Two-dimensional ultrashort echo time imaging using a spiral trajectory

Bone Biomarkers Based on Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is increasingly used to evaluate the microstructural and compositional properties of bone. MRI-based biomarkers can characterize all major compartments of bone: organic, water, fat, and mineral components. However, with a short apparent spin-spin relaxation time (T2*), bone is invisible to conventional MRI sequences that use long echo times. To address this shortcoming, ultrashort echo time MRI sequences have been developed to provide direct imaging of bone and establish a set of MRI-based biomarkers sensitive to the structural and compositional changes of bone. This review article describes the MRI-based bone biomarkers representing total water, pore water, bound water, fat fraction, macromolecular fraction in the organic matrix, and surrogates for mineral density. MRI-based morphological bone imaging techniques are also briefly described.

Ultra-short T2 components imaging of the whole brain using 3D dual-echo UTE MRI with rosette k-space pattern

PURPOSE

This study aimed to develop a new 3D dual-echo rosette k-space trajectory, specifically designed for UTE MRI applications. The imaging of the ultra-short transverse relaxation time (uT₂ ) of brain was acquired to test the performance of the proposed UTE sequence.

THEORY AND METHODS

The rosette trajectory was developed based on rotations of a "petal-like" pattern in the k_x -k_y plane, with oscillated extensions in the k_z -direction for 3D coverage. Five healthy volunteers underwent 10 dual-echo 3D rosette UTE scans with various TEs. Dual-exponential complex model fitting was performed on the magnitude data to separate uT₂ signals, with the output of uT₂ fraction, uT₂ value, and long-T₂ value.

RESULTS

The 3D rosette dual-echo UTE sequence showed better performance than a 3D radial UTE acquisition. More significant signal intensity decay in white matter than gray matter was observed along with the TEs. The white matter regions had higher uT₂ fraction values than gray matter (10.9% ± 1.9% vs. 5.7% ± 2.4%). The uT₂ value was approximately 0.10 ms in white matter .

CONCLUSION

The higher uT₂ fraction value in white matter compared to gray matter demonstrated the ability of the proposed sequence to capture rapidly decaying signals.

Making the invisible visible-ultrashort echo time magnetic resonance imaging: Technical developments and applications

Magnetic resonance imaging (MRI) uses a large magnetic field and radio waves to generate images of tissues in the body. Conventional MRI techniques have been developed to image and quantify tissues and fluids with long transverse relaxation times (T2s), such as muscle, cartilage, liver, white matter, gray matter, spinal cord, and cerebrospinal fluid. However, the body also contains many tissues and tissue components such as the osteochondral junction, menisci, ligaments, tendons, bone, lung parenchyma, and myelin, which have short or ultrashort T2s. After radio frequency excitation, their transverse magnetizations typically decay to zero or near zero before the receiving mode is enabled for spatial encoding with conventional MR imaging. As a result, these tissues appear dark, and their MR properties are inaccessible. However, when ultrashort echo times (UTEs) are used, signals can be detected from these tissues before they decay to zero. This review summarizes recent technical developments in UTE MRI of tissues with short and ultrashort T2 relaxation times. A series of UTE MRI techniques for high-resolution morphological and quantitative imaging of these short-T2 tissues are discussed. Applications of UTE imaging in the musculoskeletal, nervous, respiratory, gastrointestinal, and cardiovascular systems of the body are included.

Can Bone Erosion in Axial Spondyloarthropathy be Detected by Ultrashort Echo Time Imaging? A Comparison With Computed Tomography in the Sacroiliac Joint

BACKGROUND

Structural lesion evaluation in axial spondyloarthropathy (SpA) can improve accuracy of diagnosis. However, structural lesions (bone erosions) are difficult to be assessed using conventional MRI compared to computed tomography (CT).

PURPOSE

To evaluate the diagnostic performance of ultrashort echo time (UTE) for detecting bone erosion in axial SpA compared to T1WI and three-dimensional double-echo steady-state (3D DESS) imaging using CT as the reference standard.

STUDY TYPE

Retrospective.

POPULATION

Fourteen patients (eight females, 57.1%) and 14 healthy controls (seven females, 50.0%) who underwent sacroiliac (SI) joint MRI and CT.

FIELD STRENGTH/SEQUENCE

3 T; TSE T1WI, 3D DESS, 2D UTE.

ASSESSMENT

The bilateral SI joints were assessed for bone erosion. Three observers scored bone erosion for all three sequences of MRI. CT was used as the gold standard. Diagnostic confidence in axial SpA was measured based on a four-point confidence score.

STATISTICAL TESTS

Correlation of erosion scores between CT and MRI were evaluated using Spearman's correlation test. Sensitivity, specificity, and positive-negative predictive values were calculated. Confidence scores were compared using the Wilcoxon sum rank test. Statistical significance was set at P < 0.05.

RESULTS

Compared with erosion scores of CT, the correlation coefficients for each MRI sequence showed significant low-to-high positive correlations (0.39-0.72). UTE imaging showed the highest correlation coefficients for all observers (0.70, 0.72, and 0.67, respectively). The specificity of UTE imaging was equal or higher than those of T1WI and 3D DESS for all observers (0.86 vs. 0.71 vs. 0.57; 0.93 vs. 0.71 vs. 0.57; 0.79 vs. 0.79 vs. 0.43). All observers had the highest confidence in interpreting UTE imaging for detecting bone erosion among the three sequences (3.5, 3.4, and 3.3 for UTE; 3.1, 3.0, and 2.6 for T1WI; and 3.2, 2.7, and 2.4 for DESS).

DATA CONCLUSION

UTE imaging can detect bone erosions in patients with axial SpA and show higher specificity than conventional T1WI and 3D DESS.

EVIDENCE LEVEL

4 TECHNICAL EFFICACY: Stage 2.

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 T₂ * 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, T₂ * 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.

MRI-based mechanical competence assessment of bone using micro finite element analysis (micro-FEA): Review

Areal bone mineral density (aBMD) from dual-energy x-ray absorptiometry (DEXA) and volumetric bone mineral density (vBMD) have demonstrated limited capabilities in the evaluation of bone mechanical competence and prediction of bone fracture. Predicting the macroscopic mechanical behavior of the bone structure has been challenging because of the heterogeneous and anisotropic nature of bone, such as the dependencies on loading direction, anatomical location, and sample dimensions. Magnetic resonance imaging (MRI) has been introduced as a promising modality that can be coupled with finite element analysis (FEA) for the assessment of bone mechanical competence. This review article describes studies investigating MRI-based micro-FEA as a potential non-invasive method to predict bone mechanical competence and facilitate bone fracture risk estimation without exposure to ionizing radiation. Specifically, the steps, applications, and future potential of FEA using indirect and direct bone imaging are discussed.

