Pilot tone-based motion correction for prospective respiratory compensated cardiac cine MRI.

Prospective motion correction in functional MRI using simultaneous multislice imaging and multislice-to-volume image registration

To develop a patient-specific respiratory motion correction technique with true 100% acquisition efficiency. A short training scan consisting of a series of single heartbeat images, each acquired with a preceding diaphragmatic navigator, was performed to fit a model relating the patient-specific 3D respiratory motion of the heart-to-diaphragm position. The resulting motion model was then used to update the imaging plane in real-time to correct for translational motion based on respiratory position provided by the navigator. The method was tested in a group of 11 volunteers with 5 separate free-breathing acquisitions: FB, no motion correction; FB-TF, free breathing with a linear tracking factor; Nav Gate, navigator gating; Nav Gate-TF, navigator gating with a tracking factor; and PROCO, prospective motion correction (proposed). Each acquisition lasted for 50 accepted heartbeats, where non-gated scans had a 100% acceptance rate, and gated scans accepted data only within a ±4 mm navigator window. Retrospective image registration was used to measure residual motion and determine the effectiveness of each method. PROCO reduced the range/RMSE of residual motion to 4.08 ± 1.4/0.90 ± 0.3 mm, compared to 10.78 ± 6.9/2.97 ± 2.2 mm for FB, 5.32 ± 2.92/1.24 ± 0.8 mm for FB-TF, 4.08 ± 1.6/0.93 ± 0.4 mm for Nav Gate, and 2.90 ± 1.0/0.63 ± 0.2 mm for Nav Gate-TF. Nav Gate and Nav Gate-TF reduced scan efficiency to 48.84 ± 9.31% and 54.54 ± 10.12%, respectively. PROCO successfully limited the residual motion in single-shot imaging to the level of traditional navigator gating while maintaining 100% acquisition efficiency.

Prospective motion correction using data from optical tracking systems has been previously shown to reduce motion artifacts in MR imaging of the head. We evaluate a novel optical embedded tracking system. The home-built optical embedded tracking system performs image processing within a 7 T scanner bore, enabling high speed tracking. Corrected and uncorrected in vivo MR volumes are acquired interleaved using a modified 3D FLASH sequence, and their image quality is assessed and compared. The latency between motion and correction of the slice position was measured to be (19 ± 5) ms, and the tracking noise has a standard deviation no greater than 10 μm/0.005° during conventional MR scanning. Prospective motion correction improved the edge strength by 16 % on average, even though the volunteers were asked to remain motionless during the acquisitions. Using a novel method for validating the effectiveness of in vivo prospective motion correction, we have demonstrated that prospective motion correction using motion data from the embedded tracking system considerably improved image quality.

To prospectively evaluate the feasibility and biventricular assessment accuracy of a free-breathing cardiac cine imaging technique (RTCSCineMoCo) combined with highly accelerated real-time (RT) acquisition, compressed sensing (CS) reconstruction, and fully automated non-rigid respiratory motion correction. We evaluated 80 patients scheduled for clinical cardiac MRI. Cardiac cine images of the same long-axis and short-axis stacks were acquired using three techniques: (1) SegBH: standard segmented cine with breath-hold; (2) RTCSCineMoCo; (3) RTCSCine: single-shot RT CS cine at 3.0 T. Image quality (IQ) was evaluated using a qualitative 5-point Likert scale and the European CMR registry standardized criteria. Quantitative parameters including left (LV) and right ventricular (RV) ejection fractions (EF), end-diastolic volumes (EDV), end-systolic volumes (ESV), stroke volumes (SV), and LV mass (LVM) were measured and compared. RTCSCineMoCo and SegBH had equivalent IQ scores (4.4 ± 0.7 vs. 4.2 ± 0.8, p = 0.066), while RTCSCine had a significantly lower IQ score than SegBH (4.0 ± 0.8 vs. 4.2 ± 0.8, p = 0.031). In a quantitative analysis, RTCSCineMoCo and SegBH yielded similar measurements for all parameters, while the majority of RTCSCine parameters were significantly different compared with SegBH, except for LVEDV. RTCSCineMoCo is a promising method for robust free-breathing cardiac cine imaging, achieving better IQ and more precise quantitative analysis results for both ventricles compared with RTCSCine. • RTCSCineMoCo is a promising method for free-breathing cardiac MR cine imaging in daily practice. • RTCSCineMoCo provided better IQ and more precise quantitative measurements compared with RTCSCine, by extending RT data acquisition to multiple heartbeats, performing non-rigid respiratory motion correction, and signal averaging. • RTCSCineMoCo may be suitable for routine clinical use for vulnerable patients who may otherwise pose a challenge to image successfully with the conventional segmented cine technique.

