Orbital navigator echoes for motion measurements in magnetic resonance imaging

A single “orbital” navigator echo, that has a circular k-space trajectory, is used to simultaneously measure in-plane rotational and multi-axis translational global motion. Rotation is determined from the shift in the magnitude profile of the echo with respect to a reference echo. Displacements are calculated from the phase difference between the current echo and a reference echo. Phantom studies show that this technique can accurately measure rotation and translations. Preliminary results from adaptive motion correction studies on phantom and human subjects indicate that the orbital navigator echo is an effective method for motion measurement in MRI.     

Reduction of signal fluctuation in functional MRI using navigator echoes

Functional magnetic resonance imaging is sensitive to signal fluctuations due to physiological motion and system instability. In this paper, motion-related signal fluctuations are studied, and a method that uses navigator echoes to monitor and compensate for signal fluctuations in a gradient-echo sequence is described. The technique acquires a “navigator” signal before the application of the phase-encoding and readout gradients and corrects the phase of the subsequently acquired imaging data. This technique was implemented on a 4 Tesla whole body system and validated on normal volunteers. With this technique, temporal fluctuations in image intensity were substantially reduced and improved functional activation maps were obtained.     

Correction of physiologically induced global off-resonance effects in dynamic echo-planar and spiral functional imaging.

In functional magnetic resonance imaging, a rapid method such as echo-planar (EPI) or spiral is used to collect a dynamic series of images. These techniques are sensitive to changes in resonance frequency which can arise from respiration and are more significant at high magnetic fields. To decrease the noise from respiration-induced phase and frequency fluctuations, a simple correction of the "dynamic off-resonance in k-space" (DORK) was developed. The correction uses phase information from the center of k-space and a navigator echo and is illustrated with dynamic scans of single-shot and segmented EPI and, for the first time, spiral imaging of the human brain at 7 T. Image noise in the respiratory spectrum was measured with an edge operator. The DORK correction significantly reduced respiration-induced noise (image shift for EPI, blurring for spiral, ghosting for segmented acquisition). While spiral imaging was found to exhibit less noise than EPI before correction, the residual noise after the DORK correction was comparable. The correction is simple to apply and can correct for other sources of frequency drift and fluctuations in dynamic imaging.     

On the synchronization of transcranial magnetic stimulation and functional echo-planar imaging

PURPOSE: To minimize artifacts in echo-planar imaging (EPI) of human brain function introduced by simultaneous transcranial magnetic stimulation (TMS). MATERIALS AND METHODS: Distortions due to TMS pulses (0.25 msec, 2.0 T) were studied at 2.0 T before and during EPI. RESULTS: Best results were obtained if both the EPI section orientation and the frequency-encoding gradient were parallel to the plane of the TMS coil. Under these conditions, a TMS pulse caused image distortions when preceding the EPI sequence by less than 100 msec. Recordings with a magnetic field gradient pick-up coil revealed transient magnetic fields after TMS, which are generated by eddy currents in the TMS coil. TMS during image acquisition completely spoiled all transverse magnetizations and induced disturbances ranging from image corruption to mild image blurring, depending on the affected low and high spatial frequencies. Simultaneous TMS and radio-frequency (RF) excitation gave rise to T1-dependent signal changes that lasted for several seconds and yielded pronounced false-positive activations during functional brain mapping. CONCLUSION: To ensure reliable and robust combinations, TMS should be applied at least 100 msec before EPI while completely avoiding any pulses during imaging.     

Acoustic noise and functional magnetic resonance imaging: current strategies and future prospects

Functional magnetic resonance imaging (fMRI) has become the method of choice for studying the neural correlates of cognitive tasks. Nevertheless, the scanner produces acoustic noise during the image acquisition process, which is a problem in the study of auditory pathway and language generally. The scanner acoustic noise not only produces activation in brain regions involved in auditory processing, but also interferes with the stimulus presentation. Several strategies can be used to address this problem, including modifications of hardware and software. Although reduction of the source of the acoustic noise would be ideal, substantial hardware modifications to the current base of installed MRI systems would be required. Therefore, the most common strategy employed to minimize the problem involves software modifications. In this work we consider three main types of acquisitions: compressed, partially silent, and silent. For each implementation, paradigms using block and event-related designs are assessed. We also provide new data, using a silent event-related (SER) design, which demonstrate higher blood oxygen level-dependent (BOLD) response to a simple auditory cue when compared to a conventional image acquisition.     

