Spoiled Gradient Research Articles

Sensitivity of quantitative relaxometry and susceptibility mapping to microscopic iron distribution.

Determine the impact of the microscopic spatial distribution of iron on relaxometry and susceptibility-based estimates of iron concentration. Monte Carlo simulations and in vitro experiments of erythrocytes were used to create different microscopic distributions of iron. Measuring iron with intact erythrocyte cells created a heterogeneous distribution of iron, whereas lysing erythrocytes was used to create a homogeneous distribution of iron. Multi-echo spin echo and spoiled gradient echo acquisitions were then used to estimate relaxation parameters ( and ) and susceptibility. Simulations demonstrate that and measurements depend on the spatial distribution of iron even for the same iron concentration and volume susceptibility. Similarly, in vitro experiments demonstrate that and measurements depend on the microscopic spatial distribution of iron whereas the quantitative susceptibility mapping (QSM) susceptibility estimates reflect iron concentration without sensitivity to spatial distribution. and for iron quantification depend on the spatial distribution or iron. QSM-based estimation of iron concentration is insensitive to the microscopic spatial distribution of iron, potentially providing a distribution independent measure of iron concentration.

The advantages of radial trajectories for vessel-selective dynamic angiography with arterial spin labeling

ObjectivesTo demonstrate the advantages of radial k-space trajectories over conventional Cartesian approaches for accelerating the acquisition of vessel-selective arterial spin labeling (ASL) dynamic angiograms, which are conventionally time consuming to acquire.Materials and methodsVessel-encoded pseudocontinuous ASL was combined with time-resolved balanced steady-state free precession (bSSFP) and spoiled gradient echo (SPGR) readouts to obtain dynamic vessel-selective angiograms arising from the four main brain-feeding arteries. Dynamic 2D protocols with acquisition times of one minute or less were achieved through radial undersampling or a Cartesian parallel imaging approach. For whole-brain dynamic 3D imaging, magnetic field inhomogeneity and the high acceleration factors required rule out the use of bSSFP and Cartesian trajectories, so the feasibility of acquiring 3D radial SPGR angiograms was tested.ResultsThe improved SNR efficiency of bSSFP over SPGR was confirmed for 2D dynamic imaging. Radial trajectories had considerable advantages over a Cartesian approach, including a factor of two improvements in the measured SNR (p < 0.00001, N = 6), improved distal vessel delineation and the lack of a need for calibration data. The 3D radial approach produced good quality angiograms with negligible artifacts despite the high acceleration factor (R = 13).ConclusionRadial trajectories outperform conventional Cartesian techniques for accelerated vessel-selective ASL dynamic angiography.

Quantification of metal-induced susceptibility artifacts associated with ultrahigh-field magnetic resonance imaging of spinal implants.

Reports on spinal‐implant metallic artifacts in 7‐T magnetic resonance imaging (MRI) are lacking. Thus, we investigated the magnitude of metal artifacts derived from spinal implants in 7‐T MRI and analyzed the differences obtained with spinal rods manufactured from pure titanium, titanium alloy, and cobalt‐chrome (5.5‐mm and 6.0‐mm diameters and 50‐mm length). Following the American Society for Testing and Materials guidelines, we measured the artifact size and artifact volume ratio of each rod during image acquisition using 7‐T MRI scanners with three‐dimensional (3D) T1‐weighted and 3D T2* spoiled gradient echo (GRE), 3D T2‐weighted fast spin echo, zero echo time (ZTE), and diffusion‐weighted imaging sequences. Pure titanium and titanium alloy rods yielded significantly smaller artifacts than did cobalt‐chrome rods, with no significant difference between pure titanium and titanium alloy rods. The artifact sizes of the 5.5‐mm and 6.0‐mm diameter rods were similar. The artifact magnitude increased in the following sequence order: ZTE, 3D T2 fast spin echo, 3D T1 spoiled GRE, 3D T2* spoiled GRE, and diffusion‐weighted imaging. Artifacts obtained using the spin echo method were smaller than those obtained with the GRE method. Because the echo time in ZTE is extremely short, the occurrence of artifacts because of image distortion and signal loss caused by differences in magnetic susceptibility is minimal, resulting in the smallest artifacts. ZTE can be a clinically useful method for the postoperative evaluation of patients after instrumentation surgery, even with 7‐T MRI.

