Ultrashort echo time magnetic resonance elastography for quantification of the mechanical properties of short T2 tissues via optimal control-based radiofrequency pulses.

The aim of the current study is to demonstrate the feasibility of radiofrequency (RF) pulses generated via an optimal control (OC) algorithm to perform magnetic resonance elastography (MRE) and quantify the mechanical properties of materials with very short transverse relaxation times (T2 < 5 ms) for the first time. OC theory applied to MRE provides RF pulses that bring isochromats from the equilibrium state to a fixed target state, which corresponds to the phase pattern of a conventional MRE acquisition. Such RF pulses applied with a constant gradient allow to simultaneously perform slice selection and motion encoding in the slice direction. Unlike conventional MRE, no additional motion-encoding gradients (MEGs) are needed, enabling shorter echo times. OC pulses were implemented both in turbo spin echo (OC rapid acquisition with refocused echoes [RARE]) and ultrashort echo time (OC UTE) sequences to compare their motion-encoding efficiency with the conventional MEG encoding (classical MEG MRE). MRE experiments were carried out on agar phantoms with very short T2 values and on an ex vivo bovine tendon. Magnitude images, wave field images, phase-to-noise ratio (PNR), and shear storage modulus maps were compared between OC RARE, OC UTE, and classical MEG MRE in samples with different T2 values. Shear storage modulus values of the agar phantoms were in agreement with values found in the literature, and that of the bovine tendon was corroborated with rheometry measurements. Only the OC sequences could encode motion in very short T2 samples, and only OC UTE sequences yielded magnitude images enabling proper visualization of short T2 samples and tissues. The OC UTE sequence produced the best PNRs, demonstrating its ability to perform anatomical and mechanical characterization. Its success warrants in vivo confirmation in further studies.

Quantitative 1H Magnetic Resonance Imaging on Normal and Pathologic Rat Bones by Solid-State 1H ZTE Sequence with Water and Fat Suppression.

BACKGROUND: Osteoporosis (OP) and osteomalacia (OM) are metabolic bone diseases characterized by mineral and matrix density changes. Quantitative bone matrix density differentiates OM from OP. MRI is a noninvasive and nonionizing imaging technique that can measure bone matrix density quantitatively in ex vivo and in vivo. PURPOSE: To demonstrate water + fat suppressed 1H MRI to compute bone matrix density in ex vivo rat femurs in the preclinical model. STUDY TYPE: Prospective. ANIMAL MODEL: Fifteen skeletally mature female Sprague-Dawley rats, five per group (normal, ovariectomized (OVX), partially nephrectomized/vitamin D (Vit-D) deficient), 250-275 g, ∼15 weeks old. FIELD STRENGTH/SEQUENCE: 7T, zero echo time sequence with water + fat (VAPOR) suppression capability, µCT imaging, and gravimetric measurements. ASSESSMENT: Cortical and trabecular bone segments from normal and disease models were scanned in the same coil along with a dual calibration phantom for quantitative assessment of bone matrix density. STATISTICAL TESTS: ANOVA and linear regression were used for data analysis, with P-values <0.05 statistically significant. RESULTS: The MRI-derived three-density PEG pellet densities have a strong linear relationship with physical density measures (r2 = 0.99). The Vit-D group had the lowest bone matrix density for cortical bone (0.47 ± 0.16 g cm-3), whereas the OVX had the lowest bone matrix density for trabecular bone (0.26 ± 0.04 g cm-3). Gravimetry results confirmed these MRI-based observations for Vit-D cortical (0.51 ± 0.07 g cm-3) and OVX trabecular (0.26 ± 0.03 g cm-3) bone groups. DATA CONCLUSION: Rat femur images were obtained using a modified pulse sequence and a custom-designed double-tuned (1H/31P) transmit-receive solenoid-coil on a 7T preclinical MRI scanner. Phantom experiments confirmed a strong linear relation between MRI-derived and physical density measures and quantitative bone matrix densities in rat femurs from normal, OVX, and Vit-D deficient/partially nephrectomized animals were computed. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 2.

