From signal-based to comprehensive magnetic resonance imaging.

We present and evaluate a new insight into magnetic resonance imaging (MRI). It is based on the algebraic description of the magnetization during the transient response-including intrinsic magnetic resonance parameters such as longitudinal and transverse relaxation times (T1, T2) and proton density (PD) and experimental conditions such as radiofrequency field (B1) and constant/homogeneous magnetic field (B0) from associated scanners. We exploit the correspondence among three different elements: the signal evolution as a result of a repetitive sequence of blocks of radiofrequency excitation pulses and encoding gradients, the continuous Bloch equations and the mathematical description of a sequence as a linear system. This approach simultaneously provides, in a single measurement, all quantitative parameters of interest as well as associated system imperfections. Finally, we demonstrate the in-vivo applicability of the new concept on a clinical MRI scanner.

K-space trajectories in 3D-GRASE sequence for high resolution structural imaging.

PURPOSE: To propose and evaluate new k-space trajectories for 3D-GRASE to improve scan time over 3D-FSE/TSE for high resolution structural imaging. METHODS: Five different Cartesian k-space trajectories were developed and evaluated. They combine ideas of existing k-space trajectories for 3D-GRASE and 3D-FSE/TSE. T2 and T2* are linearly or radially modulated in k-space to achieve the desired contrast while including the autocalibration region needed for the parallel imaging reconstruction technique. Phase modulation among echoes was corrected in reconstruction to remove remaining artefacts. Simulation and in-vivo experiments on a 3T scanner were conducted to evaluate the performance of the different k-space trajectories. RESULTS: Two of the proposed k-space trajectories for high resolution structural imaging with 3D-GRASE obtained images comparable to 3D-FSE with lower specific absorption rate (PD/T2: 41%/75%) and shorter acquisition time (PD/T2: 27%/20%). CONCLUSIONS: 3D-GRASE image quality strongly depends on the k-space trajectory. With an optimal trajectory, 3D-GRASE may be preferable over 3D-FSE/TSE for structural high-resolution MRI.

Cloud-processed 4D CMR flow imaging for pulmonary flow quantification.

OBJECTIVES: In this study, we evaluated a cloud-based platform for cardiac magnetic resonance (CMR) four-dimensional (4D) flow imaging, with fully integrated correction for eddy currents, Maxwell phase effects, and gradient field non-linearity, to quantify forward flow, regurgitation, and peak systolic velocity over the pulmonary artery. METHODS: We prospectively recruited 52 adult patients during one-year period from July 2014. The 4D flow and planar (2D) phase-contrast (PC) were acquired during same scanning session, but 4D flow was scanned after injection of a gadolinium-based contrast agent. Eddy-currents were semi-automatically corrected using the web-based software. Flow over pulmonary valve was measured and the 4D flow values were compared against the 2D PC ones. RESULTS: The mean forward flow was 92 (±30) ml/cycle measured with 4D flow and 86 (±29) ml/cycle measured with 2D PC, with a correlation of 0.82 and a mean difference of -6ml/cycle (-41-29). For the regurgitant fraction the correlation was 0.85 with a mean difference of -0.95% (-17-15). Mean peak systolic velocity measured with 4D flow was 92 (±49) cm/s and 108 (±56) cm/s with 2D PC, having a correlation of 0.93 and a mean difference of 16cm/s (-24-55). CONCLUSION: 4D flow imaging post-processed with an integrated cloud-based application accurately quantifies pulmonary flow. However, it may underestimate the peak systolic velocity.

Small-tip-angle spokes pulse design using interleaved greedy and local optimization methods.

Current spokes pulse design methods can be grouped into methods based either on sparse approximation or on iterative local (gradient descent-based) optimization of the transverse-plane spatial frequency locations visited by the spokes. These two classes of methods have complementary strengths and weaknesses: sparse approximation-based methods perform an efficient search over a large swath of candidate spatial frequency locations but most are incompatible with off-resonance compensation, multifrequency designs, and target phase relaxation, while local methods can accommodate off-resonance and target phase relaxation but are sensitive to initialization and suboptimal local cost function minima. This article introduces a method that interleaves local iterations, which optimize the radiofrequency pulses, target phase patterns, and spatial frequency locations, with a greedy method to choose new locations. Simulations and experiments at 3 and 7 T show that the method consistently produces single- and multifrequency spokes pulses with lower flip angle inhomogeneity compared to current methods.

