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Contact
Martin Eschelbach |
| Address: | Max-Planck-Ring 11 72076 Tübingen |
| Room number: | 3.B.10 |
| Phone: | +49 7071 601 728 |
| Fax: | +49 7071 601 702 |
| E-Mail: |  Martin.Eschelbach |
Motion Correction and Field Monitoring
Patient or subject motion is a known source of quality degradation in MRI images. One means of tackling this problem is prospective motion correction. In this project we're applying NMR field probes [1] to assess subject head motion via position changes of the field probes. The positions of the Field probes are measured via applied gradients with known strength in every spatial direction.
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| PCB with transmit/receive electronics and field probe with attached tuning/matching board. |
Euler Angles during subject motion. Measured with three field probes. |
The NMR field probes use Hexafluorobenzene (C6F6) as an NMR active sample. The probes are connected to a tuning matching board and are tuned to the operating frequency of 376.14 MHz at the 9.4 T human MRI scanner and matched to an impedance of 50 Ohm. The sample is excited via a custom build transmit/receive-chain based on microelectronic components assembled onto a PCB board [2]. The signals are demodulated on the PCB board and transmitted to and from the board via a shielded ethernet cable. The demodulated signal is filtered and then digitized at a sampling rate of 200 kS/s using a commercial ADC (NI PCIe-6363, National Instruments, Austin, TX, USA) and the PCBs are operated via LabVIEW (National Instruments). Due to the external hardware, the setup doesn’t block any channels of the MR Scanner.
The measurements were carried out at a 9.4 T human scanner (Siemens Magnetom). The position of each probe was determined via 3 block gradients along each axis (5 mT/m, 1 ms). The phase φ of the signal is then used to determine the field strength and thus the positions of the field probes. Using at least three field probes and assuming rigid body motion, one can now calculate the rotations and translations of the attached objects.
[1] C. Barmet et al., MRM 2008; 60:187-197;
[2] J. Handwerker et al., IEEE Biomedical Circuits and Systems Conference (BiOCAS) 2013, Rotterdam, The Netherlands, ID 5027.
since 07/2013
PhD student "Motion Correction with active T/R-markers"
05/2013 - 07/2013
Internship at d-fine GmbH, Frankfurt
04/2012 - 04/2013
Diploma thesis at Max Planck Istitute for Biological Cybernetics
"NMR field probes for MRI at 9.4 T"
04/2008 - 04/2013
Diploma in Physics at Eberhard Karls Universität Tübingen
Focus: Nano technology, Medical Physics
| • | Bause J  , Ehses P , Scheffler K , Pohmann R , Aghaeifar A , Eschelbach M , In M-H and Engel E-M (October-19-2017) Abstract Talk: Distortion and prospective motion corrected zoomed functional imaging of the human brain at 9.4 Tesla, 34th Annual Scientific Meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRMB 2017), Barcelona, Spain, Magnetic Resonance Materials in Physics, Biology and Medicine30 (Supplement 1) S125-S126. Purpose/Introduction: Sub-millimeter single-shot echo-planar imaging at ultra-high field is challenging due to stronger distortions evoked by increased B0-inhomogeneities as well as increased blurring and a reduction of the echo-time yielding the maximum BOLD contrast caused by faster T2 * relaxation. Therefore, a shorter echo-train length (ETL) is required, either by utilizing partial Fourier acquisition or parallel imaging techniques. A further reduction of the ETL can be achieved with the reduced field of view (rFOV) method [1]. But even highly accelerated images can suffer from distortions which can be accurately measured and corrected by the point-spread-function (psf) mapping technique [2, 3]. Another issue of high resolution fMRI, is the increased vulnerability to subject motion. Here, we combined rFOV imaging with psf distortion correction and prospective motion correction for high-resolution functional imaging at 9.4 Tesla. Subjects and Methods: Measurements were performed on a whole body scanner (Siemens Healthcare, Germany) using a 16-Tx/31-Rx array [4]. An outer volume suppression pulse was implemented in an EPI sequence [1]. The slice positions of the psf-mapping and functional measurements were updated using data from a camera (Metria Innovation Inc., USA) which tracked a moire´ phase marker fixed to the subject’s bite bar [6]. Parameters: TR = 2000 ms, TE = 25 ms, FA = 80, GRAPPA = 3 (FLEET ACS-scan, [7]), 6/8 partial Fourier, FoV = 150 9 75 mm2, 17 slices, 0.65 mm isotropic, ETL = 36 ms. Paradigm: 20 periods, 10 s flickering circular checkerboard (8 Hz, 7 visual field) + 26.5 s isolimuninant background. Images were reconstructed using in-house developed Matlab routines. From the fMRI data, residual motion was estimated before distortion correction. Results: Although the short ETL used here resulted in already quite small distortions (Fig. 1: a,b), a clear improvement of anatomical matching can be seen when the distortion correction is applied (Fig. 1: c). According to