MR imaging

Magnetic Resonance Imaging on whole body units provides visualization of tissue inside slices with a thickness of several millimetres. The spatial resolution in the plain is often better than one millimetre so that even relatively small structures can be well depicted. However, the spatial resolution is not sufficient to resolve the microscopic structures mentioned in Section 2. Only the cross-sections of single muscles and septa from fatty tissue or cormective tissue can be visualized in MR images recorded from humans in vivo. 

Most clinical examinations apply robust spin-echo or fast spin-echo sequences. These types of sequences provide tissue contrast changes by variation of the chosen repetition time TR (time interval between succeeding RF excitations) and echo time TE (time delay between RF excitation and signal acquisition). 

Tissue contrast in sequences for standard imaging usually depends on following items  

Compared to lipids in the subcutaneous fat layer and in the bone marrow (TiR O.S s), musculature shows a clearly slower longitudinal relaxation (Ti 1.0 s). For this reason, musculature has lost more signal intensity than fat in Ty weighted images, when compared with proton density weighting (Fig. 5a 

Transverse relaxation of musculature is relatively fast compared with many other tissues. Measurements in our volunteers resulted in T2 values of approximately 40 ms, when mono-exponential fits were applied on signal intensities from images recorded with variable TE. More sophisticated approaches for relaxometry revealed a multi-exponential decay of musculature with several T2 values. Normal muscle tissue usually shows lower signal intensity than fat or free water as shown in Fig. 5c. Fatty structures inside the musculature, but also water in the intermuscular septa (Fig. 5f) appear with bright signal in T2-weighted images. 

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P. Blunder, B. Bliimich, R. E. Botto, E. Fuku-shima (eds.) 1998, Spatially Resolved Magnetic Resonance Methods, Materials, Medicine, Biology, Rheology, Geology, Ecology, Hardware, VCH, Weinheim, 774 pp. Collected lectures from an MR imaging conference, various fields, also contains chemical engineering and transport. 

Figure 2.2.3 shows a typical block diagram for a compact MRI system. The square box surrounded by the dotted lines shows the electrical sub-system, which is stored in a single portable rack as shown in Figure 2.2.2. The electrical sub-system is thus often called the (portable) MRI console. In accord with Figure 2.2.3, an MR image acquisition process is described as follows.

E. A. Zerhouni, D. M. Parrish, W. J. Rodgers, A. Yang, E. P. Shapiro 1988, (Human heart Tagging with MR imaging - a method of noninvasive assessment of myocardial motion), Radiology 169, 59. 

Fig. 5.2.2 (a) The upper section of a typical the superficial flow direction (down the col-model trickle-bed reactor used in MRI studies, umn, z) have been measured 2D slice sections (b) MR image of water flowing within a fixed through the 3D image are shown with slices bed of spherical glass beads the beads have no taken in the xy, yz and xz planes indicated. 

The resolution of the zeolite MR image is 100 x 100 x 100 gm3 and has therefore reached the resolution limit that defines NMR microscopy. For the instrumentation used for this experiment, it will take at least a few milliseconds due to the ramping time of the field gradients. If the mean displacement of the xenon atoms during this experimental time scale reaches the dimension of the voxels or pixels, the resolution limit is reached. For instance, for the aerogel experiments in Figure... 

Fig. 5.5.7 A 2D slice through a H 3D MR image of the fixed-bed of catalyst particles. The catalyst particles appear as black fluid within the inter-particle space is indicated by lighter shades. Chemical conversion within ten selected volumes within each of the three transverse sections indicated is investigated in Figures 5.5.9-5.5.11. The direction of superficial flow (z) is also shown. Reproduced with permission from Ref. [24], copyright Elsevier (2002).

Today, around 30-40% of all medical MR images are generated with the use of a contrast medium. This number is expected to increase substantially with the development of new agents and new applications. 

Fig. 26 MR images of tumors of mice after they were injected with (a) paramagnetic av[33-specific RGD-liposomes and (b) nonspecific paramagnetic RAD-liposomes. (c, d) Fluorescence microscopy of 10 pm sections from dissected tumors revealed a distinct difference between tumors of mice that were injected with RGD-liposomes (c) or RAD-liposomes (d). Vessel staining was done with an endothelial cell-specific FITC-CD31 antibody. The red fluorescence represents the liposomes and the green fluorescence represents blood vessels. RGD-liposomes were exclusively found within the vessel lumen or associated with vessel endothelial cells (c), whereas RAD-liposomes (d) were also found outside blood vessels within the tumor (Adapted from [88])...

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