3D Ultrashort Echo Time Sequence Research Articles

Assessment of ultrashort echo time (UTE) T2* mapping at 3T for the whole knee: repeatability, the effects of fat suppression, and knee position.

Knee tissues such as tendon, ligament and meniscus have short T2* relaxation times and tend to show little to no signal in conventional magnetic resonance acquisitions. An ultrashort echo time (UTE) technique offers a unique tool to probe fast-decaying signals in these tissues. Clinically relevant factors should be evaluated to quantify the sensitivity needed to distinguish diseased from control tissues. Therefore, the objectives of this study were to (I) quantify the repeatability of UTE-T2* relaxation time values, and (II) evaluate the effects of fat suppression and (III) knee positioning on UTE-T2* relaxation time quantification. A dual-echo, three-dimensional center-out radially sampling UTE and conventional gradient echo sequences were utilized to image gadolinium phantoms, one ex-vivo specimen, and five in-vivo subjects on a clinical 3T scanner. Scan-rescan images from the phantom and in-vivo experiments were used to evaluate the repeatability of T2* relaxation time values. Fat suppressed and non-suppressed images were acquired for phantoms and the ex-vivo specimen to evaluate the effect of fat suppression on T2* relaxation time quantifications. The effect of knee positioning was evaluated by imaging in-vivo subjects in extended and flexed positions within the knee coil and comparing T2* relaxation times quantified from tissues in each position. Phantom and in-vivo measurements demonstrated repeatable T2* mapping, where the percent difference between T2* relaxation time quantified from scan-rescan images was less than 8% for the phantom and knee tissues. The coefficient of variation across fat suppressed and non-suppressed images was less than 5% for the phantoms and ex-vivo knee tissues, showing that fat suppression had a minimal effect on T2* relaxation time quantification. Knee position introduced variability to T2* quantification of the anterior cruciate ligament, posterior cruciate ligament, and patellar tendon, with percent differences exceeding 20%, but the meniscus showed a percent difference less than 10%. The 3D radial UTE sequence presented in this study could potentially be used to detect clinically relevant changes in mean T2* relaxation time, however, reproducibility of these values is impacted by knee position consistency between scans.

Assessment of glenoid bone loss and other osseous shoulder pathologies comparing MR-based CT-like images with conventional CT

ObjectivesTo evaluate and compare the diagnostic performance of CT-like images based on a 3D T1-weighted spoiled gradient-echo sequence (T1 GRE), an ultra-short echo time sequence (UTE), and a 3D T1-weighted spoiled multi-echo gradient-echo sequence (FRACTURE) with conventional CT in patients with suspected osseous shoulder pathologies.Materials and methodsPatients with suspected traumatic dislocation of the shoulder (n = 46, mean age 40 ± 14.5 years, 19 women) were prospectively recruited and received 3-T MR imaging including 3D T1 GRE, UTE, and 3D FRACTURE sequences. CT was performed in patients with acute fractures and served as standard of reference (n = 25). Agreement of morphological features between the modalities was analyzed including the glenoid bone loss, Hill-Sachs interval, glenoid track, and the anterior straight-line length. Agreement between the modalities was assessed using Bland-Altman plots, Student’s t-test, and Pearson’s correlation coefficient. Inter- and intrareader assessment was evaluated with weighted Cohen’s κ and intraclass correlation coefficient.ResultsAll osseous pathologies were detected accurately on all three CT-like sequences (n = 25, κ = 1.00). No significant difference in the percentage of glenoid bone loss was found between CT (mean ± standard deviation, 20.3% ± 8.0) and CT-like MR images (FRACTURE 20.6% ± 7.9, T1 GRE 20.4% ± 7.6, UTE 20.3% ± 7.7, p > 0.05). When comparing the different measurements on CT-like images, measurements performed using the UTE images correlated best with CT.ConclusionAssessment of bony Bankart lesions and other osseous pathologies was feasible and accurate using CT-like images based on 3-T MRI compared with conventional CT. Compared to the T1 GRE and FRACTURE sequence, the UTE measurements correlated best with CT.Clinical relevance statementIn an acute trauma setting, CT-like images based on a T1 GRE, UTE, or FRACTURE sequence might be a useful alternative to conventional CT scan sparing associated costs as well as radiation exposure.Key Points• No significant differences were found for the assessment of the glenoid bone loss when comparing measurements of CT-like MR images with measurements of conventional CT images.• Compared to the T1 GRE and FRACTURE sequence, the UTE measurements correlated best with CT whereas the FRACTURE sequence appeared to be the most robust regarding motion artifacts.• The T1 GRE sequence had the highest resolution with high bone contrast and detailed delineation of even small fractures but was more susceptible to motion artifacts.

