The influence of reconstruction and attenuation correction techniques on the detection of hypoperfused lesions in brain SPECT images

Ghoorun, Shivani (2004-04)

Thesis (MScMedSc)--Stellenbosch University, 2004.

Thesis

ENGLISH ABSTRACT: Functional brain imaging using single photon emission computed tomography (SPECT) has widespread applications in the case of Alzheimers disease, acute stroke, transient ischaemic attacks, epilepsy, recurrent primary tumours and head trauma. Routine clinical SPECT imaging utilises uniform attenuation correction, assuming that the head has homogeneous attenuation properties and elliptical cross-sections. This method may be improved upon by using an attenuation map which more accurately represents the spatial distribution of linear attenuation coefficients in the brain. Reconstruction of the acquired projection data is generally performed using filtered backprojection (FBP). This is known to produce unwanted streak artifacts. Iterative techniques such as maximum likelihood (ML) methods have also been proposed to improve the reconstruction of tomographic data. However, long computation times have limited its use. In this investigation, the objective was to determine the influence of different attenuation correction and reconstruction techniques on the detection of hypoperfused lesions in brain SPECT images. The study was performed as two simulation experiments, formulated to decouple the effects of attenuation and reconstruction. In the first experiment, a high resolution SPECT phantom was constructed from four high resolution MRI scans by segmenting the MRI data into white matter, grey matter and cerebrospinal fluid (CSF). Appropriate intensity values were then assigned to each tissue type. A true attenuation map was generated by transposing the 511 keV photons of a PET transmission scan to 140 keV photons of SPECT. This method was selected because transmission scanning represents the gold standard for determining attenuation coefficients. The second experiment utilised an available digital phantom with the tissue classes already segmented. The primary difference between the two experiments was that in Experiment II, the attenuation map used for the creation of the phantom was clinically more realistic by using MRI data that were segmented into nine tissue classes. In this case, attenuation coefficients were assigned to each tissue class to create a nonuniform attenuation map. A uniform attenuation map was generated on the basis of emission projections for both experiments. Hypo-perfused lesions of varying intensities and sizes were added to the phantom. The phantom was then projected as typical SPECT projection data, taking into account attenuation and collimator blurring with the addition of Poisson noise. Each experiment employed four methods of reconstruction: (1) FBP with the uniform attenuation map; (2) FBP using the true attenuation map; (3) ML method with a uniform attenuation map; and (4) ML method with a true attenuation map. In the case of FBP methods, Chang’s first order attenuation correction was used. The analysis of the reconstructed data was performed using figures of merit such as signal to noise ratio (SNR), bias and variance. The results illustrated that uniform attenuation correction offered slight deterioration (less than 2 %) with regard to detection of lesions when compared to the ideal attenuation map, which in reality is not known. The reconstructions demonstrated that FBP methods underestimated the activity by more than 30% when compared to the true image. The iterative techniques produced superior signal to noise ratios in comparison to the FBP methods, provided that postsmoothing was applied to the data. The results also showed that the iterative methods produced lower bias at the same variance. This leads to the conclusion, that in the case of brain SPECT imaging, uniform attenuation correction is adequate for lesion detection. In addition, iterative reconstruction techniques provide enhanced lesion detection when compared to filtered backprojection methods.

