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 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.

An improved intraoral transverse loop coil design for high-resolution dental MRI.

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.

Scalable and modular 8-channel transmit and 8-channel flexible receive coil array for 19 F MRI of large animals.

PURPOSE: To introduce an RF coil system consisting of an 8-channel transmit (Tx) and 8-channel receive (Rx) coil arrays for 19 F 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 19 F Tx and Rx elements, and over 30 dB between the 1 H and 19 F 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 19 F 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 19 F coil provides homogeneous excitation and high sensitivity detection in large pig models.

RF-induced heating of interventional devices at 23.66 MHz.

OBJECTIVE: Low-field MRI systems are expected to cause less RF heating in conventional interventional devices due to lower Larmor frequency. We systematically evaluate RF-induced heating of commonly used intravascular devices at the Larmor frequency of a 0.55 T system (23.66 MHz) with a focus on the effect of patient size, target organ, and device position on maximum temperature rise. MATERIALS AND METHODS: To assess RF-induced heating, high-resolution measurements of the electric field, temperature, and transfer function were combined. Realistic device trajectories were derived from vascular models to evaluate the variation of the temperature increase as a function of the device trajectory. At a low-field RF test bench, the effects of patient size and positioning, target organ (liver and heart) and body coil type were measured for six commonly used interventional devices (two guidewires, two catheters, an applicator and a biopsy needle). RESULTS: Electric field mapping shows that the hotspots are not necessarily localized at the device tip. Of all procedures, the liver catheterizations showed the lowest heating, and a modification of the transmit body coil could further reduce the temperature increase. For common commercial needles no significant heating was measured at the needle tip. Comparable local SAR values were found in the temperature measurements and the TF-based calculations. CONCLUSION: At low fields, interventions with shorter insertion lengths such as hepatic catheterizations result in less RF-induced heating than coronary interventions. The maximum temperature increase depends on body coil design.

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.

Sub-millisecond 2D MRI of the vocal fold oscillation using single-point imaging with rapid encoding.

OBJECTIVE: The slow spatial encoding of MRI has precluded its application to rapid physiologic motion in the past. The purpose of this study is to introduce a new fast acquisition method and to demonstrate feasibility of encoding rapid two-dimensional motion of human vocal folds with sub-millisecond resolution. METHOD: In our previous work, we achieved high temporal resolution by applying a rapidly switched phase encoding gradient along the direction of motion. In this work, we extend phase encoding to the second image direction by using single-point imaging with rapid encoding (SPIRE) to image the two-dimensional vocal fold oscillation in the coronal view. Image data were gated using electroglottography (EGG) and motion corrected. An iterative reconstruction with a total variation (TV) constraint was used and the sequence was also simulated using a motion phantom. RESULTS: Dynamic images of the vocal folds during phonation at pitches of 150 and 165 Hz were acquired in two volunteers and the periodic motion of the vocal folds at a temporal resolution of about 600 µs was shown. The simulations emphasize the necessity of SPIRE for two-dimensional motion encoding. DISCUSSION: SPIRE is a new MRI method to image rapidly oscillating structures and for the first time provides dynamic images of the vocal folds oscillations in the coronal plane.

MR safety watchdog for active catheters: Wireless impedance control with real-time feedback.

PURPOSE: To dynamically minimize radiofrequency (RF)-induced heating of an active catheter through an automatic change of the termination impedance. METHODS: A prototype wireless module was designed that modifies the input impedance of an active catheter to keep the temperature rise during MRI below a threshold, ΔTmax . The wireless module (MR safety watchdog; MRsWD) measures the local temperature at the catheter tip using either a built-in thermistor or external data from a fiber-optical thermometer. It automatically changes the catheter input impedance until the temperature rise during MRI is minimized. If ΔTmax is exceeded, RF transmission is blocked by a feedback system. RESULTS: The thermistor and fiber-optical thermometer provided consistent temperature data in a phantom experiment. During MRI, the MRsWD was able to reduce the maximum temperature rise by 25% when operated in real-time feedback mode. CONCLUSION: This study demonstrates the technical feasibility of an MRsWD as an alternative or complementary approach to reduce RF-induced heating of active interventional devices. The automatic MRsWD can reduce heating using direct temperature measurements at the tip of the catheter. Given that temperature measurements are intrinsically slow, for a clinical implementation, a faster feedback parameter would be required such as the RF currents along the catheter or scattered electric fields at the tip.

Magnetic resonance imaging of the vocal fold oscillations with sub-millisecond temporal resolution.

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.

Safety of active catheters in MRI: Termination impedance versus RF-induced heating.

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.

Optimization of acoustic radiation force imaging: Influence of timing parameters on sensitivity.

PURPOSE: Optimization of timing parameters for MR-guided ARFI to achieve the highest displacement signal-to-noise ratio (SNRd ). 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 SNRd . 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 SNRd 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 SNRd 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.

Direct estimation of 17 O MR images (DIESIS) for quantification of oxygen metabolism in the human brain with partial volume correction.