High-Contrast Lumbar Spinal Bone Imaging Using a 3D Slab-Selective UTE Sequence.

Ultra-short echo time (UTE) MRI with post-processing is a promising technique in bone imaging that produces a similar contrast to computed tomography (CT). Here, we propose a 3D slab-selective ultrashort echo time (UTE) sequence together with image post-processing to image bone structures in the lumbar spine. We also explore the intermodality agreement between the UTE and CT images. The lumbar spines of two healthy volunteers were imaged with 3D UTE using five different resolutions to determine the best imaging protocol. Then, four patients with low back pain were imaged with both the 3D UTE sequence and CT to investigate agreement between the imaging methods. Two other patients with low back pain were then imaged with the 3D UTE sequence and clinical conventional T1-weighted and T2-weighted fast spin-echo (FSE) MRI sequences for qualitative comparison. The 3D UTE sequence together with post-processing showed high contrast images of bone and high intermodality agreement with CT images. In conclusion, post-processed slab-selective UTE imaging is a feasible approach for highlighting bone structures in the lumbar spine and demonstrates significant anatomical correlation with CT images.

High contrast cartilaginous endplate imaging using a 3D adiabatic inversion-recovery-prepared fat-saturated ultrashort echo time (3D IR-FS-UTE) sequence.

Ultrashort echo time (UTE) sequences can image tissues with transverse T 2 /T 2 * relaxations too short to be efficiently observed on routine clinical MRI sequences, such as the vertebral body cartilaginous endplate (CEP). Here, we describe a 3D adiabatic inversion-recovery-prepared fat-saturated ultrashort echo time (3D IR-FS-UTE) sequence to highlight the CEP of vertebral bodies in comparison to the intervertebral disc (IVD) and bone marrow fat (BF) at 3 T. The IR-FS-UTE sequence used a 3D UTE sequence combined with an adiabatic IR preparation pulse centered in the middle of the water and fat peaks, while a fat saturation module was used to suppress the signal from fat. A slab-selective half pulse was used for signal excitation, and a 3D center-out cones trajectory was used for more efficient data sampling. The 3D IR-FS-UTE sequence was applied to an ex vivo human spine sample, as well as the spines of six healthy volunteers and of three patients with back pain. Bright continuous lines representing signal from CEP were found in healthy IVDs. The measured contrast-to-noise ratio was 18.5 ± 4.9 between the CEP and BF, and 20.3 ± 4.15 between the CEP and IVD for the six volunteers. Abnormal IVDs showed CEP discontinuity or irregularity in the sample and patient studies. In conclusion, the proposed 3D IR-FS-UTE sequence is feasible for imaging the vertebral body's CEP in vivo with high contrast.

Functional MRI of the Lungs Using Single Breath-Hold and Self-Navigated Ultrashort Echo Time Sequences.

To evaluate three-dimensional (3D) ultrashort echo time (UTE) MRI regarding image quality and suitability for functional image analysis using gradient-echo sequences in breath-hold and with self-navigation. In this prospective exploratory study, 10 patients (mean age, 21 years; age range, 5-58 years; five men) and 10 healthy control participants (mean age, 25 years; age range, 10-39 years; five men) underwent 3D UTE MRI at 3.0 T. Imaging was performed with a prototypical stack-of-spirals 3D UTE sequence during single breath holds (echo time [TE], 0.05 msec) and with a self-navigated "Koosh ball" 3D UTE sequence at free breathing (TE, 0.03 msec). Image quality was rated on a four-point Likert scale. Edge sharpness was calculated. After semiautomated segmentation, fractional ventilation was calculated from MRI signal intensity (FVSI) and volume change (FVVol). The air volume fraction (AVF) was estimated from relative signal intensity (aortic blood signal intensity was used as a reference). Means were compared between techniques and participants. The Wilcoxon signed rank test and Spearman rank correlation were used for statistical analyses. Image quality ratings were equal for both techniques. However, stack-of-spirals breath-hold UTE was more susceptible to motion and aliasing artifacts. Mean FVSI was higher during breath hold than at free breathing (mean ± standard deviation in milliliters of gas per milliliters of parenchyma, 0.17 mL/mL ± 0.06 [minimum, 0.07; maximum, 0.34] vs 0.11 mL/mL ± 0.03 [minimum, 0.06; maximum, 0.17], P = .016). Mean FVSI and FVVol were in good agreement (mean difference: at breath hold, -0.008 [95% confidence interval {CI}: 0.007, -0.024]; ρ = 0.97 vs free breathing, -0.004 [95% CI: 0.007, -0.016]; ρ = 0.91). AVF correlated between both techniques (ρ = 0.94). Breath-hold and self-navigated 3D UTE sequences yield proton density-weighted images of the lungs that are similar in quality, and both techniques are suitable for functional image analysis.Supplemental material is available for this article.© RSNA, 2020.

