Optimizing Deep Learning Algorithms for Segmentation of Acute Infarcts on Non-Contrast Material-enhanced CT Scans of the Brain Using Simulated Lesions.

PURPOSE: To test the efficacy of lesion segmentation using a deep learning algorithm on non-contrast material-enhanced CT (NCCT) images with synthetic lesions resembling acute infarcts. MATERIALS AND METHODS: In this retrospective study, 40 diffusion-weighted imaging (DWI) lesions in patients with acute stroke (median age, 69 years; range, 62-76 years; 17 women; screened between 2011 and 2017) were coregistered to 40 normal NCCT scans (median age, 70 years; range, 55-76 years; 25 women; screened between 2008 and 2011), which produced 640 combinations of DWI-NCCT with and without lesions for training (n = 420), validation (n = 110), and testing (n = 110). The signal intensity on the NCCT scans was depressed by 4 HU (a 13% drop) in the region of the diffusion-weighted lesion. Two U-Net architectures (standard and symmetry aware) were trained with two different training strategies. One was a naive strategy, in which the model started training with random coefficients. The other was a progressive strategy, which started with coefficients derived from a model trained on a dataset with lesions that were depressed by 10 HU. The Dice scores from the two architectures and training strategies were compared from the test dataset. RESULTS: Dice scores of symmetry-aware U-Nets were 25% higher than those of standard U-Nets (median, 0.49 vs 0.65; P < .001). Use of a progressive training strategy had no clear effect on model performance. CONCLUSION: Symmetry-aware U-Nets offer promise for segmentation of acute stroke lesions on NCCT scans.Keywords: Adults, CT-Quantitative, StrokeSupplemental material is available for this article.© RSNA, 2021.

Marker-free optical stereo motion tracking for in-bore MRI and PET-MRI application.

PURPOSE: Prospective motion correction is arguably the "silver bullet" solution for magnetic resonance imaging (MRI) studies impacted by motion, applicable to almost any pulse sequence and immune from the spin history artifacts introduced by a moving object. In prospective motion correction, the magnetic field gradients and radio frequency waveforms are adjusted in real time in response to measured head motion so as to maintain the head in a stationary reference frame relative to the scanner. Vital for this approach are accurate and rapidly sampled head pose measurements, which may be obtained optically using cameras. However, most optical methods are limited by the need to attach physical markers to the skin, which leads to decoupling of head and marker motion and reduces the effectiveness of correction. In this work we investigate the feasibility and initial performance of a stereo-optical motion tracking method which does not require any attached markers. METHODS: The method relies on detecting distinctive natural features or amplified features (using skin stamps) directly on the forehead in multiple camera views, and then deriving pose estimates via a 3D-2D registration between the skin features and a database of forehead landmarks. To demonstrate the feasibility and potential accuracy of the marker-free method for discrete (step-wise) head motion, we performed out-of-bore and in-bore experiments using robotically and manually controlled phantoms in addition to in-bore testing on human volunteers. We also developed a convenient out-of-bore test bed to benchmark and optimize the motion tracking performance. RESULTS: For out-of-bore phantom tests, the pose estimation accuracy (compared to robotic ground truth) was 0.14 mm and 0.23 degrees for incremental translation and rotation, respectively. For arbitrary motion, the pose accuracy obtained using the smallest forehead feature patch was equivalent to 0.21 ± 0.11 mm positional accuracy in the striatum. For in-bore phantom experiments, the accuracy of rigid-body motion parameters (compared to wireless MR-sensitive markers) was 0.08-0.41 ± 0.18 mm/0.05-0.3 ± 0.12 deg and 0.14-0.16 ± 0.12 mm/0.08-0.17 ± 0.08 deg for the small and large feature patches, respectively. In vivo results in human volunteers indicated sub-millimeter and sub-degree pose accuracy for all rotations and translations except the depth direction (max error 1.8 mm) when compared to a registration-based approach. CONCLUSIONS: In both bench-top and in vivo experiments we demonstrate the feasibility of using very small feature patches directly on the skin to obtain high accuracy head pose measurements needed for motion-correction in MRI brain studies. The optical technique uses in-bore cameras and is consistent with the limited visibility of the forehead afforded by head coils used in brain imaging. Future work will focus on optimization of the technique and demonstration in prospective motion correction.

