Chemical Shift Artifact Research Articles

More accurate trabecular bone imaging using UTE MRI at the resonance frequency of fat

High-resolution magnetic resonance imaging (HR-MRI) has been increasingly used to assess the trabecular bone structure. High susceptibility at the marrow/bone interface may significantly reduce the marrow's apparent transverse relaxation time (T2*), overestimating trabecular bone thickness. Ultrashort echo time MRI (UTE-MRI) can minimize the signal loss caused by susceptibility-induced T2* shortening. However, UTE-MRI is sensitive to chemical shift artifacts, which manifest as spatial blurring and ringing artifacts partially due to non-Cartesian sampling. In this study, we proposed UTE-MRI at the resonance frequency of fat to minimize marrow-related chemical shift artifacts and the overestimation of trabecular thickness. Cubes of trabecular bone from six donors (75 ± 4 years old) were scanned using a 3 T clinical scanner at the resonance frequencies of fat and water, respectively, using 3D UTE sequences with five TEs (0.032, 1.1, 2.2, 3.3, and 4.4 ms) and a clinical 3D gradient echo (GRE) sequence at 0.2 × 0.2 × 0.4 mm3 voxel size. Trabecular bone thickness was measured in 30 regions of interest (ROIs) per sample. MRI results were compared with thicknesses obtained from micro-computed tomography (μCT) at 50 μm3 voxel size. Linear regression models were used to calculate the coefficient of determination between MRI- and μCT-based trabecular thickness. All MRI-based trabecular thicknesses showed significant correlations with μCT measurements. The correlations were higher (examined with paired Student's t-test, P < 0.01) for 3D UTE images performed at the fat frequency (R2 = 0.59–0.74, P < 0.01) than those at the water frequency (R2 = 0.18–0.52, P < 0.01) and clinical GRE images (R2 = 0.39–0.47, P < 0.01). Significantly reduced correlations were observed with longer TEs. This study highlighted the feasibility of UTE-MRI at the fat frequency for a more accurate assessment of trabecular bone thickness.

Current MR imaging of cartilage in the context of knee osteoarthritis (part1) : Principles and sequences

Magnetic resonance imaging (MRI) is the clinical method of choice for cartilage imaging in the context of degenerative and nondegenerative joint diseases. The MRI-based definitions of osteoarthritis rely on the detection of osteophytes, cartilage pathologies, bone marrow edema and meniscal lesions but currently ascientific consensus is lacking. In the clinical routine proton density-weighted, fat-suppressed 2D turbo spin echo sequences with echo times of 30-40 ms are predominantly used, which are sufficiently sensitive and specific for the assessment of cartilage. The additionally acquired T1-weighted sequences are primarily used for evaluating other intra-articular and periarticular structures. Diagnostically relevant artifacts include magic angle and chemical shift artifacts, which can lead to artificial signal enhancement in cartilage or incorrect representations of the subchondral lamina and its thickness. Although scientifically validated, high-resolution 3D gradient echo sequences (for cartilage segmentation) and compositional MR sequences (for quantification of physical tissue parameters) are currently reserved for scientific research questions. The future integration of artificial intelligence techniques in areas such as image reconstruction (to reduce scan times while maintaining image quality), image analysis (for automated identification of cartilage defects), and image postprocessing (for automated segmentation of cartilage in terms of volume and thickness) will significantly improve the diagnostic workflow and advance the field further.

Bladder involvement in placenta accreta spectrum disorder with placenta previa: MRI findings and outcomes correlation

