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van Lanen RHGJ, Wiggins CJ, Colon AJ, Backes WH, Jansen JFA, Uher D, Drenthen GS, Roebroeck A, Ivanov D, Poser BA, Hoeberigs MC, van Kuijk SMJ, Hoogland G, Rijkers K, Wagner GL, Beckervordersandforth J, Delev D, Clusmann H, Wolking S, Klinkenberg S, Rouhl RPW, Hofman PAM, Schijns OEMG. Value of ultra-high field MRI in patients with suspected focal epilepsy and negative 3 T MRI (EpiUltraStudy): protocol for a prospective, longitudinal therapeutic study. Neuroradiology 2022; 64:753-764. [PMID: 34984522 PMCID: PMC8907090 DOI: 10.1007/s00234-021-02884-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2021] [Accepted: 12/09/2021] [Indexed: 10/30/2022]
Abstract
PURPOSE Resective epilepsy surgery is a well-established, evidence-based treatment option in patients with drug-resistant focal epilepsy. A major predictive factor of good surgical outcome is visualization and delineation of a potential epileptogenic lesion by MRI. However, frequently, these lesions are subtle and may escape detection by conventional MRI (≤ 3 T). METHODS We present the EpiUltraStudy protocol to address the hypothesis that application of ultra-high field (UHF) MRI increases the rate of detection of structural lesions and functional brain aberrances in patients with drug-resistant focal epilepsy who are candidates for resective epilepsy surgery. Additionally, therapeutic gain will be addressed, testing whether increased lesion detection and tailored resections result in higher rates of seizure freedom 1 year after epilepsy surgery. Sixty patients enroll the study according to the following inclusion criteria: aged ≥ 12 years, diagnosed with drug-resistant focal epilepsy with a suspected epileptogenic focus, negative conventional 3 T MRI during pre-surgical work-up. RESULTS All patients will be evaluated by 7 T MRI; ten patients will undergo an additional 9.4 T MRI exam. Images will be evaluated independently by two neuroradiologists and a neurologist or neurosurgeon. Clinical and UHF MRI will be discussed in the multidisciplinary epilepsy surgery conference. Demographic and epilepsy characteristics, along with postoperative seizure outcome and histopathological evaluation, will be recorded. CONCLUSION This protocol was reviewed and approved by the local Institutional Review Board and complies with the Declaration of Helsinki and principles of Good Clinical Practice. Results will be submitted to international peer-reviewed journals and presented at international conferences. TRIAL REGISTRATION NUMBER www.trialregister.nl : NTR7536.
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Affiliation(s)
- R H G J van Lanen
- Department of Neurosurgery, Maastricht University Medical Center, Maastricht, the Netherlands. .,School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.
| | - C J Wiggins
- Scannexus, Ultra-High Field MRI Research Center, Maastricht, the Netherlands
| | - A J Colon
- Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands
| | - W H Backes
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Department of Radiology, Maastricht University Medical Center, Maastricht, the Netherlands
| | - J F A Jansen
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands.,Department of Radiology, Maastricht University Medical Center, Maastricht, the Netherlands.,Department of Electrical Engineering, Eindhoven University of Technology, Eindhoven, the Netherlands
| | - D Uher
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Department of Radiology, Maastricht University Medical Center, Maastricht, the Netherlands
| | - G S Drenthen
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Department of Radiology, Maastricht University Medical Center, Maastricht, the Netherlands
| | - A Roebroeck
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, the Netherlands
| | - D Ivanov
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, the Netherlands
| | - B A Poser
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, the Netherlands
| | - M C Hoeberigs
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands.,Department of Radiology, Maastricht University Medical Center, Maastricht, the Netherlands
| | - S M J van Kuijk
- Department of Clinical Epidemiology and Medical Technology Assessment, Maastricht University Medical Center, Maastricht, the Netherlands
| | - G Hoogland
- Department of Neurosurgery, Maastricht University Medical Center, Maastricht, the Netherlands.,School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands
| | - K Rijkers
- Department of Neurosurgery, Maastricht University Medical Center, Maastricht, the Netherlands.,School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands
| | - G L Wagner
- Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands
| | | | - D Delev
- Department of Neurosurgery, RWTH Aachen University Hospital, Aachen, Germany
| | - H Clusmann
