Real-time motion and dynamic receiver sensitivity correction for CEST-MRI in the human brain at 7T

Chemical Exchange Saturation Transfer (CEST) is a novel Magnetic Resonance Imaging (MRI) technique that utilises exchange reactions between metabolites and tissue water to map metabolite concentration or tissue pH noninvasively. Similarly to Magnetic Resonance Spectroscopy (MRS), CEST is able to detect many endogenous metabolites, but unlike MRS, CEST is based on imaging and thus enjoys the speed of modern MR imaging. On the other hand, CEST also suffers from the same difficulties as MRI and MRS. One of the most common source of image artifacts in MRI is subject motion during imaging. Many different motion correction methods have been devised. Recently, a novel real-time motion correction system was developed for MRS. This method is based on volumetric navigators (vNav) that are performed multiple times interleaved with the parent measurement. Navigator image comparison, affine matrix calculation, and acquisition gradient correction to correct the field of view to match subject head motion are done online and in real-time. The purpose of this thesis is to implement this real-time motion correction method to CEST-MRI and study its efficacy and correction potential in phantoms and in healthy volunteers on 7T MR scanner. Additionally, it is hypothesised that the vNav images may be used to correct for motion related receiver sensitivity (B1-) inhomogeneities. Glutamate was chosen as the metabolite of interest due to it being the most abundant neurotransmitter in the human brain and due to its involvement in both normal cognitive function as well as many brain pathologies. Since glutamate has an amine group, it undergoes chemical exchange with water and is thus a usable metabolite for CEST imaging. A glutamate phantom was constructed to show the glutamate concentration sensitivity of CEST and to test and optimise the CEST sequence. Seven healthy volunteers were imaged over a period of two months. All but one volunteer were imaged more than once (2-4 times). Subjects were measured without voluntary head motion and with controlled left-right and up-down head movements. All measurements were performed with and without motion correction to test the motion and B1- -correction methods. Additionally, three volunteers were measured with a dynamic CEST experiment to assess the reproducibility of CEST. The real-time motion correction method was found to be able to correct for small, involuntary head movements. 18 % of the CEST maps measured without motion correction were found to have motion artifacts whereas the equivalent number for maps with motion correction was 0 % (4/22 maps versus 0/18 maps). Larger (>0.7◦ or >0.7 mm in one coregistration step), voluntary head movements could not be corrected adequately. The vNav images could be used to correct for B1- -inhomogeneities. This was found to improve CEST spectra quality and to remove lateral inhomogeneities from the CEST maps. The reproducibility of the CEST-MRI could not be established, however dynamic CEST measurements were found to be stable with only small contrast fluctuation of 4 % between consecutive maps due to noise

Detrended fluctuation analysis in the presurgical evaluation of parietal lobe epilepsy patients

Objective: To examine the usability of long-range temporal correlations (LRTCs) in non-invasive localization of the epileptogenic zone (EZ) in refractory parietal lobe epilepsy (RPLE) patients. Methods: We analyzed 10 RPLE patients who had presurgical MEG and underwent epilepsy surgery. We quantified LRTCs with detrended fluctuation analysis (DFA) at four frequency bands for 200 cortical regions estimated using individual source models. We correlated individually the DFA maps to the distance from the resection area and from cortical locations of interictal epileptiform discharges (IEDs). Additionally, three clinical experts inspected the DFA maps to visually assess the most likely EZ locations. Results: The DFA maps correlated with the distance to resection area in patients with type II focal cortical dysplasia (FCD) (p < 0:05), but not in other etiologies. Similarly, the DFA maps correlated with the IED locations only in the FCD II patients. Visual analysis of the DFA maps showed high interobserver agreement and accuracy in FCD patients in assigning the affected hemisphere and lobe. Conclusions: Aberrant LRTCs correlate with the resection areas and IED locations. Significance: This methodological pilot study demonstrates the feasibility of approximating cortical LRTCs from MEG that may aid in the EZ localization and provide new non-invasive insight into the presurgical evaluation of epilepsy. (c) 2021 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Real-time motion and retrospective coil sensitivity correction for CEST using volumetric navigators (vNavs) at 7T