Articular Cartilage Assessment Using Ultrashort Echo Time MRI: A Review

Articular cartilage is a major component of the human knee joint which may be affected by a variety of degenerative mechanisms associated with joint pathologies and/or the aging process. Ultrashort echo time (UTE) sequences with a TE less than 100 µs are capable of detecting signals from both fast- and slow-relaxing water protons in cartilage. This allows comprehensive evaluation of all the cartilage layers, especially for the short T₂ layers which include the deep and calcified zones. Several ultrashort echo time (UTE) techniques have recently been developed for both morphological imaging and quantitative cartilage assessment. This review article summarizes the current catalog techniques based on UTE Magnetic Resonance Imaging (MRI) that have been utilized for such purposes in the human knee joint, such as T₁, T2∗ , T_1ρ, magnetization transfer (MT), double echo steady state (DESS), quantitative susceptibility mapping (QSM) and inversion recovery (IR). The contrast mechanisms as well as the advantages and disadvantages of these techniques are discussed.

Three-dimensional ultrashort echo time (3D UTE) magnetic resonance imaging (MRI) of the normal and degenerative disco-vertebral complex at 4.7 T: a feasibility study with longitudinal evaluation

OBJECTIVES

To assess feasibility of a three-dimensional ultrashort echo time (3D-UTE)-sequence to evaluate normal and pathological disco-vertebral complex (DVC), with assessment of its different portions in a rat model of degenerative disk disease (DDD) with histological correlation. To assess whether this sequence, in comparison with long echo time T2-weighted sequence, is able to monitor DDD with differentiation of early from chronic DVC changes in pathological mechanical conditions.

METHODS

Five rats were induced with DDD model by percutaneous disk trituration of the tail with an 18-G needle under US-guidance and imaged at 4.7 T. MRI protocol included fat-saturated-T2 (RARE) and 3D-UTE-sequences performed at baseline (day 0. n = 5 animals /10 DVC) and each week (W) from W1 to W10 postoperatively. Visual analysis and signal intensity measurements of SNR and CNR of all DVC portions were performed on RARE and UTE images. Following killing (baseline, n = 1/2 DVC; W2, n = 2/4 DVC; W10, n = 2/4 DVC), histological analysis was performed and compared with MRI.

RESULTS

In normal DVC, unlike conventional RARE-sequences, 3D-UTE allowed complete identification of DVC zonal anatomy including on visual analysis and CNR measurements. In pathological conditions, SNR and CNR measurements of the annulus fibrosus and nucleus pulposus on 3D-UTE distinguished early discitis at W1 from chronic discopathy (P < 0.001 for SNR and P < 0.001 for CNR). Neither the normal complete anatomy of the DVC nor its pathological patterns could be assessed on conventional sequences.

CONCLUSIONS

Unlike conventional sequences, 3D-UTE enables visualization of the complete normal DVC anatomy and enables monitoring of DDD differentiating between early DVC changes from chronic ones.

LEVEL OF EVIDENCE I

Diagnostic: individual cross-sectional studies with the consistently applied reference standard and blinding.

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.

Musculoskeletal MR Imaging Applications at Ultra-High (7T) Field Strength

Regulatory approval of ultrahigh field (UHF) MR imaging scanners for clinical use has opened new opportunities for musculoskeletal imaging applications. UHF MR imaging has unique advantages in terms of signal-to-noise ratio, contrast-to-noise ratio, spectral resolution, and multinuclear applications, thus providing unique information not available at lower field strengths. But UHF also comes with a set of technical challenges that are yet to be resolved and may not be suitable for all imaging applications. This review focuses on the latest research in musculoskeletal MR imaging applications at UHF including morphologic imaging, T₂, T₂∗, and T_1ρ mapping, chemical exchange saturation transfer, sodium imaging, and phosphorus spectroscopy imaging applications.

Fast quantitative three-dimensional ultrashort echo time (UTE) Cones magnetic resonance imaging of major tissues in the knee joint using extended sprial sampling

The purpose of this study is to investigate the effect of extending the spiral sampling window on quantitative 3D ultrashort echo time (UTE) Cones imaging of major knee joint tissues including articular cartilage, menisci, tendons and ligaments at 3 T. Nine cadaveric human whole knee specimens were imaged on a 3 T clinical MRI scanner. A series of quantitative 3D UTE Cones imaging biomarkers including T₂ *, T₁ , adiabatic T_1ρ , magnetization transfer ratio (MTR) and macromolecular fraction (MMF) were estimated using spiral sampling trajectories with various durations. Errors in UTE MRI biomarkers as a function of sampling time were evaluated using a nonstretched spiral trajectory as a reference standard. No significant differences were observed by increasing the spiral sampling window from 1116 to 2232 μs in the calculated T₂ *, T₁ , adiabatic T_1ρ , MTR and MMF, as all P-values were over .05 as assessed by ANOVA with two-sided Dunnett's test. Although extending the sampling window results in signal loss for short T₂ components, there was limited effect on the calculated quantitative biomarkers, with error percentages typically smaller than 5% in all the evaluated tissues. The total scan time can be reduced by up to 54% with quantification errors of less than 5% in any evaluated major tissue in the knee joint, suggesting that 3D UTE Cones MRI techniques can be greatly accelerated by using a longer spiral sampling window without causing additional quantitative bias.

Quantitative Magnetic Resonance Imaging of Cortical and Trabecular Bone

Bone is a composite material consisting of mineral, organic matrix, and water. Water in bone can be categorized as bound water (BW), which is bound to bone mineral and organic matrix, or as pore water (PW), which resides in Haversian canals as well as in lacunae and canaliculi. Bone is generally classified into two types: cortical bone and trabecular bone. Cortical bone is much denser than trabecular bone that is surrounded by marrow and fat. Magnetic resonance (MR) imaging has been increasingly used for noninvasive assessment of both cortical bone and trabecular bone. Bone typically appears as a signal void with conventional MR sequences because of its short T2*. Ultrashort echo time (UTE) sequences with echo times 100 to 1,000 times shorter than those of conventional sequences allow direct imaging of BW and PW in bone. This article summarizes several quantitative MR techniques recently developed for bone evaluation. Specifically, we discuss the use of UTE and adiabatic inversion recovery prepared UTE sequences to quantify BW and PW, UTE magnetization transfer sequences to quantify collagen backbone protons, UTE quantitative susceptibility mapping sequences to assess bone mineral, and conventional sequences for high-resolution imaging of PW as well as the evaluation of trabecular bone architecture.

