Medical Physics

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Browsing Medical Physics by Author "Ghoorun, Shivani"

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    The influence of reconstruction and attenuation correction techniques on the detection of hypoperfused lesions in brain SPECT images
    (Stellenbosch : Stellenbosch University, 2004-04) Ghoorun, Shivani; Groenewald, W. A.; Nuyts, J.; Stellenbosch University. Faculty of Medicine and Health Sciences. Dept. of Medical Imaging and Clinical Oncology. Medical Physics.
    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.