Andreas Graessl

Max-Delbrück-Centrum für Molekulare Medizin, Berlín, Berlin, Germany

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Publications (21)58.89 Total impact

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    ABSTRACT: This review documents technical progress in ophthalmic magnetic resonance imaging (MRI) at ultrahigh fields (UHF, B0 ≥ 7.0 T). The review surveys frontier applications of UHF-MRI tailored for high spatial resolution in vivo imaging of the eye, orbit and optic nerve. Early examples of clinical ophthalmic UHF-MRI including the assessment of melanoma of the choroid membrane and the characterisation of intraocular masses are demonstrated. A concluding section ventures a glance beyond the horizon and explores research promises along with future directions of ophthalmic UHF-MRI.
    Klinische Monatsblätter für Augenheilkunde 12/2014; 231(12):1187-1195. · 0.70 Impact Factor
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    ABSTRACT: Imaging of the human eye in vivo at 7 Tesla is an advanced magnetic resonance imaging application and not yet part of routine clinical practice. However, ultra highfield magnetic resonance imaging of the human eye provides benefits for in vivo evaluation of anatomy and morphology in sub-millimeter spatial resolution for today's clinical science and for future clinical applications. To this end the purpose of the study was to examine the applicability of a 6 channel transceiver radiofrequency coil array in conjunction with an optoacoustic triggering regime for imaging of the orbital and intracranial structures at 7 Tesla in vivo. Magnetic resonance imaging was performed in 7 healthy volunteers (3 female/4 male) with T1-weighted 3D fast low angle shot and 2D T2-weighted rapid acquisition with refocused echoes sequences. The six-channel coil array supports high spatial resolution imaging with an in plane resolution of 0.25 x 0.28 mm. This facilitates the depiction of anatomical details of the eye, the orbit, the optic nerve and the optical nerve sheath. Motion related artifacts could be eliminated using optoacoustic triggering regime. Our results underline the benefits of multi-element transceiver RF coil array technology and trigger protocols tailored for MRI eye applications in vivo.
    Experimental Eye Research 06/2014; · 3.03 Impact Factor
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    ABSTRACT: Imaging of the human eye in vivo at 7 Tesla is an advanced magnetic resonance imaging application and not yet part of routine clinical practice. However, ultra highfield magnetic resonance imaging of the human eye provides benefits for in vivo evaluation of anatomy and morphology in sub-millimeter spatial resolution for today's clinical science and for future clinical applications. To this end the purpose of the study was to examine the applicability of a 6 channel transceiver radiofrequency coil array in conjunction with an optoacoustic triggering regime for imaging of the orbital and intracranial structures at 7 Tesla in vivo. Magnetic resonance imaging was performed in 7 healthy volunteers (3 female/4 male) with T1-weighted 3D fast low angle shot and 2D T2-weighted rapid acquisition with refocused echoes sequences. The six-channel coil array supports high spatial resolution imaging with an in plane resolution of 0.25 x 0.28 mm. This facilitates the depiction of anatomical details of the eye, the orbit, the optic nerve and the optical nerve sheath. Motion related artifacts could be eliminated using optoacoustic triggering regime. Our results underline the benefits of multi-element transceiver RF coil array technology and trigger protocols tailored for MRI eye applications in vivo.