Design and development of a novel flexible ultra-short echo time (FUSE) sequence.

PURPOSE: To present the validation of a new Flexible Ultra-Short Echo time (FUSE) pulse sequence using a short-T2 phantom. METHODS: FUSE was developed to include a range of RF excitation pulses, trajectories, dimensionalities, and long-T2 suppression techniques, enabling real-time interchangeability of acquisition parameters. Additionally, we developed an improved 3D deblurring algorithm to correct for off-resonance artifacts. Several experiments were conducted to validate the efficacy of FUSE, by comparing different approaches for off-resonance artifact correction, variations in RF pulse and trajectory combinations, and long-T2 suppression techniques. All scans were performed on a 3 T system using an in-house short-T2 phantom. The evaluation of results included qualitative comparisons and quantitative assessments of the SNR and contrast-to-noise ratio. RESULTS: Using the capabilities of FUSE, we demonstrated that we could combine a shorter readout duration with our improved deblurring algorithm to effectively reduce off-resonance artifacts. Among the different RF and trajectory combinations, the spiral trajectory with the regular half-inc pulse achieves the highest SNRs. The dual-echo subtraction technique delivers better short-T2 contrast and superior suppression of water and agar signals, whereas the off-resonance saturation method successfully suppresses water and lipid signals simultaneously. CONCLUSION: In this work, we have validated the use of our new FUSE sequence using a short T2 phantom, demonstrating that multiple UTE acquisitions can be achieved within a single sequence. This new sequence may be useful for acquiring improved UTE images and the development of UTE imaging protocols.

Age and Gender-Dependence of Single-and Bi-Exponential T1ρ MR Parameters in Knee Ligaments.

BACKGROUND: There is limited understanding of differences in the composition and structure of ligaments between healthy males and females, and individuals of different ages. Females present higher risk for ligament injuries than males and there are conflicting reports on its cause. This study looks into T1ρ parameters for an explanation as it relates to proteoglycan, collagen, and water content in these tissues. PURPOSE: To investigate gender-related and age-related differences in T1ρ parameters in knee joint ligaments in healthy volunteers using a T1ρ -prepared zero echo-time (ZTE)-based pointwise-encoding time-reduction with radial acquisition (T1ρ -PETRA) sequence. STUDY TYPE: Prospective. POPULATION: The study group consisted of 22 healthy subjects (11 females, ages: 41 ± 18 years, and 11 males, ages: 41 ± 14 years) with no known inflammation, trauma, or pain in the knee joint. FIELD STRENGTH/SEQUENCE: A T1ρ -prepared 3D-PETRA sequence was used to acquire fat-suppressed images with varying spin-lock lengths (TSLs) of the knee joint at 3T. ASSESSMENT: Monoexponential, biexponential, and stretched-exponential 3D-PETRA-T1ρ parameters were measured in the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and patellar tendon (PT) by manually drawing ROIs over the entirety of the tissues. STATISTICAL TESTS: Mann-Whitney U-tests were used to compare 3D-PETRA-T1ρ parameters in the ACL, PCL, and PT between males and females. Spearman correlation coefficients were used to determine the association between age and T1ρ parameters. Statistical significance was defined as P < 0.05. RESULTS: Significant correlations with age were found the three ligaments with most of the measured T1ρ parameters (rs = 0.28-0.74) with the exception of the short fraction in the PCL (P = 0.18), and the short relaxation time in the ACL (P = 0.58) and in the PCL (P = 0.14). DATA CONCLUSION: 3D-PETRA-T1ρ can detect age-related differences in monoexponential, biexponential, and stretched-exponential T1ρ parameters in three ligaments of healthy volunteers, which are thought to be related to changes in tissue composition and structure during the aging process. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 1.

Single point imaging with radial acquisition and compressed sensing.

