Effect of artifacts due to flowing blood on the reproducibility of phase-contrast measurements of myocardial motion

The reproducibility of myocardial motion trajectories calculated from cine phase-contrast (PC) velocity data is reduced by artifacts due to the inconsistent motion of intracardiac blood. Spatial presaturation reduces these artifacts but requires a longer sequence TR, with a potentially negative effect on trajectory accuracy and reproducibility. We investigated the effect of spatial presaturation on trajectory reproducibility. A midventricular transaxial slice was imaged in five normal volunteers. The same slice was imaged three times each with sequences using spatial presaturation or not. Because the most serious artifacts originate in the heart chambers and propagate in the phase-encoded direction, myocardial regions that were in line with the heart chambers (in the phase-encode direction) had the highest artifact level in the scans without spatial presaturation. The reproducibility of trajectories for regions placed in these areas (the anterior wall, septum and posterior wall in the transaxial scans with phase encoding in the anterior-posterior direction) improved by a factor of two when presaturation was used (P < .001). In areas that were not in line with the heart chambers (eg, the anterior aspect of the lateral wall in the transaxial scans), the effect of presaturation was not significant. These results correlate well with the measured reduction in artifact level. The reproducibility of myocardial motion trajectories over large areas of the heart is improved to approximately 1 mm when presaturation is used. Therefore, use of presaturation is recommended for myocardial motion studies using cine PC velocity data.     

Velocity and flow quantitation in the superior sagittal sinus with ungated and cine (gated) phase-contrast MR imaging

Normal blood flow and velocity in the superior sagittal sinus were measured in 30 patients. A fast two-dimensional ungated phase-contrast (PC) pulse sequence was compared with a peripherally gated cine PC technique for velocity and flow quantitation. The same imaging parameters were used for both methods. Measured values for mean velocity and flow obtained with the two methods were compared by using regression analysis and t testing. For blood flow, the correlation coefficient was 0.976. For velocity measurements, r was 0.950. Mean flow was 285 mL/min ± 19 with the ungated PC method and 281 mL/min ± 19 with the cine PC method. The mean velocities measured with the two methods were 12.94 cm/sec ± 1.1 and 13.59 cm/sec ± 1.1, respectively. There was no significant difference (paired t test) between the methods for mean flow or velocity data. This was true even though flow in the superior sagittal sinus is moderately pulsatile, as shown with the cine PC technique. The ungated PC method provided these data in 13 seconds versus 3.5 minutes for the cine PC method.     

Retrospective reconstruction of high temporal resolution cine images from real-time MRI using iterative motion correction

Cardiac function has traditionally been evaluated using breath-hold cine acquisitions. However, there is a great need for free breathing techniques in patients who have difficulty in holding their breath. Real-time cardiac MRI is a valuable alternative to the traditional breath-hold imaging approach, but the real-time images are often inferior in spatial and temporal resolution. This article presents a general method for reconstruction of high spatial and temporal resolution cine images from a real-time acquisition acquired over multiple cardiac cycles. The method combines parallel imaging and motion correction based on nonrigid registration and can be applied to arbitrary k-space trajectories. The method is demonstrated with real-time Cartesian imaging and Golden Angle radial acquisitions, and the motion-corrected acquisitions are compared with raw real-time images and breath-hold cine acquisitions in 10 (N = 10) subjects. Acceptable image quality was obtained in all motion-corrected reconstructions, and the resulting mean image quality score was (a) Cartesian real-time: 2.48, (b) Golden Angle real-time: 1.90 (1.00-2.50), (c) Cartesian motion correction: 3.92, (d) Radial motion correction: 4.58, and (e) Breath-hold cine: 5.00. The proposed method provides a flexible way to obtain high-quality, high-resolution cine images in patients with difficulty holding their breath.     

