g., Gd 2 O 2 S:Tb) are employed in X- ray projection imaging as well as in portal imaging systems (radiotherapy), - columnar phosphors (e.g., CsI:Tl) layers are constructed mainly for mammography phosphor-based detectors, - ceramic phosphors (e.g., Gd 2 O 2 S:Pr) and storage phosphors (e.g., BaFBr:Eu) are applied in X-ray computed tomography systems, - scintillating crystals (e.g., Bi 4 Ge 3 O 12 :Ce, Lu 2 SiO 5 :Ce) are used in radionuclide imaging Phosphor materials in medical imaging

Phosphor materials are employed as radiation to light converters Phosphor materials are employed as radiation to light converters Phosphor under x-ray excitation Crystal under gamma excitation

(a) Granular phosphor materials (b) Columnar phosphor materials Phosphor materials in several forms (c) Scintillating crystals

Development of a phosphor layer

Binder material (e.g., Na 2 SiO 3 )Powder material Development of a phosphor layer

Segmentation columnHeating the samples Powder screens

Radiographic cassette

Digital detectors

Phosphor layer protective coating Optical sensor 200 μm 20 μm X-rays Detector structure

Optical sensors – digital detectors Amorphous Silicon Devices– (Flat panel) Amorphous Silicon Devices– (Flat panel) Charge Coupled Devises - (CCD) Charge Coupled Devises - (CCD)

Α) Phosphor Α) Phosphor Β) Matrix of pixels Β) Matrix of pixels Photodiode a-Si:H Photodiode a-Si:H (TFT - ) (TFT - thin-film transistor ) Phosphora-Si:H

ADVANTAGES Flat Panel  Development of large area detectors (40Χ40 cm). CCD  Development of small pixel size (less than 100 μm)

DISADVANTAGES Both  Gradually damage from the Ionizing Radiation. Flat Panel  High values of pixel size (around 100 μm).  High level of noise. CCD  Complicated electronic circuits are required.  Slow read out.

Phosphor optical matching with optical sensors

Evaluation of the optical compatibility (matching factor) : phosphor optical spectrum : optical sensor detectability : light wavelength

Detector performance – Quality of the image The Luminescence efficiency (LE) The Luminescence efficiency (LE) LE Brightness LE Brightness The Modulation of the signal (MTF) The Modulation of the signal (MTF) MTF Resolution MTF Resolution

Material - Structural Properties: Grain size Packing density (i.e., the porosity) Material - Optical properties: Light wavelength Complex refractive index (real and imaginary part) Light interactions with phosphor grains depend on: Optical diffusion

Packing density (i.e., the porosity) Material properties Complex refractive index Real part: 1.5 – 2.0 Imaginary part: 10^-6 – 10^-5 Grain size Light wavelength 400 – 700 nm

1)Light attenuation (light extinction coefficient) 2) Decision of either light scattering or light absorption (probability of light absorption) 3) Determination of the new direction in case of scattered photon (anisotropy factor) Optical diffusion

First stage: Light attenuation * High values (light does not travel deep within the material Light extinction coefficient:

Mie scattering theory: Light absorption and scattering coefficients m abs and m sct are given by the following equations (Van de Hulst, 1957): V d is the volume density of the phosphor screen V d is the volume density of the phosphor screen A is the geometrical cross-section of the grain A is the geometrical cross-section of the grain Q ext is the efficiency factor given by (Hong Du, 2004): Q ext is the efficiency factor given by (Hong Du, 2004): (1) (2)

Mie scattering theory: x x is the size parameter are the Mie coefficients given by: and (3) m is relative complex refractive index m is relative complex refractive index

Mie scattering theory: The calculation of functions and can be carried out by the following recurrences taking into account their corresponding properties: The calculation of Riccati-Bessel functions and can be carried out by the following recurrences taking into account their corresponding properties:

Mie scattering theory: Light absorption and scattering coefficients m abs and m sct are given by the following equations (Van de Hulst, 1957): and (4) Second stage: Decision of either light scattering or absorption Probability of light absorption

Third stage: Light scattering

Henyey-Greenstein distribution The cosine of the light scattering polar angle is given below : g is the anisotropy factor For g=0 isotropic distribution For g=1 sharply forward direction The probability of angle distribution of scattered light photons for different values of anisotropy factor g.

