Three dimensional magnetic resonance imaging by magnetic resonance force microscopy with a sharp magnetic needle

An electropolished magnetic needle made of Nd(2)Fe(14)B permanent magnet was used for obtaining better spatial resolution than that achieved in our previous work. We observed the magnetic field gradient |G(Z)|=80.0G/microm and the field strength B=1250G at Z approximately 8.8 microm from the top of the needle. The use of this needle for three dimensional magnetic resonance force microscopy at room temperature allowed us to achieve the voxel resolution to be 0.6 microm x 0.6 microm x 0.7 microm in the reconstructed image of DPPH phantom. The acquisition time spent for the whole data collection over 64 x 64 x 16 points, including an iterative signal average by six times per point, was about 10 days.     

3D MR microscopy with resolution 3.7 microm by 3.3 microm by 3.3 microm

The technique of magnetic resonance imaging microscopy holds promise of bringing the full capabilities of NMR to arbitrarily specified positions within spatially inhomogeneous systems, including biological cells, yet the possibilities are limited by the need for adequate sensitivity and spatial resolution. We report proton magnetic resonance images obtained by combining advances in receiver coil sensitivity, gradient strength, and pulse/gradient sequence design. We achieve resolution of 3.7 +/- 0.4 microm by 3.3 +/- 0.3 microm by 3.3 +/- 0.3 microm for a volume resolution approximately 40 femtoliters (corresponding to approximately 3 x 10(12) proton spins).     

Time-independent point-spread function for NMR microscopy.

The resolution of NMR microscopy is analyzed in terms of the point-spread function, PSF(r), and the equivalent k space modulation transfer function, MTF(k). The analysis is developed for NMR spin warp and projection reconstruction imaging experiments; however, the framework provided is quite general. Incoherent spin motion is analyzed to predict what limits, if any, on spatial resolution are imposed by diffusion. Previous estimates of diffusion limits at 1-5 microns were developed for specific imaging techniques, typically using a mean displacement argument. Although qualitatively correct, the quantitative predictions represent practical rather than fundamental limits. It is shown that diffusion-dependent "blurring" can be made arbitrarily small and that the practical limits are less stringent than previously thought. A major point illustrated by the PSF-MTF formulation is that the irreversible loss of coherence by randomly diffusing spins occurs faster than the physical displacement, thereby reducing their effect considerably on the frequency or phase of the net detected signal. The irreversible loss of signal due to diffusive motion will contribute to and possibly dominate the signal-to-noise limit of resolution. The resolution as measured by the width of the PSF and MTF for diffusion is shown to be independent of the signal acquisition time, and their functional forms allow selection of microscopic imaging parameters. An example of a three-dimensional spin-warp image of a green algae cell is shown with resolution of approximately 16 microns x 13 microns x 10 microns.     

Magnetic resonance force microscopy combined with surface topography

A new method of surface microscopy is proposed, which combines three-dimensional electron spin resonance imaging by magnetic resonance force microscopy (MRFM) and topographic imaging of the sample surface by scanning force microscopy (SFM). In order to demonstrate its potential for the identification of microscale objects, the individual and combined images are used to provide the locations, shapes and spin density distributions of target phantom objects. We report spatial resolution in MRFM of 2.8 x 2.8 x 2.0 microm(3). This could be improved to the theoretical limit of 0.08 x 0.08 x 0.04 microm(3) through reduction of the thermal noise by cooling to cryogenic temperatures approximately 0.5K. We believe that this type of microscopy will become a very useful tool for the investigation of anomalies induced in surfaces by materials buried below the surface.     

In vivo 31P echo-planar spectroscopic imaging of human calf muscle

Localized phosphorus-31 NMR spectra of human calf muscle in vivo were obtained by means of echo-planar spectroscopic imaging (EPSI) with a 1.5-T whole-body scanner. The technique permits the measurement of two-dimensional 31P SI data at a minimum acquisition time of 2.4 s (8x8 voxels, TR=300 ms). With 9.4 min measurement time (TR=1100 ms, 64 averages) and 25x25x40 mm spatial resolution in vivo the 31P NMR signal-to-noise ratio (S/N) of the phosphocreatine (PCr) resonance was about 45; the multiplets of nucleoside 5'-triphosphates were resolved. Spectral quality permits quantitative assessment of the PCr signal in a measurement time that is shorter by a factor of 2 or more than the minimum measurement time feasible with chemical-shift imaging. In a functional EPSI study with a time resolution of 20.5 s on the calf muscle of volunteers, spectra showed a 40% decrease of the PCr signal intensity (at rest: S/N congruent with12) upon exertion of the muscle.     