Noise suppression digital filter for functional magnetic resonance imaging based on image reference data

The central decision in every functional magnetic resonance imaging (fMRI) experiment is whether pixels in brain tissues are showing activation in response to neural stimulus or as a result of noise. Images are degraded not only by random (e.g., thermal) noise, but also by structured noise due to MR system characteristics, cardiac and respiratory pulsations, and patient motion. A novel digital filter has been developed to suppress cardiac and respiratory structured noise in fMRI images, using estimates of structured and random noise power spectra obtained directly from the images. It is an adaptive filter based on stationary noise statistics, and is equivalent in form to a Wiener filter. A mathematical model of the filtering process was developed to understand how the strength and distribution of structured and random noise power influenced filter performance. The filter was tested using images from an auditory activation study in ten subjects. In subjects whose structured noise power was localized to a relatively narrow frequency range, a strong relationship was found, both experimentally (R = 0.975, P < 0.0004 for H_o: R = 0) and using the model, between filter performance and the level of structured noise power contaminating the experiment frequency. The filter significantly reduced the rate of false-positive activations in the subset of subjects whose experiment frequency was relatively heavily contaminated by structured noise. Notch filters, that simply eliminate unwanted frequencies, performed poorly in all subjects. Unlike the proposed Wiener filter, these filters did not suppress structured noise power at the experiment frequency that contributes to false-positive activations.     

Functional assessment of pancreatic parenchyma after secretin administration using serial T2-weighted echo-planar magnetic resonance imaging

Signal intensity (SI) changes of pancreatic parenchyma were evaluated after intravenous administration of secretin using T2-weighted single-shot spin-echo echo-planar imaging (EPI) to assess this method as a magnetic resonance (MR) test of pancreatic exocrine function. Nine volunteers were studied with serial single-shot EPI of the pancreas for 15 minutes after the injection of secretin or saline. The normal pattern of pancreatic SI change was demonstrated after intravenous injection of secretin, a single peak at 3-4 minutes in the head, body, and tail, followed by a gradual decrease in SI. Saline injection did not induce a significant SI change. There was no statistical difference in the peak contrast ratios (first mean, 1.21-1.25, vs. second mean, 1.18-1.22) and peak times (first mean, 3.2-3.7 minutes, vs. second mean, 3.1-3.6) in a repeat study. By evaluating the pattern of time-response curves obtained from serial T2-weighted EPI after secretin injection, pancreatic exocrine function may be directly assessed at the level of the head, body, and tail.     

Magnetic resonance imaging of the upper abdomen using a free-breathing T2-weighted turbo spin echo sequence with navigator triggered prospective acquisition correction

PURPOSE: To evaluate a free-breathing navigator triggered T2-weighted turbo spin-echo sequence with prospective acquisition correction (T2w-PACE-TSE) for MRI of the upper abdomen in comparison to a conventional T2-weighted TSE (T2w-CTSE), a single-shot TSE (T2w-HASTE), and a T1-weighted gradient-echo sequence (T1w-FLASH). MATERIALS AND METHODS: A total of 40 consecutive patients were examined at 1.5 T using free-breathing T2w-PACE-TSE, free-breathing T2w-CTSE, and breath-hold T2w-HASTE and T1w-FLASH acquisition. Images were evaluated qualitatively by three radiologists regarding motion artifacts, liver-spleen contrast, depiction of intrahepatic vessels, the pancreas and the adrenal glands, and overall image quality on a four-point scale. Quantitative analysis of the liver-spleen contrast was performed. RESULTS: Depiction and sharpness of intrahepatic vessels were rated significantly better (P < 0.01) using T2w-PACE-TSE compared to T2w-CTSE and T2w-HASTE sequences. Significantly higher contrast values were measured for T2w-PACE-TSE images compared to T2w-CTSE, T2w-HASTE, and T1w-FLASH images (P < 0.01). Mean examination time of the T2w-PACE-TSE was 7.91 minutes, acquisition time of the T2w-CTSE sequence was 4.52 minutes. CONCLUSION: Prospective acquisition correction is an efficient method for reducing respiratory movement artifacts in T2w-TSE imaging of the upper abdomen. Compared to T2w-CTSE and T2w-HASTE sequences recognition of anatomical details and contrast can be significantly improved.     

