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Bayer FM, Bock M, Jezzard P, Smith AK. Unbiased signal equation for quantitative magnetization transfer mapping in balanced steady-state free precession MRI. Magn Reson Med 2021; 87:446-456. [PMID: 34331470 PMCID: PMC8951070 DOI: 10.1002/mrm.28940] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2021] [Revised: 06/12/2021] [Accepted: 07/06/2021] [Indexed: 12/20/2022]
Abstract
Purpose Quantitative magnetization transfer (qMT) imaging can be used to quantify the proportion of protons in a voxel attached to macromolecules. Here, we show that the original qMT balanced steady‐state free precession (bSSFP) model is biased due to over‐simplistic assumptions made in its derivation. Theory and Methods We present an improved model for qMT bSSFP, which incorporates finite radiofrequency (RF) pulse effects as well as simultaneous exchange and relaxation. Furthermore, a correction relating to finite RF pulse effects for sinc‐shaped excitations is derived. The new model is compared to the original one in numerical simulations of the Bloch‐McConnell equations and in previously acquired in vivo data. Results Our numerical simulations show that the original signal equation is significantly biased in typical brain tissue structures (by 7%‐20%), whereas the new signal equation outperforms the original one with minimal bias (<1%). It is further shown that the bias of the original model strongly affects the acquired qMT parameters in human brain structures, with differences in the clinically relevant parameter of pool‐size‐ratio of up to 31%. Particularly high biases of the original signal equation are expected in an MS lesion within diseased brain tissue (due to a low T2/T1‐ratio), demanding a more accurate model for clinical applications. Conclusion The improved model for qMT bSSFP is recommended for accurate qMT parameter mapping in healthy and diseased brain tissue structures.
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Affiliation(s)
- Fritz M Bayer
- Wellcome Centre for Integrative Neuroimaging, FMRIB Division, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, UK.,D-BSSE, ETH Zurich, Basel, Switzerland
| | - Michael Bock
- Department of Radiology, Medical Physics, Medical Center-University of Freiburg, Freiburg, Germany.,Faculty of Medicine, University of Freiburg, Freiburg, Germany
| | - Peter Jezzard
- Wellcome Centre for Integrative Neuroimaging, FMRIB Division, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, UK
| | - Alex K Smith
- Wellcome Centre for Integrative Neuroimaging, FMRIB Division, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, UK
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2
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Yarnykh VL, Kisel AA, Khodanovich MY. Scan-Rescan Repeatability and Impact of B 0 and B 1 Field Nonuniformity Corrections in Single-Point Whole-Brain Macromolecular Proton Fraction Mapping. J Magn Reson Imaging 2019; 51:1789-1798. [PMID: 31737961 DOI: 10.1002/jmri.26998] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2019] [Revised: 10/26/2019] [Accepted: 10/26/2019] [Indexed: 01/25/2023] Open
Abstract
BACKGROUND Single-point macromolecular proton fraction (MPF) mapping is a recent quantitative MRI method for fast assessment of brain myelination. Information about reproducibility and sensitivity of MPF mapping to magnetic field nonuniformity is important for clinical applications. PURPOSE To assess scan-rescan repeatability and a value of B0 and B1 field inhomogeneity corrections in single-point synthetic-reference MPF mapping. STUDY TYPE Prospective. POPULATION Eight healthy adult volunteers underwent two scans with 11.5 ± 2.3 months interval. FIELD STRENGTH/SEQUENCE 3T; whole-brain 3D MPF mapping protocol included three spoiled gradient-echo sequences providing T1 , proton density, and magnetization transfer contrasts with 1.25 × 1.25 × 1.25 mm3 resolution and B0 and B1 mapping sequences. ASSESSMENT MPF maps were reconstructed with B0 and B1 field nonuniformity correction, B0 - and B1 -only corrections, and without corrections. Mean MPF values were measured in automatically segmented white matter (WM) and gray matter (GM). STATISTICAL TESTS Within-subject coefficient of variation (CV), intraclass correlation coefficient (ICC), Bland-Altman plots, and paired t-tests to assess scan-rescan repeatability. Repeated-measures analysis of variance (ANOVA) to compare field corrections. RESULTS Maximal relative local MPF errors without correction in the areas of largest field nonuniformities were about 5% and 27% for B0 and B1 , respectively. The effect of B0 correction was insignificant for whole-brain WM (P > 0.25) and GM (P > 0.98) MPF. The absence of B1 correction caused a positive relative bias of 4-5% (P < 0.001) in both tissues. Scan-rescan agreement was similar for all field correction options with ICCs 0.80-0.81 for WM and 0.89-0.92 for GM. CVs were 1.6-1.7% for WM and 0.7-1.0% for GM. DATA CONCLUSION The single-point method enables high repeatability of MPF maps obtained with the same equipment. Correction of B0 inhomogeneity may be disregarded to shorten the examination time. B1 nonuniformity correction improves accuracy of MPF measurements at 3T. Reliability of whole-brain MPF measurements in WM and GM is not affected by B0 and B1 field corrections. LEVEL OF EVIDENCE 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2020;51:1789-1798.