PURPOSE: To provide a data post-processing method that corrects for partial volume effects (PVE) and fast T 2 * decay in dynamic 17 O 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 17 O gas inhalation using dynamic 17 O 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 17 O MRI data sets of a healthy volunteer acquired on a 3T MRI system. With DIESIS, 17 O MR signal time curves in selected regions were directly estimated based on parcellation of a coregistered 1 H 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 15 O-PET studies. CONCLUSION: DIESIS provided an increased separation of CMRO2 values in GM and WM brain regions and corrected for partial volume effects in 17 O-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.

Comparison of ultrashort echo time sequences for MRI of an ancient mummified human hand.

PURPOSE: To compare the three different short-echo time (TE) pulse sequences ultrashort echo time (UTE), point-wise encoding time reduction with radial acquisition (PETRA), and single point imaging (SPI) for MRI of ancient remains. METHODS: MRI of mummies is challenging due to the extremely low water content and the very short transverse relaxation times T2 *. To overcome the signal loss associated with the short T2 *, three pulse sequences with very short TEs were compared. MR images of an ancient mummified human hand were acquired at field strengths of 1.5 Tesla (T) and 3T using home-made solenoid Tx/Rx radiofrequency (RF) coils. RESULTS: In all MR images, tissues could be differentiated and anatomical structures such as bones and tendons were clearly identified. Skin with embalming resin was hyperintense in MRI, whereas it appeared iso-intense in computed tomography. PETRA has the highest signal to noise ratio. With UTE, short scan times and a homogeneous RF excitation can be achieved, and blurring is less pronounced than with PETRA. SPI shows no blurring artifacts; however, it requires long scan times. CONCLUSION: This work provided an initial analysis for the optimization of imaging protocols for paleoradiology studies with MRI, and, ultimately, for MRI of tissue with extremely short T2 *.

Ensuring safety and functionality of electroglottography measurements during dynamic pulmonary MRI.

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.

Active decoupling of RF coils using a transmit array system.

OBJECTIVE: Implementation of a decoupling method for isolation of transmit and receive radio frequency (RF) coils for concurrent excitation and acquisition (CEA) MRI in samples with ultra-short T2*. MATERIALS AND METHODS: The new phase and amplitude (PA) decoupling method is implemented in a clinical 3T-MRI system equipped with a parallel transmit array system. For RF excitation, two transmit coils are used in combination with a single receive coil. The transmit coil is geometrically decoupled from the receive coil, and the remaining B 1-induced voltages in the receive coil during CEA are minimized by the second transmit coil using a careful adjustment of the phase and amplitude settings in this coil. Isolation of the decoupling scheme and transmit noise behavior are analyzed for different loading conditions, and a CEA MRI experiment is performed in a rubber phantom with sub-millisecond T2* and in an ex vivo animal. RESULTS: Geometrical (20 dB) and PA decoupling (50 dB) provided a total isolation of 70 dB between the transmit and receive coils. Integration of a low-noise RF amplifier was necessary to minimize RF transmit noise. CEA MR images could be reconstructed from a rubber phantom and an ex vivo animal. CONCLUSION: CEA MRI can be implemented in clinical MRI systems using active decoupling with parallel transmit array capabilities with minor hardware modifications.

MR Safety of Inductively Coupled and Conventional Intraoral Coils.

PURPOSE: Intraoral coils (IOCs) in magnetic resonance imaging (MRI) significantly improve the signal-to-noise ratio compared with conventional extraoral coils. To assess the safety of IOCs, we propose a 2-step procedure to evaluate radiofrequency-induced heating of IOCs and compare maximum temperature increases in 3 different types of IOCs. METHODS: The 2-step safety assessment consists of electric field measurements and simulations to identify local hotspots followed by temperature measurements during MRI. With this method, 3 different coil types (inductively coupled IFC, transmit/receive tLoop, and receive-only tLoopRx) were tested at 1.5 T and 3 T for both tuned and detuned coil states. High SAR and regular MRI protocols were applied for 2 coil positions. RESULTS: The measured E field maps display distinct hotspots for all tuned IOCs, which were reduced by at least 40-fold when the IOCs were detuned. Maximum temperature rise was higher when the coils were positioned at the periphery of the phantom with the coil planes parallel to B0. When neither active nor passive detuning was applied, maximum temperature increase of ΔT = 1.3/0.5/1.8 K was found for IFC/tLoop/tLoopRx coils. Hotspots detected by E field measurements, and simulations were consistent. In the simulations, the results were different for homogeneous phantoms compared with full anatomical models. The 2-step test procedure is applicable to different coil types. CONCLUSIONS: The results indicate that a risk for radiofrequency-induced heating exists for tuned IOCs, so that adequate detuning circuits need to be integrated in the coils to ensure safe operation.

An Accelerated PETALUTE MRI Sequence for In Vivo Quantification of Sodium Content in Human Articular Cartilage at 3T.

