Ultrashort Echo Time Research Articles

Assessment of iron nanoparticle distribution in mouse models using ultrashort-echo-time MRI.

Microscopic magnetic field inhomogeneities caused by iron deposition or tissue‐air interfaces may result in rapid decay of transverse magnetization in MRI. The aim of this study is to detect and quantify the distribution of iron‐based nanoparticles in mouse models by applying ultrashort‐echo‐time (UTE) sequences in tissues exhibiting extremely fast transverse relaxation. In 24 C57BL/6 mice (two controls), suspensions containing either non‐oxidic Fe or AuFeO x nanoparticles were injected into the tail vein at two doses (200 μg and 600 μg per mouse). Mice underwent MRI using a UTE sequence at 4.7 T field strength with five different echo times between 100 μs and 5000 μs. Transverse relaxation times T 2* were computed for the lung, liver, and spleen by mono‐exponential fitting. In UTE imaging, the MRI signal could reliably be detected even in liver parenchyma exhibiting the highest deposition of nanoparticles. In animals treated with Fe nanoparticles (600 μg per mouse), the relaxation time substantially decreased in the liver (3418 ± 1534 μs (control) versus 228 ± 67 μs), the spleen (2170 ± 728 μs versus 299 ± 97 μs), and the lungs (663 ± 101 μs versus 413 ± 99 μs). The change in transverse relaxation was dependent on the number and composition of the nanoparticles. By pixel‐wise curve fitting, T 2* maps were calculated showing nanoparticle distribution. In conclusion, UTE sequences may be used to assess and quantify nanoparticle distribution in tissues exhibiting ultrafast signal decay in MRI.

Correlation between the elastic modulus of anterior cruciate ligament (ACL) and quantitative ultrashort echo time (UTE) magnetic resonance imaging.

Conventional magnetic resonance imaging (MRI) often acquires no signal in anterior cruciate ligament (ACL) due to the short apparent transverse relaxation time of ACL. Ultrashort echo time (UTE) MRI is capable of imaging ACL with high signal which enables quantitative ACL assessment. This study aimed to investigate the correlations of the mechanical and microstructural properties of human ACL specimens with quantitative three-dimensional UTE Cones (3D-UTE-Cones) MRI measures. ACL specimens were harvested from cadaveric knee joints of 13 (50.9 ± 21.1 years old, 11 males and 2 female) donors. Specimens were scanned using a series of quantitative 3D-UTE-Cones T2 * (UTE-T2 *), T1 (UTE-T1 ), Adiabatic T1ρ (UTE-Adiab-T1ρ ), and magnetization transfer (UTE-MT) sequences in a wrist coil on a clinical 3T scanner. ACL elastic modulus was measured using a uniaxial tensile mechanical test. Histomorphometry analysis was performed to measure the average fascicle specific surface, fascicle size, and number of cells per unit area. Spearman's rank correlations of UTE-MRI biomarkers with mechanical and histomorphometry measures were investigated. The elastic modulus of ACL showed significant moderate correlations with UTE-Adiab-T1ρ (R = -0.59, p = 0.01), macromolecular fraction from MT modeling (R = 0.54, p = 0.01), magnetization transfer ratio (R = 0.53, p = 0.01), UTE-T2* (R = -0.53, p = 0.01), and average fascicle specific surface (R = 0.54, p = 0.01). UTE-MRI showed nonsignificant correlations with histomorphometry measures. UTE-MRI biomarkers may be useful noninvasive tools for the ACL mechanical assessment.

Assessment of Achilles Tendon Changes After Long-Distance Running Using Ultrashort Echo Time Magnetization Transfer MR Imaging.