Design of a superconducting volume coil for magnetic resonance microscopy of the mouse brain

We present the design process of a superconducting volume coil for magnetic resonance microscopy of the mouse brain at 9.4T. The yttrium barium copper oxide coil has been designed through an iterative process of three-dimensional finite-element simulations and validation against room temperature copper coils. Compared to previous designs, the Helmholtz pair provides substantially higher B(1) homogeneity over an extended volume of interest sufficiently large to image biologically relevant specimens. A custom-built cryogenic cooling system maintains the superconducting probe at 60+/-0.1K. Specimen loading and probe retuning can be carried out interactively with the coil at operating temperature, enabling much higher through-put. The operation of the probe is a routine, consistent procedure. Signal-to-noise ratio in a mouse brain increased by a factor ranging from 1.1 to 2.9 as compared to a room-temperature solenoid coil optimized for mouse brain microscopy. We demonstrate images encoded at 10x10x20mum for an entire mouse brain specimen with signal-to-noise ratio of 18 and a total acquisition time of 16.5h, revealing neuroanatomy unseen at lower resolution. Phantom measurements show an effective spatial resolution better than 20mum.     

Magnetic resonance imaging of isolated single liposome by magnetic resonance force microscopy

Magnetic resonance imaging (MRI) is very useful spectroscopy to visualize a three-dimensional (3D) real structure inside the sample without physical destruction. The spatial resolution of the readily available MRI spectrometer is, however, limited by a few ten to hundreds of microns due to a technological boundary of generating larger magnetic field gradient and to the insensitivity inherent to the inductive signal detection. Magnetic resonance force microscopy (MRFM) is new alternative MRI spectroscopy which is anticipated to significantly surpass the conventional MRI in both resolution and sensitivity. We report two imaging experiments on our MRFM spectrometer operated at room temperature and in vacuum approximately 10(-3)Pa. One is for approximately 20 microm liposome membrane labeled entirely by a nitroxide imaging agent and the other for approximately 15 microm DPPH particles, both are nearly the same size as that of human cell. The reconstructed images at spatial resolution approximately 1 microm were in satisfactory agreement with the scanning electron microscope images. The potential capability of visualizing intrinsic radicals in the cell is suggested to investigate redox process from a microscopic point of view.     

NMR relaxometry and imaging of water absorbed in biodegradable polymer scaffolds

Porous substrates made of poly(3-hydroxybutyrate-3-hydroxyvalerate) (PHBHV) were prepared by a particulate leaching method. After removing the salt by extraction in water, proton nuclear magnetic resonance (NMR) relaxometry and imaging were performed on sets of PHBHV substrates immersed in phosphate-buffered solution during 3 months at different time points. Polarized optical microscopy studies were performed on thin sections, 25 and 5 mum, of the PHBHV samples. The results of NMR relaxometry showed two (1)H nuclei populations, well distinguishable on the free induction decay (FID), due to the different decay time constants, a factor of 10(2) apart. Thus, it was possible to separate the two populations, giving separate distributions of T(1) relaxation times. One population could be associated with water protons in the pores and the other to macromolecular protons. The distributions of T(1) and T(2) of the water proton shifted to lower values with increasing immersion time to a constant value after 30 days. The results obtained by NMR imaging showed an initial increase in the apparent porosity, reaching a plateau after 25 days of immersion. This increase is attributed mainly to the absorption of water in the microporosity as supported by the results of the relaxometry measurements and shown by scanning electron microscopy. The average porosity measured by NMR imaging at the plateau, 78+/-3%, is slightly higher than that determined by optical microscopy, 73+/-9%, which may be due to the fact that the latter method did not resolve the microporosity. Overall, the results suggest that at early stages after immersing the scaffolds in the aqueous medium, first 30 days approximately, NMR imaging could underestimate the porosity of the substrate.     

Stroboscopic ultrahigh-resolution full-field optical coherence tomography

We present a new technique that produces en face tomographic images with a 10-micros acquisition time per image. The setup consists of an interference microscope with stroboscopic illumination provided by a xenon arc flash lamp (10-micros flashes at 15 Hz). The tomographic images are obtained from two phase-opposed interferometric images recorded simultaneously by two synchronized CCD cameras. Transverse resolution better than 1.0 microm is achieved by use of high-numerical-aperture microscope objectives. The short coherence length of the source yields an axial resolution of 0.9 microm. 3 x 3 pixel binning leads to a detection sensitivity of 71 dB. Our system is suitable for various applications, particularly in biology for in vivo cellular-level imaging.     