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Affiliation(s)
- Vasily L Yarnykh
- Department of Radiology, University of Washington, Seattle, Washington, USA.,Research Institute of Biology and Biophysics, Tomsk State University, Tomsk, Russian Federation
| | - Alena A Kisel
- Research Institute of Biology and Biophysics, Tomsk State University, Tomsk, Russian Federation
| | - Marina Y Khodanovich
- Research Institute of Biology and Biophysics, Tomsk State University, Tomsk, Russian Federation
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Kim J, Lee S, Choi SH, Park S. Rapid framework for quantitative magnetization transfer imaging with interslice magnetization transfer and dictionary‐driven fitting approaches. Magn Reson Med 2019; 82:1671-1683. [DOI: 10.1002/mrm.27850] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2018] [Revised: 05/16/2019] [Accepted: 05/20/2019] [Indexed: 01/02/2023]
Affiliation(s)
- Jae‐Woong Kim
- Magnetic Resonance Imaging Laboratory, Department of Bio and Brain Engineering Korea Advanced Institute of Science and Technology Daejeon Korea
| | - Sul‐Li Lee
- Magnetic Resonance Imaging Laboratory, Department of Bio and Brain Engineering Korea Advanced Institute of Science and Technology Daejeon Korea
| | - Seung Hong Choi
- Department of Radiology Seoul National University College of Medicine Seoul Korea
| | - Sung‐Hong Park
- Magnetic Resonance Imaging Laboratory, Department of Bio and Brain Engineering Korea Advanced Institute of Science and Technology Daejeon Korea
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Combès B, Monteau L, Bannier E, Callot V, Labauge P, Ayrignac X, Carra Dallière C, Pelletier J, Maarouf A, de Seze J, Collongues N, Barillot C, Edan G, Ferré JC, Kerbrat A. Measurement of magnetization transfer ratio (MTR) from cervical spinal cord: Multicenter reproducibility and variability. J Magn Reson Imaging 2018; 49:1777-1785. [PMID: 30350328 DOI: 10.1002/jmri.26537] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2018] [Revised: 09/19/2018] [Accepted: 09/20/2018] [Indexed: 12/22/2022] Open
Abstract
BACKGROUND Assessing the multicenter variability of magnetization transfer ratio (MTR) measurements in the spinal cord of healthy controls is the first step toward investigating its clinical use as a biomarker. PURPOSE To analyze the between-session, between-participant, and between-scanner variability of MTR measurements in automatically extracted regions of interest in the cervical cord of healthy controls. STUDY TYPE Control study. POPULATION Forty-four participants, distributed across five MRI scanners (all from the same manufacturer). Ten participants were scanned twice in the same scanner, and 10 others were scanned twice in two different scanners. FIELD STRENGTH/SEQUENCE 3D-gradient echo images, centered on C5, without and with magnetization transfer prepulse at 3T. ASSESSMENT We calculated the mean MTR for different vertebral levels in the whole cord (WC), as well as in the white matter and gray matter, and determined the between-session, between-participant, and between-scanner variabilities. STATISTICAL TESTS Coefficients of variation and intraclass correlations (ICCs) for the different variabilities and their associated confidence intervals. RESULTS The MTR measurements for Levels C4-C6 (near the slab center) exhibited a mean value in WC of 34.6 pu and a pooled standard deviation of 0.9 pu. The between-session coefficient of variation was estimated as 2.3% (ICC = 0.63), the between-participant coefficient as 1.6% (ICC = 0.32), and the between-scanner coefficient as 0.7% (ICC = 0.05). The resulting aggregate coefficient of variation was 2.9%, which was sufficiently low to detect an MTR reduction of 1 pu between groups of about 45 participants (Type-I error rate: 0.05; Type-II error rate: 0.10). DATA CONCLUSION The good between-scanner reproducibility and low overall variability in cervical spinal cord MTR measurements in a control population might pave the way for multicenter analyses in various neurological diseases with moderate cohort sizes. LEVEL OF EVIDENCE 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;49:1777-1785.