- Department of Neurosurgery, RWTH Aachen University Hospital, Aachen, Germany
| | - S Wolking
- Department of Epileptology and Neurology, RWTH Aachen University Hospital, Aachen, Germany
| | - S Klinkenberg
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands.,Department of Neurology, Maastricht University Medical Center, Maastricht, the Netherlands
| | - R P W Rouhl
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands.,Department of Neurology, Maastricht University Medical Center, Maastricht, the Netherlands
| | - P A M Hofman
- Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands.,Department of Radiology, Maastricht University Medical Center, Maastricht, the Netherlands
| | - O E M G Schijns
- Department of Neurosurgery, Maastricht University Medical Center, Maastricht, the Netherlands.,School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, the Netherlands.,Academic Centre for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, the Netherlands
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Fritz FJ, Poser BA, Roebroeck A. MESMERISED: Super-accelerating T 1 relaxometry and diffusion MRI with STEAM at 7 T for quantitative multi-contrast and diffusion imaging. Neuroimage 2021; 239:118285. [PMID: 34147632 DOI: 10.1016/j.neuroimage.2021.118285] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2020] [Revised: 06/14/2021] [Accepted: 06/16/2021] [Indexed: 12/17/2022] Open
Abstract
There is an increasing interest in quantitative imaging of T1, T2 and diffusion contrast in the brain due to greater robustness against bias fields and artifacts, as well as better biophysical interpretability in terms of microstructure. However, acquisition time constraints are a challenge, particularly when multiple quantitative contrasts are desired and when extensive sampling of diffusion directions, high b-values or long diffusion times are needed for multi-compartment microstructure modeling. Although ultra-high fields of 7 T and above have desirable properties for many MR modalities, the shortening T2 and the high specific absorption rate (SAR) of inversion and refocusing pulses bring great challenges to quantitative T1, T2 and diffusion imaging. Here, we present the MESMERISED sequence (Multiplexed Echo Shifted Multiband Excited and Recalled Imaging of STEAM Encoded Diffusion). MESMERISED removes the dead time in Stimulated Echo Acquisition Mode (STEAM) imaging by an echo-shifting mechanism. The echo-shift (ES) factor is independent of multiband (MB) acceleration and allows for very high multiplicative (ESxMB) acceleration factors, particularly under moderate and long mixing times. This results in super-acceleration and high time efficiency at 7 T for quantitative T1 and diffusion imaging, while also retaining the capacity to perform quantitative T2 and B1 mapping. We demonstrate the super-acceleration of MESMERISED for whole-brain T1 relaxometry with total acceleration factors up to 36 at 1.8 mm isotropic resolution, and up to 54 at 1.25 mm resolution qT1 imaging, corresponding to a 6x and 9x speedup, respectively, compared to MB-only accelerated acquisitions. We then demonstrate highly efficient diffusion MRI with high b-values and long diffusion times in two separate cases. First, we show that super-accelerated multi-shell diffusion acquisitions with 370 whole-brain diffusion volumes over 8 b-value shells up to b = 7000 s/mm2 can be generated at 2 mm isotropic in under 8 minutes, a data rate of almost a volume per second, or at 1.8 mm isotropic in under 11 minutes, achieving up to 3.4x speedup compared to MB-only. A comparison of b = 7000 s/mm2 MESMERISED against standard MB pulsed gradient spin echo (PGSE) diffusion imaging shows 70% higher SNR efficiency and greater effectiveness in supporting complex diffusion signal modeling. Second, we demonstrate time-efficient sampling of different diffusion times with 1.8 mm isotropic diffusion data acquired at four diffusion times up to 290 ms, which supports both Diffusion Tensor Imaging (DTI) and Diffusion Kurtosis Imaging (DKI) at each diffusion time. Finally, we demonstrate how adding quantitative T2 and B1+ mapping to super-accelerated qT1 and diffusion imaging enables efficient quantitative multi-contrast mapping with the same MESMERISED sequence and the same readout train. MESMERISED extends possibilities to efficiently probe T1, T2 and diffusion contrast for multi-component modeling of tissue microstructure.
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Affiliation(s)
- F J Fritz
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, the Netherlands; Institut für Systemische Neurowissenschaften, Zentrum für Experimentelle Medizin, Universitätklinikum Hamburg-Eppendorf (UKE), Hamburg, Deutschland
| | - B A Poser
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, the Netherlands
| | - A Roebroeck
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, the Netherlands.