Radial k‐t FOCUSS for high‐resolution cardiac cine MRI

A compressed sensing dynamic MR technique called k‐t FOCUSS (k‐t FOCal Underdetermined System Solver) has been recently proposed. It outperforms the conventional k‐t BLAST/SENSE (Broad‐use Linear Acquisition Speed‐up Technique/SENSitivity Encoding) technique by exploiting the sparsity of x‐f signals. This paper applies this idea to radial trajectories for high‐resolution cardiac cine imaging. Radial trajectories are more suitable for high‐resolution dynamic MRI than Cartesian trajectories since there is smaller tradeoff between spatial resolution and number of views if streaking artifacts due to limited views can be resolved. As shown for Cartesian trajectories, k‐t FOCUSS algorithm efficiently removes artifacts while preserving high temporal resolution. k‐t FOCUSS algorithm applied to radial trajectories is expected to enhance dynamic MRI quality. Rather than using an explicit gridding method, which transforms radial k‐space sampling data to Cartesian grid prior to applying k‐t FOCUSS algorithms, we use implicit gridding during FOCUSS iterations to prevent k‐space sampling errors from being propagated. In addition, motion estimation and motion compensation after the first FOCUSS iteration were used to further sparsify the residual image. By applying an additional k‐t FOCUSS step to the residual image, improved resolution was achieved. In vivo experimental results show that this new method can provide high spatiotemporal resolution even from a very limited radial data set. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

In vivo time-resolved quantitative motion mapping of the murine myocardium with phase contrast MRI.

Myocardial motion of healthy mice and mice with myocardial infarction was assessed in vivo by phase contrast (PC) cine MRI. The imaging module was a segmented fast low angle shot (FLASH) sequence with velocity compensation in all three gradient directions. To accomplish additional motion encoding, the spin phase was prepared using bipolar gradient pulses, which resulted in a linear dependence between the voxel velocity and spin phase. This method provided accurate quantification of the velocity magnitude and direction of the murine myocardium at a spatial resolution of 234 microm and a temporal resolution of about 10 ms. The acquisition was EKG-gated and the mice were anesthetized by inhalation of 1.5-4.0 vol.% isoflurane at 1.5 l/min oxygen flow. To validate the MRI measurements, an experiment with a calibrated rotating phantom was performed. Deviations between MR velocity measurements and optical assessment by a light detector were lower than 1.6%. During our study, myocardial motion velocities between 0.4 cm/s and 1.7 cm/s were determined for the healthy murine myocardium across the heart cycle. Areas with myocardial infarction were clearly segmented and showed a motion velocity which was significantly reduced. In conclusion, the method is an accurate technique for the assessment of murine myocardial motion in vivo.     

Real-time imaging of skeletal muscle velocity

PURPOSE: To test the feasibility of using real-time phase contrast (PC) magnetic resonance imaging (MRI) to track velocities (1-20 cm/second) of skeletal muscle motion. MATERIALS AND METHODS: To do this we modified a fast real-time spiral PC pulse sequence to accommodate through-plane velocity encoding in the range of -20 to +20 cm/second. We successfully imaged motion of the biceps brachii and triceps brachii muscles during elbow flexion and extension in seven unimpaired adult subjects using real-time PC MRI. RESULTS: The velocity data demonstrate that the biceps brachii and the triceps brachii, antagonistic muscles, move in opposite directions during elbow flexion and extension with velocity values in the muscle tissue ranging from -10 to +10 cm/second. CONCLUSION: With further development, real-time PC MRI may provide a means to analyze muscle function in individuals with neurologic or movement disorders who cannot actively complete the repeated motions required for dynamic MRI techniques, such as cine PC MRI, that are more commonly used in musculoskeletal biomechanics applications.     

