| Summary: | Thesis (MSc)--Stellenbosch University, 2000. === ENGLISH ABSTRACT: A digital portal X-ray imaging system was developed to replace the radiographic X-ray films
currently used for patient position verification at the National Accelerator Centre (NAC)
proton therapy facility. The main advantage of a digital system is the short time in which the
image can be obtained. Other advantages include optimisation of the image display, effective
archiving of the digital images, access from various locations through data networks, and
lower operational costs. The digital system described in this thesis consists of a Gd202S:Tb
scintillator screen for converting X-rays to visible light, a protected aluminum front silvered
mirror to direct the light to a Charge Coupled Device (CCD) camera for capture and a
personal computer for data acquisition, processing and display. Compared with other digital
imaging systems, this is a simple, compact and affordable system.
The properties of the various components were investigated. The Rarex G-130 (Gd202S:Tb)
scintillation screen was chosen for its good spatial resolution, high emission efficiency and
good matching between the spectral emission wavelength peak and the quantum efficiency of
the CCD camera. The spatial resolution measured for the system with a field of view (FOV)
of 290 x 190 mnr' is 1.3 lp/mm, which can be improved by increasing the CCD chip
resolution or decreasing the field of view, since the CCD camera limits the spatial resolution.
Intrinsic detector noise determines the lower limit of the dynamic range of the detector and is
reduced by cooling the CCD camera. A dark current exposure is subtracted from the image to
remove the bias signal and background signal level mainly caused by thermal noise. Photon
noise, beam in-homogeneity and efficiency variations across the CCD chip are removed by a
flat field correction. The digital images obtained with this system compare very well with the
currently used radiographic film images and they are satisfactory for the purpose of patient
position verification. Using the digital system it is possible to reduce the patient dose by 19 %
and still obtain satisfactory image quality. === AFRIKAANSE OPSOMMING: 'n Digitale X-straalafbeeldingstelsel is ontwikkel om die radiografiese X-straalfilm wat tans
gebruik word vir die kontrolering van die pasientposisionering voor die toediening van
protonterapie by die Nasionale Versnellersentrum, te vervang. Die voordeel van die digitale
sisteem is dat die beelde feitlik onmiddellik beskikbaar is. Verdere voordele sluit die
optimisering van die vertoon van beeldkontras, effektiewe liassering, vinnige bereik deur
datanetwerke en lae lopende kostes in. Die digitale sisteem beskryf in die tesis bestaan uit 'n
gadolinium oksi-sulfied (Gd202S:Tb) sintillasieskerm wat X-strale omskakel na sigbare lig, 'n
eerste-oppervlak aluminiumspieël wat die lig na 'n digitale kamera (CCD kamera) weerkaats
en In persoonlike rekenaar vir dataverwerwing, verwerking en vertoon. Vergeleke met ander
digitale stelsels is hierdie digitale beeldingstelsel eenvoudig, kompak en bekostigbaar.
Die eienskappe van die verskillende komponente van die stelsel is ondersoek. Die Rarex G-
130 (Gd202S:Tb) sintillasieskerm IS gekies vanweë goeie resolusie, hoë
emissiedoeltreffendheid en die hoë omsettingsdoeltreffendheid van die digitale kamera by die
spektrale emissiegolflengte van dié sintillasieskerm. Die ruimtelike oplosvermoë van die
stelsel is bepaal met In veldgrootte van 290 x 190 mnr' as 1.3 lynpare per millimeter. Die
ruimtelike oplosvermoë kan verhoog word deur die kameraresolusie te verhoog of die
veldgrootte te verklein, omdat die resolusie van die kamera tans die oplosvermoë van die
stelsel beperk. Intrinsieke ruis van die detektor beperk die onderste grens van die dinamiese
reikwydte van die detektor en kan verminder word deur die kamera te verkoel. 'n
Donkerstroom-beeld word van die X-straalbeelde afgetrek om die voorspanningsein en die
agtergrondsein, wat hoofsaaklik veroorsaak word deur termiese ruis, te verwyder. Ruis wat
ontstaan as gevolg van fluktuasies in die aantal fotone, nie-homogeniteite in die bundel of
variasie van die sensitiwiteit in die skerm word verwyder met behulp van 'n plat vlak beeld.
Die digitale beelde verkry met die stelsel vergelyk goed met die beelde wat tans op film
geneem word en die beeldkwalitiet is voldoende vir die kontrolering van die pasientopstelling.
Dit is moontlik om die pasiëntdosis met 19 % te verminder en steeds voldoende
beeldkwaliteit te verkry.
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