An Update in Qualitative Imaging of Bone Using Ultrashort Echo Time Magnetic Resonance

Bone is comprised of mineral, collagenous organic matrix, and water. X-ray-based techniques are the standard approach for bone evaluation in clinics, but they are unable to detect the organic matrix and water components in bone. Magnetic resonance imaging (MRI) is being used increasingly for bone evaluation. While MRI can non-invasively assess the proton pools in soft tissues, cortical bone typically appears as a signal void with clinical MR techniques because of its short T2*. New MRI techniques have been recently developed to image bone while avoiding the ionizing radiation present in x-ray-based methods. Qualitative bone imaging can be achieved using ultrashort echo time (UTE), single inversion recovery UTE (IR-UTE), dual-inversion recovery UTE (Dual-IR-UTE), double-inversion recovery UTE (Double-IR-UTE), and zero echo time (ZTE) sequences. The contrast mechanisms as well as the advantages and disadvantages of each technique are discussed.

3D-T1ρ prepared zero echo time-based PETRA sequence for in vivo biexponential relaxation mapping of semisolid short-T2 tissues at 3 T

BACKGROUND

In addition to the articular cartilage, osteoarthritis (OA) affects several other tissues such as tendons, ligaments, and subchondral bone. T1ρ relaxation study of these short T2 tissues may provide a more comprehensive evaluation of OA.

PURPOSE

To develop a 3D spin-lattice relaxation in the rotating frame (T1ρ ) prepared zero echo time (ZTE)-based pointwise encoding time reduction with radial acquisition (3D-T1ρ -PETRA) sequence for relaxation mapping of semisolid short-T2 tissues on a clinical 3 T scanner.

STUDY TYPE

Prospective.

POPULATION

Phantom, two bovine whole knee joint and Achilles tendon specimens, 10 healthy volunteers with no known inflammation, trauma or pain in the knee or ankle.

FIELD STRENGTH/SEQUENCE

A customized PETRA sequence to acquire fat-suppressed 3D T1ρ -weighted images tissues with semisolid short T2 / T 2 * relaxation times in the knee and ankle joints at 3 T.

ASSESSMENT

Mono- and biexponential T1ρ relaxation components were assessed in the patellar tendon (PT), anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and Achilles tendon (AT).

STATISTICAL TESTS

Kruskal-Wallis with post-hoc Dunn's test for multiple pairwise comparisons.

RESULTS

Phantom and ex vivo studies showed the feasibility of T1ρ relaxation mapping using the proposed 3D-T1ρ -PETRA sequence. The in vivo study demonstrated an averaged mono-T1ρ relaxation of (median [IQR]) 15.9 [14.5] msec, 23.6 [9.4] msec, 17.4 [7.4] msec, and 5.8 [10.2] msec in the PT, ACL, PCL, and AT, respectively. The bicomponent analysis showed the short and long components (with their relative fractions) of 0.65 [1.0] msec (46.9 [15.3]%) and 37.3 [18.4] msec (53.1 [15.3]%) for PT, 1.7 [2.1] msec (42.5 [12.5]%) and 43.7 [17.8] msec (57.5 [12.5]%) for ACL, and 1.2 [1.9] msec (42.6 [14.0]%) and 27.7 [14.7] msec (57.3 [14.0]%) for PCL and 0.4 [0.02] msec (58.8 [13.3]%/) and 31.3 [10.8] msec (41.2 [13.3]%) for AT. Statistically significant (P ≤ 0.05) differences were observed in the mono- and biexponential relaxation between several regions.

DATA CONCLUSION

The 3D-T1ρ -PETRA sequence allows volumetric, isotropic (0.78 × 0.78 × 0.78 mm), biexponential T1ρ assessment with corresponding fractions of the tissues with semisolid short T2 / T 2 * .

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019;50:1207-1218.

Short-T2 MRI: Principles and recent advances

Among current modalities of biomedical and diagnostic imaging, MRI stands out by virtue of its versatile contrast obtained without ionizing radiation. However, in various cases, e.g., water protons in tissues such as bone, tendon, and lung, MRI performance is limited by the rapid decay of resonance signals associated with short transverse relaxation times T₂ or T₂*. Efforts to address this shortcoming have led to a variety of specialized short-T₂ techniques. Recent progress in this field expands the choice of methods and prompts fresh considerations with regard to instrumentation, data acquisition, and signal processing. In this review, the current status of short-T₂ MRI is surveyed. In an attempt to structure the growing range of techniques, the presentation highlights overarching concepts and basic methodological options. The most frequently used approaches are described in detail, including acquisition strategies, image reconstruction, hardware requirements, means of introducing contrast, sources of artifacts, limitations, and applications.

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.

Fast quantitative 3D ultrashort echo time MRI of cortical bone using extended cones sampling

PURPOSE

To investigate the effect of stretching sampling window on quantitative 3D ultrashort TE (UTE) imaging of cortical bone at 3 T.

METHODS

Ten bovine cortical bone and 17 human tibial midshaft samples were imaged with a 3T clinical MRI scanner using an 8-channel knee coil. Quantitative 3D UTE imaging biomarkers, including T₁ , T 2 ∗ , magnetization transfer ratio and magnetization transfer modeling, were performed using radial or spiral Cones sampling trajectories with various durations. Errors in UTE-MRI biomarkers as a function of sampling time were evaluated using radial sampling as a reference standard.

RESULTS

For both bovine and human cortical bone samples, no significant differences were observed for all UTE biomarkers (single-component T 2 ∗ , bicomponent T 2 ∗ and relative fractions, T₁ , magnetization transfer ratio, and magnetization transfer modeling of macromolecular fraction) for spiral sampling windows of 992 µs to 1600 µs compared with a radial sampling window of 688 µs.

CONCLUSION

The total scan time can be reduced by 76% with quantification errors less than 5%. Quantitative UTE-MRI techniques can be greatly accelerated using longer sampling windows without significant quantification errors.

Three-dimensional ultrashort echo time (3D UTE) MRI of Achilles tendon at 4.7T MRI with comparison to conventional sequences in an experimental murine model of spondyloarthropathy

BACKGROUND

Due to the very short T₂ of its components, the normal anatomy of Achilles enthesis is impossible to define with "conventional" long echo time (TE) T₂ sequences. However, this is a common site affected by rheumatologic disease. Early abnormalities related to inflammatory processes are impossible to detect in this location.

PURPOSE

To assess the feasibility of a 3D-UTE (ultrashort echo time) sequence to evaluate normal and pathological Achilles entheses, determining both anterior fibrocartilaginous and posterior collagenic portions at 4.7T, in a rat model of spondyloarthropathy (SpA) with histological correlation. To assess whether this sequence detects SpA enthesopathy prior to long TE T₂ sequences, enabling disease monitoring.

STUDY TYPE

Prospective case-control study.

ANIMAL MODEL

Twelve immunocompetent Wistar male rats imaged before (controls); the model was induced in eight rats (16 tendons) imaged at day 6, day 13, and day 21 with regular sacrifice for ex vivo imaging and histological correlation.

FIELD STRENGTH

4.7T Bruker Biospec Systems. 3D balanced steady-state free precession (bSSFP) and 3D-UTE sequences, performed at baseline (day 0, n = 12 animals / 24 tendons), day 6 (n = 8/16), 13 (n = 4/8), and day 21 (n = 2/4).

