In high-field magnetic resonance imaging, the radio frequency wavelength within the human body is comparable to anatomical dimensions, resulting in B1 inhomogeneity and nonuniform sensitivity patterns. Thus, this relatively short wavelength presents engineering challenges for RF coil design. In this study, a bilateral breast coil for 1H imaging at 7 T was designed and constructed using forced-current excitation. By forcing equal current through the coil elements, we reduce the effects of coupling between the elements to simplify tuning and to ensure a uniform field across both breasts. To combine the benefits of the higher power efficiency of a unilateral coil with the bilateral coverage of a bilateral coil, a switching circuit was implemented to allow the coil to be reconfigured for imaging the left, right, or both breasts.

This paper reports our results in developing a simple MRI system for teaching the basics of MR Engineering at the undergraduate or graduate level. LabVIEW data acquisition cards were used for generating and digitizing the RF signals and controlling gradients and transmit/receive and blanking switches. A very inexpensive and simple magnet reported previously by Sahakian was used to enable simple, projection reconstruction imaging. Students constructed the gradients, RF coils and did system level assembly and programming of the data acquisition system. At the end of the course students were tasked with identifying unknown imaging "phantoms" in their magnet, and then improving the image based on their knowledge.

MRI of flow remains a challenging problem despite significant improvements in imaging speeds. For periodic flow the acquisition can be gated, synchronizing data acquisition with the flow. However, this method fails to work if the flow is sufficiently fast that turbulence occurs, or when it is sufficiently fast that blurring occurs during the excitation of the spins or the acquisition of the signal. This paper describes recent progress in employing a very fast MR imaging technique, Single Echo Acquisition Imaging (SEA-MRI) and spin-tagging to visualize very rapid and turbulent flow patterns. Demonstrations are done on a separating channel phantom with input flow rates ranging from zero to over 100 cm/sec. Spin-tagging enables a "texture" to be placed on the spins, enabling clear visualization of the complex flow patterns, and in some cases measurement of the flow velocity.

In magnetic resonance imaging with array coils with many elements, as the radiofrequency (RF) coil dimensions approach the voxel dimensions, the phase gradient due to the magnetic field pattern of the coil causes signal cancellation within each voxel. In single echo acquisition (SEA) imaging with coil arrays, a gradient pulse can be applied to compensate for this effect. However, because RF coil phase varies with distance from the array and reverses on opposite sides of a dual-sided array, this method of phase compensation can be optimized for only a single slice at a time. In this study, a nonlinear gradient coil was implemented to provide spatially varying phase compensation to offset the coil phase with slice position for dual-sided arrays of narrow coils. This nonlinear gradient coil allows the use of one phase compensation pulse for imaging multiple slices through a slab, and, importantly, is shown to enable simultaneous SEA imaging from opposite sides of a sample using a dual-sided receive array.

Dynamic MRI continues to grow in interest and capability with the introduction of 64 and 128 channel receivers, and, more recently, 8 and 16 channel parallel transmitters. This talk will describe progress in developing a 64 channel transmitter and applications in high-speed MR imaging, reaching 1000 frames per second.

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Purpose Mitigating coupling effects between coil elements represents a continuing challenge. Here, we present a 16‐bowtie slot volume coil arranged in eight independent dual‐slot modules without the use of any decoupling circuits. Methods Two electrically short “bowtie” slot antennas were used to form a “module.” A bowtie configuration was chosen because electromagnetic modeling results show that bowtie slots exhibit improved B1+Pin$$ \frac{B_1^{+}}{\sqrt{P_{in}}} $$ efficiency when compared to thin rectangular slots. An eight‐module volume coil was evaluated through electromagnetic modeling, bench tests, and MRI experiments at 4.7 T. Results Bench tests indicate that worst‐case coupling between modules did not exceed −14.5 dB. MR images demonstrate well‐localized patterns about single excited modules confirming the low coupling between modules. Homogeneous MR images were acquired from a synthesized quadrature birdcage transmit mode. MRI experiments show that the RF power requirements for the proposed coil are 9.2 times more than a birdcage coil. Whereas from simulations performed to assess the proposed coil losses, the total power dissipated in the phantom was 1.1 times more for the birdcage. Simulation results at 7 T reveal an equivalent B1+ homogeneity when compared with an eight‐dipole coil. Conclusion Although exhibiting higher RF power requirements, as a transmit coil when the power availability is not a restriction, the inherently low coupling between electrically short slots should enable the use of many slot elements around the imaging volume. The slot module described in this paper should be useful in the design of multi‐channel transmit coils.

Background: Magnetic resonance elastography (MRE) measures tissue mechanical properties by applying a shear wave and capturing its propagation using magnetic resonance imaging (MRI). By using high density array coils, MRE images are acquired using single echo acquisition (SEA) and at high resolutions with significantly reduced scan times.Methods: Sixty-four channel uniplanar and 32×32 channel biplanar receive arrays are used to acquire MRE wave image sets from agar samples containing regions of varying stiffness. A mechanical actuator triggered by a stepped delay time introduces vibrations into the sample while a motion sensitizing gradient encodes micrometer displacements into the phase. SEA imaging is used to acquire each temporal offset in a single echo, while multiple echoes from the same array are employed for highly accelerated imaging at high resolutions. Additionally, stiffness variations as a function of temperature are studied by using a localized heat source above the sample. A custom insertable gradient coil is employed for phase compensation of SEA imaging with the biplanar array to allow imaging of multiple slices.Results: SEA MRE images show a mechanical shear wave propagating into and across agar samples. A set of 720 images was obtained in 720 echoes, plus a single reference scan for both harmonic and transient MRE.A set of 2,950 wave image frames was acquired from pairs of SEA images captured during heating, showing the change in mechanical wavelength with the change in agar properties. A set of 240 frames was acquired from two slices simultaneously using the biplanar array, with phase images processed into displacement maps. Combining the narrow sensitivity patterns and SNR advantage of the SEA array coil geometry allowed acquisition of a data set with a resolution of 156 µm × 125 µm × 1,000 µm in only 64 echoes, demonstrating high resolution and high acceleration factors.Conclusions: MRE using high-density arrays offers the unique ability to acquire a single frame of a propagating mechanical vibration with each echo, which may be helpful in non-repeatable or destructive testing. Highly accelerated, high resolution MRE may be enabled by the use of large arrays of coils such as used for SEA, but at lower acceleration rates supporting the higher resolution than provided by SEA imaging.