    Experimental Eye Research 06/2014; · 3.03 Impact Factor
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    ABSTRACT: Hintergrund: Die Ultra-Hochfeld-MRT (UHF-MRT) bei 7 Tesla stellt ein vielversprechendes Bildgebungsverfahren für die ophthalmologische Forschung und zukünftig auch zur Diagnostik und Differenzierung von Raumforderungen dar. Durch das höhere Signal-zu-Rausch-Verhältnis im Vergleich zu geringeren Magnetfeldstärken verfügt diese Technik über Entwicklungspotenzial hinsichtlich einer räumlich hochaufgelösten, ophthalmologischen Tumordiagnostik und der Untersuchung von Läsionen des Sehnervens. Die physikalisch bedingten, längeren Echo- und Repetitions-Zeiten und die daraus resultierenden, längeren Scanzeiten führen in der Praxis jedoch häufig zu vermehrten Bewegungsartefakten. Die Minimierung dieser Artefakte ist daher eine wesentliche Herausforderung bei der UHF-MRT am Auge. Ziel der Untersuchung war die Etablierung von Bildgebungsprotokollen mit Blickfixierungstrigger zur Reduzierung von Bewegungsartefakten bei der in vivo 7 Tesla MR-Bildgebung. Methodik: An Probanden (n = 17) wurden verschiedene MR-Bildgebungssequenzen (T1w 3D Flash, 2D T2w Turbo Spin Echo) mit einem 7 Tesla MRT und einem speziellen Triggerprotokoll durchgeführt. Dazu schauten die Probanden über eine Spiegelvorrichtung auf ein außerhalb des MRT-Scanners positioniertes Target. Auf dieses wurde eine Animation projiziert, die die Probanden für ca. 3 Sekunden fixieren mussten. Der Lidschluss und die Bewegung der Augen waren dabei nur in einer Pause, welche in die Animation eingebaut wurde, erlaubt. Während der Pause wurde die Datenaufnahme der MR-Sequenz jeweils für den gleichen Zeitraum unterbrochen und mit jedem Start der Blickfixierung fortgesetzt. Ergebnisse: Die Studie zeigt, dass das entwickelte MR-Protokoll mit einer sequenziell getriggerten Datenakquisition die Qualität der MR-Bildgebung bei 7T deutlich verbessert. Das Protokoll ermöglicht in vivo MR-Untersuchungen von Bulbus und Orbita mit einer räumlichen Auflösung von bis zu (0,2 x 0,2 x 1) mm und deutlich reduzierten Bewegungsartefakten. Schlussfolgerungen: Die UHF-MRT ermöglicht die hochauflösende, verzerrungsfreie morphologische Darstellung von Auge und Orbita im µm-Bereich. Die Technologie besitzt damit das Potenzial für klinische Anwendungen hinsichtlich verbesserter Tumordiagnostik und Bilgebung von Läsionen des Sehnervens.
    64. Tagung der Vereinigung Norddeutscher Augenärzte, Warnemünde; 05/2014
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    ABSTRACT: This study was designed to examine the feasibility of ophthalmic magnetic resonance imaging (MRI) at 7.0 T using a local 6-channel transmit/receive radiofrequency (RF) coil array in healthy volunteers and patients with intraocular masses. A novel 6-element transceiver RF coil array that makes uses of loop elements and that is customized for eye imaging at 7.0 T is proposed. Considerations influencing the RF coil design and the characteristics of the proposed RF coil array are presented. Numerical electromagnetic field simulations were conducted to enhance the RF coil characteristics. Specific absorption rate simulations and a thorough assessment of RF power deposition were performed to meet the safety requirements. Phantom experiments were carried out to validate the electromagnetic field simulations and to assess the real performance of the proposed transceiver array. Certified approval for clinical studies was provided by a local notified body before the in vivo studies. The suitability of the RF coil to image the human eye, optical nerve, and orbit was examined in an in vivo feasibility study including (a) 3-dimensional (3D) gradient echo (GRE) imaging, (b) inversion recovery 3D GRE imaging, and (c) 2D T2-weighted fast spin-echo imaging. For this purpose, healthy adult volunteers (n = 17; mean age, 34 ± 11 years) and patients with intraocular masses (uveal melanoma, n = 5; mean age, 57 ± 6 years) were investigated. All subjects tolerated all examinations well with no relevant adverse events. The 6-channel coil array supports high-resolution 3D GRE imaging with a spatial resolution as good as 0.2 × 0.2 × 1.0 mm, which facilitates the depiction of anatomical details of the eye. Rather, uniform signal intensity across the eye was found. A mean signal-to-noise ratio of approximately 35 was found for the lens, whereas the vitreous humor showed a signal-to-noise ratio of approximately 30. The lens-vitreous humor contrast-to-noise ratio was 8, which allows good differentiation between the lens and the vitreous compartment. Inversion recovery prepared 3D GRE imaging using a spatial resolution of 0.4 × 0.4 × 1.0 mm was found to be feasible. T2-weighted 2D fast spin-echo imaging with the proposed RF coil afforded a spatial resolution of 0.25 × 0.25 × 0.7 mm. This work provides valuable information on the feasibility of ophthalmic MRI at 7.0 T using a dedicated 6-channel transceiver coil array that supports the acquisition of high-contrast, high-spatial resolution images in healthy volunteers and patients with intraocular masses. The results underscore the challenges of ocular imaging at 7.0 T and demonstrate that these issues can be offset by using tailored RF coil hardware. The benefits of such improvements would be in positive alignment with explorations that are designed to examine the potential of MRI for the assessment of spatial arrangements of the eye segments and their masses with the ultimate goal to provide imaging means for guiding treatment decisions in ophthalmological diseases.