PURPOSE: To accelerate the Pointwise Encoding Time Reduction with Radial Acquisition (PETRA) sequence using compressed sensing while preserving the image quality for high-resolution MRI of tissue with ultra-short T2∗ values. METHODS: Compressed sensing was introduced in the PETRA sequence (csPETRA) to accelerate the time-consuming single point acquisition of the k-space center data. Random undersampling was applied to achieve acceleration factors up to Acc = 32. Phantom and in vivo images of the knee joint of six volunteers were measured at 3T using csPETRA sequence with Acc = 4, 8, 12, 16, 24, and 32. Images were compared against fully sampled PETRA data (Acc = 1) for structural similarity and normalized-mean-square-error. Qualitative and semi-quantitative analyses were performed to assess the effect of the acceleration on image artifacts, image quality, and delineation of anatomical structures at the knee. RESULTS: Even at high acceleration factors of Acc = 16 no aliasing artifacts were observed, and the anatomical details were preserved compared with the fully sampled data. The normalized-mean-square-error was less than 1% for Acc = 16, in which single point imaging acquisition time was reduced from 165 to 10 s, reducing the total scan time from 7.8 to 5.2 min. Semi-quantitative analyses suggest that Acc = 16 yields comparable diagnostic quality as the fully sampled data for knee imaging at a scan time of 5.2 min. CONCLUSION: csPETRA allows for ultra-short T2∗ imaging of the knee joint in clinically acceptable scan times while maintaining the image quality of original non-accelerated PETRA sequence.

High-resolution MRI of mummified tissues using advanced short-T2 methodology and hardware.

PURPOSE: Evolutionary medicine aims to study disease development from a long-term perspective, and through the analysis of mummified tissue, timescales of several thousand years are unlocked. Due to the status of mummies as ancient relics, noninvasive techniques are preferable, and, currently, CT imaging is the most widespread method. However, CT images lack soft-tissue contrast, making complementary MRI data desirable. Unfortunately, the dehydrated nature and short T2 times of mummified tissues render them practically invisible to standard MRI techniques. Specialized short-T2 approaches have therefore been used, but currently suffer severe resolution limitations. The purpose of the present study is to improve resolution in MRI of mummified tissues. METHODS: The zero-TE-based hybrid filling technique, together with a high-performance magnetic field gradient, was used to image three ancient Egyptian mummified human body parts: a hand, a foot, and a head. A similar pairing has already been shown to increase resolution and image quality in MRI of short-T2 tissues. RESULTS: MRI images of yet unparalleled image quality were obtained for all samples, reaching isotropic resolutions of 0.6 mm and SNR values above 100. The same general features as present in CT images were depicted but with different contrast, particularly for regions containing embalming substances. CONCLUSION: Mummy MRI is a potentially valuable tool for (paleo)pathological studies, as well as for investigations into ancient mummification processes. The results presented here show sufficient improvement in the depiction of mummified tissues to clear new paths for the exploration of this field.

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

The aim of this work was to improve the SNR efficiency of zero echo time (ZTE) MRI pulse sequences for faster imaging of short-T2 components at large dead-time gaps. ZTE MRI with hybrid filling (HYFI) is a strategy for retrieving inner k-space data missed during the dead-time gaps arising from radio-frequency excitation and switching in ZTE imaging. It performs hybrid filling of the inner k-space with a small single-point-imaging core surrounded by a stack of shells acquired on radial readouts in an onion-like fashion. The exposition of this concept is followed by translation into guidelines for parameter choice and implementation details. The imaging properties and performance of HYFI are studied in simulations as well as phantom, in vitro and in vivo imaging, with an emphasis on comparison with the pointwise encoding time reduction with radial acquisition (PETRA) technique. Simulations predict higher SNR efficiency for HYFI compared with PETRA at preserved image quality, with the advantage increasing with the size of the k-space gap. These results are confirmed by imaging experiments with gap sizes of 25 to 50 Nyquist dwells, in which scan times for similar image quality could be reduced by 25% to 60%. The HYFI technique provides both high SNR efficiency and image quality, thus outperforming previously known ZTE-based pulse sequences, in particular for large k-space gaps. Promising applications include direct imaging of ultrashort-T2 components, such as the myelin bilayer or collagen, T2 mapping of ultrafast relaxing signals, and ZTE imaging with reduced chemical shift artifacts.

