Feasibility of MR fingerprinting using a high-performance 0.55 T MRI system

BackgroundMR fingerprinting (MRF) is a versatile method for rapid multi-parametric quantification. The application of MRF for lower MRI field could enable multi-contrast imaging and improve exam efficiency on these systems. The purpose of this work is to demonstrate the feasibility of 3D whole-brain T1 and T2 mapping using MR fingerprinting on a contemporary 0.55 T MRI system.Materials and methodsA 3D whole brain stack-of-spirals FISP MRF sequence was implemented for 0.55 T. Quantification was validated using the NIST/ISMRM Quantitative MRI phantom, and T1 and T2 values of white matter, gray matter, and cerebrospinal fluid were measured in 19 healthy subjects. To assess MRF performance in the lower SNR regime of 0.55 T, measurement precision was calculated from 100 simulated pseudo-replicas of in vivo data and within-session measurement repeatability was evaluated.ResultsT1 and T2 values calculated by MRF were strongly correlated to standard measurements in the ISMRM/NIST MRI system phantom (R² > 0.99), with a small constant bias of approximately 5 ms in T2 values. 3D stack-of-spirals MRF was successfully applied for whole brain quantitative T1 and T2 at 0.55 T, with spatial resolution of 1.2 mm × 1.2 mm × 5 mm, and acquisition time of 8.5 min. Moreover, the T1 and T2 quantifications had precision <5%, despite the lower SNR of 0.55 T.ConclusionA 3D whole-brain stack-of-spirals FISP MRF sequence is feasible for T1 and T2 mapping at 0.55 T.     

Direct magnitude and phase imaging of myelin using ultrashort echo time (UTE) pulse sequences: A feasibility study

In this paper, we aimed to investigate the feasibility of direct visualization of myelin, including myelin lipid and myelin basic protein (MBP), using two-dimensional ultrashort echo time (2D UTE) sequences and utilize phase information as a contrast mechanism in phantoms and in volunteers. The standard UTE sequence was used to detect both myelin and long T2 signal. An adiabatic inversion recovery UTE (IR-UTE) sequence was used to selectively detect myelin by suppressing signal from long T2 water protons. Magnitude and phase imaging and T2* were investigated on myelin lipid and MBP in the forms of lyophilized powders as well as paste-like phantoms with the powder mixed with D₂O, and rubber phantoms as well as healthy volunteers. Contrast to noise ratio (CNR) between white and gray matter was measured. Both magnitude and phase images were generated for myelin and rubber phantoms as well white matter in vivo using the IR-UTE sequence. T2* values of ~ 300 μs were comparable for myelin paste phantoms and the short T2* component in white matter of the brain in vivo. Mean CNR between white and gray matter in IR-UTE imaging was increased from − 7.3 for the magnitude images to 57.4 for the phase images. The preliminary results suggest that the IR-UTE sequence allows simultaneous magnitude and phase imaging of myelin in vitro and in vivo.     

Inversion recovery ultrashort echo time magnetic resonance imaging: A method for simultaneous direct detection of myelin and high signal demonstration of iron deposition in the brain – A feasibility study

Multiple sclerosis (MS) causes demyelinating lesions in the white matter and increased iron deposition in the subcortical gray matter. Myelin protons have an extremely short T2* (< 1 ms) and are not directly detected with conventional clinical magnetic resonance (MR) imaging sequences. Iron deposition also reduces T2*, leading to reduced signal on clinical sequences. In this study we tested the hypothesis that the inversion recovery ultrashort echo time (IR-UTE) pulse sequence can directly and simultaneously image myelin and iron deposition using a clinical 3 T scanner. The technique was first validated on a synthetic myelin phantom (myelin powder in D₂O) and a Feridex iron phantom. This was followed by studies of cadaveric MS specimens, healthy volunteers and MS patients. UTE imaging of the synthetic myelin phantom showed an excellent bi-component signal decay with two populations of protons, one with a T2* of 1.2 ms (residual water protons) and the other with a T2* of 290 μs (myelin protons). IR-UTE imaging shows sensitivity to a wide range of iron concentrations from 0.5 to ~ 30 mM. The IR-UTE signal from white matter of the brain of healthy volunteers shows a rapid signal decay with a short T2* of ~ 300 μs, consistent with the T2* values of myelin protons in the synthetic myelin phantom. IR-UTE imaging in MS brain specimens and patients showed multiple white matter lesions as well as areas of high signal in subcortical gray matter. This in specimens corresponded in position to Perl's diaminobenzide staining results, consistent with increased iron deposition. IR-UTE imaging simultaneously detects lesions with myelin loss in the white matter and iron deposition in the gray matter.     

