Andrew J.M. Kiruluta

3D coronary motion tracking in swine models with MR tracking catheters

To develop MR‐tracked catheters to delineate the three‐dimensional motion of coronary arteries at high spatial and temporal resolution.

The emergence of the propagation wave vector in high field NMR: analysis and implications

At ultra high field strength (>4 T), the RF-wavelength relative to the dimension of the human body leads to significant excitation field (B1) inhomogeneity arising out of wave propagation effects. The excitation field is thus spatio-temporal and in conjunction with the increase in tissue conductivity, wave propagation phenomena can no longer be ignored. We thus find the propagation of radiation at ultra high fields new phenomena commonly observed in quantum optics but traditionally negligible in NMR such as spatio-temporal phase modulation of the excitation field such that the identity between pulse area and flip angle is no longer valid. In this paper, the emergence of field propagation phenomena in NMR experiments is analytically and experimentally demonstrated. It is shown that in addition to the well-studied dielectric resonance phenomena at high magnetic fields, propagation effects transform the excitation pulse into an adiabatic excitation. The high field strength also means that nonlinear effects such as self-induced transparency, propagation phenomena such as transient four wave mixing, are now possible in NMR experiments. It is shown and experimentally demonstrated at 7 T that additional constraints due to phase matching considerations are imposed on the formation of echoes in high field NMR.

Time-resolved contrast-enhanced magnetic resonance angiography in the investigation of suspected intracranial dural arteriovenous fistula

Cerebral angiography is widely regarded as the gold standard for the evaluation and diagnosis of neurovascular abnormalities. However, recent improvements in the spatial and temporal resolution of time-resolved magnetic resonance angiography (MRA) offer clinicians a non-invasive alternative to cerebral angiography. We explored the utility of this technique in an elderly female patient with a suspected intracranial dural arteriovenous fistula (dAVF). A product pulse sequence available from the scanners manufacturer (time-resolved imaging of contrast kinetics, TRICKS; GE Healthcare, Milwaukee, WI, USA) was used with the following parameters: TR/TE 2.832/TE 1.072 ms, flip angle 25°, receiver bandwidth 31.25 kHz, 0.75 NEX, acceleration factor (ASSET) of 2, field of view 14 cm, matrix size 96 × 96, phase-encoding left-right. Twenty overlapping 8-mm-thick slices were acquired in an axial orientation, with a slice spacing of 4mm. Images were acquired at 48 time points, with a temporal resolution of 0.3s/image. We found that all intracranial venous structures enhanced synchronously. There was no evidence of arteriovenous shunting. Retrograde venous flow explained the signal abnormality seen on time-of-flight MRA. We concluded that time-resolved MRA is useful in the investigation of suspected intracranial dAVF.

Probing restrictive diffusion dynamics at short time scales

Diffusion imaging gradients serve to spectrally filter the temporally evolving diffusion tensor. In this framework, the design of diffusion sensitizing gradients is reduced to the problem of adequately sampling q-space in the spectral domain. The practical limitations imposed by the requirement for delta-function type diffusion-sensitizing gradients to adequately sample q-space, can be relaxed if these impulse gradients are replaced with chirped oscillatory gradients. It is well known that in many systems of interest, dispersion of velocity will itself produce a peak in the velocity correlation function near w=0, while restricted diffusion will manifest itself in the dispersion spectrum at higher frequencies. In this paper, chirped diffusion-sensitizing gradients are proposed and analytically shown to yield an efficient sampling of q-space in a manner that asymptotically approaches that using delta-function diffusion-sensitizing gradient. The challenge is the consequent reduction in diffusion sensitivity as one probes higher frequency dynamics. This problem is addressed by restricting the gradient power to a spectral bandwidth corresponding to the diffusion spectral range of the underlying restrictive geometry. Simultaneous imaging of diffusion and flow at microscopic resolution and at temporally resolvable diffusion time scales thus becomes possible in vivo. Simulations and experiments validate the proposed approach.

Time-domain frequency-selective processing in nuclear magnetic resonance: a spatial-spectral holographic perspective.

The fundamental nuclear magnetic resonance (NMR) imaging equation can be derived from a spatial-spectral holographic wavefront reconstruction formulation similar to that in quantum optics. A spatial-spectral holographic interpretation arises naturally in NMR from the inhomogeneous linewidth broadening due to either an imposed set of linear orthogonal gradient fields or from the intrinsic chemical anisotropy of the spin system. We can thus think of NMR k-space as a spatial-spectral holographic grating. The spatial holographic component arises from dielectric effects at high field strength (>4 T) when the excitation wavelength is less than or commensurate with the size of the imaging sample. The holographic properties of storage, time-reversal, recognition, and triple correlations are experimentally demonstrated in an inhomogeneously broadened NMR sample. This holographic NMR interpretation has additional implications on selective radio-frequency pulse design, microscopy imaging, and the use of conjugate imaging for field inhomogeneity corrections using the time-reversed component of the readout, to be the subject of a subsequent paper.

Diffusion MR of Acute Stroke

Diffusion magnetic resonance imaging provides unique information on the state of living tissue because it provides image contrast that is dependent on the molecular motion of water. The method was introduced into clinical practice in the mid 1990s. Because it employs ultrafast, echo planar MRI scanning, it is highly resistant to patient motion, with imaging times ranging from a few seconds to 2 min. Diffusion MRI is the most reliable method for detecting acute ischemia and has assumed an essential role in the detection of acute ischemic brain infarction and in differentiating acute infarction from other disease processes.

Magnetic resonance angiography: physical principles and applications.

Magnetic resonance angiography (MRA) is the visualization of hemodynamic flow using imaging techniques that discriminate flowing spins in blood from those in stationary tissue. There are two classes of MRA methods based on whether the magnetic resonance imaging signal in flowing blood is derived from the amplitude of the moving spins, the time-of-flight methods, or is based on the phase accumulated by these flowing spins, as in phase contrast methods. Each method has particular advantages and limitations as an angiographic imaging technique, as evidenced in their application space. Here we discuss the physics of MRA for both classes of imaging techniques, including contrast-enhanced approaches and the recent rapid expansion of the techniques to fast acquisition and processing techniques using parallel imaging coils as well as their application in high-field MR systems such as 3T and 7T.

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.

Patch-Probe Excitation for Ultrahigh Magnetic Field Wide-Bore MRI

In this paper, we present the design of probe excitations for 7- and 10.5-T traveling-wave wide-bore magnetic resonance imaging systems. The probes are 297- and 447-MHz coaxially fed microstrip patches, designed to give a circularly polarized magnetic field when placed in the metallic bore waveguide. Images of a water phantom using the patch probes are obtained and compared with full-wave electromagnetic simulations. Additionally, periodic axial metal strip cylinders are inserted into the bore, resulting in improved field uniformity in a phantom with a simultaneous increase in SNR. The numerical analysis for nonquasi-static fields can be computationally intensive, and the numerical error for finite-element simulations is quantified using modified boundary conditions of the system. When the waveguide effects are taken into account, the mode content in the images compares well to full-wave simulations.