Multiple-mouse MRI with multiple arrays of receive coils

Abstract

Compared to traditional single-animal imaging methods, multiple-mouse MRI has been shown to dramatically improve imaging throughput and reduce the potentially prohibitive cost for instrument access. To date, up to a single radiofrequency coil has been dedicated to each animal being simultaneously scanned, thus limiting the sensitivity, flexibility, and ultimate throughput. The purpose of this study was to investigate the feasibility of multiple-mouse MRI with a phased-array coil dedicated to each animal. A dual-mouse imaging system, consisting of a pair of two-element phased-array coils, was developed and used to achieve acceleration factors greater than the number of animals scanned at once. By simultaneously scanning two mice with a retrospectively gated cardiac cine MRI sequence, a 3-fold acceleration was achieved with signal-to-noise ratio in the heart that is equivalent to that achieved with an unaccelerated scan using a commercial mouse birdcage coil.

Figure 1

Relationship between the two-element phased-array coil geometries and the aliasing strategies used for dual-mouse image acquisition. The full FOV (R = 1) contains the unaccelerated aliased images, making the animals appear as if they are adjacent to one another. The reduced FOVs prescribed to achieve up to four-fold accelerations are shown. Note that the FOV for R = 2 is identical to that which would be used to scan each of the two mice serially in a single-animal configuration.

Figure 2

Grid describing various imaging options and calculated throughput parameters for dual-mouse imaging. The first three rows demonstrate unaccelerated imaging of a single mouse (first row), two mice serially (second row), and two mice at once (third row). All of these imaging strategies result in throughput, T = 1, where T=R×N/F, R is the traditional PI reduction factor, N is the number of simultaneously-scanned mice, and F is the factor by which the single-animal FOV must extended to cover all animals without acceleration. The next three rows indicate accelerated (R = 2, R = 3, and R = 4) dual mouse (N = 2) imaging made possible by using multiple arrays of receive coils. Note that the accelerated imaging schemes will ultimately result in separate reconstructed images for each mouse.

Figure 3

Setup of numerical phantom simulation with respect to the phased-array coils. The gated phantom (left) is fixed with H1 in diastole while the ungated phantom (right) has H2 varied over 21 phases. Three representative phases are shown.

Figure 4

SNR maps determined from GRAPPA-reconstructed images of the originally-acquired data set (R = 1) and subsequently downsampled data sets (R = 2, R = 3, and R = 4). Note that the SNR is highest in the location corresponding to the mouse heart as indicated by the oval ROI.

Figure 5

A) Propagated motion artifact-to-noise contours from an accelerated (R = 2) GRAPPA simulation. Two contours representing potential interarray decoupling requirements for the propagated motion artifact to be equal to and one third of the noise floor are labeled. B) Reconstructions of the static numerical phantom images and difference from the ideal reconstruction are shown, windowed to emphasize noise and artifact, for two example contour points (1 and 2).

Figure 6

Representative cardiac cine images of a single slice during ventricular systole and diastole from the four accelerated dual-mouse acquisitions.