| 1. |
- Dyverfeldt, Petter, 1980-, et al.
(författare)
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Quantification of intravoxel velocity standard deviation and turbulence intensity by generalizing phase-contrast MRI
- 2006
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 0740-3194 .- 1522-2594. ; 56:4, s. 850-858
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent flow, characterized by velocity fluctuations, is a contributing factor to the pathogenesis of several cardiovascular diseases. A clinical noninvasive tool for assessing turbulence is lacking, however. It is well known that the occurrence of multiple spin velocities within a voxel during the influence of a magnetic gradient moment causes signal loss in phase-contrast magnetic resonance imaging (PC-MRI). In this paper a mathematical derivation of an expression for computing the standard deviation (SD) of the blood flow velocity distribution within a voxel is presented. The SD is obtained from the magnitude of PC-MRI signals acquired with different first gradient moments. By exploiting the relation between the SD and turbulence intensity (TI), this method allows for quantitative studies of turbulence. For validation, the TI in an in vitro flow phantom was quantified, and the results compared favorably with previously published laser Doppler anemometry (LDA) results. This method has the potential to become an important tool for the noninvasive assessment of turbulence in the arterial tree.
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| 2. |
- Golman, Klaes, et al.
(författare)
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Cardiac metabolism measured noninvasively by hyperpolarized (13)C MRI.
- 2008
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 59:5, s. 1005-1013
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Tidskriftsartikel (refereegranskat)abstract
- Pyruvate is included in the energy production of the heart muscle and is metabolized into lactate, alanine, and CO(2) in equilibrium with HCO(3) (-). The aim of this study was to evaluate the feasibility of using (13)C hyperpolarization enhanced MRI to monitor pyruvate metabolism in the heart during an ischemic episode. The left circumflex artery of pigs (4 months, male, 29-34 kg) was occluded for 15 or 45 min followed by 2 hr of reperfusion. Pigs were examined by (13)C chemical shift imaging following intravenous injection of 1-(13)C pyruvate. (13)C chemical shift MR imaging was used in order to visualize the local concentrations of the metabolites. After a 15-min occlusion (no infarct) the bicarbonate signal level in the affected area was reduced (25-44%) compared with the normal myocardium. Alanine signal level was normal. After a 45-min occlusion (infarction) the bicarbonate signal was almost absent (0.2-11%) and the alanine signal was reduced (27-51%). Due to image-folding artifacts the data obtained for lactate were inconclusive. These studies demonstrate that cardiac metabolic imaging with hyperpolarized 1-(13)C-pyruvate is feasible. The changes in concentrations of the metabolites within a minute after injection can be detected and metabolic maps constructed. Magn Reson Med 59:1005-1013, 2008. (c) 2008 Wiley-Liss, Inc.
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| 3. |
- Halle, Bertil
(författare)
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Molecular theory of field-dependent proton spin-lattice relaxation in tissue
- 2006
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 56:1, s. 60-72
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Tidskriftsartikel (refereegranskat)abstract
- A molecular theory is presented for the field-dependent spin-lattice relaxation time of water in tissue. The theory attributes the large relaxation enhancement observed at low frequencies to intermediary protons in labile groups or internal water molecules that act as relaxation sinks for the bulk water protons. Exchange of intermediary protons not only transfers magnetization to bulk water protons, it also drives relaxation by a mechanism of exchange-mediated orientational randomization (EMOR). An analytical expression for T, is derived that remains valid outside the motional-narrowing regime. Cross-relaxation between intermediary protons and polymer protons plays an important role, whereas spin diffusion among polymer protons can be neglected. For sufficiently slow exchange, the dispersion midpoint is determined by the local dipolar field rather than by molecular motions, which makes the dispersion frequency insensitive to temperature and system composition. The EMOR model differs fundamentally from previous models that identify collective polymer vibrations or hydration water dynamics as the molecular motion responsible for spin relaxation. Unlike previous models, the EMOR model accounts quantitatively for H-1 magnetic relaxation dispersion (MRD) profiles from tissue model systems without invoking unrealistic parameter values.
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| 4. |
- 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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| 5. |
- Helms, Gunther, et al.
