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- Forshult, Stig E., 1942-
(author)
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Magnetic Resonance Imaging – MRI – An Overview
- 2007
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Reports (other academic/artistic)abstract
- Magnetic Resonance Imaging or MRI is a modern diagnostic tool for acquiring information from the interior of a human body. MRI can create three-dimensional images of a human organ without hurting the organ in any way and without using any ionizing radiation.The body must be placed in a strong magnetic field; more than ten thousand times the magnetic field of earth. A radio signal is sent into the body, where it is absorbed by hydrogen atoms. The hydrogens in the body respond by sending back a signal to a detector. The strength of this signal mirrors the amount of hydrogen in various parts of the body.When creating an image of an organ, the signal must be acquired from every part of the organ point by point by a scanning procedure. To accomplish this, the magnetic field is rapidly varied with gradients in three dimensions. The monitored signal is the sum of signals from every unique volume element within the body. The instrument receives a large number of data from which the signal magnitude of every volume element can be calculated.Most parts of the body have a roughly equal concentration of hydrogen. However, signals from hydrogen atoms decay with unequal speeds depending on their various environments. Therefore the magnitude of the induced radio signal is monitored some time after the end of the initializing puls. Now various tissues show different signal strengths, from which it is possible to build an image with desired contrast. This is often excellent, even for soft tissues. It is of special interest that the signal from hydrogen in lesions and tumors often decay slower than surrounding tissues and therefore can be detected in the image.The contrast of the image is created by the experimental procedure and is not inherent in the imaged body. Thus different experimental routines will result in unequal images – not pictures – of the same object. Therefore it is crucial that the experimenter learns how to use different RF-pulse sequences and how to interpret the result.
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- Forshult, Stig E.
(author)
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Quantitative analysis with pulsed NMR and the CONTIN computer program
- 2004
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Reports (other academic/artistic)abstract
- Various substances have been analyzed by pulsed NMR at 0.47 T in order to extract quantitative information from the CPMG spin-echo signal. This signal can be considered to show the true T2 relaxation of the analyte/s. In most cases the relaxation is a first order process and the FID will be a sum of several exponentially decreasing signals. This can be resolved into its components by an inverse Laplace transform. To achieve this the CONTIN computer program has been our tool. The result is a spectrum-like graph with the peaks of the individual FIDs as functions of the time constants T2. By calculating the peak areas and divide them with the T2 of the peaks one can find the total number of protons relaxing in a T2-range. This may also bee done by drawing the graph with a logarithmic abscissa. Then the peak areas, now integrals of f(T2) with respect to lg T2, will directly show the relative amounts of protons at each T2 The spectrum will also be better resolved over a broader range of various relaxation times. However, as the peaks are assumed to have Gaussian shapes, they will be slightly distorted on a logarithmic abscissa. Thus, with an inverse Laplace transform it is possible to make calculations of the relative amount of protons in a sample without any chemical pretreatment. The result is not a full analysis of the sample but can be regarded as a characterization of its main components.
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