Time [sec

Fig. 16. Phase contrast imaging of the aorta in a human volunteer. Both the magnitude and phase images are shown. Images were acquired for 70 cardiac phases, covering approx 2.5 heartbeats. A region of interest (ROI; white box) was placed on the phase images in the thoracic aorta to determine the variation of the flow velocity in the vertical direction of the image plane. The variation of the velocity is shown in the graph.

3.7. MRI-Based Flow Velocity Measurements

A recorded MR signal can be represented in terms of a magnitude and a phase component. MRI images can elicit the spatial variations of the signal magnitudes, but it is also possible to create maps showing the spatial variations of the signal phases. It can be shown that the phase of the signal is sensitive to the velocity of tissue or blood. The so-called phase contrast MRI technique uses the phases of the signals to measure velocities. For an in-depth discussion, we refer the reader to the literature. An example of a phase contrast flow velocity measurement in an aorta is shown in Fig. 16.

3.8. Imaging of Fiber Structure

The analysis of myocardial microstructure is considered an important factor in understanding underlying pathologies because the structural fiber arrangement is modified over the time-course of various cardiac diseases. The tissue microstructure of the myocardium may be characterized by the resulting distribution of field inhomogeneities. Koehler et al. (24) showed that this structure can be observed on T2* maps of rat hearts at 11.75 T, a field strength much higher than used in current clinical MRI scanners. In this study, an in-plane resolution of 78 ^m and a slice thickness of 250 ^m were achieved, which compares quite favorably with photographs of postmortem histological sections. The field inhomogeneities revealed by T2* did provide structural information about biological tissue (both normal and scar tissue), as shown in Fig. 17. Two obvious advantages of this "NMR technique" compared to conventional histology are its noninvasive nature and speed.

How To Reduce Acne Scarring

How To Reduce Acne Scarring

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