Abstract The purpose of the present study was to implement a unique low-field open magnetic resonance scanner for perioperative imaging in neurosurgery. A paradigm was created for joint intraoperative/interventional MRI, including premises, surgical practice and an operational model. The feasibility of the paradigm was tested in clinical work. The joint use of the facilities between the Departments of Neurosurgery and Diagnostic Radiology was found to enhance the economic rationale and provide for perioperative imaging. It was also found to be organizationally viable in the long run. Intraoperative MRI was implemented and studied in connection with neuronavigation and other intraoperative instruments, tools and imaging modalities. The unique shut down possibility of the magnet enabled staged operating-imaging practice, use of non-MRI-compatible instruments and devices, multimodal imaging with navigation, and avoidance of safety risks associated with operating in magnetic fringe fields. Two dynamic contrast enhanced MR imaging sequences, which used undersampled projection reconstruction, were implemented in the low-field scanner. The applicability of these imaging sequences to follow contrast enhancement of meningiomas was studied in laboratory experiments and in two patient cases. The laboratory experiments showed a nearly linear response in signal intensity to the concentration of gadopentetate dimeglumine in purified water up to 1.25 mM. The patient cases showed results consistent with an earlier study performed at high-field strength. The potential of low-field MRI study including dynamic contrast enhanced imaging to predict surgical and histopathologic characteristics of meningiomas was studied in a series of 21 patients. Dynamic contrast enhanced imaging could be used to evaluate microvessel densities of meningiomas. Surgical bleeding, blood loss during operation, progesterone receptor expression and collagen content were statistically best correlated to the relative intensity of meningioma on FLAIR images. Tissue hardness correlated best with relative intensity on T2-weighted images.

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Neuronavigation and transcranial magnetic stimulation (TMS) are valuable tools in clinical and research environment. Neuronavigation provides visual guidance of a given instrument during procedures of neurological interventions, relative to anatomic images. In turn, TMS allows the non-invasive study of cortical brain function and to treat several neurological disorders. Despite the well-accepted importance of both techniques, high-cost of neuronavigation systems and limited spatial accuracy of TMS in targeting brain structures, limit their applications. Therefore, the aim of this thesis was to i) develop an open-source, free neuronavigation software, ii) study a possible combination of neuronavigation and 3D printing for surgical planning, and iii) construct a multi-channel TMS coil with electronic control of electric field (E-field) orientation. In the first part, we developed and characterized a neuronavigation software compatible with multiple spatial tracking devices, the InVesalius Navigator. The created co-registration algorithm enabled tracking position and orientation of instruments with an intuitive graphical interface. Measured accuracy was similar to that of commercial systems. In the second part, we created 3D printed models from patients with neurological disorders and assessed the errors of localizing anatomical landmarks during neuronavigation. Localization errors were below 3 mm, considered acceptable for clinical applications. Finally, in the last part, we combined a set of two thin, overlapping coils to allow electronic control of the E-field orientation and investigated how the motor evoked responses depend on the stimulus orientation. The developed coil enabled the stimulation of the motor cortex with high angular resolution. Motor responses showed the highest amplitude and lowest latency with E-field approximately perpendicular to the central sulcus. In summary, this thesis provides new methods to improve spatial accuracy of techniques to brain interventions. / A neuronavegação e a estimulação magnética transcraniana (EMT ou TMS, do termo em inglês transcranial magnetic stimulation) têm sido apresentadas como ferramentas valiosas em aplicações clínicas e de pesquisa. A neuronavegação possibilita a localização de instrumentos em relação a imagens anatômicas durante procedimentos de intervenção neurológica. Por sua vez, a EMT permite o estudo não invasivo da função cerebral e o tratamento de doenças neurológicas. Apesar da importância de ambas as técnicas, o alto custo dos sistemas de neuronavegação e a reduzida precisão espacial da EMT em ativar estruturas cerebrais limitam suas aplicações. Sendo assim, o objetivo desta tese foi: i) desenvolver um software de neuronavegação gratuito e de código aberto, ii) estudar a combinação entre neuronavegação e impressão 3D para planejamento cirúrgico, e iii) construir uma bobina de EMT multicanal com controle eletrônico da orientação do campo elétrico (CE). Na primeira parte, desenvolvemos e caracterizamos um software de neuronavegação compatível com vários rastreadores espaciais, o InVesalius Navigator. O algoritmo criado possibilitou o rastreamento de instrumentos por uma interface gráfica intuitiva. A precisão medida foi semelhante à de sistemas comerciais. Na segunda parte, imprimimos modelos 3D de pacientes com patologias neurológicas e avaliamos os erros de localização de marcos anatômicos durante a neuronavegação. Os erros de localização foram inferiores a 3 mm, considerados aceitáveis para aplicações clínicas. Por fim, na última parte, combinamos duas bobinas sobrepostas para controlar eletronicamente a orientação do CE, e investigamos como as respostas motoras evocadas dependem da orientação da corrente. A bobina desenvolvida possibilitou estimular o córtex motor com alta resolução angular. As respostas motoras apresentaram maior amplitude e menor latência para orientação do CE aproximadamente perpendicular ao sulco central. Em suma, esta tese fornece novos métodos para melhorar a precisão espacial de técnicas de intervenção com o cérebro.

