Zero Echo Time Imaging Research Articles

Low-rank iterative infilling for zero echo-time (ZTE) imaging.

A new referenceless low-rank reconstruction technique has been introduced to address the issue of missing samples within the Zero Echo Time (ZTE) dead-time gap. The proposed method reformulates the in-filling of the missing samples as an inverse problem subject to low-rank constraints. Its performance and robustness are evaluated through a comparative analysis that combines Monte Carlo computational simulations and data obtained from in vivo experiments. The proposed method is tested for dead-time gaps ranging up to 4.5 Nyquist dwells, across signal-to-noise ratio levels of 5, 10, 15, and 20 dB. Consistently superior performance is observed across all cases compared to algebraic and parallel imaging methods. The speed for convergence decreases exponentially as the dead-time gap expands. The proposed method enables artifact-free reconstruction up to dead-time gap of 4 Nyquist dwells and thereby supports ZTE imaging up to an imaging bandwidth of kHz (assuming transmit and receive switching less than 30 s). It demonstrates superior performance compared to algebraic and parallel imaging methods.

Zero-echo-time sequences in highly inhomogeneous fields.

Zero-echo-time (ZTE) sequences have proven a powerful tool for MRI of ultrashort tissues, but they fail to produce useful images in the presence of strong field inhomogeneities (14 000 ppm). Here we seek a method to correct reconstruction artifacts from non-Cartesian acquisitions in highly inhomogeneous , where the standard double-shot gradient-echo approach to field mapping fails. We present a technique based on magnetic field maps obtained from two geometric distortion-free point-wise (SPRITE) acquisitions. To this end, we employ three scanners with varying field homogeneities. These maps are used for model-based image reconstruction with iterative algebraic techniques (ART). For comparison, the same prior information is fed also to widely used Conjugate Phase (CP) algorithms. Distortions and artifacts coming from severe inhomogeneities, at the level of the encoding gradient, are largely reverted by our method, as opposed to CP reconstructions. This holds even close to the limit where intra-voxel bandwidths (determined by inhomogeneities, up to 1.2 kHz) are comparable to the encoding inter-voxel bandwidth (determined by the gradient fields, 625 Hz in this work). We have benchmarked the performance of a new method for ZTE imaging in highly inhomogeneous magnetic fields. For example, this can be exploited for dental imaging in affordable low-field MRI systems, and can be expanded for arbitrary pulse sequences and extreme magnet geometries, as in, for example, single-sided MRI.

A clinically-recommended MR whole lung imaging protocol using free-breathing 3D isotropic zero echo time sequence

Rationale and objectivesThis study aimed to assess the feasibility and image quality of free-breathing 3D isotropic zero echo time (ZTE) whole-lung imaging and explore a clinically appropriate protocol for MR lung imaging. Materials and methodsThe study was approved by the local ethics committee. A total of thirty healthy volunteers were enrolled in this study from October 2022 to May 2023. Free-breathing pulmonary 3D isotropic ZTE scans were implemented with various acquisition planes and the number of excitations (NEX). ZTE images were evaluated by two radiologists for the overall Image quality and visibility of intrapulmonary structures as well as the signal-to-noise ratio (SNR) of the lung parenchyma. ZTE images with different acquisition parameters were compared. For preliminary clinical visual assessment, three patients with interstitial lung disease underwent both ZTE imaging and computed tomography (CT). ResultsThe overall image quality of the lung in healthy subjects was good to excellent. The visibilities of pulmonary arteries and bronchus were up to the 7th and 5th generation, respectively. The display of lung fissures was poor. The overall image quality, the visibility of the pulmonary artery, and lung fissures in the axial acquisition were better than in the coronal acquisition (P = 0.011, 0.008, 0.010, respectively) but not statistically different from those in the sagittal acquisition (all P > 0.05). ConclusionThe free-breathing pulmonary ZTE is feasible and may serve as an alternative method in chest imaging. Either axial or sagittal ZTE image acquisition would be preferred in clinical practice.

Shoulder Bone Segmentation with DeepLab and U-Net.