ASSESSMENT

Visual analysis and signal intensity measurements (signal to noise ratio, SNR) of both bSSFP and UTE images were performed by two independent musculoskeletal radiologists at different locations of the Achilles enthesis and preinsertional area.

STATISTICAL TESTS

Normal and pathological rat values were compared by Wilcoxon signed-rank tests, as well as interobserver differences. MRI findings were compared against histological data.

RESULTS

The 3D-UTE sequence identified the anterior fibrocartilage and posterior collagenic areas of Achilles entheses in all cases. Visual analysis and signal intensity measurements distinguished SpA-affected entheses from healthy ones at days 6 and 13 (P = 0.002 and P = 0.006, respectively). Neither the normal anatomy of the enthesis nor its pathological pattern could be identified on T₂ bSSFP sequences.

DATA CONCLUSION

Unlike bSSFP T₂ sequences, 3D-UTE sequences enable visualization of normal enthesis anatomy and early detection of abnormalities in pathological conditions.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:127-135.

3D ultrashort echo time MRI of the lung using stack-of-spirals and spherical k-Space coverages: Evaluation in healthy volunteers and parenchymal diseases

BACKGROUND

Ultrashort echo time (UTE) has been shown to improve lung MRI quality in three dimensions. The evaluation of 3D-UTE stack-of-spirals VIBE (3D-USV) sequence for parenchymal diseases and a comparison of performance with that of a spherical mode of acquisition is needed.

PURPOSE

To assess MRI quality using a prototypical 3D-USV sequence and to compare performance with that of a spherical acquisition using Pointwise Encoding Time Reduction with Radial Acquisition (PETRA).

STUDY TYPE

Monocenter, prospective.

POPULATION

Twelve healthy volunteers and 32 adult patients with either cystic fibrosis (CF; n = 16) or interstitial lung disease (ILD; n = 16).

FIELD STRENGTH/SEQUENCE

Both free-breathing 3D-USV and PETRA were completed at 1.5T.

ASSESSMENT

In healthy volunteers, visual analysis of imaging quality was scored using a Likert scale. Quantitative evaluation of apparent signal ratio (Sr) and contrast ratio (Cr) was measured. Patients with CF and ILD completed both computed tomography (CT) and MRI. Depiction of structural alterations was assessed using dedicated clinical scores. All evaluations were done in consensus by two readers.

STATISTICAL TESTS

Comparison of means was assessed using the Wilcoxon signed rank test. Concordance and agreement between CT and MRI were assessed using the intraclass correlation coefficient (ICC) and kappa test.

RESULTS

In controls, 3D-USV yielded lower artifacts owing to better automatic respiratory synchronization than PETRA (P < 0.001). However, Sr and Cr of 3D-USV were found significantly lower by 2.25- and 2.36-fold, respectively (P < 0.001). In patients, 3D-USV and PETRA showed comparable performances to assess airway severity in CF (Bhalla score, ICC = 0.89 and ICC = 0.92, respectively) and presence of structural alterations in ILD such as honeycombing (kappa = 0.68 and kappa = 0.69, respectively).

DATA CONCLUSION

3D-USV enables high-resolution morphological imaging of the lung without need of an external device to compensate respiratory motions. Automation and robustness of the method may facilitate clinical application for both airway and interstitial lung investigations.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;48:1489-1497.

A double echo ultra short echo time (UTE) acquisition for respiratory motion-suppressed high resolution imaging of the lung

PURPOSE

Magnetic resonance imaging is a promising alternative to computed tomography for lung imaging. However, organ motion and poor signal-to-noise ratio, arising from short T2*, impair image quality. To alleviate these issues, a new retrospective gating method was implemented and tested with an ultra-short echo time sequence.

METHODS

A 3D double-echo ultra-short echo time sequence was used to acquire data during free breathing in ten healthy adult subjects. A self-gating method was used to reconstruct respiratory motion suppressed expiratory and inspiratory images. These images were objectively compared to uncorrected data sets using quantitative end-points (pulmonary vessel sharpness, lung-liver interface definition, signal-to-noise ratio). The method was preliminarily tested in two cystic fibrosis patients who underwent computed tomography.

RESULTS

Vessel sharpness in expiratory ultra-short echo time data sets with second echo motion detection was significantly higher (13% relative increase) than in uncorrected images while the opposite was observed in inspiratory images. The method was successfully applied in patients and some findings (e.g., hypointense areas) were similar to those from computed tomography.

CONCLUSION

Free breathing ultra-short echo time was successfully implemented, allowing flexible image reconstruction of two different respiratory states. Objective improvements in image quality were obtained with the new method and initial feasibility in a clinical setting was demonstrated. Magn Reson Med 79:2297-2305, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Measurement of bound and pore water T1 relaxation times in cortical bone using three-dimensional ultrashort echo time cones sequences

PURPOSE

We present three-dimensional ultrashort echo time Cones (3D UTE Cones) techniques for quantification of total water T1 ( T1TW), bound water T1 ( T1BW), and pore water T1 ( T1PW) in vitro and in vivo using a 3 Tesla (T) scanner.

METHODS

T1TW, T1BW, and T1PW were measured with three-dimensional (3D) Cones and adiabatic inversion recovery Cone (IR-Cone) sequences. Two-dimensional (2D) nonselective ultrashort echo time (UTE) techniques, including saturation recovery, variable repetition times (TRs), and inversion recovery (IR) preparation approaches were compared with 3D-Cones techniques on bovine cortical bone samples (n = 8). The 3D Cones sequences were used to measure T1TW, T1BW, and T1PW in the tibial midshaft of healthy volunteers (n = 8).

RESULTS

Comparable T1 images were achieved for cortical bone between 3D Cones and 2D UTE techniques as well as those published in the literature. The 3D Cones sequences showed a mean T1TW of 208 ± 22 ms, a mean T1PW of 545 ± 28 ms, and a mean T1BW of 131 ± 12 ms for bovine cortical bone; and a mean T1TW of 246 ± 32 ms, a mean T1PW of 524 ± 46 ms, and a mean T1BW of 134 ± 11 ms for the tibial midshaft of healthy volunteers.

CONCLUSIONS

The 3D Cones sequences can be used for fast volumetric assessment of bound and pore water T1 images in vitro and in vivo. Magn Reson Med 77:2136-2145, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Sodium NMR/MRI for anisotropic systems

Sodium ((23)Na) plays a central role in many physiological processes, and its high NMR sensitivity makes it an attractive nucleus for biomedical NMR and MRI research. Many biological tissues contain structures such as fibers and membranes that impose anisotropic translational and rotational motions on the sodium ions. Translational motion can be studied by diffusion measurements. Anisotropic rotational motion results in non-vanishing quadrupolar interaction that it is best studied by exploiting multiple quantum coherences for (23)Na NMR spectroscopy and MRI. The current review covers the application of the various NMR techniques to the study of (23)Na in anisotropic compartments in cartilage, tendon, intervertebral discs, red blood cells, nervous system and muscles.