Dissolved hyperpolarized xenon-129 MRI in human kidneys.

PurposeTo assess the feasibility of using dissolved hyperpolarized xenon‐129 (129Xe) MRI to study renal physiology in humans at 3 T.MethodsUsing a flexible transceiver RF coil, dynamic and spatially resolved 129Xe spectroscopy was performed in the abdomen after inhalation of hyperpolarized 129Xe gas with 3 healthy male volunteers. A transmit‐only receive‐only RF coil array was purpose‐built to focus RF excitation and enhance sensitivity for dynamic imaging of 129Xe uptake in the kidneys using spoiled gradient echo and balanced steady‐state sequences.ResultsUsing spatially resolved spectroscopy, different magnitudes of signal from 129Xe dissolved in red blood cells and tissue/plasma could be identified in the kidneys and the aorta. The spectra from both kidneys showed peaks with similar amplitudes and chemical shift values. Imaging with the purpose‐built coil array was shown to provide more than a 3‐fold higher SNR in the kidneys when compared with surrounding tissues, while further physiological information from the dissolved 129Xe in the lungs and in transit to the kidneys was provided with the transceiver coil. The signal of dissolved hyperpolarized 129Xe could be imaged with both tested sequences for about 40 seconds after inhalation.ConclusionThe uptake of 129Xe dissolved in the human kidneys was measured with spectroscopic and imaging experiments, demonstrating the potential of hyperpolarized 129Xe MR as a novel, noninvasive technique to image human kidney tissue perfusion.

Data Quality and Optimal Background Correction Order of Respiratory‐Gated k‐Space Segmented Spoiled Gradient Echo (SGRE) and Echo Planar Imaging (EPI)‐Based 4D Flow MRI

BackgroundA reduction in scan time of 4D Flow MRI would facilitate clinical application. A recent study indicates that echo‐planar imaging (EPI) 4D Flow MRI allows for a reduction in scan time and better data quality than the recommended k‐space segmented spoiled gradient echo (SGRE) sequence. It was argued that the poor data quality of SGRE was related to the nonrecommended absence of respiratory motion compensation. However, data quality can also be affected by the background offset compensation.PurposeTo compare the data quality of respiratory motion‐compensated SGRE and EPI 4D Flow MRI and their dependence on background correction (BC) order.Study TypeRetrospective.SubjectsEighteen healthy subjects (eight female, mean age 32 ± 5 years).Field Strength and Sequence1.5 T. [Correction added on July 26, 2019, after first online publication: The preceding field strength was corrected.] SGRE and EPI‐based 4D Flow MRI.AssessmentData quality was investigated visually and by comparing flows through the cardiac valves and aorta. Measurements were obtained from transvalvular flow and pathline analysis.Statistical TestsLinear regression and Bland–Altman analysis were used. Wilcoxon test was used for comparison of visual scoring. Student's t‐test was used for comparison of flow volumes.ResultsNo significant difference was found by visual inspection (P = 0.08). Left ventricular (LV) flows were strongly and very strongly associated with SGRE and EPI, respectively (R2 = 0.86–0.94 SGRE; 0.71–0.79 EPI, BC0–4). LV and right ventricular (RV) outflows and LV pathline flows were very strongly associated (R2 = 0.93–0.95 SGRE; 0.88–0.91 EPI, R2 = 0.91–0.95 SGRE; 0.91–0.93 EPI, BC1–4). EPI LV outflow was lower than the short‐axis‐based stroke volume. EPI RV outflow and proximal descending aortic flow were lower than SGREs.Data ConclusionBoth sequences yielded good internal data consistency when an adequate background correction was applied. Second and first BC order were considered sufficient for transvalvular flow analysis in SGRE and EPI, respectively. Higher BC orders were preferred for particle tracing. Level of Evidence 4 Technical Efficacy Stage 1J. Magn. Reson. Imaging 2020;51:885–896.

Ferumoxytol-enhanced MRI for surveillance of pediatric cerebral arteriovenous malformations.