AFRIKAANSE OPSOMMING: Funksionele breinbeelding deur middel van Enkel Foton Emissie Rekenaartomografie (SPECT - Single Photon Emission Computed Tomography) het veelvuldige toepassings in die geval van Alzheimer se siekte, akute beroerte, kortstondige isgemiese aanvalle, epilepsie, hervatting van primere tumore en hoofbeserings. Roetine kliniese SPECT-beelding gebruik uniforme attenuasie korreksies met die aanname dat die kop homogene attenuasie eienskappe en elliptiese dwarssnitte het. Hierdie metode kan verbeter word deur die gebruik van ‘n attenuasiekaart wat ‘n akkurater weergawe van die ruimtelike verspreiding van lineere attenuasie koeffisiente in die brein verteenwoordig. Rekonstruksie van die ingesamelde projeksiedata word gewoonlik uitgevoer deur gebruik te maak van Gefiltreerde Terugprojeksie (FBP - Filtered Backprojection). Dit is bekend dat hierdie tegniek ongewenste streep artefakte veroorsaak. Iteratiewe tegnieke soos maksimum waarskynlikheid (ML - Maximum Likelihood) metodes is ook voorgestel om die rekonstruksie van tomografiese data te verbeter. Lang berekeningstye het tot dusver die gebruik van hierdie tegnieke beperk. Die doelstelling van hierdie ondersoek was om die invloed van verskillende attenuasie korreksie en rekonstruksie tegnieke op letsels met hipo-perfusie in brein SPECT beelde te bepaal. Die ondersoek is uitgevoer in die vorm van twee simulasie eksperimente, en is geformuleer om die effekte van attenuasie en rekonstruksie te ontkoppel. In die eerste eksperiment is ‘n hoe resolusie SPECT fantoom uit vier hoe resolusie MRI (Magnetic Resonance Imaging) beelde gekonstrueer deur die MRI data in wit stof, grys stof en CSF (Cerebrospinal Fluid) te segmenteer. Geskikte intensiteitswaardes is aan elke weefseltipe toegeken. ‘n Ware attenuasiekaart is geskep deur die 511 keV fotone van ‘n PET (Positron Emission Tomography) transmissie opname na 140 keV fotone van SPECT te transponeer. Hierdie metode is gekies aangesien transmissie skandering die goue standaard vir die bepaling van attenuasie koeffisiente verteenwoordig. Die tweede eksperiment het ‘n beskikbare digitale fantoom gebruik met die weefsel soorte reeds gesegmenteer. Die primere verskil tussen die twee eksperimente was dat die attenuasiekaart gebruik in eksperiment II klinies meer realisties was, aangesien MRI data gebruik is wat reeds in nege weefselsoorte gesegmenteer is. Attenuasie koeffisiente is aan elke weefselsoort toegeken om ‘n nie-uniforme attenuasiekaart saam te stel. ‘n Uniforme attenuasiekaart gebaseer op die emissie projeksies vir beide eksperimente is saamgestel. Hipo-perfusie letsels met verskillende intensiteite en groottes is by die fantoom gevoeg. Die fantoom is daama geprojekteer as tipiese SPECT projeksiedata met inagneming van attenuasie en kollimator versluiering met die toevoeging van Poisson geraas. Elke eksperiment het vier metodes van rekonstruksie gebruik: (1) FBP met die uniforme attenuasiekaart; (2) FBP met gebruik van die ware attenuasiekaart; (3) die ML-metode met ‘n uniforme attenuasiekaart; en (4) die ML-metode met ‘n ware attenuasiekaart. Chang se eerste orde attenuasie korreksie is in die geval van die FBP-metodes gebruik. Die ontleding van die gerekonstrueerde data is gedoen deur verdienstesyfers soos sein-tot-geraas verhouding (SNR), sydigheid en variansie te gebruik. Die resultate toon dat die uniforme attenuasiekorreksie ‘n geringe verswakking (minder as 2 %) gee met betrekking tot die opsporing van letsels wanneer ‘n vergelyking met die ideale attenuasiekaart, wat nie bekend is nie, getref word. Die rekonstruksies demonstreer dat die FBP metodes die aktiwiteit met meer as 30% onderskat in vergelyking met die ware beeld. Die iteratiewe tegnieke het uitstekende sein-tot-geraas verhoudings gelewer in vergelyking met die FBP-metodes op voorwaarde dat na-vergladding op die data toegepas is. Die resultate het ook getoon dat die iteratiewe metodes laer sydigheid by dieselfde variansie lewer. Die slotsom is dat, in die geval van brein SPECT beelding, uniforme attenuasiekorreksie voldoende is vir letselopsporing. Die iteratiewe rekonstruksie tegnieke bied verder verbeterde letselopsporing in vergelyking met gefiltreerde terugprojeksie metodes.

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