Development of fast deep learning quantification for magnetic resonance fingerprinting in vivo

PurposeA deep neural network was developed for magnetic resonance fingerprinting (MRF) quantification. This study aimed at extending previous studies of deep learning MRF to in vivo applications, allowing sub-second computation time for large-scale data.MethodsWe applied the deep learning methodology based on our previously published multi-layer perceptron. The number of layers was four, which was optimized to balance the model capacity and noise robustness. The training sets were obtained from MRF dictionaries with 9000 to 28,000 atoms, depending on the desired T1 and T2 ranges. The simulated MRF undersampling artifact based on the k-space acquisition scheme and noise were both added to the training data to reduce the error in estimates.ResultsThe neural network achieved high fidelity (R2 _ 0.98) as compared to the T1 and T2 values of the ISMRM standardized phantom. In brain MRF experiment, the model trained with simulated artifacts and noise showed less error compared to that without. The in vivo application of our neural network for liver and prostate were also demonstrated. For an MRF slice with 256 _ 256 image resolution, the computation time of our neural network was 0.12 s, compared with the _ 28 s-pre-slice for the conventional dictionary matching method.ConclusionOur neural network achieved fast computation speed for MRF quantification. The model trained with simulated artifacts and noise showed less error and achieved optimal performance for phantom experiment and in vivo normal brain and liver, and prostate cancer patient.     

Reliability of the depth-dependent high-resolution BOLD hemodynamic response in human visual cortex and vicinity

Functional magnetic resonance imaging (fMRI) often relies on a hemodynamic response function (HRF), the stereotypical blood oxygen level dependent (BOLD) response elicited by a brief (< 4 s) stimulus. Early measurements of the HRF used coarse spatial resolutions (≥ 3 mm voxels) that would generally include contributions from white matter, gray matter, and the extra-pial compartment (the space between the pial surface and skull including pial blood vessels) within each voxel. To resolve these contributions, high-resolution fMRI (0.9-mm voxels) was performed at 3 T in early visual cortex and its apposed white-matter and extra-pial compartments. The results characterized the depth dependence of the HRF and its reliability during nine fMRI sessions. Significant HRFs were observed in white-matter and extra-pial compartments as well as in gray matter. White-matter HRFs were faster and weaker than in the gray matter, while extra-pial HRFs were comparatively slower and stronger. Depth trends of the HRF peak amplitude were stable throughout a broad depth range that included all three compartments for each session. Across sessions, however, the depth trend of HRF peak amplitudes was stable only in the white matter and deep-intermediate gray matter, while there were strong session-to-session variations in the superficial gray matter and the extra-pial compartment. Thus, high-resolution fMRI can resolve significant and dynamically distinct HRFs in gray matter, white matter, and extra-pial compartments.     