High-resolution line-scanning optical coherence microscopy

An optical coherence microscopy system based on line illumination and detection is demonstrated. The system uses a Linnik-type interferometer illuminated by a broadband Ti:sapphire laser and detected by a high-speed, line-scan CCD camera. This approach is less sensitive to incoherent scattering and sample motion than full-field imaging. Spatial resolutions of approximately 2 microm x approximately 3 microm(transverse x axial) are achieved. The sensitivity of the system is 93 dB with averaging over 30 line scans. En face real time, cellular-level imaging of biological tissues is demonstrated at approximately 2 frames/s.     

A closer look into DESIRE for NMR microscopy

The major challenge of nuclear magnetic resonance (NMR) microscopy at a spatial resolution of a few micrometers is to obtain a sufficiently high signal-to-noise-ratio (SNR) within a reasonable measurement time. As a particular difficulty, molecular self-diffusion poses a serious limitation to true spatial resolution and SNR if conventional Fourier encoding techniques are used. Opposed to that, the alternative DESIRE (Diffusion Enhancement of SIgnal and REsolution) approach to NMR microscopy utilises diffusion to increase the SNR. Being a real-space imaging method, spatial localisation is accomplished by saturation pulses while diffusion continuously replaces the saturated by unsaturated spins. For this technique a signal enhancement of up to three orders of magnitude has been predicted and initial experimental data have provided a proof of principle. In the present work, a detailed investigation of one-dimensional (1D) DESIRE is presented including simulations of a real implementation of the method, a quantitative experimental analysis, and basic 1D imaging. The simulations reveal the importance and provide the means of ensuring the true spatial resolution for this particular way of localisation, enable the selection of useful experimental parameters, and predict the specific image contrast to be expected around barriers restricting diffusion. Experimental data are presented with resolutions down to 3 microm and DESIRE enhancement up to 25 that are in good agreement with the simulation results. In particular, 1D DESIRE imaging in a phantom confirms the expected signal drop close to barriers due to spatially restricted diffusion.     

Operating characteristics of near-infrared self-assembled polymer microlasers

We report near-infrared laser emission from self-assembled luminescent polymer microcavities. The microrings are formed around silica optical fibers of varying diameters (80, 125, and 200 microm) and are shown to exhibit photopumped lasing at approximately 820 nm. The microrings with 200 mum inner diameter have an overall quality factor of approximately 2 x 10(3), which is limited by surface roughness and scattering. We illustrate how the laser threshold varies inversely with both the quality factor and the diameter of the microrings. The free spectral range and the intensity variation of the laser output are also presented.     

Magnetic double resonance in force microscopy

Magnetic-resonance force microscopy is combined with cross-polarization and spin-decoupling NMR techniques to obtain double-resonance NMR signals of micrometer-scaled objects. The effective one-dimensional spatial resolution obtained in our experiments performed on a KPF6 single crystal sample is approximately 0.5 microm. The spectral linewidth of 900 Hz is sample limited. The described double-resonance techniques can introduce new chemical specificity to the magnetic-force sensor.     

Molecular vibration imaging in the fingerprint region by use of coherent anti-Stokes Raman scattering microscopy with a collinear configuration

We have developed a new coherent anti-Stokes Raman scattering (CARS) microscopy system with a collinear configuration for use in the fingerprint region. The system consists of a picosecond laser system and a transmission-type laser scanning microscope without a pinhole in front of the detector. The observable Raman-shift region is 900-1750 cm(-1), the spectral resolution is 30cm(-1), and the spatial resolution is smaller than 1 mum in the lateral direction and 3.2 mum in the depth direction, with objectives with a numerical aperture of 0.65. CARS spectra and images of polystyrene beads are demonstrated, and CARS imaging of a viable yeast cell is attempted.     

Resolution enhancement in a light-sheet-based microscope (SPIM)

Light-sheet-based microscopy [single-plane illumination microscope (SPIM)] performs very well at low numerical apertures. It complements conventional (FM), confocal (CFM), and two-photon fluorescence microscopy (2hnu-FM) currently used in modern life sciences. Lateral and axial SPIM point spread function (PSF) extents are measured by using fluorescent beads to determine the 3D resolution. The results are compared with values derived from an analytical theory and numerical simulations. The discrepancies are found to be less than 5%. The axial extent of a SPIM-PSF (10x/0.3 W) is approximately 5.7 microm. This value is almost a factor of 2 smaller than in CFM, more than 2.5 times smaller than in FM, and more than three times smaller than in 2hnu-FM. SPIM outperforms 2hnu-FM and FM, while CFM has a better axial resolution at NAs above 0.8.     