Children with intracranial arteriovenous malformations (AVMs) undergo digital DSA for lesion surveillance following their initial diagnosis. However, DSA carries risks of radiation exposure, particularly for the growing pediatric brain and over lifetime. The authors evaluated whether MRI enhanced with a blood pool ferumoxytol (Fe) contrast agent (Fe-MRI) can be used for surveillance of residual or recurrent AVMs. A retrospective cohort was assembled of children with an established AVM diagnosis who underwent surveillance by both DSA and 3-T Fe-MRI from 2014 to 2016. Two neuroradiologists blinded to the DSA results independently assessed Fe-enhanced T1-weighted spoiled gradient recalled acquisition in steady state (Fe-SPGR) scans and, if available, arterial spin labeling (ASL) perfusion scans for residual or recurrent AVMs. Diagnostic confidence was examined using a Likert scale. Sensitivity, specificity, and intermodality reliability were determined using DSA studies as the gold standard. Radiation exposure related to DSA was calculated as total dose area product (TDAP) and effective dose. Fifteen patients were included in this study (mean age 10 years, range 3-15 years). The mean time between the first surveillance DSA and Fe-MRI studies was 17 days (SD 47). Intermodality agreement was excellent between Fe-SPGR and DSA (κ = 1.00) but poor between ASL and DSA (κ = 0.53; 95% CI 0.18-0.89). The sensitivity and specificity for detecting residual AVMs using Fe-SPGR were 100% and 100%, and using ASL they were 72% and 100%, respectively. Radiologists reported overall high diagnostic confidence using Fe-SPGR. On average, patients received two surveillance DSA studies over the study period, which on average equated to a TDAP of 117.2 Gy×cm2 (95% CI 77.2-157.4 Gy×cm2) and an effective dose of 7.8 mSv (95% CI 4.4-8.8 mSv). Fe-MRI performed similarly to DSA for the surveillance of residual AVMs. Future multicenter studies could further investigate the efficacy of Fe-MRI as a noninvasive alternative to DSA for monitoring AVMs in children.

Identifying cardiovascular risk factors that impact cerebrovascular reactivity: An ASL MRI study.

To maintain cerebral blood flow (CBF), cerebral blood vessels dilate and contract in response to blood supply through cerebrovascular reactivity (CR). Cardiovascular (CV) disease is associated with increased stroke risk, but which risk factors specifically impact CR is unknown. Prospective longitudinal. Fifty-three subjects undergoing carotid endarterectomy or stenting. 3T, 3D pseudo-continuous arterial spin labeling (PCASL) ASL, and T1 3D fast spoiled gradient echo (FSPGR). We evaluated group differences in CBF changes for multiple cardiovascular risk factors in patients undergoing carotid revascularization surgery. PRE (baseline), POST (48-hour postop), and 6MO (6 months postop) whole-brain CBF measurements, as 129 CBF maps from 53 subjects were modeled as within-subject analysis of variance (ANOVA). To identify CV risk factors associated with CBF change, the CBF change from PRE to POST, POST to 6MO, and PRE to 6MO were modeled as multiple linear regression with each CV risk factor as an independent variable. Statistical models were performed controlling for age on a voxel-by-voxel basis using SPM8. Significant clusters were reported if familywise error (FWE)-corrected cluster-level was P < 0.05, while the voxel-level significance threshold was set for P < 0.001. The entire group showed significant (cluster-level P < 0.001) CBF increase from PRE to POST, decrease from POST to 6MO, and no significant difference (all voxels with P > 0.001) from PRE to 6MO. Of multiple CV risk factors evaluated, only elevated systolic blood pressure (SBP, P = 0.001), chronic renal insufficiency (CRI, P = 0.026), and history of prior stroke (CVA, P < 0.001) predicted lower increases in CBF PRE to POST. Over POST to 6MO, obesity predicted lower (P > 0.001) and cholesterol greater CBF decrease (P > 0.001). The CV risk factors of higher SBP, CRI, CVA, BMI, and cholesterol may indicate altered CR, and may warrant different stroke risk mitigation and special consideration for CBF change evaluation. 1 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2020;51:734-747.

A fast volumetric 4D-MRI with sub-second frame rate for abdominal motion monitoring and characterization in MRI-guided radiotherapy.