The anisotropy factor is given by the following equation (Van de Hulst, 1957): Henyey-Greenstein distribution is the first element Mueller matrix given by: and are the complex elements of the scattering matrix (5) (6)

Henyey-Greenstein distribution The complex elements of the scattering matrix are given below (Bohren and Hoffman, 1983): The complex elements of the scattering matrix are given below (Bohren and Hoffman, 1983): The angle-dependent functions are computed by the following relations: and beginning with and (7) (8)

Literature: Books: 1) J. Beutel, H. L. Kundel, and R. L. Van Metter, Editors, Handbook of Medical Imaging. Vol. 1: Physics and Psychophysics, (SPIE, Bellingham, WA, 2000). 2) Bohren C F and Huffman D R Absorption and Scattering of Light by Small Particles, (Wiley, New York, 1983). 3) Van de Hulst H C Light Scattering by Small Particles (Wiley, New York, 1957). Articles: 1.) C. W. E. van Eijk, “Inorganic scintillators in medical imaging,” Phys Med. Biol. 47, R85-R106 (2002). 2.) P. R. Granfors, D. Albagli, “Scintillator-based flat-panel x-ray imaging detectors,” J Soc. Inf. Display. 17, (2009). 3. M. J. Yaffe and J. A Rowlands, “X-ray detectors for digital radiography,” Phys. Med. Biol. 42, 1-39 (1997). 4.) P. F. Liaparinos, ‘Optical diffusion performance of nanophosphor-based materials for use in medical imaging’, Journal of Biomedical Optics, 17, (2012).

Number of X-ray absorbed Number of X-ray absorbed Number of light emitted Number of light emitted X-ray beam X-rays: 100 X-ray energy: 15 keV Phosphor material Gd 2 O 2 S:Tb Thickness: 0.1 g/cm 2 Phosphor Evaluation

: Mass attenuation coefficient : Path length (material thickness) : Incident number of X-rays Evaluation of the X-rays absorbed by the phosphor

For every X-ray energy For every X-ray energy For every chemical compound For every chemical compound Determination of mass attenuation coefficients NIST XCOM: Photon Cross Sections DatabasePurpose: Evaluation of the X-rays absorbed by the phosphor

Therefore Thickness: x= 0.1 g/cm 2 Evaluation of the X-rays absorbed by the phosphor I= 45 (x-rays) 55 X-rays were absorbed

EAE is the total X-ray energy absorbed nc is the intrinsic conversion efficiency of the phosphor TE is the light transmission efficiency of the phosphor as is the matching factor Evaluation of the optical photons emitted and detected

EAE=55 * 15 keV = 825 keV = MeV Evaluation of the X-ray energy absorbed

nc=60000 light photons / MeV (next table) Determination of the optical parameters TE=0.80 (approximately) (depends on mext and g optical parameters) *see example final slide – Mie calculator TE is the light transmission efficiency of the phosphor nc is the intrinsic conversion efficiency of the phosphor

EAE=55 * 15 keV = 825 keV = MeV nc=60000 light photons / MeV (next table) TE=0.80 (approximately) (depends on mext and g optical parameters) *see example final slide – Mie calculator Evaluation of the light emitted Light emitted: (i.e., * *0.80)

Phosphor material: S P (λ) Optical sensor: S D (λ) Evaluation of the matching factor

Evaluation of the optical compatibility (matching factor) : phosphor optical spectrum : optical sensor detectability : light wavelength

Phosphor material: S P (λ) Optical sensor: S D (λ) Evaluation of the matching factor

Evaluation of the optical photons detected by the optical sensor LE: (light emitted) as: 0.94 LD= (light detected)

Light attenuation coefficient m ext Light attenuation coefficient m ext Optical anisotropy factor g Optical anisotropy factor g Evaluation of the optical parameters Mie calculator Reference: Prahl S A 2006 Mie Scattering Calculator. Portland, OR, Oregon Medical Laser Center.