Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Repeatability of in vivo measurement of multicomponent T2* relaxation in articular cartialges in human knee is important to clinical use. This study evaluated the repeatability of two-component T2* relaxation on seven healthy human subjects. The left knee was scanned once a day in three consecutive days, on a clinical 3T MRI scanner with eight-channel knee coil and ultrashort echo time pulse sequence at 11 echo times=0.6-40 ms. ⋯ The intersubject repeatability was also good, with CV∼8% (range 1-15%) for the short- and long-T2* relaxation time and CV∼10% (range 2-20%) for the component intensity fraction. The long-T2* component showed significantly better repeatability (CV∼8%) than the short-T2* component (CV∼12%) (P<0.005). These CV values suggest that in vivo measurement of two-component T2* relaxation in the knee cartilages is repeatable on clinical scanner at 3 T, with a signal-to-noise ratio of 90.
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Echo-planar imaging is a fast and commonly used magnetic resonance imaging technique with applications in diffusion weighted and functional MRI. Fast data acquisition in echo-planar imaging is accomplished by the extended readout, which also introduces sensitivity to off-resonance effects such as amplitude of static (polarizing) field inhomogeneities and eddy-currents. These off-resonance effects produce geometric distortions in the corresponding echo-planar images. ⋯ Additionally, a new approach to visualize and determine the distortions in a hybrid (x, y, kPSF) three-dimensional space is proposed. The accuracy and robustness of the proposed technique is demonstrated in phantom and in vivo experiments. The accuracy of the presented method here is compared to previous techniques for echo-planar imaging distortion correction such as PLACE.
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Quantification of cerebral blood flow using QUIPSSII pulsed arterial spin labeling requires that the QUIPSS saturation delay TI1 is shorter than the natural temporal bolus width. Yet the duration of the bolus of tagged spins entering the region of interest varies during vasoactive stimuli such as gaseous challenges or across subjects due to differences in blood velocity or vessel geometry. ⋯ The average bolus width measured in the right and left middle cerebral artery territories using the bolus turbo sampling technique has a repeatability coefficient of 75 ms and correlates significantly with the TI1,max determined from a novel multi-TI1 protocol (R=0.65, P<0.05). The possibility to measure the bolus width under hypercapnia is demonstrated.
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In this work, an extension of the Cartesian sensitivity encoding (SENSE) parallel imaging framework is proposed. In the well-known SENSE solution, the overdetermined reconstruction inversion problem is optimized to get the highest signal-to-noise ratio in the image. In this extension, the probability of artifacts due to incorrect knowledge of the receiver coil sensitivities is also taken into account. ⋯ This inversion problem can still be solved by a least-squares optimization without the need of any complex iterative scheme. Results in abdominal imaging show that artifact levels can be substantially reduced, at the cost of a signal-to-noise ratio penalty. The size of the signal-to-noise ratio penalty depends on the assumed inaccuracy of the coil sensitivities, sensitivity encoding acceleration factor, and coil configuration.