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Affiliation(s)
- Benoit Combès
- Univ Rennes, Inria, CNRS, Inserm, IRISA UMR 6074, Visages, U1128, France
| | - Laureline Monteau
- Univ Rennes, Inria, CNRS, Inserm, IRISA UMR 6074, Visages, U1128, France.,CHU Rennes, Radiology Department, F-35033, Rennes, France
| | - Elise Bannier
- Univ Rennes, Inria, CNRS, Inserm, IRISA UMR 6074, Visages, U1128, France.,CHU Rennes, Radiology Department, F-35033, Rennes, France
| | - Virginie Callot
- AP-HM, Pôle d'imagerie médicale, Hôpital de la Timone, CEMEREM, Marseille, France.,Aix-Marseille Université, CNRS, UMR 7339, CRMBM, Marseille, France
| | | | | | | | - Jean Pelletier
- AP-HM, Pôle d'imagerie médicale, Hôpital de la Timone, CEMEREM, Marseille, France.,AP-HM, CHU Timone, Pole de Neurosciences Cliniques, Department of Neurology, Marseille, France
| | - Adil Maarouf
- AP-HM, Pôle d'imagerie médicale, Hôpital de la Timone, CEMEREM, Marseille, France.,AP-HM, CHU Timone, Pole de Neurosciences Cliniques, Department of Neurology, Marseille, France
| | - Jerome de Seze
- Strasbourg University Hospital, France; CIC Strasbourg INSERM 1434, Strasbourg, France
| | - Nicolas Collongues
- Strasbourg University Hospital, France; CIC Strasbourg INSERM 1434, Strasbourg, France
| | - Christian Barillot
- Univ Rennes, Inria, CNRS, Inserm, IRISA UMR 6074, Visages, U1128, France
| | - Gilles Edan
- Univ Rennes, Inria, CNRS, Inserm, IRISA UMR 6074, Visages, U1128, France.,Neurology Department, Rennes University Hospital, France
| | - Jean Christophe Ferré
- Univ Rennes, Inria, CNRS, Inserm, IRISA UMR 6074, Visages, U1128, France.,CHU Rennes, Radiology Department, F-35033, Rennes, France
| | - Anne Kerbrat
- Univ Rennes, Inria, CNRS, Inserm, IRISA UMR 6074, Visages, U1128, France.,Neurology Department, Rennes University Hospital, France
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Groeschel S, Hagberg GE, Schultz T, Balla DZ, Klose U, Hauser TK, Nägele T, Bieri O, Prasloski T, MacKay AL, Krägeloh-Mann I, Scheffler K. Assessing White Matter Microstructure in Brain Regions with Different Myelin Architecture Using MRI. PLoS One 2016; 11:e0167274. [PMID: 27898701 PMCID: PMC5127571 DOI: 10.1371/journal.pone.0167274] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2016] [Accepted: 11/13/2016] [Indexed: 12/13/2022] Open
Abstract
OBJECTIVE We investigate how known differences in myelin architecture between regions along the cortico-spinal tract and frontal white matter (WM) in 19 healthy adolescents are reflected in several quantitative MRI parameters that have been proposed to non-invasively probe WM microstructure. In a clinically feasible scan time, both conventional imaging sequences as well as microstructural MRI parameters were assessed in order to quantitatively characterise WM regions that are known to differ in the thickness of their myelin sheaths, and in the presence of crossing or parallel fibre organisation. RESULTS We found that diffusion imaging, MR spectroscopy (MRS), myelin water fraction (MWF), Magnetization Transfer Imaging, and Quantitative Susceptibility Mapping were myelin-sensitive in different ways, giving complementary information for characterising WM microstructure with different underlying fibre architecture. From the diffusion parameters, neurite density (NODDI) was found to be more sensitive than fractional anisotropy (FA), underlining the limitation of FA in WM crossing fibre regions. In terms of sensitivity to different myelin content, we found that MWF, the mean diffusivity and chemical-shift imaging based MRS yielded the best discrimination between areas. CONCLUSION Multimodal assessment of WM microstructure was possible within clinically feasible scan times using a broad combination of quantitative microstructural MRI sequences. By assessing new microstructural WM parameters we were able to provide normative data and discuss their interpretation in regions with different myelin architecture, as well as their possible application as biomarker for WM disorders.
