- Email: [email protected]
Multimodal imaging: Simultaneous EEG in a 3T Hybrid MR–PET system
Nuclear Instruments and Methods in Physics Research A 702 (2013) 37–38
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Multimodal imaging: Simultaneous EEG in a 3T Hybrid MR–PET system I. Neuner a,b,d,n, T. Warbrick a, L. Tellmann a, E. Rota Kops a, J. Arrubla a, F. Boers a, H. Herzog a, N.J. Shah a,c,d a
Institute of Neuroscience and Medicine (INM-4), Forschungszentrum J¨ ulich GmbH, Germany Department of Psychiatry, Psychotherapy and Psychosomatics, RWTH Aachen University, Germany c Department of Neurology, RWTH Aachen University, Germany d JARA BRAIN—Translational Medicine, Germany b
a r t i c l e i n f o
abstract
Available online 18 August 2012
The new generation of integrated MR–PET systems allows the simultaneous acquisition of MR and PET data. While MR delivers structural data with an excellent spatial resolution, the advantage of PET is its information on a molecular level. However, both modalities have a low temporal resolution. Thus, for pharmacological studies or patients who suffer from treatment resistant epilepsy the combination of yet another modality such as EEG could be desirable. We tested the feasibility of evoked visual potentials in a 3T Hybrid MR–PET system (Siemens Germany) in comparison to a standalone 3T Trio System (Siemens Germany). A T2*-weighted EPI sequence was used: TR: 2.2 s, TE: 30 ms, FOV: 200 mm, slice thickness 3, 36 slices in a healthy volunteer (male, 27 years old) using an MR-compatible 32-channel EEG system (Brainproducts, Munich, Germany). We applied 200 trials of visual stimulation from a white and black checkerboard. Visual evoked potentials were analyzed using Brain Vision Analyzer (Brainproducts, Munich, Germany). Gradient correction and cardioballistic artefact correction were performed as implemented in Vision Analyzer. Visual event related potentials were successfully recorded at the 3T Hybrid MR–PET system. Both curves differ slightly in shape and latency due to the following factors: the distance from the screen varies slightly and the size of the field of view of the subjects is smaller in the 3T MR–PET system in comparison to the 3T stand alone system. Extending the 3T MR–PET Hybrid system to 3T Hybrid MR–PET–EEG is feasible and adds another tool to clinical neuroimaging and research. & 2012 Elsevier B.V. All rights reserved.
Keywords: Positron Emission Tomography (PET) Electrophysiology Temporal resolution Hybrid MR–PET system
1. Introduction Hybrid MR–PET allows the simultaneous acquisition of MR and PET data for clinical and research applications. Within a time frame of 50 min a detailed assessment in brain tumor patients including structural and functional MRI, diffusion tensor imaging and FET– PET [1]. Research questions e.g. assessing cognitive functions such as working memory could be addressed using a constant bolusinfusion model applying 11C-raclopride in combination with structural imaging (MPRAGE, diffusion tensor imaging) and functional imaging probing working memory via n-back working task. The simultaneous approach is an important research tool for pharmacological challenges in cognitive functions where a cognitive or otherwise designed task could not be presented twice without confounding the potentials results by the second run per se. Whereas MRI has the advantage in the high spatial resolution and its multiple sequences assessing structure, delivering functional and metabolic information via spectroscopy is rather slow. Its temporal
n ¨ Corresponding author at: Forschungszentrum Julich GmbH, Institute of ¨ Neuroscience and Medicine (INM-4), Leo-Brant-Strasse, Julich, Germany. E-mail address: [email protected] (I. Neuner).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.08.022