Long-distance running is a common cause of Achilles tendinopathy. A reliable magnetic resonance imaging (MRI) technique to track early changes in the tendon caused by running could facilitate more effective interventions to combat progression. To evaluate an ultrashort echo time sequence with magnetization transfer preparation (UTE-MT) in the detection of changes in Achilles tendons of amateur marathon runners before and after long-distance running. Prospective. Thirty-two runners (19 enrolled for full marathons and 13 enrolled for half-marathons) and 5 healthy non-runners. 3.0 T; UTE-MT and dual-echo UTE for T2* assessment (UTE-T2*). MRI was performed 1-week pre-race, 2-days post-race, and 4-weeks post-race. UTE-MT ratio (UTE-MTR) and UTE-T2* of tendon were measured by two independent radiologists who were blinded to the scan time point and participant data. The Achilles tendon was divided into six regions of interest (ROIs) for data analysis, namely the insertion part (INS), middle part (MID), muscle-tendon junction (MTJ), tendon-bone insertion (TBI), tendon-muscle insertion (TMI), and whole tendon (bulk). Analysis of variance and Friedman's rank tests were used to evaluate changes in UTE-MTR and UTE-T2* between time points. Tukey test and Bonferroni method were used for further comparisons. P < 0.05 was considered significant. The UTE-MTR values of most tendon ROIs changed significantly between the measured time points, except for the INS region (P=0.1977). Conversely, the UTE-T2* values only showed significant changes in the MID and TBI regions. Paired comparisons showed that the UTE-MTR decreases in the MTJ, MID, TMI, and bulk regions at 2-days post-race were significant compared to measures taken pre-race and 4-weeks post-race. For UTE-T2* measurements, significant differences were observed only for the MID region between pre-race and 2-days post-race (P=0.0408, 95% CI: 0.0061, 0.1973), and for the TBI region between pre-race and 4-weeks post-race (P=0.0473, 95% CI: 0.0013, 0.1766). The UTE-MT sequence is able to detect biochemical changes in the Achilles tendon after long-distance running. 2 TECHNICAL EFFICACY: Stage 1.

Pulmonary Magnetic Resonance Imaging of Ex-Preterm Children with and without Bronchopulmonary Dysplasia.

Rationale: Children born prematurely, particularly those with bronchopulmonary dysplasia, have persisting lung abnormalities requiring longitudinal monitoring. Pulmonary ultrashort echo time magnetic resonance imaging (MRI) measurements may provide sensitive markers of persisting lung abnormalities and have not been evaluated in school-aged children born prematurely. Objectives: To compare pulmonary MRI and pulmonary function test measurements in preterm-born school-aged children with and without bronchopulmonary dysplasia. Methods: Children aged 7-9 years, born extremely preterm, with and without bronchopulmonary dysplasia, were recruited from three centers. Participants underwent pulmonary ultrashort echo time MRI and pulmonary function tests. Primary outcomes included total proton density and proton density at full expiration, measured using MRI. Multiple linear regression analysis was performed, adjusting for gestational age and bronchopulmonary dysplasia. Associations between MRI and pulmonary function were tested. Results: Thirty-five children were included in the primary analysis (24 with bronchopulmonary dysplasia, 11 without); 29 completed pulmonary function tests, of whom 11 (38%) had airflow limitation. Children with bronchopulmonary dysplasia had 44% (95% confidence interval [CI], 10-66%) lower mean total proton density (mean ± standard deviation, 3.6 ± 2.6) than those without (6.1 ± 4.0). Those with bronchopulmonary dysplasia had 25% (95% CI, 3-42%) lower proton density at full expiration than those without. Lower total proton density and proton density at full expiration were moderately correlated with greater residual volume, residual volume/total lung capacity, and lung clearance index (Spearman correlations for total proton density: -0.42, -0.57, and -0.53, respectively. Spearman correlations for proton density at full expiration: -0.28, -0.57, and -0.45, respectively). Conclusions: School-aged preterm-born children with bronchopulmonary dysplasia have parenchymal tissue abnormalities measured using ultrashort MRI proton density, compared with those without. MRI proton density correlated with pulmonary function measures indicative of gas trapping. Clinical trial registered with www.clinicaltrials.gov (NCT02921308).

Estimation of Contrast Agent Concentration in DCE-MRI Using 2 Flip Angles.