BackgroundPlacental accreta spectrum (PAS) disorder with bladder involvement can be associated with maternal and neonatal morbidity. Magnetic resonance imaging (MRI) may provide accurate preoperative diagnoses. ObjectiveThis study had 2 aims: to retrospectively review the MRI findings for bladder involvement in PAS with placental previa and to correlate bladder involvement with maternal and neonatal outcomes. Materials and methodsMRI images of 48 patients with severe PAS (increta and percreta) with placenta previa/low-lying placenta were evaluated by 2 experienced radiologists blinded to the final diagnoses. Nine MRI findings and stepwise logistic regression analysis were assessed to identify predictive MRI findings for bladder involvement. The correlations between PAS patients with bladder involvement and clinical outcomes were analyzed using Fisher’s exact test. ResultsOf the 48 patients, 27 did not have bladder involvement, while 21 did. Logistic regression analysis identified 2 predictive MRI features for bladder involvement. They were abnormal vascularization (OR,6.94; 95 %CI,1.05–45.75) and loss of the chemical shift line at the uterovesical interface (OR, 4.41; 95 %CI, 0.63–30.98). The sensitivity and specificity of the combined MRI features were 38.1 % and 100 %, respectively (p = 0.001). A significant correlation was found between bladder involvement and massive blood loss during surgery (p = 0.022). ConclusionsPAS with bladder involvement was significantly correlated with massive surgical blood loss. Prenatally, the disorder was predicted with high specificity by the combination of loss of chemical shift artifacts in the steady-state free precession sequence and abnormal vascularization at the uterovesical interface on MRI.

A signature of structural MRI features at 3 Tesla allows an accurate characterization of orbital cavernous venous malformation.

To differentiate OCVM from other orbital lesions using structural MRI. This IRB-approved a historical-prospective cohort single-center analysis of a prospective cohort that included consecutive adult patients presenting with an orbital lesion undergoing a 3T MRI before surgery from December 2015 to May 2021. Two readers blinded to all data read all MRIs assessing structural MRI characteristics. A univariate analysis followed by a stepwise multivariate analysis identified structural MRI features showing the highest sensitivity and specificity when diagnosing OCVM. One hundred ninety-one patients with 30/191 (16%) OCVM and 161/191 (84%) other orbital lesions were included. OCVM were significantly more likely to present with a higher signal intensity than that of the cortex on T2WI: 26/29 (89.7%) versus 28/160 (17.5%), p < 0.001, or with a chemical shift artifact (CSA): 26/29 (89.7%) versus 16/155 (10.3%), p < 0.001, or to present with a single starting point of enhancement, as compared to other orbital lesions: 18/29 (62.1%) versus 4/159 (2.5%), p = 0.001. The step-wise analysis identified 2 signatures increasing performances. Signature 1 combined a higher signal intensity than that of the cortex on T2WI and a CSA. Signature 2 included these two features and the presence of a single starting point of enhancement. Sensitivity, specificity, and accuracy were 0.83, 0.94, and 0.92 for signature 1 and 0.97, 0.93, and 0.93 for signature 2, respectively. Structural MRI yields high sensitivity and specificity when diagnosing OCVM. • Structural MRI shows high sensitivity and specificity when diagnosing orbital cavernous venous malformation. • We identified two signatures combining structural MRI features which might be used easily in routine clinical practice. • The combination of higher signal intensity of the lesion as compared to the cortex on T2WI and of a chemical shift artifact yields a sensitivity and specificity of 0.83 and 0.94 for the diagnosis of orbital cavernous venous malformation, respectively.

Clinical practice guidelines for endometriosis in Japan (The 3rd edition).

Clinical practice guidelines for endometriosis in Japan (The 3rd edition).

Interpretation of Cartilage Damage at Routine Clinical MRI: How to Match Arthroscopic Findings.

This review is intended to aid in the interpretation of damage to the articular cartilage at routine clinical MRI to improve clinical management. Relevant facets of the histologic and biochemical characteristics and clinical management of cartilage are discussed, as is MRI physics. Characterization of damage to the articular cartilage with MRI demands a detailed understanding of the normal and damaged appearance of the osteochondral unit in the context of different sequence parameters. Understanding the location of the subchondral bone plate is key to determining the depth of the cartilage lesion. Defining the bone plate at MRI is challenging because of the anisotropic fibrous organization of articular cartilage, which is susceptible to the "magic angle" phenomenon and chemical shift artifacts at the interface with the fat-containing medullary cavity. These artifacts may cause overestimation of the thickness of the subchondral bone plate and, therefore, overestimation of the depth of a cartilage lesion. In areas of normal cartilage morphology, isolated hyperintense and hypointense lesions often represent degeneration of cartilage at arthroscopy. Changes in the subchondral bone marrow at MRI also increase the likelihood that cartilage damage will be visualized at arthroscopy, even when a morphologic lesion cannot be resolved, and larger subchondral lesions are associated with higher grades at arthroscopy. The clinical significance of other secondary features of cartilage damage are also reviewed, including osteophytes, intra-articular bodies, and synovitis. Online supplemental material is available for this article. Work of the U.S. Government published under an exclusive license with the RSNA.