resolution in functional MRI (fMRI), based on BOLD (blood oxygenation level dependent), allows a response in the order of 4–6 s. The spatial resolution of PET is in the range of 4–6 mm depending on the system used [2]. However in comparison to MRI it does not allow for a very detailed anatomical analysis. The strength of PET lies in providing highly specific data on a molecular level. Thus, MR and PET are complimentary with regard to spatial resolution and specificity of molecular information. However, both methods share a weakness in the temporal domain. A method that has an excellent temporal resolution in the domain of milliseconds is electrophysiology. Different electrode set ups are possible, i.e. electroencephalography (EEG) with 32, 64, 128 or 256 channels. EEG could provide information of visual or auditory evoked potentials, P300 and also late evoked potentials reflecting different stages of sensory and cognitive processing. The combination of electrophysiology and fMRI has been established over the last decade with current commercially available MR-compatible electrode caps in different designs. The combination of fMRI and EEG in a MR-scanner brings along two kinds of artefacts that must be addressed before analyzing the data. The first one is the gradient artefact, which is caused during the switching of the gradients within the EPI-sequence. Since it is a very consistent artefact, it can be quite easily removed using a template subtraction method [3] as implemented in the software
38
I. Neuner et al. / Nuclear Instruments and Methods in Physics Research A 702 (2013) 37–38
Brain Vision Analyzer 2.0 (Brainproducts, Munich, Germany) and also in EEGLAB [4], an open source software developed by the team of Delorme and Makeig. The other artefact, the cardioballistic artefact has its source in the ejection of blood from the left ventricle during a heart cycle within the magnetic field. Since it is a variable artefact, it is much more difficult to correct. Different approaches have been attempted, ranging from template creation to independent component analysis (ICA) in order to successfully remove the artefact in a sufficient way. The aim of this pre-study was to investigate whether the EEGsystem setup as used in a standalone MR-system would also work within a 3T Hybrid MR–PET system and deliver meaningful results.
2. Material and methods 2.1. 3T MR–PET system The prototype 3T MR–PET consists of a commercially available 3T Siemens Tim Trio MR System and a newly developed PET insert for brain imaging (Siemens, Erlangen, Germany). The hybrid MR– PET is a compact cylinder with a length of 72 cm and an outer diameter of 60 cm. 32 copper-shielded detector cassettes with 6 comprehensive detector modules which form the BrainPET-ring. The field-of-view (FOV) is 31.4 cm in diameter and its length is 19.2 cm. The front end of the detector module is a 12 12 matrix of individual LSO crystals coupled to a 3 3 array of Avalanche photo diodes (APDs) avoiding detector sensitivities to the magnetic field of the MR system. The small volume of the LSO crystals measuring 2.5 2.5 20 mm3 results in a central image resolution of about 3 mm. This implicates that the BrainPET together with the HRRT PET-only equipment offer the best PET image resolution presently available for human brain studies. The inner diameter of the BrainPET is 36 cm offering space for the MR head coil. For functional MRI, a custom-built mirror system is clipped on to the receive coil of the MR-head coil which allows for visual stimulus presentation. A custom-made pillow with holes for the occipital electrodes was placed in the head coil for accommodation of the head of the volunteer with the EEG cap on. EEG-cables were guided through the back of the open MR-coil.
Fig. 1. EEG signals in occipital channels O1 and O2 after correction and averaging. Black curve corresponds to recording inside 3T MR Trio scanner and red curve corresponds to recording inside the 3T MR–PET Hybrid scanner.
basis functions describing temporal variation in the artefact using temporal Principal Components Analysis (PCA). These basis functions are then fitted to and subtracted from the EEG. After correction for cardioballistic artefact data were downsampled to 250 Hz and exported again to BrainVision Analyzer 2 (Brain Products, Munich, Germany) for further processing and averaging. fMRI data were acquired with spin echo EPI using echo and repetition time, TE/TR, of 30 ms/2000 ms. A total of 180 measurements were performed for 36 slices and slice thickness of 3 mm.
3. Results Visual event related potentials were successfully recorded at the 3T Hybrid MR–PET system (Fig. 1). Both signal curves recorded inside the 3T MR–PET showed some differences in shape and latency compared to the signal recorded inside the 3T MR Trio.