The aim of this study was to investigate the feasibility of using 2 flip angles (FAs) with an ultrashort echo time during dynamic contrast-enhanced (DCE)-magnetic resonance imaging (MRI) for estimation of plasma gadolinium (Gd) concentration without using a precontrast longitudinal relaxation time T1 (T10) measurement. T1-weighted DCE-MRI experiments were carried out with C57BL/6J mice using the scan protocol with 2 FAs over 3 sequential segments during 1 scan. The data with 2 FAs were used to estimate T10 (T1T) during conversion of a time-intensity curve to the time-concentration curve. Three dosages of gadolinium-based contrast agent were used to achieve a wide range of variability in Gd concentrations when measured at 10 minutes postinjection: 0.05 mmol/kg (n = 6), 0.1 mmol/kg (n = 11), and 0.15 mmol/kg (n = 7). For comparison, the signal-to-concentration conversion was also conducted using the T10 measured from the precontrast scan (T1M) as well as a constant T10 (2.1 seconds) from the literature (T1C). The Gd concentrations ([Gd]) estimated using DCE-MRI data for the time of retro-orbital blood collection ([Gd]T1T, [Gd]T1M, and [Gd]T1C, respectively) were compared against the [Gd] of the blood samples measured by inductively coupled plasma mass spectrometry ([Gd]MS). In addition, contrast kinetic model analysis was conducted on mice with GL261 brain tumors (n = 5) using the 3 different methods for T10. T1T strongly correlated with T1M (r = 0.81). [Gd]T1M and [Gd]T1T were significantly different from [Gd]T1C. [Gd]T1M and [Gd]T1T were in good agreement with [Gd]MS with strong correlations (mean percentage error ± standard deviation) of r = 0.70 (16% ± 56%) and r = 0.85 (15% ± 44%), respectively. In contrast, [Gd]T1C had a weak correlation of r = 0.52 with larger errors of 33% ± 24%. The contrast kinetic model parameters of GL261 brain tumors using T1T were not significantly different from those using T1M. This study substantiates the feasibility of using the 2-FA approach during DCE-MRI scan to estimate [Gd] in the plasma without using an extra scan to perform precontrast T1 measurements.

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.

Assessment of pulmonary arteriovenous malformation with ultra-short echo time magnetic resonance imaging

PurposeThe purpose of this study was to evaluate the diagnostic performance of ultra-short echo time magnetic resonance imaging (UTE MRI) in the assessment of pulmonary arteriovenous malformation (PAVM). MethodsEighteen consecutive patients (mean [± standard deviation] age, 48.6 ± 16.8 years) with 46 untreated PAVMs who underwent and thin-section computed tomography (CT) and UTE MRI with a 1.5-Tesla and 3-Tesla unit were retrospectively assessed. Two radiologists evaluated the diagnostic capabilities of UTE MRI for the detection and classification of PAVMs with reference to CT. Sensitivity, specificity, and kappa statistics were calculated with reference to CT. We also compared the differences in PAVM measurements between CT and MRI. ResultsThe sensitivity and specificity of UTE-MRI for the detection of PAVMs were 89.1% and 100%, respectively, for reader 1 and 87.0% and 100%, respectively, for reader 2. In the classification of PAVMs, inter-modality agreement in reader 1 and 2 were both substantial (κ = 0.78 and 0.69, respectively). The measurements of the PAVM feeding artery and sac on CT and MRI were strongly correlated in both readers 1 and 2 (R2 = 0.981 and 0.983, respectively). Both readers 1 and 2 slightly underestimated the diameter of the PAVM feeding artery and sac on UTE MRI (p < 0.001). ConclusionThis study indicates that UTE MRI is a feasible and promising modality for noninvasive assessment of PAVMs.

Imaging gravity-induced lung water redistribution with automated inline processing at 0.55 T cardiovascular magnetic resonance

