Flexible retrospective selection of temporal resolution in real-time speech MRI using a golden-ratio spiral view order

Abstract

In speech production research using real-time magnetic resonance imaging (MRI), the analysis of articulatory dynamics is performed retrospectively. A flexible selection of temporal resolution is highly desirable because of natural variations in speech rate and variations in the speed of different articulators. The purpose of the study is to demonstrate a first application of golden-ratio spiral temporal view order to real-time speech MRI and investigate its performance by comparison with conventional bit-reversed temporal view order. Golden-ratio view order proved to be more effective at capturing the dynamics of rapid tongue tip motion. A method for automated blockwise selection of temporal resolution is presented that enables the synthesis of a single video from multiple temporal resolution videos and potentially facilitates subsequent vocal tract shape analysis.

Figure 1

k-space trajectories for (a) conventional bit-reversed 13-interleaf UDS and (b) golden-ratio spiral view order when samples from 13 consecutive TRs are combined. Temporal view orders are marked with the numbers on each end of the spiral interleaves. In (a), the angle spacing between spatially adjacent spiral interleaves is uniform with an angle of 360°/13. In (b), the angle spacing between spatially adjacent spiral interleaves is not uniform but the angle increment in successive view numbers is uniform with an angle of $360°·2/(\sqrt{5}+1)\approx 222.4969°$.

Figure 2

Retrospective selection of temporal resolution: (a) Comparison of unaliased FOV between the golden-ratio view order and conventional bit-reversed 13-interleaf UDS sampling. (b) The enlargement of the region within the green rectangle in (a). The blue shaded region in (a,b) indicates that unaliased FOV varies in conventional bit-reversed 13-interleaf UDS when the number of TRs is less than 10. The black solid line illustrates a linear relationship between unaliased FOV and temporal resolution when UDS trajectories are designed under the constraints of the same spatial resolution and readout duration. Note that the unaliased FOV is fixed as 20 cm for a temporal window length (≥13-TR) for the conventional bit-reversed 13-interleaf UDS, but it increases with temporal window length for the golden-ratio view order.

Figure 3

Midsagittal images with a large reconstruction field-of-view (FOV) of 38 × 38 cm² reconstructed from the data acquired in static posture. (a–c) Conventional bit-reversed 13-interleaf UDS. (a) image from coil 1, (b) image from coil 2, (c) root sum-of-squares (SOS) of the coil 1 and coil 2 images. The region within the dashed box in (c) is the vocal tract regions of interest (ROIs). For speed-up of the spiral acquisition, the FOV of the 13-interleaf UDS is typically chosen to be small such that aliasing artifacts are not observed in the vocal tract ROIs. (d–f) Root SOS of the coil 1 and coil 2 images reconstructed from data acquired using the spiral golden-ratio acquisition: reconstruction from (d) 13-TR, (e) 21-TR, and (f) 34-TR data. Spatial aliasing artifacts are completely removed in (f) because of larger FOV available from the golden-ratio method. The image SNR in (f) is higher than that in (d) and (e).

Figure 4

Gridding reconstructed dynamic frames and time intensity profiles from (a,c) bit-reversed 13-interleaf uniform density spiral data and (b,d) spiral golden-ratio view order data. A 40 × 40 matrix containing only the vocal tract region of interest is shown. Retrospective selection of temporal resolution is performed using 8-TR and 13-TR in (a), and 8-TR, 13-TR, and 21-TR in (b). Frame update rate was 1-TR = 6.164 ms. Two example frames (frame 1, 6) are shown (frame 1 is relatively stationary and frame 6 is captured during rapid tongue tip motion). Time intensity profiles from the image column indicated by the dashed lines are shown for (c) bit-reversed 13-interleaf UDS and (d) golden-ratio view order data.

Figure 5

Blockwise temporal resolution selection and synthesis of a single video from four temporal resolution videos. Five representative frames are shown that are captured when the subject produced /bono/. (a) Normalized time difference energy (TDE) map. (b) Temporal resolution selection map [white: 49 ms (= 8-TR) temporal resolution, bright gray: 80 ms (= 13-TR) temporal resolution, dark gray: 129 ms (= 21-TR) temporal resolution, black: 210 ms (= 34-TR) temporal resolution]. (c) Synthesized temporal resolution frames based on the temporal resolution selection map in (b). (d) 49 ms (= 8-TR) temporal resolution frames. (e) 210 ms (= 34-TR) temporal resolution frames.