A within-coil optical prospective motion-correction system for brain imaging at 7T.

PURPOSE: Motion artifact limits the clinical translation of high-field MR. We present an optical prospective motion correction system for 7 Tesla MRI using a custom-built, within-coil camera to track an optical marker mounted on a subject. METHODS: The camera was constructed to fit between the transmit-receive coils with direct line of sight to a forehead-mounted marker, improving upon prior mouthpiece work at 7 Tesla MRI. We validated the system by acquiring a 3D-IR-FSPGR on a phantom with deliberate motion applied. The same 3D-IR-FSPGR and a 2D gradient echo were then acquired on 7 volunteers, with/without deliberate motion and with/without motion correction. Three neuroradiologists blindly assessed image quality. In 1 subject, an ultrahigh-resolution 2D gradient echo with 4 averages was acquired with motion correction. Four single-average acquisitions were then acquired serially, with the subject allowed to move between acquisitions. A fifth single-average 2D gradient echo was acquired following subject removal and reentry. RESULTS: In both the phantom and human subjects, deliberate and involuntary motion were well corrected. Despite marked levels of motion, high-quality images were produced without spurious artifacts. The quantitative ratings confirmed significant improvements in image quality in the absence and presence of deliberate motion across both acquisitions (P < .001). The system enabled ultrahigh-resolution visualization of the hippocampus during a long scan and robust alignment of serially acquired scans with interspersed movement. CONCLUSION: We demonstrate the use of a within-coil camera to perform optical prospective motion correction and ultrahigh-resolution imaging at 7 Tesla MRI. The setup does not require a mouthpiece, which could improve accessibility of motion correction during 7 Tesla MRI exams.

Improved Segmentation and Detection Sensitivity of Diffusion-weighted Stroke Lesions with Synthetically Enhanced Deep Learning.

PURPOSE: To compare the segmentation and detection performance of a deep learning model trained on a database of human-labeled clinical stroke lesions on diffusion-weighted (DW) images to a model trained on the same database enhanced with synthetic stroke lesions. MATERIALS AND METHODS: In this institutional review board-approved study, a stroke database of 962 cases (mean patient age ± standard deviation, 65 years ± 17; 255 male patients; 449 scans with DW positive stroke lesions) and a normal database of 2027 patients (mean age, 38 years ± 24; 1088 female patients) were used. Brain volumes with synthetic stroke lesions on DW images were produced by warping the relative signal increase of real strokes to normal brain volumes. A generic three-dimensional (3D) U-Net was trained on four different databases to generate four different models: (a) 375 neuroradiologist-labeled clinical DW positive stroke cases (CDB); (b) 2000 synthetic cases (S2DB); (c) CDB plus 2000 synthetic cases (CS2DB); and (d) CDB plus 40 000 synthetic cases (CS40DB). The models were tested on 20% (n = 192) of the cases of the stroke database, which were excluded from the training set. Segmentation accuracy was characterized using Dice score and lesion volume of the stroke segmentation, and statistical significance was tested using a paired two-tailed Student t test. Detection sensitivity and specificity were compared with labeling done by three neuroradiologists. RESULTS: The performance of the 3D U-Net model trained on the CS40DB (mean Dice score, 0.72) was better than models trained on the CS2DB (Dice score, 0.70; P < .001) or the CDB (Dice score, 0.65; P < .001). The deep learning model (CS40DB) was also more sensitive (91% [95% confidence interval {CI}: 89%, 93%]) than each of the three human readers (human reader 3, 84% [95% CI: 81%, 87%]; human reader 1, 78% [95% CI: 75%, 81%]; human reader 2, 79% [95% CI: 76%, 82%]), but was less specific (75% [95% CI: 72%, 78%]) than each of the three human readers (human reader 3, 96% [95% CI: 94%, 98%]; human reader 1, 92% [95% CI: 90%, 94%]; human reader 2, 89% [95% CI: 86%, 91%]). CONCLUSION: Deep learning training for segmentation and detection of stroke lesions on DW images was significantly improved by enhancing the training set with synthetic lesions.Supplemental material is available for this article.© RSNA, 2020.