Evaluation of 3D bone morphology of the glenohumeral joint is necessary for pre-surgical planning. Zero echo time (ZTE) magnetic resonance imaging (MRI) provides excellent bone contrast and can potentially be used in place of computed tomography. Segmentation of shoulder anatomy, particularly humeral head and acetabulum, is needed for detailed assessment of each anatomy and for pre-surgical preparation. In this study we compared performance of two popular deep learning models based on Google's DeepLab and U-Net to perform automated segmentation on ZTE MRI of human shoulders. Axial ZTE images of normal shoulders (n=31) acquired at 3-Tesla were annotated for training with a DeepLab and 2D U-Net, and the trained model was validated with testing data (n=13). While both models showed visually satisfactory results for segmenting the humeral bone, U-Net slightly over-estimated while DeepLab under-estimated the segmented area compared to the ground truth. Testing accuracy quantified by Dice score was significantly higher (p<0.05) for U-Net (88%) than DeepLab (81%) for the humeral segmentation. We have also implemented the U-Net model onto an MRI console for a push-button DL segmentation processing. Although this is an early work with limitations, our approach has the potential to improve shoulder MR evaluation hindered by manual post-processing and may provide clinical benefit for quickly visualizing bones of the glenohumeral joint.

Evaluation of glenoid morphology and bony Bankart lesion in shoulders with traumatic anterior instability using zero echo time magnetic resonance imaging

BackgroundPreoperative computed tomography (CT) evaluation of bone morphometry aids in determining treatment strategies for shoulder instability. The use of zero echo time (ZTE) sequence in magnetic resonance imaging (MRI), a new bone cortex imaging technique, may help reduce radiation exposure and medical costs. Therefore, this study aimed to evaluate the glenoid morphology and detect the presence of bony Bankart lesion using ZTE MRI in shoulders with anterior instability and compare its diagnostic accuracy with that of CT. MethodsThirty-six patients (36 shoulders) with anterior instability who underwent preoperative CT and MRI examinations between April 2019 and October 2021 were retrospectively analyzed. The percentages of glenoid bone defects on three-dimensional (3D) CT and ZTE images were determined, and the correlation between these percentages was evaluated. The number of cases with bony Bankart lesion on CT and two types of ZTE (3D and CT-like) images was determined, and the diagnostic accuracy of ZTE for detecting bony Bankart lesion was assessed, with CT as the gold standard. Patients with bony Bankart lesion on CT images were divided into two groups based on whether the lesion was detectable on 3D ZTE or CT-like images. The longer diameters of bony Bankart lesion were compared between the groups. ResultsThe median percentage of glenoid bone loss was 12.1% (range, 1.3%–45.9%) and 12.3% (range, 0%–46.6%) on 3D CT and 3D ZTE images, respectively. The Spearman’s rank correlation coefficient was 0.89. Bony Bankart lesion was detected in 18, 13, and eight shoulders of the 36 patients on CT, 3D ZTE, and CT-like images, respectively. The overall diagnostic accuracy of the CT-like and 3D ZTE images for detecting bony Bankart lesion was 86.1% and 72.2%, respectively. A significant difference was observed between the groups with and without bony Bankart lesion on CT-like images in terms of the long diameter of the bone fragments on CT (p < 0.01). ConclusionsZTE MRI demonstrated high reproducibility for the evaluation of glenoid bone defect in shoulders with anterior instability. Although no significant difference in the measurement was observed compared with that on CT, the ability of ZTE MRI to delineate bone Bankart lesion remains limited.