MRF-ZOOM for the unbalanced steady-state free precession (ubSSFP) magnetic resonance fingerprinting

In magnetic resonance fingerprinting (MRF), tissue parameters are determined by finding the best-match to the acquired MR signal from a predefined signal dictionary. This dictionary searching (DS) process is generally performed in an exhaustive manner, which requires a large predefined dictionary and long searching time. A fast MRF DS algorithm, MRF-ZOOM, was recently proposed based on DS objective function optimization. As a proof-of-concept study, MRF-ZOOM was only tested with one of the earliest MRF sequences but not with the recently more popular unbalanced steady state free precession MRF sequence (MRF-ubSSFP, or MRF-FISP). Meanwhile noise effects on MRF and MRF-ZOOM have not been examined. The purpose of this study was to address these open questions and to verify whether MRF-ZOOM can be combined with a dictionary-compression based method to gain further speed. Numerical simulations were performed to evaluate the DS objective function properties, noise effects on MRF, and to compare MRF-ZOOM with other methods in terms of speed and accuracy. In-vivo experiments were performed as well. Evaluation results showed that premises of MRF-ZOOM held for MRF-FISP; noise did not affect MRF-ZOOM more than the conventional MRF method; when SNR ≥ 1, MRF quantification yielded accurate results. Dictionary compression introduced quantification errors more to T2 quantification. MRF-ZOOM was thousands of times faster than the conventional MRF method. Combining MRF-ZOOM with dictionary compression showed no benefit in terms of fitting speed. In conclusion, MRF-ZOOM is valid for MRF- FISP, and can remarkably save MRF dictionary generation and searching time without sacrificing matching accuracy.     

Effects of RF pulse profile and intra-voxel phase dispersion on MR fingerprinting with balanced SSFP readout

PurposeTo investigate possible errors in T1 and T2 quantification via MR fingerprinting with balanced steady-state free precession readout in the presence of intra-voxel phase dispersion and RF pulse profile imperfections, using computer simulations based on Bloch equations.Materials and methodsA pulse sequence with TR changing in a Perlin noise pattern and a nearly sinusoidal pattern of flip angle following an initial 180-degree inversion pulse was employed. Gaussian distributions of off-resonance frequency were assumed for intra-voxel phase dispersion effects. Slice profiles of sinc-shaped RF pulses were computed to investigate flip angle profile influences. Following identification of the best fit between the acquisition signals and those established in the dictionary based on known parameters, estimation errors were reported. In vivo experiments were performed at 3 T to examine the results.ResultsSlight intra-voxel phase dispersion with standard deviations from 1 to 3 Hz resulted in prominent T2 under-estimations, particularly at large T2 values. T1 and off-resonance frequencies were relatively unaffected. Slice profile imperfections led to under-estimations of T1, which became greater as regional off-resonance frequencies increased, but could be corrected by including slice profile effects in the dictionary. Results from brain imaging experiments in vivo agreed with the simulation results qualitatively.ConclusionMR fingerprinting using balanced SSFP readout in the presence of intra-voxel phase dispersion and imperfect slice profile leads to inaccuracies in quantitative estimations of the relaxation times.     

Velocity encoding and velocity compensation for multi-spoke RF excitation

PurposeTo investigate velocity encoded and velocity compensated variants of multi-spoke RF pulses that can be used for flip-angle homogenization at ultra-high fields (UHF). Attention is paid to the velocity encoding for each individual spoke pulse and to displacement artifacts that arise in Fourier transform imaging in the presence of flow.Theory and methodsA gradient waveform design for multi-spoke excitation providing an algorithm for minimal TE was proposed that allows two different encodings. Such schemes were compared to an encoding approach that applies an established scheme to multi-spoke excitations. The impact on image quality and quantitative velocity maps was evaluated in phantoms using single- and two-spoke excitations. Additional validation measurements were obtained in-vivo at 7 T.ResultsPhantom experiments showed that keeping the first gradient moment constant for all k-space lines eliminates any displacements in phase-encoding and slice-selection direction for all spoke pulses but leads to artifacts for non-zero velocity components along readout direction. Introducing variable but well-defined first gradient moments in the phase-encoding direction creates displacements along the velocity vector and thus minimizes velocity-induced geometrical distortions. Phase-resolved mean volume flow in the ascending and descending aorta obtained from two-spoke excitation showed excellent agreement with single-spoke excitation over the cardiac cycle (mean difference 0.8 ± 16.2 ml/s).ConclusionsThe use of single- and multi-spoke RF pulses for flow quantification at 7 T with controlled displacement artifacts has been successfully demonstrated. The presented techniques form the basis for correct velocity quantification and compensation not only for conventional but also for multi-spoke RF pulses allowing in-plane B₁⁺ homogenization using parallel transmission at UHF.     