Nonuniform and multidimensional Shinnar-Le Roux RF pulse design method.

The Shinnar-Le Roux (SLR) radiofrequency (RF) pulse design algorithm is widely used for designing slice-selective RF pulses due to its intuitiveness, optimality, and speed. SLR is limited, however, in that it is only capable of designing one-dimensional pulses played along constant gradients. We present a nonuniform SLR RF pulse design framework that extends most of the capabilities of classical SLR to nonuniform gradient trajectories and multiple dimensions. Specifically, like classical SLR, the new method is a hard pulse approximation-based technique that uses filter design relationships to produce the lowest power RF pulse that satisfies target magnetization ripple levels. The new method is validated and compared with methods conventionally used for nonuniform and multidimensional large-tip-angle RF pulse design.

Fast radiofrequency flip angle calibration by Bloch-Siegert shift.

In a recent work, we presented a novel method for B 1+ field mapping based on the Bloch-Siegert shift. Here, we apply this method to automated fast radiofrequency transmit gain calibration. Two off-resonance radiofrequency pulses were added to a slice-selective spin echo sequence. The off-resonance pulses induce a Bloch-Siegert phase shift in the acquired signal that is proportional to the square of the radiofrequency field magnitude B(1) (2) . The signal is further spatially localized by a readout gradient, and the signal-weighted average B(1) field is calculated. This calibration from starting system transmit gain to average flip angle is used to calculate the transmit gain setting needed to produce a desired imaging sequence flip angle. A robust implementation is demonstrated with a scan time of 3 s. The Bloch-Siegert-based calibration was used to predict the transmit gain for a 90° radiofrequency pulse and gave a flip angle of 88.6 ± 3.42° when tested in vivo in 32 volunteers.

Local specific absorption rate in high-pass birdcage and transverse electromagnetic body coils for multiple human body models in clinical landmark positions at 3T.

PURPOSE: To use electromagnetic (EM) simulations to study the effects of body type, landmark position, and radiofrequency (RF) body coil type on peak local specific absorption rate (SAR) in 3T magnetic resonance imaging (MRI). MATERIALS AND METHODS: Numerically computed peak local SAR for four human body models (HBMs) in three landmark positions (head, heart, pelvic) were compared for a high-pass birdcage and a transverse electromagnetic 3T body coil. Local SAR values were normalized to the IEC whole-body average SAR limit of 2.0 W/kg for normal scan mode. RESULTS: Local SAR distributions were highly variable. Consistent with previous reports, the peak local SAR values generally occurred in the neck-shoulder area, near rungs, or between tissues of greatly differing electrical properties. The HBM type significantly influenced the peak local SAR, with stockier HBMs, extending extremities towards rungs, displaying the highest SAR. There was also a trend for higher peak SAR in the head-centric and heart-centric positions. The impact of the coil types studied was not statistically significant. CONCLUSION: The large variability in peak local SAR indicates the need to include more than one HBM or landmark position when evaluating safety of body coils. It is recommended that an HBM with arms near the rungs be included to create physically realizable high-SAR scenarios.

B1 mapping by Bloch-Siegert shift.

A novel method for amplitude of radiofrequency field (B1+) mapping based on the Bloch-Siegert shift is presented. Unlike conventionally applied double-angle or other signal magnitude-based methods, it encodes the B(1) information into signal phase, resulting in important advantages in terms of acquisition speed, accuracy, and robustness. The Bloch-Siegert frequency shift is caused by irradiating with an off-resonance radiofrequency pulse following conventional spin excitation. When applying the off-resonance radiofrequency in the kilohertz range, spin nutation can be neglected and the primarily observed effect is a spin precession frequency shift. This shift is proportional to the square of the magnitude of B1(2). Adding gradient image encoding following the off-resonance pulse allows one to acquire spatially resolved B(1) maps. The frequency shift from the Bloch-Siegert effect gives a phase shift in the image that is proportional to B(1)(2). The phase difference of two acquisitions, with the radiofrequency pulse applied at two frequencies symmetrically around the water resonance, is used to eliminate undesired off-resonance effects due to amplitude of static field inhomogeneity and chemical shift. In vivo Bloch-Siegert B(1) mapping with 25 sec/slice is demonstrated to be quantitatively comparable to a 21-min double-angle map. As such, this method enables robust, high-resolution B(1)(+) mapping in a clinically acceptable time frame.

