MRI Principles

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MRI Principles
2010-06-15 00:13:19
MRI Cognitive Neuroscience

MRI Terms
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  1. 3D acquisition
    Technique of volumetric imaging instead of acquistion of individual slices. Accomplished by performing phase encoding in two directions (phase-encoding and slice-selection gradients). Advantages: Good SNR, very thing slices can be obtained, excellent raw data set for secondary reconstruction.
  2. Active shielding
    Technique for containment of the fringe fields of an MR magnet. An actively shielded magnet consists of a set of two coils, an inner coil to generate the magnetic field and an outer coil to provide return paths for the magnetic field lines.
  3. B0
    The static external magnetic field of an MR scanner. The field strength in clinical MR imaging ranges from 0.064-3.0 tesla (up to 9T in experimental applications).
  4. Body coil
    The integrated RF coil in an MR scanner.
  5. Bound protons
    Water protons not freely mobile in a tissue. They are macromolecular water protons bound by hydration. The incorporate water protons are restricted in their mobility and thus exchange less energy with their surroundings (long T1) while their fixed structure promotes their exchange with each other (extremely short T2 of <0.1 msec). This is why bound protons do not contribute to the MR signal.
  6. B-value
  7. Chemical shift
    Describes the fact that the resonant frequency of protons varies with their molecular environment. The chemical shift most important in clinical MR imaging is sthat between protons in fat and water. As a result of the chemical shift, the protons of fat and water which coexist in the same voxel may be alternately in phase, i.e. their transverse magnetization vectors add together, or out of phase (opposed phase), i.e. their magnetization vectors point in opposite directions. This phenomenon can be exploited to differeniate fatty tissue (signal drop on image acquired while fat and water are out of phase) from other tissue (no signal drop on out-of-phase image).
  8. Centric k-space ordering
  9. Coil
    Component of an MR scanner which serves to transmit RF pulses and/or receive MR signals.
  10. Coil array
    Arrangement of several surface coils placed side by side for simultaenous signal collection in parallel imaging.
  11. Contrast-to-noise ratio (CNR)
    Measure of the ability to differentiate two adjacent anatomic structures in an MR image on the basis of their signal intensities in relation to image noise.
  12. Cross-talk
    Interference resulting from the unintended excitation of adjacent slices which overlap at their edges due to imperfect, nonrectangular slice profiles. Cross-talk decreases SNR.
  13. Echo planar imaging (EPI)
  14. Echo time (TE)
    The interval between excitation of a spin system and collection of the MR signal. TE predominately determines the amount of T2 contrast of the resultant image. Tissues with a shorter T2 appear dark on T2-weighted imaging (longer TE), tissues with longer T2 appear bright.
  15. Echo train length (ETL)
  16. Effective echo time
    In an FSE sequence, the time between the excitation pulse and the echo which primarily determines T2 contrast because it produces the strongest signal.
  17. Ernst angle
    The flip angle at which the maximum signal is generated for a given TR and TE.
  18. Fast spin echo sequence (FSE)
    A spin echo sequence run more rapidly than usual; also known as a turbo spin echo or RARE. This technique shortens scan time by generating up to 16 echoes with a series of 180o pulses. FSE sequences have the same image quality as conventional SE sequences and are nearly as fast as GRE sequences.
  19. Field of view (FOV)
    The area of anatomy covered in an image. The FOV is usually square, though a rectangular FOV may be chosen to reduce scan time. A smaller FOV improves spatial resoluation but decreases SNR.
  20. Flip angle (excitation angle)
    The angle by which magnetization is tilted when a spin system is excited by an RF pulse. The angle can be varied freely by changing the strength and duration of the excitation pulse applied. A flip ange of exactly 90o deflects all longitudinal magnetization (Mz) into the transverse plane (xy-plane). The flip angle is always 90o in a spin echo sequence with a gradient echo sequence can be acquired with different flip angles, e.g. 30o. The flip angle ddetermines the amount of T1 weigghting of an MR image.
