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 T 2 ∗ 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 T 2 ∗ imaging of the knee joint in clinically acceptable scan times while maintaining the image quality of original non-accelerated PETRA sequence.

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.

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.

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.

Three-dimensional ultrashort echo time cones T1ρ (3D UTE-cones-T1ρ ) imaging

We report a novel three-dimensional (3D) ultrashort echo time (UTE) sequence employing Cones trajectory and T_1ρ preparation (UTE-Cones-T_1ρ ) for quantitative T_1ρ assessment of short T₂ tissues in the musculoskeletal system. A basic 3D UTE-Cones sequence was combined with a spin-locking preparation pulse for T_1ρ contrast. A relatively short TR was used to decrease the scan time, which required T₁ measurement and compensation using 3D UTE-Cones data acquisitions with variable TRs. Another strategy to reduce the total scan time was to acquire multiple Cones spokes (N_sp ) after each T_1ρ preparation and fat saturation. Four spin-locking times (TSL = 0-20 ms) were acquired over 12 min, plus another 7 min for T₁ measurement. The 3D UTE-Cones-T_1ρ sequence was compared with a two-dimensional (2D) spiral-T_1ρ sequence for the imaging of a spherical CuSO₄ phantom and ex vivo meniscus and tendon specimens, as well as the knee and ankle joints of healthy volunteers, using a clinical 3-T scanner. The CuSO₄ phantom showed a T_1ρ value of 76.5 ± 1.6 ms with the 2D spiral-T_1ρ sequence, as well as 85.7 ± 3.6 and 89.2 ± 1.4 ms for the 3D UTE-Cones-T_1ρ sequences with N_sp of 1 and 5, respectively. The 3D UTE-Cones-T_1ρ sequence provided shorter T_1ρ values for the bovine meniscus sample relative to the 2D spiral-T_1ρ sequence (10-12 ms versus 16 ms, respectively). The cadaveric human Achilles tendon sample could only be imaged with the 3D UTE-Cones-T_1ρ sequence (T_1ρ  = 4.0 ± 0.9 ms), with the 2D spiral-T_1ρ sequence demonstrating near-zero signal intensity. Human studies yielded T_1ρ values of 36.1 ± 2.9, 18.3 ± 3.9 and 3.1 ± 0.4 ms for articular cartilage, meniscus and the Achilles tendon, respectively. The 3D UTE-Cones-T_1ρ sequence allows volumetric T_1ρ measurement of short T₂ tissues in vivo.

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.

ZTE imaging with long-T2 suppression

Three-dimensional radial zero echo time (ZTE) imaging enables efficient direct MRI of tissues with rapid transverse relaxation. Yet, the feature of capturing signals with a wide range of T2 and T2 * values is accompanied by a lack of contrast between the corresponding tissues. In particular, the targeted short-T2 tissues may not be easily identified, and various approaches have been proposed to generate T2 contrast by reducing the long-T2 signal of water and/or fat. The aim of this work was to provide efficient long-T2 suppression for selective direct MRI of short-T2 tissues using the ZTE technique. For magnetization preparation, suppression pulses for water and fat were designed to provide both good T2 selectivity and off-resonance performance. To obtain high efficiency at short TRs, the pulses were applied in a segmented sequence scheme with minimized timing overhead, thus leading to a quasi-steady state of magnetization. The sequence timing was adjusted for optimal tissue contrast in musculoskeletal applications by means of simulations and experiments, incorporating both T2 and T1 of the involved tissues. The developed technique was employed for imaging of a lamb joint sample at 4.7 T. ZTE images were obtained with effective suppression of signals from tissues with long-T2 water, such as muscle or articular spaces, and fat. Hence, primarily short-T2 tissues were visible, such as bone and tendon. The MR image intensity of bone showed strong similarity with bone density imaged with micro-computed tomography.

