Prospective motion correction using tracking coils

Intracavity imaging coils provide higher signal‐to‐noise than surface coils and have the potential to provide higher spatial resolution in shorter acquisition times. However, images from these coils suffer from physiologically induced motion artifacts, as both the anatomy and the coils move during image acquisition. We developed prospective motion‐correction techniques for intracavity imaging using an array of tracking coils. The system had <50 ms latency between tracking and imaging, so that the images from the intracavity coil were acquired in a frame of reference defined by the tracking array rather than by the system's gradient coils. Two‐dimensional gradient‐recalled and three‐dimensional electrocardiogram‐gated inversion‐recovery‐fast‐gradient‐echo sequences were tested with prospective motion correction using ex vivo hearts placed on a moving platform simulating both respiratory and cardiac motion. Human abdominal tests were subsequently conducted. The tracking array provided a positional accuracy of 0.7 ± 0.5 mm, 0.6 ± 0.4 mm, and 0.1 ± 0.1 mm along the X, Y, and Z directions at a rate of 20 frames‐per‐second. The ex vivo and human experiments showed significant image quality improvements for both in‐plane and through‐plane motion correction, which although not performed in intracavity imaging, demonstrates the feasibility of implementing such a motion‐correction system in a future design of combined tracking and intracavity coil. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Motion correction in magnetic resonance spectroscopy

In vivo proton magnetic resonance spectroscopy and spectroscopic imaging (MRS/MRSI) are valuable tools to study normal and abnormal human brain physiology. However, they are sensitive to motion, due to strong crusher gradients, long acquisition times, reliance on high magnetic field homogeneity, and particular acquisition methods such as spectral editing. The effects of motion include incorrect spatial localization, phase fluctuations, incoherent averaging, line broadening, and ultimately quantitation errors. Several retrospective methods have been proposed to correct motion-related artifacts. Recent advances in hardware also allow prospective (real-time) correction of the effects of motion, including adjusting voxel location, center frequency, and magnetic field homogeneity. This article reviews prospective and retrospective methods available in the literature and their implications for clinical MRS/MRSI. In combination, these methods can attenuate or eliminate most motion-related artifacts and facilitate the acquisition of high-quality data in the clinical research setting.     

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. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Advantages and limitations of prospective head motion compensation for MRI using an optical motion tracking device

RATIONALE AND OBJECTIVES: Subject motion appears to be a limiting factor in numerous magnetic resonance (MR) imaging (MRI) applications. In particular, head tremor, which often accompanies stroke, may render certain high-resolution two- (2D) and three-dimensional (3D) techniques inapplicable. The reason for that is head movement during acquisition. The study objective is to achieve a method able to compensate for complete motion during data acquisition. The method should be usable for every sequence and easily implemented on different MR scanners. MATERIALS AND METHODS: The possibility of interfacing the MR scanner with an external optical motion-tracking system capable of determining the object's position with submillimeter accuracy and an update rate of 60 Hz is shown. Movement information on the object position (head) is used to compensate for motion in real time by updating the field of view (FOV) by recalculating the gradients and radiofrequency parameter of the MR scanner during acquisition of k-space data, based on tracking data. RESULTS: Results of rotation phantom, in vivo experiments, and implementation of three different MRI sequences, 2D spin echo, 3D gradient echo, and echo planar imaging, are presented. Finally, the proposed method is compared with the prospective motion correction software available on the scanner software. CONCLUSION: A prospective motion correction method that works in real time only by updating the FOV of the MR scanner is presented. Results show the feasibility of using an external optical motion-tracking system to compensate for strong and fast subject motion during acquisition.     

Navigator accuracy requirements for prospective motion correction

Prospective motion correction in MRI is becoming increasingly popular to prevent the image artifacts that result from subject motion. Navigator information is used to update the position of the imaging volume before every spin excitation so that lines of acquired k‐space data are consistent. Errors in the navigator information, however, result in residual errors in each k‐space line. This paper presents an analysis linking noise in the tracking system to the power of the resulting image artifacts. An expression is formulated for the required navigator accuracy based on the properties of the imaged object and the desired resolution. Analytical results are compared with computer simulations and experimental data. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Real‐time optical motion correction for diffusion tensor imaging

Head motion is a fundamental problem in brain MRI. The problem is further compounded in diffusion tensor imaging because of long acquisition times, and the sensitivity of the tensor computation to even small misregistration. To combat motion artifacts in diffusion tensor imaging, a novel real‐time prospective motion correction method was introduced using an in‐bore monovision system. The system consists of a camera mounted on the head coil and a self‐encoded checkerboard marker that is attached to the patient's forehead. Our experiments showed that optical prospective motion correction is more effective at removing motion artifacts compared to image‐based retrospective motion correction. Statistical analysis revealed a significant improvement in similarity between diffusion data acquired at different time points when prospective correction was used compared to retrospective correction (P < 0.001). Magn Reson Med, 2010. © 2011 Wiley‐Liss, Inc.     