2.2. Data acquisition and processing
4. Discussion
One healthy volunteer (1 male, 27 years old) was measured with the same sequence parameters in the 3T MR–PET system and in the 3T MR Trio system. The volunteer was wearing a 32-channel MR-compatible EEG-cap. The EEG cap (BrainCap MR, EasyCap GmbH, Breitbrunn, Germany) consisted of 29 scalp electrodes distributed according to the 10–20 system and three additional electrodes, one of which was attached to the back of the subject for recording the electrocardiogram, while the others were attached on the outer canthus of each eye for detection of ocular artefacts. Data were recorded relative to a Cz reference and a ground electrode was located at Iz [5]. Data were sampled at 5000 Hz, with a bandpass of 0.016–250 Hz. Impedance at all recording electrodes was less than 10 kO. We applied 200 trials of visual stimulation from a white and black checkerboard. Cardioballistic artefact correction starts with the accurate detection of R-peaks in the electrocardiographic signal. This was accomplished using the semi-automatic peak detection method in BrainVision Analyzer 2 (Brain Products, Gilching, Germany). Raw EEG data were then exported and corrected for cardioballistic artefact using an Optimal Basis Set (OBS) [6], which is available as a function in EEGLAB [4]. The OBS method involves identifying
Visual event related potentials were successfully recorded at the 3T Hybrid MR–PET system. Both curves differed slightly in shape and latency. One important factor influencing those differences is the setup of visual stimulation. The distance between the mirror and the screen in the 3T MR–PET Hybrid system is 10% higher than that in the 3T MR Trio scanner, which affects the size of the field of view and modifies slightly the primary visual sensorial processing [7]. Extending the 3T MR–PET Hybrid system to 3T Hybrid MR– PET–EEG is feasible and adds another tool to clinical neuroimaging and research. References [1] I. Neuner, et al., European Radiology, http://dx.doi.org/10.1007/ s00330-012-2543-x, in press. [2] F.H. Van Velden, et al., Journal of Nuclear Medicine 50 (5) (2009) 693. [3] P.J. Allen, et al., Neuroimage 12 (2) (2000) 230. [4] A. Delorme, S. Makeig, Neuroscience Methods 134 (1) (2004) 9. [5] R. Oostenveld, P. Praamstra, Clinical Neurophysiology 112 (4) (2001) 713. [6] R.K. Niazy, et al., Neuroimage 28 (3) (2005) 720. [7] L. Ciga´nek, Experimental Brain Research 4 (2) (1967) 118.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Multimodal imaging: Simultaneous EEG in a 3T Hybrid MR–PET system I. Neuner a,b,d,n, T. Warbrick a, L. Tellmann a, E. Rota Kops a, J. Arrubla a, F. Boers a, H. Herzog a, N.J. Shah a,c,d a
Institute of Neuroscience and Medicine (INM-4), Forschungszentrum J¨ ulich GmbH, Germany Department of Psychiatry, Psychotherapy and Psychosomatics, RWTH Aachen University, Germany c Department of Neurology, RWTH Aachen University, Germany d JARA BRAIN—Translational Medicine, Germany b
a r t i c l e i n f o
abstract
Available online 18 August 2012
The new generation of integrated MR–PET systems allows the simultaneous acquisition of MR and PET data. While MR delivers structural data with an excellent spatial resolution, the advantage of PET is its information on a molecular level. However, both modalities have a low temporal resolution. Thus, for pharmacological studies or patients who suffer from treatment resistant epilepsy the combination of yet another modality such as EEG could be desirable. We tested the feasibility of evoked visual potentials in a 3T Hybrid MR–PET system (Siemens Germany) in comparison to a standalone 3T Trio System (Siemens Germany). A T2*-weighted EPI sequence was used: TR: 2.2 s, TE: 30 ms, FOV: 200 mm, slice thickness 3, 36 slices in a healthy volunteer (male, 27 years old) using an MR-compatible 32-channel EEG system (Brainproducts, Munich, Germany). We applied 200 trials of visual stimulation from a white and black checkerboard. Visual evoked potentials were analyzed using Brain Vision Analyzer (Brainproducts, Munich, Germany). Gradient correction and cardioballistic artefact correction were performed as implemented in Vision Analyzer. Visual event related potentials were successfully recorded at the 3T Hybrid MR–PET system. Both curves differ slightly in shape and latency due to the following factors: the distance from the screen varies slightly and the size of the field of view of the subjects is smaller in the 3T MR–PET system in comparison to the 3T stand alone system. Extending the 3T MR–PET Hybrid system to 3T Hybrid MR–PET–EEG is feasible and adds another tool to clinical neuroimaging and research. & 2012 Elsevier B.V. All rights reserved.