BackgroundQuantitative assessment of dynamic lung water accumulation is of interest to unmask latent heart failure. We develop and validate a free-breathing 3D ultrashort echo time (UTE) sequence with automated inline image processing to image changes in lung water density (LWD) using high-performance 0.55 T cardiovascular magnetic resonance (CMR).MethodsQuantitative lung water CMR was performed on 15 healthy subjects using free-breathing 3D stack-of-spirals proton density weighted UTE at 0.55 T. Inline image reconstruction and automated image processing was performed using the Gadgetron framework. A gravity-induced redistribution of LWD was provoked by sequentially acquiring images in the supine, prone, and again supine position. Quantitative validation was performed in a phantom array of vials containing mixtures of water and deuterium oxide.ResultsThe phantom experiment validated the capability of the sequence in quantifying water density (bias ± SD 4.3 ± 4.8%, intraclass correlation coefficient, ICC = 0.97). The average global LWD was comparable between imaging positions (supine 24.7 ± 3.4%, prone 22.7 ± 3.1%, second supine 25.3 ± 3.6%), with small differences between imaging phases (first supine vs prone 2.0%, p < 0.001; first supine vs second supine − 0.6%, p = 0.001; prone vs second supine − 2.7%, p < 0.001). In vivo test–retest repeatability in LWD was excellent (− 0.17 ± 0.91%, ICC = 0.97). A regional LWD redistribution was observed in all subjects when repositioning, with a predominant posterior LWD accumulation when supine, and anterior accumulation when prone (difference in anterior–posterior LWD: supine − 11.6 ± 2.7%, prone 5.5 ± 2.7%, second supine − 11.4 ± 2.9%). Global LWD maps were calculated inline within 23.2 ± 0.3 s following the image reconstruction using the automated pipeline.ConclusionsRedistribution of LWD due to gravitational forces can be depicted and quantified using a validated free-breathing 3D proton density weighted UTE sequence and inline automated image processing pipeline on a high-performance 0.55 T CMR system.

Ultrashort Echo Time (UTE) MRI Porosity Index (PI) and Suppression Ratio (SR) Correlate with the Cortical Bone Microstructural and Mechanical Properties: Ex Vivo Study

Ultrashort Echo Time (UTE) MRI Porosity Index (PI) and Suppression Ratio (SR) Correlate with the Cortical Bone Microstructural and Mechanical Properties: Ex Vivo Study

Comprehensive assessment of in vivo lumbar spine intervertebral discs using a 3D adiabatic T1ρ prepared ultrashort echo time (UTE-Adiab-T1ρ) pulse sequence.

T1ρ has been extensively reported as a sensitive biomarker of biochemical changes in the nucleus pulposus (NP) and annulus fibrosis of intervertebral discs (IVDs). However, no T1ρ study of cartilaginous endplates (CEPs) has yet been reported because the relatively long echo times (TEs) of conventional clinical T1ρ sequences cannot effectively capture the fast-decaying magnetic resonance signals of CEPs, which have very short T2/T2*s. This can be overcome by using ultrashort echo time (UTE) T1ρ acquisitions. Seventeen subjects underwent UTE with adiabatic T1ρ preparation (UTE-Adiab-T1ρ) and T2-weighted fast spin echo imaging of their lumbar spines. Each IVD was manually segmented into seven regions (i.e., outer anterior annulus fibrosis, inner anterior annulus fibrosis, outer posterior annulus fibrosis, inner posterior annulus fibrosis, superior CEP, inferior CEP, and NP). T1ρ values of these sub-regions were correlated with IVD modified Pfirrmann grades and subjects' ages. In addition, T1ρ values were compared in subjects with and without low back pain (LBP). Correlations of T1ρ values of the outer posterior annulus fibrosis, superior CEP, inferior CEP, and NP with modified Pfirrmann grades were significant (P<0.05) with R values of 0.51, 0.36, 0.38, and -0.94, respectively. Correlations of T1ρ values of the outer anterior annulus fibrosis, outer posterior annulus fibrosis, and NP with ages were significant with R equal to 0.52, 0.71, and -0.76, respectively. T1ρ differences of the outer posterior annulus fibrosis, inferior CEP, and NP between the subjects with and without LBP were significant (P=0.005, 0.020, and 0.000, respectively). The UTE-Adiab-T1ρ sequence can quantify T1ρ of whole IVDs including CEPs. This is an advance, and of value for comprehensive assessment of IVD degeneration.

