The diminishing variance algorithm for real-time reduction of motion artifacts in MRI

A technique has been developed whereby motion can be detected in real time during the acquisition of data. This enables the implementation of several algorithms to reduce or eliminate motion effects from an image as it is being acquired. One such algorithm previously described is the acceptance/rejection method. This paper deals with another real-time algorithm called the diminishing variance algorithm (DVA). With this method, a complete set of preliminary data is acquired along with information about the relative motion position of each frame of data. After all the preliminary data are acquired, the position information is used to determine which data frames are most corrupted by motion. Frames of data are then reacquired, starting with the most corrupted one. The position information is continually updated in an iterative process; therefore, each subsequent reacquisition is always done on the worst frame of data. The algorithm has been implemented on several different types of sequences. Preliminary in vivo studies indicate that motion artifacts are dramatically reduced.     

Body MRI artifacts in clinical practice: A physicist's and radiologist's perspective

The high information content of MRI exams brings with it unintended effects, which we call artifacts. The purpose of this review is to promote understanding of these artifacts, so they can be prevented or properly interpreted to optimize diagnostic effectiveness. We begin by addressing static magnetic field uniformity, which is essential for many techniques, such as fat saturation. Eddy currents, resulting from imperfect gradient pulses, are especially problematic for new techniques that depend on high performance gradient switching. Nonuniformity of the transmit radiofrequency system constitutes another source of artifacts, which are increasingly important as magnetic field strength increases. Defects in the receive portion of the radiofrequency system have become a more complex source of problems as the number of radiofrequency coils, and the sophistication of the analysis of their received signals, has increased. Unwanted signals and noise spikes have many causes, often manifesting as zipper or banding artifacts. These image alterations become particularly severe and complex when they are combined with aliasing effects. Aliasing is one of several phenomena addressed in our final section, on artifacts that derive from encoding the MR signals to produce images, also including those related to parallel imaging, chemical shift, motion, and image subtraction. J. Magn. Reson. Imaging 2013;38:269–287. © 2013 Wiley Periodicals, Inc.     

Reduction of motion artifacts in cine MRI using variable-density spiral trajectories

Dynamic cardiac imaging in MRI is a very challenging task. To obtain high spatial resolution, temporal resolution, and signalto-noise ratio (SNR), single-shot imaging is not sufficient Use of multishot techniques resolves this problem but can cause motion artifacts because of data inconsistencies between views. Motion artifacts can be reduced by signal averaging at some cost in increased scan time. However, for the same increase in scan time, other techniques can be more effective than simple averaging in reducing the artifacts. If most of the energy of the inconsistencies is limited to a certain region of k-space, increased sampling density (oversampling) in this region can be especially effective in reducing motion artifacts. In this work, several variable-density spiral trajectories are designed and tested. Their efficiencies for artifact reduction are evaluated in computer simulations and in scans of normal volunteers. The SNR compromise of these trajectories is also investigated. The authors conclude that variable-density spiral trajectories can effectively reduce motion artifacts with a small loss in SNR as compared with a uniform density counterpart.     

Magnetic field threshold for accurate electrocardiography in the MRI environment

Although the electrocardiogram is known to be nondiagnostic within the bore of any high‐field magnet due to the magnetohydrodynamic effect, there are an increasing number of applications that require accurate electrocardiogram monitoring of a patient inside the MRI room but outside of the magnet bore. Magnetohydrodynamic effects on the ST segment of the electrocardiogram waveform were investigated in six subjects at magnetic field strengths ranging from 6.4 mT to 652 mT at the aortic midarch, and the electrocardiogram was found to be accurate at magnetic fields below 70 mT. This corresponds to a distance of 160 cm from the isocenter and 80 cm from the bore entrance for the 1.5‐T MRI system used in this study. These results can be translated to any MRI system, with knowledge of the fringe field. Accurate electrocardiogram monitoring is feasible in close proximity to the MRI magnet, such as during and after pharmacologic or exercise stress, or interventional or surgical procedures performed in the MRI room. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Pulse sequences and system interfaces for interventional and real-time MRI

The use of MRI for intervention and real-time imaging has seen many changes since its inception in the late 1980s. Initial interventional MRI researchers made great strides in building this new specialty, creating devices, sequences, and applications to push the field forward. More recently, researchers have gained more access to the systems themselves, and have taken advantage of this situation to create truly interactive interventional systems. Techniques such as fully interactive scan adjustments and device tracking can be accomplished in real time due to increased transparency between vendors and researchers. Additionally, pulse sequences have undergone an evolution as well, with the constant emergence of novel acquisition schemes to generate image contrast quickly, increase temporal resolution and cover k-space with nonrectilinear trajectories. We will look at both emerging system interface concepts and novel pulse sequences that we believe will continue to push innovation in the field of interventional MRI.     