Spiral imaging artifact reduction: a comparison of two k-trajectory measurement methods.

PURPOSE: To compare an external sensor-based k-space calibration technique with a routine precalibration method for quantification of method accuracy and reduction of spiral imaging artifacts to obtain improved image quality. MATERIALS AND METHODS: Recently, magnetic field monitoring (MFM) has been introduced as a new calibration technique of gradient field-related imperfections. External sensors are placed near the observed object to measure magnetic field variations during image acquisition. The measured field data are used to determine the actual k-space trajectory and for image reconstruction to reduce artifacts. In the past, precalibration techniques have been proposed where the k-space trajectory is measured by means of special calibration sequences directly in the object of interest. In this study, MFM is introduced as an effective correction technique for spiral imaging. On the basis of a comparison, whether MFM is as viable as the chosen reference method presented by Duyn et al (J Magn Reson [1998] 132:150-153) is analyzed in terms of detecting imperfections of spatial encoding gradients in order to correct for these in image reconstruction. As this technique is used as a reference method, it is given the acronym Duyn calibration technique (DCT). MFM and DCT are compared and timing delays, k-space offsets, eddy current effects, k-trajectory error propagation, image distortions, and signal-to-noise ratio were determined for different spiral sequences in two different phantoms. RESULTS: Both techniques effectively detect k-space offsets and k-trajectory error propagation and correct for general error sources like timing delays. In object border areas, artifacts such as deformations and blurring were dramatically reduced. Within all tested categories, MFM performed as well as DCT. In terms of k-trajectory error propagation and image distortion quantification, MFM was more accurate. CONCLUSION: We introduce MFM as an effective and accurate correction technique for spiral imaging, where a comparison of MFM and DCT has shown that both techniques are accurate correction techniques for spiral imaging.

Design of phase-modulated broadband refocusing pulses.

Broadband linear-phase refocusing pulses were designed with the Shinnar-Le Roux (SLR) transformation and verified experimentally. The design works in several steps: initially, a linear-phase B polynomial is created with the Parks-McClellan/Remez exchange algorithm. The complementary A polynomial required for the SLR transformation is generated with the Hilbert transformation, yielding the minimum-phase response. The phase response of the A polynomial is altered by zero-flipping, which changes the overall pulse shape while retaining its refocusing profile. Optimal pulses in terms of minimal B(1max) and hence broadest bandwidth were found with non-linear optimisation of the zero-flipping pattern. These pulses are generally phase modulated with a time-symmetric amplitude and anti-symmetric phase modulation. In this work, a whole range of pulses were designed to demonstrate the underlying relationships. Five exemplary pulses were implemented into a PRESS sequence and validated by acquiring images of a water-oil phantom and lactate spectra at TE = 144 ms.

Importance of bone-conducted sound transmission on patient hearing in the MR scanner.

PURPOSE: to evaluate the influence of bone-conduction in the MR environment compared to a standardized acoustic environment. MATERIALS AND METHODS: Acoustic noise is an unwanted side effect of MRI that is commonly tackled with passive hearing protection. In an MR scanner, however, with the patient completely surrounded by the MR sounds and in close contact with the vibrating MR table and gantry, bone-conduction may increase subjective sound levels, restricting the efficacy of passive protection that reduces air-conducted noise only. A total of 10 volunteers were subjected to pure MR tones, covering the frequency range relevant for hearing and at 60 dB, generated through the MR system's gradient coils. Bone-conduction was determined for various passive damping conditions in an MR scanner and was compared to that acquired in an acoustically-calibrated environment. The contribution of mechanical vibrations to bone-conduction was determined. Also, with a microphone in the ear canal, the objective efficacy of the passive protection was measured. RESULTS: We found no difference between the bone-conduction experiments executed inside the imager and in the acoustically-controlled environment. The overall insertion loss of the passive hearing protectors was over 20 dB with strongest effects at 0.4 and 2.5 kHz. CONCLUSION: As bone-conduction is not more pronounced inside the MR scanner than outside, the previous reports on the subjective evaluation of protection devices in MRI hold their validity.