Bone-Selective MRI as a Nonradiative Alternative to CT for Craniofacial Imaging

Computed tomography (CT) is the clinical gold-standard for high-resolution 3D visualization of cortical bone structures. However, ionizing radiation is of concern, particularly for pediatric patients. This study evaluates the feasibility of producing 3D human skull renderings using a novel bone-selective magnetic resonance imaging technique. A dual-radiofrequency pulse, dual-echo, 3D ultrashort echo time sequence was applied for scanning of a cadaver skull and five healthy adult subjects. Scans were each completed within 6 minutes. Semiautomatic segmentation of bone voxels was performed using ITK-SNAP software, leading to 3D renderings of the skulls. For comparison, thin-slice head CT scans were performed. Mimics software was used to measure eight anatomic distances from 3D renderings. Lin's Concordance Correlation test was applied to assess agreement between measurements from MR-based and CT-based 3D skull renderings. The 3D rendered MR images depict most craniofacial features (e.g., zygomatic arch), although some voxels were erroneously included or excluded in the renderings. MR-based measurements differed from CT-based measurements by mean percent difference ranging from 2.3%-5.0%. Lin's Concordance Correlation Coefficients for MR-based vs CT-based measurements ranged from 0.998-1.000. The proposed dual-radiofrequency dual-echo 3D ultrashort echo time imaging technique produces high-resolution bone-specific images within a clinically feasible imaging time, leading to clear visualization of craniofacial skeletal structures. Concordance coefficients suggest good reliability of the method compared to CT. The method is currently limited by time and manual input necessary for segmentation correction. Further investigation is needed for more accurate 3D renderings and for scanning of pediatric patients.

Three-dimensional quantitative MRI of aerosolized gadolinium-based nanoparticles and contrast agents in isolated ventilated porcine lungs.

The objective of this study is to evaluate the suitability and performance of ultra-short echo time (UTE) sequences for imaging and quantifying the deposition of nebulized MRI contrast agents in human-sized lungs. Nebulization of clinically used contrast agent or gadolinium-based nanoparticles were performed using a commercial jet nebulizer in isolated and ventilated porcine lungs connected to a 3D-printed human upper airways replica. MR images of isolated lungs were acquired on a 3T clinical MR scanner using 3D UTE sequences at different flip angles. 3D acquisitions with isotropic millimetric resolution were obtained in less than 4 min. Images exhibit homogeneous and large MR signal enhancement (above 200%) following nebulization of both types of aerosols. Deposition of aerosol down to the level of the bronchi of secondary lobules was visualized. T1 values and the concentration of nanoparticles obtained by MRI were found to correlate with the amount of nebulized gadolinium3+ ions. The distribution of aerosolized gadolinium-based contrast agent or nanoparticles can be visualized and quantified using UTE MRI in large animal ventilated lung model on a clinical MRI scanner. This protocol can be used for assessing and quantifying aerosol regional deposition with high spatial resolution (1 mm 3D isotropic) without ionizing radiation and could be applied in the future for diagnostic or therapeutic applications in patients.

Patients with Type 2 Diabetes Exhibit a More Mineralized Deep Cartilage Layer Compared with Nondiabetic Controls: A Pilot Study.

To assess differences in biochemical composition of the deep cartilage layer in subjects with type 2 diabetes mellitus (T2DM) and nondiabetic controls using UTE (ultra-short echo time) T2* mapping and to investigate the association of vascular health and UTE T2* measurements. Ten subjects with T2DM matched for age, sex, and body mass index with 10 nondiabetic controls. A 3D UTE sequence with 6 echo times was acquired using 3T magnetic resonance imaging of the knee. For UTE T2* analysis, the deep cartilage layer was segmented and analyzed in 5 compartments (patella, medial, and lateral femur and tibia). The ankle brachial index (ABI) was obtained in all subjects. Linear regression analyses were used to assess associations of T2DM and UTE T2* relaxation times and the associations of ABI measurements and UTE measurements. Compared with nondiabetic controls, T2DM subjects had significantly lower mean T2*-UTE in the patella (mean difference 4.87 ms; 95% confidence interval [CI] 1.09-8.65; P = 0.015), the lateral tibia (mean difference 2.26 ms; 95% CI 0.06-4.45; P = 0.045), and the lateral femur (mean difference 4.96 ms; 95% CI 0.19-9.73; P = 0.043). Independent of diabetic status, subjects with higher ABI values, indicating better vascular health, had higher T2*-UTE of the patella (coefficient 15.2; 95% CI 3.3-21.4; P = 0.017), the medial tibia (coefficient 9.8; 95% CI 1.0-18.6; P = 0.031), and the lateral femur (coefficient 18.8; 95% CI 3.3-34.3; P = 0.021). T2*-UTE measurements of the deep cartilage layer were consistently lower in subjects with T2DM and in subjects with impaired vascular health, likely indicating increased mineralization of this layer.

