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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.)

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.