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

[摘要] 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, assumingthat the head has homogeneous attenuation properties and elliptical cross-sections.This method may be improved upon by using an attenuation map which moreaccurately represents the spatial distribution of linear attenuation coefficients in thebrain.Reconstruction of the acquired projection data is generally performed using filteredbackprojection (FBP). This is known to produce unwanted streak artifacts. Iterativetechniques such as maximum likelihood (ML) methods have also been proposed toimprove the reconstruction of tomographic data. However, long computation timeshave limited its use.In this investigation, the objective was to determine the influence of differentattenuation correction and reconstruction techniques on the detection of hypoperfusedlesions in brain SPECT images.The study was performed as two simulation experiments, formulated to decouple theeffects of attenuation and reconstruction. In the first experiment, a high resolutionSPECT phantom was constructed from four high resolution MRI scans by segmentingthe 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 PETtransmission scan to 140 keV photons of SPECT. This method was selected becausetransmission scanning represents the gold standard for determining attenuationcoefficients.The second experiment utilised an available digital phantom with the tissue classesalready segmented. The primary difference between the two experiments was that inExperiment II, the attenuation map used for the creation of the phantom was clinicallymore realistic by using MRI data that were segmented into nine tissue classes. In thiscase, attenuation coefficients were assigned to each tissue class to create a nonuniformattenuation map. A uniform attenuation map was generated on the basis ofemission 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 intoaccount attenuation and collimator blurring with the addition of Poisson noise.Each experiment employed four methods of reconstruction: (1) FBP with the uniformattenuation map; (2) FBP using the true attenuation map; (3) ML method with auniform attenuation map; and (4) ML method with a true attenuation map. In the caseof FBP methods, Chang's first order attenuation correction was used.The analysis of the reconstructed data was performed using figures of merit such assignal to noise ratio (SNR), bias and variance. The results illustrated that uniformattenuation correction offered slight deterioration (less than 2 %) with regard to detection of lesions when compared to the ideal attenuation map, which in reality isnot known.The reconstructions demonstrated that FBP methods underestimated the activity bymore than 30% when compared to the true image. The iterative techniques producedsuperior signal to noise ratios in comparison to the FBP methods, provided thatpostsmoothing was applied to the data. The results also showed that the iterativemethods produced lower bias at the same variance.This leads to the conclusion, that in the case of brain SPECT imaging, uniformattenuation correction is adequate for lesion detection. In addition, iterativereconstruction techniques provide enhanced lesion detection when compared tofiltered backprojection methods.

[发布机构] Stellenbosch University