High resolution myelin water imaging incorporating local tissue susceptibility analysis

Quantitative myelin water imaging (MWI) from signal T₂* decay acquired with multiple Gradient-Recalled Echo (mGRE) sequence has been widely used since its first report. A recent study showed that with low resolution data (2 mm isotropic voxels), direct application of complex fitting to a three-pool WM model with frequency shift terms could produce more stable parameter estimation for myelin water fraction mapping. MWI maps of higher spatial resolution resulting in more detailed tissue structures and reduced partial volume effects around white matter/gray matter (WM/GM) interface, however, is more desirable. Furthermore, as signal-to-noise ratio (SNR) of original images decreases due to reduced voxel size, the direct complex fitting procedure of myelin water imaging becomes more prone to systematic errors which severely compromised stability and reliability of the result. Instead of using the original part of T₂* decay, this work presents a new method based on the WM-induced phase from tissue susceptibility calculated with the same mGRE dataset, in a three-pool WM model (water of myelin, axonal and extracellular water), to improve high resolution MWI. Compared with direct complex fitting for the higher spatial resolution case, the proposed method is shown to provide a more stable and accurate estimation of MWI parameters, and finer details near WM/GM boundaries with greatly reduced partial volume effects.     

Simultaneous measurement of total water content and myelin water fraction in brain at 3 T using a T2 relaxation based method

PurposeThis work demonstrates the in vivo application of a T₂ relaxation based total water content (TWC) measurement technique at 3 T in healthy human brain, and evaluates accuracy using simulations that model brain tissue. The benefit of using T₂ relaxation is that it provides simultaneous measurements of myelin water fraction, which correlates to myelin content.MethodsT₂ relaxation data was collected from 10 healthy human subjects with a gradient and spin echo (GRASE) sequence, along with inversion recovery for T₁ mapping. Voxel-wise T₂ distributions were calculated by fitting the T₂ relaxation data with a non-negative least squares algorithm incorporating B₁⁺ inhomogeneity corrections. TWC was the sum of the signals in the T₂ distribution, corrected for T₁ relaxation and receiver coil inhomogeneity, relative to either an external water standard or cerebrospinal fluid (CSF). Simulations were performed to determine theoretical errors in TWC.ResultsTWC values measured in healthy human brain relative to both external and CSF standards agreed with literature values. Simulations demonstrated that TWC could be measured to within 3–4% accuracy.ConclusionIn vivo TWC measurement using T₂ relaxation at 3 T works well and provides a valuable tool for studying neurological diseases with both myelin and water changes.     

Bi-phase age-related brain gray matter magnetic resonance T1ρ relaxation time change in adults

ObjectivesTo investigate normative value and age-related change of brain magnetic resonance T1ρ relaxation at 1.5 T.MethodsThis study was approved by the local ethical committee with participants' written consent obtained. There were 42 adults healthy volunteers, including 20 males (age: 41 ± 16 (mean ± standard deviation) years, range: 22–68 years,) and 22 females (age: 39 ± 15 years, range: 21–62 years). MRI was performed at 1.5 T using 3D fluid suppressed turbo spin echo sequence. Regions-of-interests (ROIs) were obtained by atlas-based tissue segmentation and T1ρ was calculated by fitting the mean value to mono-exponential model. Correlation between T1ρ relaxation of brain gray matter regions and age was investigated.ResultsA regional difference among individual gray matter areas was noted; the highest values were observed in the hippocampus (98.37 ± 5.37 ms, median: 97.88 ms) and amygdala (94.95 ± 4.34 ms, median: 94.73 ms), while the lowest values were observed in the pallidum (83.81 ± 5.49 ms, median: 83.77 ms) and putamen (83.93 ± 4.76 ms, median: 83.99 ms). Gray matter T1ρ values decreased slowly (mean slope: − 0.256) and significantly (p < 0.05) with age in gray matter for subjects younger than 40 years old, while for subjects older than 40 years old there was no apparent correlation between T1ρ relaxation and age. Global white matter measured T1ρ value of 88.65 ± 3.47 ms (median: 87.86 ms), and the correlation with age was not significant (p = 0.18).ConclusionGray matter T1ρ relaxation demonstrates a bi-phase change with age in adults of 22–68 years.     

