Search

To maximize acquisition bandwidth in zero echo time (ZTE) sequences, readout gradients are already switched on during the RF pulse, creating unwanted slice selectivity. The resulting image distortions are amplified especially when the anatomy of interest is not located at the isocenter. We aim to characterize off-center ZTE MRI of extremities such as the shoulder, knee, and hip, adjusting the carrier frequency of the RF pulse excitation for each TR. In ZTE MRI, radial encoding schemes are used, where the distorted slice profile due to the finite RF pulse length rotates with the k-space trajectory. To overcome these modulations for objects far away from the magnet isocenter, the frequency of the RF pulse is shifted for each gradient setting so that artifacts do not occur at a given off-center target position. The sharpness of the edges in the images were calculated and the ZTE acquisition with off-center excitation was compared to an acquisition with isocenter excitation both in phantom and in vivo off-center MRI of the shoulder, knee, and hip at 1.5 and 3T MRI systems. Distortion and blurriness artifacts on the off-center MRI images of the phantom, in vivo shoulder, knee, and hip images were mitigated with off-center excitation without time or noise penalty, at no additional computational cost. The off-center excitation allows ZTE MRI of the shoulder, knee, and hip for high-bandwidth image acquisitions for clinical settings, where positioning at the isocenter is not possible. [ABSTRACT FROM AUTHOR]

Purpose: To improve intraoral transverse loop coil design for high‐resolution dental MRI. Methods: The transverse intraoral loop coil (tLoop) was modified (mtLoop) by overlapping the feed port conductors, bending the posterior section, introducing a parallel plate capacitor, optimizing the insulation thickness, and using it in receive‐only mode. In addition, an MR‐silent insulation was introduced. The performances of the mtLoop and tLoop coils were compared in terms of sensitivity, image SNR, and eddy currents using electromagnetic simulations and MRI measurements at 3T. Results: The receive‐only mode of the mtLoop increases the sensitivity at the apices of the roots, and the overlapped feed port design eliminated signal voids along the incisors. The bent posterior section with the parallel plate capacitor reduced the unwanted signal of the tongue by a factor of 2.3 in the selected region off interest and lowered the eddy currents by 10%. The proposed new coil provided higher SNR by elevenfold and 2.5‐fold at the incisors and apices of the molar roots within the selected regions of interest, respectively, in the experiments, as well as improved comfort. Optimal insulation thickness was determined as 1 mm. With the mtLoop, a (250 μm)3 isotropic resolution of the dental arch could be realized using a UTE sequence within 2 min total acquisition time. A T2‐SPACE protocol with (350 μm)2 in‐plane resolution was also demonstrated. Conclusion: The proposed new coil offers higher SNR at the incisors and apices of the molar roots, less unwanted signals from tongue, lower eddy currents, and improved patient comfort. [ABSTRACT FROM AUTHOR]

Purpose: To introduce an RF coil system consisting of an 8‐channel transmit (Tx) and 8‐channel receive (Rx) coil arrays for 19F MRI of large animals. Methods: The Tx efficiency and homogeneity of the 8‐element loop coil array (loop size: 6 × 15 cm2) were simulated for two different pig models rendered from MR images. An 8‐channel Rx coil array consisting of a flexible 6‐channel posterior and a 2‐channel planar anterior array was designed to fit on the abdomen of an average‐sized pig in supine position. Measurements were performed in a grid phantom and ex vivo on a pig model with perfluoroctylbromide (PFOB)–filled tubes inserted in the thorax. Results: Measured and simulated Tx efficiency and homogeneity for the 8‐channel and 5‐channel arrays were in good agreement: 1.87 ± 0.22μT/√kW versus 1.96 ± 0.29μT/√kW, and 2.29 ± 0.39μT/√kW versus 2.41 ± 0.37μT/√kW. An isolation of 38 ± 8 dB is achieved between the 19F Tx and Rx elements, and over 30 dB between the 1H and 19F elements. The PFOB‐filled vials could be clearly identified within the cadaver abdomen with an SNR of 275 ± 51 for a 3D gradient‐echo sequence with 2‐mm isotropic resolution and 12 averages, acquired in 9:52 min:s. Performance of the Tx array was robust against phase and amplitude mismatches at the input ports. Conclusions: A modular and scalable Tx array offers improved Tx efficiency in 19F MRI of large animals with various sizes. Although conventional birdcage coils have superior Tx efficiency within the target region of interest, scalability of the Tx array to animal size is a major benefit. The described 19F coil provides homogeneous excitation and high sensitivity detection in large pig models. [ABSTRACT FROM AUTHOR]

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 kx–ky plane, with oscillated extensions in the kz‐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. [ABSTRACT FROM AUTHOR]

