Nanosecond Freezing of Water at High Pressures: Nucleation and Growth near the Metastability Limit.

The fundamental study of phase transition kinetics has motivated experimental methods toward achieving the largest degree of undercooling possible, more recently culminating in the technique of rapid, quasi-isentropic compression. This approach has been demonstrated to freeze water into the high-pressure ice VII phase on nanosecond timescales, with some experiments undergoing heterogeneous nucleation while others, in apparent contradiction, suggest a homogeneous nucleation mode. In this study, we show through a combination of theory, simulation, and analysis of experiments that these seemingly contradictory results are in agreement when viewed from the perspective of classical nucleation theory. We find that, perhaps surprisingly, classical nucleation theory is capable of accurately predicting the solidification kinetics of ice VII formation under an extremely high driving force (|Δµ/k_{B}T|≈1) but only if amended by two important considerations: (i) transient nucleation and (ii) separate liquid and solid temperatures. This is the first demonstration of a model that is able to reproduce the experimentally observed rapid freezing kinetics.

AEGIS autonomous targeting for ChemCam on Mars Science Laboratory: Deployment and results of initial science team use.

Limitations on interplanetary communications create operations latencies and slow progress in planetary surface missions, with particular challenges to narrow-field-of-view science instruments requiring precise targeting. The AEGIS (Autonomous Exploration for Gathering Increased Science) autonomous targeting system has been in routine use on NASA's Curiosity Mars rover since May 2016, selecting targets for the ChemCam remote geochemical spectrometer instrument. AEGIS operates in two modes; in autonomous target selection, it identifies geological targets in images from the rover's navigation cameras, choosing for itself targets that match the parameters specified by mission scientists the most, and immediately measures them with ChemCam, without Earth in the loop. In autonomous pointing refinement, the system corrects small pointing errors on the order of a few milliradians in observations targeted by operators on Earth, allowing very small features to be observed reliably on the first attempt. AEGIS consistently recognizes and selects the geological materials requested of it, parsing and interpreting geological scenes in tens to hundreds of seconds with very limited computing resources. Performance in autonomously selecting the most desired target material over the last 2.5 kilometers of driving into previously unexplored terrain exceeds 93% (where ~24% is expected without intelligent targeting), and all observations resulted in a successful geochemical observation. The system has substantially reduced lost time on the mission and markedly increased the pace of data collection with ChemCam. AEGIS autonomy has rapidly been adopted as an exploration tool by the mission scientists and has influenced their strategy for exploring the rover's environment.

Design and calibration of electric field probes in the range 10-120 MHz.

In view of potential thermal hazards, there is a need to determine the specific absorption rate (SAR) distributions associated with radiofrequency coils used in magnetic resonance imaging (MRI) (typically 10-120 MHz). Electric field (E-field) distributions in tissue-equivalent phantoms may be determined using a probe comprising a dipole antenna and a detector. The geometry of the dipole dictates the sensitivity of the device, thus two designs are discussed in this paper. Both probes are compact, have a spatial resolution of 2.5 cm3, operate at MR frequencies and have a response independent of the dielectric characteristics of the phantom material. Calibration of these probes requires a system capable of producing a known E-field both in air and in a tissue-like medium at frequencies between 10 and 120 MHz. Transverse electromagnetic wave (TEM) cells answering these specifications are described and the calibration procedure outlined. Accurately calibrated E-field probes can make field measurements in phantoms which can be used to verify predictions from numerical models. These numerical techniques may then be used to predict E-fields, and hence SAR, in patients.

Experimental verification of numerically predicted electric field distributions produced by a radiofrequency coil.

There are safety issues regarding energy deposition within tissues due to radiofrequency fields used in some magnetic resonance (MR) procedures. Procedures should be compliant with guidelines that specify limits to temperature elevation and specific absorption rate (SAR). In general, direct measurement of these quantities in patients is impractical and an alternative approach is to determine SAR from the electric field (E-field) distributions predicted by numerical models. In this initial study the E-field distribution in a tissue-simulating phantom due to a square coil driven at 31 MHz is predicted using a finite-difference time domain (FDTD) solution to Maxwell's equations. An experimental arrangement of the same problem was constructed and the resulting E-field distribution was measured using a calibrated minimally perturbing E-field probe. A comparison between experimentally and theoretically derived data showed that the numerically predicted E-fields were within +/-1 dB of the fields measured with the E-field probe in the phantom material. The results provide confidence in the use of the FDTD algorithm to determine quantitatively accurate E-field distributions arising from square radiofrequency (RF) coils used in MR procedures. The accuracy of numerical models of other coil designs such as bird cages, saddles and surface coils can be investigated in the same manner. Future studies will evaluate the exposure of patients to these RF fields.

An in vivo 13C magnetic resonance spectroscopic study of the relationship between diet and adipose tissue composition.

