Introduction MRI images obtained at 7 Tesla from a small diameter loop-resonator can exhibit excitation and reception that are many coil diameters removed from the conducting elements of the resonator. Webb and coworkers recently demonstrated this by obtaining images of the entire human leg using an 8 cm diameter single loop H (300 MHz) RF coil [1], and postulated the existence of traveling dielectric waves [2] as the mechanism for the extended coverage. A potentially important application for such an RF coil is when used in combination with a single-tuned heteronuclear RF coil. In such a configuration the small coil could provide high-resolution ‘scout’ images to improve heteronuclear acquisition planning, B0 optimization, and post-acquisition registration with a dedicated independently acquired high-quality H MRI dataset. Accurate coregistration is an important step to properly attribute signal to the appropriate tissue type in large heteronuclear voxels. This method does not require careful subject positioning procedures when employing separate heteronulcear and proton coils or complicated dual tuned resonators. Here, we describe the results of combining a small loop H RF coil with a P single tuned RF coil at 7T. Methods The P resonance frequency at 7T is 3 MHz below H at 3T. A simple retuning of an old 12 leg high pass birdcage coil for 3T H allowed spectroscopic imaging on a Siemens 7T Tim Trio imaging system. An 8 cm loop resonator tuned to 297 MHz was constructed following the design published by Webb [1]. The loop coil can be oriented such that the loop is normal or parallel to the polarizing B0 field direction. Results and Discussion Preliminary experiments with the 8 cm loop coil were performed to assess the range of coverage that could be obtained. The first tests on a phantom consisting of a tube 1 meter in length and 9 cm in diameter containing 50 mM saline solution showed coverage expected by the familiar pattern for the RF near field produced by a thin loop when oriented both parallel and perpendicular to B0 and placed near the phantom. The parallel orientation, which is not normally used for MRI reception, does offer significant near field excitation from the radial components of the RF field. Only by replacing the 70 mM saline with pure water did we observe extended signal coverage beyond that expected from the near field excitation profiles. These observations are in accord with similar results that demonstrate the damping of standing wave patterns in a spherical phantom of water by the addition of saline [3]. The human body of course does not have the dielectric properties of saline and presents a more complicated geometry than a simple tube. Placement of the loop coil for acceptable H density maps of the human head was determined experimentally. The configuration chosen is illustrated in Figure 1, where the loop is positioned just above the head and oriented axially. An example of a typical proton density map obtained from this configuration and used for registration of P spectroscopic imaging is given in Figure 2. The image is a 2D FLASH with TE of 2.5 ms, TR of 100 ms. The resolution is 0.9mm x 0.9mm x 5mm, 32 slices with a 1 min acquisition time. The FLASH image shows extended excitation can be obtained from an axially oriented loop coil. The excitation pattern also does not show a dark central area characteristic of an excitation pattern expected from a loop where the radial component of the near field RF vanishes. The intensity remains fairly constant as a function of distance from the loop plane and remains significant even at the brain stem. A more complete analysis of the intensity variations is in progress. We note that the presence of the loop did not interfere with the operation of the P coil, suggesting negligible coupling between both structures. This is due to the isolation produced by frequency difference as well as geometry. The small loop coil used in the configuration shown in Figure 1 provides an important set of in situH MRI scout images that greatly facilitate co-registration of P MRSI data set with high quality H MRI acquired in an independent scanning session (see Figure 3 – which shows an overlay of P MRSI data and H MRI). References [1] Webb MRM 63: 297-302(2010) [2] Brunner Nature 457: 994-997(2009) [3] Yang MRM 47:982-989(2002).

Standard phase‐contrast flow quantification (PC‐FQ) using radiofrequency (RF) spoiled steady‐state (SS) incoherent gradient‐echo sequences have a relatively low signal‐to‐noise ratio (SNR). Unspoiled SS coherent (SSC) gradient‐echo sequences have a higher intrinsic SNR and are T2/T1 weighted so that blood has a relatively large signal compared to other tissues. An SSC sequence that was modified to allow in‐plane velocity encoding is presented. Velocity encoding was achieved by inverting the readout gradients. This offers the benefit that there is no resultant increase in repetition time (TR), which avoids increased sensitivity to off‐resonance artifacts when conventional velocity‐encoding methods using separate velocity‐encoding gradients are extended to SSC sequences. The results of standard PC‐FQ and the new method from in vitro experiments of constant and sinusoidal flow, and in vivo imaging of the carotid artery were compared. Vector field maps and paths obtained from particle‐tracking calculations based on the velocity‐encoded images were used to visualize the velocity data. The technique has the potential to increase the precision of PC‐FQ measurements. Magn Reson Med 54:138–145, 2005. © 2005 Wiley‐Liss, Inc.

