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Parallel imaging

Published on Sunday 15 February 2009 by Denis Hoa

Contents

Parallel imaging

  1. Introduction
  2. Phased array coils
  3. Reconstruction in the image domain
  4. Reconstruction in the frequency domain
  5. Choice of reconstruction algorithm
  6. Benefits of parallel imaging
  7. Drawbacks
  8. Applications
  9. Parallel RF transmission

Learning objectives

After reading this chapter, you should be able:

  • To present the particularities of coils and the radiofrequency system for parallel imaging
  • Explain the increased signal-to-noise ratio using phased array coils
  • Describe the principles of acceleration in parallel acquisition
  • Compare the 2 main groups of reconstruction algorithms
  • List the advantages and drawbacks of parallel acquisition and its clinical applications

Key points

  • Parallel imaging exploits the multiple elements of a phased array coil. Each element is associated with a dedicated radiofrequency channel whose signals can be processed and combined together.
  • This improves the signal-to-noise ratio compared to a standard single-element coil covering the same explored volume.
  • The spatial data yielded by the array of coil elements can be used for partial phase encoding only, to speed up acquisition. The acceleration factors routinely employed at 1.5 T range from 2 to 3. At higher fields, these can be higher.
  • Reconstruction algorithms for the intermediary, undersampled phase images are divided into 2 groups:
    • Those which reconstruct the global image before Fourier transformation: these operate in the frequency domain (SMASH, GRAPPA)
    • Those which reconstruct the image after Fourier transformation, in the image domain (SENSE, mSENSE)
  • These algorithms require calibration data for the volume explored by each coil element and the signal produced. Calibration is carried out before or during the imaging sequence (self-calibration).
  • The objective is either to recreate the missing k-space lines (SMASH, GRAPPA), or to “unfold” the image due to aliasing (SENSE).
  • The advantages of these techniques are numerous: reduced acquisition time (fast imaging, breath-hold, temporal resolution, perfusion and functional imaging etc.) reduced TE of sequences with echo train, reduction of certain artifacts …
  • Their main drawback is a decrease in signal to noise ratio compared to a non-accelerated sequence (fewer measurements, g factor etc.)


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References

  1. Glockner, Hu. Parallel MR imaging: a user's guide. Radiographics. 2005 Sep-Oct;25(5):1279-97.
  2. Bammer and Schoenberg. Current concepts and advances in clinical parallel magnetic resonance imaging. Top Magn Reson Imaging. 2004 Jun;15(3):129-58.
  3. Blaimer, Breuer. SMASH, SENSE, PILS, GRAPPA: how to choose the optimal method. Top Magn Reson Imaging. 2004 Aug;15(4):223-36.
  4. Pruessmann. Parallel imaging at high field strength: synergies and joint potential. Top Magn Reson Imaging. 2004 Aug;15(4):237-44.
  5. Katscher, Bornert. Transmit SENSE. Magn Reson Med. 2003 Jan;49(1):144-50.
  6. Katscher and Bornert. Parallel RF transmission in MRI. NMR in biomedicine. 2006 May;19(3):393-400.
  7. Zhang, Yip. Reduction of transmitter B(1) inhomogeneity with transmit SENSE slice-select pulses. Magn Reson Med. 2007 May;57(5):842-7.

« Image quality and artifacts Introduction »


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