Simultaneous Excitation

Can you excite two slices at once?  

Yes, it is indeed possible to use a single composite RF-pulse to excite several slices at once, which are then unaliased during image reconstruction. Most manufacturers offer such an option under different names: POMP (GE), Simultaneous excitation (Siemens), Multi-slice (Philips), Dual-slice (Hitachi), and QuadScan (Canon). Newer methods applicable to multicoil arrays include the MultiBand (MB) technique, described in a later Q&A.

In all these methods an initial complex RF-pulse simultaneously stimulate two or more slices that are encoded using a binary (Hadamard) scheme or view-dependent phase modulatation. The oldest commercial version is GE's POMP ("Phase-Offset MultiPlanar") technique, illustrated below. Here the RF- pulse is a multiplex of the individual RF-pulses needed to stimulate each of the two slices, phase shifted from each other so that the resulting images do not overlap. 

Phase-offset multiplanar (POMP) imaging.
A complex RF-pulse simultaneously stimulates two slices,
which are phase offset from one another by an amount
that depends on the phase-encoding step (here 180°)

To distinguish between signal components from each of the two excited slices, the FOV in the phase-encoding direction must be doubled.  This process is similar to that used in the no-phase-wrap technique. To preserve imaging time and SNR, the number of excitations is cut in half.  The resultant data set then contains two images with the desired spatial resolution.  The phase offset for the images and composite RF pulses will vary with the phase-encode step.  At step 1, for example, the first image is phase encoded over a range of 0° to 180°, whereas the second image is encoded over a range of 180° to 360°. 

Despite these theoretical advantages, POMP and other simultaneous multi-slice excitation schemes are hampered because noise and phase-encoding artifacts (e.g., flow, wrap-around) may be propagated from one image to the other.  Furthermore, because they cannot be used in conjunction with a no-phase-wrap option, they can only be applied in situations when the imaged object does not exceed the FOV in the phase-encode direction. 

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Many of these limitations have been  eliminated through the use of multi-coil receiver arrays and parallel imaging. Specifically, the relatively new MultiBand (MB) technique, described in the next Q&A, does precisely that. 

Advanced Discussion (show/hide)»

The POMP technique is the oldest of a class of techniques that utilize time-multiplexing to acquire MR signals simultaneously from two or more slices. The classic POMP method, described in the Q&A above, is an example of simultaneous echo/image refocusing.

A second different approach combines slice excitation at different off-resonance frequencies with demultiplexing based on spatial sensitivity differences of RF receiver coils. This method is known as Multi-Band (MB) excitation. The MB method allows 3 or more slices to be simultaneously excited, increasing imaging efficiency.

The MR signals from MB slices are unfolded by using spatial sensitivity information in each RF channel similar to techniques employed in parallel imaging. As each receiver coil records a combination of MR signals from all excited slices weighted by sensitivities of that specific coil. A matrix inversion procedure can thus decode locations and reconstruct images of the respective slices.

The simultaneous image refocusing and MB methods are not mutually exclusive, and can be combined for improved results. See paper by Feinberg et al below.

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References
     Feinberg DA, Moeller S, Smith SM et al. Multiplexed echo planar imaging for sub-second whole brain FMRI and fast diffusion imaging. PLoS ONE 2010; 5(12):e15710.    
     Glover GH.  Phase-offset multiplanar (POMP) volume imaging: a new technique.  J Magn Reson Imaging 1991; 1:457-461.
     Larkman DJ, Hajnal JV, Herlihy AH, et al. Use of multicoil arrays for separation of signal from multiple slices simultaneously excited. J Magn Reson Imaging 2001; 13:313–317. 
     Lee KJ, Wild JM, Griffiths PD, Paley MNJ. Simultaneous multislice imaging with slice multiplexed RF pulses. Mag Reson Med 2005; 54:755–760.     
     Souza SP, Szumowski J, Dumoulin CL, et al. SIMA: simultaneous multislice acquisition of MR images by Hadamard-encoded excitation. J Comput Assist Tomogr 1988; 12:1026–1030.
     Weaver JB. Simultaneous multislice acquisition of MR images. Magn Res Med 1988; 8:275–284.

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Related Questions
     How does frequency-encoding let you stimulate a given slice? 
     What is MultiBand imaging and how does it work?

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