    Investigative radiology 03/2014; · 4.85 Impact Factor
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    ABSTRACT: A combination of magnetic resonance images with real-time high-resolution ultrasound known as fusion imaging may improve ophthalmologic examination. This study was undertaken to evaluate the feasibility of orbital high-field magnetic resonance and real-time colour Doppler ultrasound image fusion and navigation. This case study, performed between April and June 2013, included one healthy man (age, 47 years) and two patients (one woman, 57 years; one man, 67 years) with choroidal melanomas. All cases underwent 7.0-T magnetic resonance imaging using a custom-made ocular imaging surface coil. The Digital Imaging and Communications in Medicine volume data set was then loaded into the ultrasound system for manual registration of the live ultrasound image and fusion imaging examination. Data registration, matching and then volume navigation were feasible in all cases. Fusion imaging provided real-time imaging capabilities and high tissue contrast of choroidal tumour and optic nerve. It also allowed adding a real-time colour Doppler signal on magnetic resonance images for assessment of vasculature of tumour and retrobulbar structures. The combination of orbital high-field magnetic resonance and colour Doppler ultrasound image fusion and navigation is feasible. Multimodal fusion imaging promises to foster assessment and monitoring of choroidal melanoma and optic nerve disorders. • Orbital magnetic resonance and colour Doppler ultrasound real-time fusion imaging is feasible • Fusion imaging combines the spatial and temporal resolution advantages of each modality • Magnetic resonance and ultrasound fusion imaging improves assessment of choroidal melanoma vascularisation.
    European Radiology 02/2014; · 4.34 Impact Factor
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    Proc. Intl. Soc. Mag. Reson. Med.; 01/2014
  • Experimental Eye Research 01/2014; 125:89–94. · 3.03 Impact Factor
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    ABSTRACT: To design and evaluate a modular transceiver coil array with 32 independent channels for cardiac MRI at 7.0T. The modular coil array comprises eight independent building blocks, each containing four transceiver loop elements. Numerical simulations were used for B1 (+) field homogenization and radiofrequency (RF) safety validation. RF characteristics were examined in a phantom study. The array's suitability for accelerated high spatial resolution two-dimensional (2D) FLASH CINE imaging of the heart was examined in a volunteer study. Transmission field adjustments and RF characteristics were found to be suitable for the volunteer study. The signal-to-noise intrinsic to 7.0T together with the coil performance afforded a spatial resolution of 1.1 × 1.1 × 2.5 mm(3) for 2D CINE FLASH MRI, which is by a factor of 6 superior to standardized CINE protocols used in clinical practice at 1.5T. The 32-channel transceiver array supports one-dimensional acceleration factors of up to R = 4 without impairing image quality significantly. The modular 32-channel transceiver cardiac array supports accelerated and high spatial resolution cardiac MRI. The array is compatible with multichannel transmission and provides a technological basis for future clinical assessment of parallel transmission techniques at 7.0T. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 07/2013; · 3.27 Impact Factor
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    ABSTRACT: BACKGROUND: Functional and morphologic assessment of the right ventricle (RV) is of clinical importance. Cardiovascular magnetic resonance (CMR) at 1.5T has become gold standard for RV chamber quantification and assessment of even small wall motion abnormalities, but tissue analysis is still hampered by limited spatial resolution. CMR at 7T promises increased resolution, but is technically challenging. We examined the feasibility of cine imaging at 7T to assess the RV. METHODS: Nine healthy volunteers underwent CMR at 7T using a 16-element TX/RX coil and acoustic cardiac gating. 1.5T served as gold standard. At 1.5T, steady-state free-precession (SSFP) cine imaging with voxel size (1.2x1.2x6) mm3 was used; at 7T, fast gradient echo (FGRE) with voxel size (1.2x1.2x6) mm3 and (1.3x1.3x4) mm3 were applied. RV dimensions (RVEDV, RVESV), RV mass and RV function (RVEF) were quantified in transverse slices. Overall image quality, image contrast and image homogeneity were assessed in transverse and sagittal views. RESULTS: All scans provided diagnostic image quality. Overall image quality and image contrast of transverse RV views were rated equally for SSFP at 1.5T and FGRE at 7T with voxel size (1.3x1.3x4)mm3. FGRE at 7T provided significantly lower image homogeneity compared to SSFP at 1.5T. RVEDV, RVESV, RVEF and RVM did not differ significantly and agreed close between SSFP at 1.5T and FGRE at 7T (p=0.5850; p=0.5462; p=0.2789; p=0.0743). FGRE at 7T with voxel size (1.3x1.3x4) mm3 tended to overestimate RV volumes compared to SSFP at 1.5T (mean difference of RVEDV 8.2+/-9.3ml) and to FGRE at 7T with voxel size (1.2x1.2x6) mm3 (mean difference of RVEDV 9.3+/-8.6ml). CONCLUSIONS: FGRE cine imaging of the RV at 7T was feasible and provided good image quality. RV dimensions and function were comparable to SSFP at 1.5T as gold standard.