Quantitative 31P magnetic resonance imaging on pathologic rat bones by ZTE at 7T

BackgroundOsteoporosis is characterized by low bone mineral density (BMD), which predisposes individuals to frequent fragility fractures. Quantitative BMD measurements can potentially help distinguish bone pathologies and allow clinicians to provide disease-relieving therapies. Our group has developed non-invasive and non-ionizing magnetic resonance imaging (MRI) techniques to measure bone mineral density quantitatively. Dual-energy X-ray Absorptiometry (DXA) is a clinically approved non-invasive modality to diagnose osteoporosis but has associated disadvantages and limitations. PurposeEvaluate the clinical feasibility of phosphorus (31P) MRI as a non-invasive and non-ionizing medical diagnostic tool to compute bone mineral density to help differentiate between different metabolic bone diseases. Materials and methodsFifteen ex-vivo rat bones in three groups [control, ovariectomized (osteoporosis), and vitamin-D deficient (osteomalacia - hypo-mineralized) were scanned to compute BMD. A double-tuned (1H/31P) transmit-receive single RF coil was custom-designed and in-house-built with a better filling factor and strong radiofrequency (B1) field to acquire solid-state 31P MR images from rat femurs with an optimum signal-to-noise ratio (SNR). Micro-computed tomography (μCT) and gold-standard gravimetric analyses were performed to compare and validate MRI-derived bone mineral densities. ResultsThree-dimensional 31P MR images of rat bones were obtained with a zero-echo-time (ZTE) sequence with 468 μm spatial resolution and 12–17 SNR on a Bruker 7 T Biospec having multinuclear capability. BMD was measured quantitatively on cortical and trabecular bones with a known standard reference. A strong positive correlation (R = 0.99) and a slope close to 1 in phantom measurements indicate that the densities measured by 31P ZTE MRI are close to the physical densities in computing quantitative BMD. The 31P NMR properties (resonance linewidth of 4 kHz and T1 of 67 s) of ex-vivo rat bones were measured, and 31P ZTE imaging parameters were optimized. The BMD results obtained from MRI are in good agreement with μCT and gravimetry results. ConclusionQuantitative measurements of BMD on ex-vivo rat femurs were successfully conducted on a 7 T preclinical scanner. This study suggests that quantitative measurements of BMD are feasible on humans in clinical MRI with suitable hardware, RF coils, and pulse sequences with optimized parameters within an acceptable scan time since human femurs are approximately ten times larger than rat femurs. As MRI provides quantitative in-vivo data, various systemic musculoskeletal conditions can be diagnosed potentially in humans.

Utility of Zero Echo Time MRI for the Diagnosis and Characterization of Ankle Fractures.

Background: Zero echo time (ZTE) imaging is a relatively new magnetic resonance (MR) pulse sequence that provides bone-soft tissue contrast similar to that of computed tomography (CT). Purpose: We sought to (1) determine the accuracy of ZTE MRI for the diagnosis of common ankle fractures and (2) investigate whether ZTE imaging sequences are equivalent to the gold standard of CT for the characterization of fracture fragments. Methods: We conducted a prospective case series of 54 patients with acute ankle trauma, in whom ZTE MRI was performed, followed by surgical reduction. Fractures on the ZTE sequence were correlated with the operative report as the reference standard. Raw agreement (%) and correlation (κ) were calculated. Selected fracture fragments were measured in 2 dimensions (anterior-posterior and superior-inferior) on corresponding sagittal ZTE and CT images by 3 independent radiologists to determine reliability. Results: The ZTE sequence demonstrated 47 distal fibular, 17 medial malleolar, 24 posterior malleolar, 5 anterior talofibular ligament avulsion, and 4 distal tibial fractures on the 54 cases. Raw agreement with operative findings was 95% (range: 86%-100%) and correlation almost perfect (0.960 [0.926-0.995]). Fragment characterization was accurate and repeatable. Intraobserver and interobserver agreement was excellent. Conclusions: Our case series suggests that the use of the MRI ZTE sequence may provide images with CT-like contrast for characterizing acute ankle fractures.

Comprehensive dose evaluation of a Deep Learning based synthetic Computed Tomography algorithm for pelvic Magnetic Resonance-only radiotherapy