Ghost artifact reduction for echo planar imaging using image phase correction

An algorithm is described for reducing ghost artifacts in echo planar imaging (EPI) using phase corrections derived from images reconstructed using only even or odd k-space lines. The N/2 ghost, that arises principally from time-reversal of alternate k-space lines, was significantly reduced by this algorithm without the need for a calibration scan. In images obtained in eight subjects undergoing EPI for auditory functional MRI (fMRI) experiments, N/2 ghost intensity was reduced from 10.3% – 2.1% (range: 7.9–14.1%) to 4.5% ± 0.2% (range: 4.1–4.9%) of parent image intensity, corresponding to a percent reduction in ghost intensity of 54% ± 9% (range: 43–65%), and the algorithm restored this intensity to the parent image. It provided a significant improvement in image appearance, and increased the correlation coefficients related to neural activation in functional MRI studies. The algorithm provided reduction of artifacts from all polynomial orders of spatial phase errors in both spatial directions. The algorithm did not eliminate N/2 ghost intensity contributed by field inhomogeneities, susceptibility, or chemical shift.     

Single-shot magnetic field mapping embedded in echo-planar time-course imaging.

A technique for acquiring magnetic field maps simultaneously with gradient-recalled echo-planar time-course data is described. This technique uses a trajectory in which the central part of k-space is collected twice. For a 64 x 64 image acquired with a 125-kHz bandwidth, a field map suitable for geometric correction can be collected simultaneously with the echo-planar time-course data in <70 ms. The field maps generated by this technique are registered with the magnitude images because they are calculated using the same data. They do not suffer from errors due to subject motion, or from different geometric distortions that can result from using different pulse sequences. In addition to correcting geometric distortions that resulted from dynamic magnetic field perturbations, this method was used to measure field shifts arising from respiration and jaw motion across five subjects. Values ranged from 0.035 to 0.165 parts per million (ppm).     

Improved bulk myocardial motion suppression for navigator-gated coronary magnetic resonance imaging

PURPOSE: To evaluate the impact of a new, cross-correlation based method for compensation of respiratory induced motion of the heart using an individually adapted three-dimensional (3D) translation or affine transformation approach. MATERIALS AND METHODS: A total of 32 patients underwent a routine cardiac MR examination. In each patient, a calibration scan was performed during free-breathing to register breathing-related motion within a 3D ellipsoid registration kernel covering the entire heart. Three navigators were employed for all three spatial dimensions (feet-head, anterior-posterior, and left-right) and the optimal translatory correction factors for each spatial dimension were determined. In addition, the cross-correlations for different motion models (no compensation, fixed 1D-translation, adapted 3D-translation, and affine transformation) were calculated. RESULTS: The mean correction factor for the feet-head direction was 0.45 +/- 0.13. Though the mean correction factors for the anterior-posterior and left-right direction were nearly zero (-0.01 +/- 0.08 and 0.02 +/- 0.09, respectively), the correction factors exceeded the amount of 0.1 in 12 (19%) and in 19 patients (30%), respectively. All motion compensation models showed significantly higher cross-correlations when compared to "no compensation" (P < 0.05). In particular, the affine transformation algorithm achieved the highest cross-correlation values (88.3 +/- 5.1%) with a significant increase compared to fixed 1D translation (84.7 +/- 6.5%, P < 0.05). CONCLUSION: A considerable number of patients demonstrated relevant breathing-related movement of the heart in the anterior-posterior or left-right direction in addition to the predominant breathing-related movement in the feet-head direction. Thus, it is recommended to compensate for all three spatial dimensions. The affine transformation algorithm combined with three navigators significantly improved breathing-related cardiac motion compensation when compared to the conventionally applied 1D translation with a fixed correction factor.     

Time-to-echo optimization for spin echo magnetic resonance imaging of liver metastasis using superparamagnetic iron oxide particles.

Superparamagnetic iron oxide (SPIO) particles are used as a contrast agent in liver magnetic resonance imaging (MRI). SPIO particles exert their greatest influence on T2-weighted MR signal intensity. The time-to-echo (TE) value that provides optimal contrast has not been systematically studied over the range of clinically relevant field strengths. The purpose of this study was to quantitatively evaluate the TE dependence of the post-SPIO tumor to liver contrast-to-noise ratio (CNR). The hypothesis was that there is a TE that provides an optimal CNR. Subjects having probable metastatic hepatic lesions secondary to colorectal carcinoma were studied. Pre- and post-SPIO images were acquired at TE-effective (TE(eff)) equal to 46, 76, and 106 msec by using a turbo spin echo pulse sequence at 0.2 T and 1.5 T. The CNR for all lesions greater than 1 cm in diameter was determined in pre- and post-SPIO images. A paired statistical design was used to identify TE-related CNR dependencies. The primary findings were as follows. (1) CNR differences attributable to TE(eff) variation over the range of 46-106 msec were less than 34%. For 0.2 T, TE(eff) = 46 msec yielded a statistically significantly greater CNR than did TE(eff) = 76 or 106 msec. The same was true at the higher field strength, but differences were not significant. (2) Signal-to-noise measures suggested that SPIO reduced the lesion signal. (3) Post-SPIO CNR was significantly greater at 1.5 T than at 0.2 T. The observations indicate that over the field strength range of 0.2-1.5 T, CNR differences attributable to the TE(eff) variation, while being statistically significant in some cases, are small relative to those resulting from the SPIO administration.     

Real-time adaptive motion correction in functional MRI

Functional magnetic resonance imaging (fMRI) of the brain is often degraded by bulk head motion. Algorithms that address this by retrospective re-registration of images in an fMRI time series are all fundamentally limited by any motion that occurs through-plane. Here, a technique is described that can account for such motion by prospective. correction in real time. A navigator echo is used before every image acquisition to detect superior/inferior displacements of the head. The displacement information is then used to adjust the plane of excitation of the ensuing single-shot echo-planar fMRI axial image. These correction updates can be completed in 100 ms with motion sensitivity at least as small as 0.5 mm. The efficacy of this method is documented in phantom and human studies.     

Spatially filtering functional magnetic resonance imaging data

When constructing MR images from acquired spatial frequency data, it can be beneficial to apply a low-pass filter to remove high frequency noise from the resulting images. This amounts to attenuating high spatial frequency fluctuations that can affect detected MR signal. A study is presented of spatially filtering MR data and possible ramifications on detecting regionally specific activation signal. It is shown that absolute activation levels are strongly dependent on the parameters of the filter used in image construction and that significance of an activation signal can be enhanced through appropriate filter selection. A comparison is made between spatially filtering MR image data and applying a Gaussian convolution kernel to statistical parametric maps.     

Real-time autoshimming for echo planar timecourse imaging.

Head motion within an applied magnetic field alters the effective shim within the brain, causing geometric distortions in echo planar imaging (EPI). Even if subtle, change in shim can lead to artifactual signal changes in timecourse EPI acquisitions, which are typically performed for functional MRI (fMRI) or diffusion tensor imaging. Magnetic field maps acquired before and after head motions of clinically realistic magnitude indicate that motion-induced changes in magnetic field may cause translations exceeding 3 mm in the phase-encoding direction of the EPI images. The field maps also demonstrate a trend toward linear variations in shim changes as a function of position within the head, suggesting that a real-time, first-order correction may compensate for motion-induced changes in magnetic field. This article presents a navigator pulse sequence and processing method, termed a "shim NAV," for real-time detection of linear shim changes, and a shim-compensated EPI pulse sequence for dynamic correction of linear shim changes. In vivo and phantom experiments demonstrate the detection accuracy of shim NAVs in the presence of applied gradient shims. Phantom experiments demonstrate reduction of geometric distortion and image artifact using shim-compensated EPI in the presence of applied gradient shims. In vivo experiments with intentional interimage subject motion demonstrate improved alignment of timecourse EPI images when using the shim NAV-detected values to update the shim-compensated EPI acquisition in real time.     

Artifact and noise suppression in GRAPPA imaging using improved k-space coil calibration and variable density sampling.

A parallel imaging technique, GRAPPA (GeneRalized Auto-calibrating Partially Parallel Acquisitions), has been used to improve temporal or spatial resolution. Coil calibration in GRAPPA is performed in central k-space by fitting a target signal using its adjacent signals. Missing signals in outer k-space are reconstructed. However, coil calibration operates with signals that exhibit large amplitude variation while reconstruction is performed using signals with small amplitude variation. Different signal variations in coil calibration and reconstruction may result in residual image artifact and noise. The purpose of this work was to improve GRAPPA coil calibration and variable density (VD) sampling for suppressing residual artifact and noise. The proposed coil calibration was performed in local k-space along both the phase and frequency encoding directions. Outer k-space was acquired with two different reduction factors. Phantom data were reconstructed by both the conventional GRAPPA and the improved technique for comparison at an acceleration of two. Under the same acceleration, optimal sampling and calibration parameters were determined. An in vivo image was reconstructed in the same way using the predetermined optimal parameters. The performance of GRAPPA was improved by the localized coil calibration and VD sampling scheme.     

功能磁共振成像在脑膜瘤患者运动功能区的应用

目的：探讨血氧水平依赖功能磁共振技术(BOLD-fMRI)在脑膜瘤患者运动功能区术前定位的价值。方法收集10例经术后病理证实的靠近运动区的大脑凸面脑膜瘤患者,采用概率独立成分分析(PICA),进行 ICA 分析。术前术后行远期生活质量评估(KPS)生活状态评分来评价患者的状态。结果双侧主要运动皮层及辅助运动皮层均出现运动功能激活簇,其中患侧激活区与对侧激活区相比较为对称的有6例,出现明显推压移位的有4例,患者运动功能激活区均被肿瘤挤压导致功能区向前或向后移位,并且出现拉伸变形。结论 BOLD-fMRI 能够有效对脑肿瘤患者进行术前运动功能区定位,对脑膜瘤患者术前手术计划的制定能够提供帮助。     

Interleaved echo-planar imaging for fast multiplanar magnetic resonance temperature imaging of ultrasound thermal ablation therapy

PURPOSE: To develop a multiplanar magnetic resonance temperature imaging (MRTI) technique based on interleaved gradient-echo echo-planar imaging (EPI), verify in phantom, develop software tools to process and display data on a clinical scanner in near real-time, and demonstrate feasibility to monitor ultrasound thermal ablation therapy in vivo. MATERIALS AND METHODS: Temperature estimation used complex phase-difference subtraction of the EPI MRTI data to indirectly measure the temperature-dependent water proton-resonance-frequency shift. Software tools were developed to run on a clinical 1.5-T MR scanner that processed and displayed relevant temperature and thermal dosimetry data during the course of thermal ablation treatments in canine brain and prostate in vivo. RESULTS: EPI MRTI provided multi-planar acquisitions and increased temperature sensitivity and lipid suppression. Relative to a single-plane fast gradient-echo MRTI sequence at comparable spatial and temporal resolutions in phantom, EPI MRTI demonstrated a three-fold increase in sensitivity and slice coverage per TR. In vivo monitoring of ultrasound thermal ablation therapy in canine brain and prostate demonstrated the usefulness of the temperature and thermal dose information. CONCLUSION: Multi-planar MRTI allowed progression of thermal damage to be monitored and treatment parameters adjusted in near real-time (less than five second delay). EPI MRTI is an effective multi-planar monitoring method during ultrasound thermal ablation procedures.     

3D MR angiography of pulmonary arteries using realtime navigator gating and magnetization preparation

An ECG-triggered magnetization-prepared segmented 3D fast gradient echo sequence was developed to perform pulmonary arterial MR angiography. A selective inversion recovery pulse was used in the magnetization preparation to suppress venous vasculature. A real-time gating technique based on navigator echoes was implemented to reduce respiration effects. Pencil-beam navigator echoes were acquired immediately before and after the readout train and processed in real-time to dynamically measure the diaphragm position, which was used to control data acquisition with an accept-or-reject-reacquire logic. In a study of 10 volunteers, a gated 3D acquisition with 28 slices required on average approximately 4 min of acquisition time, and six to seven segmental arteries related to the interlobar trunk of the pulmonary artery were depicted. The use of SIR pulse reduced venous signal by 99%. The gated acquisitions were superior to the ungated acquisitions (n = 10, P < 0.005). The real-time navigator gating technique is effective for reduction of respiration effects and thereby makes high resolution 3D MRA of the pulmonary arteries feasible.