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Affiliation(s)
| | - Gisela E. Hagberg
- High Field Magnetic Resonance, Max-Planck Institute for Biological Cybernetics, Tübingen, Germany
- Biomedical Magnetic Resonance, University Hospital Tübingen, Germany
| | - Thomas Schultz
- Institute of Computer Science, University of Bonn, Germany
| | - Dávid Z. Balla
- Department Physiology of Cognitive Processes, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
| | - Uwe Klose
- Department of Diagnostic and Interventional Neuroradiology, University Hospital, Tübingen, Germany
| | - Till-Karsten Hauser
- Department of Diagnostic and Interventional Neuroradiology, University Hospital, Tübingen, Germany
| | - Thomas Nägele
- Department of Diagnostic and Interventional Neuroradiology, University Hospital, Tübingen, Germany
| | - Oliver Bieri
- Radiological Physics, University of Basel, Basel, Switzerland
| | | | | | | | - Klaus Scheffler
- High Field Magnetic Resonance, Max-Planck Institute for Biological Cybernetics, Tübingen, Germany
- Biomedical Magnetic Resonance, University Hospital Tübingen, Germany
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6
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Garcia M, Gloor M, Bieri O, Radue EW, Lieb JM, Cordier D, Stippich C. Imaging of Primary Brain Tumors and Metastases with Fast Quantitative 3-Dimensional Magnetization Transfer. J Neuroimaging 2015; 25:1007-14. [PMID: 25702714 DOI: 10.1111/jon.12222] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2014] [Revised: 12/04/2014] [Accepted: 12/10/2014] [Indexed: 11/27/2022] Open
Abstract
BACKGROUND AND PURPOSE This study assesses whether magnetization transfer (MT) imaging provides additive information to conventional MRI in brain tumors. METHODS MT data of 26 patients with neoplastic and metastatic brain tumors were analyzed at 1.5 T. For the 3 largest tumor groups investigated in this study--glioblastoma multiforme (GBM), meningiomas, and metastases-statistical comparisons were performed. Analyzed MT parameters included the magnetization transfer ratio (MTR) and 4 quantitative MT parameters (qMT): Relaxation times (T1, T2), exchange rate (kf), and macromolecular content (F). Total imaging time of high-resolution whole brain MTR and qMT imaging with balanced steady-state free precession required 9 minutes. Five ROIs were chosen: Contrast-enhancing (T1W-CE), noncontrast-enhancing (T1W-non-CE), proximal hyperintensity (T2W-pSI), distal hyperintensity (T2W-dSI), and a reference (ref). RESULTS Pathologies showed significant (P < .05) MT changes (MTR and qMT) compared to the reference. The T1W-CE, T1W-non-CE, and T2W-pSI ROIs of GBMs, meningiomas, and metastases showed significant differences in MTR and qMT estimates. Similar MTR with significant different qMT values were observed in several ROIs among different lesions. MT maps (MTR and qMT) indicated changes in tissue appearing unaffected on MRI in most glial tumors. CONCLUSIONS MTR and qMT imaging enables a better differentiation between brain tumors and provides additive information to MRI.
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Affiliation(s)
- Meritxell Garcia
- Division of Diagnostic & Interventional Neuroradiology, Department of Radiology, Clinic for Radiology & Nuclear Medicine, University of Basel Hospital, Basel, Switzerland
| | - Monika Gloor
- Division of Radiological Physics, Department of Radiology, Clinic for Radiology & Nuclear Medicine, University of Basel Hospital, Basel, Switzerland
| | - Oliver Bieri
- Division of Radiological Physics, Department of Radiology, Clinic for Radiology & Nuclear Medicine, University of Basel Hospital, Basel, Switzerland
| | - Ernst-Wilhelm Radue
- Medical Imaging Analysis Center, University of Basel Hospital, Basel, Switzerland
| | - Johanna M Lieb
- Division of Diagnostic & Interventional Neuroradiology, Department of Radiology, Clinic for Radiology & Nuclear Medicine, University of Basel Hospital, Basel, Switzerland
| | - Dominik Cordier
- Department of Neurosurgery, University of Basel Hospital, Basel, Switzerland
| | - Christoph Stippich
- Division of Diagnostic & Interventional Neuroradiology, Department of Radiology, Clinic for Radiology & Nuclear Medicine, University of Basel Hospital, Basel, Switzerland
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Comparison between balanced steady-state free precession and standard spoiled gradient echo magnetization transfer ratio imaging in multiple sclerosis: methodical and clinical considerations. Neuroimage 2014; 108:87-94. [PMID: 25536494 DOI: 10.1016/j.neuroimage.2014.12.045] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2014] [Revised: 12/12/2014] [Accepted: 12/16/2014] [Indexed: 11/22/2022] Open
Abstract
Different pathological processes like demyelination and axonal loss can alter the magnetisation transfer ratio (MTR) in brain tissue. The standard method to measure this effect is to scan the respective tissue twice, one with and one without a specific saturation pulse. A major drawback of this technique based on spoiled gradient echo (GRE) sequences relates to its long acquisition time due to the saturation pulses. Recently, an alternative concept for MT imaging based on balanced steady state free precession (bSSFP) has been proposed. Modification of the duration of the radiofrequency pulses for imaging allows scanning MT sensitive and non-sensitive images. The steady-state character of bSSFP with high intrinsic signal-to-noise ratio (SNR) allows three-dimensional (3D) whole brain MTR at high spatial resolution within short and thus clinically feasible acquisition times. In the present study, both bSSFP-MT and 2D GRE-MT imaging were used in a cohort of 31 patients with multiple sclerosis (MS) to characterize different normal appearing (NA) and pathological brain structures. Under the constraint of identical SNR and scan time, a 3.4 times higher voxel size could be achieved with bSSFP. This increased resolution allowed a more accurate delineation of the different brain structures, especially of cortex, hippocampus and MS lesions. In a multiple linear regression model, we found an association between MTR of cortical lesions and a clinical measure of disability (r= -0.407, p=0.035) in the bSSFP dataset only. The different relaxation weighting of the base images (T2/T1 in bSSFP, proton density in GRE) had no effects besides a larger spreading of the MTR values of the different NA structures. This was demonstrated by the nearly perfect linearity between the NA matter MTR of both techniques as well as in the absolute MTR differences between NA matter and the respective lesions.