Rigid Motion Correction for Brain PET/MR Imaging using Optical Tracking.

A significant challenge during high-resolution PET brain imaging on PET/MR scanners is patient head motion. This challenge is particularly significant for clinical patient populations who struggle to remain motionless in the scanner for long periods of time. Head motion also affects the MR scan data. An optical motion tracking technique, which has already been demonstrated to perform MR motion correction during acquisition, is used with a list-mode PET reconstruction algorithm to correct the motion for each recorded event and produce a corrected reconstruction. The technique is demonstrated on real Alzheimer's disease patient data for the GE SIGNA PET/MR scanner.

Prospective motion correction using coil-mounted cameras: Cross-calibration considerations.

PURPOSE: Optical prospective motion correction substantially reduces sensitivity to motion in neuroimaging of human subjects. However, a major barrier to clinical deployment has been the time-consuming cross-calibration between the camera and MRI scanner reference frames. This work addresses this challenge. METHODS: A single camera was mounted onto the head coil for tracking head motion. Two new methods were developed: (1) a rapid calibration method for camera-to-scanner cross-calibration using a custom-made tool incorporating wireless active markers, and (2) a calibration adjustment method to compensate for table motion between scans. Both methods were tested at 1.5T and 3T in vivo. Simulations were performed to determine the required mechanical tolerance for repositioning of the camera. RESULTS: The rapid calibration method is completed in a short (<30 s) scan, which is carried out only once per installation. The calibration adjustment method requires no extra scan time and runs automatically whenever the system is used. The mechanical tolerance analysis indicates that most motion (90% reduction in voxel displacement) could be corrected even with far larger camera repositioning errors than are observed in practice. CONCLUSION: The methods presented here allow calibration of sufficient quality to be carried out and maintained with no additional technologist workload. Magn Reson Med 79:1911-1921, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Extended hybrid-space SENSE for EPI: Off-resonance and eddy current corrected joint interleaved blip-up/down reconstruction.

INTRODUCTION: Geometric distortions along the phase encode direction caused by off-resonant spins are still a major issue in EPI based functional and diffusion imaging. If the off-resonance map is known it is possible to correct for distortions. Most correction methods operate as a post-processing step on the reconstructed magnitude images. THEORY AND METHODS: Here, we present an algebraic reconstruction method (hybrid-space SENSE) that incorporates a physics based model of off-resonances, phase inconsistencies between k-space segments, and T2*-decay during the acquisition. The method can be used to perform a joint reconstruction of interleaved acquisitions with normal (blip-up) and inverted (blip-down) phase encode direction which results in reduced g-factor penalty. RESULTS: A joint blip-up/down simultaneous multi slice (SMS) reconstruction for SMS-factor 4 in combination with twofold in-plane acceleration leads to a factor of two decrease in maximum g-factor penalty while providing off-resonance and eddy-current corrected images. CONCLUSION: We provide an algebraic framework for reconstructing diffusion weighted EPI data that in addition to the general applicability of hybrid-space SENSE to 2D-EPI, SMS-EPI and 3D-EPI with arbitrary k-space coverage along z, allows for a modeling of arbitrary spatio-temporal effects during the acquisition period like off-resonances, phase inconsistencies and T2*-decay. The most immediate benefit is a reduction in g-factor penalty if an interleaved blip-up/down acquisition strategy is chosen which facilitates eddy current estimation and ensures no loss in k-space encoding in regions with strong off-resonance gradients.

Prospective motion correction for 3D pseudo-continuous arterial spin labeling using an external optical tracking system.