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van Lanen RHGJ, Colon AJ, Wiggins CJ, Hoeberigs MC, Hoogland G, Roebroeck A, Ivanov D, Poser BA, Rouhl RPW, Hofman PAM, Jansen JFA, Backes W, Rijkers K, Schijns OEMG. Ultra-high field magnetic resonance imaging in human epilepsy: A systematic review. Neuroimage Clin 2021; 30:102602. [PMID: 33652376 PMCID: PMC7921009 DOI: 10.1016/j.nicl.2021.102602] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/05/2020] [Revised: 02/12/2021] [Accepted: 02/13/2021] [Indexed: 12/15/2022]
Abstract
RATIONALE Resective epilepsy surgery is an evidence-based curative treatment option for patients with drug-resistant focal epilepsy. The major preoperative predictor of a good surgical outcome is detection of an epileptogenic lesion by magnetic resonance imaging (MRI). Application of ultra-high field (UHF) MRI, i.e. field strengths ≥ 7 Tesla (T), may increase the sensitivity to detect such a lesion. METHODS A keyword search strategy was submitted to Pubmed, EMBASE, Cochrane Database and clinicaltrials.gov to select studies on UHF MRI in patients with epilepsy. Follow-up study selection and data extraction were performed following PRISMA guidelines. We focused on I) diagnostic gain of UHF- over conventional MRI, II) concordance of MRI-detected lesion, seizure onset zone and surgical decision-making, and III) postoperative histopathological diagnosis and seizure outcome. RESULTS Sixteen observational cohort studies, all using 7T MRI were included. Diagnostic gain of 7T over conventional MRI ranged from 8% to 67%, with a pooled gain of 31%. Novel techniques to visualize pathological processes in epilepsy and lesion detection are discussed. Seizure freedom was achieved in 73% of operated patients; no seizure outcome comparison was made between 7T MRI positive, 7T negative and 3T positive patients. 7T could influence surgical decision-making, with high concordance of lesion and seizure onset zone. Focal cortical dysplasia (54%), hippocampal sclerosis (12%) and gliosis (8.1%) were the most frequently diagnosed histopathological entities. SIGNIFICANCE UHF MRI increases, yet variably, the sensitivity to detect an epileptogenic lesion, showing potential for use in clinical practice. It remains to be established whether this results in improved seizure outcome after surgical treatment. Prospective studies with larger cohorts of epilepsy patients, uniform scan and sequence protocols, and innovative post-processing technology are equally important as further increasing field strengths. Besides technical ameliorations, improved correlation of imaging features with clinical semiology, histopathology and clinical outcome has to be established.
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Affiliation(s)
- R H G J van Lanen
- Department of Neurosurgery, Maastricht University Medical Center, Maastricht, the Netherlands; School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, The Netherlands.
| | - A J Colon
- Academic Center for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, The Netherlands
| | - C J Wiggins
- Scannexus, Ultra High Field MRI Research Center, Maastricht, The Netherlands
| | - M C Hoeberigs
- Department of Radiology, Maastricht University Medical Center, Maastricht, The Netherlands
| | - G Hoogland
- Department of Neurosurgery, Maastricht University Medical Center, Maastricht, the Netherlands; School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, The Netherlands; Academic Center for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, The Netherlands
| | - A Roebroeck
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - D Ivanov
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - B A Poser
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - R P W Rouhl
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, The Netherlands; Academic Center for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, The Netherlands; Department of Neurology, Maastricht University Medical Center, Maastricht, The Netherlands
| | - P A M Hofman
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, The Netherlands; Department of Radiology, Maastricht University Medical Center, Maastricht, The Netherlands
| | - J F A Jansen
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, The Netherlands; Department of Radiology, Maastricht University Medical Center, Maastricht, The Netherlands; Department of Electrical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| | - W Backes
- School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, The Netherlands; Department of Radiology, Maastricht University Medical Center, Maastricht, The Netherlands
| | - K Rijkers
- Department of Neurosurgery, Maastricht University Medical Center, Maastricht, the Netherlands; School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, The Netherlands; Academic Center for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, The Netherlands
| | - O E M G Schijns
- Department of Neurosurgery, Maastricht University Medical Center, Maastricht, the Netherlands; School for Mental Health and Neuroscience (MHeNs), Maastricht University, Maastricht, The Netherlands; Academic Center for Epileptology, Kempenhaeghe/Maastricht University Medical Center, Heeze/Maastricht, The Netherlands
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4
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Sengupta S, Fritz FJ, Harms RL, Hildebrand S, Tse DHY, Poser BA, Goebel R, Roebroeck A. High resolution anatomical and quantitative MRI of the entire human occipital lobe ex vivo at 9.4T. Neuroimage 2017; 168:162-171. [PMID: 28336427 PMCID: PMC5862655 DOI: 10.1016/j.neuroimage.2017.03.039] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2016] [Revised: 03/17/2017] [Accepted: 03/19/2017] [Indexed: 11/06/2022] Open
Abstract