Impact of acceleration on bone depiction quality by ultrashort echo time magnetic resonance bone imaging sequences in medication-related osteonecrosis of the jaw

ObjectivesTo assess the impact on bone depiction quality by decreasing number of radial acquisitions (RA) of a UTE MR bone imaging sequence in MRONJ. Material and methodsUTE MR bone imaging sequences using pointwise encoding time reduction with RA (PETRA) with 60’000, 30’000 and 10’000 RA were acquired in 16 patients with MRONJ and 16 healthy volunteers. Blinded readout sessions were performed by two radiologists. Qualitative analysis compared the detection of osteolytic lesions and productive bony changes in the PETRA sequences of the patients with MRONJ. Quantitative analysis assessed the differences in image artifacts, contrast-to-noise ratio (CNR) and image noise. ResultsAcquisition times were reduced from 315 to 165 and 65 s (60’000, 30’000, 10’000 RA, respectively), resulting in a fewer number of severe motion artifacts. Bone delineation was increasingly blurred when reducing the number of RA but without any trade-off in terms of diagnostic performance. Interreader agreement for the detection of pathognomonic osteolysis was moderate (κ = 0.538) for 60’000 RA and decreased to fair (κ = 0.227 and κ = 0.390) when comparing 30’000 and 10’000 RA, respectively. Image quality between sequences was comparable regarding CNR, image noise and artifact dimensions without significant differences (all P > 0.05). ConclusionsUTE MR bone imaging sequences with a lower number of RA provide sufficient image quality for detecting osteolytic lesions and productive bony changes in MRONJ subjects at faster acquisition times compared to the respective standard UTE MR bone imaging sequence.

MR Imaging of the Pelvic Bones: The Current and Cutting-Edge Techniques.

GRE, SWI, UTE, ZTE MRI and synthetic CT sequences depict the cortical outline of the bones better in comparison to conventional MR images.MRI-based synthetic CT can create HU maps and allows for automated segmentation of pelvic bones.The current and cutting-edge MR techniques for bone imaging are complementary in the characterization of a variety of musculoskeletal disorders.

Applying Compressed Sensing Volumetric Interpolated Breath-Hold Examination and Spiral Ultrashort Echo Time Sequences for Lung Nodule Detection in MRI.

This prospective study aimed to investigate the ability of spiral ultrashort echo time (UTE) and compressed sensing volumetric interpolated breath-hold examination (CS-VIBE) sequences in magnetic resonance imaging (MRI) compared to conventional VIBE and chest computed tomography (CT) in terms of image quality and small nodule detection. Patients with small lung nodules scheduled for video-assisted thoracoscopic surgery (VATS) for lung wedge resection were prospectively enrolled. Each patient underwent non-contrast chest CT and non-contrast MRI on the same day prior to thoracic surgery. The chest CT was performed to obtain a standard reference for nodule size, location, and morphology. The chest MRI included breath-hold conventional VIBE and CS-VIBE with scanning durations of 11 and 13 s, respectively, and free-breathing spiral UTE for 3.5–5 min. The signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and normal structure visualizations were measured to evaluate MRI quality. Nodule detection sensitivity was evaluated on a lobe-by-lobe basis. Inter-reader and inter-modality reliability analyses were performed using the Cohen κ statistic and the nodule size comparison was performed using Bland–Altman plots. Among 96 pulmonary nodules requiring surgery, the average nodule diameter was 7.7 ± 3.9 mm (range: 4–20 mm); of the 73 resected nodules, most were invasive cancer (74%) or pre-invasive carcinoma in situ (15%). Both spiral UTE and CS-VIBE images achieved significantly higher overall image quality scores, SNRs, and CNRs than conventional VIBE. Spiral UTE (81%) and CS-VIBE (83%) achieved a higher lung nodule detection rate than conventional VIBE (53%). Specifically, the nodule detection rate for spiral UTE and CS-VIBE reached 95% and 100% for nodules >8 and >10 mm, respectively. A 90% detection rate was achieved for nodules of all sizes with a part-solid or solid morphology. Spiral UTE and CS-VIBE under-estimated the nodule size by 0.2 ± 1.4 mm with 95% limits of agreement from −2.6 to 2.9 mm and by 0.2 ± 1.7 mm with 95% limits of agreement from −3.3 to 3.5 mm, respectively, compared to the reference CT. In conclusion, chest CT remains the gold standard for lung nodule detection due to its high image resolutions. Both spiral UTE and CS-VIBE MRI could detect small lung nodules requiring surgery and could be considered a potential alternative to chest CT; however, their clinical application requires further investigation.

