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- Tapper, Sofie, et al.
(författare)
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Retrospective artifact elimination in MEGA-PRESS using a correlation approach
- 2019
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Ingår i: Magnetic Resonance in Medicine. - : John Wiley & Sons. - 0740-3194 .- 1522-2594. ; 81:4, s. 2223-2237
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Tidskriftsartikel (refereegranskat)abstract
- PurposeTo develop a method for retrospective artifact elimination of MRS data. This retrospective method was based on an approach that combines jackknife analyses with the correlation of spectral windows, and therefore termed “JKC.”MethodsTwelve healthy volunteers performed 3 separate measurement protocols using a 3T MR system. One protocol consisted of 2 cerebellar MEGA‐PRESS measurements: 1 reference and 1 measurement including head movements. One‐third of the artifact‐influenced datasets were treated as training data for the implementation the JKC method, and the rest were used for validation.ResultsThe implemented JKC method correctly characterized most of the validation data. Additionally, after elimination of the detected artifacts, the resulting concentrations were much closer to those computed for the reference datasets. Moreover, when the JKC method was applied to the reference data, the estimated concentrations were not affected, compared with standard averaging.ConclusionThe implemented JKC method can be applied without any extra cost to MRS data, regardless of whether the dataset has been contaminated by artifacts. Furthermore, the results indicate that the JKC method could be used as a quality control of a dataset, or as an indication of whether a shift in voxel placement has occurred during the measurement.
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| 2. |
- Helms, Gunther, et al.
(författare)
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Removing bias and increasing dynamic range in DREAM flip angle mapping at 7T
- 2019
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Ingår i: Removing bias and increasing dynamic range in DREAM flip angle mapping at 7T. - 1545-4428. ; 27
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Konferensbidrag (refereegranskat)abstract
- DREAM is an ultra-fast multi-slice B1+-mapping technique based on the single-shot STEAM sequence. To study noise and bias related to slice-profiles, DREAM B1+-maps at 3.75 mm resolution were acquired at 7T in phantoms and in human brain with nominal flip angles (FA) between 20° and 90° of the two STEAM preparation pulses. B1+ was decreasing at actual FAs above 50°; noise became apparent at actual FAs below 20° reducing dynamic range. By varying the preparation FA, this reliable range (20°
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| 3. |
- Wiggermann, Vanessa, et al.
(författare)
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In vivo investigation of the multi‐exponential T2 decay in human white matter at 7 T: Implications for myelin water imaging at UHF
- 2021
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Ingår i: NMR in Biomedicine. - : Wiley. - 0952-3480 .- 1099-1492. ; 34:2
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Tidskriftsartikel (refereegranskat)abstract
- IntroductionMulti‐component T2 mapping using a gradient‐ and spin‐echo (GraSE) acquisition has become standard for myelin water imaging at 3 T. Higher magnetic field strengths promise signal‐to‐noise ratio benefits but face specific absorption rate limits and shortened T2 times. This study investigates compartmental T2 times in vivo and addresses advantages and challenges of multi‐component T2 mapping at 7 T.MethodsWe acquired 3D multi‐echo GraSE data in seven healthy adults at 7 T, with three subjects also scanned at 3 T. Stimulated echoes arising from B1+ inhomogeneities were accounted for by the extended phase graph (EPG) algorithm. We used the computed T2 distributions to determine T2 times that identify different water pools and assessed signal‐to‐noise and fit‐to‐noise characteristics of the signal estimation. We compared short T2 fractions and T2 properties of the intermediate water pool at 3 T and 7 T.ResultsFlip angle mapping confirmed that EPG accurately determined the larger B1+ inhomogeneity at 7 T. Multi‐component T2 analysis demonstrated shortened T2 times at 7 T compared with 3 T. Fit‐to‐noise and signal‐to‐noise ratios were improved at 7 T but depended on B1+ homogeneity. Adjusting the shortest T2 to 8 ms and the T2 threshold that separates different water compartments to 20 ms yielded short T2 fractions at 7 T that conformed to 3 T data. Short T2 fractions in myelin‐rich white matter regions were lower at 7 T than at 3 T, and higher in iron‐rich structures.DiscussionAdjusting the T2 compartment boundaries was required due to the shorter T2 relaxation times at 7 T. Shorter echo spacing would better sample the fast decaying signal but would increase peripheral nerve stimulation. Multi‐channel transmission will improve T2 measurements at 7 T.ConclusionWe used a multi‐echo 3D GraSE sequence to characterize the multi‐exponential T2 decay at 7 T. We adapted T2 parameters for evaluation of the short T2 fraction. Obtained 7 T multi‐component T2 maps were in good agreement with 3 T data.