To propose a fast volumetric 4D-MRI based on 3D pulse sequence acquisition for abdominal motion monitoring and characterization in MRI-guided radiotherapy (MRgRT). A 3D spoiled gradient echo sequence volumetric interpolated breath-hold examination (VIBE) [repetition time/echo time (TR/TE) =0.53/1.57 ms, flip-angle =5°, receiver bandwidth (RBW) =1,400 Hz/voxel] based 4D-MRI acquisition, accelerated by 4-fold controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA), named CAIPIRINHA-VIBE 4D-MRI, was implemented on a 1.5T MRI simulator (MR-sim) and applied for abdominal imaging of nine healthy volunteers under free breathing. One hundred and forty-four dynamics of the entire abdomen volume (56 slices), in total 8,064 (144×56) images with a voxel size of 2.7×2.7×4.0 mm3, were acquired in 89 s for 4D-MRI. This CAIPIRINHA-VIBE 4D-MRI was qualitatively compared with a 2D half-Fourier acquisition single-shot turbo spin-echo (2D-HASTE) based 4D-MRI. The motions of liver dome, kidney and spleen were analyzed using the CAIPIRINHA-VIBE 4D-MRI data. The kidney motion was quantitatively characterized in terms of motion range and the correlations between left and right kidneys. CAIPIRINHA-VIBE 4D-MRI was successfully conducted in all subjects. CAIPIRINHA-VIBE 4D-MRI exhibited much higher effective volumetric temporal resolution (0.615 vs. ~5 s/volume) and better reconstructed volume consistency than 2D-HASTE 4D-MRI. CAIPIRINHA-VIBE 4D-MRI was able to characterize the respiratory motion of abdominal organs simultaneously in three orthogonal directions, and could potentially be used for whole abdomen deformable motion tracking. Renal motion range was most pronounced in superior-inferior (SI) direction (L: 10.03±2.65 mm; R: 10.38±2.80 mm), significantly larger (P<0.001) than that in anterior-posterior (AP) and the least in left-right (LR) directions. Right kidney had significantly larger mobility (4.18±2.19 vs. 2.32±1.34 mm, P=0.045) than left kidney in AP, but not in LR and SI directions. The Pearson correlation coefficients r between left and right kidney motion were 0.5063 (P=0.164), 0.6624 (P=0.052) and 0.5752 (P=0.105) in LR, AP and SI correspondingly. The correlation of renal motion in SI and AP was found significant in right kidney (r=0.843, P=0.004) but not in left kidney (r=0.467, P=0.205). A fast volumetric 4D-MRI was implemented for abdominal motion monitoring in MRgRT. A sub-second volumetric temporal resolution of 0.615 s, covering the entire abdomen, was demonstrated for respiratory motion monitoring and characterization. This technique holds potentials for MRgRT applications.

Quantitative magnetization transfer imaging of the human locus coeruleus

The locus coeruleus (LC) is the major origin of norepinephrine in the central nervous system, and is subject to age-related and neurodegenerative changes, especially in disorders such as Parkinson's disease and Alzheimer's disease. Previous studies have shown that neuromelanin (NM)-sensitive MRI can be used to visualize the LC, and it is hypothesized that magnetization transfer (MT) effects are the primary source of LC contrast. The aim of this study was to characterize the MT effects in LC imaging by applying high spatial resolution quantitative MT (qMT) imaging to create parametric maps of the macromolecular content of the LC and surrounding tissues. Healthy volunteers (n = 26; sex = 17 F/9M; age = 41.0 ± 19.1 years) underwent brain MRI on a 3.0 T scanner. qMT data were acquired using a 3D MT-prepared spoiled gradient echo sequence. A traditional NM scan consisting of a T1-weighted turbo spin echo sequence with MT preparation was also acquired. The pool-size ratio (PSR) was estimated for each voxel using a single-point qMT approach. The LC was semi-automatically segmented on the MT-weighted images. The MT-weighted images provided higher contrast-ratio between the LC and surrounding pontine tegmentum (PT) (0.215 ± 0.031) than the reference images without MT-preparation (−0.005 ± 0.026) and the traditional NM images (0.138 ± 0.044). The PSR maps showed significant differences between the LC (0.090 ± 0.009) and PT (0.188 ± 0.025). The largest difference between the PSR values in the LC and PT was observed in the central slices, which also correspond to those with the highest contrast-ratio. These results highlight the role of MT in generating NM-related contrast in the LC, and should serve as a foundation for future studies aiming to quantify pathological changes in the LC and surrounding structures in vivo.

Rapid framework for quantitative magnetization transfer imaging with interslice magnetization transfer and dictionary-driven fitting approaches.

