Week 10 B Magnetic Resonance Imaging & Bone Densitometry

Week 10 B Magnetic Resonance Imaging & Bone...

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Week 10 A Magnetic Resonance Imaging Material from Clinical Imaging with Skeletal, Chest and Abdomen Pattern Differentials by Dennis M. Marchiori
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Magnetic Resonance Imaging Developed from Nuclear Magnetic Resonance used in laboratories to evaluate composition of laboratory samples. Raymond Damadien used the technology to produce a crude image of a rat tumor in 1974. He produced an image of a full body in 1977. His machine is now in the Smithsonian.
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MRI Benefits MRI has superior tissue contrast compared to computed tomography and radiography. Tissue contrast in CT is based upon attenuation properties where MRI uses properties of tissue nuclei, principally hydrogen. This information is more sensitive than x-ray attenuation.
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MRI Benefits MRI does not use ionizing radiation so it is not associated with ionizing radiation hazards. In general the parameters of MRI are without significant health risks but it has not been studies enough to assume that it is absolutely safe.
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MRI MRI used a strong magnetic fields and radiofrequencies to analyze the magnetic spin properties of hydrogen nuclei. Principle components of the MRI scanner include: A large homogeneous magnetic field Gradient Magnetic coils Radiofrequency coils Computer system.
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MRI Looks like a CT scanner. The gantry is much longer than a CT unit. The machine is composed of: A gantry containing the primary magnet A couch for the patient A computer system
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Types of Magnets There are three principle types of magnets used to generate the magnetic fields needed for MRI. Superconducting magnets Permanent magnets Resistive electromagnets
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Superconductive Magnets These magnets consist of a primary magnetic coils that are super-cooled by cryogens such as liquid helium or liquid nitrogen. Super-cooling dramatically reduces electrical resistance.
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Superconductive Magnets At infinite conductivity, the primary coils no longer requires a power supply. The magnetic field can be disrupted only by ramping down the magnet by expelling the cryogens.
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Superconductive Magnets The operating cost is high due to cost of cryogens. Able to make higher field strengths 1.5 T and higher for better signal to noise ration and greater detailed images.3 T becoming standard. A problem is the large fringe field that can effect medical pacemakers and other devices.
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Permanent magnets Permanent magnets are constructed of bricks of ferromagnetic material. As a result, they can be constructed as open units.
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They cannot obtain the high field strengths of super conductive units. Typical field strength is 0.2 tesla or less. Less problems with
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Week 10 B Magnetic Resonance Imaging & Bone...

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