Reynolds Stress Tensor Measurements In A Shear-Thinning Fluid Using Magnetic Resonance Velocimetry: A Parameter Study With The FDA Benchmark Nozzle

Compared to refractive index matching in laser optical measurement techniques, magnetic resonance velocimetry (MRV) is assumed to be comparatively robust to changes in fluid composition when the principal components of the mixture provide a measurable MRI signal. In previous studies, MRV measurements were performed with different mixtures of water and glycerol, and no significant errors arose regardless of the mixing ratio. MRV is therefore proposed as a promising method for providing experimental data for a wide range of fluids that may also replicate different viscosity models. However, MRV can suffer from so-called chemical shift artifacts caused by slight variations in resonance frequency of differently bound hydrogen protons. These effects may arise when additional components are added to the flow medium. In this study, the ability of MRV to measure different flow mixtures is investigated. The long-term goal is to provide comprehensive experimental validation data for viscosity models in numerical simulations. Measurements were performed in stationary fluid samples as well as in flow measurements at the sudden expansion of the FDA benchmark nozzel. It is shown that MRV can provide reliable results for a Newtonian fluid and a shear-thinning fluid.

Automatic Artifact Detection Algorithm in Fetal MRI.

Fetal MR imaging is subject to artifacts including motion, chemical shift, and radiofrequency artifacts. Currently, such artifacts are detected by the MRI operator, a process which is subjective, time consuming, and prone to errors. We propose a novel algorithm, RISE-Net, that can consistently, automatically, and objectively detect artifacts in 3D fetal MRI. It makes use of a CNN ensemble approach where the first CNN aims to identify and classify any artifacts in the image, and the second CNN uses regression to determine the severity of the detected artifacts. The main mechanism in RISE-Net is the stacked Residual, Inception, Squeeze and Excitation (RISE) blocks. This classification network achieved an accuracy of 90.34% and a F1 score of 90.39% and outperformed other state-of-the-art architectures, such as VGG-16, Inception, ResNet-50, ReNet-Inception, SE-ResNet, and SE-Inception. The severity regression network had an MSE of 0.083 across all classes. The presented algorithm facilitates rapid and accurate fetal MRI quality assurance that can be implemented into clinical use.

ORYX‐MRSI: A fully‐automated open‐source software for proton magnetic resonance spectroscopic imaging data analysis

AbstractProton magnetic resonance spectroscopic imaging (1H‐MRSI) provides a noninvasive, spatially resolved evaluation of brain metabolism. However, there are some limitations of 1H‐MRSI preventing its wider use in the clinics, including the spectral quality issues, partial volume effect and chemical shift artifact. Additionally, it is necessary to create metabolite maps for analyzing spectral data along with other MRI modalities. In this study, a MATLAB‐based open‐source data analysis software for three‐dimensional 1H‐MRSI, called Oryx‐MRSI, which includes modules for visualization of raw 1H‐MRSI data and LCModel outputs, chemical shift correction, tissue fraction calculation, metabolite map production, and registration onto standard MNI152 brain atlas while providing automatic spectral quality control, is presented. Oryx‐MRSI implements region of interest analysis at brain parcellations defined on MNI152 brain atlas. All generated metabolite maps are stored in NIfTI format. Oryx‐MRSI is publicly available at https://github.com/Computational-Imaging-LAB/Oryx-MRSI along with six example datasets.