Accelerated phase-contrast cine MRI using k-t SPARSE-SENSE

Phase-contrast (PC) cine MRI is a promising method for assessment of pathologic hemodynamics, including cardiovascular and hepatoportal vascular dynamics, but its low data acquisition efficiency limits the achievable spatial and temporal resolutions within clinically acceptable breath-hold durations. We propose to accelerate PC cine MRI using an approach which combines compressed sensing and parallel imaging (k-t SPARSE-SENSE). We validated the proposed 6-fold accelerated PC cine MRI against 3-fold accelerated PC cine MRI with parallel imaging (generalized autocalibrating partially parallel acquisitions). With the programmable flow pump, we simulated a time varying waveform emulating hepatic blood flow. Normalized root mean square error between two sets of velocity measurements was 2.59%. In multiple blood vessels of 12 control subjects, two sets of mean velocity measurements were in good agreement (mean difference = -0.29 cm/s; lower and upper 95% limits of agreement = -5.26 and 4.67 cm/s, respectively). The mean phase noise, defined as the standard deviation of the phase in a homogeneous stationary region, was significantly lower for k-t SPARSE-SENSE than for generalized autocalibrating partially parallel acquisitions (0.05 ± 0.01 vs. 0.19 ± 0.06 radians, respectively; P < 0.01). The proposed 6-fold accelerated PC cine MRI pulse sequence with k-t SPARSE-SENSE is a promising investigational method for rapid velocity measurement with relatively high spatial (1.7 mm × 1.7 mm) and temporal (～35 ms) resolutions.     

Validation of cine phase-contrast MR imaging for motion analysis

The accuracy of cine phase-contrast magnetic resonance (MR) imaging for motion analysis was evaluated. By using a rotating phantom and postprocessing algorithm for phase tracking, errors arising during data acquisition were identified and compensation methods were developed. A spatially varying background phase offset in the velocity images was found to be due to eddy current-induced fields. The magnitude of the offset was in the range of 0–20 cm/sec, which is of the same order of magnitude as cardiac contractile velocities. Background offset is thus an important source of error in tracking cardiac motion. Study of different tracking algorithms revealed the need for an integration scheme using motion terms higher than velocity. Also, considerable improvement in the accuracy and stability of the predicted trajectories was obtained by averaging the trajectories proceeding both forward and backward in time from the starting point. With the algorithm developed, the motion of the phantom was tracked through a complete rotation of the phantom to an accuracy of 2 pixels.     

3D cine displacement‐encoded MRI of pulsatile brain motion

Pulsatile brain motion is considered to be an important mechanical link between blood and cerebrospinal fluid (CSF) dynamics. Like many severe brain diseases, different types of hydrocephalus are associated with impairment of these dynamics. In this work a cine displacement‐encoded imaging method employing stimulated echoes (DENSE) and a three‐dimensional (3D) segmented echo‐planar imaging (EPI) readout for brain motion measurements in all three spatial directions is presented. Displacement‐encoded data sets of 12 healthy volunteers were analyzed with respect to reproducibility, periodicity, and intra‐ as well as intersubject physiological consistency. In addition, displacement values were compared with data derived from phase‐contrast (PC) velocity measurements in a subset of all measured subjects. Using DENSE, displacements as low as 0.01 mm could be detected and observation of the 3D pulse pressure wave propagation was possible. Among other parameters, peak displacements in the central brain regions were measured: feet–head (FH): thalamus (0.13 ± 0.01 mm); right–left (RL): thalamus (0.06 ± 0.01 mm); and anterior–posterior (AP): caudate nucleus (0.05 ± 0.01 mm). Magn Reson Med 61:153–162, 2009. © 2008 Wiley‐Liss, Inc.     

Cine phase-contrast magnetic resonance imaging as a tool for quantification of skeletal muscle motion

In recent years, biomechanics researchers have increasingly used dynamic magnetic resonance imaging techniques, such as cine phase contrast (cine PC), to study muscle and bone motion in vivo. Magnetic resonance imaging provides a non-invasive tool to visualize the anatomy and measure musculoskeletal tissue velocities during joint motion. Current application of cine PC magnetic resonance imaging in biomechanics includes study of knee joint kinematics, tendon strain, and skeletal muscle displacement and shortening. This paper article reviews the use of cine PC magnetic resonance imaging for quantification of skeletal muscle motion. The imaging studies presented examine the relative motion of the knee flexor and extensor muscles after orthopedic surgery and examine the uniformity of shortening within the biceps brachii muscle. The current challenges and limitations of using cine PC magnetic resonance imaging in biomechanics research are addressed as well as opportunities for future studies of skeletal muscle motion using dynamic magnetic resonance imaging.     