Ultrashort Echo Time Magnetic Resonance Imaging Techniques: Met and Unmet Needs in Musculoskeletal Imaging.

This review article summarizes recent technical developments in ultrashort echo time (UTE) magnetic resonance imaging of musculoskeletal (MSK) tissues with short-T2 relaxation times. A series of contrast mechanisms are discussed for high-contrast morphological imaging of short-T2 MSK tissues including the osteochondral junction, menisci, ligaments, tendons, and bone. Quantitative UTE mapping of T1, T2*, T1ρ, adiabatic T1ρ, magnetization transfer ratio, MT modeling of macromolecular proton fraction, quantitative susceptibility mapping, and water content is also introduced. Met and unmet needs in MSK imaging are discussed. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 3.

Evaluation of enzymatic proteoglycan loss and collagen degradation in human articular cartilage using ultrashort echo time-based biomarkers: A feasibility study.

The objective of the current study was to investigate the feasibility of quantitative 3D ultrashort echo time (UTE)-based biomarkers in detecting proteoglycan (PG) loss and collagen degradation in human cartilage. A total of 104 cartilage samples were harvested for a trypsin digestion study (n = 44), and a sequential trypsin and collagenase digestion study (n = 60), respectively. Forty-four cartilage samples were randomly divided into a trypsin digestion group (tryp group) and a control group (phosphate-buffered saline [PBS] group) (n = 22 for each group) for the trypsin digestion experiment. The remaining 60 cartilage samples were divided equally into four groups (n = 15 for each group) for sequential trypsin and collagenase digestion, including PBS + Tris (incubated in PBS, then Tris buffer solution), PBS + 30 U col (incubated in PBS, then 30 U/ml collagenase [30 U col] with Tris buffer solution), tryp + 30 U col (incubated in trypsin solution, then 30 U/ml collagenase with Tris buffer solution), and tryp + Tris (incubated in trypsin solution, then Tris buffer solution). The 3D UTE-based MRI biomarkers included T1 , multiecho T2 *, adiabatic T1ρ (AdiabT1ρ ), magnetization transfer ratio (MTR), and modeling of macromolecular proton fraction (MMF). For each cartilage sample, UTE-based biomarkers (T1 , T2 *, AdiabT1ρ , MTR, and MMF) and sample weight were evaluated before and after treatment. PG and hydroxyproline assays were performed. Differences between groups and correlations were assessed. All the evaluated biomarkers were able to differentiate between healthy and degenerated cartilage in the trypsin digestion experiment, but only T1 and AdiabT1ρ were significantly correlated with the PG concentration in the digestion solution (p=0.004 and p= 0.0001, respectively). In the sequential digestion experiment, no significant differences were found for T1 and AdiabT1ρ values between the PBS + Tris and PBS + 30 U col groups (p=0.627 and p= 0.877, respectively), but T1 and AdiabT1ρ values increased significantly in the tryp + Tris (p=0.031 and p= 0.024, respectively) and tryp + 30 U col groups (both p<0.0001). Significant decreases in MMF and MTR were found in the tryp + 30 U col group compared with the PBS + Tris group (p=0.002 and p= 0.001, respectively). It was concluded that AdiabT1ρ and T1 have the potential for detecting PG loss, while MMF and MTR are promising for the detection of collagen degradation in articular cartilage, which could facilitate earlier, noninvasive diagnosis of osteoarthritis.

