Simple through-plane spatial filter for 2D MRI projections.

A simple spatial filter for 2D projection MR imaging is introduced. It works in the third (unresolved) direction to eliminate uniform or slowly varying interfering background signals. A constant amplitude gradient pulse in the unresolved direction is applied at the same time as the usual phase encode gradient during 2D acquisition. The filter is demonstrated for root imaging in soil, where background soil water signals can be troublesome. The filter suppresses the soil water signal while preserving the desired signal of plant roots. Fundamental to the operation of the filter is that the roots are sparse in the image domain, meaning there are relatively few pixels with multiple roots present. The performance of the through-plane filter is demonstrated and compares favorably to more conventional in-plane spatial filtering.

Bioenergy sorghum stem growth regulation: intercalary meristem localization, development, and gene regulatory network analysis.

Bioenergy sorghum is a highly productive drought tolerant C4 grass that accumulates 80% of its harvestable biomass in approximately 4 m length stems. Stem internode growth is regulated by development, shading, and hormones that modulate cell proliferation in intercalary meristems (IMs). In this study, sorghum stem IMs were localized above the pulvinus at the base of elongating internodes using magnetic resonance imaging, microscopy, and transcriptome analysis. A change in cell morphology/organization occurred at the junction between the pulvinus and internode where LATERAL ORGAN BOUNDARIES (SbLOB), a boundary layer gene, was expressed. Inactivation of an AGCVIII kinase in DDYM (dw2) resulted in decreased SbLOB expression, disrupted IM localization, and reduced internode cell proliferation. Transcriptome analysis identified approximately 1000 genes involved in cell proliferation, hormone signaling, and other functions selectively upregulated in the IM compared with a non-meristematic stem tissue. This cohort of genes is expressed in apical dome stem tissues before localization of the IM at the base of elongating internodes. Gene regulatory network analysis identified connections between genes involved in hormone signaling and cell proliferation. The results indicate that gibberellic acid induces accumulation of growth regulatory factors (GRFs) known to interact with ANGUSTIFOLIA (SbAN3), a master regulator of cell proliferation. GRF:AN3 was predicted to induce SbARF3/ETT expression and regulate SbAN3 expression in an auxin-dependent manner. GRFs and ARFs regulate genes involved in cytokinin and brassinosteroid signaling and cell proliferation. The results provide a molecular framework for understanding how hormone signaling regulates the expression of genes involved in cell proliferation in the stem IM.

Low-cost gradient amplifiers for small MRI systems.

We describe low-cost gradient amplifiers for small MRI systems, such as those based on small/medium permanent magnets. The requirements for MRI gradient amplifiers are quite similar to those of modestly priced audio stereo power amplifiers. Such amplifiers for sound service can be modified to be DC coupled, extending their response down to zero frequency, as needed for MRI gradient service. We describe such modifications to one unit (Samson Servo 600) and mention a commercially available modification of another. The 600 is capable of an output greater than ±8 A and ±60 V, much greater than our needs for a greenhouse MRI. Audio amplifiers can be used this way as controlled voltage (CV) gradient amplifiers, with acceptable performance because the gradient coils typically have short L/R time constants. Superior performance can be had by using a controlled current (CC) "front end"; the circuit of our simple front end is included. The complete 3-axis CC system costs about $1200.

NMR spectroscopy of coin cell batteries with metal casings.

Battery cells with metal casings are commonly considered incompatible with nuclear magnetic resonance (NMR) spectroscopy because the oscillating radio-frequency magnetic fields ("rf fields") responsible for excitation and detection of NMR active nuclei do not penetrate metals. Here, we show that rf fields can still efficiently penetrate nonmetallic layers of coin cells with metal casings provided "B1 damming" configurations are avoided. With this understanding, we demonstrate noninvasive high-field in situ 7Li and 19F NMR of coin cells with metal casings using a traditional external NMR coil. This includes the first NMR measurements of an unmodified commercial off-the-shelf rechargeable battery in operando, from which we detect, resolve, and separate 7Li NMR signals from elemental Li, anodic ß-LiAl, and cathodic LixMnO2 compounds. Real-time changes of ß-LiAl lithium diffusion rates and variable ß-LiAl 7Li NMR Knight shifts are observed and tied to electrochemically driven changes of the ß-LiAl defect structure.