In this work, we demonstrate the sodium magnetic resonance imaging (MRI) capabilities of a three-dimensional (3D) dual-echo ultrashort echo time (UTE) sequence with a novel rosette petal trajectory (PETALUTE), in comparison to the 3D density-adapted (DA) radial spokes UTE sequence. We scanned five healthy subjects using a 3D dual-echo PETALUTE acquisition and two comparable implementations of 3D DA-radial spokes acquisitions, one matching the number of k-space projections (Radial-Matched Trajectories) and the other matching the total number of samples (Radial-Matched Samples) acquired in k-space. The PETALUTE acquisition enabled equivalent sodium quantification in articular cartilage volumes of interest (168.8 ± 29.9 mM) to those derived from the 3D radial acquisitions (171.62 ± 28.7 mM and 149.8 ± 22.2 mM, respectively). We achieved a shorter scan time of 2:06 for 3D PETALUTE, compared to 3:36 for 3D radial acquisitions. We also evaluated the feasibility of further acceleration of the PETALUTE sequence through retrospective compressed sensing with 2× and 4× acceleration of the first echo and showed structural similarity of 0.89 ± 0.03 and 0.87 ± 0.03 when compared to non-retrospectively accelerated reconstruction. Together, these results demonstrate improved scan time with equivalent performance of the PETALUTE sequence compared to the 3D DA-radial sequence for sodium MRI of articular cartilage.

Quantification of regional CMRO2 in human brain using dynamic 17O-MRI at 3T.

OBJECTIVE: To investigate the feasibility of cerebral metabolic rate of oxygen consumption (CMRO2) measurements with MRI at 3 Tesla in different brain regions. METHODS: CMRO2 represents a key indicator of the physiological state of brain tissue. Dynamic 17O-MRI with inhalation of isotopically enriched 17O gas has been used to quantify global CMRO2 in brain white (WM) and gray matter (GM). However, global CMRO2 can only reflect the overall oxygen metabolism of the brain and cannot provide enough information on local tissue oxygen metabolism. To investigate the feasibility of determination of regional CMRO2 at a clinical 3 T MRI system, CMRO2 values in frontal, parietal and occipital WM and GM were determined in 5 healthy volunteers and compared to evaluate the regional differences of oxygen metabolism in WM and GM. Additionally, regional CMRO2 values were determined in deep brain structures including thalamus, dorsal striatum, caudate nucleus and insula cortex and in the cerebella, and compared with literature values from 15O-PET studies. RESULTS: In cortical GM the determined CMRO2 values were in good agreement with the literature, whereas values in WM were about 32-48% higher than literature values. Regional analysis revealed a significantly higher CMRO2 in the occipital GM compared to the frontal and parietal GM. By contrast, no significant difference of CMRO2 was observed across the WM. In addition, CMRO2 in deep brain structures was lower compared to literature values and in the cerebella a good hemispheric symmetry of the tissue oxygen metabolism was found. CONCLUSION: Dynamic 17O-MRI enables direct, non-invasive determination of regional CMRO2 in brain structures in healthy volunteers at 3T.

High-resolution 3D ultra-short echo time MRI with Rosette k-space pattern for brain iron content mapping.

BACKGROUND: The iron concentration increases during normal brain development and is identified as a risk factor for many neurodegenerative diseases, it is vital to monitor iron content in the brain non-invasively. PURPOSE: This study aimed to quantify in vivo brain iron concentration with a 3D rosette-based ultra-short echo time (UTE) magnetic resonance imaging (MRI) sequence. METHODS: A cylindrical phantom containing nine vials of different iron concentrations (iron (II) chloride) from 0.5 millimoles to 50 millimoles and six healthy subjects were scanned using 3D high-resolution (0.94 ×0.94 ×0.94 mm3) rosette UTE sequence at an echo time (TE) of 20 µs. RESULTS: Iron-related hyperintense signals (i.e., positive contrast) were detected based on the phantom scan, and were used to establish an association between iron concentration and signal intensity. The signal intensities from in vivo scans were then converted to iron concentrations based on the association. The deep brain structures, such as the substantia nigra, putamen, and globus pallidus, were highlighted after the conversion, which indicated potential iron accumulations. CONCLUSION: This study suggested that T1-weighted signal intensity could be used for brain iron mapping.

Characterizing Off-center MRI with ZTE.

PURPOSE: 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. METHODS: 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. RESULTS: 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. CONCLUSION: 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.

Differences of respiratory kinematics in female and male singers - A comparative study using dynamic magnetic resonance imaging.

Breath control is an important factor for singing voice production, but pedagogic descriptions of how a beneficial movement pattern should be performed vary widely and the underlying physiological processes are not understood in detail. Differences in respiratory movements during singing might be related to the sex of the singer. To study sex-related differences in respiratory kinematics during phonation, 12 singers (six male and six female) trained in the Western classical singing tradition were imaged with dynamic magnetic resonance imaging. Singers were asked to sustain phonation at five different pitches and loudness conditions, and cross-sectional images of the lung were acquired. In each dynamic image frame the distances between anatomical landmarks were measured to quantify the movements of the respiratory apparatus. No major difference between male and female singers was found for the general respiratory kinematics of the thorax and the diaphragm during sustained phonation. However when compared to sole breathing, male singers significantly increased their thoracic movements for singing. This behavior could not be observed in female singers. The presented data support the hypothesis that professional singers follow sex-specific breathing strategies. This finding may be important in a pedagogical context where the biological sex of singer and student differ and should be further investigated in a larger cohort.