Single-spoke binning: Reducing motion artifacts in abdominal radial stack-of-stars imaging

Abstract

Purpose: To increase the effectiveness of respiratory gating in radial stack-of-stars MRI, particularly when imaging at high spatial resolutions or with multiple echoes.

Methods: Free induction decay (FID) navigators were integrated into a three-dimensional gradient echo radial stack-of-stars pulse sequence. These navigators provided a motion signal with a high temporal resolution, which allowed single-spoke binning (SSB): each spoke at each phase encode step was sorted individually to the corresponding motion state of the respiratory signal. SSB was compared with spoke-angle binning (SAB), in which all phase encode steps of one projection angle were sorted without the use of additional navigator data. To illustrate the benefit of SSB over SAB, images of a motion phantom and of six free-breathing volunteers were reconstructed after motion-gating using either method. Image sharpness was quantitatively compared using image gradient entropies.

Results: The proposed method resulted in sharper images of the motion phantom and free-breathing volunteers. Differences in gradient entropy were statistically significant (p = 0.03) in favor of SSB. The increased accuracy of motion-gating led to a decrease of streaking artifacts in motion-gated four-dimensional reconstructions. To consistently estimate respiratory signals from the FID-navigator data, specific types of gradient spoiler waveforms were required.

Conclusion: SSB allowed high-resolution motion-corrected MR imaging, even when acquiring multiple gradient echo signals or large acquisition matrices, without sacrificing accuracy of motion-gating. SSB thus relieves restrictions on the choice of pulse sequence parameters, enabling the use of motion-gated radial stack-of-stars MRI in a broader domain of clinical applications.

Keywords: FID navigators; motion binning; motion correction; radial sampling; respiratory gating.

FIGURE 1

Schematic drawing of the two motion-binning strategies for a golden-angle radial stack-of-stars acquisition for three motion phases (indicated by colors) and three spoke angles. In spoke-angle binning (SAB), all readout lines corresponding to a spoke angle are assigned to the same motion phase. In single-spoke binning (SSB) each readout line is individually assigned to a motion phase. Thus, different readouts acquired at the same spoke angle may be assigned to different motion phases.

FIGURE 2

Diagram of the modified multi-echo golden-angle radial stack-of-stars spoiled gradient echo pulse sequence. Excitation is performed using a 1-1 binomial slab-selective water-excitation pulse. A short, nonlocalized free induction decay acquisition between slab-selection rephasing and slice encoding acts as respiratory navigator. Small triangles on axes G_x and G_y represent the blip gradients, which added slight rotations between subsequent gradient echo readouts. Spoiler gradients were timed with slight offsets to avoid exceeding gradient stimulation limits.

FIGURE 3

Simulated distributions of the number of spokes across 10 unique motion phases and 32 partitions for spoke-angle binning (left), single-spoke binning (SSB) with linear k_z-reordering (center), and SSB with random k_z-reordering (right). Linear k_z-reordering results in strongly varying numbers of spokes between partitions in any given motion phase. This effect can be suppressed by sampling partitions in random order (right).

FIGURE 4

Free induction decay (FID) navigator signal magnitudes over time from three separate measurements of a stationary phantom with T_R = 10 ms with angle-dependent (black), fixed (blue), and random (red) spoiling schemes (A). Each data point represents the sum of the magnitude values of the 22 central samples of each FID readout from the same, single receiver coil. The different spoiling schemes did not lead to observable disparities in the resulting images (B).

FIGURE 5

Respiratory signals (A) and nonuniform Fourier transform (iNUFFT) reconstructions (B) of the motion phantom data with both spoke-angle binning (SAB) and single-spoke binning (SSB). The blue, green, and dotted red lines in (a) represent the motion phantom’s reference (MP), self-gated (SG), and free induction decay (FID)-navigated respiratory signals, respectively. For coronal reconstructions, the vertical axes of images correspond to the slice encoding (z) direction. An animated version of (B) is available as Video S1.

FIGURE 6

Coronal view of the iterative reconstructions from volunteer 1 for self-gated spoke-angle binning (SAB) and single-spoke binning (SSB). The top two rows display the in- and exhalation phases in coronal views with the vertical axis as the phase encoding (z) direction. The SSB reconstructions show considerably less motion artifacts than those of SAB. Animated reconstructions for this volunteer are available as Videos S2 and S3.

FIGURE 7

Root sum-of-squares-combined multi-echo single-spoke binning (SSB) inverse nonuniform Fourier transform (iNUFFT) reconstructions of the exhalation phase of volunteer 6 (A), and examples of the five individual echoes used to produce these images (B). The echo-combined motion-resolved iNUFFT images resulted in reduced aliasing artifacts compared to the single-echo images, enabling the use of the gradient entropy as a quantitative measure of image sharpness without having to use regularized reconstructions.

FIGURE 8

Gradient entropies for self-gated spoke-angle binning (SAB) (solid green line) and single-spoke binning (SSB) (dotted red line) across three ROIs in transversal (top row), coronal (middle row), and sagittal (bottom row) orientations of volunteer 2’s motion phase 4. The transversal, coronal, and sagittal ROIs are located at the left kidney, the liver dome, and the lung vessels, respectively. The columns labeled SAB and SSB are the gray-level images reconstructed with self-gated SAB and SSB, g_SAB and g_SSB represent the corresponding image gradients.

FIGURE 9

Gradient entropies H(g) for self-gated spoke-angle binning (SAB) (solid green line) and single-spoke binning (SSB) (dotted red line) for volunteers 1–6. Each data point represents the average value of H(g) across 70 central coronal slices. H(g) was lower for SSB than for SAB in all volunteers and motion phases, indicating an overall increase in image sharpness using SSB.