Ultrashort echo time imaging of the lungs under high-frequency noninvasive ventilation: A new approach to lung imaging.

BackgroundAlthough ultrashort echo time (UTE) sequences allow excellent assessment of lung parenchyma, image quality remains lower than that of computed tomography (CT).PurposeTo investigate a high‐frequency noninvasive ventilation (HF‐NIV) technique allowing a stabilized inspiration and to compare image quality with current dedicated MR sequences.Study TypeProspective.PopulationTen healthy volunteers.Field Strength/Sequence3D radial UTE sequence at 1.5T.AssessmentUTE‐HF‐NIV sequence was compared with UTE‐free‐breathing (UTE‐FB), reconstructed at end expiration (UTE‐Exp) and average (UTE‐Avg), and breath‐hold VIBE sequences. The distance from lung apex to the dome of the right hemidiaphragm was measured. Visual assessment of the visibility and sharpness of normal anatomical structures was carried out. Dedicated software also quantitatively evaluated vessel–lung and right lung–liver interface sharpness. Apparent signal ratio (Sr) and contrast ratios (Cr) were quantitatively evaluated.Statistical TestsWilcoxon signed rank test for visual scores, paired t‐test for continuous variables, significance at P < 0.05.ResultsThe distance between apex and the right hemidiaphragmatic dome was significantly larger (P < 0.001) with UTE‐HF‐NIV compared with UTE‐FB and VIBE acquisitions. Vessel and airway visibility had identical median visual scores with all UTE methods. Median visual scores for sharpness of vessels and airways were significantly higher (P < 0.001) with HF‐NIV (vessels = 3; airways = 2) than in UTE‐FB (vessels = 2; airways = 1) and VIBE (vessels = 1; airways = 1). Software‐based vessel sharpness evaluation resulted in larger values in 8/10 volunteers with UTE‐HF‐NIV (67.3 ± 9.8) compared with UTE‐Avg (62.3 ± 12.6) but the average difference was not significant (P = 0.28). The sharpness of the lung–liver interface was significantly higher (P < 0.001) with HF‐NIV (17.3 ± 5.3) compared with UTE‐Avg (14.1 ± 3.9). Significantly higher values (P < 0.01) of Sr and Cr were observed with UTE‐HF‐NIV compared with UTE‐FB and VIBE.Data ConclusionHF‐NIV allowing acquisition at full inspiration significantly improves image quality for lung imaging. This could offer the option to alternate some follow‐up CT studies by using this technique. Level of Evidence: 2 Technical Efficacy: Stage 1J. Magn. Reson. Imaging 2019;50:1789–1797.

Early Achilles Enthesis Involvement in a Murine Model of Spondyloarthropathy: Morphological Imaging with Ultrashort Echo-Time Sequences and Ultrasmall Superparamagnetic Iron Oxide (USPIO) Particle Evaluation in Macrophagic Detection.

Purpose To confirm the interest of 3-dimensional ultrashort echo-time (3D-UTE) sequences to assess morphologic aspects in normal and pathological Achilles entheses in a rat model of spondyloarthropathy (SpA) with histological correlations, in comparison with conventional RARE T2 Fat-Sat sequences, and, furthermore, to evaluate the feasibility of a 3D multiecho UTE sequence performed before and after the intravenous injection of ultrasmall superparamagnetic iron oxide (USPIO) particles to assess macrophagic involvement in the Achilles enthesis in the same rat model of SpA. Materials and Methods Fourteen rats underwent in vivo MRI of the ankle at 4.7 T, including a 3D RARE T2 Fat-Sat sequence and a 3D ultrashort echo-time (UTE) sequence for morphologic assessment at baseline and day 3 after induction of an SpA model, leading to Achilles enthesopathy in the left paw (right paw serving as a control). A 3D multiecho UTE sequence was also performed at day 3 before and then 24 (4 rats) and 48 (2 rats) hours after intravenous injection of USPIO. Visual analysis and signal intensity measurements of all images were performed at different locations of the Achilles enthesis and preinsertional area. Visual analysis and T2∗ measurements were performed before and after USPIO injection, on the 3D multiecho UTE sequence in the same locations. Normal and pathological values were compared by Wilcoxon signed-rank tests. MR findings were compared against histological data. Results 3D-UTE sequences enabled morphologic identification of the anterior fibrocartilage and posterior collagenic areas of the Achilles enthesis. Visual analysis and signal intensity measurements distinguished SpA-affected entheses from healthy ones at day 3 (P=0.02). After administration of USPIO, no differences in signals were detected. Similarly, both visual analysis and signal T2∗ measurements in the enthesis were unable to distinguish the SpA-affected tendons from healthy ones (P=0.914). Neither the normal anatomy of the enthesis nor its pathological pattern could be distinguished using the standard RARE sequence. Histology confirmed the absence of USPIO in Achilles entheses, despite marked signs of inflammation. Conclusion Unlike conventional RARE T2 Fat-Sat sequences, 3D-UTE sequences enable morphologic assessment of normal enthesis anatomy and early detection of abnormalities in pathological conditions. However, 3D multiecho UTE sequences combined with USPIO injections with T2∗ measurements were unable to detect macrophagic involvement in these pathological conditions.

