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Basic MRI I

Basic MRI I. Chapter 2. The Magnet. The heart of the system Induces tissue magnetization Induces resonance with a specific frequency Frequency is proportional to field strength. Frequency is proportional to field strength. Freq = gyromagnetic ratio x field strength

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Basic MRI I

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  1. Basic MRI I Chapter 2

  2. The Magnet • The heart of the system • Induces tissue magnetization • Induces resonance with a specific frequency • Frequency is proportional to field strength

  3. 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

  4. Magnetic field • Magnetic fields have strength and direction – they are vectors • Strength measured in Gauss or in Tesla • 1 Tesla = 1 T = 10,000 Gauss = 10,000 G • 1000 G = 1 kG • Direction is defined as the “z-direction” in MR, in 3D Cartesian coordinates

  5. Type of Magnets

  6. Gradients • Intro to gradient concept • Linear gradients • Non-linear gradients

  7. Gradients • Orientation • Definition of orthogonal = “ at right angles” • Functions • To get images—all types • To get specialized images (gradient echo) • To reduce artifacts • Strength • Units of mT/m • High value = ~ 20 mT/m

  8. Gradients • Rise time = time for gradient to reach its maximum, like 0.20 msec • Slew rate = rate of gradient change, like 100 mT/m/msec

  9. Eddy currents • Electrical currents induced or generated in conducting materials, in metal in the machine, minimized by gradients

  10. Shimming • To achieve homogeneous field strength

  11. Shielding • Passive—large pieces of ferromagnetic metal like iron • Active—electromagnets to decrease the extent of the field

  12. RF system • Coils • Body coils • Surface coils • Special coils • TMJ • Head • Wrist • Linear, quadrature, CP, phased array

  13. RF shielding • Farraday cage • To decrease stray RF energy from lights and motors outside the magnet from getting into the receive coil (antenna) • Copper screen • Sheet metal

  14. Computer • To store and acquire signals in k-space • To reconstruct images • To perform Fourier Transform • Storage, retrieval, transmission of images • Post Processing, zoom, magnification, contrast/windowing, 3D reconstructions

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