Exploring the bandwidth limits of ZTE imaging: Spatial response, out-of-band signals, and noise propagation

PURPOSE

Zero echo time (ZTE) imaging with single-pulse excitation is a fast, robust, and silent three-dimensional (3D) method for MRI of short T2 tissues. In this technique, algebraic reconstruction serves to fill gaps in the center of k-space due to finite acquisition dead time. The purpose of this study was to investigate the effect of this operation on depiction characteristics, noise behavior, and achievable bandwidth.

METHODS

The spatial response function (SRF) and noise covariance resulting from ZTE reconstruction were studied using formal analysis, simulations, and phantom experiments.

RESULTS

Three prominent limiting phenomena were identified: SRF behavior within the field of view, heightened sensitivity to out-of-band signal sources, and noise amplification. The related errors all appear as image distortions of low spatial frequency and are strongly attenuated upon the transition from one-dimensional projections to 3D image data. Relying on these observations, ZTE imaging was accomplished with a previously unreached gap size, permitting the depiction of a solid sample with T2 ≈ 25 µs at a bandwidth of 500 kHz.

CONCLUSION

The tightest bandwidth limits in ZTE arise from background signal and radiofrequency (RF) switching transients. Significant advances in ZTE performance will be afforded by faster transmit-receive (T/R) switching with negligible transients and RF coils free of background signal.

ZTE imaging with enhanced flip angle using modulated excitation

PURPOSE

Zero echo time (ZTE) imaging is a fast, robust, and silent three-dimensional technique for direct MRI of tissues with rapid transverse relaxation. It is conventionally performed with hard, block-shaped excitation pulses short enough to excite spins uniformly over a large bandwidth. With this approach, the achievable flip angle (FA) is limited by the available B1 amplitude. The purpose of this work is to accomplish ZTE imaging with larger FAs by combined amplitude and frequency modulation of the excitation pulse while keeping the pulse duration short enough to limit acquisition dead time.

METHODS

Quantitative performance criteria for FA yield and uniformity of radio frequency (RF) pulses were developed and used to optimize hyperbolic secant pulse shapes. The RF pulses were implemented on a 4.7 T animal MRI system, included in algebraic image reconstruction, and tested in experiments on phantoms and tissue samples.

RESULTS

The optimized modulated pulses provide considerably improved performance with respect to uniformity and mean FA as compared with block-shaped counterparts of the same maximum length. Using these pulses, ZTE images of excellent uniformity were obtained with enhanced FA and thus expanded contrast versatility.

CONCLUSION

The performance of ZTE imaging can be significantly improved by employing optimized short amplitude- and frequency-modulated RF pulses.

Rapid and robust pulmonary proton ZTE imaging in the mouse

Pulmonary MRI is challenging because of the low proton density and rapid transverse relaxation in the lung associated with microscopic magnetic field inhomogeneities caused by tissue-air interfaces. Therefore, low signal is obtained in gradient and spin echo proton images. Alternatively, non-proton MRI using hyperpolarized gases or radial techniques with ultrashort or zero TE have been proposed to image the lung. Also with the latter approach, the general challenge remains to provide full coverage of the lung at sufficient spatial resolution, signal-to-noise ratio (SNR) and image quality within a reasonable scan time. This task is further aggravated by physiological motion and is particularly demanding in small animals, such as mice. In this work, three-dimensional (3D) zero echo time (ZTE) imaging is employed for efficient pulmonary MRI. Four protocols with different averaging and respiratory triggering schemes are developed and compared with respect to image quality and SNR. To address the critical issue of background signal in ZTE images, a subtraction approach is proposed, providing images virtually free of disturbing signal from nearby hardware parts. The protocols are tested for pulmonary MRI in six mice at 4.7 T, consistently providing images of high quality with a 3D isotropic resolution of 313 µm and SNR values in the lung between 8.0 and 18.5 within scan times between 1 min 21 s and 4 min 44 s. A generally high robustness of the ZTE approach against motion is observed, whilst respiratory triggering further improves the SNR and visibility of image details. The developed techniques are expected to enable efficient preclinical animal studies in the lung and will also be of importance for human applications. Further improvements are expected from radiofrequency (RF) coils with increased SNR and reduced background signal.