Free-breathing 2D radial cine MRI with respiratory auto-calibrated motion correction (RAMCO)

Abstract

Purpose: To develop a free-breathing (FB) 2D radial balanced steady-state free precession cine cardiac MRI method with 100% respiratory gating efficiency using respiratory auto-calibrated motion correction (RAMCO) based on a motion-sensing camera.

Methods: The signal from a respiratory motion-sensing camera was recorded during a FB retrospectively electrocardiogram triggered 2D radial balanced steady-state free precession acquisition using pseudo-tiny-golden-angle ordering. With RAMCO, for each acquisition the respiratory signal was retrospectively auto-calibrated by applying different linear translations, using the resulting in-plane image sharpness as a criterium. The auto-calibration determines the optimal magnitude of the linear translations for each of the in-plane directions to minimize motion blurring caused by bulk respiratory motion. Additionally, motion-weighted density compensation was applied during radial gridding to minimize through-plane and non-bulk motion blurring. Left ventricular functional parameters and sharpness scores of FB radial cine were compared with and without RAMCO, and additionally with conventional breath-hold Cartesian cine on 9 volunteers.

Results: FB radial cine with RAMCO had similar sharpness scores as conventional breath-hold Cartesian cine and the left ventricular functional parameters agreed. For FB radial cine, RAMCO reduced respiratory motion artifacts with a statistically significant difference in sharpness scores (P < 0.05) compared to reconstructions without motion correction.

Conclusion: 2D radial cine imaging with RAMCO allows evaluation of left ventricular functional parameters in FB with 100% respiratory efficiency. It eliminates the need for breath-holds, which is especially valuable for patients with no or impaired breath-holding capacity. Validation of the proposed method on patients is warranted.

Keywords: Cardiac MRI; Free-breathing Cine; Non-cartesian acquisition; respiratroy motion correction.

FIGURE 1

Schematic diagram of the proposed respiratory motion‐corrected FB radial cine reconstruction framework using RAMCO. Respiratory motion artifacts caused by in‐plane bulk motion were minimized with linear translations based on the auto‐calibrated respiratory signal from a motion sensing camera. Through‐plane motion and residual in‐plane motion were minimized with respiratory motion‐weighted density compensation applied in the gridding. FB, free‐breathing; RAMCO, respiratory auto‐calibrated motion correction.

FIGURE 2

Overview of the auto‐calibration and linear translation components of RAMCO. By minimizing $E_s$, an optimal scaling factor for each slice in 1 cardiac phase is identified. This scaling factors ($f(s)$) are then used to scale the respiratory signal and in turn used to perform linear translation of data corresponding to all cardiac phases. Es, motion‐artifact metric

FIGURE 3

The effect of auto‐calibration and linear translations for a representative volunteer. (A) Images of a midventricular slice in end‐diastolic phase, reconstructed with different linear translations in (A.1) SI and (B.1) AP directions, respectively. The corresponding scaling factors (${f_m,f}_m^{\prime }$) and gradient entropy scores ($E\left[f_m\right],E\left[f_m^{\prime }\right]$) are displayed on top of the images. Bar plots of gradient entropy score versus scaling factors in different slices are displayed for (A.2) SI and (B.2) AP directions. For each slice, a scaling factor with the lowest gradient entropy score was assumed as an optimal scaling factor to convert unitless respiratory signal to a signal with physical units. (C) End‐diastolic and end‐systolic images from the mid‐ventricular slice with no motion correction, linear translation only in SI direction, and linear translation in SI and AP directions based on the calibrated respiratory signal are shown in the first, second, and third rows, respectively. Note the progressive reduction in motion artifacts AP, anteroposterior, SI, superoinferior.

FIGURE 4

The effect of motion‐weighted density compensation on a representative volunteer. (A) Normalized respiratory signal obtained from the patient sensing camera and (B) the initial weights corresponding to the end‐diastolic images shown in (E). (C) Conventional DCF (D) $\mathit{\text{mwDCF}}$ computed using the iterative algorithm with initial weights according to (B). (E) Representative images with no respiratory motion correction, auto‐calibrated linear translations in SI direction, and RAMCO combined with $\mathit{\text{mwDCF}}$. The through‐phase (cardiac) profile shown in the last row of (E) is obtained from an oblique line profile in end‐diastole indicated by a white dotted line through the left ventricle as shown in the third row. Note the progressive reduction in respiratory motion‐artifacts from the left to the right column, indicated by red arrows DCF, density compensation function; mwDCF, motion‐weighted density compensation function.

FIGURE 5

Comparison of cine images obtained with the standard BH ^cart to the proposed FB ^rad ‐NoMC and with FB ^rad –MC (RAMCO) on a volunteer. (A) Representative images in end‐diastole and end‐systole showing the entire FOV. (B) Through‐phase profile of pixel intensities along the white dotted line in the left ventricle shown in (A), and (C) orthogonal 4‐chamber MPR images of the 3 datasets. Note the significant reduction of motion artifacts in the images of FB ^rad –MC compared to FB ^rad –NoMC . The sharpness of FB ^rad –MC (RAMCO) is comparable to that of BH ^cart , as indicated by green arrows. Whereas RAMCO reduced motion artifacts in the heart, it introduced some blurring in regions outside the heart as indicated by red arrows BH ^cart , breath‐holds Cartesian cine; FB ^rad –MC , FB radial cine with RAMCO motion correction; FB ^rad ‐NoMC , FB radial cine without respiratory motion correction.

FIGURE 6

Image sharpness scores. (A) Bar plots of average sharpness scores of the standard BH ^cart , FB ^rad –NoMC, and with FB ^rad ‐MC (RAMCO) obtained from 9 volunteers. Statistically significant differences (P < 0.05) are indicated by (*). (B) Images obtained with the 3 methods from 4 different volunteers in ED and ES, along with the corresponding sharpness score printed at the bottom of each image ED, end‐diastolic phase; ES, end‐systolic phase.

FIGURE 7

Comparison of the standard BH ^cart to the proposed FB ^rad ‐NoMC and with FB ^rad –MC on a representative volunteer. (A) Representative slices in end‐systolic phase and (B) end‐diastolic phase shown for the 3 methods. (C) Through‐cardiac phase profile along a white dotted line for the 3 methods. FB ^rad –MC corrected for most of the local blurring due to respiratory motion and had similar spatial and temporal quality as the BH ^cart as indicated by red arrows. (D) Through‐slice profiles along the oblique axis of the heart shown by white dotted line in (A). Slice mis‐alignments due to inconsistent breath‐holds are clearly visible in BH ^cart that are not present in FB ^rad as indicated by yellow arrows

FIGURE 8

Bland–Altman plots comparing LV functional assessment parameters between (A–D) standard BH ^cart and FB ^rad ‐NoMC: and between (E–H) BH ^cart and FB ^rad ‐MCLV, left ventricular.