Precision current regulator for NMR electromagnets.

When using electromagnets or non-persistent superconducting magnets, the field stability can be directly proportional to the current stability. We present here a simple design for a current regulator based upon a high-precision, dc-dc fluxgate transformer. The output current and NMR frequency are stabilized by about a factor of 50, over times of order one hour. The method can be applied to essentially any current supply.

Low-field magnetic resonance imaging of roots in intact clayey and silty soils.

The development of a robust method to non-invasively visualize root morphology in natural soils has been hampered by the opaque, physical, and structural properties of soils. In this work we describe a novel technology, low field magnetic resonance imaging (LF-MRI), for imaging energy sorghum (Sorghum bicolor (L.) Moench) root morphology and architecture in intact soils. The use of magnetic fields much weaker than those used with traditional MRI experiments reduces the distortion due to magnetic material naturally present in agricultural soils. A laboratory based LF-MRI operating at 47 mT magnetic field strength was evaluated using two sets of soil cores: 1) soil/root cores of Weswood silt loam (Udifluventic Haplustept) and a Belk clay (Entic Hapluderts) from a conventionally tilled field, and 2) soil/root cores from rhizotrons filled with either a Houston Black (Udic Haplusterts) clay or a sandy loam purchased from a turf company. The maximum soil water nuclear magnetic resonance (NMR) relaxation time T2 (4 ms) and the typical root water relaxation time T2 (100 ms) are far enough apart to provide a unique contrast mechanism such that the soil water signal has decayed to the point of no longer being detectable during the data collection time period. 2-D MRI projection images were produced of roots with a diameter range of 1.5-2.0 mm using an image acquisition time of 15 min with a pixel resolution of 1.74 mm in four soil types. Additionally, we demonstrate the use of a data-driven machine learning reconstruction approach, Automated Transform by Manifold Approximation (AUTOMAP) to reconstruct raw data and improve the quality of the final images. The application of AUTOMAP showed a SNR (Signal to Noise Ratio) improvement of two fold on average. The use of low field MRI presented here demonstrates the possibility of applying low field MRI through intact soils to root phenotyping and agronomy to aid in understanding of root morphology and the spatial arrangement of roots in situ.

NMR of petrochemical-type chemical reactions.

A simple technique is presented for NMR of chemically reacting systems at conditions of high temperature and pressure. The method can follow reactions that are typical of refinery operations - hydrogenation, transfer dehydrogenation, methanol synthesis, and isomerization. All of the reacting materials are flame-sealed into a glass capillary. Gaseous agents such as O2 and CO are loaded into the capillary by condensation at liquid N2 temperature. H2 is provided by loading LiAlH4. The capillary holds the high pressure, up to 7 MPa, so the NMR probe can be a simple design with hot air flowing over the sample tube, up to 350 °C. Example reaction results are presented, including hydrogenation of benzene, hydrogenation/dehydrogenation of cyclohexene to benzene and cyclohexane (a disproportionation), and synthesis of methane, methanol and dimethyl ether from CO and H2. In this work we present a simple, inexpensive method with rapid temperature response for tracking chemical reactions in real-time at high temperature and high pressure.

RF shielding and eddy currents in NMR probes.