Characterization of and correction for artifacts in linogram MRI

Chemical shift differences, field inhomogeneity, and gradient nonlinearity result in artifacts in magnetic resonance imaging. Three artifacts are characterized for linogram imaging and it is shown that, based on computer simulations and theory, linogram MRI behaves similarly to 2DFT. A correction technique similar to a scheme for 2DFT imaging based on the Dixon technique and coordinate transform methods is proposed. The algorithm is applied to correct for field inhomogeneity and gradient nonlinearity-induced artifacts in both simulations and images of a clinical phantom. The results show good correlation with the theory. It is concluded that linogram imaging offers certain attractive features of both 2DFT and PR imaging techniques, and is a potentially viable alternative to PR imaging in the presence of field inhomogeneity.     

Dynamic coil selection for real-time imaging in interventional MRI.

MR-guided intravascular interventions require image update rates of up to 10 images per second, which can be achieved using parallel imaging. However, parallel imaging requires many coil elements, which increases reconstruction times and thus compromises real-time image reconstruction. In this study a dynamic coil selection (DCS) algorithm is presented that selects a subset of receive coils to reduce image reconstruction times. The center-of-sensitivity coordinates and the relative signal intensities are determined for each coil in a prescan. During the intervention m coils are selected for reconstruction using a coil ranking based on the distance to the current slice or catheter position. In a phantom experiment for m = 6, an optimal signal-to-background ratio (SBR) was achieved and foldover artifacts were avoided. In three animal experiments involving catheter manipulation in the aorta and the right heart chamber, the anatomy was successfully visualized at frame rates of about 5 Hz using active catheter tracking.     

Automatic correction of echo-planar imaging (EPI) ghosting artifacts in real-time interactive cardiac MRI using sensitivity encoding

PURPOSE: To develop a method that automatically corrects ghosting artifacts due to echo-misalignment in interleaved gradient-echo echo-planar imaging (EPI) in arbitrary oblique or double-oblique scan planes. MATERIALS AND METHODS: An automatic ghosting correction technique was developed based on an alternating EPI acquisition and the phased-array ghost elimination (PAGE) reconstruction method. The direction of k-space traversal is alternated at every temporal frame, enabling lower temporal-resolution ghost-free coil sensitivity maps to be dynamically estimated. The proposed method was compared with conventional one-dimensional (1D) phase correction in axial, oblique, and double-oblique scan planes in phantom and cardiac in vivo studies. The proposed method was also used in conjunction with two-fold acceleration. RESULTS: The proposed method with nonaccelerated acquisition provided excellent suppression of ghosting artifacts in all scan planes, and was substantially more effective than conventional 1D phase correction in oblique and double-oblique scan planes. The feasibility of real-time reconstruction using the proposed technique was demonstrated in a scan protocol with 3.1-mm spatial and 60-msec temporal resolution. CONCLUSION: The proposed technique with nonaccelerated acquisition provides excellent ghost suppression in arbitrary scan orientations without a calibration scan, and can be useful for real-time interactive imaging, in which scan planes are frequently changed with arbitrary oblique orientations.     

Band artifacts due to bulk motion.