Dynamic range extension of cine velocity measurements using motion registered spatiotemporal phase unwrapping

A motion-registered spatiotemporal phase-unwrapping method for extending the dynamic range of cine magnetic resonance phase velocity measurements is presented. The interframe cardiac movement is estimated from the magnitude image derived from the velocity encoded raw data, which ensures that the phase signal is unwrapped in the temporal direction with reference to pixels belonging to the same anatomic flow region. An extra step of spatial phase correction is then used to further eliminate any residual errors. The combination of spatial and temporal information for phase unwrapping provides a robust way of extending the dynamic range of cine velocity data in the presence of large phase aliasing errors.     

Correction of motion artifacts from cardiac cine magnetic resonance images

RATIONALE AND OBJECTIVES: An image registration method was developed to automatically correct motion artifacts, mostly from breathing, from cardiac cine magnetic resonance (MR) images. MATERIALS AND METHODS: The location of each slice in an image stack was optimized by maximizing a similarity measure of the slice with another image slice stack. The optimization was performed iteratively and both image stacks were corrected simultaneously. Two procedures to optimize the similarity were tested: standard gradient optimization and stochastic optimization in which one slice is chosen randomly from the image stacks and its location is optimized. In this work, cine short- and long-axis images were used. In addition to visual inspection results from real data, the performance of the algorithm was evaluated quantitatively by simulating the movements in four real MR data sets. The mean error and standard deviation were defined for 50 simulated movements as each slice was randomly displaced. The error rate, defined as the percentage of non-satisfactory registration results, was evaluated. The paired t-test was used to evaluate the statistical difference between the tested optimization methods. RESULTS: The algorithm developed was successfully applied to correct motion artifacts from real and simulated data. The results, where typical motion artifacts were simulated, indicated an error rate of about 3%. Subvoxel registration accuracy was also achieved. When different optimization methods were compared, the registration accuracy of the stochastic approach proved to be superior to the standard gradient technique (P < 10(-9)). CONCLUSIONS: The novel method was capable of robustly and accurately correcting motion artifacts from cardiac cine MR images.     

Motion correction using coil arrays (MOCCA) for free‐breathing cardiac cine MRI

In this study, we present a motion correction technique using coil arrays (MOCCA) and evaluate its application in free‐breathing respiratory self‐gated cine MRI. Motion correction technique using coil arrays takes advantages of the fact that motion‐induced changes in k‐space signal are modulated by individual coil sensitivity profiles. In the proposed implementation of motion correction technique using coil arrays self‐gating for free‐breathing cine MRI, the k‐space center line is acquired at the beginning of each k‐space segment for each cardiac cycle with 4 repetitions. For each k‐space segment, the k‐space center line acquired immediately before was used to select one of the 4 acquired repetitions to be included in the final self‐gated cine image by calculating the cross correlation between the k‐space center line with a reference line. The proposed method was tested on a cohort of healthy adult subjects for subjective image quality and objective blood‐myocardium border sharpness. The method was also tested on a cohort of patients to compare the left and right ventricular volumes and ejection fraction measurements with that of standard breath‐hold cine MRI. Our data indicate that the proposed motion correction technique using coil arrays method provides significantly improved image quality and sharpness compared with free‐breathing cine without respiratory self‐gating and provides similar volume measurements compared with breath‐hold cine MRI. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Generalized self-navigated motion detection technique: Preliminary investigation in abdominal imaging.