Shielding of the NMR sample by a thin coating of metal between the sample and the rf coil arises in some geometries. The shielding may be used for rf heating of the sample tube or for a high infrared reflectivity coating in cryoprobe applications. An important application is for a shield that prevents noise from entering the rf probe circuit while allowing pulsed magnetic field gradients or field modulation to pass. We show by simple, approximate derivations that the criterion for shielding is not whether the coating exceeds the classical electromagnetic skin depth δ at the operating frequency (as is often stated), but whether the geometric mean between the thickness and an appropriate radius r exceeds δ. Thus, because r is typically much larger than δ, conducting layers substantially thinner than δ may still be good shields. Measurements are performed at high audio frequencies to confirm the calculations, using geometries relevant to rf saddle coils and to rf solenoids. Measurements of the slowing of the edges of a pulsed field gradient are also in accord with the calculations.

Helicopter-borne NMR for detection of oil under sea-ice.

Mobile nuclear magnetic resonance (NMR) operating in Earth's magnetic field is adapted to detect leaked or spilled oil trapped in or under sea ice without the need to place any personnel on the ice. A helicopter placed a 6-meter diameter NMR coil system weighing approximately 1000 kg on 92 cm-thick ice surrogate and detected the equivalent of 1 cm thick oil under the ice surrogate in 3-1/2 min.

Circuit filling factor (CFF) for multiply tuned probes, revisited.

The concept of circuit filling factor (CFF) is re-examined for multi-tuned, multi-inductor probe circuits. The CFF is the fraction of magnetic stored energy residing in the NMR coil. The CFF theorem states that the CFF sums to unity across all the resonant normal modes. It dictates that improved performance from a large CFF in one mode comes at the expense of CFF (and performance) at the other mode(s). Simple analytical calculations of two-mode circuits are used to demonstrate and confirm the CFF theorem. A triple-resonance circuit is calculated to show the large trade-offs involved there. The theorem can provide guidance for choosing the best circuit and relative inductances in multi-nuclear probes. The CFF is directly accessible from ball frequency-shift measurements. We give experimental measures of the CFF from ball shifts and compare to calculated values of the CFF, with good agreement.

3He diffusion MRI in human lungs.

Hyperpolarized 3He gas allows the air spaces of the lungs to be imaged via MRI. Imaging of restricted diffusion is addressed here, which allows the microstructure of the lung to be characterized through the physical restrictions to gas diffusion presented by airway and alveolar walls in the lung. Measurements of the apparent diffusion coefficient (ADC) of 3He at time scales of milliseconds and seconds are compared; measurement of acinar airway sizes by determination of the microscopic anisotropy of diffusion is discussed. This is where Dr. JJH Ackerman's influence was greatest in aiding the formation of the Washington University 3He group, involving early a combination of physicists, radiologists, and surgeons, as the first applications of 3He ADC were to COPD and its destruction/modification of lung microstructure via emphysema. The sensitivity of the method to early COPD is demonstrated, as is its validation by direct comparison to histology. More recently the method has been used broadly in adult and pediatric obstructive lung diseases, from severe asthma to cystic fibrosis to bronchopulmonary dysplasia, a result of premature birth. These applications of the technique are discussed briefly.

Coil extensions improve line shapes by removing field distortions.

The static magnetic susceptibility of the rf coil can substantially distort the field B0 and be a dominant source of line broadening. A scaling argument shows that this may be a particular problem in microcoil NMR. We propose coil extensions to reduce the distortion. The actual rf coil is extended to a much longer overall length by abutted coil segments that do not carry rf current. The result is a long and nearly uniform sheath of copper wire, in terms of the static susceptibility. The line shape improvement is demonstrated at 43.9 MHz and in simulation calculations.

Earth's field NMR detection of oil under arctic ice-water suppression.

Earth's field NMR has been developed to detect oil trapped under or in Arctic sea-ice. A large challenge, addressed here, is the suppression of the water signal that dominates the oil signal. Selective suppression of water is based on relaxation time T1 because of the negligible chemical shifts in the weak earth's magnetic field, making all proton signals overlap spectroscopically. The first approach is inversion-null recovery, modified for use with pre-polarization. The requirements for efficient inversion over a wide range of B1 and subsequent adiabatic reorientation of the magnetization to align with the static field are stressed. The second method acquires FIDs at two durations of pre-polarization and cancels the water component of the signal after the data are acquired. While less elegant, this technique imposes no stringent requirements. Similar water suppression is found in simulations for the two methods. Oil detection in the presence of water is demonstrated experimentally with both techniques.