Free-running cardiac magnetic resonance fingerprinting: Joint T1/T2 map and Cine imaging

PurposeTo develop and evaluate a novel non-ECG triggered 2D magnetic resonance fingerprinting (MRF) sequence allowing for simultaneous myocardial T₁ and T₂ mapping and cardiac Cine imaging.MethodsCardiac MRF (cMRF) has been recently proposed to provide joint T₁/T₂ myocardial mapping by triggering the acquisition to mid-diastole and relying on a subject-dependent dictionary of MR signal evolutions to generate the maps. In this work, we propose a novel “free-running” (non-ECG triggered) cMRF framework for simultaneous myocardial T₁ and T₂ mapping and cardiac Cine imaging in a single scan. Free-running cMRF is based on a transient state bSSFP acquisition with tiny golden angle radial readouts, varying flip angle and multiple adiabatic inversion pulses. The acquired data is retrospectively gated into several cardiac phases, which are reconstructed with an approach that combines parallel imaging, low rank modelling and patch-based high-order tensor regularization. Free-running cMRF was evaluated in a standardized phantom and ten healthy subjects. Comparison with reference spin-echo, MOLLI, SASHA, T₂-GRASE and Cine was performed.ResultsT₁ and T₂ values obtained with the proposed approach were in good agreement with reference phantom values (ICC(A,1) > 0.99). Reported values for myocardium septum T₁ were 1043 ± 48 ms, 1150 ± 100 ms and 1160 ± 79 ms for MOLLI, SASHA and free-running cMRF respectively and for T₂ of 51.7 ± 4.1 ms and 44.6 ± 4.1 ms for T₂-GRASE and free-running cMRF respectively. Good agreement was observed between free-running cMRF and conventional Cine 2D ejection fraction (bias = −0.83%).ConclusionThe proposed free-running cardiac MRF approach allows for simultaneous assessment of myocardial T₁ and T₂ and Cine imaging in a single scan.     

Motion corrected DWI with integrated T2-mapping for simultaneous estimation of ADC,T2-relaxation and perfusion in prostate cancer

ObjectiveMultiparametric magnetic resonance imaging (MRI) and PI-RADS (Prostate Imaging – Reporting and Data System) has become the standard to determine a probability score for a lesion being a clinically significant prostate cancer. T2-weighted and diffusion-weighted imaging (DWI) are essential in PI-RADS, depending partly on visual assessment of signal intensity, while dynamic-contrast enhanced imaging is less important. To decrease inter-rater variability and further standardize image evaluation, complementary objective measures are in need.MethodsWe here demonstrate a sequence enabling simultaneous quantification of apparent diffusion coefficient (ADC) and T2-relaxation, as well as calculation of the perfusion fraction f from low b-value intravoxel incoherent motion data. Expandable wait pulses were added to a FOCUS DW SE-EPI sequence, allowing the effective echo time to change at run time. To calculate both ADC and f, b-values 200 s/mm² and 600 s/mm² were chosen, and for T2-estimation 6 echo times between 64.9 ms and 114.9 ms were used.ResultsThree patients with prostate cancer were examined and all had significantly decreased ADC and T2-values, while f was significantly increased in 2 of 3 tumors. T2 maps obtained in phantom measurements and in a healthy volunteer were compared to T2 maps from a SE sequence with consecutive scans, showing good agreement. In addition, a motion correction procedure was implemented to reduce the effects of prostate motion, which improved T2-estimation.ConclusionsThis sequence could potentially enable more objective tumor grading, and decrease the inter-rater variability in the PI-RADS classification.     