Moving silicone oil particles in the ventricle: a case report and updated review

BackgroundThe movement of intraventricular silicone oil observed in the supine position is extremely rare. Herein, we describe a patient who presented with dynamically moving silicone oil particles in the ventricle when changing position and provide an updated review of this phenomenon.Case presentationWe report a case of a 70-year-old woman who presented with intraventricular hyperdensities that were occasionally found on brain computed tomography (CT). Initial nonenhanced brain CT demonstrated nondependent hyperdensities in the bilateral anterior horns of the lateral ventricles, the third ventricle, and the right suprasellar cistern, mimicking an intraventricular hemorrhage. Further brain magnetic resonance imaging (MRI) in the supine position revealed abnormal signals in the bilateral anterior horns of the lateral ventricles, the posterior horn of the right lateral ventricle, the third ventricle, the right suprasellar cistern, and the bilateral eyeballs, with isosignal intensities surrounded by low-signal chemical shift artifacts on T1-weighted imaging and variable signals (hypo- or hyperintensity) on T2-weighted imaging. The lesion in the anterior horn of the right ventricle largely moved to the posterior horn of the ipsilateral ventricle. The final craniocervical CT angiography showed that the lesion in the posterior horn had moved back to the anterior horn of the right lateral ventricle. These features were consistent with intraventricular silicone oil migration. The final spinal MRI did not demonstrate a migration of silicone oil into the spinal subarachnoid space.Discussion and conclusionsThis case report describes a dynamic process of silicone oil displacement in the supine position and provides a comprehensive imaging presentation. The moving pattern and a characteristic chemical shift artifact on MRI are key to the diagnosis and may help prevent unnecessary examinations or intervention.

3.0-Tesla MRI of normal canine adrenal glands.

Adrenal disease is a common problem in dogs and MRI is increasingly being used as an adjunctive diagnostic test. To date, the MRI features of normal canine adrenal glands have only been reported using 1.5-Tesla (T) MRI. The aims of this prospective, methods-comparison, exploratory study were to evaluate the effects of pulse sequence on the appearance of normal canine adrenal glands using 3 Tesla MRI. Six research beagle dogs were sampled and the following pulse sequences were acquired for each: (1) T2-weighted images using two-dimensional (2D) turbo spin-echo (TSE), single-shot spin-echo (SSTSE), and three-dimensional (3D) TSE, (2) T1-weighted images using 2D TSE, 3D TSE, and 3D turbo field echo sequences, (3) post-contrast T1-weighted images, and (4) chemical shift imaging. The signal-to-noise ratio and contrast-to-noise ratio were measured for each dog and each pulse sequence. The signal intensity, clarity of the contour, distinction of the corticomedullary junction, degrees of motion, partial volume, and chemical shift artifact, and homogeneity of the contrast enhancement were evaluated qualitatively. In all sequences, except for chemical shift imaging, the adrenal glands were visualized in both planes with successful control of motion artifacts by manual ventilation. The adrenal contour was considered to be most clearly visualized with 2D TSE. Adrenal images were acquired within the shortest time using SSTSE although the contour was less clearly visualized than with TSE. Findings from this study in normal dogs can serve as background for further 3.0-T MRI studies of dogs with adrenal disease.

Right adrenal vein identification using unenhanced magnetic resonance imaging.

Unenhanced magnetic resonance imaging (MRI) is known to be useful in characterizing adrenal adenomas through the implementation of in-phase (IPI) and opposed-phase imaging (OPI) based on chemical shift artifacts. However, whether unenhanced MRI can contribute to the identification of right adrenal vein (RAV) remains unclear. The aim of this study was to evaluate the feasibility of unenhanced MRI for the identification of RAV. This retrospective study reviewed 30 patients (16 men; median age 60 years; range 34-76 years) who underwent MRI and subsequent adrenal venous sampling (AVS). Chemical shift MRI was acquired using echo times of 2.3 ms (OPI) and 4.6 ms (IPI) with a slice thickness of 3 mm and a gap of 1 mm. T2-weighted imaging (T2WI) was also performed. Identification of RAVs was performed by 2 independent radiologists. Inter-observer agreement on a 3-point rating scale was evaluated using κ statistics. The identification rate of RAVs was compared between OPI, IPI, and T2WI using McNemar's test. Good inter-observer agreement was found for the OPI (κ = 0.744), whereas fair agreement was obtained for both other sequences (IPI: κ = 0.375; T2WI: 0.348). For both raters, the identification rate of RAVs was higher with OPI (36/60; 60.0%) than with other sequences (IPI: 16/60, 26.7%; T2WI: 9/60, 15.0%; p < 0.05, each). OPI may play a screening role in the identification of RAVs preceding AVS, which could reduce the required radiation exposure and doses of contrast agent.

Broadband selective excitation radiofrequency pulses for optimized localization in vivo.