Advances in MRI of the myelin bilayer.

Myelin plays a key role in the function of the central nervous system and is involved in many neurodegenerative diseases. Hence, depiction of myelin is desired for both research and diagnosis. However, MRI of the lipid bilayer constituting the myelin membrane is hampered by extremely rapid signal decay and cannot be accomplished with conventional sequences. Dedicated short-T2 techniques have therefore been employed, yet with extended sequence timings not well matched to the rapid transverse relaxation in the bilayer, which leads to signal loss and blurring. In the present work, capture and encoding of the ultra-short-T2 signals in the myelin bilayer is considerably improved by employing advanced short-T2 methodology and hardware, in particular a high-performance human-sized gradient insert. The approach is applied to tissue samples excised from porcine brain and in vivo in a human volunteer. It is found that the rapidly decaying non-aqueous components in the brain can indeed be depicted with MRI at useful resolution. As a considerable fraction of these signals is related to the myelin bilayer, the presented approach has strong potential to contribute to myelin research and diagnosis.

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

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

Characterizing a short T2 * signal component in the liver using ultrashort TE chemical shift-encoded MRI at 1.5T and 3.0T.

PURPOSE: Recent studies have suggested the presence of short-T2 * signals in the liver, which may confound chemical shift-encoded (CSE) fat quantification when using short echo times (TEs). The purpose of this study was to characterize the liver signal at short echo times and to determine its impact on liver fat quantification. METHODS: An ultrashort echo time (UTE) chemical shift-encoded MRI (CSE-MRI) technique and a multicomponent reconstruction were developed to characterize short-T2 * liver signals. Subsequently, liver fat fraction was quantified using a short-TE (first TE = 0.7 ms) and UTE CSE-MRI acquisitions and compared with a standard CSE-MRI (first TE = 1.2 ms). RESULTS: Short-T2 * signals were consistently observed in the liver of all healthy volunteers imaged at both 1.5T and 3.0T. At 3.0T, short-T2 * signal fractions of 9.6 ± 1.5%, 7.0 ± 1.7%, and 7.4 ± 1.7% with T2 * of 0.23 ± 0.05 ms, 0.20 ± 0.05 ms, and 0.10 ± 0.02 ms were measured in healthy volunteers, patients with liver cirrhotic disease, and patients with hepatic steatosis (but no cirrhosis), respectively. For proton density fat fraction (PDFF) estimation, 1.7% (P < .01) and 3.4% (P < .01) biases were observed in subjects imaged using short-TE CSE-MRI and using UTE CSE-MRI at 1.5T, respectively. The biases were reduced to 0.4% and -0.7%, respectively, by excluding short echoes less than 1 ms. A 3.2% bias (P < .01) was observed in subjects imaged using UTE CSE-MRI at 3.0T, which was reduced to 0.1% by excluding short echoes <1 ms. CONCLUSIONS: A liver short-T2 * signal component was consistently observed and was shown to confound liver fat quantification when short echo times were used with CSE-MRI.

Rapid dual-RF, dual-echo, 3D ultrashort echo time craniofacial imaging: A feasibility study.

PURPOSE: To develop a dual-radiofrequency (RF), dual-echo, 3D ultrashort echo-time (UTE) pulse sequence and bone-selective image reconstruction for rapid high-resolution craniofacial MRI. METHODS: The proposed pulse sequence builds on recently introduced dual-RF UTE imaging. While yielding enhanced bone specificity by exploiting high sensitivity of short T2 signals to variable RF pulse widths, the parent technique exacts a 2-fold scan time penalty relative to standard dual-echo UTE. In the proposed method, the parent sequence's dual-RF scheme was incorporated into dual-echo acquisitions while radial view angles are varied every pulse-to-pulse repetition period. The resulting 4 echoes (2 for each RF) were combined by view-sharing to construct 2 sets of k-space data sets, corresponding to short and long TEs, respectively, leading to a 2-fold increase in imaging efficiency. Furthermore, by exploiting the sparsity of bone signals in echo-difference images, acceleration was achieved by solving a bone-sparsity constrained image reconstruction problem. In vivo studies were performed to evaluate the effectiveness of the proposed acceleration approaches in comparison to the parent method. RESULTS: The proposed technique achieves 1.1-mm isotropic skull imaging in 3 minutes without visual loss of image quality, compared to the parent technique (scan time = 12 minutes). Bone-specific images and corresponding 3D renderings of the skull were found to depict the expected craniofacial anatomy over the entire head. CONCLUSION: The proposed method is able to achieve high-resolution volumetric craniofacial images in a clinically practical imaging time, and thus may prove useful as a potential alternative to computed tomography.