    Journal of Cardiovascular Magnetic Resonance 03/2013; 15(1):23. · 4.44 Impact Factor
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    Journal of Cardiovascular Magnetic Resonance 01/2013; 15(1). · 4.44 Impact Factor
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    ABSTRACT: This work demonstrates the feasibility of a hybrid radiofrequency (RF) applicator that supports magnetic resonance (MR) imaging and MR controlled targeted RF heating at ultrahigh magnetic fields (B0≥7.0T). For this purpose a virtual and an experimental configuration of an 8-channel transmit/receive (TX/RX) hybrid RF applicator was designed. For TX/RX bow tie antenna electric dipoles were employed. Electromagnetic field simulations (EMF) were performed to study RF heating versus RF wavelength (frequency range: 64 MHz (1.5T) to 600 MHz (14.0T)). The experimental version of the applicator was implemented at B0 = 7.0T. The applicators feasibility for targeted RF heating was evaluated in EMF simulations and in phantom studies. Temperature co-simulations were conducted in phantoms and in a human voxel model. Our results demonstrate that higher frequencies afford a reduction in the size of specific absorption rate (SAR) hotspots. At 7T (298 MHz) the hybrid applicator yielded a 50% iso-contour SAR (iso-SAR-50%) hotspot with a diameter of 43 mm. At 600 MHz an iso-SAR-50% hotspot of 26 mm in diameter was observed. RF power deposition per RF input power was found to increase with B0 which makes targeted RF heating more efficient at higher frequencies. The applicator was capable of generating deep-seated temperature hotspots in phantoms. The feasibility of 2D steering of a SAR/temperature hotspot to a target location was demonstrated by the induction of a focal temperature increase (ΔT = 8.1 K) in an off-center region of the phantom. Temperature simulations in the human brain performed at 298 MHz showed a maximum temperature increase to 48.6C for a deep-seated hotspot in the brain with a size of (19×23×32)mm(3) iso-temperature-90%. The hybrid applicator provided imaging capabilities that facilitate high spatial resolution brain MRI. To conclude, this study outlines the technical underpinnings and demonstrates the basic feasibility of an 8-channel hybrid TX/RX applicator that supports MR imaging, MR thermometry and targeted RF heating in one device.
    PLoS ONE 01/2013; 8(4):e61661. · 3.53 Impact Factor
  • ISMRM, Salt Lake City; 01/2013
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    Journal of Cardiovascular Magnetic Resonance 01/2013; 15(1). · 4.44 Impact Factor
  • ISMRM, Salt Lake City; 01/2013
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    Journal of Cardiovascular Magnetic Resonance 01/2013; 15(1). · 4.44 Impact Factor
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    ABSTRACT: a b s t r a c t A growing number of reports eloquently speak about explorations into cardiac magnetic resonance (CMR) at ultrahigh magnetic fields (B 0 P 7.0 T). Realizing the progress, promises and challenges of ultrahigh field (UHF) CMR this perspective outlines current trends in enabling MR technology tailored for cardiac MR in the short wavelength regime. For this purpose many channel radiofrequency (RF) technology con-cepts are outlined. Basic principles of mapping and shimming of transmission fields including RF power deposition considerations are presented. Explorations motivated by the safe operation of UHF-CMR even in the presence of conductive implants are described together with the physics, numerical simulations and experiments, all of which detailing antenna effects and RF heating induced by intracoronary stents at 7.0 T. Early applications of CMR at 7.0 T and their clinical implications for explorations into cardiovas-cular diseases are explored including assessment of cardiac function, myocardial tissue characterization, MR angiography of large and small vessels as well as heteronuclear MR of the heart and the skin. A con-cluding section ventures a glance beyond the horizon and explores future directions. The goal here is not to be comprehensive but to inspire the biomedical and diagnostic imaging communities to throw further weight behind the solution of the many remaining unsolved problems and technical obstacles of UHF-CMR with the goal to transfer MR physics driven methodological advancements into extra clinical value.
