Broadband multicoil imaging using multiple demodulation hardware: a feasibility study.

Multiple receiver-coil data collection is an effective approach to reduce scan time. There are many parallel imaging techniques that reduce scan time using multiple receiver coils. One of these methods, partially parallel imaging with localized sensitivities (PILS), utilizes the localized sensitivity of each coil. The advantages of PILS over other parallel imaging methods include the simplicity of the algorithm, good signal-to-noise ratio (SNR) properties, and the fact that there is no additional complexity involved in applying the algorithm to arbitrary k-space trajectories. This PILS method can be further improved to provide truly parallel broadband imaging with the use of multiple-demodulation hardware. By customizing the demodulation based on each coil’s location, the k-space sampling rate can be chosen based on each coil’s localized sensitivity region along the readout direction. A simulated demodulation of data from 2D Fourier transform (FT) and spiral trajectories is shown to demonstrate the method’s feasibility. Magn Reson Med 54: 669 – 676, 2005. © 2005 Wiley-Liss, Inc. Image acquisition and reconstruction methods using multiple receiver coils are very effective ways to improve the signal-to-noise ratio (SNR) or reduce scan time. Traditionally, phased-array imaging methods (1) were commonly used to improve SNR (2). In phased-array imaging, SNR is improved by reducing the noise volume of each coil. Recently, various methods designed to decrease scan time using multiple receiver coils (3) were developed under the name of parallel imaging. These methods include sensitivity encoding (SENSE) (4), simultaneous acquisition of spatial harmonics (SMASH) (5), and partially parallel imaging with localized sensitivities (PILS) (6). The main difference between phased-array imaging and parallel imaging is the fact that parallel imaging utilizes the coils’ sensitivity pattern as an encoding scheme. The sensitivity provides additional encoding on top of the gradient encoding. This allows the process of encoding to be more parallel since the multiple sensitivity-encoded data are acquired simultaneously using multiple coils. SENSE is a method that utilizes the precise knowledge of the receiver coils’ sensitivity function as spatial encoding information. The sensitivity of the coil is a smooth function that can be measured with low-resolution scans and can then be used as an additional encoding method. By measuring this sensitivity and using it to invert the encoding matrix to reconstruct the image, the number of samples needed for Fourier encoding can be decreased. The SMASH method is similar to SENSE in that it uses the sensitivity of the coil for encoding; however, SMASH uses this information in a very different way. SMASH relies on making sinusoids out of the coils’ sensitivity patterns. Therefore, it tries to fill in the missing k-space lines by making sinusoids that normally would have been generated through gradients. In other words, it tries to interpolate data in k-space by assuming that the sensitivity pattern mimics a sinusoidal pattern. In contrast, the PILS method is a simpler method that utilizes the fact that each coil can be sensitive to a different region of the imaging FOV, so that images from different coils can be simply cut and pasted together. This method can be treated as a special case of SENSE when the sensitivity of the coils are completely orthogonal to each other. In this paper, a method to improve the PILS technique is proposed along with a new hardware configuration that will allow such a method to be implemented. THEORY