The aim of the study is to optimize the performance of localized 1 H MRS sequences at 3T, using the entire spin system of N-acetyl aspartate (NAA) as an example of the large chemical shift spread of all the metabolites routinely detected in vivo, including the amide region. We specifically focus on the design of the suitable broadband excitation radiofrequency (RF) pulses to minimize chemical shift artifacts. The performance of the excitation and refocusing pulse shapes is evaluated with respect to NAA localization. Two new excitation RF pulses are developed to achieve optimized performance in the brain using single-voxel 1 H MRS at 3T. Numerical simulations and in vivo experiments are carried out to demonstrate the performance of the RF pulses. New excitation RF pulses with the same B1 requirements but larger excitation bandwidth (up to a factor of 2) are shown to significantly reduce localization artifacts. The large frequency spread of the entire NAA spin system necessitates the use of broadband excitation and refocusing pulses for MRS at 3T. To minimize chemical shift artifacts of metabolic compounds with spins in the amide area (>5 ppm) at 3T it is important to use broadband excitation and refocusing pulses.

Quantitative susceptibility mapping of the head-and-neck using SMURF fat-water imaging with chemical shift and relaxation rate corrections.

PurposeTo address the challenges posed by fat-water chemical shift artifacts and relaxation rate discrepancies to quantitative susceptibility mapping (QSM) outside the brain, and to generate accurate susceptibility maps of the head-and-neck at 3 and 7 Tesla.MethodsSimultaneous Multiple Resonance Frequency (SMURF) imaging was extended to 7 Tesla and used to acquire head-and-neck gradient echo images at both 3 and 7 Tesla. Separated fat and water images were corrected for Type 1 (displacement) and Type 2 (phase discrepancy) chemical shift artefacts, and for the bias resulting from differences in T1 and relaxation rates, recombined and used as the basis for QSM. A novel phase signal-based masking approach was used to generate head-and-neck masks.ResultsSMURF generated well-separated fat and water images of the head-and-neck. Corrections for chemical shift artefacts and relaxation rate differences removed overestimation of the susceptibility values, blurring in the susceptibility maps, and the disproportionate influence of fat in mixed voxels. The resulting susceptibility maps showed high correspondence between the paramagnetic areas and the locations of fatty tissues and the susceptibility estimates were similar to literature values. The proposed masking approach was shown to provide a simple means of generating head-and-neck masks.ConclusionCorrections for Type 1 and Type 2 chemical shift artefacts and for fat-water relaxation rate differences, mainly in T1, were shown to be required for accurate susceptibility mapping of fatty-body regions. SMURF made it possible to apply these corrections and generate high-quality susceptibility maps of the entire head-and-neck at both 3 and 7 Tesla.

Fat-containing pelvic lesions in females.

Pelvic tumors are common in females and have a broad differential diagnosis. The clinical management of pelvic tumors varies widely-from observation to surgical resection-and imaging plays a pivotal role in diagnosis and clinical decision-making in these cases. In particular, imaging can help determine the organ of origin and tissue content of these tumors, which are the most important steps to narrowing the differential diagnosis. Fat has a characteristic appearance and is often easily identified on ultrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI). The amount and distribution of intralesional fat varies in different types of tumors. Macroscopic intralesional fat is often easily recognized by its hyperechoic appearance on US and low attenuation on CT similar to subcutaneous fat. On MRI, macroscopic fat is hyperintense on T1-weighted (T1W) images, with characteristic signal loss on fat-saturated sequences and India-ink artifact on opposed-phase T1W images. Macroscopic fat is the hallmark of teratomas, which are the most common ovarian neoplasms. Uterine lipoleiomyomas, peritoneal loose bodies, intraperitoneal and extraperitoneal primary lipomatous tumors such as lipoma and liposarcomas, and extra-adrenal myelolipomas are other pelvic masses distinguished by the presence of macroscopic fat. However, the imaging diagnosis of pelvic masses containing minimal or microscopic fat, such as immature ovarian teratomas, steroid cell ovarian neoplasms, and extramedullary hematopoiesis, can present a diagnostic challenge owing to their nonspecific appearance on US or CT. Obtaining MRI with in-phase and opposed-phase dual-echo T1W sequences and depicting chemical shift artifacts can be helpful in distinguishing these lesions.

