Alexey Tonyushkin

Observation of saturation of fidelity decay with an atom interferometer.

We experimentally investigate the effect of atomic δ-kicked rotor potentials on the mutual coherence between wavepackets in an atom interferometer. The differential action of the kicked rotor degrades the mutual coherence, leading to a reduction of the interferometry fringe visibility; however, when the repetition rate of the kicked rotor is at or near the quantum resonance, we observe revival of matter-wave coherence as the number of kicks increases, resulting in non-vanishing coherence in the large kick number limit. This coherence saturation effect reflects a saturation of fidelity decay due to momentum displacements in deep quantum regime. The saturation effect is accompanied with an invariant distribution of matter-wave coherence under the kicked rotor

Straight macroscopic magnetic guide for cold atom interferometer

We demonstrate a macroscopic magnetic guide for cold atom interferometry, where the magnetic guiding field is generated by a symmetrical array of racetrack coils of copper tape. This system represents a conceptual advance over previous guided atom interferometers based on nonsymmetrical geometries because the symmetry provides a much lower magnetic field curvature per fixed length than equivalent nonsymmetrical geometries, permitting a decrease in system length without increasing the decoherence rate associated with field curvature. We realized a magnetic guide a few cm away from each coil, where smooth translation of the guided atoms is achieved by changing the currents in second array of the multiple-conductor tape.We demonstrate a macroscopic magnetic guide for cold atoms with suppressed longitudinal field curvature which is highly desired for atom interferometry. The guide is based on macroscopic copper tape coils in a copropagating currents geometry, where the atoms are located between the coils few cm away from each surface. The symmetric geometry provides a much lower magnetic field curvature per fixed length that promises longer coherence time for atom interferometers. A double-tape design of each coil allows a smooth translation of guided atoms without addition of an external bias field. The guide is also immune from the current and thermal noise by virtue of the turns averaging and a large working distance, respectively. We present the experimental results of guide application to atom interferometry.

Demonstration of a multipulse interferometer for quantum kicked-rotor studies

We demonstrate perfect coherence preservation in an atom in terferometer perturbed by kicks from offresonant standing wave pulses. Under most conditions, the d ecoherence induced by the pulses reduces the signal; however, the coherence is perfectly preserved when the kicking period is equal to the rational fraction of the inverse atomic recoil frequency, independent of the num ber or the randomness of the strength of the applied kicks. The width narrowing of coherence revival as a functio n of increasing kick number and strength provides a new accurate measurement of the recoil frequency.

Quantitative study of prostate cancer using three dimensional fiber tractography

AIM To investigate feasibility of a quantitative study of prostate cancer using three dimensional (3D) fiber tractography. METHODS In this institutional review board approved retrospective study, 24 men with biopsy proven prostate cancer underwent prostate magnetic resonance imaging (MRI) with an endorectal coil on a 1.5 T MRI scanner. Single shot echo-planar diffusion weighted images were acquired with b = 0.600 s/mm(2), six gradient directions. Open-source available software TrackVis and its Diffusion Toolkit were used to generate diffusion tensor imaging (DTI) map and 3D fiber tracts. Multiple 3D spherical regions of interest were drawn over the areas of tumor and healthy prostatic parenchyma to measure tract density, apparent diffusion coefficient (ADC) and fractional anisotropy (FA), which were statistically analyzed. RESULTS DTI tractography showed rich fiber tract anatomy with tract heterogeneity. Mean tumor region and normal parenchymal tract densities were 2.53 and 3.37 respectively (P < 0.001). In the tumor, mean ADC was 0.0011 × 10(-3) mm(2)/s vs 0.0014 × 10(-3) mm(2)/s in the normal parenchyma (P < 0.001). The FA values for tumor and normal parenchyma were 0.2047 and 0.2259 respectively (P = 0.3819). CONCLUSION DTI tractography of the prostate is feasible and depicts congregate fibers within the gland. Tract density may offer new biomarker to distinguish tumor from normal tissue.

RF excitation in 7 tesla MRI systems using monofilar axial-mode helical antenna

We present a novel RF exciter for traveling-wave magnetic resonance imaging (MRI) systems based on a monofilar axial-mode helical antenna. In specific, we present a bore-extended, subject-loaded monofilar helical antenna for 7-T (ultra-high magnetic field) MRI systems. By means of rigorous full-wave electromagnetic modeling, we confirm clear advantages of the novel exciter over the existing TW excitation methods, and its excellent performance in terms of circular polarization and uniformity of the RF magnetic field in simple phantoms inside MRI bores at 7 T.

Travelling-wave excitation for 16.4T small-bore MRI

In this paper, we present the design of a probe for a travelling-wave 16.4T small-bore animal research MRI system. The probe is a 698-MHz coaxially-fed microstrip patch designed to give a circularly polarized magnetic field when placed in the bore cavity. Images of a water phantom using the patch probe are obtained and compared with simulations. Additionally, a periodic axial strip cylinder is inserted into the bore, resulting in a 7-fold increase in SNR, and enabling both gradient recalled echo and spin echo imaging of the phantom. The modified mode content in the image is compared to full-wave simulations.