Motion detection and correction for carotid MRI using a markerless optical system

BackgroundCine cardiovascular magnetic resonance (CMR) is challenging in patients who cannot perform repeated breath holds. Real-time, free-breathing acquisition is an alternative, but image quality is typically inferior. There is a clinical need for techniques that achieve similar image quality to the segmented cine using a free breathing acquisition. Previously, high quality retrospectively gated cine images have been reconstructed from real-time acquisitions using parallel imaging and motion correction. These methods had limited clinical applicability due to lengthy acquisitions and volumetric measurements obtained with such methods have not previously been evaluated systematically.MethodsThis study introduces a new retrospective reconstruction scheme for real-time cine imaging which aims to shorten the required acquisition. A real-time acquisition of 16-20s per acquired slice was inputted into a retrospective cine reconstruction algorithm, which employed non-rigid registration to remove respiratory motion and SPIRiT non-linear reconstruction with temporal regularization to fill in missing data. The algorithm was used to reconstruct cine loops with high spatial (1.3-1.8 × 1.8-2.1 mm2) and temporal resolution (retrospectively gated, 30 cardiac phases, temporal resolution 34.3 ± 9.1 ms). Validation was performed in 15 healthy volunteers using two different acquisition resolutions (256 × 144/192 × 128 matrix sizes). For each subject, 9 to 12 short axis and 3 long axis slices were imaged with both segmented and real-time acquisitions. The retrospectively reconstructed real-time cine images were compared to a traditional segmented breath-held acquisition in terms of image quality scores. Image quality scoring was performed by two experts using a scale between 1 and 5 (poor to good). For every subject, LAX and three SAX slices were selected and reviewed in the random order. The reviewers were blinded to the reconstruction approach and acquisition protocols and scores were given to segmented and retrospective cine series. Volumetric measurements of cardiac function were also compared by manually tracing the myocardium for segmented and retrospective cines.ResultsMean image quality scores were similar for short axis and long axis views for both tested resolutions. Short axis scores were 4.52/4.31 (high/low matrix sizes) for breath-hold vs. 4.54/4.56 for real-time (paired t-test, P = 0.756/0.011). Long axis scores were 4.09/4.37 vs. 3.99/4.29 (P = 0.475/0.463). Mean ejection fraction was 60.8/61.4 for breath-held acquisitions vs. 60.3/60.3 for real-time acquisitions (P = 0.439/0.093). No significant differences were seen in end-diastolic volume (P = 0.460/0.268) but there was a trend towards a small overestimation of end-systolic volume of 2.0/2.5 ml, which did not reach statistical significance (P = 0.052/0.083).ConclusionsReal-time free breathing CMR can be used to obtain high quality retrospectively gated cine images in 16-20s per slice. Volumetric measurements and image quality scores were similar in images from breath-held segmented and free breathing, real-time acquisitions. Further speedup of image reconstruction is still needed.

The aim of this study was to develop a method for tracking respiratory motion throughout full MR or PET/MR studies that requires only minimal additional hardware and no modifications to the sequences. Patient motion that is caused by respiration affects the quality of the signal of the individual radiofrequency receive coil elements. This effect can be detected as a modulation of a monofrequent signal that is emitted by a small portable transmitter placed inside the bore (Pilot Tone). The frequency is selected such that it is located outside of the frequency band of the actual MR readout experiment but well within the bandwidth of the radiofrequency receiver, that is, the oversampling area. Temporal variations of the detected signal indicate motion. After extraction of the signal from the raw data, principal component analysis was used to identify respiratory motion. The approach and potential applications during MR and PET/MR examinations that rely on a continuous respiratory signal were validated with an anthropomorphic, PET/MR-compatible motion phantom as well as in a volunteer study. Respiratory motion detection and correction were presented for MR and PET data in phantom and volunteer studies. The Pilot Tone successfully recovered the ground-truth respiratory signal provided by the phantom. The presented method provides reliable respiratory motion tracking during arbitrary imaging sequences throughout a full PET/MR study. All results can directly be transferred to MR-only applications as well.

To demonstrate for the first time the feasibility of performing prospective motion correction using spherical navigators (SNAVs). SNAVs were interleaved in a 3D FLASH sequence with an additional short baseline scan (6.8 s) for fast rotation estimation. Assessment of SNAV-based prospective motion correction was performed in six volunteers. Participant motion was guided using randomly generated stepwise prompts as well as prompts derived from real motion cases. Experiments were performed on a 3 T MRI scanner using a 32-channel head coil. When optimized for real-time application, SNAV-based motion estimates were computed in 25.8 ± 1.3 ms. Phantom-based quantification of rotation and translation accuracy indicated mean absolute errors of 0.10 ± 0.09° and 0.25 ± 0.14 mm, respectively. Implementing SNAV-based motion estimates for prospective motion correction led to a clear improvement in image quality with minimal increase in scan time (<5%). Optimization of SNAV processing for real-time application enables prospective motion correction with low latency and minimal scan time requirements.