To develop a rapid framework for quantitative magnetization transfer (qMT) imaging based on the 2D interslice MT and dictionary-driven fitting approaches. Sequential balanced steady-state free precession (bSSFP) scanning was performed on a whole human brain in a total of 12 conditions from six different interslice gaps and two different flip angles. To obtain qMT maps, the acquired 12 datasets were fitted to a dictionary predefined by using Bloch equation simulations based on the two-pool MT model. The proposed qMT method was compared to the conventional qMT methods, in terms of qMT parameter maps and processing time. The proposed method yielded qMT maps similar to those of the conventional method, indicating feasibility of modulating MT saturation frequency and power with the interslice gap and flip angle. The whole-brain qMT imaging could be completed in 8min because of the absence of separate MT pulses. The time for processing qMT parameters was significantly reduced through the dictionary-driven approach; it is 1000 times shorter than that without the dictionary-driven approach and 3 times shorter than that with the spoiled gradient recalled echo-qMT method that uses an analytical solution. The proposed dictionary-driven interslice qMT imaging provides qMT maps close to those from the conventional method with significantly reduced scan time and postprocessing time, which can make qMT imaging more clinically acceptable.

Normal T1 relaxometry and extracellular volume of the pancreas in subjects with no pancreas disease: correlation with age and gender.

Determine normal T1 and extracellular volume (ECV) of the pancreas in subjects with no pancreas disease and correlate with age and gender. We imaged 120 healthy subjects (age range 20-78years) who are on annual screening with MRI/MRCP for the possibility of pancreatic cancer. Subjects had a predisposition to develop pancreatic cancer, but no history of pancreas disease or acute symptoms. Equal number (n = 60) of subjects were scanned on either 1.5 T or 3 T scanner using dual flip angle spoiled gradient echo technique incorporating fat suppression and correction for B1 field inhomogeneity. Optimization of imaging parameters was performed using a T1 phantom. ECV was calculated using pre- and post-contrast T1 of the pancreas and plasma. Regression analysis and Mann-Whitney tests were used for statistical analysis. Median T1 on 1.5 T was 654ms (IQR 608-700); median T1 on 3 T was 717ms (IQR 582-850); median ECV on 1.5 T was 0.28 (IQR 0.21-0.33), and median ECV on 3 T was 0.25 (IQR 0.19-0.28). Age had a mild positive correlation with T1 (r = 0.24, p = 0.009), but not with ECV (r = 0.06, p = 0.54). T1 and ECV were similar in both genders (p > 0.05). This study measured the median T1 and ECV of the pancreas in subjects with no pancreas disease. Pancreas shows longer T1 relaxation times in older population, whereas extracellular fraction remains unchanged. Median T1 values were different between two magnet strengths; however, no difference was seen between genders and ECV fractions.

MRI-derived regional flow-volume loop parameters detect early-stage chronic lung allograft dysfunction.

Chronic lung allograft dysfunction (CLAD) is a major cause for the low long-term survival rates after lung transplantation (LTx). Early detection of CLAD may enable providing medical treatment before a nonreversible graft dysfunction has occurred. MRI is advantageous to pulmonary function testing (PFT) in the ability to assess regional function changes, and thus have the potential in detecting very early stages of CLAD before changes in global forced expiratory volume during the first second (FEV1%) occur. To examine whether early stages of CLAD (diagnosed based on PFT values) could also be detected using MRI-derived parameters of regional flow-volume dynamics. Retrospective. 62 lung transplantation recipients were included in the study, 29 of which had been diagnosed with CLAD at various stages. MRI datasets were acquired with a 1.5T Siemens scanner using a spoiled gradient echo sequence. MRI datasets were retrospectively preprocessed and analyzed by a blinded radiologist according to the phase resolved functional lung MRI (PREFUL-MRI) approach, resulting in fractional ventilation (FV) maps and regional flow-volume loops (rFVL). FV- and rFVL-based parameters of regional lung ventilation were estimated. Differences between groups were compared by Mann-Whitney U-test with a Bonferroni correction for multiple comparisons (n = 2). rFVL-CC-based parameters discriminated significantly between the presence or absence of CLAD (P < 0.003). Using the contrast media-free PREFUL-MRI technique, parameters of ventilation dynamics and its regional heterogeneity were shown to be sensitive for the detection of early CLAD stages. 3 TECHNICAL EFFICACY: Stage 3 J. Magn. Reson. Imaging 2019;50:1873-1882.

