A computerized tomography (CT) scan combines a series of X-ray images taken from different angles around your body and uses computer processing to create cross-sectional images (slices) of the bones, blood vessels and soft tissues inside your body. CT scan images provide more-detailed information than plain X-rays do.

A CT scan has many uses, but it’s particularly well-suited to quickly examine people who may have internal injuries from car accidents or other types of trauma. A CT scan can be used to visualize nearly all parts of the body and is used to diagnose disease or injury as well as to plan medical, surgical or radiation treatment.

The workflow of signal detection and processing in CT is shown in a simplified form in Fig. 1. In the X-ray CT process chain, the detection step is carried out by means of physical detectors, which capture the attenuated X-ray beam and convert it into electrical signals, which are subsequently converted into binary coded information. As depicted in Fig. 1, after several processing steps, the results are available for final visualisation and interpretation on a computer.

In conventional attenuation-based CT, the X-rays are attenuated due to absorption or scattering during their travel through the material. The attenuation is then measured by capturing the attenuated X-rays using an X-ray detector. Over the years, there has been a considerable improvement in the detection technology. The detector captures the X-ray beam and converts it into electrical signals. This conversion of X-ray energy to electrical signals is primarily based on two principles: gas ionisation detectors and scintillation (solid-state) detectors. A schematic representation of both detection principles is shown in Fig. 2. Gas ionisation detectors convert X-ray energy directly into electrical energy, whereas scintillation detectors convert X-rays into visible light and then the light is converted into electrical energy;

When the X-rays strike the scintillator crystals, they are converted into long-wave radiation (visible light) within the scintillation media. The light is then directed to an electronic light sensor such as a photomultiplier tube. The photomultiplier absorbs the light emitted by the scintillator and ejects electrons via the photoelectric effect. Electrons are released when the light strikes the photocathode and then the electrons cascade through a series of dynodes, maintained at different potentials, to result in an output signal.

The choice of scintillator material is critical and depends on the desired quantum efficiency for the X-ray-to-light conversion, and on the time constant for the conversion process, which determines the “afterglow” of the detector (i.e. the persistence of the image even after turning off the radiation, as described in the following section). Modern sub-second scanners require a very small time constant (or very fast fluorescence decay). Typical materials for scintillators are sodium iodide doped with thallium caesium iodide cadmium tungstate (CdWO4), zinc sulphide (ZnS), naphthalene, anthracene and other compounds based on rare earth elements, or gadolinium oxysulfide (Gd2O2S).

Types of scintillator crystal applied in Medical CT

Ce:YAG

Wavelength(Max. emission) (nm)550
Decay time (ns)70
Light yield (photons/keV)35
Refractive index1.82@550nm
Radiation length (cm)3.5

The YAP(Ce) shows better performances with respect to the YAG(Ce) in terms of maximum acquisition rate. but, the YAG(Ce) crystal was chosen because its emitted light spectrum fifits better with the sensitivity of the photodiode. The calorimeter has to work in particle accelerator treatment rooms, where the presence of a magnetic fifield could be not negligible, so the use of a photodiode in the readout system is preferable to the standard photomultiplier necessary to YAP(Ce) crystal. Moreover, using photodiodes the calorimeter results more compact and permits to reduce the sizes of whole pCT apparatus.

References
[1] Design and characterisation of a YAG(Ce) calorimeter for proton Computed Tomography application
[2] Characterization of a Silicon Strip Detector and a YAG:Ce Calorimeter for a Proton Computed Radiography Apparatus
[3] A High Speed Functional MicroCT Detector For Small Animal Studies

Ce:YAP

Wavelength(Max. emission) (nm)370
Decay time (ns)28
Light yield (photons/keV)25
Light output relative to Nal(Tl)  (%)60-70
Refractive index1.95@370nm

YAP(Ce)s are generally used as scintillators with photons in medical equipment and environmental monitoring detectors due to their high light yield, good resolution, hardness, and non-hydroscopic properties. Unfortunately, this kind of crystal has a slow decay time in scintillation light. This limits their high-frequency application. In this case people investigated the use of this crystal as a calorimeter for protons at a high rate of incoming particles, such as proton computed tomography.