Prospective head‐movement correction for high‐resolution MRI using an in‐bore optical tracking system

In MRI of the human brain, subject motion is a major cause of magnetic resonance image quality degradation. To compensate for the effects of head motion during data acquisition, an in‐bore optical motion tracking system is proposed. The system comprises two MR‐compatible infrared cameras that are fixed on a holder right above and in front of the head coil. The resulting close proximity of the cameras to the object allows precise tracking of its movement. During image acquisition, the MRI scanner uses this tracking information to prospectively compensate for head motion by adjusting the gradient field direction and radio frequency (RF) phases and frequencies. Experiments performed on subjects demonstrate robust system performance with translation and rotation accuracies of 0.1 mm and 0.15°, respectively. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Prospective respiratory motion correction for coronary MR angiography using a 2D image navigator

Respiratory motion remains the major impediment in a substantial amount of patients undergoing coronary magnetic resonance angiography. Motion correction in coronary magnetic resonance angiography is typically performed with a diaphragmatic 1D navigator (1Dnav) assuming a constant linear relationship between diaphragmatic and cardiac respiratory motion. In this work, a novel 2D navigator (2Dnav) is proposed, which prospectively corrects for translational motion in foot–head and left–right direction. First, 1Dnav‐ and 2Dnav‐based motion correction are compared in 2D real time imaging experiments, by evaluating the residual respiratory motion in 10 healthy subjects as well as in a moving vessel phantom. Subsequently, 1Dnav and 2Dnav corrected high‐resolution 3D coronary MR angiograms were acquired, and both objective and subjective image quality were assessed. For a gating window of 10 mm, 1Dnav and 2Dnav performed equally well; however, without any respiratory gating, the 1Dnav had a lower visual score for all coronary arteries compared with 10 mm gating, whereas the 2Dnav without gating performed similar to 1Dnav with 10 mm gating. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Prospective optical motion correction for 3D time‐of‐flight angiography

Magnetic resonance angiograms are often nondiagnostic due to patient motion. In clinical practice, the available time to repeat motion‐corrupted scans is very limited—especially in patients who suffer from acute cerebrovascular conditions. Here, the feasibility of an optical motion correction system to prospectively correct patient motion for 3D time‐of‐flight magnetic resonance angiography was investigated. Experiments were performed on five subjects with and without parallel imaging (SENSE R = 2) on a 1.5 T unit. Two human readers assessed the data and were in good agreement (kappa: 0.77). The results from this study indicate that the optical motion correction system greatly reduces motion artifacts when motion was present and did not impair the image quality in the absence of motion. Statistical analysis showed no significant difference between the (vendor‐provided) SENSE and the nonaccelerated acquisitions. In conclusion, the optical motion correction system tested in this study has the potential to greatly improve 3D time‐of‐flight angiograms regardless of whether it is used with or without SENSE. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Prospective motion correction with continuous gradient updates in diffusion weighted imaging

Despite the existence of numerous motion correction methods, head motion during MRI continues to be a major source of artifacts and can greatly reduce image quality. This applies particularly to diffusion weighted imaging, where strong gradients are applied during long encoding periods. These are necessary to encode microscopic movements. However, they also make the technique highly sensitive to bulk motion. In this work, we present a prospective motion correction method where all applied gradients are adjusted continuously to compensate for changes of the object position and ensure the desired phase evolution in the image coordinate frame. Additionally, in phantom experiments this new technique is used to reproduce motion artifacts with high accuracy by changing the position of the imaging frame relative to the measured object. In vivo measurements demonstrate the validity of the new correction method.     

Prospective real‐time correction for arbitrary head motion using active markers

Patient motion during an MRI exam can result in major degradation of image quality, and is of increasing concern due to the aging population and its associated diseases. This work presents a general strategy for real‐time, intraimage compensation of rigid‐body motion that is compatible with multiple imaging sequences. Image quality improvements are established for structural brain MRI acquired during volunteer motion. A headband integrated with three active markers is secured to the forehead. Prospective correction is achieved by interleaving a rapid track‐and‐update module into the imaging sequence. For every repetition of this module, a short tracking pulse‐sequence remeasures the marker positions; during head motion, the rigid‐body transformation that realigns the markers to their initial positions is fed back to adaptively update the image‐plane—maintaining it at a fixed orientation relative to the head—before the next imaging segment of k‐space is acquired. In cases of extreme motion, corrupted lines of k‐space are rejected and reacquired with the updated geometry. High‐precision tracking measurements (0.01 mm) and corrections are accomplished in a temporal resolution (37 ms) suitable for real‐time application. The correction package requires minimal additional hardware and is fully integrated into the standard user interface, promoting transferability to clinical practice. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.