Fat-suppressed T2 mapping of human knee femoral articular cartilage: comparison with conventional T2 mapping

BackgroundThere is paucity of studies applying fat suppressed (FS) technique to T2 mapping to overcome chemical shift artifacts. The purpose of the study is to difference between FS T2 and conventional T2 mapping and reproducibility of FS T2 mapping in the femoral articular cartilage.MethodsEighteen patients who had normal-looking femoral cartilage and underwent knee MRI with conventional T2 and FS T2 mapping were included. T2 values of each mapping were measured by two readers independently from nine regions in the medial femoral condyle (MFC) and lateral femoral condyle (LFC). Each anatomical region was divided by lines at ± 10°, 30°, 50°, 70°, 90°, and 110°. Comparisons of T2 values between conventional and FS T2 mapping were statistically analyzed. The T2 values between FS and conventional T2 mapping in the anterior, central and posterior femoral condyles were compared.ResultsThe overall femoral condyle T2 values from the FS T2 map were significantly lower than those from the conventional T2 map (48.5ms vs. 51.0ms, p < 0.001). The differences in the T2 values between the two maps were significantly different among the three divisions of the LFC (p = 0.009) and MFC (p = 0.031). The intra-class correlation coefficients indicated higher agreement in the FS T2 map than in the conventional T2 map (0.943 vs. 0.872).ConclusionsThe T2 values of knee femoral cartilage are significantly lower on FS T2 mapping than on conventional T2 mapping. FS T2 mapping is a more reproducible method for evaluating knee femoral cartilage.

MRI chemical shift artifact produced by center-out radial sampling of k-space: a potential pitfall in clinical diagnosis.

Center-out radial sampling of k-space in magnetic resonance imaging employs a different direction for each readout. Off-resonance artifacts (including those produced by chemical shift between water and fat) found with this type of sampling are usually described as blurring, however more specific characterization of these artifacts can be ascertained from the fact that their point spread function is ring-shaped. This produces effects that differ from those seen with Cartesian sampling of k-space. Experiments were designed to demonstrate the origin of these artifacts and a volunteer was imaged to show them. Two phantoms containing oil in a syringe and an annulus of oil surrounded by water were scanned with a range of bandwidths from 62.5 down to 4 kHz. In a human volunteer, head, pelvis and spine images were obtained with bandwidths of 62.5 and 4 kHz. The two phantoms showed displacement of the oil signal away from the center into the region of the surrounding water. The effect increased as the bandwidth was decreased. In the head of the volunteer, signal from fat in red bone marrow in the skull was displaced centrally and peripherally relative to water within the marrow, and appeared in the region between the skull and the brain, as well as in the surrounding scalp. Displacements of the former type simulated subdural hematomas. Displacement of perivesical fat signal centrally over the wall of the bladder simulated bladder tumor, and displacement of fat signal from red bone marrow in the lumbar spine to the intervertebral discs simulated their cartilaginous endplates. Center-out radial artifacts are important to recognize on clinical images since they may mimic anatomy and simulate pathology. The article shows how these artifacts originate, includes examples, and describes how the artifacts differ from Cartesian chemical shift artifacts.

Controlling Through-Slice Chemical-Shift Artifacts for Improved Non-Fat-Suppressed Musculoskeletal Turbo-Spin-Echo Magnetic Resonance Imaging at 7 T.