Ultrashort TE evaluation of the osteochondral junction in vivo: a feasibility study

OBJECTIVE

To determine whether differences in the osteochondral junction (OCJ) in two different age groups could be detected with ultrashort time-to-echo (UTE) MRI in vivo.

METHODS

10 healthy controls aged 20-30 years (Group 1) and 10 patients aged 40-50 years with knee pain but no evidence of osteoarthritis (OA) (Group 2) underwent 3-T MRI of the knee using a two-dimensional UTE sequence. Evaluation of the presence/absence of a high-signal-intensity (SI) line at the medial tibial/lateral tibial (MT/LT) OCJ was performed. Regions of interest (ROI) were created at the MT OCJ and LT OCJ. The peak OCJ SI ratio was calculated by measuring peak OCJ SI using averaged craniocaudal SI profiling, then dividing by the mean SI of a background ROI to standardize between studies. Groups were compared using χ(2) and Mann-Whitney U tests.

RESULTS

A high SI line was present in significantly more Group 1 subjects at both MT (p < 0.001) and LT (p = 0.002). There was a significant difference (p < 0.001) in MT peak OCJ SI ratio between Group 1 (mean ± SD = 0.85 ± 0.10) and Group 2 (0.77 ± 0.12). There was no significant difference (p = 0.19) in LT peak OCJ SI ratio between Group 1 (0.81 ± 0.23) and Group 2 (0.80 ± 0.13).

CONCLUSION

Significant differences in the UTE MR characteristics of the OCJ were demonstrated between the two age groups.

ADVANCES IN KNOWLEDGE

Alterations in the UTE appearances of the tibial plateau OCJ in the older group with knee pain compared with a younger, healthy control cohort may reflect the very early stages of OA.

Ultrashort echo time and zero echo time MRI at 7T

OBJECTIVE

Zero echo time (ZTE) and ultrashort echo time (UTE) pulse sequences for MRI offer unique advantages of being able to detect signal from rapidly decaying short-T2 tissue components. In this paper, we applied 3D ZTE and UTE pulse sequences at 7T to assess differences between these methods.

MATERIALS AND METHODS

We matched the ZTE and UTE pulse sequences closely in terms of readout trajectories and image contrast. Our ZTE used the water- and fat-suppressed solid-state proton projection imaging method to fill the center of k-space. Images from healthy volunteers obtained at 7T were compared qualitatively, as well as with SNR and CNR measurements for various ultrashort, short, and long-T2 tissues.

RESULTS

We measured nearly identical contrast-to-noise and signal-to-noise ratios (CNR/SNR) in similar scan times between the two approaches for ultrashort, short, and long-T2 components in the brain, knee and ankle. In our protocol, we observed gradient fidelity artifacts in UTE, and our chosen flip angle and readout also resulted in shading artifacts in ZTE due to inadvertent spatial selectivity. These can be corrected by advanced reconstruction methods or with different chosen protocol parameters.

CONCLUSION

The applied ZTE and UTE pulse sequences achieved similar contrast and SNR efficiency for volumetric imaging of ultrashort-T2 components. Key differences include that ZTE is limited to volumetric imaging, but has substantially reduced acoustic noise levels during the scan. Meanwhile, UTE has higher acoustic noise levels and greater sensitivity to gradient fidelity, but offers more flexibility in image contrast and volume selection.

Ultrashort echo time (UTE) imaging using gradient pre-equalization and compressed sensing

Ultrashort echo time (UTE) imaging is a well-known technique used in medical MRI, however, the implementation of the sequence remains non-trivial. This paper introduces UTE for non-medical applications and outlines a method for the implementation of UTE to enable accurate slice selection and short acquisition times. Slice selection in UTE requires fast, accurate switching of the gradient and r.f. pulses. Here a gradient "pre-equalization" technique is used to optimize the gradient switching and achieve an effective echo time of 10μs. In order to minimize the echo time, k-space is sampled radially. A compressed sensing approach is used to minimize the total acquisition time. Using the corrections for slice selection and acquisition along with novel image reconstruction techniques, UTE is shown to be a viable method to study samples of cork and rubber with a shorter signal lifetime than can typically be measured. Further, the compressed sensing image reconstruction algorithm is shown to provide accurate images of the samples with as little as 12.5% of the full k-space data set, potentially permitting real time imaging of short T2(*) materials.

UTE imaging in the musculoskeletal system

Tissues, such as bone, tendon, and ligaments, contain a high fraction of components with "short" and "ultrashort" transverse relaxation times and therefore have short mean transverse relaxation times. With conventional magnetic resonance imaging (MRI) sequences that employ relatively long echo times (TEs), there is no opportunity to encode the decaying signal of short and ultrashort T2 /T2 * tissues before it has reached zero or near zero. The clinically compatible ultrashort TE (UTE) sequence has been increasingly used to study the musculoskeletal system. This article reviews the UTE sequence as well as various modifications that have been implemented since its introduction. These modifications have been used to improve efficiency or contrast as well as provide quantitative analysis. This article reviews several clinical musculoskeletal applications of UTE.

UTE MRI of the Osteochondral Junction

The osteochondral junction is composed of numerous tissue components and serves important functions relating to structural stability and proper nutrition in joints such as the knee and spine. Conventional MR techniques have been inadequate at imaging the tissues of the osteochondral junction primarily because of the intrinsically short T2 nature of these tissues, rendering them "invisible" with the standard acquisitions. Ultrashort time to echo (UTE) MR techniques acquire sufficient MR signal of osteochondral tissues, thereby allowing direct evaluation. This article reviews the anatomy of the osteochondral junction of the knee and the spine, technical aspects of UTE MRI, and the application of UTE MRI for evaluation of the osteochondral junction.

Ultrashort echo time (UTE) magnetic resonance imaging of the short T2 components in white matter of the brain using a clinical 3T scanner

White matter of the brain contains a majority of long T2 components as well as a minority of short T2 components. These are not detectable using clinical magnetic resonance imaging (MRI) sequences with conventional echo times (TEs). In this study we used ultrashort echo time (UTE) sequences to investigate the ultrashort T2 components in white matter of the brain and quantify their T2*s and relative proton densities (RPDs) (relative to water with a proton density of 100%) using a clinical whole body 3T scanner. An adiabatic inversion recovery prepared dual echo UTE (IR-dUTE) sequence was used for morphological imaging of the ultrashort T2 components in white matter. IR-dUTE acquisitions at a constant TR of 1000 ms and a series of TIs were performed to determine the optimal TI which corresponded to the minimum signal to noise ratio (SNR) in white matter of the brain on the second echo image. T2*s of the ultrashort T2 components were quantified using mono-exponential decay fitting of the IR-dUTE signal at a series of TEs. RPD was quantified by comparing IR-dUTE signal of the ultrashort T2 components with that of a rubber phantom. Nine healthy volunteers were studied. The IR-dUTE sequence provided excellent image contrast for the ultrashort T2 components in white matter of the brain with a mean signal to noise ratio of 18.7 ± 3.7 and a contrast to noise ratio of 14.6 ± 2.4 between the ultrashort T2 white matter and gray matter in a 4.4 min scan time with a nominal voxel size of 1.25 × 1.25 × 5.0mm(3). On average a T2* value of 0.42 ± 0.08 ms and a RPD of 4.05 ± 0.88% were demonstrated for the ultrashort T2 components in white matter of the brain of healthy volunteers at 3T.

