Synopsis
Functional connectivity fMRI studies in animal models of epilepsy allow assessment of
reorganization of networks already before occurrence of spontaneous seizures.
Extreme care has to be taken when planning fMRI experiment especially regarding
anesthesia and physiological monitoring. New technological advances such as
implantable RF-coils, and radial zero echo time imaging provide solutions to
many of the existing problems and make awake fMRI approaches more accessible
TARGET AUDIENCE
Investigators who are using or planning to use resting
state fMRI in their preclinical epilepsy researchOBJECTIVES
To understand how functional connectivity can be measured
in preclinical settings, and how it can be used in the context of epilepsy and formation
of epileptogenic zone.INTRODUCTION
Large scale network alterations have gained momentum as a key concept in
epilepsy (Gotman et al 2008), mainly due to advent of resting state fMRI which
can assess functional connectivity in the brain. There is the growing body of
evidence showing that the interaction and dysfunction of networks plays a key
role both in focal and generalized epilepsy (Laufs et al 2012, Maneshi et al
2014). While there is a substantial body of research into functional
connectivity changes in epileptic patients (Halasz 2010), there is an almost
complete void of studies relating connectivity changes to epileptogenesis
before the occurrence of seizures. This is mostly because long-term follow-up
studies are very difficult and expensive to perform in patients. On the other
hand, longitudinal animal resting state fMRI studies with EEG recordings have
been technically challenging in animals and are potentially influenced by
anesthesia.METHODOLOGICAL CHALLENGES AND SOLUTIONS
Typically animals are anesthetized for fMRI which
influences physiology and alters both brain function and hemodynamic coupling. Understanding
the influence of anesthesia on resting state fMRI measurements (Paasonen et al 2018) and use of careful physiological monitoring is
essential for producing reliable functional connectivity results in preclinical
settings. Recently also awake fMRI
protocols have been introduced to avoid adverse effect of anesthesia. Awake
fMRI requires use of restrainer and habituation period in mock scanner (King et
al 2005, Steenroos et al 2018, Gao et al 2017), and when performed correctly can
highlight connections, such ae cortico-thalamic connection, that are often
suppressed in anesthetized animals. If fMRI is combined, simultaneously or interleaved,
with EEG with chronically implanted electrodes, technical challenge to obtain
high quality functional connectivity data becomes even more evident. Head
implant prevents using standard head coils, which can be circumvented by using
implantable RF-coils (Pirttimäki 2016), or large transceiver surface coils.
Surgical procedures and/or deep electrodes cause magnetic susceptibility
artefacts that can prevent using conventional echo planar imaging (EPI) based fMRI
approaches. Recently novel zero-echo time method MB-SWIFT was introduced with
minimal susceptibility artefacts allowing to collect high quality fMRI data in
the presence of metallic electrodes (Lehto et al 2017) and with minimal artefacts
for EEG recordings (Paasonen et al 2020).REVIEW OF RESULTS
Mostly due the technical challenges, relatively few
resting state fMRI studies in preclinical epilepsy models exists (Bertoglio et al, 2017). In a recent study, rats with increased
seizure susceptibility following lateral fluid percussion injury, a traumatic
brain injury model, were used (Mishra et
al., 2014).
The group statistics revealed decreased connectivity between the
ipsilateral and contralateral parietal cortex and between the parietal cortex
and hippocampus on the side of injury as compared to sham-operated animals.
Injured animals also had abnormal negative connectivity between the ipsilateral
and contralateral parietal cortex and other regions. Another work utilized graph theory analysis of functional connectivity
data in a rat model of mild facial seziures caused by injection of tetanus toxin into the right primary
motor cortex. The results indicated that, despite
the locality of the epileptogenic area, epileptic brains exhibit a different
global network topology, connectivity, and structural integrity than healthy
brains (Otte
et al., 2012). Consistently,
a recent study investigated the graph topological properties of brain networks
during chronic epilepsy in kainic acid injected rats. The authors reported
extensive
disruptions in the functional brain networks of epileptic rats compared to
control animals (Gill et al., 2017). Similarly, in status epilepticus model Bertoglio et al.
(2019 ) found a wide-spread network connectivity hyposynchrony 2 weeks post-SE
and aseverely affected functional connectivity in several regions
of the DMN,including cingulate, parietal association and posterior parietal
cortex. Interestingly, subjects
with a delayed epilepsy onset demonstrated significantly lower synchronicity
compared to controls and the epileptic group at 4 weeks post-SE. Interestingly, network
connectivity at 4 weeks was found to correlate with seizure onset and disease severity measured over 12 weeks,
suggesting a possible network strengthening upon seizure reoccurrence.