Quantitative Gd-DOTA-based aerosol deposition mapping in the lungs of asthmatic rats using 3D UTE-MRI.

Asthma is a chronic respiratory disease, commonly treated with inhaled therapy. Better understanding of the mechanisms of aerosol deposition is required to improve inhaled drug delivery. Three-dimensional ultrashort echo time (UTE) MRI acquisitions at 1.5T were combined with spontaneous nose-only inhalation of aerosolized gadolinium (Gd) to map the aerosol deposition and to characterize signal enhancement in asthmatic rat lungs. The rats were sensitized to ovalbumin (OVA) to develop asthmatic models and challenged before imaging by nebulization of OVA to trigger asthmatic symptoms. The negative controls were not sensitized or challenged by nebulization of saline. The animal lungs were imaged before and after administration of Gd-based aerosol in freely breathing rats, by using a T1 -weighted 3D UTE sequence. A contrast-enhanced quantitative analysis was performed to assess regional concentration. OVA-sensitized rats had lower signal enhancement and lower deposited aerosol concentration. Their enhancement dynamics showed large inter-subject variability. The signal intensity was homogeneously enhanced for controls while OVA-sensitized rats showed heterogeneous enhancement. Contrast-enhanced 3D UTE was applied with aerosolized Gd to efficiently measure spatially resolved deposition in asthmatic lungs. The small administered dose (around 1μmol/kg body weight) and the use of standard clinical MRI suggest a potential application for the exploration of asthma.

Uteroplacental and Fetal 4D Flow MRI in the Pregnant Rhesus Macaque.

Pregnancy complications are often associated with poor uteroplacental vascular adaptation and standard diagnostics are unable to reliably quantify flow in all uteroplacental vessels and have poor sensitivity early in gestation. To investigate the feasibility of using 4D flow MRI to assess total uteroplacental blood flow in pregnant rhesus macaques as a precursor to human studies. Retrospective feasibility study. Fifteen healthy, pregnant rhesus macaques ranging from the 1st trimester to 3rd trimester of gestation. Abdominal 4D flow MRI was performed on a 3.0T scanner with a radially undersampled phase contrast (PC) sequence. Reference ferumoxytol-enhanced angiograms were acquired with a 3D ultrashort echo time sequence with a center-out radial trajectory. Repeatability of flow measurements was assessed with scans performed same-day and on consecutive days in the uterine arteries and ovarian veins. In-flow was compared against out-flow in the uterus, umbilical cord, and fetal heart with a conservation of mass analysis. Conspicuity of uteroplacental vessels was qualitatively compared between PC angiograms derived from 4D flow data and ferumoxytol-enhanced angiograms. Bland-Altman analysis was used to quantify same-day and consecutive-day repeatability. Same-day flow measurements showed an average difference between scans of 13% in both the uterine arteries and ovarian veins, while consecutive-day measurements showed average differences of 22% and 24%, respectively. Comparisons of in-flow and out-flow showed average differences of 15% in the uterus, 8% in fetal heart, and 15% in the umbilical cord. PC angiograms showed similar depiction of main uteroplacental vessels as high-resolution, ferumoxytol-enhanced angiograms. 4D flow MRI could be used in the rhesus macaque for repeatable flow measurements in the uteroplacental and fetal vasculature, setting the stage for future human studies. 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:534-545.

Simultaneous quantitative susceptibility mapping (QSM) and R2* for high iron concentration quantification with 3D ultrashort echo time sequences: An echo dependence study.

