By William Oldendorf M.D., William Oldendorf Jr. (auth.)
This ebook isn't really meant as a common textual content on MRI. it really is written as an intro duction to the sector, for nonexperts. We current right here an easy exposition of convinced features of MRI which are vital to appreciate to exploit this worthy diagnostic software intelligently in a scientific environment. the fundamental rules are awarded nonmathematically, utilizing no equations and no less than symbols and abbreviations. For these requiring a deeper realizing of MRI, this booklet can help facilitate the transition to plain texts. Chapters 1 via four supply a normal advent to the phenomenon of nuclear magnetic resonance and the way it really is utilized in imaging. bankruptcy 1 discus ses magnetic resonance, utilizing a compass needle for instance. In bankruptcy 2, the transition to the magnetic resonance of the atomic nucleus is made. bankruptcy three describes the rules of imaging. In bankruptcy four, the phrases T 1 and T 2 are defined and their dating to tissue characterization; the joys damental position of thermal magnetic noise in T 1 and T 2 is discussed.
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This is indeed the case. For reasons explained in the Appendix, magnetic fluctuations at this particular frequency cause the population of 40 4. Tissue Characterization: T, and T2 NMR BAND -----... HIGH VISCOSITY 10 100 1000 FREQUENCY, MEGAHERTZ 10000 Figure 23. The two curves represent the frequency distribution of the magnetic fluctuations due to thermal motion. At low viscosity (or high temperature), the thermal motion is faster; there are more high frequency components. The area under the two curves is the same.
T:2 is defined as the time for 63% of the signal to be lost due to dephasing. 5-3 seconds. T:2 must always be shorter than or equal to T 1> since phase relationship is of no interest once the nuclei have all lost their energy. FACTORS AFFECTING Tl AND T2 The local fluctuations in magnetic field strength that alter magnetic behavior of hydrogen nuclei come from thermal motion of magnetic particles. Thermal Motion The environment of the nucleus is a world of violent motion. At body temperature, water and other molecules that make up the magnetic environment are in constant motion, colliding randomly with each other.
We would expect the motion of all the compass needles to decay at the same rate, and their combined signal to fall off at the same rate as that produced by a single compass, but this is not the case. The combined signal falls off much faster than expected, due to the phenomenon of dephasing. Although the compass needles would all initially be swinging together (in phase), unavoidable local variations in magnetic field strength cause them to swing at slightly different rates; after a few seconds they are no longer in phase.