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.

A transmit-receive array for brain imaging with a high-performance gradient insert.

PURPOSE: The aim of this work was to provide parallel imaging capability for the human head in a gradient insert of 33-cm inner diameter within the related constraints of space, encoding ambiguity, and eddy current immunity. METHODS: Eddy current behavior of the 8-channel transmit-receive array coil was investigated via heating and field deviation measurements. RF performance was evaluated using S-parameters, noise statistics, B1 maps, and g-factor maps. In vivo images of a human head and knee were acquired with Cartesian readout and TE below 0.45 ms. RESULTS: Under intense gradient use, the shield was heated up to 55°C and other coil structures to 40°C. After standard preemphasis calibration, eddy current-related field distortions caused by the developed RF coil were smaller than for a commercial receive-only coil. In the ambiguous regions of the gradient, B1+ is 20 dB lower than in the center of the FOV. Coupling between elements is below -15 dB, and noise correlation is less than 0.31 when the coil is loaded with a head. Power efficiency was 0.52 ± 0.02 µT/√W, and the SD of the flip angle was below 10% in central slices of the brain. 2D, up to fourfold acceleration causes less than 30% noise amplification. The RF coil can be used during full gradient performance. CONCLUSION: Based on the described features, the presented coil enables parallel imaging in the high-performance gradient insert.

High-resolution short-T2 MRI using a high-performance gradient.

PURPOSE: To achieve high resolution in imaging of short-T2 materials and tissues by using a high-performance human-sized gradient insert with strength up to 200 mT/m and 100% duty cycle. METHODS: Dedicated short-T2 methodology and hardware are used, such as the pointwise encoding time reduction with radial acquisition (PETRA) technique with modulated excitation pulses, optimized radio-frequency hardware, and a high-performance gradient insert. A theoretical analysis of actual spatial resolution and SNR is provided to support the choice of scan parameters and interpretation of the results. Imaging is performed in resolution phantoms, animal specimen, and human volunteers at both conventional and maximum available gradient strengths and compared using image subtraction. RESULTS: Calculations suggest that increasing gradient strength beyond conventional values considerably improves both actual resolution and SNR efficiency in short-T2 imaging. Resolution improvements are confirmed in all investigated samples, in particular 2 mm slots were resolved in a hard-plastic plate with T2 ≈ 10 µs and in vivo musculoskeletal images were acquired at isotropic 200 µm resolution. Expected improvements in signal yield are realized in fine structures benefitting from high resolution but to less extent in regions of low contrast where decay-related blurring leads to signal overlap between neighboring locations. CONCLUSION: Strong gradients with high duty cycle enable short-T2 imaging at unprecedentedly high resolution, holding the potential for improving MRI of, eg, bone, tendon, lung, or teeth. Moreover, it allows direct access of tissues with T2 of tens of microseconds such as myelin or collagen.

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.

(39) K and (23) Na relaxation times and MRI of rat head at 21.1 T.

At ultrahigh magnetic field strengths (B0 ≥ 7.0 T), potassium ((39) K) MRI might evolve into an interesting tool for biomedical research. However, (39) K MRI is still challenging because of the low NMR sensitivity and short relaxation times. In this work, we demonstrated the feasibility of (39) K MRI at 21.1 T, determined in vivo relaxation times of the rat head at 21.1 T, and compared (39) K and sodium ((23) Na) relaxation times of model solutions containing different agarose gel concentrations at 7.0 and 21.1 T. (39) K relaxation times were markedly shorter than those of (23) Na. Compared with the lower field strength, (39) K relaxation times were up to 1.9- (T1 ), 1.4- (T2S ) and 1.9-fold (T2L ) longer at 21.1 T. The increase in the (23) Na relaxation times was less pronounced (up to 1.2-fold). Mono-exponential fits of the (39) K longitudinal relaxation time at 21.1 T revealed T1 = 14.2 ± 0.1 ms for the healthy rat head. The (39) K transverse relaxation times were 1.8 ± 0.2 ms and 14.3 ± 0.3 ms for the short (T2S ) and long (T2L ) components, respectively. (23) Na relaxation times were markedly longer (T1 = 41.6 ± 0.4 ms; T2S = 4.9 ± 0.2 ms; T2L = 33.2 ± 0.2 ms). (39) K MRI of the healthy rat head could be performed with a nominal spatial resolution of 1 × 1 × 1 mm(3) within an acquisition time of 75 min. The increase in the relaxation times with magnetic field strength is beneficial for (23) Na and (39) K MRI at ultrahigh magnetic field strength. Our results demonstrate that (39) K MRI at 21.1 T enables acceptable image quality for preclinical research. Copyright © 2016 John Wiley & Sons, Ltd.

In vivo 39K MR imaging of human muscle and brain.

PURPOSE: To implement potassium 39 ((39)K) magnetic resonance (MR) imaging with a 7-T MR imaging system and to evaluate its feasibility for in vivo imaging of human muscle and brain. MATERIALS AND METHODS: Three healthy volunteers were examined with approval of the ethical review board of Heidelberg University. Written informed consent was obtained from all volunteers. Because the available 7-T MR imaging system did not support (39)K, a frequency conversion scheme was developed and connected to the imaging unit. The standard X-nucleus frequency of lithium 7 (115 MHz) was converted to the frequency of (39)K at 7 T (14 MHz). Relaxation times of healthy thigh muscle and brain tissue were estimated by using multiple-echo and inversion-recovery sequences. Data analysis was conducted with a nonlinear least squares curve fitting tool. In vivo (39)K MR imaging of healthy human thigh muscle and brain was performed. RESULTS: With use of the custom-built frequency conversion scheme, (39)K MR imaging is feasible with a commercially available 7-T MR imaging system. Nominal spatial resolutions of 8 × 8 × 16 mm(3) and 9.5 × 9.5 × 9.5 mm(3) were achieved for human thigh muscle and brain, respectively. Acquisition time was 30 minutes for both muscle and brain tissue. The measured potassium concentration (uncorrected for fat fraction) of thigh muscle tissue (112-124 mmol/L) lies within the expected range. CONCLUSION: In vivo (39)K MR imaging in humans can be performed in clinically feasible measurement times (approximately 30 minutes) with voxel sizes of approximately 1 mL.