Band artifacts due to bulk motion were investigated in images acquired with fast gradient echo sequences. A simple analytical calculation shows that the width of the artifacts has a square-root dependence on the velocity of the imaged object, the time taken to acquire each line of k-space and the field of view in the phase-encoding direction. The theory furthermore predicts that the artifact width can be reduced using parallel imaging by a factor equal to the square root of the acceleration parameter. The analysis and results are presented for motion in the phase- and frequency-encoding directions and comparisons are made between sequential and centric ordering. The theory is validated in phantom experiments, in which bulk motion is simulated in a controlled and reproducible manner by rocking the scan table back and forth along the bore axis. Preliminary cardiac studies in healthy human volunteers show that dark bands may be observed in the endocardium in images acquired with nonsegmented fast gradient echo sequences. The fact that the position of the bands changes with the phase-encoding direction suggests that they may be artifacts due to motion of the heart walls during the image acquisition period.     

On the nature and reduction of plaque-mimicking flow artifacts in black blood MRI of the carotid bifurcation

Cardiac-gated black blood MRI of the carotid artery bifurcation in normal human subjects shows signal within the lumen suggesting wall thickening or atherosclerotic plaque. This signal was believed to be artifactual, arising from complex flow patterns present at the carotid bifurcation. Computer simulation of the hemodynamics and black blood multislice image acquisition in a model of the carotid bifurcation showed that these artifacts arise from spins recovering their signal within the slow, recirculating flow of the carotid bulb. The computed hemodynamics also suggested that these artifacts could be minimized or eliminated entirely by gating the acquisition of slices in the most artifact-prone region of the carotid bulb within a 250-ms window after peak systole. Application of these predictions to studies of normal volunteers showed that, in most cases, these flow artifacts in black blood MRI can be eliminated simply by altering the phase of the cardiac cycle to which the image acquisition is gated. The observation that the size and placement of the saturation slabs had little effect on these artifacts suggested that, in those cases in which recirculation persists throughout the cardiac cycle, either inversion-recovery or presaturation within the bulb itself would be required to suppress them.     

On the heating of inductively coupled resonators (stents) during MRI examinations.

Stents that have been implanted to preserve the results of vascular dilatation are frequently affected by in-stent restenosis, which ideally should be followed up by a noninvasive diagnostic modality. Active MRI stents can enable this kind of follow-up, while normal metallic stents can not. The prototype stents investigated in this study were designed as electric resonating circuits without a direct connection to the MR imager, and function as inductively coupled transmit coils. The model of a long solenoid coil is used to describe the additional power loss caused by such resonators. The theoretically estimated temperature increase is verified by measurements for different resonators and discussed for worst-case conditions. The RF power absorption of an active resonator is negligible compared to the total power absorbed during MRI. The local temperature increase observed for prototypes embedded in phantoms is in a range that excludes direct tissue damage. However, ruptures in the conducting structure of a resonator can cause hot spots, which may establish a high local temperature. This hazard can be reduced by designing resonators with a low quality (Q) factor or by setting the circuit slightly off resonance; however, this would lower the nominal amplification for which the resonator was designed.     

Passive catheter tracking during interventional MRI using hyperpolarized 13C.

Interventional procedures in MRI can be performed preclinically using active or passive catheter-tracking methods. A novel passive nonproton technique is suggested that uses a catheter filled with a hyperpolarized (13)C contrast agent. A prototype three-lumen catheter was built with two closed lumens containing a flowing hyperpolarized (13)C contrast agent. Entire-length (13)C catheter projection visualization could be performed in vivo with a catheter SNR of approximately 80, one dual projection frame per approximately 700 ms, and an in-plane resolution of 2 x 2 mm(2) while traveling through the aorta of a pig. The traveling path of the (13)C catheter was visualized after back-projection catheter reconstruction and after image fusion with an anatomical offline proton road map. Catheter length visualization was aided by an oblique planar visualization mode. The high catheter signal demonstrated, together with the entire catheter length visualization and high surrounding soft-tissue contrast, warrants further development into a real-time technique.     