Background and PurposeMagnetic Resonance (MR)-only radiotherapy enables the use of MR without the uncertainty of MR–Computed Tomography (CT) registration. This requires a synthetic CT (sCT) for dose calculations, which can be facilitated by a novel Zero Echo Time (ZTE) sequence where bones are visible and images are acquired in 65 seconds. This study evaluated the dose calculation accuracy for pelvic sites of a ZTE-based Deep Learning sCT algorithm developed by GE Healthcare. Materials and MethodsZTE and CT images were acquired in 56 pelvic radiotherapy patients in the radiotherapy position. A 2D U-net convolutional neural network was trained using pairs of deformably registered CT and ZTE images from 36 patients. In the remaining 20 patients the dosimetric accuracy of the sCT was assessed using cylindrical dummy Planning Target Volumes (PTVs) positioned at four different central axial locations, as well as the clinical treatment plans (for prostate (n = 10), rectum (n = 4) and anus (n = 6) cancers). The sCT was rigidly and deformably registered, the plan recalculated and the doses compared using mean differences and gamma analysis. ResultsMean dose differences to the PTV D98% were ≤ 0.5% for all dummy PTVs and clinical plans (rigid registration). Mean gamma pass rates at 1%/1 mm were 98.0 ± 0.4% (rigid) and 100.0 ± 0.0% (deformable), 96.5 ± 0.8% and 99.8 ± 0.1%, and 95.4 ± 0.6% and 99.4 ± 0.4% for the clinical prostate, rectum and anus plans respectively. ConclusionsA ZTE-based sCT algorithm with high dose accuracy throughout the pelvis has been developed. This suggests the algorithm is sufficiently accurate for MR-only radiotherapy for all pelvic sites.

Low field slice-selective ZTE imaging of ultra-short T_2 tissues based on spin-locking

Magnetic Resonance Imaging of hard biological tissues is very challenging due to small proton abundance and ultra-short T_2 decay times, especially at low magnetic fields, where sample magnetization is weak. While several pulse sequences, such as Ultra-short Echo Time (UTE), Zero Echo Time (ZTE) and SWeep Imaging with Fourier Transformation (SWIFT), have been developed to cope with ultra-short lived MR signals, only the latter two hold promise of imaging tissues with sub-millisecond T_2 times at low fields. All these sequences are intrinsically volumetric, thus 3D, because standard slice selection using a long soft radio-frequency pulse is incompatible with ultra-short lived signals. The exception is UTE, where double half pulses can perform slice selection, although at the cost of doubling the acquisition time. Here we demonstrate that spin-locking is a versatile and robust method for slice selection for ultra-short lived signals, and present three ways of combining this pulse sequence with ZTE imaging of the selected slice. With these tools, we demonstrate slice-selected 2D ex vivo imaging of the hardest tissues in the body at low field (260 mT) within clinically acceptable times.

Optimization of the Grashey View Radiograph for Critical Shoulder Angle Measurement: A Reliability Assessment With Zero Echo Time MRI.

Background:Suboptimal positioning on Grashey view radiographs may limit the prognosticating potential of the critical shoulder angle (CSA) for shoulder disorders.Purpose:To investigate whether radiography optimized according to the latest research is reliable for measuring CSA in comparison with magnetic resonance imaging (MRI) featuring 3-dimensional (3D) zero echo time (ZTE) sequencing, which accentuates the contrast between cortical bone and surrounding soft tissue with high fidelity.Study Design:Cohort study (diagnosis); Level of evidence, 2.Methods:Patients with shoulder pain were prospectively and consecutively enrolled. All patients had Grashey view radiographs as well as 3.0-T MRI scans with isotropic 3D ZTE sequencing. Acceptable positioning on the radiographs was determined using the ratio of the transverse to longitudinal (RTL) diameter of the lateral glenoid outline; radiographs with an RTL ≥0.25 were repeated. Two observers independently measured the CSA on the radiographs and the coronal oblique reformatted ZTE images, the latter including verification of measurement points by cross-referencing against images from other planes. Reliability of measurements between observers and modalities was analyzed with the intraclass correlation coefficient (ICC). The paired-samples t test was used to compare the differences between imaging modalities.Results:Enrolled were 65 patients (35 female and 30 male; mean age, 40.2 years; range, 25-49 years). Radiographs with optimal positioning (RTL < 0.25) were attained after a mean of 1.6 exposures (range, 1-4); the mean RTL was 0.09 (range, 0-0.20). Interobserver agreement of CSA was excellent for radiographs (ICC = 0.91; 95% CI, 0.84-0.94) and good for ZTE MRI scans (ICC = 0.85; 95% CI, 0.71-0.92). Intermodality agreement of CSA between radiographs and ZTE MRI scans was moderate (ICC = 0.66; 95% CI, 0.48-0.73). The CSA was significantly different between an optimal radiograph (30.7° ± 4.3°) and ZTE MRI scan (31.8° ± 3.8) (P = .005). Subgroup analysis revealed no significant differences in CSA measurement between ZTE MRI scans and Grashey view radiographs with an RTL of <0.1 (P = .08).Conclusion:CSA measurement on ZTE MRI scans with anatomic point cross-referencing was significantly different from that on Grashey view radiographs, even with optimal positioning, and radiography may necessitate more than 1 exposure. An RTL of <0.1 ensured reliability of radiographs when other standards of sufficient x-ray exposure were met.