To evaluate the echo dependence of 3D ultrashort echo time (TE) quantitative susceptibility mapping (3D UTE-QSM) and effective transverse relaxation rate ( R2*) measurement in the setting of high concentrations of iron oxide nanoparticles. A phantom study with iron concentrations ranging from 2 to 22 mM was performed using a 3D UTE Cones sequence. Simultaneous QSM processing with morphology-enabled dipole inversion (MEDI) and R2* single exponential fitting was conducted offline with the acquired 3D UTE data. The dependence of UTE-QSM and R2* on echo spacing (ΔTE) and first TE (TE1 ) was investigated. A linear relationship was observed between UTE-QSM measurement and iron concentration up to 22 mM only, with the minimal TE1 of 0.032 ms and ΔTE of less than 0.1 ms. A linear relationship was observed between R2* and iron concentration up to 22 mM only when TE1 was less than 0.132 ms and ΔTE was less than 1.2 ms. UTE-QSM with MEDI processing showed strong dependence on ΔTE and TE1 , especially at high iron concentrations. UTE-QSM is more sensitive than R2* measurement to TE selection. Both an ultrashort TE1 and a small ΔTE are needed to achieve accurate QSM for high iron concentrations. Magn Reson Med 79:2315-2322, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Motion robust high resolution 3D free-breathing pulmonary MRI using dynamic 3D image self-navigator.

To achieve motion robust high resolution 3D free-breathing pulmonary MRI utilizing a novel dynamic 3D image navigator derived directly from imaging data. Five-minute free-breathing scans were acquired with a 3D ultrashort echo time (UTE) sequence with 1.25 mm isotropic resolution. From this data, dynamic 3D self-navigating images were reconstructed under locally low rank (LLR) constraints and used for motion compensation with one of two methods: a soft-gating technique to penalize the respiratory motion induced data inconsistency, and a respiratory motion-resolved technique to provide images of all respiratory motion states. Respiratory motion estimation derived from the proposed dynamic 3D self-navigator of 7.5 mm isotropic reconstruction resolution and a temporal resolution of 300 ms was successful for estimating complex respiratory motion patterns. This estimation improved image quality compared to respiratory belt and DC-based navigators. Respiratory motion compensation with soft-gating and respiratory motion-resolved techniques provided good image quality from highly undersampled data in volunteers and clinical patients. An optimized 3D UTE sequence combined with the proposed reconstruction methods can provide high-resolution motion robust pulmonary MRI. Feasibility was shown in patients who had irregular breathing patterns in which our approach could depict clinically relevant pulmonary pathologies. Magn Reson Med 79:2954-2967, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Assessment of different fitting methods for in-vivo bi-component T2* analysis of human patellar tendon in magnetic resonance imaging.

To investigate the robustness of four fitting methods for bi-component effective spin-spin T2 (T2*) relaxation time analysis of human patellar tendon. A three-dimensional (3D) cone ultra-short echo-time (UTE) sequence was performed on the knees of ten healthy volunteers at 3.0T. Four fitting methods incorporating either Gaussian or Rician noise distribution were used for voxel-by-voxel bi-component T2* analysis of the patellar tendon. The T2* for the short relaxing (T**,s ) and long relaxing (T*2,l ) water components and the fraction of the short relaxing water component (fs ) were measured, and different fitting methods were compared using Friedman's and Wilcoxon signed rank tests. A numerical simulation study was also performed to predict the accuracy and precision of bi-component T2* parameter estimation in tendon at different signal-to-noise ratios (SNR) levels. The average T*2,s , T*2,l , fs of human patellar tendon were 1.5ms, 30ms, and 80% respectively. Incorporating different noise models and fitting methods influenced the measured bi-component T2* parameters. Fitting methods incorporating Rician noise were superior to traditional fitting methods for bi-component T2* analysis especially at lower SNR. fs and T*2,s were less sensitive than T*2,1 to noise at even moderate and low SNR. The result of the in-vivo bi-component T2* analysis of tendon agreed well with numerical simulations. Our study demonstrated the use of a 3D cone UTE sequence to perform in vivo voxel-by-voxel bi-component T2* analysis of human patellar tendon. Incorporating Rician noise was useful for improving bi-component T2* analysis especially at lower SNR. IV.

Time Efficient 3D Radial UTE Sampling with Fully Automatic Delay Compensation on a Clinical 3T MR Scanner

This work’s aim was to minimize the acquisition time of a radial 3D ultra-short echo-time (UTE) sequence and to provide fully automated, gradient delay compensated, and therefore artifact free, reconstruction. The radial 3D UTE sequence (echo time 60 μs) was implemented as single echo acquisition with center-out readouts and improved time efficient spoiling on a clinical 3T scanner without hardware modifications. To assess the sequence parameter dependent gradient delays each acquisition contained a quick calibration scan and utilized the phase of the readouts to detect the actual k-space center. This calibration scan does not require any user interaction. To evaluate the robustness of this automatic delay estimation phantom experiments were performed and 19 in vivo imaging data of the head, tibial cortical bone, feet and lung were acquired from 6 volunteers. As clinical application of this fast 3D UTE acquisition single breath-hold lung imaging is demonstrated. The proposed sequence allowed very short repetition times (TR~1ms), thus reducing total acquisition time. The proposed, fully automated k-phase based gradient delay calibration resulted in accurate delay estimations (difference to manually determined optimal delay −0.13 ± 0.45 μs) and allowed unsupervised reconstruction of high quality images for both phantom and in vivo data. The employed fast spoiling scheme efficiently suppressed artifacts caused by incorrectly refocused echoes. The sequence proved to be quite insensitive to motion, flow and susceptibility artifacts and provides oversampling protection against aliasing foldovers in all directions. Due to the short TR, acquisition times are attractive for a wide range of clinical applications. For short T2* mapping this sequence provides free choice of the second TE, usually within less scan time as a comparable dual echo UTE sequence.

