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The AAPM/RSNA physics tutorial for residents. Contrast mechanisms in gradient-echo imaging and an introduction to fast imaging.
Price RR.
Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN 37232-2675.
Radiographics. 1995 Jan;15(1):165-78; quiz 149-50
The strengths of spin-echo sequences in clinical magnetic resonance imaging are its relative insensitivity to magnetic field inhomogeneities and its versatility; however, they require relatively long imaging times. Use of gradient-echo sequences largely circumvents the problem. A gradient echo is created by means of a gradient reversal following a single radio-frequency (RF) pulse, typically with a flip angle that is less than 90 degrees, compared with a spin-echo sequence, which requires at least two RF pulses, typically a 90 degree excitation pulse followed by a 180 degree refocusing pulse. The shortened repetition times (TRs) in gradient-echo sequences together with small flip angles enable maintaining adequate image signal-to-noise ratios while still achieving a short imaging time. Numerous gradient-echo sequences have been developed, with the most common being (a) fast low angle shot (FLASH), which uses small flip angles and short TRs; (b) gradient-recalled acquisition in the steady state and fast imaging with steady-state precession (GRASS/FISP); (c) magnetization-prepared rapid gradient echo (MP-RAGE); and (d) echo planar. The shorter imaging times made possible through gradient-echo sequences are used clinically to minimize patient motion artifacts and to allow isotropic three-dimensional imaging with acceptable imaging times.
Posted via PubMed for educational and discussion purposes only.
Link to PubMed Reference
Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN 37232-2675.
Radiographics. 1995 Jan;15(1):165-78; quiz 149-50
The strengths of spin-echo sequences in clinical magnetic resonance imaging are its relative insensitivity to magnetic field inhomogeneities and its versatility; however, they require relatively long imaging times. Use of gradient-echo sequences largely circumvents the problem. A gradient echo is created by means of a gradient reversal following a single radio-frequency (RF) pulse, typically with a flip angle that is less than 90 degrees, compared with a spin-echo sequence, which requires at least two RF pulses, typically a 90 degree excitation pulse followed by a 180 degree refocusing pulse. The shortened repetition times (TRs) in gradient-echo sequences together with small flip angles enable maintaining adequate image signal-to-noise ratios while still achieving a short imaging time. Numerous gradient-echo sequences have been developed, with the most common being (a) fast low angle shot (FLASH), which uses small flip angles and short TRs; (b) gradient-recalled acquisition in the steady state and fast imaging with steady-state precession (GRASS/FISP); (c) magnetization-prepared rapid gradient echo (MP-RAGE); and (d) echo planar. The shorter imaging times made possible through gradient-echo sequences are used clinically to minimize patient motion artifacts and to allow isotropic three-dimensional imaging with acceptable imaging times.
Posted via PubMed for educational and discussion purposes only.
Link to PubMed Reference
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