Direct imaging of white matter ultrashort [formula omitted] components at 7 Tesla

PurposeTo demonstrate direct imaging of the white matter ultrashort T2∗ components at 7 Tesla using inversion recovery (IR)-enhanced ultrashort echo time (UTE) MRI. To investigate its characteristics, potentials and limitations, and to establish a clinical protocol. Material and methodsThe IR UTE technique suppresses long T2∗ signals within white matter by using adiabatic inversion in combination with dual-echo difference imaging. Artifacts arising at 7 T from long T2∗ scalp fat components were reduced by frequency shifting the IR pulse such that those frequencies were inverted likewise. For 8 healthy volunteers, the T2∗ relaxation times of white matter were then quantified. In 20 healthy volunteers, the UTE difference and fraction contrast were evaluated. Finally, in 6 patients with multiple sclerosis (MS), the performance of the technique was assessed. ResultsA frequency shift of −1.2 ppm of the IR pulse (i.e. towards the fat frequency) provided a good suppression of artifacts. With this, an ultrashort compartment of (68 ± 6) % with a T2∗ time of (147 ± 58) μs was quantified with a chemical shift of (−3.6 ± 0.5) ppm from water. Within healthy volunteers' white matter, a stable ultrashort T2∗ fraction contrast was calculated. For the MS patients, a significant fraction reduction in the identified lesions as well as in the normal-appearing white matter was observed. ConclusionsThe quantification results indicate that the observed ultrashort components arise primarily from myelin tissue. Direct IR UTE imaging of the white matter ultrashort T2∗ components is thus feasible at 7 T with high quantitative inter-subject repeatability and good detection of signal loss in MS.

Can magnetic resonance imaging accurately and reliably measure humeral cortical thickness?

BackgroundHistorically, imaging osseous detail in three dimensions required a computed tomography (CT) scan with ionizing radiation that poorly visualizes the soft tissues. The purpose of this study was to determine the accuracy and reliability of ultrashort echo time (UTE) magnetic resonance imaging (MRI) in measuring humeral cortical thickness and cancellous density as compared with CT.MethodsThis was a comparative radiographic study in nine cadavers, each of which underwent CT and UTE MRI. On images aligned to the center of the humeral shaft, anterior, posterior, medial, and lateral humeral cortical thickness was measured 5, 10, and 15 cm distal to the top of the head. Cancellous density was measured as signal within a 1-cm diameter region of interest in the center of the head, the subtuberosity head, the subarticular head, and the subarticular glenoid vault. Glenoid cortical thickness was measured at the center of the glenoid. Cortical measurements were compared using mean differences and 95% confidence intervals, paired Student’s t-tests, and intraclass correlation coefficients (ICCs). We compared cancellous measurements using Pearson’s correlation coefficients. For all measurements, we calculated interobserver and intraobserver reliability using ICCs with 0.75 as the lower limit for acceptability.ResultsWith regard to accuracy, for humeral cortical thickness measurements, there were no significant differences between MRI and CT measures, and ICCs were >0.75. The glenoid cortical thickness ICC was <0.75. There was no significant correlation between the cancellous signal on MRI and on CT in any region. For both MRI and CT, interobserver reliability and intraobserver reliability were acceptable (ie, >0.75) for almost all humeral cortical thickness measures.ConclusionUTE MRI can reliably and accurately measure humeral cortical thickness, but cannot accurately measure cancellous density or accurately and reliably measure glenoid cortical thickness.

Spatially resolved free-induction decay spectroscopy using a 3D ultra-short echo time multi-echo imaging sequence with systematic echo shifting and compensation of B0 field drifts.

Biologically interesting signals can exhibit fast transverse relaxation and frequency shifts compared to free water. For spectral assignment, a ultra-short echo time (UTE) imaging sequence was modified to provide pixel-wise free-induction decay (FID) acquisition. The UTE-FID approach presented relies on a multi-echo 3D spiral UTE sequence with six echoes per radiofrequency (RF) excitation (TEmin 0.05ms, echo spacing 3ms). A complex pixel-wise raw data set for FID spectroscopy is obtained by several multi-echo UTE measurements with systematic shifting of the readout by 0.25 or 0.5ms, until the time domain is filled for 18 or 45ms. B0 drifts are compensated by mapping and according phase correction. Autoregressive extrapolation of the signal is performed before Gaussian filtering. This method was applied to a phantom containing collagen-water solutions of different concentrations. To calculate the collagen content, a 19-peak collagen model was extracted from a non-selective FID spectrum (50% collagen solution). Proton-density-collagen-fraction (PDCF) was calculated for 10 collagen solutions (2%-50%). Furthermore, an in vivo UTE-FID spectrum of adipose tissue was recorded. UTE-FID signal patterns agreed well with the non-spatially selective pulse-acquire FID spectrum from a sphere filled with 50% collagen. Differentiation of collagen solution from distilled water in the PDCF map was possible from 4% collagen concentration for a UTE-FID sequence with 128 × 128 × 64 matrix (voxel size 1 × 1 × 2.85mm3 ). The mean values of the PDCF correlate linearly with collagen concentration. The presented UTE-FID approach allows pixel-wise raw data acquisition similar to non-spatially selective pulse-acquire spectroscopy. Spatially resolved applications for assessment of spectra of rapidly decaying signals seem feasible.