Evaluation of Cartilaginous End Plate and Annulus Fibrosus in Lumbar Disc Degeneration using OFF Resonant Saturation Ratios Assessed by a 3D Ultra Short Echo Time Sequence at 3 Tesla

Introduction Ultrashort echo time (UTE) imaging in combination with off-resonant saturation ratios (OSR) is increasingly used for characterization of tissue with fast signal decay, such as the annulus fibrosus (AF) as well as the upper and lower cartilaginous end plate (EP). The aim of this study was to assess effects of age, gender, and location of OSR measurement on evaluation of lumbar disc degeneration. Materials and Methods A total of 35 lumbar intervertebral discs of 12 fresh-frozen cadaveric specimens (median age 66 years, range 49–75 years) were examined in a whole-body MR scanner using a sagittal 3D UTE sequence with an isotropic submillimeter resolution of 0.7 mm (TE 0.07 milliseconds, TR 10.3 milliseconds, FoV 128 mm, and FA 10 degrees) with and without off-resonant saturation preparation pulse at 3 T. The off-resonance frequency varied between 1 and 5 kHz. OSRs of images with and without preparation pulse were calculated for upper and lower EP as well as for AF. Intervertebral discs were classified healthy (CO, n = 20), mild degeneration (MIL, n = 7), and severe degeneration (SEV, n = 8) concerning morphology and signal alteration in conventional MR sequences and UTE imaging. A nonparametric U-test reports level of significance. Correlation between age and OSRs is calculated using Spearman rank correlation coefficient. Effects of different off-resonance frequencies and measurement locations on lumbar disc degeneration are investigated using logistic regression and ROC analysis. Cut-off values, sensitivity, and specificity are given. Results Best differentiation between mild and severe degeneration was seen at an off-resonance frequency of 1 kHz for AF (sensitivity 87.5%/specificity 87.5%/cut-off 0.34) and the lower EP at 1.5 kHz (sensitivity 100%/specificity 85.7%/cut-off 0.29). Comparison of CO and MIL groups revealed best differentiation at 1 kHz for upper EP (sensitivity 100%/specificity 80%/cut-off 0.35). A reasonable correlation between age and OSRs in healthy lumbar discs was seen in AF for all investigated frequencies, ρ = 0.49 (5 kHz) to 0.61 (2 kHz), and in lower EP for frequencies above 2 kHz, ρ = 0.51 (3 kHz) to 0.71 (5 kHz). Significant differences between OSR measurements were seen in AF of healthy female and male lumbar discs for all investigated sequences and in upper EP at 1 kHz. Conclusion OSR assessment might be a suitable method for detection of early stage lumbar disc degeneration and should be preferably performed using off-resonance frequencies between 1 and 2 kHz. Age might influence OSR measurement in AF and lower EP. Gender seems to influence results of OSRs, especially in evaluation of AF. [Figure: see text]

Effects of Lumbar Disc Degeneration at the Cartilaginous End Plate and the Annulus Fibrosus on Off-Resonant Saturation Ratios Assessed by a 3D Ultrashort Echo Time Sequence at 3 T