13C magnetic resonance spectroscopy (MRS) is a noninvasive technique used in the study of lipids. We applied 13C MRS to assess the effects of long-term dietary variation on adipose tissue composition in humans. In vivo 13C MRS was used to analyze the fatty acid composition of adipose tissue in 88 healthy volunteers with significantly different diets (38 vegans, 11 vegetarians, and 39 omnivores) assessed by analysis of dietary records. Results were compared with the serum lipid profile. 13C MRS revealed clear differences in the adipose tissue composition of vegans, which contained more unsaturated (P < 0.01) and fewer saturated fatty acids (P < 0.01) compared with omnivores and vegetarians. The vegan subjects had a significantly lower intake of saturated fatty acids and higher intake of polyunsaturated fatty acids than either the omnivore or the vegetarian groups (P < 0.01). These findings were associated with significantly lower levels of serum total cholesterol and low density lipoprotein-cholesterol in the vegan group compared with the omnivores. Our results demonstrate the use of 13C MRS for the noninvasive study of adipose tissue composition and its application to the study of the interaction between long-term dietary and metabolic risk factors in humans.

Examples of the design of screened and shielded RF receiver coils.

Three methods of screening RF surface coil receivers are discussed. The aim is to design coils with very much greater sensitivity on one side than on the other so that signal and noise contributions from unwanted regions of the body can be minimized. These methods can usefully be applied to situations wherein data are only wanted from one leg, although the coil is most conveniently located between the two, or from an arm lying adjacent to the body.

Tuned fiducial markers to identify body locations with minimal perturbation of tissue magnetization.

A fiducial marker is described in which a small cell containing a short T1 solution is surrounded by single- or quadrature-tuned windings. It is shown how these can be used to enhance small externally applied RF pulses, which do not significantly perturb the magnetization of the tissue, to excite the fiducial sample, and get substantial signals from it. The fiducials can be used to mark locations on the body unambiguously for, for example, frameless stereotaxy, and to identify coils, catheters, or other inserted devices.

Comparison of the performance of different discriminant algorithms in analyte discrimination tasks using an array of carbon black--polymer composite vapor detectors.

An array of 20 compositionally different carbon black--polymer composite chemiresistor vapor detectors was challenged under laboratory conditions to discriminate between a pair of extremely similar pure analytes (H2O and D2O), compositionally similar mixtures of pairs of compounds, and low concentrations of vapors of similar chemicals. Several discriminant algorithms were utilized, including k nearest neighbors (kNN, with k = 1), linear discriminant analysis (LDA, or Fisher's linear discriminant), quadratic discriminant analysis (QDA), regularized discriminant analysis (RDA, a hybrid of LDA and QDA), partial least squares, and soft independent modeling of class analogy (SIMCA). H2O and D2O were perfectly classified by most of the discriminants when a separate training and test set was used. As expected, discrimination performance decreased as the analyte concentration decreased, and performance decreased as the composition of the analyte mixtures became more similar. RDA was the overall best-performing discriminant, and LDA was the best-performing discriminant that did not require several cross-validations for optimization.

Effects of cervical collars on standing balance.

The effect of cervical collars on standing balance in two age groups was examined. Twenty healthy women aged 60 to 78 years and 20 healthy women aged 18 to 29 years stood on a Kistler force platform with and without a cervical collar. Total, lateral and anteroposterior sway velocity were measured in each of three positions; long-base stance and eyes open, wide-base stance and eyes open, and wide-base stance and eyes closed. Analysis of variance showed no significant difference (p < .05) between the collar and no collar conditions for any of the standing balance measures. Older women had significantly more sway velocity (p < .001) than younger women during long-base standing both with and without the collar. Also they showed significantly more sway velocity in both the total (p < .01) and the anteroposterior (AP) directions (p < .001) of wide-base standing. There were no significant lateral sway velocity differences (p < .05). With eyes closed, sway velocity was greater in all age groups. In the wide-based condition there were significant differences in sway velocity between AP (p < .001) and total and lateral (p < .01). These results indicate that a cervical collar does not disturb standing balance in healthy women in the age groups tested.

Comparative efficiency of different pulse sequences in MR imaging.

This article examines the relative performance of various sequences in terms of a figure of merit. The range of possible options is large but there are also major constraints on possible improvements by manipulating sequences. The performance of inversion recovery and partial saturation techniques is better than is often suggested.

Electromagnetic and thermal modeling of SAR and temperature fields in tissue due to an RF decoupling coil.

The finite difference time domain method is used to calculate the specific absorption rate (SAR) due to a butterfly surface coil in a realistic tissue model of the leg. The resulting temperature distribution and temperature changes are found using a finite difference solution to the bioheat transfer equation. Reasonable agreement is found between predicted temperature changes and those measured in vivo provided that the resulting hyperthermia does not induce noticeable changes in perfusion. The method is applicable to radiofrequency dosimetry problems associated with high Bo field magnetic resonance systems and where knowledge of spatial variation in SAR is important in assessing the safety of new magnetic resonance procedures.

High resolution magnetic resonance imaging of the anal sphincter using an internal coil.