DISCUSSION
Imaging studies in animal models of epilepsy allow assessment of
reorganization of networks already before occurrence of spontaneous seizures.
Extreme care has to be taken when planning fMRI experiment especially regarding
anesthesia and physiological monitoring. New technological advances such as
implantable RF-coils, and radial zero echo time imaging provide solutions to
many of the existing problems and make awake fMRI approaches more accessibleAcknowledgements
Academy of FinlandReferences
Bertoglio D, Jonckers E, Ali
I, Verhoye M, Van der Linden A, Dedeurwaerdere S. In vivo measurement of brain
network connectivity reflects progression and intrinsic disease severity in a
model of temporal lobe epilepsy. Neurobiol Dis. 2019 Jul;127:45-52.
Bertoglio D, Verhaeghe J,
Dedeurwaerdere S, Gröhn O. Neuroimaging in animal models of epilepsy.
Neuroscience. 2017 Sep 1;358:277-299
Gao YR, Ma Y, Zhang Q, Winder AT, Liang Z, Antinori
L, Drew PJ, Zhang N. Time to wake up: Studying neurovascular coupling and
brain-wide circuit function in the un-anesthetized animal. Neuroimage. Jun;153:382-398,
2017
Gill RS, Mirsattari SM, Leung
LS. Resting state functional network disruptions in a kainic acid model of
temporal lobe epilepsy. Neuroimage Clin. 2016;13:70–81.
Gotman J. Epileptic networks studied with EEG-fMRI.
Epilepsia. 2008;49 Suppl 3:42-51
Halasz P. The concept of epileptic networks. Part2.
Ideggyogy Sz 2010, 63(11–12):377–84
King J, Garelick T, Brevard M, Chen W, Messenger T,
Duong T.et al. Procedure for minimizing stress for fMRI studies in conscious
rats. J. Neurosci. Methods 148, 154–16, 2005
Laufs H. Functional imaging of seizures and epilepsy:
evolution from zones to networks. 2012 CurrOpinNeurol) 25(2):194–200
Lehto LJ,
Idiyatullin D, Zhang J, et al. MB-SWIFT functional MRI during deep brain
stimulation in rats. Neuroimage. 2017;159:443–448.
Maneshi M, Vahdat S, Fahoum F, Grova C, Gotman J.
Specific resting-state brain networks in mesial temporal lobe epilepsy. Front
Neurol. 2014 Jul 14;5:127
Mishra
AM, Bai X, Sanganahalli BG, Waxman SG, Shatillo O, Grohn O, Hyder F, Pitkänen
A, Blumenfeld H. Decreased resting functional connectivity after traumatic
brain injury in the rat. PLoS One. 2014 Apr 18;9(4):e95280
Otte WM,
Dijkhuizen RM, van Meer MP, van der Hel WS, Verlinde SA, van Nieuwenhuizen O,
Viergever MA, Stam CJ, Braun KP. Characterization of functional and structural
integrity in experimental focal epilepsy: reduced network efficiency coincides
with white matter changes. PLoS One. 2012;7(7):e39078
Paasonen
J, Stenroos P, Salo RA, Kiviniemi V, Gröhn O. Functional connectivity under six
anesthesia protocols and the awake condition in rat brain. Neuroimage. 2018 May
15;172:9-20
Paasonen
J, Laakso H, Pirttimäki T, Stenroos P, Salo RA, Zhurakovskaya E, Lehto LJ,
Tanila H, Garwood M, Michaeli S, Idiyatullin D, Mangia S, Gröhn O. Multi-band
SWIFT enables quiet and artefact-free EEG-fMRI and awake fMRI studies in rat.
Neuroimage. 2020 Feb 1;206:
Pirttimäki
T, Salo RA, Shatillo A, Kettunen MI, Paasonen J, Sierra A, Jokivarsi K,
Leinonen V, Andrade P, Quittek S, Pitkänen A, Gröhn O. Implantable RF-coil with
multiple electrodes for long-term EEG-fMRI monitoring in rodents. J Neurosci
Methods. 2016 Dec 1;274:154-163.
Stenroos P, Paasonen J, Salo R ,
Jokivarsi K, Shatillo A, Tanila H, Gröhn
O. Awake Rat Brain Functional Magnetic Resonance Imaging Using Standard
Radio Frequency Coils and a 3D Printed Restraint Kit. Front Neurosci, 12, 548 2018