Zero Echo Time Musculoskeletal MRI: Technique, Optimization, Applications, and Pitfalls.

Zero echo time (ZTE) imaging is an MRI technique that produces images similar to those obtained with radiography or CT. In ZTE MRI, the very short T2 signal from the mineralized trabecular bone matrix and especially cortical bone-both of which have a low proton density (PD)-is sampled in a unique sequence setup. Additionally, the PD weighting of the ZTE sequence results in less contrast between soft tissues. Therefore, along with gray-scale inversion from black to white and vice versa, ZTE imaging provides excellent contrast between cortical bone and soft tissues similar to that of radiography and CT. However, despite isotropic or near-isotropic three-dimensional (3D) imaging capabilities of the ZTE sequence, spatial resolution in this technique is still inferior to that of radiography and CT, and 3D volume renderings are currently time-consuming and require postprocessing software that features segmentation and manual contouring. Optimization of ZTE MRI mostly entails adjustments of bandwidth, flip angle, field of view, and image matrix. A wide range of structural abnormalities and disease or healing processes in the musculoskeletal system are well delineated with ZTE MRI, including conditions that involve bone-based morphometric analyses (which aid diagnosis, help prognostication, and guide surgery), impaction, avulsion and stress fractures, loose bodies or erosions in and around joints, soft-tissue calcifications and ossifications, and bone tumors (including treatment response). The pitfalls of ZTE imaging include mimics of foci of calcification or ossification such as intra-articular gas and susceptibility artifacts from surgical materials and hemosiderin deposition, which can be avoided in many instances by cross-referencing images obtained with other MRI sequences. Online supplemental material is available for this article. ©RSNA, 2022.

Detailed bone assessment of the sacroiliac joint in a prospective imaging study: comparison between computed tomography, zero echo time, and black bone magnetic resonance imaging

ObjectivesTo compare the value of zero echo time (ZTE) and gradient echo “black bone” (BB) MRI sequences for bone assessment of the sacroiliac joint (SI) using computed tomography (CT) as the reference standard.Materials and methodsBetween May 2019 and January 2021, 79 patients prospectively underwent clinically indicated 3-T MRI including ZTE and BB imaging. Additionally, all patients underwent a CT scan covering the SI joints within 12 months of the MRI examination. Two blinded readers performed bone assessment by grading each side of each SI joint qualitatively in terms of seven features (osteophytes, subchondral sclerosis, erosions, ankylosis, joint irregularity, joint widening, and gas in the SI joint) using a 4-point Likert scale (0 = no changes–3 = marked changes). Scores were compared between all three imaging modalities.ResultsInterreader agreement was largely good (k values: 0.5–0.83). Except for the feature “gas in SI joint” where ZTE exhibited significantly lower scores than CT (p < 0.001), ZTE and BB showed similar performance relative to CT for all other features (p > 0.52) with inter-modality agreement being substantial to almost perfect (Krippendorff’s alpha coefficients: 0.724–0.983). When combining the data from all features except for gas in the SI joint and when binarizing grading scores, combined sensitivity/specificity was 76.7%/98.6% for ZTE and 80.8%/99.1% for BB, respectively, compared to CT.ConclusionsThe performance of ZTE and BB sequences was comparable to CT for bone assessment of the SI joint. These sequences may potentially serve as an alternative to CT yet without involving exposure to ionizing radiation.