    Journal of Magnetic Resonance 11/2012; · 2.30 Impact Factor
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    ABSTRACT: To design, evaluate, and apply a 2D 16-channel transmit/receive (TX/RX) coil array tailored for cardiac magnetic resonance imaging (MRI) at 7.0 T. The cardiac coil array consists of two sections each using eight elements arranged in a 2 × 4 array. Radiofrequency (RF) safety was validated by specific absorption rate (SAR) simulations. Cardiac imaging was performed using 2D CINE FLASH imaging, T 2* mapping, and fat-water separation imaging. The characteristics of the coil array were analyzed including parallel imaging performance, left ventricular chamber quantification, and overall image quality. RF characteristics were found to be appropriate for all subjects included in the study. The SAR values derived from the simulations fall well within the limits of legal guidelines. The baseline signal-to-noise ratio (SNR) advantage at 7.0 T was put to use to acquire 2D CINE images of the heart with a very high spatial resolution of (1 × 1 × 4) mm(3) . The proposed coil array supports 1D acceleration factors of up to R = 4 without significantly impairing image quality. The 16-channel TX/RX coil has the capability to acquire high contrast and high spatial resolution images of the heart at 7.0 T. J. Magn. Reson. Imaging 2012;36:847-857. © 2012 Wiley Periodicals, Inc.
    Journal of Magnetic Resonance Imaging 06/2012; 36(4):847-57. · 2.57 Impact Factor
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    ABSTRACT: To implement, examine, and compare three multichannel transmit/receive coil configurations for cardiovascular MR (CMR) at 7T. Three radiofrequency transmit-receive (TX/RX) coils with 4-, 8-, and 16-coil elements were used. Ten healthy volunteers (seven males, age 28 ± 4 years) underwent CMR at 7T. For all three RX/TX coils, 2D CINE FLASH images of the heart were acquired. Cardiac chamber quantification, signal-to-noise ratio (SNR) analysis, parallel imaging performance assessment, and image quality scoring were performed. Mean total examination time was 29 ± 5 min. All images obtained with the 8- and 16-channel coils were diagnostic. No significant difference in ejection fraction (EF) (P > 0.09) or left ventricular mass (LVM) (P > 0.31) was observed between the coils. The 8- and 16-channel arrays yielded a higher mean SNR in the septum versus the 4-channel coil. The lowest geometry factors were found for the 16-channel coil (mean ± SD 2.3 ± 0.5 for R = 4). Image quality was rated significantly higher (P < 0.04) for the 16-channel coil versus the 8- and 4-channel coils. All three coil configurations are suitable for CMR at 7.0T under routine circumstances. A larger number of coil elements enhances image quality and parallel imaging performance but does not impact the accuracy of cardiac chamber quantification. KEY POINTS : • Cardiac chamber quantification using 7.0T magnetic resonance imaging is feasible. • Examination times for cardiac chamber quantification at 7.0T match current clinical practice. • Multichannel transceiver RF technology facilitates improved image quality and parallel imaging performance. • Increasing the number of RF channels does not influence cardiac chamber quantification.
    European Radiology 06/2012; 22(10):2211-20. · 4.34 Impact Factor
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    ABSTRACT: The sensitivity gain of ultrahigh field Magnetic Resonance (UHF-MR) holds the promise to enhance spatial and temporal resolution. Such improvements could be beneficial for cardiovascular MR. However, intracoronary stents used for treatment of coronary artery disease are currently considered to be contra-indications for UHF-MR. The antenna effect induced by a stent together with RF wavelength shortening could increase local radiofrequency (RF) power deposition at 7.0 T and bears the potential to induce local heating, which might cause tissue damage. Realizing these constraints, this work examines RF heating effects of stents using electro-magnetic field (EMF) simulations and phantoms with properties that mimic myocardium. For this purpose, RF power deposition that exceeds the clinical limits was induced by a dedicated birdcage coil. Fiber optic probes and MR thermometry were applied for temperature monitoring using agarose phantoms containing copper tubes or coronary stents. The results demonstrate an agreement between RF heating induced temperature changes derived from EMF simulations versus MR thermometry. The birdcage coil tailored for RF heating was capable of irradiating power exceeding the specific-absorption rate (SAR) limits defined by the IEC guidelines by a factor of three. This setup afforded RF induced temperature changes up to +27 K in a reference phantom. The maximum extra temperature increase, induced by a copper tube or a coronary stent was less than 3 K. The coronary stents examined showed an RF heating behavior similar to a copper tube. Our results suggest that, if IEC guidelines for local/global SAR are followed, the extra RF heating induced in myocardial tissue by stents may not be significant versus the baseline heating induced by the energy deposited by a tailored cardiac transmit RF coil at 7.0 T, and may be smaller if not insignificant than the extra RF heating observed under the circumstances used in this study.
    PLoS ONE 01/2012; 7(11):e49963. · 3.53 Impact Factor