Head motion is an unsolved problem in magnetic resonance imaging (MRI) studies of the brain. Real-time tracking using a camera has recently been proposed as a way to prevent head motion artifacts. As compared to navigator-based approaches that use MRI data to detect and correct motion, optical motion correction works independently of the MRI scanner, thus providing low-latency real-time motion updates without requiring any modifications to the pulse sequence. The purpose of this study was two-fold: 1) to demonstrate that prospective optical motion correction using an optical camera mitigates artifacts from head motion in three-dimensional pseudo-continuous arterial spin labeling (3D PCASL) acquisitions and 2) to assess the effect of latency differences between real-time optical motion tracking and navigator-style approaches (such as PROMO). An optical motion correction system comprising a single camera and a marker attached to the patient's forehead was used to track motion at a rate of 60fps. In the presence of motion, continuous tracking data from the optical system was used to update the scan plane in real-time during the 3D-PCASL acquisition. Navigator-style correction was simulated by using the tracking data from the optical system and performing updates only once per repetition time. Three normal volunteers and a patient were instructed to perform continuous and discrete head motion throughout the scan. Optical motion correction yielded superior image quality compared to uncorrected images or images using navigator-style correction. The standard deviations of pixel-wise CBF differences between reference and non-corrected, navigator-style-corrected and optical-corrected data were 14.28, 14.35 and 11.09mL/100g/min for continuous motion, and 12.42, 12.04 and 9.60mL/100g/min for discrete motion. Data obtained from the patient revealed that motion can obscure pathology and that application of optical prospective correction can successfully reveal the underlying pathology in the presence of head motion.

Trade-off between angular and spatial resolutions in in vivo fiber tractography.

Tractography is becoming an increasingly popular method to reconstruct white matter connections in vivo. The diffusion MRI data that tractography is based on requires a high angular resolution to resolve crossing fibers whereas high spatial resolution is required to distinguish kissing from crossing fibers. However, scan time increases with increasing spatial and angular resolutions, which can become infeasible in clinical settings. Here we investigated the trade-off between spatial and angular resolutions to determine which of these factors is most worth investing scan time in. We created a unique diffusion MRI dataset with 1.0 mm isotropic resolution and a high angular resolution (100 directions) using an advanced 3D diffusion-weighted multi-slab EPI acquisition. This dataset was reconstructed to create subsets of lower angular (75, 50, and 25 directions) and lower spatial (1.5, 2.0, and 2.5 mm) resolution. Using all subsets, we investigated the effects of angular and spatial resolutions in three fiber bundles-the corticospinal tract, arcuate fasciculus and corpus callosum-by analyzing the volumetric bundle overlap and anatomical correspondence between tracts. Our results indicate that the subsets of 25 and 50 directions provided inferior tract reconstructions compared with the datasets with 75 and 100 directions. Datasets with spatial resolutions of 1.0, 1.5, and 2.0 mm were comparable, while the lowest resolution (2.5 mm) datasets had discernible inferior quality. In conclusion, we found that angular resolution appeared to be more influential than spatial resolution in improving tractography results. Spatial resolutions higher than 2.0 mm only appear to benefit multi-fiber tractography methods if this is not at the cost of decreased angular resolution.

Contact-free physiological monitoring using a markerless optical system.

PURPOSE: Physiological noise remains a major problem in MRI, particularly at higher imaging resolutions and field strengths. The aim of this work was to investigate the feasibility of using an MR-compatible in-bore camera system to perform contactless monitoring of cardiac and respiratory information during MRI of human subjects. METHODS: An MR-compatible camera was mounted on an eight-channel head coil. Video data of the skin was processed offline to derive cardiac and respiratory signals from the pixel signal intensity and from head motion in the patient head-feet direction. These signals were then compared with data acquired simultaneously from the pulse oximeter and the respiratory belt. RESULTS: The cardiac signal computed using the average image pixel intensity closely resembled the signal obtained using the pulse oximeter. Trigger intervals obtained from both systems matched to within 50 ms (one standard deviation). The respiratory signal computed from small in-plane movements closely matched the signal obtained from the respiratory belt. Simultaneous MR imaging did not appear to have an effect on the physiological signals acquired by means of the contact-free monitoring system. CONCLUSION: Contact-free monitoring of human subjects to obtain cardiac and respiratory information is feasible using a small camera and light emitting diode mounted on the head coil of an MRI scanner.