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 / T2* 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 / T2* . LEVEL OF EVIDENCE: 2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019;50:1207-1218.

Filling the dead-time gap in zero echo time MRI: Principles compared.

PURPOSE: MRI of tissues with short coherence lifetimes T2 or T2* can be performed efficiently using zero echo time (ZTE) techniques such as algebraic ZTE, pointwise encoding time reduction with radial acquisition (PETRA), and water- and fat-suppressed proton projection MRI (WASPI). They share the principal challenge of recovering data in central k-space missed due to an initial radiofrequency dead time. The purpose of this study was to compare the three techniques directly, with a particular focus on their behavior in the presence of ultra-short-lived spins. METHODS: The most direct comparison was enabled by aligning acquisition and reconstruction strategies of the three techniques. Image quality and short- T2* performance were investigated using point spread functions, 3D simulations, and imaging of phantom and bone samples with short (<1 ms) and ultra-short (<100 µs) T2*. RESULTS: Algebraic ZTE offers favorable properties but is limited to k-space gaps up to approximately three Nyquist dwells. At larger gaps, PETRA enables robust imaging with little compromise in image quality, whereas WASPI may be prone to artifacts from ultra-short T2* species. CONCLUSION: For small k-space gaps (<4 dwells) and T2* much larger than the dead time, all techniques enable artifact-free short- T2* MRI. However, if these requirements are not fulfilled careful consideration is needed and PETRA will generally achieve better image quality. Magn Reson Med 79:2036-2045, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Three-dimensional ultrashort echo time MRI and Short T2 images generated from subtraction for determination of tumor burden in lung cancer: Preclinical investigation in transgenic mice.

PURPOSE: To investigate the potential of 3D ultrashort echo time MRI and short T2 images generated by subtraction for determination of total tumor burden in lung cancer. METHODS: As an animal model of spontaneously developing non-small cell lung cancer, the K-rasLA1 transgenic mouse was used. Three-dimensional MR imaging was performed with radial k-space acquisition and echo times of 20 µs and 1 ms. For investigation of the short T2 component in the recorded signal, subtraction images were generated from these data sets and used for consensus identification of tumors. Next, manual segmentation was performed on all MR images by two independent investigators. MRI data were compared with the results from histologic investigations and among the investigators. RESULTS: Tumor number and total tumor burden from imaging experiments correlated strongly with the results of histologic investigations. Intra- and interuser comparison showed highest correlations between the individual measurements for ultra-short TE MRI. CONCLUSIONS: Three-dimensional MRI protocols facilitate accurate tumor identification in mice harboring lung tumors. Ultrashort TE MRI is the superior imaging strategy when investigating lung tumors of miscellaneous size with 3D MR imaging strategies. Magn Reson Med 79:1052-1060, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Quantitative magnetization transfer ultrashort echo time imaging using a time-efficient 3D multispoke Cones sequence.