the camera data, the subject presented here moved less than 0.24 mm/0.57. However, the retrospective estimate was even smaller (max. 0.21 mm/0.09). Discussion/Conclusion: We demonstrated anatomically matching sub-millimeter fMRI with reduced interpolation blurring due to avoiding retrospective motion correction. In addition, the echo-train length was shortened in order to limit T2 * -blurring. However, the increased SAR caused by the additional outer volume suppression required a reduction of the number of slices. To reduce SAR, implementation of static B1 +-shimming or even 2D-selective excitation may help [7]. Nevertheless, the presented approach can probably help to investigate the actual resolution of single-shot EPI at high field and can possibly be also used for measuring differences in functional layers.   CiteID: BauseAIEEESP2017 |
| • | Scheffler K , Aghaeifar A and Eschelbach M (September-10-2017) Abstract Talk: Multi-Modality Prospective Motion Correction of the Human Head, ISMRM Workshop on Motion Correction in MRI & MRS (MoCor 2017), Cape Town, South Africa. CiteID: AghaeifarES2017 |
| • | Bause J , Scheffler K , Thielscher A , Aghaeifar A and Eschelbach M (April-24-2017) Abstract Talk: AMoCo, a software package for prospective motion correction, 25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017), Honolulu, HI, USA(0305) 219. Long scan time makes MRI prone to subject motion which can result in image artifacts. Here we introduce a library for advanced motion correction (AMoCo) for Siemens platforms which can be embedded in any sequence and enables connecting to any tracking device. The library is programmed in a modular way that allows user to customize the correction procedure. The library is integrated with EPI, GRE, and FLASH sequences and tested with various tracking devices. CiteID: AghaeifarEBTS2017 |
| • | Scheffler K , Eschelbach M and Aghaeifar A (September-2016) Abstract Talk: Real time communications over UDP protocol, European IDEA Users Group Meeting 2016, Maastricht, The Netherlands. CiteID: AghaeifarES2016 |
| • | Thielscher A , Scheffler K , Henning A , Eschelbach M , Loktyushin A , Handwerker J, Anders J and Chang P (May-10-2016) Abstract Talk: A Comparison of 19F NMR Field Probes and an Optical Camera System for Motion Tracking, 24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016), Singapore(0340) . The goal of this study is to evaluate and compare motion tracking with two different modalities: NMR field probes and an optical MPT (Moiré Phase Tracking) camera system. This was done by simultaneously measuring the manually induced motion of a spherical phantom with both systems. Our experimental results indicate that the motion patterns measured with both methods are in good agreement. However, the accuracy of the motion estimates from the field probe measurements are of an order of magnitude worse than the camera's tracking results. CiteID: EschelbachLCHAHTS2016 |
| • | Pohmann R , Scheffler K , Mirkes C , Eschelbach M , Bause J and Engel E-M (October-2015) Abstract Talk: Ultrahigh resolution anatomical brain imaging at 9.4 T using prospective motion correction, 32nd Annual Scientific Meeting ESMRMB 2015, Edinburgh, UK, Magnetic Resonance Materials in Physics, Biology and Medicine28 (1 Supplement) S155. Purpose/Introduction: One of the main goals at ultra-high magnetic fields is to take advantage of the increased SNR to improve the spatial resolution [1]. However, the long scan durations together with the high resolution amplify the problem of involuntary subject motion, even for experienced subjects. For further boosts of the voxel size to below 10 nl, some way to correct for motion is required. Subjects and Methods: An optical tracking system (Metria Innovations), based on an in-bore camera and a single motion marker which allows for detection of all translation and rotation parameters was used [2]. Five subjects were placed inside a 16 channel transmit/31 channel receive coil array [3] and padded to avoid movements to a large degree. The motion correction marker was attached to a bitebar [4], custom-made for each volunteer and placed to protrude from the coil housing. Two ultra-high resolution 3D gradient echo experiments with acquisition weighting were performed with a spatial resolution of (120 9 150 9 500) lm3 (9 nl). Each scan acquired two echoes with echo times of 7.4 ms and 15.8 ms. A total of 62,400 scans were recorded within 26 min. Motion correction was applied for only the second image. Results: The motion tracks of the subject with the largest motion are shown in Fig. 1. Even though motion was suppressed to a large extent by efficient padding and the use of experienced subjects, the remaining variations only due to breathing had an amplitude of up to 0.8 mm in the direction of strongest motion. In the resulting images (Fig. 2), this already lead to strong artifacts. All images showed sufficient SNR in spite of the high resolution. The long TE-data also has excellent contrast and shows remarkable details, while the low TE images, in spite of the higher SNR, are of weaker quality due to a significant lack of contrast. The images with motion correction are largely free of artifacts. Discussion/Conclusion: The high SNR at ultra-high field makes it possible to obtain images with very high spatial resolutions, showing an unprecedented amount of detail in brain structures. Although we were able to obtain artifact-free images from some of our most experienced subjects even without motion correction, the data shown here indicate that reliable imaging with these resolution requires an efficient motion correction technique. CiteID: PohmannBMeES2015 |