Feasibility study of highly accelerated phase-sensitive inversion recovery myocardial viability imaging using simultaneous multislice and parallel imaging techniques.

Phase-sensitive inversion recovery (PSIR) is a powerful cardiac MRI method to assess myocardial viability, which can eliminate the background phase and preserve the sign of the desired magnetization during inversion recovery. To shorten the acquisition time of myocardial viability imaging by introducing both simultaneous multislice (SMS) and parallel imaging (PI) into PSIR without additional acquisitions for calibration data. Prospective study. A high-resolution phantom and three vials with doped solutions matching typical postcontrast T1 and T2 values of scar, healthy myocardium, and blood; 18 patients (six with known myocardial infarction) were included in this study. 3T/segmented fast spoiled gradient echo pulse sequence. Phantom and in vivo experiments were performed to compare the performance of conventional PSIR, SMS accelerated PSIR (SMS-PSIR, 2× acceleration), and SMS as well as PI accelerated PSIR (SMS + PI-PSIR, 4× acceleration). In phantom experiments, the error maps, local signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) were calculated. In in vivo experiments, the image quality and artifact level of each study were qualitatively graded (by three radiologists). G-factor maps were calculated. The infarct size presented as a percentage of the left ventricle was measured (full-width half-maximum). Acquisition time of each study was recorded. One-way analysis of variance, Kruskal-Wallis test. In phantom experiments, SNR and CNR were well preserved for SMS-PSIR, while they dropped for SMS + PI-PSIR, as expected. In 15 subjects, the overall image quality scores were not significantly different among conventional PSIR (3.70 ± 1.06), SMS-PSIR (3.78 ± 0.99), and SMS + PI-PSIR (3.47 ± 0.94; P = 0.20). The artifact level scores were also comparable among conventional PSIR (3.67 ± 1.04), SMS-PSIR (3.77 ± 1.03), and SMS + PI-PSIR (3.45 ± 1.00; P = 0.22). SMS-PSIR achieved negligible g-factor noise amplification (1.04 ± 0.03) and SMS + PI-PSIR showed higher g-factors (2.83 ± 0.48). The infarct size was consistent among conventional PSIR (22.51 ± 25.05%) and SMS-PSIR (22.98 ± 26.19%), as well as SMS + PI-PSIR (22.93 ± 25.68%; P = 0.98). The acquisition time of two short-axis slices for SMS-PSIR (17.6 ± 1.7 sec, 16 heartbeats) and SMS + PI-PSIR (9.8 ± 1.9 sec, 8 heartbeats) was 30% and 17% of that for conventional PSIR (56.2 ± 8.5 sec, 32 heartbeats), respectively. SMS can be implemented in PSIR without additional reference scan. The image quality is comparable with conventional PSIR, while the acquisition time is much shorter. The proposed method is also compatible with PI to further reduce the scan time. 2 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2019;50:1964-1972.

Optimized and accelerated 19 F-MRI of inhaled perfluoropropane to assess regional pulmonary ventilation.

PurposeTo accelerate 19F‐MR imaging of inhaled perfluoropropane using compressed sensing methods, and to optimize critical scan acquisition parameters for assessment of lung ventilation properties.MethodsSimulations were performed to determine optimal acquisition parameters for maximal perfluoropropane signal‐to‐noise ratio (SNR) in human lungs for a spoiled gradient echo sequence. Optimized parameters were subsequently employed for 19F‐MRI of inhaled perfluoropropane in a cohort of 11 healthy participants using a 3.0 T scanner. The impact of 1.8×, 2.4×, and 3.0× undersampling ratios on 19F‐MRI acquisitions was evaluated, using both retrospective and prospective compressed sensing methods.Results3D spoiled gradient echo 19F‐MR ventilation images were acquired at 1‐cm isotropic resolution within a single breath hold. Mean SNR was 11.7 ± 4.1 for scans acquired within a single breath hold (duration = 18 s). Acquisition of 19F‐MRI scans at shorter scan durations (4.5 s) was also demonstrated as feasible. Application of both retrospective (n = 8) and prospective (n = 3) compressed sensing methods demonstrated that 1.8× acceleration had negligible impact on qualitative image appearance, with no statistically significant change in measured lung ventilated volume. Acceleration factors of 2.4× and 3.0× resulted in increasing differences between fully sampled and undersampled datasets.ConclusionThis study demonstrates methods for determining optimal acquisition parameters for 19F‐MRI of inhaled perfluoropropane and shows significant reduction in scan acquisition times (and thus participant breath hold duration) by use of compressed sensing.