Purpose: The temporal resolution of the MRI acquisition is intrinsically limited by the duration of the spatial encoding, which is typically on the order of milliseconds. Faster motion such as the vibration of the vocal folds during phonation cannot be imaged with conventional MRI as this would require sampling frequencies in the kilo‐Hertz range. Here, a faster MRI acquisition strategy is presented that encodes a 1D periodic motion at a temporal resolution that is an order of magnitude higher compared to conventional MRI. Methods: The proposed method encodes the position of an object moving along 1 dimension by applying very short phase encoding gradients along the same direction. This reduces the temporal resolution from the repetition time (TR) to the duration of the phase encoding gradients, which in this work was well below 1 ms. The technique is applied to the vocal fold oscillations and the position of the vocal folds is measured simultaneously using electroglottography (EGG). Simulations of the point spread function for regular encoding and the proposed method are performed as well. Results: With this new phase, encoding strategy oscillations of the human vocal folds up to a frequency of 145 Hz could be dynamically imaged at 10 images per cycle. Simulations show the advantage of this method over conventional imaging of fast moving objects. Conclusion: A new method for MR imaging of fast moving spins is presented allowing a temporal resolution below 1 ms at a spatial resolution below 1 mm, circumventing TR as the limit for temporal resolution. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the effect of the termination impedance on the RF‐induced heating of active catheters using analytical modeling. Theory and Methods: Interaction of an arbitrary electric (E) field and an isolated transmission line (TL) embedded in cascaded lossy media was analytically modeled. Termination impedances at the tip and the input sides were expressed as distinct parameters in the current and voltage distribution formulae that are obtained by solving the inhomogeneous wave equations using the Green's function approach. The tip specific absorption rate (SAR) was calculated for different E field configurations. The tip SAR was displayed on a color‐coded Smith chart in terms of the normalized input reflection coefficient. Results of the analytical calculations were compared to transfer function (TF) measurements. An input impedance control unit that is integrated to the interface circuit was introduced. Results: TFs from analytical model and measurements exhibited similar behaviors. Color‐coded Smith charts shows that the analytical model and measured TF‐based tip SAR depends strongly on the input reflection coefficient. Both for measured and analytical TFs, SAR can deviate up to 70% from the mean value for different input impedance values. Conclusion: This study shows that it is possible to change the RF‐induced heating characteristics of an active catheter by adjusting the input impedance, and the presented analytical model is in good agreement with TF measurements. [ABSTRACT FROM AUTHOR]

Purpose: To provide a data post‐processing method that corrects for partial volume effects (PVE) and fast T2* decay in dynamic 17O MRI for the mapping of cerebral metabolic rates of oxygen consumption (CMRO2). Methods: CMRO2 is altered in neurodegenerative diseases and tumors and can be measured after 17O gas inhalation using dynamic 17O MRI. CMRO2 quantification is difficult because of PVE. To correct for PVE, a direct estimation of the MR images (DIESIS) method is proposed and used in 4 dynamic 17O MRI data sets of a healthy volunteer acquired on a 3T MRI system. With DIESIS, 17O MR signal time curves in selected regions were directly estimated based on parcellation of a coregistered 1H MPRAGE image. Results: Profile likelihood analysis of the DIESIS method showed identifiability of CMRO2. In white matter (WM), DIESES reduced CMRO2 from 0.97 ± 0.25 µmol/gtissue/min with Kaiser‐Bessel gridding reconstruction to 0.85 ± 0.21 µmol/gtissue/min, whereas in gray matter (GM) it increases from 1.3 ± 0.31 µmol/gtissue/min to 1.86 ± 0.36 µmol/gtissue/min; both values are closer to the literature values from the 15O‐PET studies. Conclusion: DIESIS provided an increased separation of CMRO2 values in GM and WM brain regions and corrected for partial volume effects in 17O‐MRI inhalation experiments. DIESIS could also be applied to more heterogeneous tissues such as glioblastomas if subregions of the tumor can be represented as additional parcels. [ABSTRACT FROM AUTHOR]

Purpose Optimization of timing parameters for MR-guided ARFI to achieve the highest displacement signal-to-noise ratio ( SNR d). Theory and Methods In MR-guided ARFI the phase change induced by motion encoding gradients (MEGs) is measured to assess tissue displacement. The sensitivity of this encoding procedure depends on several timing parameters, such as the MEG duration and the offset time between ultrasound (US) and MEG. Furthermore, mechanical and MR tissue constants and MEG schemes (bipolar or three-lobed) influence SNR d. Optimal timing parameters were determined in simulations for bipolar and three-lobed MEGs, and the results were compared with measurements. To provide clinically usable timing parameters, physiologically relevant ranges of tissue constants were considered. Results For the considered ranges of tissue constants, optimal timing parameters provide only 6% higher SNR d for bipolar than for three-lobed MEG. Three-lobed MEG is less sensitive to motion as confirmed in phantom experiments. Bipolar MEG can use approximately 1.5-fold shorter MEG durations. Conclusion Both bipolar and three-lobed MEGs can yield approximately the same SNR d if the optimal timing parameters are chosen. Bipolar MEG allows for shorter durations, which is preferable if deposition of US energy needs to be minimized, and three-lobed MEG is more suitable when residual motion compensation is necessary. Magn Reson Med 79:981-986, 2018. © 2017 International Society for Magnetic Resonance in Medicine. [ABSTRACT FROM AUTHOR]

Purpose To combine vocal tract measurements with dynamic MRI of the lungs to provide fundamental insights into the lung physiology during singing. Methods To analyze vocal fold oscillatory patterns during dynamic lung MRI, an electroglottography (EGG) system was modified to allow for simultaneous EGG measurements during MR image acquisitions. A low-pass filter was introduced to suppress residual radiofrequency (RF) coupling into the EGG signal. RF heating was tested in a gel phantom to ensure MR safety, and functionality of the EGG device was assessed in a volunteer experiment at singing frequencies from A5 to A3. In the recorded EGG signals, remaining RF interferences were removed by independent component analysis post processing, and standard EGG parameters such as fundamental frequency, contact quotient and jitter were calculated. In a second volunteer experiment, EGG recordings were compared with lung diameter measurements from 2D time-resolved trueFISP acquisitions. Results RF heating measurements resulted in less than 1.2°C temperature increase in the gel phantom. EGG parameters measured during MR imaging are within the range of ideal values. In the lung measurement, both the lung diameter and the EGG recordings could be successfully performed with only minimal interference. Conclusion EGG recording is pos sible during dynamic lung MRI, and glottal activity can be studied safely at 1.5T. Magn Reson Med 76:1629-1635, 2016. © 2015 International Society for Magnetic Resonance in Medicine [ABSTRACT FROM AUTHOR]