A Comparison of Available MRI Array Coil Configurations

March 1999

General Comparison:

There are different solutions in the market for the technical implementation of phased array coils. These different solutions are emphasizing different aspects in clinical application as well as in technical realization. At the moment there are four versions of phased array configurations in the market.
     
    Version 1
         Four high speed CP channels    		 Version 2
       One high speed LP channel & three low speed  LP channels
    Version 3
        One CP channel multiplexed     		 Version 4
        Two / four high speed CP channels

Technical Comparison:

Version 1
Up to four CP elements of one phased array coil can be activated. Each element has its own channel to the image reconstruction. All channels are identical and provide high speed data transfer for EPI applications. This version is realized in Siemens Magnetom Vision and Expert.

Version 2
Up to four LP elements of one array coil can be activated. Each element has its own channel but only one channel is for EPI speed, the other three are for standard imaging techniques. LP technology provides only 70% of SNR compared to CP. New developments may also provide CP or quadrature technology. I believe this is what is used on GE SIGNA with LP arrays and Picker.

Version 3
There is only one channel available and all selected elements (CP or LP) are scanned by a multiplexer. The more elements are activated the slower the data acquisition becomes. EPI speed can be realized only with one single element. This solution is more multi coil switch than a phased array platform.  This version has been offered by Toshiba and others.

Version 4
 Up to 4 or 8 CP elements from different phased array coils are connected to 2 (IPA) or 4 (Advanced IPA) high speed data channels. The very flexible distribution of the available channels to the active elements is determined by a matrix and depends on the number of elements selected. SNR slightly drops when the number of active elements execs the number of available channels because of analogue combination.. All channels provide EPI speed.  This version is realized in Siemens Magnetom Harmony and Symphony.

Clinical Comparison:

The different technical solutions for a phased array platform each have benefits and restrictions in the clinical applications.

Version 1
Supports all clinical examinations with no restrictions in terms of speed and SNR.  The elements of one coil can be used at a time.

  • maximum image quality - no combination of coil elements
  • full EPI compatible
    • Version 2
    Supports all clinical examinations as well but the slower channels limit fast imaging techniques. Abdominal, cardiac and peripheral angiographic applications requiring very short Te and TR may be hindered.  Single breath-hold imaging and high resolution matrices in large FOVs are only possible but will be limited by the speed of the slower channels and the ability of the system to handle the rapid flow of data inherent in shorter TR and TE sequences. The linear polarization of the phased array elements achieves only 70% of the image quality level of CP or quadrature polarization. Version 3
    Supports all standard clinical applications with limitations. The more elements are activated the slower becomes the imaging. Even in standard sequences like SE, TurboSE or gradient echo the choice of  TE and TR are restricted when more than just one element is active for a large imaging volume. Version 4
    Supports all clinical examinations with in terms of speed and SNR. Imaging sequences starting from standard SE via single breath-hold livers and cardiac to single shot EPI are compatible with phased array coils. The flexibility in the selection of elements to be combined for an examination is not limited by receiver chain issues. One can select elements not only from one phased array coil but from several to participate in an anatomical volume. If imaging volumes are covered with more elements than channels available a small sacrifice in SNR must be made. This type of phased array platform is expandable while keeping the flexibility and the application for high speed imaging. The ability to select more than one coil element per channel affords greater flexibility in crafting a coil set for teh examination.

    Effects on Speed and SNR

    The influence of the number of active elements selected to the data acquisition can be also translated into a graph which shows the interdependence of speed, SNR and elements.
     
    Version 1   Doesn't show any deterioration in speed and SNR with the number of elements selected, but only 4 elements can be used. 

    Version 2   Provides the same speed as version 1 if just one element is selected. As soon as one further element is used the speed drops because  the low speed channels are now in use. Lower SNR due to the LP technology. In quadrature (CP) solutions the SNR becomes the same as in Version 1. 

    Version 3  Shows the reduction in speed the more elements are selected because of the multiplexing. 

    Version 4  Achieves the same speed and SNR as version 1 as long as the number of the selected elements does not exceed the number of available channels. If there are more elements selected, the resulting SNR slightly drops but the speed is maintained. 
     
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