Evaluation of four injection profiles for uniform contrast-enhanced signal intensity profiles in MR angiography.

Gadolinium concentration variation during acquisition of contrast-enhanced MR angiography (CE-MRA) may lead to artifacts. To compare signal intensity (SI) profiles of four different contrast agent injection strategies during CE-MRA with the goal of minimizing SI variation during acquisition. Prospective. Forty subjects randomized to receive one of four injection profiles of gadobenate dimeglumine (0.1 mmol/kg), either undiluted (0.5 M) or diluted to 40 ml total volume. Tested profiles: 1) nondiluted single-phase ("standard" NS; 1.6 ml/s), 2) diluted single-phase (DS; 1.6 ml/s), 3) diluted biphasic (DB; 9 ml @ 3.3 ml/s, 29 ml @ 1.4 ml/s), 4) patient-tailored protocol using linear prediction (DT). Time-resolved SI measured at 3T with spoiled gradient echo sequences having analogous parameters to those of CE-MRA. Plateau arrival time, rise time, duration, peak and tail SI, plateau quality (sum of squared residuals; SSR), average SI for each injection type derived were used. Two-tailed t-test. Peak SI, arrival, and rise times were not significantly different between groups, excepting peak SI DB slightly > DS (P = 0.042). Duration of NS vs. the diluted groups was significantly shorter (all P < 0.0001), and DS duration was significantly shorter than that of DT and DB (NS 11.4 ± 3.5 vs. DS 22.9 ± 4.3, DB 25.4 ± 2.3, DT 28.3 ± 4.1 sec). Quality (SSR) of the 20-second plateau was significantly better for DS, DB, DT as compared with NS (all P < 0.001). Three different strategies to power-inject diluted gadobenate dimeglumine targeting a 20-second plateau produced SI profiles with longer duration, more consistent plateau, and no significant loss in peak SI. Such injection profiles may provide more uniform SI during CE-MRA, potentially reducing blurring artifacts. 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;50:1808-1816.

Toward "plug and play" prospective motion correction for MRI by combining observations of the time varying gradient and static vector fields.

The efficacy of a Wireless Radio frequency triggered Acquisition Device (WRAD) is evaluated for high frequency (50 Hz) prospective motion correction in a 3-dimensional spoiled gradient echo pulse sequence. The device measures the rate of change in the gradient vector fields (slew) using a 3-dimensional assembly of Printed Circuit Board (PCB) inductors and the direction of the static magnetic field using a 3-axis Hall effect magnetometer. The slew vector encoding is highly efficient, because the Maxwell-term position encoding is observable, allowing overconstrained pose measurement using 3 sinusoidal gradient pulses lasting 880 μs. Since small offsets in the magnetometer can introduce bias into the pose estimates, sensor/system biases are tracked using a lightweight Kalman filter. The only calibration required is determining a geometric scaling factor for the pickup coils, which is specific to the device and will therefore be valid in any scanner. The device was used to perform prospective motion correction in 3 subjects, resulting in an increase in Average Edge Strength (AES) for involuntary and deliberate motion. The WRAD is simple to set up and use, with well-defined measurement variance. This could enable "plug and play" prospective motion correction if pulse sequence independence is achieved.

Update on MR imaging of the pulmonary vasculature

Magnetic resonance imaging (MRI) plays an increasingly important role in the non-invasive evaluation of the pulmonary vasculature. MR angiographic (MRA) techniques provide morphological information, while MR perfusion techniques provide functional information of the pulmonary vasculature. Contrast-enhanced MRA can be performed at high spatial resolution using 3D T1-weighted spoiled gradient echo sequence or at high temporal resolution using time-resolved techniques. Non-contrast MRA can be performed using 3D steady state free precession, double inversion fast spin echo, time of flight or phase contrast sequences. MR perfusion can be done using dynamic contrast-enhanced technique or using non-contrast techniques such as arterial spin labelling and time-resolved imaging of lungs during free breathing with Fourier decomposition analysis. MRI is used in the evaluation of acute and chronic pulmonary embolism, pulmonary hypertension and other vascular abnormalities, congenital anomalies and neoplasms. In this article, we review the different MR techniques used in the evaluation of pulmonary vasculature and its clinical applications.

