Qualitative and Quantitative Evaluation of BlobBa

Authors: Eva L Leemans Fotis Kotasidis Michael Wissmeyer Valentina Garibotto Habib Zaidi

Publish Date: 2015/01/30

Volume: 17, Issue: 5, Pages: 704-713

PDF Link

Abstract

Many neurological diseases affect small structures in the brain and as such reliable visual evaluation and accurate quantification are required Recent technological developments made the clinical use of hybrid positron emission tomography/magnetic resonance PET/MR systems possible providing both functional and anatomical information in a single imaging session Nevertheless there is a tradeoff between spatial resolution and image quality contrast and noise which is dictated mainly by the chosen acquisition and reconstruction protocols Image reconstruction algorithms using spherical symmetric basis functions blobs for image representation have a number of additional parameters that impact both the qualitative and quantitative image characteristics Hence a detailed investigation of the blobbased reconstruction characteristics using different parameters is needed to achieve optimal reconstruction resultsThis work evaluated the impact of a range of blob parameters on image quality and quantitative accuracy of brain PET images acquired on the Ingenuity TimeofFlight TOF PET/MR system Two different phantoms were used to simulate brain imaging applications Image contrast and noise characteristics were assessed using an image quality phantom Quantitative performance in a clinical setting was investigated using the Hoffman 3D brain phantom at various count levels Furthermore the visual quality of four clinical studies was scored blindly by two experienced physicians to qualitatively evaluate the influence of different reconstruction protocols hereby providing indications on parameters producing the best image qualityQuantitative evaluation using the image quality phantom showed that larger basis function radii result in lower contrast recovery ∼2  and lower variance levels ∼15  The brain phantom and clinical studies confirmed these observations since lower contrast was seen between anatomical structures High and low count statistics gave comparable values The qualitative evaluation of the clinical studies based on the assessment performed by the physicians showed a preference towards lower image variance levels with a slightly lower contrast favoring higher radii and four iterationsThis study systematically evaluated a number of basis function parameters and their quantitative and qualitative effect within PET image reconstruction in the context of brain imaging A range of blob parameters can minimize error and provided optimal image quality where the anatomical structures could be recognized but the exact delineation of these structures is simplified in scans with lower image variance levels and thus higher radii should be preferred With the optimization of blob parameters the reconstructed images were found to be qualitatively improved optimum parameters d = 20375 alpha = 104101 radius = 39451 as assessed by the physicians compared to the current clinical protocol However this qualitative improvement is task specific depending on the desired image characteristics to be extracted Finally this work could be used as a guide for applicationspecific optimal parameter selectionA number of neurological diseases such as epilepsy dementia or brain tumors produce important alterations in small brain structures that are frequently not detectable using conventional anatomical imaging techniques 1 Both magnetic resonance imaging MRI and xray computed tomography CT imaging provide detailed anatomic information with CT being faster easily accessible and able to visualize bone structures however MRI has several advantages over CT including the higher soft tissue contrast of MRI and the absence of radiation exposure in addition to providing useful functional information 2 3 Positron emission tomography PET imaging provides biochemical and molecular information at the cellular level Therefore hybrid or fusion PET/MR imaging could become the de facto standard procedure offering unique capabilities for the clinical neuroimaging community and neuroscience research at large 4The recent development of MRcompatible PET components made hybrid PET/MR systems a reality These systems facilitate coregistration of structural and functional images and enable simultaneous in vivo assessment of multimodality imaging probes Such acquisition protocols potentially create a more convenient workflow for the patients as they undergo two examinations within a single scanning session Moreover multiple studies have shown potential clinical benefits of such hybrid imaging protocols such as improved diagnostic accuracy 2 3 4 However the actual combination of PET and MRI faces two major technical challenges namely reducing the potential interference between the two systems and developing reliable and robust MRbased attenuation correction MRAC schemes 5 6Throughout this work the Philips Ingenuity TOF PET/MR system combining the Gemini TOF PET and the Achieva 3T Xseries MRI scanners and arranged in a tandem geometry with a ∼3 m physical separation was used allowing sequential acquisition of PET and MR images To minimize the interference between the two components additional shielding of the photodetectors was used MRIbased attenuation correction is performed through intensitybased tissue segmentation and classification into three categories air lung and soft tissue 7 Combination of these modalities in a single device could alter image characteristics such as spatial resolution due to the magnetic field or bias due to the MR derived attenuation correction 8 However previous studies using phantom and clinical data have shown that the performance of the PETsubsystem was comparable to the Gemini TOF PETCT system 9

Keywords: