Crusher Gradient Research Articles

Filter exchange imaging with crusher gradient modelling detects increased blood–brain barrier water permeability in response to mild lung infection

Blood–brain barrier (BBB) dysfunction occurs in many brain diseases, and there is increasing evidence to suggest that it is an early process in dementia which may be exacerbated by peripheral infection. Filter-exchange imaging (FEXI) is an MRI technique for measuring trans-membrane water exchange. FEXI data is typically analysed using the apparent exchange rate (AXR) model, yielding estimates of the AXR. Crusher gradients are commonly used to remove unwanted coherence pathways arising from longitudinal storage pulses during the mixing period. We first demonstrate that when using thin slices, as is needed for imaging the rodent brain, crusher gradients result in underestimation of the AXR. To address this, we propose an extended crusher-compensated exchange rate (CCXR) model to account for diffusion-weighting introduced by the crusher gradients, which is able to recover ground truth values of BBB water exchange (kin) in simulated data. When applied to the rat brain, kin estimates obtained using the CCXR model were 3.10 s−1 and 3.49 s−1 compared to AXR estimates of 1.24 s−1 and 0.49 s−1 for slice thicknesses of 4.0 mm and 2.5 mm respectively. We then validated our approach using a clinically relevant Streptococcus pneumoniae lung infection. We observed a significant 70 ± 10% increase in BBB water exchange in rats during active infection (kin = 3.78 ± 0.42 s−1) compared to before infection (kin = 2.72 ± 0.30 s−1; p = 0.02). The BBB water exchange rate during infection was associated with higher levels of plasma von Willebrand factor (VWF), a marker of acute vascular inflammation. We also observed 42% higher expression of perivascular aquaporin-4 (AQP4) in infected animals compared to non-infected controls, while levels of tight junction proteins remain consistent between groups. In summary, we propose a modelling approach for FEXI data which removes the bias in estimated water-exchange rates associated with the use of crusher gradients. Using this approach, we demonstrate the impact of peripheral infection on BBB water exchange, which appears to be mediated by endothelial dysfunction and associated with an increase in perivascular AQP4.

The Utility of Arterial Transit Time Measurement for Evaluating the Hemodynamic Perfusion State of Patients with Chronic Cerebrovascular Stenosis or Occlusive Disease: Correlative Study between MR Imaging and 15O-labeled H2O Positron Emission Tomography.

To verify whether arterial transit time (ATT) mapping can correct arterial spin labeling-cerebral blood flow (ASL-CBF) values and to verify whether ATT is a parameter that correlates with positron emission tomography (PET)-oxygen extraction fraction (OEF) and PET-mean transit time (MTT). Eleven patients with unilateral major cerebral artery stenosis or occlusion underwent MRI and PET in the chronic or asymptomatic phase. ASL-MRI acquisitions were conducted with each of two post-label delay (PLD) settings (0.7s and 2.0s) using a pseudo-continuous ASL pulse sequence and 3D-spin echo spiral readout with vascular crusher gradient. ATT maps were obtained using a low-resolution pre-scan approach with five PLD settings. Using the ASL perfusion images and ATT mapping, ATT-corrected ASL-CBF images were obtained. Four kinds of ASL-CBF methods (PLD 0.7s with or without ATT correction and PLD 2.0s with or without ATT correction) were compared to PET-CBF, using vascular territory ROIs. ATT and OEF were compared for all ROIs, unaffected side ROIs, and affected side ROIs, respectively. ATT and MTT were compared by the ratio of the affected side to the unaffected side. Transit time-based ROIs were used for the comparison with ATT. Comparing ASL-CBF and PET-CBF, the correlation was higher with ATT correction than without correction, and for a PLD of 2.0s compared with 0.7s. The best correlation was for PLD of 2.0s with ATT correction (R2 = 0.547). ROIs on the affected side showed a low but significant correlation between ATT and PET-OEF (R2 = 0.141). There was a low correlation between the ATT ratio and the MTT ratio (R2 = 0.133). Low-resolution ATT correction may increase the accuracy of ASL-CBF measurements in patients with unilateral major cerebral artery stenosis or occlusion. In addition, ATT itself might have a potential role in detecting compromised hemodynamic state.

Strategies to improve intratrain prospective motion correction for turbo spin-echo sequences with constant flip angles.

To investigate the effects of prospective motion correction on turbo spin echo sequences and optimize motion correction approaches, mitigating signal dropout artifacts caused by the imperfections of motion tracking data. Signal dropout artifacts caused by undesired phase deviations introduced by tracking errors are analyzed theoretically. To reduce the adverse effect of such deviations, two approaches are proposed: (1) freezing the correction for example, for even-numbered or higher number of echoes and (2) shifting the correction event prior to the left crusher gradient preceding the refocusing pulse. A comprehensive analysis is presented, including both signal simulations and experimental verifications in phantoms and in vivo. Performance of the proposed approach is validated in two healthy volunteers imaged under two types of motion conditions simulating inadvertent fast motions associated with discomfort and continuous large motions. The results show that the proposed optimization is able to efficiently correct for the motion artifacts and at the same time avoid signal dropout artifacts. Specifically, performing correction every 4th echo prior to the left crusher gradient was shown to improve image quality. An optimization approach is proposed to exploit the potential of external tracking for intra-echo-train motion artifact correction for turbo spin echo sequences.

Simultaneous acoustic radiation force imaging and MR thermometry based on a coherent echo-shifted sequence.

Simultaneous magnetic resonance (MR) acoustic radiation force imaging (ARFI) and MR thermometry (MRT) (STARFI) based on coherent echo-shifted (cES) sequence was proposed and comprehensively compared to radiofrequency (RF)-spoiled gradient echo (spGRE) STARFI. Through use of delicately designed gradients, a collection of echoes was delayed by one repetition time (TR) cycle. The crusher gradient after readout (RO) was used as the displacement encoding gradient (DEG). The sequence was intrinsically sensitive to temperature. High-intensity focused ultrasound (HIFU) pulses were interleaved ON/OFF in successive TRs to separate the phase changes induced by displacement due to acoustic radiation force (ARF) impulses and temperature. Bloch simulation was performed to study the phase sensitivity to displacement of the proposed cES STARFI and spGRE STARFI. The proposed cES sequence was evaluated and compared to spGRE STARFI in ex vivo porcine muscle and ex vivo porcine brain. The minimally achievable TR of cES STARFI was shorter than that of spGRE STARFI, indicating that the cES sequence was more time efficient. It was verified through Bloch simulation and ex vivo experiments that the phase sensitivity to displacement of cES STARFI was higher than that of spGRE STARFI. The optimal trigger delays of cES STARFI and spGRE STARFI in ex vivo porcine muscle were toffset =-2 and -1 ms, respectively. The displacement-induced phase change to acoustic pressure slopes of cES STARFI were 0.079, 0.079, and 0.047 rad/Mpa across the three muscle samples, while the slopes of spGRE STARFI were only 0.047, 0.052, and 0.027 rad/Mpa. The maximum temperature difference between cES STARFI and spGRE STARFI was 1.1 °C. In ex vivo porcine brain, both the displacement-induced phase-to-noise ratio (PNRd) and the temperature uncertainty of cES STARFI were better than those of spGRE STARFI (P<0.05). The temperature and displacement-induced phase change maps of cES STARFI and spGRE STARFI during HIFU treatment were in good accordance in time and spatial location. The cES STARFI sequence can provide simultaneous MR-ARFI and temperature measurements during pulsed HIFU applications. Though the exact displacement cannot be quantified directly, the sequence showed increased phase sensitivity compared with the spGRE sequence and provided efficient visualization of the focal spot. cES STARFI could therefore be a desirable alternative to spGRE STARFI in practical applications.

Diffusion sensitivity of 3D-GRASE in arterial spin labeling perfusion.

To evaluate the role of true diffusion and flow-related pseudodiffusion in cerebral blood flow (CBF) quantification using arterial spin labeling (ASL) with single-shot or segmented 3D gradient and spin echo (GRASE) readouts. The extended phase graph (EPG) algorithm, originally designed to model the effects of T1 /T2 relaxation and true diffusion in MRI acquisitions utilizing multiple refocusing RF pulses, was augmented (aEPG). This augmentation accounted for flow-related pseudodiffusion attenuation of intravascular MRI signal in the k-space domain during 3D-GRASE acquisition, which leads to blur along the partition direction in the image domain. Blurring of ASL signal into neighboring voxels can lead to underestimation of CBF in small, high-flow structures such as cortical gray matter (GM). The diffusion sensitivity of 3D-GRASE was evaluated through aEPG simulations and in vivo experiments in 13 healthy subjects. The CBF estimation bias for different postlabeling delays, crusher gradient strengths, and segmentation factors along the partition (PAR) and phase-encoding (PE) directions was numerically assessed by simulations and experimentally validated. In vivo experiments demonstrated systematic underestimation of mean GM CBF measured with segmented 3D-GRASE. The GM CBF underestimation depended on ASL preparation and imaging parameters; it reached up to 25% at low-segmentation schemes (1PAR × 2PE ) but was considerably lower at high-segmentation schemes (4PAR × 2PE or 8PAR × 2PE ). Theoretical predictions showed that conventional T1 /T2 relaxation and true diffusion may account for at most ∼25% of GM CBF estimation bias, whereas the pseudodiffusion effect constituted the major fraction in a typical ASL experiment. The pseudodiffusion effect leads to considerable estimation bias in ASL-based CBF quantification using 3D-GRASE readouts. This bias can be substantially reduced by increasing the segmentation factors. Magn Reson Med 80:736-747, 2018. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.

Effects of eddy currents on selective spectral editing experiments at 3T.

To investigate frequency-offset effects in edited magnetic resonance spectroscopy (MRS) experiments arising from B0 eddy currents. Macromolecule-suppressed (MM-suppressed) γ-aminobutyric acid (GABA)-edited experiments were performed at 3T. Saturation-offset series of MEGA-PRESS experiments were performed in phantoms, in order to investigate different aspects of the relationship between the effective editing frequencies and eddy currents associated with gradient pulses in the sequence. Difference integrals were quantified for each series, and the offset dependence of the integrals was analyzed to quantify the difference in frequency (Δf) between the actual vs. nominal expected saturation frequency. Saturation-offset N-acetyl-aspartate-phantom experiments show that Δf varied with voxel orientation, ranging from 10.4 Hz (unrotated) to 6.4 Hz (45° rotation about the caudal-cranial axis) and 0.4 Hz (45° rotation about left-right axis), indicating that gradient-related B0 eddy currents vary with crusher-gradient orientation. Fixing the crusher-gradient coordinate-frame substantially reduced the orientation dependence of Δf (to ∼2 Hz). Water-suppression crusher gradients also introduced a frequency offset, with Δf = 0.6 Hz ("excitation" water suppression), compared to 10.2 Hz (no water suppression). In vivo spectra showed a negative edited "GABA" signal, suggesting Δf on the order of 10 Hz; with fixed crusher-gradient coordinate-frame, the expected positive edited "GABA" signal was observed. Eddy currents associated with pulsed field gradients may have a considerable impact on highly frequency-selective spectral-editing experiments, such as MM-suppressed GABA editing at 3T. Careful selection of crusher gradient orientation may ameliorate these effects. 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:673-681.

Sensitivity analysis of magnetic field measurements for magnetic resonance electrical impedance tomography (MREIT).

Clinical use of magnetic resonance electrical impedance tomography (MREIT) still requires significant sensitivity improvements. Here, the measurement of the current-induced magnetic field (ΔBz,c ) is improved using systematic efficiency analyses and optimization of multi-echo spin echo (MESE) and steady-state free precession free induction decay (SSFP-FID) sequences. Considering T1 , T2 , and T2* relaxation in the signal-to-noise ratios (SNRs) of the MR magnitude images, the efficiency of MESE and SSFP-FID MREIT experiments, and its dependence on the sequence parameters, are analytically analyzed and simulated. The theoretical results are experimentally validated in a saline-filled homogenous spherical phantom with relaxation parameters similar to brain tissue. Measurement of ΔBz,c is also performed in a cylindrical phantom with saline and chicken meat. The efficiency simulations and experimental results are in good agreement. When using optimal parameters, ΔBz,c can be reliably measured in the phantom even at injected current strengths of 1 mA or lower for both sequence types. The importance of using proper crusher gradient selection on the phase evolution in a MESE experiment is also demonstrated. The efficiencies observed with the optimized sequence parameters will likely render in-vivo human brain MREIT feasible. Magn Reson Med 79:748-760, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

SU‐G‐IeP1‐12: Size Selective Arterial Cerebral Blood Volume Mapping Using Multiple Inversion Time Arterial Spin Labeling

Purpose:To demonstrate the feasibility of a novel method for size specific arterial cerebral blood volume (aCBV) mapping using pseudo‐continuous arterial spin labeling (PCASL), with multiple TI.Methods:Multiple PCASL images were obtained from a subject with TI of [300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500, 3000, 3500, 4000] ms. Each TI pair was averaged six times. Two scans were performed: one without a flow crusher gradient and the other with a crusher gradient (10cm/s in three directions) to remove signals from large arteries. Scan times were 5min. without a crusher gradient and 5.5 min with a crusher gradient. Non‐linear fitting algorithm finds the minimum mean squared solution of per‐voxel based aCBV, cerebral blood flow, and arterial transit time, and fits the data into a hemodynamic model that represents superposition of blood volume and flow components within a single voxel.Results:aCBV maps with a crusher gradient represent signals from medium and small sized arteries, while those without a crusher gradient represent signals from all sized arteries, indicating that flow crusher gradients can be effectively employed to achieve size‐specific aCBV mapping. Regardless of flow crusher, the CBF and ATT maps are very similar in appearance.Conclusion:Quantitative size selective blood volume mapping controlled by a flow crusher is feasible without additional information because the ASL quantification process doesn't require an arterial input function measured from a large artery. The size specific blood volume mapping is not interfered by sSignals from large arteries do not interfere with size specific aCBV mapping in the applications of interest in for applications in which only medium or small arteries are of interest.

Suppression of skeletal muscle signal using a crusher coil: A human cardiac (31) p-MR spectroscopy study at 7 tesla.

PurposeThe translation of sophisticated phosphorus MR spectroscopy (31P‐MRS) protocols to 7 Tesla (T) is particularly challenged by the issue of radiofrequency (RF) heating. Legal limits on RF heating make it hard to reliably suppress signals from skeletal muscle that can contaminate human cardiac 31P spectra at 7T. We introduce the first surface‐spoiling crusher coil for human cardiac 31P‐MRS at 7T.MethodsA planar crusher coil design was optimized with simulations and its performance was validated in phantoms. Crusher gradient pulses (100 μs) were then applied during human cardiac 31P‐MRS at 7T.ResultsIn a phantom, residual signals were 50 ± 10% with BISTRO (B1‐insensitive train to obliterate signal), and 34 ± 8% with the crusher coil. In vivo, residual signals in skeletal muscle were 49 ± 4% using BISTRO, and 24 ± 5% using the crusher coil. Meanwhile, in the interventricular septum, spectral quality and metabolite quantification did not differ significantly between BISTRO (phosphocreatine/adenosine triphosphate [PCr/ATP] = 2.1 ± 0.4) and the crusher coil (PCr/ATP = 1.8 ± 0.4). However, the specific absorption rate (SAR) decreased from 96 ± 1% of the limit (BISTRO) to 16 ± 1% (crusher coil).ConclusionA crusher coil is an SAR‐efficient alternative for selectively suppressing skeletal muscle during cardiac 31P‐MRS at 7T. A crusher coil allows the use of sequence modules that would have been SAR‐prohibitive, without compromising skeletal muscle suppression. Magn Reson Med 75:962–972, 2016. © 2015 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.

Optimal control of the signal-to-noise ratio per unit time of a spin 1/2 particle: The crusher gradient and the radiation damping cases

We show to which extent the signal to noise ratio per unit time of a spin 1/2 particle can be maximized. We consider a cyclic repetition of experiments made of a measurement followed by a radio-frequency magnetic field excitation of the system, in the case of unbounded amplitude. In the periodic regime, the objective of the control problem is to design the initial state of the system and the pulse sequence which leads to the best signal to noise performance. We focus on two specific issues relevant in nuclear magnetic resonance, the crusher gradient and the radiation damping cases. Optimal control techniques are used to solve this non-standard control problem. We discuss the optimality of the Ernst angle solution, which is commonly applied in spectroscopic and medical imaging applications. In the radiation damping situation, we show that in some cases, the optimal solution differs from the Ernst one.

PET/MR synchronization by detection of switching gradients.

The full potential of simultaneous PET and MRI image acquisition, such as dynamic studies or motion compensation, can only be explored if the data of both modalities are temporally synchronized. These hybrid imaging systems are often realized as custom made PET inserts for commercially available MRI scanner. Unfortunately, the standard MRIs do not always offer easily programmable synchronization outputs, nor can they be modified. Here, we demonstrate a simple method for temporal synchronization that does not require a connection to the MRI. It uses the normally undesired effect of induced voltages on the PET electronics by switching MRI gradients. In our preclinical PET/RF inserts the PET detector electronics are located in RF-tight housings between the gradient coil and the RF coil. Unlike RF fields, gradient fields penetrate the housings due to their relatively low frequencies. A dedicated coil system was made from traces and vias on the PET detector board. It captures induced voltages from the switching gradients and a time-stamped gradient trigger message is added to the PET data stream. Temporal alignment between PET and MRI data can be made at the beginning of each MRI sequence by detecting its first gradient or by recognizing its preparation phase. From then on, the MRI time can be translated to the PET time. Detecting the preparation phase of an MRI sequence was tested with a standard survey sequence (for brain) using the PET/RF insert “Hyperion I” inside a Philips 3T Achieva MRI. The three crusher gradient pulses (slew rate = 25 mT/m/ms) used during noise level determination can clearly be recognized in the data from the sensors. As a first application the sensors were used to synchronize PET and MRI time to facilitate MRI based PET motion compensation of a deforming breathing phantom.

Proton CSI without solvent suppression with strongly reduced field gradient related sideband artifacts

Non-water-suppressed MRSI (magnetic resonance spectroscopy imaging) offers a number of advantages; however, spectra are hampered by the sideband artifacts. The origin of those is associated with the vibration of the gradient coils, and most of the sidebands are assumed to be related to the crusher gradients. The aim was to examine the dependency between the physical direction of the crushers and the sidebands. Additionally, the possibilities of optimization of the point resolved spectroscopy sequence (PRESS) were investigated. For the assessment of the sidebands, spectra at short echo time (TE) were collected at 3 T from standard water phantom. A homemade agar phantom was used to test the optimal strength of the crusher gradients. Optimized PRESS sequence was tested in vivo. The greatest sidebands were found to be associated with the crusher gradient in x-direction. Agar phantom and in vivo measurements revealed that reduction of the crusher's strength to 5 mT/m could provide a significant minimization of the sidebands without raising the unwanted signals produced by volume selection. This study demonstrates that crusher gradients in different directions produce a unique pattern of the sidebands. Moreover, optimization of the strength of crushers has been found to decrease sidebands so, the remaining part could be reduced in postprocessing.

Steady state effects in PGSE NMR diffusion experiments

The consequences of performing the Hahn spin-echo based PGSE sequence in steady state mode are investigated. Theory suggests that addition of a crusher gradient pulse during the prescan delay should result in a steady state of magnetisation at the beginning of each instance of the pulse sequence regardless of the length of the recycle delay. Although theoretical analysis reveals that substantially shorter recycle delays can be used, experiments reveal that very short recycle delays result in significant deviation in the measured diffusion coefficient as determined by a standard PGSE experiment with a recycle delay sufficient for full thermal relaxation (normally ∼5 × T 1).

Reducing cardiac noise in BOLD‐weighted voxel time courses in an fMRI dataset by increasing TR and/or applying a crusher gradient in an EPI acquisition pulse

Reducing cardiac noise in BOLD‐weighted voxel time courses in an fMRI dataset by increasing TR and/or applying a crusher gradient in an EPI acquisition pulse

Resurrection of crushed magnetization and chaotic dynamics in solution NMR spectroscopy.

We show experimentally and theoretically that two readily observed effects in solution nuclear magnetic resonance (NMR)-radiation damping and the dipolar field-combine to generate bizarre spin dynamics (including chaotic evolution) even with extraordinarily simple sequences. For example, seemingly insignificant residual magnetization after a crusher gradient triggers exponential regrowth of the magnetization, followed by aperiodic turbulent spin motion. The estimated Lyapunov exponent suggests the onset of spatial-temporal chaos and the existence of chaotic attractors. This effect leads to highly irreproducible experimental decays that amplify minor nonuniformities such as temperature gradients. Imaging applications and consequences for other NMR studies are discussed.

B(0)-fluctuation-induced temporal variation in EPI image series due to the disturbance of steady-state free precession.

Steady-state free precession (SSFP) can develop under a train of RF pulses, given the condition TR < T(2). SSFP in multi-shot imaging sequences has been well studied. It is shown here that serial single-shot echo-planar imaging (EPI) acquisition can also develop SSFP, and the SSFP can be disturbed by B(0) fluctuation, causing voxel-wise temporal variation. This SSFP disturbance is predominantly present in cerebrospinal fluid (CSF) regions due to the long T(2) value. By applying a sufficiently strong crusher gradient in the EPI pulse sequence, the temporal variation induced by SSFP disturbance can be suppressed due to diffusion. Evidence is provided to indicate that physiological motions such as cardiac pulsation and respiration could affect the voxel-wise time courses through the mechanism of SSFP disturbance. It is advised that if the disturbance is observed in serial EPI images, the crusher should be made stronger to eliminate the unwanted temporal variation.

Three-Dimensional Triple-Quantum-Filtered Imaging of 0.012 and 0.024 M Sodium-23 Using Short Repetition Times

The gradient-selected triple-quantum-filtered (GS3Q) esperiment, developed to improve the contrast in NMR imaging of sodium-23 (23Na) in the human brain, is limited by low signal-to-noise ratio (SNR). We have analyzed the GS3Q experiment and show here that the improvement in GS3Q-filtered 32Na SNR as the repetition time (TR) decreases is accompanied by the appearance of spurious one-quantum (1Q) 23Na signals. An improved filter with better suppression of spurious 1Q 23Na signals is obtained by adding a preparatory crusher gradient and two-step phase cycling to a conventional GS3Q filter. The relative contributions of 3Q coherence and spurious 1Q coherences to the conventional and modified-GS3Q-filtered signals are calculated, providing a measure of the effectiveness of each GS3Q filter. The filters were implemented on a 2.35 T medium-bore spectrometer and their predicted properties verified. SNR measurements from GS3Q-filtered three-dimensional images of an agarose gel phantom indicate that 0.012 M23Na images in the human brain can be acquired with 8 cm3 voxels and SNR of 10 in 30 minutes at 2.35 T, assuming similar relaxation times.

Effects of hyperoxia on T and resonance frequency weighted magnetic resonance images of rodent tumours

Experiments were performed to determine whether T2* and resonance frequency weighted MR images are sensitive to effects of hyperoxia on model tumors. Hyperoxia can increase tumor oxygen tension and thus affect T2* and/or the average resonance frequency within each image voxel due to the paramagnetism of oxygen itself or through modulation of the oxidation state of hemoglobin. Alternatively, changes in T2* during hyperoxia may reflect changes in tumor water content due to changes in systemic blood pressure. Mammary adenocarcinomas implanted in the flanks of rats were studied. Imaging sequences were preceded by two 90 degrees pulses separated by an evolution period of 50 or 75 ms and followed by a crusher gradient to eliminate transverse magnetization. This pulse sequence produced images which were sensitized to both T2* and the average resonance frequency of each voxel. Images were produced at 2 T using a gradient echo imaging method with a TR of 3 s. Images obtained during inhalation of air and 100% O2 were compared. Significant increases in image intensity were observed in most tumors during hyperoxia, particularly at the tumor center. The increase was accentuated when the evolution period was increased and greatly reduced when a 180 degrees refocusing pulse was placed at the center of the evolution period. These results suggest that hyperoxia reduces local magnetic susceptibility gradients leading to an increase in T2* or causes a shift in resonance frequency. The magnitude of this change may be a function of the rate at which oxygen is delivered to and metabolized by tumors and may also reflect tumor oxygen tension under normoxic conditions.(ABSTRACT TRUNCATED AT 250 WORDS)