MR in the far field: From mode transformation and holography to quasi-optics

Fundamental properties of holography such as storage, recall and matched filtering, arising out of momentum matching considerations of the propagating excitation fields, have been experimentally demonstrated for the first time in MR. As the wavelength of the MR scanner becomes commensurate or smaller than the geometrical thickness of the sample, new phenomena common in quantum optics but hitherto unknown in MR. Far-field concept such as interference and diffraction, will become prevalent with the use of propagating excitation fields in MRI. This realization of holographic principles in MR can be fruitful in designing MR imaging and spectroscopic techniques such as phase conjugate imaging for correcting image distortions caused by field inhomogeneities, as well as new spatial encoding schemes based on a holographic grating encoding. In addition, it has potential to lead to new concepts for information storage and processing at MR frequencies. For example, the use of convolution operations opens the possibility of applying spectral filters directly to the hologram as part of the readout while holographic recording has the potential to increase resolution in MR limited only by the fringe spacing and T2 of the sample. Our analysis shows that for a Larmor frequency of 300 MHz in a 7.0 T whole-body scanner, traveling wave modes in dielectric samples within the range of biological tissues can be sufficient to support imaging of the body parts. The modes diversity depends on the tissue efficient diameter, relative permittivity, conductivity, and the Larmor frequency. The imaging contrast will depend on the particular modes that have been excited in the tissue. A more complicated case of heterogeneous axial symmetric dielectric can be also analyzed using effective permittivity with our approach of mode transformation.

Effect of androgen deprivation and radiation therapy on MRI fiber tractography in prostate cancer: Can we assess treatment response on imaging?

OBJECTIVE: To evaluate quantitative changes in Diffusion Tensor Magnetic Resonance Tractography in prostate cancer following androgen deprivation and radiation therapy. METHODS: 22 patients with elevated PSA and biopsy proven prostate carcinoma who underwent MRI of the prostate at 1.5 T with an endorectal coil were included. Group A) was the study group (n = 11), participants who underwent androgen deprivation and/or radiation therapy and group B) were Gleason-matched control group (n = 11) participants who did not undergo such therapy. Diffusion weighted images were used to generate three-dimensional (3D) map of fiber tracts from DTI. 3D regions of interest (ROI) were drawn over the tumor and healthy prostatic parenchyma in both groups to record tract number and tract density. Tumor region and normal parenchymal tract densities within each group were compared. RESULTS: Mean tract density in the tumor region and normal parenchyma was 2.3 and 3.3 in study group (tract numbers: 116.6 and 170.2 respectively) and 1.6 and 2.7 in the control group respectively (tract numbers: 252.5 and 346.3 respectively). The difference between these values was statistically significant for the control group (p = 0.0018) but not for the study group (p = 0.11). The difference between the tract numbers of tumor and normal parenchyma appears to narrow following therapy. CONCLUSION: The study demonstrated utility in using tractography as a biomarker in prostate cancer patients post treatment. ADVANCES IN KNOWLEDGE: Quantitative DTI fiber tractography is a promising imaging biomarker to quantitatively assess treatment response in the setting of post-androgen deprivation and radiation therapy for prostate cancer.

Direct imaging of radio-frequency modes via traveling wave magnetic resonance imaging

We demonstrate an experimental method for direct 2D and 3D imaging of magnetic radio-frequency (rf) field distribution in metal-dielectric structures based on traveling wave (TW) magnetic resonance imaging (MRI) at ultra-high field (>7 T). The typical apparatus would include an ultra-high field whole body or small bore MRI scanner, waveguide elements filled with MRI active dielectrics with predefined electric and magnetic properties, and TW rf transmit-receive probes. We validated the technique by obtaining TW MR images of the magnetic field distribution of the rf modes of circular waveguide filled with deionized water in a 16.4 T small-bore MRI scanner and compared the MR images with numerical simulations. Our MRI technique opens up a practical non-perturbed way of imaging of previously inaccessible rf field distribution of modes inside various shapes metal waveguides with inserted dielectric objects, including waveguide mode converters and transformers.

Improving traveling-wave RF fields inside magnetic resonance imaging bores by incorporating dielectric loadings

The next-generation magnetic resonance imaging (MRI) systems at ultra-high static magnetic fields (magnetic flux densities), B 0 > 3 T, and ultra-high Larmor frequencies, ƒ 0 > 127.8 MHz, utilize RF excitation magnetic fields, B 1 , in the form of traveling waves (TWs) in the MRI bore. Hence, the images of subjects are generated and received by far-field coils, namely, by excitation probes that essentially operate as antennas, in place of the traditional quasi-static, near-field RF coils used in 3-T clinical MRI scanners (e.g., birdcage coils). When compared to traditional, quasi-static, MRI systems, TW MRI systems can provide more homogeneous B 1 field distribution, better signal-to-noise ratio, larger field of view, more comfort for patients, etc. Moreover, it is possible to potentially benefit from the advantages of TW concepts also at relatively lower (but still considered high) field strengths (e.g., B 0 = 3 T; ƒ 0 = 127.8 MHz), in order to address challenges and enable substantial improvements of current clinical MRI scanners at 3 T.