Motion Artifact Reduction Using a Convolutional Neural Network for Dynamic Contrast Enhanced MR Imaging of the Liver.

Purpose:To improve the quality of images obtained via dynamic contrast enhanced MRI (DCE-MRI), which contain motion artifacts and blurring using a deep learning approach.Materials and Methods:A multi-channel convolutional neural network-based method is proposed for reducing the motion artifacts and blurring caused by respiratory motion in images obtained via DCE-MRI of the liver. The training datasets for the neural network included images with and without respiration-induced motion artifacts or blurring, and the distortions were generated by simulating the phase error in k-space. Patient studies were conducted using a multi-phase T1-weighted spoiled gradient echo sequence for the liver, which contained breath-hold failures occurring during data acquisition. The trained network was applied to the acquired images to analyze the filtering performance, and the intensities and contrast ratios before and after denoising were compared via Bland–Altman plots.Results:The proposed network was found to be significantly reducing the magnitude of the artifacts and blurring induced by respiratory motion, and the contrast ratios of the images after processing via the network were consistent with those of the unprocessed images.Conclusion:A deep learning-based method for removing motion artifacts in images obtained via DCE-MRI of the liver was demonstrated and validated.

MRI artifact simulation for clinically relevant MRI sequences for guidance of prostate HDR brachytherapy

For the purpose of magnetic resonance imaging (MRI) guidance of prostate high-dose-rate (HDR) brachytherapy, this paper presents a study on the potential of clinically relevant MRI sequences to facilitate tracking or localization of brachytherapy devices (HDR source/titanium needle), and which could simultaneously be used to visualize the anatomy. The tracking or localization involves simulation of the MRI artifact in combination with a template matching algorithm. Simulations of the MRI artifacts induced by an HDR brachytherapy source and a titanium needle were implemented for four types of sequences: spoiled gradient echo, spin echo, balanced steady-state free precession (bSSFP) and bSSFP with spectral attenuated inversion recovery (SPAIR) fat suppression. A phantom study was conducted in which mentioned sequences (in 2D) as well as the volumetric MRI sequences of the current clinical scan protocol were applied to obtain the induced MRI artifacts for an HDR source and a titanium needle. Localization of the objects was performed by a phase correlation based template matching algorithm. The simulated images demonstrated high correspondences with the acquired MR images, and allowed localization of the objects. A comparison between the object positions obtained for all applied MRI sequences showed deviations (from the average position) of 0.2–0.3 mm, proving that all MRI sequences were suitable for localization of the objects, irrespective of their 2D or volumetric nature. This study demonstrated that the MRI artifact induced by an HDR source or a titanium needle could be simulated for the four investigated types of MRI sequences (spoiled gradient echo, spin echo, bSSFP and bSSFP-SPAIR), valuable for real-time object localization in clinical practice. This leads to more flexibility in the choice of MRI sequences for guidance of HDR brachytherapy, as they are suitable for both object localization and anatomy visualization.

Magnetohydrodynamic flow imaging of ionic solutions using electrical current injection and MR phase measurements.

In this study, a method is proposed to image magnetohydrodynamic (MHD) flow of ionic solutions, which is caused by externally injected electrical current to an imaging media, during MRI scans. A multi-physics (MP) model is created by using the electrical current, laminar flow, and MR equations. The conventional spoiled gradient echo MRI pulse sequence with bipolar flow encoding gradients is utilized to encode the MHD flow. Using the MP model and the MRI pulse sequence, relationship between the MHD flow related phase in the acquired MR signal, the injection current, and the MRI pulse sequence parameters is stated. Numerical simulations and physical experiments are performed to validate the proposed method. The simulation and experimental results are in agreement and show that the MHD flow related MR phase depends on the amplitude and duration of the flow encoding gradient and the injected current. This method may be used to evaluate the MHD flow of conductive liquid media during MRI scans with simultaneous electrical current injections. The MHD flow related MR phase is 1.5 radian for an injected current of 1 mA amplitude, 30 ms duration and a flow encoding gradient amplitude of 24 mT/m. This large MR phase range exhibits potential use of this method for clinical applications such as investigation of highly conductive cerebrospinal fluid (CSF) during clinical use of electrical current based neuromodulation in MRI. However, very high and time varying velocities of typical CSF flow compared to the MHD flow velocities should also be considered.