Simultaneous quantification of SPIO and gadolinium contrast agents using MR fingerprinting

PurposeDevelop a magnetic resonance fingerprinting (MRF) methodology with R₂¹ quantification, intended for use with simultaneous contrast agent concentration mapping, particularly gadolinium (Gd) and iron labelled CD8+ T cells.MethodsVariable-density spiral SSFP MRF was used, modified to allow variable TE, and with an exp.(−TE·R₂¹) dictionary modulation. In vitro phantoms containing SPIO labelled cells and/or gadolinium were used to validate parameter maps, probe undersampling capacity, and verify dual quantification capabilities. A C57BL/6 mouse was imaged using MRF to demonstrate acceptable in vivo resolution and signal at 8× undersampling necessary for a 25-min scan.ResultsStrong agreement was found between conventional and MRF-derived values for R₁, R₂, and R₂¹. Expanded MRF allowed quantification of iron-loaded CD8+ T cells. Results were robust to 8× undersampling and enabled recreation of relaxation profiles for both a Gd agent and iron labelled cells simultaneously. In vivo data demonstrated sufficient SNR in undersampled data for parameter mapping to visualise key features.ConclusionMRF can be expanded to include R₁, R₂, and R₂¹ mapping required for simultaneous quantification of gadolinium and SPIO in vitro, allowing for potential implementation of a variety of future in vivo studies using dual MR contrast agents, including molecular imaging of labelled cells.     

Quantifying cortical bone free water using short echo time (STE-MRI) at 1.5 T

PurposeThe purpose of our study was to use Dual-TR STE-MR protocol as a clinical tool for cortical bone free water quantification at 1.5 T and validate it by comparing the obtained results (MR-derived results) with dehydration results.MethodsHuman studies were compliant with HIPPA and were approved by the institutional review board. Short Echo Time (STE) MR imaging with different Repetition Times (TRs) was used for quantification of cortical bone free water T₁ (T₁free) and concentration (ρ_free). The proposed strategy was compared with the dehydration technique in seven bovine cortical bone samples. The agreement between the two methods was quantified by using Bland and Altman analysis. Then we applied the technique on a cross-sectional population of thirty healthy volunteers (18F/12M) and examined the association of the biomarkers with age.ResultsThe mean values of ρ_free for bovine cortical bone specimens were quantified as 4.37% and 5.34% by using STE-MR and dehydration techniques, respectively. The Bland and Altman analysis showed good agreement between the two methods along with the suggestion of 0.99% bias between them. Strong correlations were also reported between ρ_free (r² = 0.62) and T_1free and age (r² = 0.8). The reproducibility of the method, evaluated in eight subjects, yielded an intra-class correlation of 0.95.ConclusionSTE-MR imaging with dual-TR strategy is a clinical solution for quantifying cortical bone ρ_free and T_1free.     

Automated analysis of immediate reliability of T2 and T2* relaxation times of hip joint cartilage from 3 T MR examinations