Monitoring tissue coagulation during thermoablative treatment by using a novel magnetic resonance imaging contrast agent.

INTRODUCTION: We tested the feasibility of using a novel contrast agent, MS-325, as a marker of coagulating tissue during thermoablative treatment. MATERIALS AND METHODS: In vivo, we created coagulated lesions in porcine muscle tissue under 3 different conditions: MS-325 (n = 5), gadolinium-DTPA (n = 5), or no contrast agent (n = 9) present during laser thermoablation. At the same time, we performed continuous T1-weighted magnetic resonance imaging at 1.5 T. We quantified the change in signal intensity during treatment expressed as relative enhancement, and compared the 3 groups by using Mann-Whitney analysis. RESULTS: MS-325 resulted in a more than 3.2-fold increase in relative enhancement over the gadolinium-DTPA and noncontrast control groups (P < 0.008). CONCLUSION: MS-325 appears to be a valid marker for coagulating tissue and significantly increased relative enhancement of the treated lesions when compared with both Gd-DTPA and noncontrast-enhanced conditions. MS-325 thus has potential for monitoring of thermoablative treatment.

Use of fast spin echo for phase shift magnetic resonance thermometry.

PURPOSE: To propose a modified fast spin echo (FSE) magnetic resonance imaging sequence for MR thermometry, employing the proton resonance frequency (PRF) shift by means of MR phase maps. Despite their obvious advantages of speed and high signal-to-noise ratio (SNR), FSE sequences have not until now been used for this purpose due to the restraints imposed by the Carr-Purcell-Meiboom-Gill (CPMG) conditions. MATERIALS AND METHODS: The new FSE combines a new phase modulation scheme that maintains magnetization that ordinarily is destroyed under CPMG conditions, while employing conventional FSE gradient waveforms. The echoes are read in a single shot using 128 readouts in 650 msec, with a phase sensitive preparation using an optional time shift tau before the start of the refocusing gradient waveforms. This feature allows the quantification of temperature dependent phase shifts. We tested the sequence by imaging a heated agar gel phantom while cooling, using different values for tau. RESULTS: There was good correlation between FSE and fiberoptic-based temperature measurements in the phantom(r(2) >or= 0.95). Temperature sensitivity could be adjusted by varying the tau value. CONCLUSION: With the proposed non-CPMG FSE sequence it is feasible to quantify temperature changes by means of the PRF shift.

Efficacy of passive acoustic screening: implications for the design of imager and MR-suite.

PURPOSE: To investigate the efficacy of passive acoustic screening in the magnetic resonance (MR) environment by reducing direct and indirect MR-related acoustic noise, both from the patient's and health worker's perspective. MATERIALS AND METHODS: Direct acoustic noise refers to sound originating from the inner and outer shrouds of the MR imager, and indirect noise to acoustic reflections from the walls of the MR suite. Sound measurements were obtained inside the magnet bore (patient position) and at the entrance of the MR imager (health worker position). Inner and outer shrouds and walls were lined with thick layers of sound insulation to eliminate the direct and indirect acoustic pathways. Sound pressure levels (SPLs) and octave band frequencies were acquired during various MR imaging sequences at 1.5 T. RESULTS: Inside the magnet bore, direct acoustic noise radiating from the inner shroud was most relevant, with substantial reductions of up to 18.8 dB when using passive screening of the magnetic bore. At the magnet bore entrance, blocking acoustic noise from the outer shroud and reflections showed significant reductions of 4.5 and 2.8 dB, respectively, and 9.4 dB when simultaneously applied. Inner shroud coverage contributed minimally to the overall SPL reduction. CONCLUSION: Maximum noise reduction by passive acoustic screening can be achieved by reducing direct sound conduction through the inner and outer shrouds. Additional measures to optimize the acoustic properties of the MR suite have only little effect.