Motion corrected silent ZTE neuroimaging.

PurposeTo develop self‐navigated motion correction for 3D silent zero echo time (ZTE) based neuroimaging and characterize its performance for different types of head motion.MethodsThe proposed method termed MERLIN (Motion Estimation & Retrospective correction Leveraging Interleaved Navigators) achieves self‐navigation by using interleaved 3D phyllotaxis k‐space sampling. Low resolution navigator images are reconstructed continuously throughout the ZTE acquisition using a sliding window and co‐registered in image space relative to a fixed reference position. Rigid body motion corrections are then applied retrospectively to the k‐space trajectory and raw data and reconstructed into a final, high‐resolution ZTE image.ResultsMERLIN demonstrated successful and consistent motion correction for magnetization prepared ZTE images for a range of different instructed motion paradigms. The acoustic noise response of the self‐navigated phyllotaxis trajectory was found to be only slightly above ambient noise levels (<4 dBA).ConclusionSilent ZTE imaging combined with MERLIN addresses two major challenges intrinsic to MRI (i.e., subject motion and acoustic noise) in a synergistic and integrated manner without increase in scan time and thereby forms a versatile and powerful framework for clinical and research MR neuroimaging applications.

Assessment of lung density in pediatric patients using three-dimensional ultrashort echo-time and four-dimensional zero echo-time sequences

PurposeLung magnetic resonance imaging (MRI) using conventional sequences is limited due to strong signal loss by susceptibility effects of aerated lung. Our aim is to assess lung signal intensity in children on ultrashort echo-time (UTE) and zero echo-time (ZTE) sequences. We hypothesize that lung signal intensity can be correlated to lung physical density.Materials and methodsLung MRI was performed in 17 children with morphologically normal lungs (median age: 4.7 years, range 15 days to 17 years). Both lungs were manually segmented in UTE and ZTE images and the average signal intensities were extracted. Lung-to-background signal ratios (LBR) were compared for both sequences and between both patient groups using non-parametric tests and correlation analysis. Anatomical region-of-interest (ROI) analysis was performed for the normal cohort for assessment of the anteroposterior lung gradient.ResultsThere was no significant difference between LBR of normal lungs using UTE and ZTE (p < 0.05). Both sequences revealed a LBR age-dependency with a high negative correlation for UTE (Rs = – 0.77; range 2.98–1.41) and ZTE (Rs = – 0.82; range 2.66–1.38)). Signal-to-noise (SNR) and contrast-to-noise ratios (CNR) were age-dependent for both sequences. SNR was higher for children up to 2 years old with 3D UTE Cones while for the rest it was higher with 4D ZTE. CNR was similar for both sequences. Posterior lung areas exhibited higher signal intensity compared to anterior ones (UTE 9.4% and ZTE 12% higher), both with high correlation coefficients (R2UTE = 0.94, R2ZTE = 0.97).ConclusionThe ZTE sequence can measure signal intensity similarly to UTE in pediatric patients. Both sequences reveal an age- and gravity-dependency of LBR.

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.

Automated 3D MRI rendering of the craniofacial skeleton: using ZTE to drive the segmentation of black bone and FIESTA-C images