Qualitative and quantitative ultrashort-TE MRI of cortical bone

Osteoporosis causes over 1.5 million fractures per year, costing about $15 billion annually in the USA. Current guidelines utilize bone mineral density (BMD) to assess fracture risk; however, BMD alone only accounts for 30-50% of fractures. The other two major components of bone, organic matrix and water, contribute significantly to bone mechanical properties, but cannot be assessed with conventional imaging techniques in spite of the fact that they make up about 57% of cortical bone by volume. Conventional clinical MRI usually detects signals from water in tissues without difficulty, but cannot detect the water bound to the organic matrix, or the free water in the microscopic pores of the Haversian and the lacunar-canalicular system of cortical bone, because of their very short apparent transverse relaxation times (T2 *). In recent years, a new class of sequences, ultrashort-TE (UTE) sequences, with nominal TEs of less than 100 µs, which are much shorter than the TEs available with conventional sequences, have received increasing interest. These sequences can detect water signals from within cortical bone and provide an opportunity to study disease of this tissue in a new way. This review summarizes the recent developments in qualitative UTE imaging (techniques and contrast mechanisms to produce bone images with high contrast) and quantitative UTE imaging (techniques to quantify the MR properties, including T1 , T2 * and the magnetization transfer ratio, and tissue properties, including bone perfusion, as well as total, bound and free water content) of cortical bone in vitro and in vivo. The limitations of the current techniques for clinical applications and future directions are also discussed.

Preliminary use of a double-echo pulse sequence with 3D ultrashort echo time in the MRI of bones and joints

The aim of the present study was to investigate the application of a double-echo pulse sequence with 3D ultrashort echo time (UTE) in the magnetic resonance imaging (MRI) of bones and joints. In total, 7 healthy volunteers and 1 volunteer with a suspected tear of the lateral meniscus of the left knee joint underwent MRI with a double-echo pulse sequence and 3D UTE. The imaging was performed on the tibial diaphysis, knee joint and ankle of the volunteers and on a segment of porcine fibula in vitro. The echo time of echo 1 (TE1) of the UTE images for the achilles tendon of the ankle joint were set as 0.08, 0.16, 0.24 and 0.35 msec. The maximum intensity projection (MIP) of the difference images created from the primary double-echo images with a TE1 of 0.08 msec were performed on the tendons of the ankle to display their 3D structure. The data were analyzed with a one-way ANOVA and paired-sample t-test. The 3D distribution of the tendons was displayed through MIPs of the difference images created from the primary double-echo images. The cortical bones, periosteum, tendons and menisci of the 8 volunteers appeared as high signal intensities in the UTE pulse sequence. Multiplanar reconstruction followed by subtraction of the primary double-echo images raised the image signal-to-noise (S/N) ratio from 2.80±0.75 to 3.76±0.88 (t=−4.851, P<0.01). The artifacts appeared more marked as the TE1 was prolonged. A double pulse sequence MRI with 3D UTE may display the short T₂ components which are not displayed with a conventional clinical MRI sequence, therefore creating a basis for the further quantification of these tissues.

Characterization and mapping of dipolar interactions within macromolecules in tissues using a combination of DQF, MT and UTE MRI

This study shows that by combining a double-quantum filtered magnetization transfer (DQF-MT) with an ultra-short TE (UTE) MRI that it is possible to obtain contrast between tissue compartments based on the following characteristics: (a) the residual dipolar coupling interaction within the biomacromolecules, which depends on their structure, (b) residual dipolar interactions within water molecules, and (c) the magnetization exchange rate between biomacromolecules and water. The technique is demonstrated in rat-tail specimens, where the collagenous tissue such as tendons and the annulus pulposus of the disc are highlighted in these images, and their macromolecular properties along with those of bones and muscles can be characterized. DQF-MT UTE MRI also holds promise because collagenous tissues that are typically invisible in conventional MRI experiments produce significant signal intensities using this approach.

Ultrashort-echo time MR imaging of the patella with bicomponent analysis: correlation with histopathologic and polarized light microscopic findings

PURPOSE

To correlate short and long T2* water fractions, derived from ultrashort-echo time (TE) magnetic resonance (MR) imaging, with semiquantitative histopathologic and polarized light microscopic (PLM) assessment of human cadaveric patellae cartilage.

MATERIALS AND METHODS

Twenty human cadaveric patellae were evaluated by using ultrashort-TE imaging, spin-echo imaging, histopathologic analysis, and PLM, with institutional review board approval. Short and long T2* water components were evaluated for each patella by using bicomponent fitting of ultrashort-TE signal decay. Four to six regions of interest (ROIs) within each patella were chosen for correlation between ultrashort-TE bicomponent analysis, histopathologic grading (Mankin score), and PLM grading (Vaudey score).

RESULTS

Ultrashort-TE imaging with bicomponent analysis showed two distinct water components with a short T2* and a longer T2* in all patellae. ROI analysis showed that the short T2* fraction was correlated significantly with the Mankin (ρ = 0.66, P < .001) and Vaudey (ρ = 0.68, P < .001) scores. The Mankin scores were weakly positively correlated with T2 (ρ = 0.28, P = .13) and short T2* (ρ = 0.24, P = .14) but were negatively correlated with long T2* (ρ = -0.55, P < .01). The Vaudey scores were weakly positively correlated with T2 (ρ = 0.18, P = .16) and short T2* (ρ = 0.22, P = .14) but were negatively correlated with long T2* (ρ = -0.55, P < .01).

CONCLUSION

Short T2* water fraction derived from ultrashort-TE imaging with bicomponent analysis correlates significantly with both the Mankin and Vaudey scores and may serve as a biomarker of cartilage degeneration.

Conventional and ultrashort time-to-echo magnetic resonance imaging of articular cartilage, meniscus, and intervertebral disk

Magnetic resonance imaging (MRI) examination of musculoskeletal tissues is being performed routinely for diagnoses of injury and diseases. Although conventional MRI using spin echo sequences has been effective, a number of important musculoskeletal soft tissues remain "magnetic resonance-invisible" because of their intrinsically short T2 values resulting in a rapid signal decay. This makes visualization and quantitative characterization difficult. With the advent and refinement of ultrashort time-to-echo (UTE) MRI techniques, it is now possible to directly visualize and quantitatively characterize these tissues. This review explores the anatomy, conventional MRI, and UTE MRI of articular cartilage, meniscus of the knee, and intervertebral disks and provides a survey of magnetic resonance studies used to better understand tissue structure, composition, and function, as well as subtle changes in diseases.