Purpose To explore the impact of temporal motion-induced coil sensitivity changes on CEST-MRI at 7T and its correction using interleaved volumetric EPI navigators, which are applied for real-time motion correction. Methods Five healthy volunteers were scanned via CEST. A 4-fold correction pipeline allowed the mitigation of (1) motion, (2) motion-induced coil sensitivity variations, Delta B1-, (3) motion-induced static magnetic field inhomogeneities, Delta B-0, and (4) spatially varying transmit RF field fluctuations, Delta B1+. Four CEST measurements were performed per session. For the first 2, motion correction was turned OFF and then ON in absence of voluntary motion, whereas in the other 2 controlled head rotations were performed. During post-processing Delta B1- was removed additionally for the motion-corrected cases, resulting in a total of 6 scenarios to be compared. In all cases, retrospective increment B-0 and -Delta B1+ corrections were performed to compute artifact-free magnetization transfer ratio maps with asymmetric analysis (MTRasym). Results Dynamic Delta B1- correction successfully mitigated signal deviations caused by head motion. In 2 frontal lobe regions of volunteer 4, induced relative signal errors of 10.9% and 3.9% were reduced to 1.1% and 1.0% after correction. In the right frontal lobe, the motion-corrected MTRasym contrast deviated 0.92%, 1.21%, and 2.97% relative to the static case for Delta omega = 1, 2, 3 +/- 0.25 ppm. The additional application of Delta B1- correction reduced these deviations to 0.10%, 0.14%, and 0.42%. The fully corrected MTRasym values were highly consistent between measurements with and without intended head rotations. Conclusion Temporal Delta B1- cause significant CEST quantification bias. The presented correction pipeline including the proposed retrospective Delta B1- correction significantly reduced motion-related artifacts on CEST-MRI.Peer reviewe

Detrended fluctuation -analyysi parietaalilohkoepilepsiapotilaiden prekirurgisessa arvioinnissa

Epilepsia on maailmanlaajuisesti yksi yleisimmistä neurologisista sairauksista. Noin viidesosalla epilepsiaa sairastavista todetaan lääkeresistentti epilepsia. Heistä osa hyötyy epilepsiakirurgisesta hoidosta. Ennen kirurgista hoitoa epileptogeenisen alueen (EZ) paikka tulee selvittää tarkasti hyvän kirurgisen lopputuloksen saavuttamiseksi. Uusia menetelmiä EZ:n paikantamiseen kehitetään jatkuvasti. Tutkimuksen tarkoituksena oli selvittää, voidaanko pitkän kantaman temporaalisia korrelaatioita (LRTC) käyttää EZ:n ei-invasiiviseen paikallistamiseen potilailla, joilla on lääkehoidolle resistentti epilepsia. Tutkimukseen osallistui 10 epilepsiapotilasta, jotka olivat edeltävästi läpikäyneet epilepsialeikkauksen sekä magneettienkefalografiatutkimuksen. Hyödynsimme detrended fluctuation analysis (DFA) -menetelmää kvantifioidaksemme LRTC:t neljällä eri taajuuskaistalla 200 aivokuorialueella, jotka perustuivat potilaiden yksilökohtaisiin lähdemalleihin. Korreloimme DFA-arvot etäisyyksiin leikkausalueista sekä interiktaalisten epileptiformisten purkausten (IED) kortikaalisista sijainneista. Lisäksi kolme kliinistä asiantuntijaa tarkasteli DFA-karttoja visuaalisesti tunnistaakseen todennäköisimmät EZ-alueet. Tutkimuksessa havaittiin, että DFA-kartat korreloivat merkitsevästi etäisyyden kanssa leikkausalueesta vain potilailla, joilla oli tyypin II fokaalinen kortikaalinen dysplasia (FCD II), ja olivat yhteydessä IED-alueisiin vain FCD II -potilailla. Lisäksi DFA-karttojen visuaalinen analyysi osoitti, että FCD-potilailla havaitsijoiden välinen yksimielisyys ja tarkkuus oli suuri, kun määritettiin vaurioitunutta aivopuoliskoa ja -lohkoa. Nämä havainnot osoittavat, että LRTC:t voisivat tarjota ei-invasiivisen menetelmän EZ:n tunnistamiseksi ja epilepsiapotilaiden leikkausta edeltävän arvioinnin helpottamiseksi.Epilepsy is one of the most common neurological diseases worldwide. Around one-fifth of people with epilepsy are diagnosed with drug-resistant epilepsy. Some of them benefit from epilepsy surgery. Before surgical treatment, the location of the epileptogenic zone (EZ) must be carefully identified to achieve a good surgical outcome. New methods for locating the EZ are constantly being developed. The aim of this study was to investigate whether long-range temporal correlations (LRTC) can be used for non-invasive localization of the EZ in patients with drug-resistant epilepsy. The study involved 10 patients with epilepsy who had previously undergone epilepsy surgery and an MRI. We used the detrended fluctuation analysis (DFA) method to quantify the LRTCs in four different frequency bands in 200 cortical regions based on the patients' individual source reconstructions. We correlated the DFA values with distances from the resection areas and cortical locations of interictal epileptiform discharges (IEDs). In addition, DFA maps were visually inspected by three clinicians to identify the most likely EZs. The study found that DFA maps were significantly correlated with distance from the location of the resection only in the patients with type II focal cortical dysplasia (FCD II) and were associated with IED sites only in the FCD II patients. In addition, visual analysis of the DFA maps showed that in the FCD patients, there was a high interobserver agreement and accuracy in defining the affected hemisphere and lobe. These findings suggest that LRTCs could provide a non-invasive method to identify EZ and facilitate the preoperative evaluation of epilepsy patients