Patient motion remains a primary obstacle to diagnostic image quality, especially in the abdomen, despite the existence of various motion artifact reduction techniques. This work presents a self-navigated motion detection method that can be generalized for most pulse sequences and k-space trajectories. Motion information is extracted directly from raw MR data, requiring no additional gradient or RF pulses, no physiologic monitoring equipment, and minimal--if any--impact on scan time. Initial feasibility results with a two-dimensional gradient echo sequence are shown in phantom studies and in vivo volunteer abdominal studies, demonstrating the sensitivity of the method to both respiratory motion and cardiovascular pulsatility. Prospectively gated images were acquired using the self-navigated data to synchronize image acquisition with motion. These preliminary results suggest that the self-navigated method is a promising technique for reducing motion artifacts in clinical abdominal and cardiac applications.     

电影相位对比与屏气二维相位对比MRI对肝硬化门静脉血流的对照研究

目的探讨MRI电影法相位对比（Cine PC）与屏气二维相位对比（2D PC）在肝硬化患者和正常志愿者门静脉血流测量中的诊断价值。资料与方法对照组为82名志愿者,男45名,女37名,平均年龄26.65岁;肝硬化组24例,男14例,女10例,平均年龄42.00岁。空腹状态下采用CinePC技术对门静脉血流定量测量,同期采用2DPC技术3种不同屏气状态（正常吸气屏气,呼气屏气,平静呼吸屏气）与其进行比较。结果肝硬化患者门静脉血流速度较对照组略减低,除2D PC平静呼吸法,余方法两组差异均无统计学意义,肝硬化组门静脉血流量与对照组比较明显增大（P〈0.01）,但2DPC吸气屏气技术两者差异无统计学意义。采用相关性分析对屏气2DPC（不同呼吸状态）与Cine PC MRI方法定量门静脉血流速度、血流量,显示两种技术相关性很好（r〉0.8;P〈0.01）,但在对照组行相关性分析,仅呼气后屏气2D PC与Cine PC显示中度相关（r〉0.5;P〈0.01）,吸气后屏气与CinePC相关性较差（r〈0.4）。结论MRI Cine PC在正常呼吸情况下对门静脉血流行定量测量,接近人体生理状态,适合门静脉血流测量;正常呼吸状态屏气对肝硬化患者门静脉血流影响较正常人小,在肝硬化血流测量中,MRI Cine PC与屏气2DPCMRI法显示很好相关性。屏气2DPCMRI技术提供了一种简便、实用、相对准确的肝硬化门静脉血流测量方法。     

Quantification of vessel wall motion and cyclic strain using cine phase contrast MRI: in vivo validation in the porcine aorta.

Artery wall motion and strain play important roles in vascular remodeling and may be important in the pathogenesis of vascular disease. In vivo observations of circumferentially nonuniform wall motion in the human aorta suggest that nonuniform strain may contribute to the localization of vascular pathology. A velocity-based method to investigate circumferential strain variations was previously developed and validated in vitro; the current study was undertaken to determine whether accurate displacement and strain fields can be calculated from velocity data acquired in vivo. Wall velocities in the porcine thoracic aorta were quantified with PC-MRI and an implanted coil and were then time-integrated to compute wall displacement trajectories and cyclic strain. Displacement trajectories were consistent with observed aortic wall motion and with the displacements of markers in the aortic wall. The mean difference between velocity-based and marker-based trajectory points was 0.1 mm, relative to an average pixel size of 0.4 mm. Propagation of error analyses based on the precision of the computed displacements were used to demonstrate that 10% strain results in a standard deviation of 3.6%. This study demonstrates that it is feasible to accurately quantify strain from low wall velocities in vivo and that the porcine thoracic aorta does not deform uniformly.     

Rapid measurement of pulse wave velocity via multisite flow displacement.