OBJECT The aim of this study was to evaluate the feasibility of complex intraaneurysmal flow visualization with the currently available phase-contrast magnetic resonance (MR) imaging modality. METHODS A geometrically realistic in vitro aneurysm model, in which detailed flow velocity analysis had already been conducted using laser Doppler velocimetry was used for this in vitro hemodynamic simulation, so that the results of phase-contrast velocity measurements could be compared with the previous reliable results. On a 1.5-tesla unit, three orthogonal components of velocity were obtained using a standard two-dimensional fast low-angle shot flow quantification sequence. Three-dimensional (3D) intraaneurysmal flow structures recorded during one cardiac cycle were depicted in one midsagittal and three axial cross-sectional planes with the aid of gray scale phase-contrast velocity maps. Isovelocity contour maps and secondary flow vectors were also created based on the phase-contrast velocity maps by using MATLAB software. The isovelocity contours in those three axial sections could demonstrate the shapes of inward and outward flow areas and their alternation over one cardiac cycle. The secondary flow vectors demonstrated twin vortices within the outward flow area adjacent to the boundary layer of inward and outward flow in all axial planes. CONCLUSIONS The phase-contrast MR imaging method was able to depict the complex 3D intraaneurysmal flow structures in the in vitro aneurysm model. Detailed 3D intraaneurysmal flow information will be obtainable in vivo after improvements are made in spatial resolution, which is expected in the near future. The capability to visualize intraaneurysmal flow structures directly with the use of noninvasive MR imaging technology will have a positive impact on future clinical practice.

PURPOSE:To develop and compare phase-contrast (PC) and spin-tag (ST) MR imaging techniques for accurate quantification of velocity and displacement distribution in the muscle tendon complex of the lower leg during isometric contractions under in vivo conditions, in healthy subjects and subjects with atrophy. MATERIALS AND METHODS:Techniques were developed to acquire PC and ST dynamic images, gated to the force exerted by a subject during isometric contraction. Algorithms were optimized for correction of phase shading errors. Flow velocity quantification was validated in phantoms and ex vivo rabbit muscles. Trajectories of pixels calculated from PC images were compared with those in ST images. Velocity distributions were determined in healthy muscles, those atrophied by four weeks of suspension, and during physiotherapy-induced recovery. RESULTS:The technique developed allowed acquisition of images retrospectively gated to the isometric contraction performed with the subject in the scanner. Significant phase shading errors in PC images (approximately 3 cm/second over the field of view) were reduced to the background noise level by the correction algorithm. Tissue trajectories calculated from PC images agreed very well with those from ST images both in human and excised animal tissues. Peak velocities in atrophied muscles were significantly lower compared to the preatrophy state but recovered to baseline values after six weeks of therapy. CONCLUSION:We show the feasibility of monitoring muscle velocity and tissue displacement during voluntary contractions in humans under in vivo conditions using MR tissue motion mapping methods. The clinical feasibility of this technique in monitoring atrophied muscle is also demonstrated.

The principle of energy conservation is used to represent the behaviour of ice during ship ramming. The summation of all ship and ice, local and global energy dissipations throughout a ram was done using a time-step digital simulation model. Theoretical derivations of six local ice energies are described: bending, cracking, crushing, flaking, ship/ice frictional and removal. The most important local ice and global ship energies are calculated as percentages of the total input energy (ship kinetic plus propulsion energies) during a ram.

With an overall objective of establishing a physiologic measure of neuroplasticity in infant human brain, fMRI studies were conducted to isolate regions of the brain specific for phoneme processing and language processing. The neuronal auditory response to 20 sec blocks of noise, human-produced nonsense speech, and short meaningful phrases in the infant’s mother tongue was studied in 5 sleeping neonates. Strong activity was observed in Broca’s and Wernicke’s area, with bilateral activity seen in the primary auditory cortex, with a strong left hemisphere bias. The results indicate that infants show lateralized activation in language areas in response to speech, suggesting that language lateralization is innate and established from birth.