Neuronavigation and transcranial magnetic stimulation (TMS) are valuable tools in clinical and research environment. Neuronavigation provides visual guidance of a given instrument during procedures of neurological interventions, relative to anatomic images. In turn, TMS allows the non-invasive study of cortical brain function and to treat several neurological disorders. Despite the well-accepted importance of both techniques, high-cost of neuronavigation systems and limited spatial accuracy of TMS in targeting brain structures, limit their applications. Therefore, the aim of this thesis was to i) develop an open-source, free neuronavigation software, ii) study a possible combination of neuronavigation and 3D printing for surgical planning, and iii) construct a multi-channel TMS coil with electronic control of electric field (E-field) orientation. In the first part, we developed and characterized a neuronavigation software compatible with multiple spatial tracking devices, the InVesalius Navigator. The created co-registration algorithm enabled tracking position and orientation of instruments with an intuitive graphical interface. Measured accuracy was similar to that of commercial systems. In the second part, we created 3D printed models from patients with neurological disorders and assessed the errors of localizing anatomical landmarks during neuronavigation. Localization errors were below 3 mm, considered acceptable for clinical applications. Finally, in the last part, we combined a set of two thin, overlapping coils to allow electronic control of the E-field orientation and investigated how the motor evoked responses depend on the stimulus orientation. The developed coil enabled the stimulation of the motor cortex with high angular resolution. Motor responses showed the highest amplitude and lowest latency with E-field approximately perpendicular to the central sulcus. In summary, this thesis provides new methods to improve spatial accuracy of techniques to brain interventions. / A neuronavegação e a estimulação magnética transcraniana (EMT ou TMS, do termo em inglês transcranial magnetic stimulation) têm sido apresentadas como ferramentas valiosas em aplicações clínicas e de pesquisa. A neuronavegação possibilita a localização de instrumentos em relação a imagens anatômicas durante procedimentos de intervenção neurológica. Por sua vez, a EMT permite o estudo não invasivo da função cerebral e o tratamento de doenças neurológicas. Apesar da importância de ambas as técnicas, o alto custo dos sistemas de neuronavegação e a reduzida precisão espacial da EMT em ativar estruturas cerebrais limitam suas aplicações. Sendo assim, o objetivo desta tese foi: i) desenvolver um software de neuronavegação gratuito e de código aberto, ii) estudar a combinação entre neuronavegação e impressão 3D para planejamento cirúrgico, e iii) construir uma bobina de EMT multicanal com controle eletrônico da orientação do campo elétrico (CE). Na primeira parte, desenvolvemos e caracterizamos um software de neuronavegação compatível com vários rastreadores espaciais, o InVesalius Navigator. O algoritmo criado possibilitou o rastreamento de instrumentos por uma interface gráfica intuitiva. A precisão medida foi semelhante à de sistemas comerciais. Na segunda parte, imprimimos modelos 3D de pacientes com patologias neurológicas e avaliamos os erros de localização de marcos anatômicos durante a neuronavegação. Os erros de localização foram inferiores a 3 mm, considerados aceitáveis para aplicações clínicas. Por fim, na última parte, combinamos duas bobinas sobrepostas para controlar eletronicamente a orientação do CE, e investigamos como as respostas motoras evocadas dependem da orientação da corrente. A bobina desenvolvida possibilitou estimular o córtex motor com alta resolução angular. As respostas motoras apresentaram maior amplitude e menor latência para orientação do CE aproximadamente perpendicular ao sulco central. Em suma, esta tese fornece novos métodos para melhorar a precisão espacial de técnicas de intervenção com o cérebro.