Real-time multislice-to-volume motion correction for task-based EPI-fMRI at 7T.

Purpose: Current hepatic dynamic contrast enhanced CT (DCE‐CT) protocols rely on patients holding their breath either once or multiple times. We propose that hepatic CT perfusion imaging is feasible using a free breathing DCE‐CT scanning protocol without prior coaching on breathing and automatic post acquisition image registration to eliminate motion induced artifacts. Methods: 26 patients (21 males, 5 females, average age 71 years) with primary or metastatic hepatomas were scanned with a free‐ breathing axial shuttle DCE‐CT. One‐dimensional respiratory motion correction in the axial direction was performed automatically by registering high contrast liver features in every image to that of a chosen reference image.CT Perfusion (GE Healthcare) was extended to calculate root mean squared deviation (RMSD) maps to quantify the amount of respiratory motion before and after motion correction. Functional parameters were generated for both corrected and uncorrected scans to determine the effect of motion induced artifacts. Results: Within the liver, background levels of RMSD due to tissue inhomogeneity, imaging noise and reconstruction artifacts was found to be less than 20 HU. A threshold of 200 HU was chosen to indicate organ motion. The mean fraction of voxels in the liver with RMSD above 200 HU (VF) decreased from 8.0% to 4.0% after correction (p=0.01). A strong correlation was found between initial VF (VFI) and reduction in VF (DVF) due to motion correction (DVF = 0.94*VFI − 3.66, R=0.97). Total blood flow was 30 ml/min/kg (p=0.08) higher in tumour than normal tissue prior to correction and 40 ml/min/kg (p=0.008) after correction. Conclusion: Free breathing DCE‐CT in liver is feasible and using a posteriori motion correction reduces potential motion artifacts in functional maps. Motion correction is beneficial in cases where initial VF is greater than 5% or an ROI can not be drawn that includes the portal vein in all volumes. “This research is funded by the Canadian Cancer Society (grant #700386)”.

Localized shimming in single-voxel MRS often results in large B0 inhomogeneity outside the volume-of-interest. This causes unacceptable degradation in motion navigator images. Switching back and forth between whole-brain shim and localized shim is possible for linear shims, but not for higher-order shims. Here we propose motion navigators largely insensitive to B0 inhomogeneity for prospective motion-corrected MRS with localized higher-order shimming. A recent fast high-resolution motion navigator based on spiral-in/out k-space trajectories and multislice-to-volume registration was modified by splitting the readout into multiple shot interleaves which shortened the echo time and reduced the effect of B0 inhomogeneity. The performance of motion correction was assessed in healthy subjects in the prefrontal cortex using a sLASER sequence at 3T (N = 5) and 7T (N = 5). With multiple spatial interleaves, excellent quality navigator images were acquired in the whole brain in spite of large B0 inhomogeneity outside the MRS voxel. The total duration of the navigator in sLASER remained relatively short even with multiple shots (3T: 10 spatial interleaves 94 ms per slice; 7T: 15 spatial interleaves 103 ms per slice). Prospective motion correction using the multi-shot navigators yielded comparable spectral quality (water linewidth and metabolite SNR) with and without subject motion. B0-insensitive motion navigators enable prospective motion correction for MRS with all first- and second-order shims adjusted in the MRS voxel, providing optimal spectral linewidth.

The purpose of this work is to describe a T1 -weighted fluid-attenuated inversion recovery (FLAIR) sequence that is able to produce sharp magnetic resonance images even if the subject is moving their head throughout the acquisition. The robustness to motion artifacts and retrospective motion correction capabilities of the PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) trajectory were combined with prospective motion correction. The prospective correction was done using an intelligent marker attached to the subject. This marker wirelessly synchronizes to the pulse sequence to measure the directionality and magnitude of the magnetic fields present in the MRI machine during a short navigator, thus enabling it to determine its position and orientation in the scanner coordinate frame. Three approaches to incorporating the marker-navigator into the PROPELLER sequence were evaluated. The specific absorption rate, and subsequent scan time, of the T1 -weighted FLAIR PROPELLER sequence, was reduced using a variable refocusing flip-angle scheme. Evaluations of motion correction performance were done with 4 volunteers and 3 types of head motion. During minimal out-of-plane movement, retrospective PROPELLER correction performed similarly to the prospective correction. However, the prospective clearly outperformed the retrospective correction when there was out-of-plane motion. Finally, the combination of retrospective and prospective correction produced the sharpest images even during large continuous motion. Prospective motion correction of a PROPELLER sequence makes it possible to handle continuous, large, and high-speed head motions with only minor reductions in image quality.