Adaptive retrospective correction of motion artifacts in cranial MRI with multicoil three‐dimensional radial acquisitions

Despite reduction in imaging times through improved hardware and rapid acquisition schemes, motion artifacts can compromise image quality in magnetic resonance imaging, especially in three‐dimensional imaging with its prolonged scan durations. Direct extension of most state‐of‐the‐art two‐dimensional rigid body motion compensation techniques to the three‐dimensional case is often challenging or impractical due to a significant increase in sampling requirements. This article introduces a novel motion correction technique that is capable of restoring image quality in motion corrupted two‐dimensional and three‐dimensional radial acquisitions without a priori assumptions about when motion occurs. The navigating properties of radial acquisitions—corroborated by multiple receiver coils—are exploited to detect actual instances of motion. Pseudorandom projection ordering provides flexibility of reconstructing navigator images from the obtained motion‐free variable‐width subsets for subsequent estimation of rigid body motion parameters by coregistration. The proposed approach does not require any additional navigators or external motion estimation schemes. The capabilities and limitations of the method are described and demonstrated through simulations and representative volunteer cranial acquisitions. Magn Reson Med 69:1094–1103, 2013. © 2012 Wiley Periodicals, Inc.     

Active delivery cable tuned to device deployment state: Enhanced visibility of nitinol occluders during preclinical interventional MRI

Purpose:

To develop an active delivery system that enhances visualization of nitinol cardiac occluder devices during deployment under real‐time magnetic resonance imaging (MRI).

Materials and Methods:

We constructed an active delivery cable incorporating a loopless antenna and a custom titanium microscrew to secure the occluder devices. The delivery cable was tuned and matched to 50Ω at 64 MHz with the occluder device attached. We used real‐time balanced steady state free precession in a wide‐bore 1.5T scanner. Device‐related images were reconstructed separately and combined with surface‐coil images. The delivery cable was tested in vitro in a phantom and in vivo in swine using a variety of nitinol cardiac occluder devices.

Results:

In vitro, the active delivery cable provided little signal when the occluder device was detached and maximal signal with the device attached. In vivo, signal from the active delivery cable enabled clear visualization of occluder device during positioning and deployment. Device release resulted in decreased signal from the active cable. Postmortem examination confirmed proper device placement.

Conclusion:

The active delivery cable enhanced the MRI depiction of nitinol cardiac occluder devices during positioning and deployment, both in conventional and novel applications. We expect enhanced visibility to contribute to the effectiveness and safety of new and emerging MRI‐guided treatments. J. Magn. Reson. Imaging 2012;36:972–978. © 2012 Wiley Periodicals, Inc.     

2D locally focused MRI: Applications to dynamic and spectroscopic imaging

Conventional magnetic resonance images have uniform spatial resolution across the entire field of view. A method of creating MR images with user-specified spatial resolution along one dimension of the field of view was described recently by the authors. This paper presents the 2D generalization of this technique, which allows the user to specify arbitrary spatial resolution in arbitrary 2D regions. These images are reconstructed from signals that sparsely sample the k-space representation of the image. Therefore, locally focused images can be acquired in less time than that required by Fourier imaging with uniformly high resolution. In this paper the authors show how to increase the temporal resolution of dynamic imaging (e.g., interventional imaging) by using high resolution in areas of expected change and lower resolution elsewhere. Alternatively, by matching the local spatial resolution to the expected edge content of the image, it is possible to avoid the localized truncation artifacts that mark Fourier images reconstructed from the same number of signals. For example, the authors show how proton spectroscopic images of the head may be improved by using high resolution in the neighborhood of scalp lipids that might otherwise cause truncation artifacts.     

实时功能磁共振成像对运动皮层的定位

目的：探讨研究RTIP-fMRI对“手结节”的功能定位及正常和病理状态下对指运动时运动皮层的功能改变。方法：采用GE1：5TMRI扫描机及具有RTIP功能的工作站，成像序列为单次激发梯度回波平面回波成像。10例右手利健康志愿者及3例额顶叶病例作为研究对象，运动激活由大拇指和其它四指的依次对指运动所组成，数据分析采用“相关系数”几何算法，结果：（1）RTIP-fMRI能对“手结节”准确定位，并且功能图与解剖图上“手结节”有很好的对应关系；（2）对指运动时运动皮层的激活包括对侧M1/S1区（M1区包括大部分“手结节”），SMA区及同侧少量M1区；（3）时间过程图呈“城垛样”改变，相关系数范围为0.51-0.88:(4)对于额顶叶占位病变，RTIP-fMRI能准确显示M1区的移位变形及受累。结论：RTIP-fMRI能对运动皮层准确定位，具有其它fMRI技术无法比拟的优越性，可广泛应用于脑功能开发研究。