PurposeAutomated bone segmentation from MRI datasets would have a profound impact on clinical utility, particularly in the craniofacial skeleton where complex anatomy is coupled with radiosensitive organs. Techniques such as gradient echo black bone (GRE-BB) and short echo time (UTE, ZTE) have shown potential in this quest. The objectives of this study were to ascertain (1) whether the high-contrast of zero echo time (ZTE) could drive segmentation of high-resolution GRE-BB data to enhance 3D-output and (2) if these techniques could be extrapolated to ZTE driven segmentation of a routinely used non bone-specific sequence (FIESTA-C).MethodsEleven adult volunteers underwent 3T MRI examination with sequential acquisition of ZTE, GRE-BB and FIESTA-C imaging. Craniofacial bone segmentation was performed using a fully automated segmentation algorithm. Segmentation was completed individually for GRE-BB and a modified version of the algorithm was subsequently implemented, wherein the bone mask yielded by ZTE segmentation was used to initialise segmentation of GRE-BB. The techniques were subsequently applied to FIESTA-C datasets. The resulting 3D reconstructions were evaluated for areas of unexpected bony defects and discrepancies.ResultsThe automated segmentation algorithm yielded acceptable 3D outputs for all GRE-BB datasets. These were enhanced with the modified algorithm using ZTE as a driver, with improvements in areas of air/bone interface and dense muscular attachments. Comparable results were obtained with ZTE+FIESTA-C.ConclusionAutomated 3D segmentation of the craniofacial skeleton is enhanced through the incorporation of a modified segmentation algorithm utilising ZTE. These techniques are transferrable to FIESTA-C imaging which offers reduced acquisition time and therefore improved clinical utility.

Value of zero echo time MR imaging and CT in diagnosis of bone destructions of bone tumors and tumor-like lesions

Bone tumors and tumor-like lesions require that comprehensive imaging contains soft tissues and bone structures. There are so many kinds of examinations that the patients with bone tumors and tumor-like lesions should be performed. The objective of this study was to demonstrate the clinical applicability of zero echo time (ZTE) magnetic resonance (MR) imaging in osseous bone tumor imaging and evaluate the image quality between computed tomography (CT) and ZTE imaging of bone tumors and tumor-like lesions. Thirty-six patients included 18 male patients (mean age: 35 ± 20 years) and 18 female patients (mean age: 42 ± 20 years) were recruited from a clinical cohort of patients undergoing MR and CT. We presented a novel scanning technique for MR scanners (750 W MR, GE Medical Systems) that could obtain ZTE images. ZTE images were acquired after standard-of-care MR imaging. Quality comparison of osseous findings was performed between ZTE and CT imaging using a five-point grading scale. The differences of image quality between ZTE and CT imaging were investigated using Mann–Whitney test. A significance level of P < 0.05 was used for all statistical analyses. Agreement was assessed between the raters and Weight Kappa statistics were used. Interpretation of agreement was based on published standards: 0.00–0.20 indicated slight; 0.21–0.40, fair; 0.41–0.60, moderate; 0.61–0.80, substantial; and 0.81–1, perfect agreement. There were 13 kinds of bone tumors and tumor-like lesions. The differences of image quality between ZTE (4.47 ± 0.76) and CT (4.97 ± 0.16) imaging were not significant (Z = − 1.83, P = 0.067). It showed good agreement between the two raters (K = 1, K = 0.84) in the bone destruction between CT and ZTE. CT and ZTE-MR had similar displays in lesion’s boundary, periosteal reaction, bone sclerosis, bone destructions, calcifications, and soft tissues. Our results confirm that ZTE-MR imaging provides accurate imaging of bone morphology with CT-like contrast that is not available with the standard MR sequences.

Evaluation of Osseous Morphology of the Hip Using Zero Echo Time Magnetic Resonance Imaging

Background: Femoroacetabular impingement syndrome (FAIS) is a common disorder of the hip resulting in groin pain and ultimately osteoarthritis. Radiologic assessment of FAI morphologies, which may present with overlapping radiologic features of hip dysplasia, often requires the use of computed tomography (CT) for evaluation of osseous abnormality, owing to the difficulty of direct visualization of cortical and subchondral bone with conventional magnetic resonance imaging (MRI). The use of a zero echo time (ZTE) MRI pulse sequence may obviate the need for CT by rendering bone directly from MRI. Purpose/Hypothesis: The purpose was to explore the application of ZTE MRI to the assessment of osseous FAI and dysplasia morphologies of the hip. It was hypothesized that angular measurements from ZTE images would show significant agreement with measurements obtained from CT images. Study Design: Cohort study (diagnosis); Level of evidence, 2. Methods: Thirty-eight hips from 23 patients were imaged with ZTE MRI and CT. Clinically relevant angular measurements of hip morphology were made in both modalities and compared to assess agreement. Measurements included coronal and sagittal center-edge angles, femoral neck-shaft angle, acetabular version (at 1-, 2-, and 3-o’clock positions), Tönnis angle, alpha angle, and modified-beta angle. Interrater agreement was assessed for a subset of 10 hips by 2 raters. Intermodal agreement was assessed on the complete cohort and a single rater. Results: Interrater agreement was demonstrated in both CT and ZTE, with intraclass correlation coefficient values ranging from 0.636 to 0.990 for ZTE and 0.747 to 0.983 for CT, indicating “good” to “excellent” agreement. Intermodal agreement was also shown to be significant, with intraclass correlation coefficients ranging from 0.618 to 0.904. Conclusion: Significant agreement of angular measurements for hip morphology exists between ZTE MRI and CT imaging. ZTE MRI may be an effective method to quantitatively evaluate osseous hip morphology.

