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Basic MRI I. Chapter 3 Notes. Terms to know. Resonant Characteristic-- able to have resonance Magnetized—makes a net magnetization vector in a magnetic field Magnetization—process of forming a net magnetization vector Resonance—capacity to absorb and re-radiate radiofrequency energy.
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Basic MRI I Chapter 3 Notes
Terms to know • Resonant Characteristic-- able to have resonance • Magnetized—makes a net magnetization vector in a magnetic field • Magnetization—process of forming a net magnetization vector • Resonance—capacity to absorb and re-radiate radiofrequency energy
Two ways to display MR signals • MR images • MR spectra
Magnetic nuclei • Have magnetic “moment” • Act like small magnets • Have an odd number of protons and neutrons • Have the quantum property of “spin” • Are sometimes called “spins”
RF signal strength • Most significant factor in determining image quality and the time it takes to acquire an image
Relative signal strength • Tissue concentration of the element • Isotopic abundance • Sensitivity of the specific nuclide
Tissue concentration • For hydrogen nuclei, or protons, this is the “proton density” or “PD” • For other nuclei, referred to as “spin density”
Isotopic abundance • H-1 – 99.985% • H-2 – 0.015% “deuterium” • H-3 -- <0.0011% “tritium”
Sensitivity • Signal strength produced by an equal number of nuclei • H-1 is the strongest, considered 1.0 or 100% • See table 3 in your book.
Why proton imaging is so good • High tissue concentration • Most abundant magnetic isotope of hydrogen (H-1) is magnetic!
RF energy • RF pulses (transmitted)– usually 90 degrees or 180 degrees, but can be of any angle • RF signal (received)– received by coils that act as antennae • Stored in k-space • Fourier transform (FT) to make images • Received as echoes
Nuclear magnetic interactions • Nuclear alignment—only a few of every million nuclei align with the magnet, but this is enough to form a net magnetization vector • The rest are tumbling randomly, due to “thermal activity.”
Precession and resonance • The rate of precession is the same as the resonant frequency, called the Larmor Frequency
Excitation • Tip the net magnetization vector away from the z direction…any component in the x-y plane generates a signal that can be detected.
Relaxation • The net magnetization vector re-forms, lines up with the z direction, and the individual spins dephase.
Frequency is proportional to field strength • Freq = gyromagnetic ratio x field strength • Freq = “gamma” x B-naught • Freq = γHB0 • Gyromagnetic ratio = γH = 43.58 MHz/T • Field strength = B0