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- Dechent, Peter, et al.
(författare)
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Basal cerebral blood volume during the poststimulation undershoot in BOLD MRI of the human brain
- 2011
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Ingår i: Journal of Cerebral Blood Flow and Metabolism. - : SAGE Publications. - 1559-7016 .- 0271-678X. ; 31:1, s. 82-89
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Tidskriftsartikel (refereegranskat)abstract
- One of the characteristics of the blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) response to functional challenges of the brain is the poststimulation undershoot, which has been suggested to originate from a delayed recovery of either cerebral blood volume (CBV) or cerebral metabolic rate of oxygen to baseline. Using bolus-tracking MRI in humans, we recently showed that relative CBV rapidly normalizes after the end of stimulation. As this observation contradicts at least part of the blood-pool contrast agent studies performed in animals, we reinvestigated the CBV contribution by dynamic T1-weighted three-dimensional MRI (8 seconds temporal resolution) and Vasovist at 3 T (12 subjects). Initially, we determined the time constants of individual BOLD responses. After injection of Vasovist, CBV-related T1-weighted signal changes revealed a signal increase during visual stimulation (1.7%±0.4%), but no change relative to baseline in the poststimulation phase (0.2%±0.3%). This finding renders the specific nature of the contrast agent unlikely to be responsible for the discrepancy between human and animal studies. With the assumption of normalized cerebral blood flow after stimulus cessation, a normalized CBV lends support to the idea that the BOLD MRI undershoot reflects a prolonged elevation of oxidative metabolism.
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| 6. |
- Helms, Gunther, et al.
(författare)
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Identification of signal bias in the variable flip angle method by linear display of the algebraic Ernst equation
- 2011
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 66:3, s. 669-677
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Tidskriftsartikel (refereegranskat)abstract
- A novel linear parameterization for the variable flip angle method for longitudinal relaxation time T1 quantification from spoiled steady state MRI is derived from the half angle tangent transform, t, of the flip angle. Plotting the signal S at coordinates x=St and y=S/t, respectively, establishes a line that renders signal amplitude and relaxation term separately as y-intercept and slope. This representation allows for estimation of the respective parameter from the experimental data. A comprehensive analysis of noise propagation is performed. Numerical results for efficient optimization of longitudinal relaxation time and proton density mapping experiments are derived. Appropriate scaling allows for a linear presentation of data that are acquired at different short pulse repetition times, TR << T1 thus increasing flexibility in the data acquisition by removing the limitation of a single pulse repetition time. Signal bias, like due to slice-selective excitation or imperfect spoiling, can be readily identified by systematic deviations from the linear plot. The method is illustrated and validated by 3T experiments on phantoms and human brain.
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| 7. |
- Helms, Gunther, et al.
(författare)
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Modeling the influence of TR and excitation flip angle on the magnetization transfer ratio (MTR) in human brain obtained from 3D spoiled gradient echo MRI
- 2010
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 64:1, s. 177-185
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Tidskriftsartikel (refereegranskat)abstract
- Attempts to optimize the magnetization transfer ratio (MTR) obtained from spoiled gradient echo MRI have focused on the properties of the magnetization transfer pulse. In particular, continuous-wave models do not explicitly account for the effects of excitation and relaxation on the MTR. In this work, these were modeled by an approximation of free relaxation between the radiofrequency pulses and of an instantaneous saturation event describing the magnetization transfer pulse. An algebraic approximation of the signal equation can be obtained for short pulse repetition time and small flip angles. This greatly facilitated the mathematical treatment and understanding of the MTR. The influence of inhomogeneous radiofrequency fields could be readily incorporated. The model was verified on the human brain in vivo at 3 T by variation of flip angle and pulse repetition time. The corresponding range in MTR was similar to that observed by a 4-fold increase of magnetization transfer pulse power. Choice of short pulse repetition time and larger flip angles improved the MTR contrast and reduced the influence of radiofrequency inhomogeneity. Optimal contrast is obtained around an MTR of 50%, and noise progression is reduced when a high reference signal is obtained.
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