Adiabatic sweep pulses for earth's field NMR with a surface coil.

Adiabatic NMR sweep pulses are described for inversion and excitation in very low magnetic fields B0 and with broad distribution of excitation field amplitude B1. Two aspects distinguish the low field case: (1) when B1 is comparable to or greater than B0, the rotating field approximation fails and (2) inversion sweeps cannot extend to values well below the Larmor frequency because they would approach or pass through zero frequency. Three approaches to inversion are described. The first is a conventional tangent frequency sweep down to the Larmor frequency, a 180° phase shift, and a sweep back up to the starting frequency. The other two are combined frequency and amplitude sweeps covering a narrower frequency range; one is a symmetric sweep from above to below the Larmor frequency and the other uses a smooth decrease of B1 immediately before and after the 180° phase shift. These two AM/FM sweeps show excellent inversion efficiencies over a wide range of B1, a factor of 30 or more. We also demonstrate an excitation sweep that works well in the presence of the same wide range of B1. We show that the primary effect of the counter-rotating field (i.e., at low B0) is that the magnetization suffers large, periodic deviations from where it would be at large B0. Thus, successful sweep pulses must avoid any sharp features in the amplitude, phase, or frequency.

A flow-through, elevated-temperature and -pressure NMR apparatus for in-situ CO2 sequestration studies.

We report an apparatus for in-situ nuclear magnetic resonance (NMR) studies of chemical reactions of dissolved 13CO2 with minerals (rock or powder) under continuous flow. The operating range of the apparatus is 18-150°C and 1-140bar. A flow pump is used to circulate a CO2-water solution, with a heated mixing vessel where CO2 gas equilibrates with a water solution. The NMR probe is built around a strong zirconia ceramic vessel, with o-ring sealed connections; the mineral is contained inside. The horizontal orientation of the zirconia vessels allows use of a radio frequency (rf) solenoid for improved spin sensitivity.

A combined experimental setup for OP and ODNMR.

Instrumentation for optically-pumped and optically-detected nuclear magnetic resonance (OPNMR and ODNMR) has been developed and implemented as a single experimental apparatus to study semiconductors such as GaAs and CdTe. These two measurement schemes use many of the same components for experiments. Here we describe, in two parts, the apparatus that can record such measurements and give examples of representative data. In Part 1, the radio-frequency probe and low-temperature cryostat are described, including single-channel and two-channel static cryogenic probes that both incorporate a modified solenoid coil that permits better optical access. In Part 2, the optical bench is described in detail, which uses a set of experiments (magneto-photoluminescence, photoluminescence excitation, detection of polarized photoluminescence) as important input for ODNMR. We are able to portray a robust design that encompasses multiple measurement modalities, along with the ability to change many experimental parameters with ease.

Pre-polarization fields for earth's field NMR: Fast discharge for use with short T1 and large coils.

The sensitivity of earth's field NMR is greatly increased by the use of a pre-polarizing field Bp. When used with short T1 samples, the field must be decreased rapidly to avoid loss of the pre-polarized magnetization by relaxation. Such a rapid decrease in the field requires rapid discharge (∼10ms) of a large stored magnetic field energy (∼700J). In addition, in order that the full pre-polarized magnetization be available for the subsequent pulse sequence, the field discharge should be adiabatic. This requirement is difficult to fulfill in cases where Bp is not everywhere parallel to the earth's field, such as with a large surface coil. Circuitry for rapid and controlled discharge is presented. Simulations and experiments confirm the importance of both of these conditions.

What makes a good pediatric transplant lung: Insights from in vivo lung morphometry with hyperpolarized 3 He magnetic resonance imaging.