Understanding the genetics of coronary artery disease through the lens of noninvasive imaging

Coronary artery disease is a common condition with a known heritable component that has spurred interest in genetic research for decades, resulting in a handful of candidate genes and an appreciation for the complexity of its genetic contributions. Recent advances in sequencing technologies have resulted in large-scale association studies, possibly adding to our current understanding of the genetics of coronary artery disease. Sifting through the statistical noise, however, requires the selection of effective phenotypic markers. New imaging technologies have improved our ability to detect subclinical atherosclerosis in a safe and reproducible manner in large numbers of patients. In this article, we propose that advances in imaging technology have generated improved phenotypic markers for genetic association studies of coronary artery disease.

Ultrashort echo time spectroscopic imaging (UTESI): an efficient method for quantifying bound and free water

Biological tissues usually contain distinct water compartments with different transverse relaxation times. In this study, two-dimensional, multi-slice, ultrashort echo time spectroscopic imaging (UTESI) was used with bi-component analysis to detect bound and free water components in musculoskeletal tissues. Feasibility studies were performed using numerical simulation. Imaging was performed on bovine cortical bone, human cadaveric menisci and the Achilles' tendons of volunteers. The simulation study demonstrated that UTESI, together with bi-component analysis, could reliably quantify both T(2)* and fractions of the short and long (2)* components. The in vitro and in vivo studies each took less than 14 min. The bound water components showed a short T(2)* of ~0.3 ms for bovine bone, ~1.8 ms for meniscus and ~0.6 ms for the Achilles' tendon. The free water components showed about an order of magnitude longer T(2)* values, with ~2 ms for bovine bone, ~14 ms for meniscus and ~8 ms for the Achilles' tendon. Bound water fractions of up to ~76% for bovine bone, 50% for meniscus and ~75% for the Achilles' tendon were measured. The corresponding free water components were up to ~24% for bovine bone, 50% for meniscus and ~25% for the Achilles' tendon by volume. These results demonstrate that UTESI, combined with bi-component analysis, can quantify the bound and free water components in musculoskeletal tissues in clinically realistic times.

Review. The Agfa Mayneord lecture: MRI of short and ultrashort T₂ and T₂* components of tissues, fluids and materials using clinical systems

A variety of techniques are now available to directly or indirectly detect signal from tissues, fluids and materials that have short, ultrashort or supershort T₂ or T₂* components. There are also methods of developing image contrast between tissues and fluids in the short T₂ or T₂* range that can provide visualisation of anatomy, which has not been previously seen with MRI. Magnetisation transfer methods can now be applied to previously invisible tissues, providing indirect access to supershort T₂ components. Particular methods have been developed to target susceptibility effects and quantify them after correcting for anatomical distortion. Specific methods have also been developed to image the effects of magnetic iron oxide particles with positive contrast. Major advances have been made in techniques designed to correct for loss of signal and gross image distortion near metal. These methods are likely to substantially increase the range of application for MRI.

Short T2 contrast with three-dimensional ultrashort echo time imaging

There is increasing interest in imaging short T2 species which show little or no signal with conventional magnetic resonance (MR) pulse sequences. In this paper, we describe the use of three-dimensional ultrashort echo time (3D UTE) sequences with TEs down to 8 μs for imaging of these species. Image contrast was generated with acquisitions using dual echo 3D UTE with echo subtraction, dual echo 3D UTE with rescaled subtraction, long T2 saturation 3D UTE, long T2 saturation dual echo 3D UTE with echo subtraction, single adiabatic inversion recovery 3D UTE, single adiabatic inversion recovery dual echo 3D UTE with echo subtraction and dual adiabatic inversion recovery 3D UTE. The feasibility of using these approaches was demonstrated in in vitro and in vivo imaging of calcified cartilage, aponeuroses, menisci, tendons, ligaments and cortical bone with a 3-T clinical MR scanner. Signal-to-noise ratios and contrast-to-noise ratios were used to compare the techniques.

RF coil considerations for short-T2 MRI

With continuing hardware and pulse sequence advancements, modern MRI is gaining sensitivity to signals from short-T(2) (1)H species under practical experimental conditions. However, conventional MRI coils are typically not designed for this type of application, as they often contain proton-rich construction materials that may contribute confounding (1)H background signal during short-T(2) measurements. An example of this is shown herein. Separately, a loop-gap style coil was used to compare different coil construction materials and configurations with respect to observed (1)H background signal sizes in a small animal imaging system. Background signal sources were spatially identified and quantified in a number of different coil configurations. It was found that the type and placement of structural coil materials around the loop-gap resonator, as well as the coil's shielding configuration, are critical determinants of the coil's background signal size. Although this study employed a loop-gap resonator design, these findings are directly relevant to standard volume coils commonly used for MRI.

Qualitative and quantitative ultrashort echo time (UTE) imaging of cortical bone

We describe the use of two-dimensional ultrashort echo time (2D UTE) sequences with minimum TEs of 8 μs to image and quantify cortical bone on a clinical 3T scanner. An adiabatic inversion pulse was used for long T(2) water and fat signal suppression. Adiabatic inversion prepared UTE acquisitions with varying TEs were used for T(2) measurement. Saturation recovery UTE acquisitions were used for T(1) measurement. Bone water concentration was measured with the aid of an external reference phantom. UTE techniques were evaluated on cadaveric specimens and healthy volunteers. A signal-to-noise ratio of around 30, contrast-to-noise ratio of around 27/20 between bone and muscle/fat were achieved in tibia in vivo with a nominal voxel size of 0.23 × 0.23 × 6.0 mm(3) in a scan time of 5 min. A mean T(1) of 223 ± 11 ms and mean T(2) of 390 ± 19 μs were found. Mean bone water concentrations of 23.3 ± 1.6% with UTE and 21.7 ± 1.3% with adiabatic inversion prepared UTE sequences were found in tibia in five normal volunteers. The results show that in vivo qualitative and quantitative evaluation of cortical bone is feasible with 2D UTE sequences.