  21. Fourier transform
    Mathematical operation needed to reconstruct MR images from raw data. The Fourier transform decomposes the measured MR signal into its frequency spectrum. In medical MR imaging, two-dimensional and three-dimensional Fourier transforms (2D-FT, 3D-FT) are used for image reconstruction.
  22. Fractional echo imaging
    Technique used to reduce scan time. Only half (or slightly more than half) the lines of k-space in the frequency-encoding direction are filled. Also known as partial echo imaging.
  23. Free induction decay (FID)
    Signal loss that occurs at a characteristic time constant T2* without any external influence.
  24. Free protons
    The free protons (protons in free water) of a tissue interact frequently with their environment (short T1) but rarely with each other (long T2). Only free protons contribute to the MR signal.
  25. Frequency encoding
    Part of spatial encoding of an MR signal. While the echo is being sampled, a gradient field is switched on in one dimension, imparting different precessional frequencies to the nuclear spins along that dimension. In this way, a spectrum of resonance frequencies is obtained instead of a single frequency. The frequency information serves to locate the individual signal components in space along the gradient.
  26. Frequency-encoding gradient
    The gradient fiel that is switched on while the MR signal is being collected, hence it is also called readout gradient. It is needed for frequency encoding of the MR signal.
  27. Gradient
    Defines the strength of the change of a quantity in a specific spatial direction. A magnetic field strength created along the x-, y-, or z-axis of the stationary magnetic field. Such gradients are needed for slice selection and spatial encoding and are generated using dedicated coils built into the scanner. In a more general sense, the term "gradients" is also used to denote the gradient coils.
  28. Gradient echo sequence (GRS)
    Pulse sequence which differs from a sin echo sequence in that no 180o refocusing pulse is applied. Magnetic field inhomogeneities and the phase differences imparted by the gradient are not compensated for and the MR signal decays with T2* instead of T2. Advantages: shorter scan time.
  29. GRASE (Gradient and spin echo)
  30. Inversion recovery sequence
    Spin echo sequence with an additional 180o inversion pulse preceding the usual excitation and refocusing pulses.
  31. Inversion time (TI)
    The interval between the 180o inversion pulse an the 90o excitation pulse in an inversion recovery sequence. The T1 can be selected to null the signal from a specific tissue such as fat, which is done by applying the 90o RF pulse when the magnetization of that tissue is zero.
  32. Isocenter
    The geometric center of the main magnetic field of an MR scanner where the field strength is not affected by any of the three gradients.
  33. K-space
    The mathematical space for storage of the measured raw data before the MR image is reconstructed by applying 2D or 3D Fourier transform. The center lines of k-space predominantly determine image contrast while the peripheral lines mainly affect spacial resolution.
  34. Larmor frequency
    The frequency at which spins precess about a magnetic field. The precession or resonance frequency is proportional to the strength of the magnetic field applied.
  35. Magnetization transfer
    Describes the transfer of magnetic saturation from bound macromolecular protons to free protons. This phenomenon reduces the signal intensity of free water.
  36. Matrix
    A two-dimensional grid consisting of rows and columns in which each square is a pixel (picture element). The matrix determines the number of pixels that make up an image.
  37. MIP (Maximum intensity projection)
  38. NEX (Number of excitations); NSA (Number of signal averages)
    Denotes how often a signal from a given slice is measured per phase encoding. An increase in NEX usually improves SNR.
  39. Parallel imaging
  40. Partial Fourier imaging
    Technique of k-space filling in which only slightly more than half the k-space lines in the phase-encoding direction are actually sampled and the unfilled lines are interpolated. The scan time is thus reduced by almost 50% while resolution is the same but noise is somewhat increased.
  41. Partial k-space acquisition
    General term for different techniques employed to reduce scan time by incomplete sampling of the lines of k-space.
  42. Partial volume effect
    The loss of contrast between two adjacent tissues with different signal intensities caused by insufficient resolution when both tissues are in the same voxel.