Through-slice chemical shift artifacts in state-of-the-art turbo-spin-echo (TSE) images can be significantly more severe at 7 T than at lower field strengths. In musculoskeletal applications, these artifacts appear similar to bone fractures or neoplastic bone marrow disease. The objective of this work was to explore and reduce through-slice chemical shift artifacts in 2-dimensional (2D) TSE imaging at 7 T. This prospective study was approved by the local ethics board. The bandwidths of the excitation and refocusing radiofrequency (RF) pulses of a prototype 2D TSE sequence were individually modified and their effect on the slice profiles and relative slice locations of water and fat spins was assessed in an oil-water phantom. Based on these results, it was hypothesized that the combination of matched and increased excitation and refocusing RF pulse bandwidths ("MIB") of 1500 Hz would enable 2D TSE imaging with significantly reduced chemical shift artifacts compared with a state-of-the-art sequence with unmatched and moderate RF pulse bandwidths ("UMB") of 1095 and 682 Hz.A series of T1-weighted sagittal knee examinations in 10 healthy human subjects were acquired using the MIB and UMB sequences and independently evaluated by 2 radiologists. They measured the width of chemical shift artifacts at 2 standardized locations and graded the perceived negative effect of chemical shift artifacts on image quality in the bones and in the whole gastrocnemius muscle on a 5-point scale. Similar knee, wrist, and foot images were acquired in a single subject. Signal-to-noise ratios in the femoral bone marrow were computed between the UMB and MIB sequences. Phantom measurements confirmed the expected spatial separation of simultaneously affected water and fat slices between 40% and 200% of the prescribed slice thickness for RF pulse bandwidths between 2500 and 500 Hz. Through-slice chemical shift artifacts at the bone-cartilage interface were significantly smaller with MIB than with UMB (location 1: 0.35 ± 0.20 mm vs 1.27 ± 0.27 mm, P < 0.001; location 2: 0.25 ± 0.13 mm vs 1.48 ± 0.46 mm, P < 0.001; intraclass correlation coefficient = 0.98). The negative effect of chemical shift artifacts on image quality was significantly smaller with MIB than with UMB (bone: 2 ± 0 vs 4 ± 1, P < 0.004 [both readers]; muscle: 3 ± 0 vs 2 ± 0, P < 0.004 [both readers]; κ = 0.69). The signal-to-noise ratio of the UMB and MIB sequences was comparable, with a ratio of 99 ± 7%. Images acquired using the UMB sequence displayed numerous artifactual hyperintensities and diffuse, as well as locally severe, fat signal loss in all examined regions, whereas the MIB sequence consistently yielded high image quality with bright T1-weighted fat signal and excellent depiction of fine tissue structures. On 7 T systems, the selection of high and matched RF bandwidths for excitation and refocusing pulses for 2D TSE imaging without fat suppression showed consistently better image quality than state-of-the-art sequences with unmatched lower RF pulse bandwidths.

HYFI: Hybrid filling of the dead-time gap for faster zero echo time imaging.

The aim of this work was to improve the SNR efficiency of zero echo time (ZTE) MRI pulse sequences for faster imaging of short‐T 2 components at large dead‐time gaps. ZTE MRI with hybrid filling (HYFI) is a strategy for retrieving inner k‐space data missed during the dead‐time gaps arising from radio‐frequency excitation and switching in ZTE imaging. It performs hybrid filling of the inner k‐space with a small single‐point‐imaging core surrounded by a stack of shells acquired on radial readouts in an onion‐like fashion. The exposition of this concept is followed by translation into guidelines for parameter choice and implementation details. The imaging properties and performance of HYFI are studied in simulations as well as phantom, in vitro and in vivo imaging, with an emphasis on comparison with the pointwise encoding time reduction with radial acquisition (PETRA) technique. Simulations predict higher SNR efficiency for HYFI compared with PETRA at preserved image quality, with the advantage increasing with the size of the k‐space gap. These results are confirmed by imaging experiments with gap sizes of 25 to 50 Nyquist dwells, in which scan times for similar image quality could be reduced by 25% to 60%. The HYFI technique provides both high SNR efficiency and image quality, thus outperforming previously known ZTE‐based pulse sequences, in particular for large k‐space gaps. Promising applications include direct imaging of ultrashort‐ components, such as the myelin bilayer or collagen, T 2 mapping of ultrafast relaxing signals, and ZTE imaging with reduced chemical shift artifacts.

Very low field 19F MRI of perfluoro-octylbromide: Minimizing chemical shift effects and signal loss due to scalar coupling

19F images have been obtained from perflurooctylbromide (PFOB) at very low magnetic field (50 mT). The small spectral dispersion (in Hz) means that all fluorine nuclei contribute to the signal without chemical shift artifacts or the need for specialized imaging sequences. Turbo spin echo trains with short interpulse intervals and full 180° refocussing pulses suppress scalar coupling, leading to long apparent T2 values and highly efficient data collection. Overall, the detection efficiency of PFOB is very similar that of water in tissue.