A novel RF coil configuration for in-vivo and ex-vivo imaging of arthritic rabbit knee joints

The purpose of this study was to design and build an optimized Radio Frequency (RF) coil configuration, that would facilitate the acquisition of high resolution 3-dimensional (3D) images of arthritic and normal rabbit knees. A surface coil transmit surface coil receive configuration was built, in order to ensure adequate B(1) homogeneity over the imaging volume and maximum filling factor, and hence to maximize the Signal to Noise ratio (SNR) and resolution of the 3-dimensional images. The two coils were passively decoupled using crossed diodes and lambda/4 lines, both during the transmit and receive phases of the imaging experiment. A specialized animal bed, to optimize the use of the coils and minimize the experiment setup time was designed and constructed. Three dimensional images of resolution 156 x 156 x 468 microm, were acquired in 20 min; the results, in terms both of the high resolution images and the ease with which the experimental setup could be reproduced, demonstrated that this configuration is ideal for imaging rabbit knee joints.     

Heating of thin foils with a relativistic-intensity short-pulse laser

K-shell x-ray spectroscopy of sub-100 nm Al foils irradiated by high contrast, spatially uniform, 150 fs, Ilambda (2)=2 x 10(18) W microm(2)/cm(2), laser pulses is obtained with 500 fs time resolution. Two distinct phases occur: At /=500 fs the resonance transitions dominate. Initial satellites arise from a large area, high density, low temperature (approximately 100 eV) plasma created by fast electrons. Thus, contrary to predictions, a short, high intensity laser incident on a thin foil does not create a uniform, hot dense plasma.     

Spatial and temporal resolution effects on dynamic contrast-enhanced magnetic resonance mammography

We tested the hypothesis that partial volume effects due to poor in-plane resolution and/or low temporal resolution used in clinical dynamic contrast-enhanced magnetic resonance imaging results in erroneous diagnostic information based on inaccurate estimates of tumor contrast agent extravasation and tested whether reduced encoding techniques can correct for dynamic data volume averaging. Image spatial resolution was reduced from 469 x 469 microm2 to those reported below by selecting a subset of k-space data. We then compared the top five K(trans)/V(T) "hot spots" obtained from the original data set, 469 x 469-microm in-plane spatial resolution and an 18-s temporal resolution processed by fast Fourier transform (FFT), with values obtained from data sets having in-plane spatial resolutions of 938 x 938, 1875 x 1875 and 2500 x 2500 microm2 and a temporal resolution of 18 s, or data sets with temporal resolutions of 36, 54 and 72 and a spatial resolution of 469 x 469 microm2, and found them to statistically differ from the parent data sets. We then tested four different post processing methods for improving the spatial resolution without sacrificing temporal resolution: zero-filled FFT, keyhole, reduced-encoding imaging by generalized-series reconstruction (RIGR) and two-reference RIGR (TRIGR). The top five values of K(trans)/V(T) obtained from data sets, the in-plane spatial resolutions of which were improved to 469 x 469 microm2 by zero-filling FFT, Keyhole and RIGR, statistically differed from those obtained from the original 469 x 469 microm2 FFT parent image data set. Only the 938 x 938 and 1875 x 1875 microm2 data sets reconstructed to 469 x 469 microm2 with TRIGR reconstruction method yielded values of the top five K(trans)/V(T) hot spots statistically the same as the original parent data set, 469 x 469 microm2 in-plane spatial and 18-s temporal-resolution FFT. That is, partial volume effects from data sets of different in-plane spatial resolution resulted in statistically different values of the top five K(trans)/V(T) hot spots relative to a high spatial and temporal resolution data set, and TRIGR reconstruction of these low resolution data sets to high resolution images provided statistically similar values with a savings in temporal resolution of 2 to 4 times.     

Two-dimensional synthetic aperture imaging in the optical domain

In scan-mode synthetic aperture imaging radar, spatial resolution in a range is given by a frequency-swept waveform, whereas resolution in the orthogonal direction is derived from the record of phase as the beam footprint executes linear motion over the object. We demonstrate here what is to our knowledge the first two-dimensional imaging that uses exactly this process in the optical domain for a 1 cm x 1 cm object with 90 mumx170 mum resolution.     

Flow measurements below 50 mum: NMR microscopy experiments in lithographic model pore spaces

Quasi two-dimensional random site percolation model objects have been prepared using a synchrotron radiation lithography technique with a spatial resolution better than 50 microm and an aspect ratio of up to 17. Flow of water through the pore space was studied with the aid of an NMR velocity mapping method and compared with a computational fluid dynamics simulation. In order to be able to measure and map widely distributed flow velocities with microscopic resolution (typically 40 x 40 microm), an experimental protocol that permits one to cover an effectively very wide velocity field of view (0.6-10 mm/s) had to be developed.