| • | Thielscher A , Eschelbach M , Kühne M, Ergin MA, Klare S, Aghaeifar A and Peer A (May-2015) Abstract Talk: Towards an MR-compatible Haptic Interface with Seven Actuated Degrees of Freedom, IEEE International Conference on Robotics and Automation (ICRA 2015), Seattle, WA, USA. CiteID: KuhneEKEATP2015 |
| • | Scheffler K , Eschelbach M , Handwerker J, Hoffmann A, Ortmanns M and Anders L (October-9-2014) Abstract Talk: A distributed active NMR sensor array for artifact correction in ultra high field MRI applications, 48th Annual Conference of the German Society for Biomedical Engineering (BMT 2014), Hannover, Germany, Biomedical Engineering / Biomedizinische Technik59 (Supplement 1) S530. Introduction We present a distributed sensor array for the real-time monitoring of magnetic field imperfections in magnetic resoncance imaging (MRI) scanners. These imperfections occur due to hardware limitations originating from non-ideal gradient coils as well as patient motion and lead to artifacts which limit the achievable imaging quality especially for ultra high field scanners. By monitoring these imperfections, the artifacts can be corrected by either a predistortion of the gradient waveforms or during image reconstruction. Methods The presented sensor array consists of four active transmit/receive (TX/RX) field probes and signal conditioning electronics on a printed circuit board (PCB). The field probes consist of a glass capillary (din = 800 μm) filled with a liquid NMR sample surrounded by a solenoid TX/RX coil which is connected via a tuning/matching network to a homodyne quadrature transceiver. The proposed system is an extension of the work presented in [1] to an array of sensors which allows for an artifact correction based on first order spherical harmonic base functions. Furthermore, we use a 19F instead of a 1H NMR sample to reduce coupling between sensor and imaging object and a significantly enhanced the transceiver architecture and layout. The field probes are connected using differential, impedance-matched cables to the signal conditioning board which provides line drivers and anti-aliasing filters and interfaces to a commercial data acquisition system (USB-6366, National Instruments) with 2 MS/s and 16 bit resolution. Results The sensor array has an input amplitude ranging from <2.2 μVRMS - 78.4 mVRMS and accepts input frequencies between 175 MHz - 660 MHz, corresponding to field strengths of 4.4 T - 16.4 T for 19F samples. The detector gain can be adjusted between 21 dB and 81 dB with a noise figure of 2.74 dB for quadrature detection. The on-board transmitter generates a peak power of 18.7 dBm, resulting in a 90° pulse time <10 μs. The sensor array was successfully tested in a 9.4 T wholebody scanner and a 11.7 T small animal scanner and achieved a frequency resolution <5 ppb. Conclusion In contrast to previously published RX-only [2] and TX/RX [3] field probes, the active field probe array presended here eliminates the need for long RF cables inside the scanner due to a local generation of the RF signal required for excitation and downconversion of the NMR signal, reducing the crosstalk with the imaging experiment and therefore improving the accuracy of the recorded data. Currently, we are working on an implementation of the field probe electronics as a custom designed integrated circuit to further reduce crosstalk and power consumption and improve system performance. CiteID: HandwerkerHESOA2014 |
| • | Scheffler K , Handwerker J, Ortmanns M, Anders J, Eschelbach M and Bonehi V (September-19-2013) Abstract Talk: An active transmit/receive NMR magnetometer for field monitoring in ultra high field MRI scanners, Dreiländertagung der Deutschen, Schweizerischen und Österreichischen Gesellschaft für Biomedizinische Technik (BMT 2013), Graz, Austria, Biomedical Engineering / Biomedizinische Technik58 (Supplement 1) 305-306. We present a miniaturized active nuclear magnetic resonance (NMR) magnetometer consisting of a susceptibility matched field probe and a PCB based RF transceiver. Thanks to its transmit-receive (TX/RX) capabilities, the system can be used to monitor the spatio-temporal field evolution during a magnetic resonance imaging (MRI) scan and thereby allows for a correction of gradient field imperfections to improve image quality. The magnetometer can be tuned for magnetic fields ranging from 7 T to 15.5 T and achieves a resolution of 14.8 nT measured in a 9.4 T whole body scanner. CiteID: HandwerkerBESOA2013 |
Last updated: Monday, 22.05.2017