Several magnetic resonance imaging (MRI) contrasts are sensitive to myelin content in gray matter in vivo which has ignited ambitions of MRI-based in vivo cortical histology. Ultra-high field (UHF) MRI, at fields of 7 T and beyond, is crucial to provide the resolution and contrast needed to sample contrasts over the depth of the cortex and get closer to layer resolved imaging. Ex vivo MRI of human post mortem samples is an important stepping stone to investigate MRI contrast in the cortex, validate it against histology techniques applied in situ to the same tissue, and investigate the resolutions needed to translate ex vivo findings to in vivo UHF MRI. Here, we investigate key technology to extend such UHF studies to large human brain samples while maintaining high resolution, which allows investigation of the layered architecture of several cortical areas over their entire 3D extent and their complete borders where architecture changes. A 16 channel cylindrical phased array radiofrequency (RF) receive coil was constructed to image a large post mortem occipital lobe sample (~80×80×80 mm3) in a wide-bore 9.4 T human scanner with the aim of achieving high-resolution anatomical and quantitative MR images. Compared with a human head coil at 9.4 T, the maximum Signal-to-Noise ratio (SNR) was increased by a factor of about five in the peripheral cortex. Although the transmit profile with a circularly polarized transmit mode at 9.4 T is relatively inhomogeneous over the large sample, this challenge was successfully resolved with parallel transmit using the kT-points method. Using this setup, we achieved 60μm anatomical images for the entire occipital lobe showing increased spatial definition of cortical details compared to lower resolutions. In addition, we were able to achieve sufficient control over SNR, B0 and B1 homogeneity and multi-contrast sampling to perform quantitative T2* mapping over the same volume at 200 μm. Markov Chain Monte Carlo sampling provided maximum posterior estimates of quantitative T2* and their uncertainty, allowing delineation of the stria of Gennari over the entire length and width of the calcarine sulcus. We discuss how custom RF receive coil arrays built to specific large post mortem sample sizes can provide a platform for UHF cortical layer-specific quantitative MRI over large fields of view. Custom-built 16 channel 9.4 T RF-coil to image large post mortem samples at high resolution. Parallel transmit techniques allow homogenization of B1+ for 3D GRE imaging at UHF. 60 μm anatomical MRI of the entire human occipital lobe. 200 μm isotropic quantitative T2* mapping of the entire human occipital lobe. A platform for future UHF cortical layer specific qMRI over large FoVs.
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Affiliation(s)
- S Sengupta
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands.
| | - F J Fritz
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - R L Harms
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - S Hildebrand
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - D H Y Tse
- Department of Neuropsychology and Psychopharmacology, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - B A Poser
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - R Goebel
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - A Roebroeck
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
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5
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Herbst M, Poser BA, Singh A, Deng W, Knowles B, Zaitsev M, Stenger VA, Ernst T. Motion correction for diffusion weighted SMS imaging. Magn Reson Imaging 2016; 38:33-38. [PMID: 27988191 DOI: 10.1016/j.mri.2016.12.013] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2016] [Revised: 12/13/2016] [Accepted: 12/13/2016] [Indexed: 10/20/2022]
Affiliation(s)
- M Herbst
- Department of Medicine, John A. Burns School of Medicine, University of Hawaii, Honolulu, HI, USA; Department of Radiology, Medical Physics, Medical Center University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany.
| | - B A Poser
- Maastricht Brain Imaging Centre, Faculty of Psychology & Neuroscience, Maastricht University, Netherlands
| | - A Singh
- Department of Medicine, John A. Burns School of Medicine, University of Hawaii, Honolulu, HI, USA
| | - W Deng
- Department of Medicine, John A. Burns School of Medicine, University of Hawaii, Honolulu, HI, USA
| | - B Knowles
- Department of Radiology, Medical Physics, Medical Center University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
| | - M Zaitsev
- Department of Radiology, Medical Physics, Medical Center University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
| | - V A Stenger
- Department of Medicine, John A. Burns School of Medicine, University of Hawaii, Honolulu, HI, USA
| | - T Ernst
- Department of Medicine, John A. Burns School of Medicine, University of Hawaii, Honolulu, HI, USA
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Orzada S, Maderwald S, Poser BA, Johst S, Kannengiesser S, Ladd ME, Bitz AK. Time-interleaved acquisition of modes: an analysis of SAR and image contrast implications. Magn Reson Med 2011; 67:1033-41. [PMID: 21858867 DOI: 10.1002/mrm.23081] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2010] [Revised: 03/30/2011] [Accepted: 06/13/2011] [Indexed: 11/08/2022]
Abstract
As the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole-body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called time-interleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B 1+ modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time-interleaved acquisition of mode on image contrast as well as on time-averaged specific absorption rate is addressed in detail. Time-interleaved acquisition of mode is superior in B 1+ homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time-interleaved acquisition of modes can enable almost homogeneous high-field imaging throughout the entire field of view in PD, T(2) , and T(2) *-weighted imaging and, if a specified homogeneity criterion is met, in T(1) -weighted imaging as well.
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Affiliation(s)
- S Orzada
- University Duisburg-Essen, Essen, Germany.
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