A MR method is presented for measuring pulse wave velocity (PWV) and its application to assessing stiffness in the human thoracic aorta. This one-dimensional (1D) flow displacement method applies a single RF comb excitation to the vessel, followed by an oscillating frequency encoding gradient, each oscillation providing a 1D projection of the vessel, enabling one to track fluid motion. The currently implemented sequence excites nine slices within a 20-cm length of vessel and has a temporal resolution of 2.03 msec and a total acquisition time of 140 msec. Offline-reconstructed position-versus-time plots show curvilinear flow displacement trajectories corresponding to fluid motion at each of the excitation positions. The PWV can be reliably calculated by curve-fitting these trajectories to a model. In vitro studies using compliant tubes demonstrate no significant difference between results obtained using this method and those directly obtained using pressure transducers. Compared to another MR method previously developed in our laboratory, the proposed method displays improved temporal resolution and enhanced ability to extract PWV from vessels exhibiting low peak flow velocity. Preliminary data suggest that this method is feasible for in vivo application and may provide a more accurate estimation of aortic wave velocity among subjects exhibiting low peak flow velocity, such as the elderly or those with impaired cardiac function.     

MRI电影相位对比法液体流速与信号强度的实验研究

目的：评估磁共振电影相位对比法对液体流速的实验价值。材料和方法：在１．５Ｔ磁共振机上利用磁共振电影相位对比法,在高压注射器分别匀速注射为０．１ｍｌ／ｓ、０．２ｍｌ／ｓ及０．３ｍｌ／ｓ时,对三根截面积分别为４ｍｍ２、５ｍｍ２及１３ｍｍ２的管道内液体流动情况进行测定,在工作站上根据不同流动速度测得相应的信号强度值。结果：三组数值经计算机处理后Ｒ２＝０．９９８,显示相关性极佳;信号强度值（ｙ）与流速（ｘ）之间公式为：ｙ＝－０．０１４１ｘ＋５１．７３５ｘ＋２７．４７８。结论：在实验测定液体流速与信号强度数值时,ＭＲ电影相位对比法是一种有效的方法。     

Three-dimensional magnetic resonance myocardial motion tracking from a single image plane.

Three-dimensional imaging for the quantification of myocardial motion is a key step in the evaluation of cardiac disease. A tagged magnetic resonance imaging method that automatically tracks myocardial displacement in three dimensions is presented. Unlike other techniques, this method tracks both in-plane and through-plane motion from a single image plane without affecting the duration of image acquisition. A small z-encoding gradient is subsequently added to the refocusing lobe of the slice-selection gradient pulse in a slice following CSPAMM acquisition. An opposite polarity z-encoding gradient is added to the orthogonal tag direction. The additional z-gradients encode the instantaneous through plane position of the slice. The vertical and horizontal tags are used to resolve in-plane motion, while the added z-gradients is used to resolve through-plane motion. Postprocessing automatically decodes the acquired data and tracks the three-dimensional displacement of every material point within the image plane for each cine frame. Experiments include both a phantom and in vivo human validation. These studies demonstrate that the simultaneous extraction of both in-plane and through-plane displacements and pathlines from tagged images is achievable. This capability should open up new avenues for the automatic quantification of cardiac motion and strain for scientific and clinical purposes.     

Unsupervised estimation of myocardial displacement from tagged MR sequences using nonrigid registration.

We propose a fully automatic cardiac motion estimation technique that uses nonrigid registration between temporally adjacent images to compute the myocardial displacement field from tagged MR sequences using as inputs (sources) both horizontally and vertically tagged images. We present a new multisource nonrigid registration algorithm employing a semilocal deformation model that provides controlled smoothness. The method requires no segmentation. We apply a multiresolution optimization strategy for better speed and robustness. The accuracy of the algorithm is assessed on experimental data (animal model) and healthy volunteer data by calculating the root mean square (RMS) difference in position between the estimated tag trajectories and manual tracings outlined by an expert. For the approximately 20000 tag lines analyzed (45 slices over 20-40 time frames), the RMS difference between the automatic tag trajectories and the manually segmented tag trajectories was 0.51 pixels (0.25 mm) for the animal data and 0.49 pixels (0.49 mm) for the human volunteer data. The RMS difference in the separation between adjacent tag lines (RMS_TS) was also assessed, resulting in an RMS_TS of 0.40 pixels (0.19 mm) in the experimental data and 0.52 pixels (0.56 mm) in the volunteer data. These results confirm the subpixel accuracy achieved using the proposed methodology.