Keywords: Positron Emission Tomography (PET) Electrophysiology Temporal resolution Hybrid MR–PET system
1. Introduction Hybrid MR–PET allows the simultaneous acquisition of MR and PET data for clinical and research applications. Within a time frame of 50 min a detailed assessment in brain tumor patients including structural and functional MRI, diffusion tensor imaging and FET– PET [1]. Research questions e.g. assessing cognitive functions such as working memory could be addressed using a constant bolusinfusion model applying 11C-raclopride in combination with structural imaging (MPRAGE, diffusion tensor imaging) and functional imaging probing working memory via n-back working task. The simultaneous approach is an important research tool for pharmacological challenges in cognitive functions where a cognitive or otherwise designed task could not be presented twice without confounding the potentials results by the second run per se. Whereas MRI has the advantage in the high spatial resolution and its multiple sequences assessing structure, delivering functional and metabolic information via spectroscopy is rather slow. Its temporal
n ¨ Corresponding author at: Forschungszentrum Julich GmbH, Institute of ¨ Neuroscience and Medicine (INM-4), Leo-Brant-Strasse, Julich, Germany. E-mail address: [email protected] (I. Neuner).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.08.022
resolution in functional MRI (fMRI), based on BOLD (blood oxygenation level dependent), allows a response in the order of 4–6 s. The spatial resolution of PET is in the range of 4–6 mm depending on the system used [2]. However in comparison to MRI it does not allow for a very detailed anatomical analysis. The strength of PET lies in providing highly specific data on a molecular level. Thus, MR and PET are complimentary with regard to spatial resolution and specificity of molecular information. However, both methods share a weakness in the temporal domain. A method that has an excellent temporal resolution in the domain of milliseconds is electrophysiology. Different electrode set ups are possible, i.e. electroencephalography (EEG) with 32, 64, 128 or 256 channels. EEG could provide information of visual or auditory evoked potentials, P300 and also late evoked potentials reflecting different stages of sensory and cognitive processing. The combination of electrophysiology and fMRI has been established over the last decade with current commercially available MR-compatible electrode caps in different designs. The combination of fMRI and EEG in a MR-scanner brings along two kinds of artefacts that must be addressed before analyzing the data. The first one is the gradient artefact, which is caused during the switching of the gradients within the EPI-sequence. Since it is a very consistent artefact, it can be quite easily removed using a template subtraction method [3] as implemented in the software
38
I. Neuner et al. / Nuclear Instruments and Methods in Physics Research A 702 (2013) 37–38
Brain Vision Analyzer 2.0 (Brainproducts, Munich, Germany) and also in EEGLAB [4], an open source software developed by the team of Delorme and Makeig. The other artefact, the cardioballistic artefact has its source in the ejection of blood from the left ventricle during a heart cycle within the magnetic field. Since it is a variable artefact, it is much more difficult to correct. Different approaches have been attempted, ranging from template creation to independent component analysis (ICA) in order to successfully remove the artefact in a sufficient way. The aim of this pre-study was to investigate whether the EEGsystem setup as used in a standalone MR-system would also work within a 3T Hybrid MR–PET system and deliver meaningful results.
2. Material and methods 2.1. 3T MR–PET system The prototype 3T MR–PET consists of a commercially available 3T Siemens Tim Trio MR System and a newly developed PET insert for brain imaging (Siemens, Erlangen, Germany). The hybrid MR– PET is a compact cylinder with a length of 72 cm and an outer diameter of 60 cm. 32 copper-shielded detector cassettes with 6 comprehensive detector modules which form the BrainPET-ring. The field-of-view (FOV) is 31.4 cm in diameter and its length is 19.2 cm. The front end of the detector module is a 12 12 matrix of individual LSO crystals coupled to a 3 3 array of Avalanche photo diodes (APDs) avoiding detector sensitivities to the magnetic field of the MR system. The small volume of the LSO crystals measuring 2.5 2.5 20 mm3 results in a central image resolution of about 3 mm. This implicates that the BrainPET together with the HRRT PET-only equipment offer the best PET image resolution presently available for human brain studies. The inner diameter of the BrainPET is 36 cm offering space for the MR head coil. For functional MRI, a custom-built mirror system is clipped on to the receive coil of the MR-head coil which allows for visual stimulus presentation. A custom-made pillow with holes for the occipital electrodes was placed in the head coil for accommodation of the head of the volunteer with the EEG cap on. EEG-cables were guided through the back of the open MR-coil.