Silent volumetric multi-contrast 7 Tesla MRI of ocular tumors using Zero Echo Time imaging.

Magnetic Resonance Imaging (MRI) has become a valuable imaging modality in ophthalmology, especially for the diagnosis and treatment planning of patients with uveal melanoma, the most common primary intra-ocular tumor. We aim to develop and evaluate the value of silent Zero Echo Time (ZTE) MRI to image patients with ocular tumors at 7Tesla. Therefore, ZTE and different types of magnetization-prepared ZTE (FLAIR, SPIR, T2 and Saturation recovery), have been developed. After an initial validation with 7 healthy subjects, nine patients with an eye tumor have been evaluated. The ZTE scans were compared to their Cartesian equivalent in terms of contrast, motion-sensitivity, diagnostic quality and patient comfort. All volunteers and especially the patients reported a more comfortable experience during the ZTE scans, which had at least a 10 dB lower sound pressure. The image contrast in the native ZTE was poor, but in the different magnetization-prepared ZTE, the eye lens, cornea and retina were clearly discriminated. Overall the T2-prepared scan yielded the best contrast, especially between tumor and healthy tissue, and proved to be robust against eye motion. Although the intrinsic 3D nature of the ZTE-technique provides an accurate analysis of the tumor morphology, the quality of the ZTE-images is lower than their Cartesian equivalent. In conclusion, the quality of magnetization-prepared ZTE images is sufficient to assess the 3D tumor morphology, but insufficient for more detailed evaluations. As such this technique can be an option for patients who cannot comply with the sound-levels of Cartesian scans, but for other patients the conventional Cartesian scans offer a better image quality.

Long-T2 -suppressed zero echo time imaging with weighted echo subtraction and gradient error correction.

To perform direct, selective MRI of short-T2 tissues using zero echo time (ZTE) imaging with weighted echo subtraction (WSUB). Radial imaging was performed at 7T, acquiring both ZTE and gradient echo (GRE) signals created by bipolar gradients. Long-T2 suppression was achieved by weighted subtraction of ZTE and GRE images. Special attention was given to imperfections of gradient dynamics, to which radial GRE imaging is particularly susceptible. To compensate for gradient errors, matching of gradient history was combined with data correction based on trajectory measurement. The proposed approach was first validated in phantom experiments and then demonstrated in musculoskeletal (MSK) imaging. Trajectory analysis and phantom imaging demonstrated that gradient imperfections were successfully addressed. Gradient history matching enabled consistency between antiparallel projections as required for deriving zeroth-order eddy current dynamics. Trajectory measurement provided individual echo times per projection that showed considerable variation between gradient directions. In in vivo imaging of knee, ankle, and tibia, the proposed approach enabled high-resolution 3D depiction of bone, tendons, and ligaments. Distinct contrast of these structures indicates excellent selectivity of long-T2 suppression. Clarity of depiction also confirmed sufficient SNR of short-T2 tissues, achieved by high baseline sensitivity at 7T combined with high SNR efficiency of ZTE acquisition. Weighted subtraction of ZTE and GRE data reconciles robust long-T2 suppression with fastest k-space coverage and high SNR efficiency. This approach enables high-resolution imaging with excellent selectivity to short-T2 tissues, which are of major interest in MSK and neuroimaging applications.