An internal receiver coil was used to obtain high resolution transverse and oblique coronal magnetic resonance images of the anal sphincter in five normal volunteers and five patients. The internal sphincter had a high signal intensity on T1 weighted, T2 weighted, and STIR sequences whereas the conjoined longitudinal muscle and external sphincter had a low signal intensity. The internal sphincter (but not the external sphincter) showed contrast enhancement after administration of intravenous gadopentetate dimeglumine. The oblique coronal plane was particularly useful for showing the thickness and the relations of the external sphincter. Sphincteric abscesses as well as muscle defects, hypertrophy, and atrophy were clearly shown. The coil was well tolerated by most subjects. It has considerable potential for improving the diagnosis of anorectal disease.

MR-guided biopsy of the breast after lumpectomy and radiation therapy using two methods of immobilization in the lateral decubitus position.

We describe two methods of breast immobilization using the lateral decubitus position to increase patient comfort and access to the axillary tail for MR-guided biopsy in the postsurgical or irradiated breast. The first method uses a compression device with good immobilization but poor patient tolerance. The second approach uses a thermoplastic mesh material to form a rigid exoskeleton around the breast: immobilization is adequate and patient acceptability is good. The latter method is preferred and requires formal evaluation in larger scale trials.

Twisted-pair RF coil suitable for locating the track of a catheter.

An RF coil, made by twisting a conventional wire loop, was tuned to form an extended sensor to provide an effective method of showing the track of a catheter. A twisted-pair coil can be made small enough in diameter to pass through needles in common clinical use. The coil has a very small field of view and in transmit/receive operation has minimal effect on the magnetization of the surrounding tissue.

Imaging of the brain by nuclear magnetic resonance.

A nuclear magnetic resonance (NMR) machine constructed by Thorn-EMI Ltd was used to produce tomographic images of the brain in eight volunteers and fourteen patients. The use of an inversion recovery technique designed to emphasise variations in the spin-lattice time constant (T1) resulted in remarkable differentiation between grey and white matter in all subjects examined. White matter was seen both centrally and peripherally to subcortical level and the basal ganglia were clearly demarcated by the surrounding white matter and ventricular system. The posterior fossa was visualised with substantially less artefact than with X-ray computed tomography (CT) and both the brainstem and middle cerebellar peduncle were clearly shown. Pathological appearances in patients with glioblastoma multiforme, cerebral infarction, and cerebral aneurysm were demonstrated and compared with those seen with CT. The technique will require thorough clinical evaluation but appears to have considerable potential in the diagnosis of neurological disease.

Initial clinical evaluation of a whole body nuclear magnetic resonance (NMR) tomograph.

A nuclear magnetic resonance (NMR) imaging system is described, and preliminary results from its clinical use are presented. The properties and detection of the magnetisation due to hydrogen protons are outlined, and a rotating frame is introduced to describe the motion of the magnetisation. Radiofrequency (RF) pulses are used to rotate the magnetisation, and slice selection is achieved using a 90 degree RF pulse and a magnetic field gradient. Data acquisition and image reconstruction are explained. Three scanning sequences are described: repeated free induction decay (FID), inversion - recovery, and spin-echo. These sequences produce images whose pixel values have different dependencies on hydrogen proton density, T1 and T2. Inversion-recovery images show striking differentiation between grey and white matter in the brain. The absence of bone artifact is a significant advantage over X-ray computed tomography in the posterior fossa, where rapid repeated FID sequences can also be used to demonstrate flow effects. The considerable soft tissue contrast available with NMR is of value in demonstrating disease within the liver where T1 appears to be sensitive but relatively nonspecific diagnostic parameter. High resolution scans are of value in demonstrating the adrenal gland and spinal cord.

Magnetic resonance properties of hydrogen: imaging the posterior fossa.

Posterior fossa scans were performed on five healthy volunteers using a nuclear magnetic resonance (NMR) machine constructed by Thorn-EMI Ltd. Three different NMR scanning sequences were used. In the first, a type of saturation-recovery technique was used to produce images strongly dependent on the density of hydrogen nuclei, but with some dependence on the spin-lattice relaxation time (T1). In the second, an inversion-recovery technique was used to produce images with a stronger dependence on the spin-lattice relaxation time. In the third, a spin-echo technique was used to obtain images with a dependence on the spin-spin relaxation time (T2). All three types of NMR image were unaffected by bone artifact. Visualization of brain adjacent to the skull base was obtained without loss of detail due to partial-volume effect from bone. The saturation-recovery images highlighted arteries and veins that were clearly visible without the use of contrast agents. The inversion-recovery images showed remarkable gray-white matter differentiation enabling internal structure to be seen within the brainstem and cerebellum. The trigeminal nerve and ganglion were also seen outside the brain. Experience with the spin-echo technique is limited, but the images at the base of the brain show considerable soft-tissue detail. The NMR images of the posterior fossa in this study were comparable in quality to those obtained from a new rotate-rotate x-ray computed tomography machine and were superior in several respects.