The interface region between articular cartilage and bone by μMRI and PLM at microscopic resolutions.

This dual-modality microscopic imaging study quantifies the interface region between the noncalcified cartilage and the subchondral bone plate, which includes the deep portion of the noncalcified articular cartilage and the zone of calcified cartilage (ZCC). This interface region is typically not visible in routine MRI but becomes visible in MRI with the application of an ultra-short echo time (UTE) sequence. A number of cartilage-bone blocks from a well-documented canine humeral head were harvested for imaging by microscopic MRI (μMRI) and PLM (polarized light microscopy). In μMRI, T2 anisotropic images were acquired by 2D gradient-echo, magnetization-prepared spin-echo and UTE sequences at the 0° and 55° (the magic angle) orientations at 11.7μm/pixel resolution. In PLM, quantitative optical retardation (nm) and collagen orientation (°) were mapped from the thin sections from the same μMRI specimens at 0.5-2μm pixel resolutions. The orientational and organizational architecture of the collagen matrix in this interface region was quantified and correlated between the complementary imaging. The magic angle effect as seen in the noncalcified cartilage was statistically confirmed in ZCC in μMRI, which was further supported by quantitative PLM. With an enhanced understanding of the tissue properties in this important interface region, it will potentially be possible to monitor the changes of this tissue region which is instrumental to the initiation and development of osteoarthritis and other joint diseases.

Detection of lung lesions in breath-hold VIBE and free-breathing Spiral VIBE MRI compared to CT

BackgroundDetection of pulmonary nodules in MRI requires fast imaging strategies without respiratory motion impairment, such as single-breath-hold Cartesian VIBE. As patients with pulmonary diseases have limited breath-hold capacities, this study investigates the clinical feasibility of non-Cartesian Spiral VIBE under free-breathing compared to CT as the gold standard.MethodsProspective analysis of 27 oncological patients examined in PET/CT and PET/MR. A novel motion-robust 3D ultrashort-echo-time (UTE) MR sequence was evaluated in comparison with CT and conventional breath-hold MR. CT scans were performed under breath-hold in end-expiratory and end-inspiratory position (CT ex, CT in). MR data was acquired with non-contrast-enhanced breath-hold Cartesian VIBE followed by a free-breathing 3D UTE Spiral VIBE. Impact of respiratory motion on pulmonary evaluation was investigated by two readers in Cartesian VIBE, followed by UTE Spiral VIBE and CT ex and the reference standard of CT in. Diagnostic accuracy was calculated, and visual image quality assessed.ResultsHigher detection rate and sensitivity of pulmonary nodules in free-breathing UTE Spiral VIBE in comparison with breath-hold Cartesian VIBE were found for lesions > 10 mm (UTE Spiral VIBE/VIBE/CT ex): 93%/54%/100%; Lesions 5–10 mm: 67%/25%/ 92%; Lesions < 5 mm: 11%/11%/78%. Lobe-based analysis revealed sensitivities and specificities of 64%/96%/41% and 96%/93%/100% for UTE Spiral VIBE/VIBE/CT ex.ConclusionFree-breathing UTE Spiral VIBE indicates higher sensitivity for detection of pulmonary nodules than breath-hold Cartesian VIBE and is a promising but time-consuming approach. However, sensitivity and specificity of inspiratory CT remain superior in comparison and should be preferred for detection of pulmonary lesions.