Introduction Biological disc replacement demands a distinct understanding of referring pathologies and states of degeneration. Clinical imaging modalities are needed to apply such measurements to potential patients. The introduction of UTE sequences to whole body magnetic resonance imaging (MRI) allows for quantitative characterization of tissue consisting of components with fast signal decay. We hypothesize that UTE will enable us to differentiate states of degeneration within the EP and the AF. The aim of this study was to investigate effects of LDD on ORS ratios in the cartilaginous EP and the AF. Materials and Methods A total of 20 human lumbar motion segments of seven fresh-frozen cadaveric specimens (median age 66, range 49-66 years) were assessed using a whole-body MRI scanner. A sagittal 3D UTE sequence with an isotropic submillimeter resolution of 0.7 mm (TE 0.07 ms, TR 10.3 ms, FoV 128 mm, FA 10 degrees) with and without off-resonant saturation preparation pulse at 3 T was used. The off-resonance frequency was varied between 1 and 5 kHz. OSRs of images with and without preparation pulse were calculated for the upper and the lower EP as well as for the AF. Intervertebral discs were classified healthy (CO, n = 9), mildly degenerated (MIL, n = 6) and severely degenerated (SEV, n = 8) concerning morphology and signal alteration in conventional MR sequences. A nonparametric U-test was performed to report the level of significance. The relationship between OSR and grade of degeneration was estimated using multiple regression analysis in respect to age and body mass index (BMI). Results Upper and lower EP as well as AF in controls revealed no significant differences concerning OSR from 1 to 5 kHz. Regarding all regions (EPs and AF), the SEV group showed significantly increased OSRs ranging from 0.37 ± 0.01 (1 kHz) to 0.22 ± 0.01 (5 kHz), compared with the CO group, ranging from 0.33 ± 0.01 (1 kHz) to 0.2 ± 0.01 (5 kHz), and the MIL group, ranging from 0.32 ± 0.01 (1 kHz) to 0.2 ± 0.01 (5 kHz) ( p &lt; 0.05). The MIL group showed a tendency toward lower OSR compared with the CO group. This was underlined by the results of multiple regression analysis after adjusting for age (Table A). [Table: see text] Conclusion UTE sequences reveal clear images of human lumbar EP and AF and allow for quantitative assessment of free water. EP and AF of lumbar discs with major degeneration showed a loss of free water content compared with healthy and slightly degenerated discs, indicating a change toward a more condensed structure. EP and AF of slightly degenerated discs, however, showed a tendency to higher OSR, indicating an increase of free water. This might have occurred due to edema or focal destruction of EP and AF in early stages. Focused repair strategies and potential outcome predictors may result from this imaging modality. Disclosure of Interest None declared

Weighted subtraction in 3D ultrashort echo time (UTE) imaging for visualization of short T2 tissues of the knee

Ultrashort echo time (UTE) image to directly visualize short T2 tissues requires postprocessing for the suppression of the surrounding long T2 tissues in a clinical magnetic resonance imaging (MRI) scanner. Weighted subtraction of UTE images with an optimal weighting factor could provide high positive contrast with adequate suppression. To demonstrate in-vivo UTE MRI with weighted subtractions of dual echo UTE imaging using a 3 T clinical MRI and to determine the optimal weighting factors through the analyses of signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs). Forty-seven consecutive knee MR including dual echo 3D UTE sequence (TE of 70 µs and 3.82 ms) were reviewed. Seven patients with normal findings in the knee MR were included in the current study. For variably weighted subtraction images, the initial UTE image was rescaled relative to that of the second echo image with weighting factors from 0.1 to 4.0. Thirty-five pairs of the ROI measurements of the UTE image and second long echo images were repeated after 2 weeks. For quantitative assessment, SNR and CNR were measured from the ROIs on the patellar tendon, cartilage, cortical bone, meniscus, and infrapatellar fat pad. Intra-observer agreement was assessed by using both Cohen's Kappa and Bland-Altman approach. The short T2 tissue could be visualized with adequate suppression on the subtraction images. Considering the CNRs and SNRs, the optimal ranges of the weighting factors could be suggested: 0.3 for the tendon, 0.4 for the cortical bone, and 1.0 for the meniscus. The 35 paired measurements showed excellent agreement with statistical significance (P < 0.05). The 3D UTE MRI provides imaging of short T2 tissues which cannot be visualized by conventional MRI. Using weighted subtractions with optimal weighting values, each tissue can be optimally depicted by overcoming the reduced T2 contrast.

Magnetic resonance imaging of solid dental restoration materials using 3D UTE sequences: visualization and relaxometry of various compounds

Due to an increasing scientific interest in MR-imaging of carious lesions and teeth, an accurate signal characterization of dental restoration materials is necessary for optimization of MR sequence protocols and evaluation of material degradation. Therefore, signal yield and relaxation behavior of common dental restoration materials in comparison to those of dentine of extracted human teeth were assessed in vitro by ultrashort echo time (UTE) sequences. Eighteen material samples and dentine of two freshly extracted human teeth were investigated on a 3T whole-body clinical MR-scanner. Transverse (T2*) and longitudinal relaxation times (T1) were quantified using a recently published modified Ernst equation that takes relevant in-pulse relaxation effects into account. All investigated samples could be successfully visualized but maximum signal yield was highly variable between samples. T1-values of the investigated dental restoration materials ranged between 28 and 365 ms, whereas T2*-values ranged between 96 and 917 μs. In contrast, T1-values of dentine (T1=545 ms±299 ms) were higher, while T2*-values (T2*=478 μs±271 μs) showed similar values. Dental restoration materials and dentine of extracted human teeth can be visualized by UTE sequences and show a broad range of signal yield and relaxation times.