PURPOSE: To accelerate the quantitative ultrashort echo time imaging using a time-efficient 3D multispoke Cones sequence with magnetization transfer (3D UTE-Cones-MT) and signal modeling. THEORY AND METHODS: A 3D UTE-Cones-MT acquisition scheme with multispoke per MT preparation and a modified rectangular pulse (RP) approximation was developed for two-pool MT modeling of macromolecular and water components including their relative fractions, relaxation times and exchange rates. Numerical simulation and cadaveric specimens, including human Achilles tendon and bovine cortical bone, were investigated using a clinical 3T scanner. RESULTS: Numerical simulation showed that the modified RP model provided accurate estimation of MT parameters when multispokes were acquired per MT preparation. For the experiment with the Achilles tendon and cortical bone samples, the macromolecular fractions were 20.4 ± 2.0% and 59.4 ± 5.3%, respectively. CONCLUSION: The 3D multispoke UTE-Cones-MT sequence can be used for fast volumetric assessment of macromolecular and water components in short T2 tissues. Magn Reson Med 79:692-700, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

A diffusion-based method for long-T2 suppression in steady state sequences: Validation and application for 3D-UTE imaging.

PURPOSE: To introduce a novel method for long-T2 signal physical suppression in steady-state based on configuration states combination and modulation using diffusion weighting. Its efficiency in yielding a high contrast in short-T2 structures using an ultrashort echo time acquisition module (Diff-UTE) is compared to the adiabatically prepared Inversion-Recovery-UTE sequence (IR-UTE). THEORY AND METHODS: Using a rectangular-pulse prepared 3D-UTE sequence, the possibility of long-T2 component signal cancellation through diffusion effects is addressed, and the condition met for sets of sequence parameters. Simultaneously, the short-T2 component signal is maximized using a Bloch equation-based optimization process. The method is evaluated from simulations, and experiments are conducted on a phantom composed of short and long-T2 components, as well as on an ex vivo mouse head. RESULTS: Within equal scan times, the proposed method allowed for an efficient long-T2 signal suppression, and expectedly yielded a higher signal to noise ratio in short-T2 structures compared to the IR-UTE technique, although an intrinsic short-T2 signal loss is expected through the preparation module. CONCLUSION: The Diff-UTE method represents an interesting alternative to the IR-UTE technique. Diffusion weighting allowing for a long-T2 suppression results in a less penalizing method to generate a high and selective contrast in short-T2 components. Magn Reson Med 80:548-559, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Imaging and T2 relaxometry of short-T2 connective tissues in the knee using ultrashort echo-time double-echo steady-state (UTEDESS).

PURPOSE: To develop a radial, double-echo steady-state (DESS) sequence with ultra-short echo-time (UTE) capabilities for T2 measurement of short-T2 tissues along with simultaneous rapid, signal-to-noise ratio (SNR)-efficient, and high-isotropic-resolution morphological knee imaging. METHODS: THe 3D radial UTE readouts were incorporated into DESS, termed UTEDESS. Multiple-echo-time UTEDESS was used for performing T2 relaxometry for short-T2 tendons, ligaments, and menisci; and for Dixon water-fat imaging. In vivo T2 estimate repeatability and SNR efficiency for UTEDESS and Cartesian DESS were compared. The impact of coil combination methods on short-T2 measurements was evaluated by means of simulations. UTEDESS T2 measurements were compared with T2 measurements from Cartesian DESS, multi-echo spin-echo (MESE), and fast spin-echo (FSE). RESULTS: UTEDESS produced isotropic resolution images with high SNR efficiency in all short-T2 tissues. Simulations and experiments demonstrated that sum-of-squares coil combinations overestimated short-T2 measurements. UTEDESS measurements of meniscal T2 were comparable to DESS, MESE, and FSE measurements while the tendon and ligament measurements were less biased than those from Cartesian DESS. Average UTEDESS T2 repeatability variation was under 10% in all tissues. CONCLUSION: The T2 measurements of short-T2 tissues and high-resolution morphological imaging provided by UTEDESS makes it promising for studying the whole knee, both in routine clinical examinations and longitudinal studies. Magn Reson Med 78:2136-2148, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

A virtually 1H-free birdcage coil for zero echo time MRI without background signal.