Motion artifacts in MRI: A complex problem with many partial solutions.

Subject motion during magnetic resonance imaging (MRI) has been problematic since its introduction as a clinical imaging modality. While sensitivity to particle motion or blood flow can be used to provide useful image contrast, bulk motion presents a considerable problem in the majority of clinical applications. It is one of the most frequent sources of artifacts. Over 30 years of research have produced numerous methods to mitigate or correct for motion artifacts, but no single method can be applied in all imaging situations. Instead, a "toolbox" of methods exists, where each tool is suitable for some tasks, but not for others. This article reviews the origins of motion artifacts and presents current mitigation and correction methods. In some imaging situations, the currently available motion correction tools are highly effective; in other cases, appropriate tools still need to be developed. It seems likely that this multifaceted approach will be what eventually solves the motion sensitivity problem in MRI, rather than a single solution that is effective in all situations. This review places a strong emphasis on explaining the physics behind the occurrence of such artifacts, with the aim of aiding artifact detection and mitigation in particular clinical situations.

Comparison of optical and MR-based tracking.

PURPOSE: The goal of this study was to compare the accuracy of two real-time motion tracking systems in the MR environment: MR-based prospective motion correction (PROMO) and optical moiré phase tracking (MPT). METHODS: Five subjects performed eight predefined head rotations of 8° ± 3° while being simultaneously tracked with PROMO and MPT. Structural images acquired immediately before and after each tracking experiment were realigned with SPM8 to provide a reference measurement. RESULTS: Mean signed errors (MSEs) in MPT tracking relative to SPM8 were less than 0.3 mm and 0.2° in all 6 degrees of freedom, and MSEs in PROMO tracking ranged up to 0.2 mm and 0.3°. MPT and PROMO significantly differed from SPM8 in y-translation and y-rotation values (P < 0.05). Maximum absolute errors ranged up to 2.8 mm and 2.1° for MPT, and 2.2 mm and 2.9° for PROMO. CONCLUSION: This study presents the first in vivo comparison of MPT and PROMO tracking. Our data show that two methods yielded similar performances (within 1 mm and 1° standard deviation) relative to reference image registration. Tracking errors of both systems were larger than offline tests. Future work is required for further comparison of two methods in vivo with higher precision.

Prevention of motion-induced signal loss in diffusion-weighted echo-planar imaging by dynamic restoration of gradient moments.

PURPOSE: Head motion is a significant problem in diffusion-weighted imaging as it may cause signal attenuation due to residual dephasing during strong diffusion encoding gradients even in single-shot acquisitions. Here, we present a new real-time method to prevent motion-induced signal loss in DWI of the brain. METHODS: The method requires a fast motion tracking system (optical in the current implementation). Two alterations were made to a standard diffusion-weighted echo-planar imaging sequence: first, real-time motion correction ensures that slices are correctly aligned relative to the moving brain. Second, the tracking data are used to calculate the motion-induced gradient moment imbalance which occurs during the diffusion encoding periods, and a brief gradient blip is inserted immediately prior to the signal readout to restore the gradient moment balance. RESULTS: Phantom experiments show that the direction as well as magnitude of the gradient moment imbalance affects the characteristics of unwanted signal attenuation. In human subjects, the addition of a moment-restoring blip prevented signal loss and improved the reproducibility and reliability of diffusion tensor measures even in the presence of substantial head movements. CONCLUSION: The method presented can improve robustness for clinical routine scanning in populations that are prone to head movements, such as children and uncooperative adult patients.

Reproduction of motion artifacts for performance analysis of prospective motion correction in MRI.