Fig. 1. EEG signals in occipital channels O1 and O2 after correction and averaging. Black curve corresponds to recording inside 3T MR Trio scanner and red curve corresponds to recording inside the 3T MR–PET Hybrid scanner.
basis functions describing temporal variation in the artefact using temporal Principal Components Analysis (PCA). These basis functions are then fitted to and subtracted from the EEG. After correction for cardioballistic artefact data were downsampled to 250 Hz and exported again to BrainVision Analyzer 2 (Brain Products, Munich, Germany) for further processing and averaging. fMRI data were acquired with spin echo EPI using echo and repetition time, TE/TR, of 30 ms/2000 ms. A total of 180 measurements were performed for 36 slices and slice thickness of 3 mm.
3. Results Visual event related potentials were successfully recorded at the 3T Hybrid MR–PET system (Fig. 1). Both signal curves recorded inside the 3T MR–PET showed some differences in shape and latency compared to the signal recorded inside the 3T MR Trio.
2.2. Data acquisition and processing
4. Discussion
One healthy volunteer (1 male, 27 years old) was measured with the same sequence parameters in the 3T MR–PET system and in the 3T MR Trio system. The volunteer was wearing a 32-channel MR-compatible EEG-cap. The EEG cap (BrainCap MR, EasyCap GmbH, Breitbrunn, Germany) consisted of 29 scalp electrodes distributed according to the 10–20 system and three additional electrodes, one of which was attached to the back of the subject for recording the electrocardiogram, while the others were attached on the outer canthus of each eye for detection of ocular artefacts. Data were recorded relative to a Cz reference and a ground electrode was located at Iz [5]. Data were sampled at 5000 Hz, with a bandpass of 0.016–250 Hz. Impedance at all recording electrodes was less than 10 kO. We applied 200 trials of visual stimulation from a white and black checkerboard. Cardioballistic artefact correction starts with the accurate detection of R-peaks in the electrocardiographic signal. This was accomplished using the semi-automatic peak detection method in BrainVision Analyzer 2 (Brain Products, Gilching, Germany). Raw EEG data were then exported and corrected for cardioballistic artefact using an Optimal Basis Set (OBS) [6], which is available as a function in EEGLAB [4]. The OBS method involves identifying
Visual event related potentials were successfully recorded at the 3T Hybrid MR–PET system. Both curves differed slightly in shape and latency. One important factor influencing those differences is the setup of visual stimulation. The distance between the mirror and the screen in the 3T MR–PET Hybrid system is 10% higher than that in the 3T MR Trio scanner, which affects the size of the field of view and modifies slightly the primary visual sensorial processing [7]. Extending the 3T MR–PET Hybrid system to 3T Hybrid MR– PET–EEG is feasible and adds another tool to clinical neuroimaging and research. References [1] I. Neuner, et al., European Radiology, http://dx.doi.org/10.1007/ s00330-012-2543-x, in press. [2] F.H. Van Velden, et al., Journal of Nuclear Medicine 50 (5) (2009) 693. [3] P.J. Allen, et al., Neuroimage 12 (2) (2000) 230. [4] A. Delorme, S. Makeig, Neuroscience Methods 134 (1) (2004) 9. [5] R. Oostenveld, P. Praamstra, Clinical Neurophysiology 112 (4) (2001) 713. [6] R.K. Niazy, et al., Neuroimage 28 (3) (2005) 720. [7] L. Ciga´nek, Experimental Brain Research 4 (2) (1967) 118.