References
[1] YAP(Ce) crystal characterization with proton beam up to 60 MeV[2] Mcps-range photon-counting X-ray computed tomography system utilizing an oscillating linear-YAP(Ce) photon detector
[3] Dark-count-less photon-counting X-ray computed tomography system using a YAP-MPPC detecto

CsI

Wavelength(Max. emission) (nm)410
Decay time (ns)40
Light yield (photons/keV)25
Light output relative to Nal(Tl)  (%)75
Refractive index1.82@410nm

Compared to the conventional aS1000 model, which consists of a 0.4 mm–thick layer of gadolinium oxysulfide scintillator, the CsI detector has an 8 mm–thick CsI crystal, which has a higher X-ray absorption efficiency.

References
[1] A High Speed Functional MicroCT Detector For Small Animal Studies

Ce:LYSO

Wavelength(Max. emission) (nm)410
Decay time (ns)40
Light yield (photons/keV)25
Light output relative to Nal(Tl)  (%)75
Refractive index1.82@410nm

LYSO:Ce is single crystal non-hy-groscopic scintillators of high density, high light yield and short decay time, which has been successfully used in medical CT imagers.

References
[1] YAP(Ce) crystal characterization with proton beam up to 60 MeV
[2] Luminescence Emission Properties of (Lu; Y)2SiO5:Ce (LYSO:Ce) and (Lu; Y)AlO3:Ce (LuYAP:Ce) Single Crystal Scintillators Under Medical Imaging Conditions
[3] A comparative study of the luminescence properties of LYSO:Ce, LSO:Ce, GSO:Ce and BGO single crystal scintillators for use in medical X-ray imaging

Ce:LuAG

Wavelength (Max. Emission) (nm)535
Wavelength range (nm)475-800
Decay time (ns)70
Light yield (photons/keV)25
Refractive index1.84@633nm

High detection efficiency can be achieved by using materials with high density and high atomic number. Fast 5d-to-4f luminescence of Ce3+ with distinctive decay time of 20–60 ns is usually exploited to provide an efficient emission atroom temperature. Ce-doped Lu3Al5O12 (LuAG:Ce) is a well-known scintillator that exhibits desirable physical and scintillation properties.

References
[1] Crystal growth and characterization of LuAG:Ce:Tb scintillator
[2] Epitaxial growth of LuAG:Ce and LuAG:Ce,Pr films and their scintillation properties

BGO

Wavelength(Max. Emission) (nm)480
Decay time (ns)300
Light yield (photons/keV)8~10
Refractive index2.15
Energy resolution (%)12

Bismuth germanate (BGO :Bi4Ge3O12) has high X-ray stopping power, but its relatively low scintillation efficiency[8-16% of NaI(TI)] and its long scintillation decay time (300 ns) require efficient optical coupling to photomultiplier tubes in order to obtain good time resolution and reasonable enegy resolution.

References

[1] A QUAD BGO DETECTOR AND ITS TIMING AND POSITIONING DISCRIMINATION FOR POSITRON COMPUTED TOMOGRAPHY
[2] Plastic scintillation detectors for high resolution emission computed tomography.
[3] YAP(Ce) crystal characterization with proton beam up to 60 MeV

CdWO4

Wavelength(Max. emission) (nm)490
Decay time (ns)14000
Light yield (photons/keV)12~15
Light output relative to Nal(Tl) (%)50
Refractive index2.2-2.3

Single crystals of cadmium tungstate (CWO) find broad applications in X-ray computer tomography and introscopy, in spectrometric and radiometric devices, for creation of small-sized detectors with photodiodes or photoelectronic amplifiers and multielement detecting assemblies for computer tomography (CT).

References
[1] A bench-top megavoltage fan-beam CT using CdWO4-photodiode detectors. I. System description and detector characterization
[2] Production ofthe high-quality CdWO4 single crystals for application in CT and radiometric monitoring
[3] MONOLITHICALLY INTEGRATED X-RAY DETECTOR ARRAYS FOR COMPUTED TOMOGRAPHY

LaBr3

Wavelength(Max. emission) (nm)380
Decay time (us)25
Light yield(photons/keV)63
Radiation length(cm)1.881
Reflection loss/surface(%)6.8

LaBr3(Ce) – Lanthanum Bromide -is a salt compound of lanthanum and bromine and one of a new generation of inorganic scintillator based gamma radiation detectors. LaBr3(Ce) scintillators have fast light output decay times provide excellent energy resolution performance.

References
[1]A Study on Determination of an Optimized Detector for Single Photon Emission Computed Tomography

GOS

Wavelength(Max. emission) (nm)510
Decay time (us)5.5
Relative light output (%)80
Afterglow (%)< 0.01
Luminescence intensity (keV)27.5

References
[1]Improvements in the X-ray Characteristics of Gd2O2S:Pr Ceramic Scintillators