Direct imaging and quantification of carotid plaque calcification

Carotid plaque calcification normally appears as a signal void with clinical MR sequences. Here, we describe the use of an adiabatic inversion recovery prepared two-dimensional ultrashort echo time sequence to image and characterize carotid plaque calcification using a clinical 3-T scanner. T(1), T 2*, and free water content were measured for seven carotid samples, and the results were compared with micro-CT imaging. Conventional gradient echo and fast spin echo images were also acquired for comparison. Correlations between T(1), T 2*, free water concentration, and mineral density were performed. There was a close correspondence between inversion recovery prepared two-dimensional ultrashort echo time morphologic and micro-CT appearances. Carotid plaque calcification varied significantly from sample to sample, with T(1) s ranging from 94 ± 19 to 328 ± 21 msec, T 2*s ranging from 0.31 ± 0.12 to 2.15 ± 0.25 msec, and free water concentration ranging from 5.7 ± 2.3% to 16.8 ± 3.4%. There was a significant positive correlation between T(1)(R = 0.709; P < 0.074), T 2* (R = 0.816; P < 0.025), and free water concentration, a negative correlation between T(1) (R = 0.773; P < 0.042), T 2* (R = 0.948; P < 0.001) and CT measured mineral density, and a negative correlation between free water concentration (R = 0.936; P < 0.002) and mineral density.

Effects of in-pulse transverse relaxation in 3D ultrashort echo time sequences: analytical derivation, comparison to numerical simulation and experimental application at 3T

The introduction of ultrashort-echo-time-(UTE)-sequences to clinical whole-body MR scanners has opened up the field of MR characterization of materials or tissues with extremely fast signal decay. If the transverse relaxation time is in the range of the RF-pulse duration, approximation of the RF-pulse by an instantaneous rotation applied at the middle of the RF-pulse and immediately followed by free relaxation will lead to a distinctly underestimated echo signal. Thus, the regular Ernst equation is not adequate to correctly describe steady state signal under those conditions. The paper presents an analytically derived modified Ernst equation, which correctly describes in-pulse relaxation of transverse magnetization under typical conditions: The equation is valid for rectangular excitation pulses, usually applied in 3D UTE sequences. Longitudinal relaxation time of the specimen must be clearly longer than RF-pulse duration, which is fulfilled for tendons and bony structures as well as many solid materials. Under these conditions, the proposed modified Ernst equation enables adequate and relatively simple calculation of the magnetization of materials or tissues. Analytically derived data are compared to numerical results obtained by using an established Runge-Kutta-algorithm based on the Bloch equations. Validity of the new approach was also tested by systematical measurements of a solid polymeric material on a 3T whole-body MR scanner. Thus, the presented modified Ernst equation provides a suitable basis for T1 measurements, even in tissues with T2 values as short as the RF-pulse duration: independent of RF-pulse duration, the 'variable flip angle method' led to consistent results of longitudinal relaxation time T1, if the T2 relaxation time of the material of interest is known as well.

Dual inversion recovery, ultrashort echo time (DIR UTE) imaging: creating high contrast for short-T(2) species

Imaging of short-T(2) species requires not only a short echo time but also efficient suppression of long-T(2) species in order to maximize the short-T(2) contrast and dynamic range. This paper introduces a method of long-T(2) suppression using two long adiabatic inversion pulses. The first adiabatic inversion pulse inverts the magnetization of long-T(2) water and the second one inverts that of fat. Short-T(2) species experience a significant transverse relaxation during the long adiabatic inversion process and are minimally affected by the inversion pulses. Data acquisition with a short echo time of 8 mus starts following a time delay of inversion time (TI1) for the inverted water magnetization to reach a null point and a time delay of TI2 for the inverted fat magnetization to reach a null point. The suppression of long-T(2) species depends on proper combination of TI1, TI2, and pulse repetition time. It is insensitive to radiofrequency inhomogeneities because of the adiabatic inversion pulses. The feasibility of this dual inversion recovery ultrashort echo time technique was demonstrated on phantoms, cadaveric specimens, and healthy volunteers, using a clinical 3-T scanner. High image contrast was achieved for the deep radial and calcified layers of articular cartilage, cortical bone, and the Achilles tendon.

Ultrashort TE imaging with off-resonance saturation contrast (UTE-OSC)

Short T(2) species such as the Achilles tendon and cortical bone cannot be imaged with conventional MR sequences. They have a much broader absorption lineshape than long T(2) species, therefore they are more sensitive to an appropriately placed off-resonance irradiation. In this work, a technique termed ultrashort TE (UTE) with off-resonance saturation contrast (UTE-OSC) is proposed to image short T(2) species. A high power saturation pulse was placed +1 to +2 kHz off the water peak to preferentially saturate signals from short T(2) species, leaving long T(2) water and fat signals largely unaffected. The subtraction of UTE images with and without an off-resonance saturation pulse effectively suppresses long T(2) water and fat signals, creating high contrast for short T(2) species. The UTE-OSC technique was validated on a phantom, and applied to bone samples and healthy volunteers on a clinical 3T scanner. High-contrast images of the Achilles tendon and cortical bone were generated with a high contrast-to-noise ratio (CNR) of the order of 12 to 20 between short T(2) and long T(2) species within a total scan time of 4 to 10 min.

Orientational analysis of the Achilles tendon and enthesis using an ultrashort echo time spectroscopic imaging sequence

Tendons and entheses are magnetic resonance (MR) "invisible" when imaged with conventional clinical pulse sequences. When the highly ordered, collagen-rich fibers in tendons and entheses are placed at the magic angle, dipolar interactions are decreased and their T2s are often considerably increased. The bulk magnetic susceptibility of tendons and entheses also varies with orientation to B(0), leading to a direction-dependent resonance frequency shift. Ultrashort echo time (UTE) sequences with a minimum TE of 8 mus provide high signal from both tendons and entheses. The combination of a UTE sequence with an interleaved undersampled variable TE acquisition scheme provides a new approach for fast spectroscopic imaging of short T2 tissues. This UTE spectroscopic imaging (UTESI) technique provides quantitative information including T2, chemical shift and resonance frequency shift due to bulk susceptibility effect. In this article, the orientational effects on tendons and entheses were investigated using a UTESI sequence on a clinical 3-T scanner. T2 was found to increase fivefold for tendons and twofold for entheses due to the magic angle effect. A resonance frequency shift up to 1.2 ppm was observed for both tendons and entheses due to the bulk susceptibility effect when their orientation was changed from 0 degree to 90 degrees relative to B(0).

Advanced techniques for MRI of atherosclerotic plaque

This review examines the state of the art in vessel wall imaging by magnetic resonance imaging (MRI) with an emphasis on the biomechanical assessment of atherosclerotic plaque. Three areas of advanced techniques are discussed. First, alternative contrast mechanisms, including susceptibility, magnetization transfer, diffusion, and perfusion, are presented as to how they facilitate accurate determination of plaque constituents underlying biomechanics. Second, imaging technologies including hardware and sequences, are reviewed as to how they provide the resolution and signal-to-noise ratio necessary for determining plaque structure. Finally, techniques for combining MRI data into an overall assessment of plaque biomechanical properties, including wall shear stress and internal plaque strain, are presented. The paper closes with a discussion of the extent to which these techniques have been applied to different arteries commonly targeted by vessel wall MRI.