Motion-induced artefacts in MRI are a common occurrence but can obscure pathologies or be falsely identified as pathological. Reacquiring motion-corrupted scans is expensive, and thus retrospective and prospective motion correction methods have been introduced. Although motion correction shows promise, there is a lack of exhaustive testing on its efficacy with respect to full clinical cerebral MRI protocols. Here we present a dataset (n = 22) to facilitate future research, which includes data with and without intentional motion, and with and without prospective motion correction, across six MRI sequences included in a full clinical cerebral MRI protocol. Motion was captured by an external tracking device, and the dataset includes the motion data as derived motion transforms. For standardization, all image data are fully BIDS-compliant. Raw k-space data are available as well. As the dataset pairs motion-free data with motion-corrupted data, it can be used to develop or test different motion-correction or k-space reconstruction methods.

In liver diffusion-weighted imaging (DWI), single-shot echo-planar imaging (SS-EPI) sequences are susceptible to motion artifacts, resulting in image blurring and decreased lesion detection rates. This study aimed to develop and optimize a motion-corrected (MOCO) technique for liver DWI at 3 Tesla (3T). The technique incorporates motion correction, complex averaging, and a combination of a reparametrized sinc fatsat pulse with an optimized water excitation pulse. This prospective cross-sectional study performed at Fujian Medical University Union Hospital included 42 healthy volunteers who underwent four SS-EPI DWI sequences on a 3T magnetic resonance imaging (MRI) system between January 2023 and March 2023. The sequences included a navigator-triggered (NT) MOCO-DWI, two free-breathing (FB) MOCO-DWI, and an FB conventional DWI (FB cDWI) sequence. Motion correction and complex averaging were performed for both MOCO-DWI sequences, and fat suppression was achieved using either a sinc fatsat pulse with optimized water excitation or a conventional spectral attenuated inversion recovery (SPAIR) pulse. Liver signal-to-noise ratio (SNR) was measured at b=1,000 s/mm2. Qualitative parameters were independently evaluated by three radiologists using 5-point Likert scales. Quantitative parameters were assessed using the Kolmogorov-Smirnov test, and variance homogeneity was assessed using Levene's test. Regarding the qualitative analysis, the Friedman test was used to compare subjective scores among the four techniques. The SNRs of the liver were significantly higher with FB MOCO-DWI compared to the other EPI DWI sequences at b=1,000 s/mm2 (P<0.05). In the superior-inferior direction, the SNRs of the inferior level of the liver were higher than those of the superior level in NT MOCO-DWI. The qualitative results showed significantly higher ratings for NT MOCO-DWI and FB MOCO-DWI compared to the other EPI DWI sequences at b=1,000 s/mm2 (P<0.05). Regarding the apparent diffusion coefficient (ADC) quantification, the ADC values of the left lobe were higher than those of the right lobe in all four techniques. The proposed EPI DWI technique, incorporating motion correction, complex averaging, and a modified fat suppression scheme using spectral fat saturation and binomial water excitation, was found to be clinically feasible for liver MRI. The FB MOCO-DWI sequence, with its superior SNR and excellent image quality, is recommended for liver DW imaging at 3T in clinical routine.

To develop an efficient navigator-based motion and temporal B0-shift correction technique for 3D multi-echo gradient-echo (ME-GRE) MRI for quantitative susceptibility mapping (QSM) and mapping. A dual-echo 3D stack-of-spiral navigator was designed to interleave with the Cartesian multi-echo gradient-echo acquisitions, allowing the acquisition of both low-echo and high-echo time signals. We additionally designed a novel conjugate phase-based reconstruction method for the joint correction of motion and temporal B0 shifts. We performed numerical simulation, phantom scans,and in vivo human scans to assess the performance of the methods. Numerical simulation and human brain scans demonstrated that the proposed technique successfully corrected artifacts induced by both head motions and temporal B0 changes. Efficient B0-change correction with conjugate-phase reconstruction can be performed on fewer than 10 clustered k-space segments. In vivo scans showed that combining temporal B0 correction with motion correction further reduced artifacts and improved image quality in both and QSM images. Our proposed approach of using 3D spiral navigators and a novel conjugate-phase reconstruction method can improve susceptibility-related measurements using MR.