(författare)
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Quantitative FLASH MRI at 3T using a rational approximation of the Ernst equation
- 2008
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 59:4, s. 667-672
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Tidskriftsartikel (refereegranskat)abstract
- From the half-angle substitution of trigonometric terms in the Ernst equation, rational approximations of the flip angle dependence of the FLASH signal can be derived. Even the rational function of the lowest order was in good agreement with the experiment for flip angles up to 20°. Three-dimensional maps of the signal amplitude and longitudinal relaxation rates in human brain were obtained from eight subjects by dual-angle measurements at 3T (nonselective 3D-FLASH, 7° and 20° flip angle, TR=30ms, isotropic resolution of 0.95mm, each 7:09 min). The corresponding estimates of T1 and signal amplitude are simple algebraic expressions and deviated about 1% from the exact solution. They are ill-conditioned to estimate the local flip angle deviation but can be corrected post hoc by division of squared RF maps obtained by independent measurements. Local deviations from the nominal flip angles strongly affected the relaxation estimates and caused considerable blurring of the T1 histograms.
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| 6. |
- Helms, Gunther, et al.
(författare)
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Rapid radiofrequency field mapping in vivo using single-shot STEAM MRI
- 2008
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 60:3, s. 739-743
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Tidskriftsartikel (refereegranskat)abstract
- Higher field strengths entail less homogeneous RF fields. This may influence quantitative MRI and MRS. A method for rapidly mapping the RF field in the human head with minimal distortion was developed on the basis of a single-shot stimulated echo acquisition mode (STEAM) sequence. The flip angle of the second RF pulse in the STEAM preparation was set to 60° and 100° instead of 90°, inducing a flip angle-dependent signal change. A quadratic approximation of this trigonometric signal dependence together with a calibration accounting for slice excitation-related bias allowed for directly determining the RF field from the two measurements only. RF maps down to the level of the medulla could be obtained in less than 1 min and registered to anatomical volumes by means of the T2-weighted STEAM images. Flip angles between 75% and 125% of the nominal value were measured in line with other methods.
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| 7. |
- Kjølby, Birgitte F, et al.
(författare)
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Analysis of partial volume effects on arterial input functions using gradient echo: a simulation study.
- 2009
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Ingår i: Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. - : Wiley. - 1522-2594. ; 61:6, s. 1300-9
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Tidskriftsartikel (refereegranskat)abstract
- Absolute blood flow and blood volume measurements using perfusion weighted MRI require an accurately measured arterial input function (AIF). Because of limited spatial resolution of MR images, AIF voxels cannot be placed completely within a feeding artery. We present a two-compartment model of an AIF voxel including the relaxation properties of blood and tissue. Artery orientations parallel and perpendicular to the main magnetic field were investigated and AIF voxels were modeled to either include or be situated close to a large artery. The impact of partial volume effects on quantitative perfusion metrics was investigated for the gradient echo pulse sequence at 1.5 T and 3.0 T. It is shown that the tissue contribution broadens and introduces fluctuations in the AIF. Furthermore, partial volume effects bias perfusion metrics in a nonlinear fashion, compromising quantitative perfusion estimates and profoundly effecting local AIF selection.
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| 8. |
- Lagerstrand, Kerstin M, et al.
(författare)
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Flow-induced disturbances in balanced steady-state free precession images: means to reduce or exploit them.
- 2009
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Ingår i: Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. - : Wiley. - 1522-2594. ; 61:4, s. 893-8
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Tidskriftsartikel (refereegranskat)abstract
- In this work computer simulations and phantom measurements are presented that show the effect of flow on in-plane balanced steady-state free precession images. The images were studied for various flow velocities, excitation regions, relaxation times, RF-pulse angles, and off-resonance frequencies. The work shows that flow-induced disturbances are present in the images, but can be reduced by the application of inhomogeneous excitation regions. Also, a velocity quantification method that utilizes the disturbances was developed and proved to quantify flow velocities accurately. The work concluded that the flow-induced disturbances can be reduced to improve image quality, but can also be exploited to quantify the flow velocity.
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| 9. |
- Magnusson, Peter, et al.
(författare)
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Passive catheter tracking during interventional MRI using hyperpolarized 13C.
- 2007
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 57:6, s. 1140-1147
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Tidskriftsartikel (refereegranskat)abstract
- nterventional procedures in MRI can be performed preclinically using active or passive catheter-tracking methods. A novel passive nonproton technique is suggested that uses a catheter filled with a hyperpolarized C-13 contrast agent. A prototype three-lumen catheter was built with two closed lumens containing a flowing hyperpolarized C-13 contrast agent. Entire-length C-13 catheter projection visualization could be performed in vivo with a catheter SNR of similar to 80, one dual projection frame per similar to 700 ms, and an in-plane resolution of 2 x 2 mm(2) while traveling through the aorta of a pig. The traveling path of the C-13 catheter was visualized after back-projection catheter reconstruction and after image fusion with an anatomical offline proton road map. Catheter length visualization was aided by an oblique planar visualization mode. The high catheter signal demonstrated, together with the entire catheter length visualization and high surrounding soft-tissue contrast, warrants further development into a real-time technique.
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| 10. |
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