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Walker-Samuel S, Ramasawmy R, Torrealdea F, Rega M, Rajkumar V, Johnson SP, Richardson S, Gonçalves M, Parkes HG, Arstad E, Thomas DL, Pedley RB, Lythgoe MF, Golay X. In vivo imaging of glucose uptake and metabolism in tumors. Nat Med 2013; 19:1067-72. [PMID: 23832090 PMCID: PMC5275770 DOI: 10.1038/nm.3252] [Citation(s) in RCA: 368] [Impact Index Per Article: 33.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2012] [Accepted: 10/26/2012] [Indexed: 01/01/2023]
Abstract
Tumors have a greater reliance on anaerobic glycolysis for energy production than normal tissues. We developed a noninvasive method for imaging glucose uptake in vivo that is based on magnetic resonance imaging and allows the uptake of unlabeled glucose to be measured through the chemical exchange of protons between hydroxyl groups and water. This method differs from existing molecular imaging methods because it permits detection of the delivery and uptake of a metabolically active compound in physiological quantities. We show that our technique, named glucose chemical exchange saturation transfer (glucoCEST), is sensitive to tumor glucose accumulation in colorectal tumor models and can distinguish tumor types with differing metabolic characteristics and pathophysiologies. The results of this study suggest that glucoCEST has potential as a useful and cost-effective method for characterizing disease and assessing response to therapy in the clinic.
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Affiliation(s)
- Simon Walker-Samuel
- University College London Centre for Advanced Biomedical Imaging, Division of Medicine and Institute of Child Health, London, UK.
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Fast high-resolution brain imaging with balanced SSFP: Interpretation of quantitative magnetization transfer towards simple MTR. Neuroimage 2012; 59:202-11. [DOI: 10.1016/j.neuroimage.2011.07.038] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2011] [Accepted: 07/12/2011] [Indexed: 11/17/2022] Open
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10
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Bieri O. Superbalanced steady state free precession. Magn Reson Med 2011; 67:1346-54. [PMID: 22173718 DOI: 10.1002/mrm.23122] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2011] [Revised: 05/30/2011] [Accepted: 07/08/2011] [Indexed: 11/08/2022]
Abstract
Steady state free precession (SSFP) signal theory is commonly derived in the limit of quasi-instantaneous radiofrequency (RF) excitation. SSFP imaging protocols, however, are frequently set up with minimal pulse repetition times and RF pulses can thus constitute a considerable amount to the actual pulse repetition time. As a result, finite RF pulse effects can lead to 10-20% signal deviation from common SSFP theory in the transient and in the steady state which may impair the accuracy of SSFP-based quantitative imaging techniques. In this article, a new and generic approach for intrinsic compensation of finite RF pulse effects is introduced. Compensation is based on balancing relaxation effects during finite RF excitation, similar to flow or motion compensation of gradient moments. RF pulse balancing, in addition to the refocusing of gradient moments with balanced SSFP, results in a superbalanced SSFP sequence free of finite RF pulse effects in the transient and in the steady state; irrespective of the RF pulse duration, flip angles, relaxation times, or off-resonances. Superbalancing of SSFP sequences can be used with all quantitative SSFP techniques where finite RF pulse effects are expected or where elongated RF pulses are used.
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Affiliation(s)
- Oliver Bieri
- Division of Radiological Physics, Department of Medical Radiology, University of Basel Hospital, Basel, Switzerland.
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