PURPOSE: Despite numerous publications describing the ability of prospective motion correction to improve image quality in magnetic resonance imaging of the brain, a reliable approach to assess this improvement is still missing. A method that accurately reproduces motion artifacts correctable with prospective motion correction is developed, and enables the quantification of the improvements achieved. METHODS: A software interface was developed to simulate rigid body motion by changing the scanning coordinate system relative to the object. Thus, tracking data recorded during a patient scan can be used to reproduce the prevented motion artifacts on a volunteer or a phantom. The influence of physiological motion on image quality was investigated by filtering these data. Finally, the method was used to reproduce and quantify the motion artifacts prevented in a patient scan. RESULTS: The accuracy of the method was tested in phantom experiments and in vivo. The calculated quality factor, as well as a visual inspection of the reproduced artifacts shows a good correspondence to the original. CONCLUSION: Precise reproduction of motion artifacts assists qualification of prospective motion correction strategies. The presented method provides an important tool to investigate the effects of rigid body motion on a wide range of sequences, and to quantify the improvement in image quality through prospective motion correction.

Knee cartilage MRI with in situ mechanical loading using prospective motion correction.

PURPOSE: To assess the feasibility of high resolution knee cartilage MRI with in situ mechanical loading using optical tracking to compensate for motion. METHODS: In vivo cartilage MRI with in situ mechanical loading is demonstrated on a clinical 3T system for the patellofemoral as well as for the tibiofemoral knee joint using a T1-weighted spoiled three-dimensional gradient-echo sequence. Prospective motion correction is performed with a moiré phase tracking system consisting of an in-bore camera and a single tracking marker attached to the skin. RESULTS: Rigid-body approximation required for prospective correction with optical motion tracking is fulfilled well enough for the patellofemoral as well as for the tibiofemoral joint when the tracking marker is attached to the knee cap and the shin, respectively. Presaturation proves to be efficient in suppressing pulsation artifacts from the popliteal artery and residual motion artifacts primarily arising from nonrigid motion of the posterior knee compartment. CONCLUSION: The proposed technique enables knee cartilage imaging under in situ mechanical loading with submillimeter spatial resolution devoid of significant motion artifacts and thus appropriate for cartilage volumetry. It has the potential to provide new insight into the biomechanics of the knee and might complement the panoply of diagnostic MR methods for osteoarthritis.

Reliability of brain volume measurements: a test-retest dataset.

Evaluation of neurodegenerative disease progression may be assisted by quantification of the volume of structures in the human brain using magnetic resonance imaging (MRI). Automated segmentation software has improved the feasibility of this approach, but often the reliability of measurements is uncertain. We have established a unique dataset to assess the repeatability of brain segmentation and analysis methods. We acquired 120 T1-weighted volumes from 3 subjects (40 volumes/subject) in 20 sessions spanning 31 days, using the protocol recommended by the Alzheimer's Disease Neuroimaging Initiative (ADNI). Each subject was scanned twice within each session, with repositioning between the two scans, allowing determination of test-retest reliability both within a single session (intra-session) and from day to day (inter-session). To demonstrate the application of the dataset, all 3D volumes were processed using FreeSurfer v5.1. The coefficient of variation of volumetric measurements was between 1.6% (caudate) and 6.1% (thalamus). Inter-session variability exceeded intra-session variability for lateral ventricle volume (P<0.0001), indicating that ventricle volume in the subjects varied between days.

Prospective motion correction using inductively coupled wireless RF coils.

PURPOSE: A novel prospective motion correction technique for brain MRI is presented that uses miniature wireless radio-frequency coils, or "wireless markers," for position tracking. METHODS: Each marker is free of traditional cable connections to the scanner. Instead, its signal is wirelessly linked to the MR receiver via inductive coupling with the head coil. Real-time tracking of rigid head motion is performed using a pair of glasses integrated with three wireless markers. A tracking pulse-sequence, combined with knowledge of the markers' unique geometrical arrangement, is used to measure their positions. Tracking data from the glasses is then used to prospectively update the orientation and position of the image-volume so that it follows the motion of the head. RESULTS: Wireless-marker position measurements were comparable to measurements using traditional wired radio-frequency tracking coils, with the standard deviation of the difference < 0.01 mm over the range of positions measured inside the head coil. Wireless-marker safety was verified with B1 maps and temperature measurements. Prospective motion correction was demonstrated in a 2D spin-echo scan while the subject performed a series of deliberate head rotations. CONCLUSION: Prospective motion correction using wireless markers enables high quality images to be acquired even during bulk motions. Wireless markers are small, avoid radio-frequency safety risks from electrical cables, are not hampered by mechanical connections to the scanner, and require minimal setup times. These advantages may help to facilitate adoption in the clinic.