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| 4. |
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| 5. |
- Dathe, Henning, et al.
(författare)
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Exact algebraization of the signal equation of spoiled gradient echo MRI
- 2010
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 55, s. 4231-4245
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Tidskriftsartikel (refereegranskat)abstract
- The Ernst equation for Fourier transform nuclear magnetic resonance (MR) describes the spoiled steady-state signal created by periodic partial excitation. In MR imaging (MRI), it is commonly applied to spoiled gradient-echo acquisition in the steady state, created by a small flip angle α at a repetition time TR much shorter than the longitudinal relaxation time T1. We describe two parameter transformations of α and TR/T1, which render the Ernst equation as a low-order rational function. Computer algebra can be readily applied for analytically solving protocol optimization, as shown for the dual flip angle experiment. These transformations are based on the half-angle tangent substitution and its hyperbolic analogue. They are monotonic and approach identity for small α and small TR/T1 with a third-order error. Thus, the exact algebraization can be readily applied to fast gradient echo MRI to yield a rational approximation in α and TR/T1. This reveals a fundamental relationship between the square of the flip angle and TR/T1 which characterizes the Ernst angle, constant degree of T1-weighting and the influence of the local radiofrequency field.
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| 8. |
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| 9. |
- Helms, Gunther, et al.
(författare)
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Cross-vendor transfer and RF coil comparison of a high-resolution MP2RAGE protocol for brain imaging at 7T
- 2020
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Ingår i: Acta Scientiarum Lundensia. - 1651-5013. ; 2020:001, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- Abstract. An established MP2RAGE protocol for semi-quantitative structural brain MRI was transferred from one 7T MR scanner to that of another vendor featuring comparable hardware, but opposite polarity. On this system, the scan time could be reduced from 11 to 8 minutes by elliptical k-space sampling. Three configurations of radio-frequency (RF) transmission were compared (single channel with quadrature slitting, two and eight channels of fixed phase delays). Data processing for offline calculation MP2RAGE images is described in detail. The DICOM scaling of scanner B entails a loss of precision and accuracy and stronger artifacts in regions of low RF field, which can be improved by dielectric pads. For comparison and multi-site studies, though, use of an identical scan protocol and identical RF hardware is recommended.
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| 10. |
- Helms, Gunther, et al.
(författare)
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High-resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T1 relaxation obtained from 3D FLASH MRI
- 2008
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 60:6, s. 1396-1407
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Tidskriftsartikel (refereegranskat)abstract
- An empirical equation for the magnetization transfer (MT) FLASH signal is derived by analogy to dual-excitation FLASH, introducing a novel semiquantitative parameter for MT, the percentage saturation imposed by one MT pulse during TR. This parameter is obtained by a linear transformation of the inverse signal, using two reference experiments of proton density and T1 weighting. The influence of sequence parameters on the MT saturation was studied. An 8.5-min protocol for brain imaging at 3T was based on nonselective sagittal 3D-FLASH at 1.25mm isotropic resolution using partial acquisition techniques (TR/TE/flipangle = 25ms/4.9ms/5° or 11ms/4.9ms/15° for the T1 reference). A 12.8 ms Gaussian MT pulse was applied 2.2 kHz off-resonance with 540° flip angle. The MT saturation maps showed an excellent contrast in the brain due to clearly separated distributions for white and gray matter and cerebrospinal fluid. Within the limits of the approximation (excitation <15°, TR/T1<<1) the MT term depends mainly on TR, the energy and offset of the MT pulse, but hardly on excitation and T1 relaxation. It is inherently compensated for inhomogeneities of receive and transmit RF fields. The MT saturation appeared to be a sensitive parameter to depict MS lesions and alterations of normal-appearing white matter.
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