PURPOSE: MRI of tissues with rapid transverse relaxation can be performed efficiently using the zero echo time (ZTE) technique. At high bandwidths leading to large relative initial radiofrequency (RF) dead times, the method becomes increasingly sensitive to artifacts related to signal stemming from outside the field of view, particularly from the RF coils. Therefore, in this work, a birdcage coil was designed that is virtually free of 1H signal. METHODS: A transmit-receive birdcage RF coil for MRI of joints at 7T was designed by rigorously avoiding materials containing 1H nuclei, by using purely mechanical connections without glue, and by spoiling of unwanted signal by application of ferromagnetic materials. The coil was tested for residual 1H signal using ZTE phantom and in vivo joint imaging. RESULTS: In standard ZTE imaging, no 1H signal was detected above noise level. Only at extreme averaging, residual signal was observed close to conductors associated with 1H-containing molecules at adjacent glass surfaces. Phantom images with dead times up to 3.8 Nyquist dwells were obtained with only negligible background artifacts. Furthermore, high-quality ZTE images of human joints were acquired. CONCLUSION: A virtually 1H-free birdcage coil is presented, thus enabling in vivo ZTE MRI practically free of background signal, even at high bandwidths. Magn Reson Med 78:399-407, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Quantitative ultrashort TE imaging of the short-T2 components in skeletal muscle using an extended echo-subtraction method.

PURPOSE: To introduce an ultrashort echo time (UTE) based method for quantitative mapping of short-T2 signals in skeletal muscle (SKM) in the presence of fat, with the aim of monitoring SKM fibrosis. METHODS: From a set of at least five UTE images of the same slice, a long- T2* map, a fat-fraction map, and a map of short-T2 -signal fraction are extracted. The method was validated by numerical simulations and in vitro studies on collagen solutions. Finaly, the method was applied to image the short-T2 signals in the leg of eight healthy volunteers. RESULTS: The imaged short-T2 -signal fractions in the collagen solutions correlated with their respective collagen concentrations ( R=0.999,  P=0.009). Short-T2 tissues such as cortical bone and fasciae were highlighted in the resulting short-T2 fraction maps. A significant fraction of short-T2 signal was systematically observed in the skeletal muscle of all of the subjects (4.5±1.2%). CONCLUSION: The proposed method allows the quantitative imaging of short-T2 components in tissues containing fat. By also having the fat-fraction and T2* maps as outcomes, long-T2 suppression is accomplished without requiring modifications to the basic UTE sequence. Although the hypersignal observed in the fasciae suggests that the short-T2 signal observed in SKM might arise from interstitial connective tissue, further investigation is necessary to confirm this statement. Magn Reson Med 78:997-1008, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Selective in vivo bone imaging with long-T2 suppressed PETRA MRI.

PURPOSE: To design and evaluate an optimized PETRA (point-wise encoding time reduction with radial acquisition) sequence with long-T2 suppression at 3 Tesla. METHODS: An adiabatic inversion recovery-based scheme was used to null the long-T2 signal. To minimize scan time, the signal was sampled multiple times after each inversion with variable excitation flip angles designed to yield constant short-T2 signal amplitude. The excitation pulses were phase-modulated, allowing for increased flip angle and higher signal-to-noise ratio (SNR). A fast, noniterative image reconstruction algorithm was designed to minimize image artifacts due to nonuniform excitation profile. RESULTS: Phase-modulated pulse excitation, along with the noniterative reconstruction algorithm, allows the use of larger radiofrequency pulse flip angles, resulting in effective suppression of long-T2 protons and improved image SNR without causing image artifacts. Midtibia images representative of collagen-bound water yielded SNR of 15 at 1-mm isotropic resolution in 6.5 minutes with a standard extremity coil. Further, the technology is shown to be suited for generating multi-angle projection images of bone akin to X-ray images displaying subtle anatomic detail. CONCLUSION: Optimized long-T2 suppressed PETRA allows imaging of bone matrix water unencumbered by long-T2 soft tissue and pore water protons, opening up new possibilities for anatomic bone imaging at isotropic resolution and quantification in clinically practical scan times. Magn Reson Med 77:989-997, 2017. © 2016 International Society for Magnetic Resonance in Medicine.