BackgroundMagnetic resonance (MR) T2 and T2* mapping sequences allow in vivo quantification of biochemical characteristics within joint cartilage of relevance to clinical assessment of conditions such as hip osteoarthritis (OA).PurposeTo evaluate an automated immediate reliability analysis of T2 and T2* mapping from MR examinations of hip joint cartilage using a bone and cartilage segmentation pipeline based around focused shape modelling.Study typeTechnical validation.Subjects17 asymptomatic volunteers (M: F 7:10, aged 22–47 years, mass 50–90 kg, height 163-189 cm) underwent unilateral hip joint MR examinations. Automated analysis of cartilage T2 and T2* data immediate reliability was evaluated in 9 subjects (M: F 4: 5) for each sequence.Field strength/sequenceA 3 T MR system with a body matrix flex-coil was used to acquire images with the following sequences: T2 weighted 3D-trueFast Imaging with Steady-State Precession (water excitation; 10.18 ms repetition time (TR); 4.3 ms echo time (TE); Voxel Size (VS): 0.625 × 0.625 × 0.65 mm; 160 mm field of view (FOV); Flip Angle (FA): 30 degrees; Pixel Bandwidth (PB): 140 Hz/pixel); a multi-echo spin echo (MESE) T2 mapping sequence (TR/TE: 2080/18–90 ms (5 echoes); VS: 4 × 0.78 × 0.78 mm; FOV: 200 mm; FA: 180 degrees; PB: 230 Hz/pixel) and a MESE T2* mapping sequence (TR/TE: 873/3.82–19.1 ms (5 echoes); VS: 3 × 0.625 × 0.625 mm; FOV: 160 mm; FA: 25 degrees; PB: 250 Hz/pixel).AssessmentAutomated cartilage segmentation and quantitative analysis provided T2 and T2* data from test-retest MR examinations to assess immediate reliability.Statistical testsCoefficient of variation (CV) and intraclass correlations (ICC_{2, 1}) to analyse automated T2 and T2* mapping reliability focusing on the clinically important superior cartilage regions of the hip joint.ResultsComparisons between test-retest T2 and (T2*) data revealed mean CV's of 3.385% (1.25%), mean ICC_{2, 1}′s of 0.871 (0.984) and median mean differences of −1.139ms (+0.195ms).ConclusionThe T2 and T2* times from automated analyses of hip cartilage from test-retest MR examinations had high (T2) and excellent (T2*) immediate reliability.     

Quantitative O imaging towards oxygen consumption study in tumor bearing mice at 7 T

¹⁷O magnetic resonance imaging (MRI) using a conventional pulse sequence was explored as a method of quantitative imaging towards regional oxygen consumption rate measurement for tumor evaluation in mice. At 7 T, fast imaging with steady state (FISP) was the best among gradient echo, fast spin echo and FISP for the purpose. The distribution of natural abundance H₂¹⁷O in mice was visualized under spatial resolution of 2.5 × 2.5 mm² by FISP in 10 min. The signal intensity by FISP showed a linear relationship with ¹⁷O quantity both in phantom and mice. Following the injection of 5% ¹⁷O enriched saline, ¹⁷O re-distribution was monitored in temporal resolution down to 5 sec with an image quality sufficient to distinguish each organ. The image of labeled water produced from inhaled ¹⁷O₂ gas was also obtained. The present method provides quantitative ¹⁷O images under sufficient temporal and spatial resolution for the evaluation of oxygen consumption rate in each organ. Experiments using various model compounds of R-OH type clarified that the signal contribution of body constituents other than water in the present in vivo¹⁷O FISP image was negligible.     

Feasibility of free-breathing T1-weighted 3D radial VIBE for fetal MRI in various anomalies

Rationale and objectivesIn magnetic resonance (MR) fetal imaging, the image quality acquired by the traditional Cartesian-sampled breath-hold T1-weighted (T1W) sequence may be degraded by motion artifacts arising from both mother and fetus. The radial VIBE sequence is reported to be a viable alternative to conventional Cartesian acquisition for both pediatric and adult MR, yielding better image quality. This study evaluated the role of radial VIBE in fetal MR imaging and compared its image quality and motion artifacts with those of the Cartesian T1W sequence.Materials and methodsWe included 246 pregnant women with 50 lesions on 1.5-T MR imaging. Image quality and lesion conspicuity were evaluated by two radiologists, blinded to the acquisition schemes used, using a five-point scale, where a higher score indicated a better trajectory method. Mixed-model analysis of variance and interobserver variability assessment were performed.ResultsThe radial VIBE sequence showed a significantly better performance than conventional T1W imaging in the head and neck, fetal body, and placenta region: 3.92 ± 0.88 vs 3 ± 0.74, p < 0.001, 3.8 ± 0.94 vs 3.15 ± 0.87, p < 0.001, and 4.17 ± 0.63 vs 3.12 ± 0.72, p < 0.001, respectively. Additionally, fewer motion artifacts were observed in all regions with the radial VIBE sequence (p < 0.01). Of 50 lesions, 49 presented better lesion conspicuity on radial VIBE images than on T1W images (4.34 ± 0.91 vs 3.48 ± 1.46, p < 0.001).ConclusionFor fetal imaging, the radial VIBE sequences yielded better image quality and lesion conspicuity, with fewer motion artifacts, than conventional breath-hold Cartesian-sampled T1W sequences.     