Obtaining information on transplanted lung microstructure is an important part of the current care for monitoring transplant recipients. However, until now this information was only available from invasive lung biopsy. The objective of this study was to evaluate the use of an innovative non-invasive technique, in vivo lung morphometry with hyperpolarized ³He MRI-to characterize lung microstructure in the pediatric lung transplant population. This technique yields quantitative measurements of acinar airways' (alveolar ducts and sacs) parameters, such as acinar airway radii and alveolar depth. Six pediatric lung transplant recipients with cystic fibrosis underwent in vivo lung morphometry MRI, pulmonary function testing, and quantitative CT. We found a strong correlation between lung lifespan and alveolar depth-patients with more shallow alveoli were likely to have a negative outcome sooner than those with larger alveolar depth. Combining morphometric results with CT, we also determined mean alveolar wall thickness and found substantial increases in this parameter in some patients that negatively correlated with DLCO. In vivo lung morphometry uniquely provides previously unavailable information on lung microstructure that may be predictive of a negative outcome and has a potential to aid in lung selection for transplantation.

Experimental evidence of age-related adaptive changes in human acinar airways.

The progressive decline of lung function with aging is associated with changes in lung structure at all levels, from conducting airways to acinar airways (alveolar ducts and sacs). While information on conducting airways is becoming available from computed tomography, in vivo information on the acinar airways is not conventionally available, even though acini occupy 95% of lung volume and serve as major gas exchange units of the lung. The objectives of this study are to measure morphometric parameters of lung acinar airways in living adult humans over a broad range of ages by using an innovative MRI-based technique, in vivo lung morphometry with hyperpolarized (3)He gas, and to determine the influence of age-related differences in acinar airway morphometry on lung function. Pulmonary function tests and MRI with hyperpolarized (3)He gas were performed on 24 healthy nonsmokers aged 19-71 years. The most significant age-related difference across this population was a 27% loss of alveolar depth, h, leading to a 46% increased acinar airway lumen radius, hence, decreased resistance to acinar air transport. Importantly, the data show a negative correlation between h and the pulmonary function measures forced expiratory volume in 1 s and forced vital capacity. In vivo lung morphometry provides unique information on age-related changes in lung microstructure and their influence on lung function. We hypothesize that the observed reduction of alveolar depth in subjects with advanced aging represents a remodeling process that might be a compensatory mechanism, without which the pulmonary functional decline due to other biological factors with advancing age would be significantly larger.

Characterization of a Mixture of CO2 Adsorption Products in Hyperbranched Aminosilica Adsorbents by (13)C Solid-State NMR.

Hyperbranched amine polymers (HAS) grown from the mesoporous silica SBA-15 (hereafter "SBA-15-HAS") exhibit large capacities for CO2 adsorption. We have used static in situ and magic-angle spinning (MAS) ex situ (13)C nuclear magnetic resonance (NMR) to examine the adsorption of CO2 by SBA-15-HAS. (13)C NMR distinguishes the signal of gas-phase (13)CO2 from that of the chemisorbed species. HAS polymers possess primary, secondary, and tertiary amines, leading to multiple chemisorption reaction outcomes, including carbamate (RnNCOO(-)), carbamic acid (RnNCOOH), and bicarbonate (HCO3(-)) moieties. Carbamates and bicarbonate fall within a small (13)C chemical shift range (162-166 ppm), and a mixture was observed including carbamic acid and carbamate, the former disappearing upon evacuation of the sample. By examining the (13)C-(14)N dipolar coupling through low-field (B0 = 3 T) (13)C{(1)H} cross-polarization MAS NMR, carbamate is confirmed through splitting of the (13)C resonance. A third species that is either bicarbonate or a second carbamate is evident from bimodal T2 decay times of the ∼163 ppm peak, indicating the presence of two species comprising that single resonance. The mixture of products suggests that (1) the presence of amines and water leads to bicarbonate being present and/or (2) the multiple types of amine sites in HAS permit formation of chemically distinct carbamates.