Ballistocardiographic artifact removal from simultaneous EEG-fMRI using an optical motion-tracking system.

The combination of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) allows the investigation of neuronal activity with high temporal and spatial resolution. While much progress has been made to overcome the multiple technical challenges associated with the recording of EEG inside the MR scanner, the ballistocardiographic (BCG) artifact, which is caused by cardiac-related motion inside the magnetic field, remains a major issue affecting EEG quality. The BCG is difficult to remove by standard average artifact subtraction (AAS) methods due to its variability across cardiac cycles. We thus investigate the possibility of directly recording the BCG motion using an optical motion-tracking system. In 5 subjects, the system is shown to accurately measure BCG motion. Regressing out linear and quadratic functions of the measured motion parameters resulted in a significant reduction (p<0.05) in root-mean-square (RMS) amplitudes across cardiac cycles compared to AAS. A further significant RMS reduction was obtained when applying the regression and AAS methods sequentially, resulting in RMS amplitudes that were not significantly different from those of EEG recorded outside the scanner, although with higher residual variability. The large contributions of pure translational parameters and of non-linear terms to the BCG waveforms indicate that non-rigid motion of the EEG wires (originating from rigid head motion) is likely an important cause of the artifact.

Prospective motion correction in brain imaging: a review.

Motion correction in magnetic resonance imaging by real-time adjustment of the imaging pulse sequence was first proposed more than 20 years ago. Recent advances have resulted from combining real-time correction with new navigator and external tracking mechanisms capable of quantifying rigid-body motion in all 6 degrees of freedom. The technique is now often referred to as "prospective motion correction." This article describes the fundamentals of prospective motion correction and reviews the latest developments in its application to brain imaging and spectroscopy. Although emphasis is placed on the brain as the organ of interest, the same principles apply whenever the imaged object can be approximated as a rigid body. Prospective motion correction can be used with most MR sequences, so it has potential to make a large impact in clinical routine. To maximize the benefits obtained from the technique, there are, however, several challenges still to be met. These include practical implementation issues, such as obtaining tracking data with minimal delay, and more fundamental problems, such as the magnetic field distortions caused by a moving object. This review discusses these challenges and summarizes the state of the art. We hope that this work will motivate further developments in prospective motion correction and help the technique to reach its full potential.

Measurement and correction of microscopic head motion during magnetic resonance imaging of the brain.

Magnetic resonance imaging (MRI) is a widely used method for non-invasive study of the structure and function of the human brain. Increasing magnetic field strengths enable higher resolution imaging; however, long scan times and high motion sensitivity mean that image quality is often limited by the involuntary motion of the subject. Prospective motion correction is a technique that addresses this problem by tracking head motion and continuously updating the imaging pulse sequence, locking the imaging volume position and orientation relative to the moving brain. The accuracy and precision of current MR-compatible tracking systems and navigator methods allows the quantification and correction of large-scale motion, but not the correction of very small involuntary movements in six degrees of freedom. In this work, we present an MR-compatible tracking system comprising a single camera and a single 15 mm marker that provides tracking precision in the order of 10 m and 0.01 degrees. We show preliminary results, which indicate that when used for prospective motion correction, the system enables improvement in image quality at both 3 T and 7 T, even in experienced and cooperative subjects trained to remain motionless during imaging. We also report direct observation and quantification of the mechanical ballistocardiogram (BCG) during simultaneous MR imaging. This is particularly apparent in the head-feet direction, with a peak-to-peak displacement of 140 m.