The prognostic value of proton magnetic resonance spectroscopy in term newborns treated with therapeutic hypothermia following asphyxia

ObjectiveThe purpose of this study was to correlate brain metabolism assessed shortly after therapeutic hyperthermia by ¹H magnetic resonance spectroscopy (MRS), with neurodevelopmental outcome.MethodsAt the age of 6.0 ± 1.8 days, brain metabolites of 35 term asphyxiated newborns, treated with therapeutic hypothermia, were quantified by multivoxel proton MRS of a volume cranial to the corpus callosum, containing both gray and white matter. At the age of 30 months the Bayley Scale of Infant Development-III was performed.ResultsInfants that died had lower gray matter NAA levels than infants that survived (P = 0.005). In surviving infants (28 of 35) there was a trend of negative correlation between gray matter choline levels and gross motor outcome (r = − 0.45). In the white matter, choline correlated negatively with fine motor skills (r = − 0.40), and creatine positively with gross motor skills (r = 0.58, P = 0.02). There was no relationship between lactate levels and outcome.ConclusionMRS of asphyxiated neonates treated by therapeutic hypothermia can serve as predictor of outcome. Unlike previously reported associations in untreated asphyxiates, lactate levels had no relationship with outcome, which indicates that one of the working mechanisms of therapeutic hypothermia is reduction of the metabolic rate.     

Combined modified-Dixon and PROPELLER method with low refocusing flip angle for contrast-enhanced fat-suppressed T1-weighted MRI: A prospective cross-sectional study

PurposeTo propose the combined modified-Dixon and PROPELLER sequence with low refocusing flip angle (RFA) and investigate whether this sequence can acquire clinical contrast-enhanced (CE), fat-suppressed T1-weighted (T1W) images of the head and neck.MethodsThe optimal RFA for T1W imaging was investigated in the brain of a healthy volunteer. The motion artifacts, water–fat separation error, contrast ratio (CR), and comprehensive quality were evaluated through comparison with a standard Cartesian modified-Dixon sequence in 50 patients. Two radiologists independently scored motion artifacts and water–fat separation error using a 4-point scale (1, unacceptable; 4, excellent) and comprehensive quality using a 5-point scale (1, substantially inferior; 5, substantially superior). The CR between CE lesions and non-CE muscle was calculated.ResultsThe optimal RFA of 40° was determined. In the motion artifact assessment, ratings of 3 or 4 points were assigned to 83% (observer-1, 42/50; observer-2, 41/50) and 99% (50/50; 49/50) of cases for the standard and proposed sequences, respectively (p < 0.001; p < 0.001). For the water–fat separation error assessment, ratings of 3 or 4 points were assigned to 100% (50/50; 50/50) and 97% (48/50; 49/50) of cases, respectively (p < 0.001; p = 0.02). In comprehensive evaluation, the proposed sequence was equal, slightly superior, or substantially superior to the standard sequence in 85% (39/50; 46/50). The CR was significantly higher with the proposed sequence [2.27 (1.99–2.97) vs. 2.08 (1.88–2.42), p < 0.001].ConclusionThe proposed sequence acquired stable fat-suppressed CE T1W images without motion artifacts and yielded superior overall image quality compared with the standard sequence.