NMR Imaging and MRI

At location A on the image, signals identified in the embryo spectrum include ATP, glycine (Gly), and creatine (Cr) as well as large signals from lipids (Lip). (Courtesy of Dr. Christian Bock, Aifred-Wegener-lnstitute tor Poiarand Marine Research, Bremerhaven, Germany www.awi.de.) 

MRI permits the noninvasive imaging of the interior of solid objects. This has been successfully utilized in the study of extruded polymers and foams and the study of spatial distributions of porosity in porous materials. The structure of ice in polar ice cores has been studied as noted earlier unlike optical imaging, MRI is nondestructive. Ice has sufficient mobile protons to be imaged with conventional MRI. 

In conventional MRI, RF excitation and signal acquisition are separated by an echo time greater than 1 ms. By the time this echo delay is completed, hard materials with fast transverse relaxation times have already decayed and cannot be measured. In SWIFT, the echo time approaches zero because the acquisition begins within a few microseconds of excitation. 

X-rays have been used extensively for noninvasive medical imaging studies in the past, but the contrast between different forms of soft tissue is low and therefore abnormalities in soft tissues are hard to visualize using X-rays. Also, X-rays are ionizing radiation and can cause tissue damage at high exposure levels. In contrast, MRI, also a noninvasive procedure, has essentially no side effects and can more readily visualize very small differences in soft tissue. MRI can monitor in vivo concentrations of biologically important molecules like adenosine triphosphate (ATP) in a noninvasive manner this permits 

Another example of the use of MRI is seen in Fig. 3.57. This is the image of a fractured hip the fracmre is in the hip bone on the left side of the image and appears as the 

There are two major limitations to NMR (1) it is limited to the measurement of nuclei with magnetic moments and (2) it may be less sensitive than other spectroscopic and chromatographic methods of analyses. As we have seen, although most elements have at least one nucleus that responds in NMR, that nucleus is often of low natural abundance and may have a small magnetogyric ratio, reducing sensitivity. The proton, H, and fluorine, are the two most sensitive elements. 

Elements in the ionic state do not respond in NMR, but the presence of ions in a sample contributes to unacceptable line broadening. Paramagnetic contaminants such as iron and dissolved oxygen also broaden NMR lines. Nuclei with quadrupole moments, such as Br, broaden the NMR signal. Line broadening in general reduces the NMR signal and hence the sensitivity. 

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Voidage. Single- and two-phase flows in fixed-bed reactors were visualized by three-dimensional NMR imaging and MRI velocimetry. The fluid velocity vector is determined at a pore-scale resolution of 156 pm. Characteristics of... 

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