Dean O. Kuethe

Effects of turbulence on signal intensity in gradient echo images.

Although the appearance of laminar vascular flow in magnetic resonance (MR) images has been characterized, there is no general agreement about the effect of turbulent flow on MR signal intensity. This study uses a fast scan gradient echo pulse sequence to evaluate nonpulsatile turbulent flow in two different models. The first model simulated flow in normal vascular structure. It generated nonpulsatile, laminar and turbulent flow in straight, smooth-walled Plexiglas tubes. The second model simulated flow through a vascular stenosis. It generated nonpulsatile, laminar, and turbulent flow through an orifice. Velocities and flow rates ranged from low physiologic to well above the physiologic range (velocity = .3 to 280 cm/second, flow rate from .15 to 40 L/minute). Transition from laminar to turbulent flow was observed with dye streams. Turbulent flow in straight, smooth-walled vessels was not associated with a decrease in MR signal intensity even at the highest velocities and flow rates studied. The transition from laminar to turbulent flow through an orifice is not associated with a decrease in gradient echo signal intensity. As the intensity of the turbulent flow increases, however, there is a threshold above which signal intensity decreases linearly as turbulence increases (r = .97). This study suggests that flow in normal vascular structures should not be associated with decreased signal intensity in gradient echo images. Turbulent flow through areas such as valves, valvular lesions or vascular stenoses, may be associated with a decrease in gradient echo signal intensity.

Short data-acquisition times improve projection images of lung tissue.

MR images of laboratory rat lungs that resolve the thin membranes that separate lung lobes are presented. It appears that the capabilities of in vivo small‐animal pulmonary MRI may rival those of in vivo small‐animal X‐ray CT. Free induction decay (FID)‐projection imaging was employed with particular attention to the choice of acquisition time. For a given nominal resolution, one obtains optimal point discrimination when the acquisition time Tacq normalized by the signal decay time constant T  2* is approximately 0.8–0.9, although a better signal‐to‐noise ratio (SNR) is obtained when this quotient is 1.6. Currently available equipment should be able to even exceed the results presented herein. Magn Reson Med 57:1058–1064, 2007.

Volume of rat lungs measured throughout the respiratory cycle using 19F NMR of the inert gas SF6.

The lung volumes of mechanically ventilated rats were measured over the course of the respiratory cycle using the NMR signal strength from inhaled sulfurhexafluoride. Rats with elastase‐induced emphysema showed larger lung volumes and slower exhalation than control rats. For humans the technique should be able to provide lung volume measurements at least 20 times a second. Magn Reson Med 48:547–549, 2002.

Quantitative mapping of ventilation-perfusion ratios in lungs by 19F MR imaging of T1 of inert fluorinated gases.

A new method is presented for quantitative mapping of ventilation‐to‐perfusion ratios (VA/Q) in the lung: MRI of the 19F longitudinal relaxation time (T1) of an inert fluorinated gas at thermal polarization. The method takes advantage of the dependence of the 19F T1 on the local SF6 partial pressure, which depends on the local value of VA/Q. In contrast to hyperpolarized noble gases, with very long T1s, the T1 of SF6 in mammal lungs is 0.8–1.3 ms. Thus, rapid signal averaging overcomes the low thermal equilibrium polarization. T1 imaging of a phantom consisting of four different SF6/air mixtures with known T1 values validates the modified Look‐Locker T1 imaging sequence. To demonstrate the method in vivo, partial obstruction of the left bronchus was attempted in three rats; 3D free induction decay (FID)‐projection T1 images (2 mm isotropic resolution) revealed obstructed ventilation in two of the animals. In those images, ≈1700 lung voxels contained sufficient SF6 for analysis and T1 was determined in each voxel with a standard error of 8–10%. For comparison, independent VA/Q images of the same animals were obtained using a previously described SF6 MRI technique, and good agreement between the two techniques was obtained. Relative to the previous technique the resolution achieved using the T1 method is lower (for similar VA/Q precision and imaging time); however, the T1 method offers the potential advantages of eliminating the need for image coregistration and allowing patients with impaired lung function to breathe a 70% O2 gas mixture during the entire imaging procedure. Magn Reson Med 59:739–746, 2008.

Characterization of partially sintered ceramic powder compacts using fluorinated gas NMR imaging

We use nuclear magnetic resonance (NMR) imaging of C2F6 gas to characterize porosity, mean pore size, and permeability of partially sintered ceramic (Y-TZP Yttria-stabilized tetragonal-zirconia polycrystal) samples. Conventional measurements of these parameters gave porosity values from 0.18 to 0.4, mean pore sizes from 10 nm to 40 nm, and permeability from 4 nm(2) to 25 nm(2). The NMR methods are based on relaxation time measurements (T(1)) and the time dependent diffusion coefficient D(Delta). The relaxation time of C2F6 gas is longer in pores than in bulk gas and it increases as the pore sizes decrease. NMR yielded accurate porosity values after correcting for surface adsorption effects. A model for T(1) dependence on pore size that accounts for collisions between gas molecules and walls as well as surface adsorption effects is proposed. The model fits the experimental data well. Finally, the long time limit of D(Delta)/D(o), where D(o) is the bulk gas diffusion coefficient is useful for measuring tortuosity, while the short time limit was not achieved experimentally and could not be used for calculating surface-area to volume (S/V) ratios.

Studies of porous media by thermally polarized gas NMR: current status

Three examples of thermally polarized gas NMR performed at New Mexico Resonance are presented to demonstrate its unique advantages in porous media studies. 1) In-vivo animal lung imaging by Kuethe et al., in which useful quality 3D images of rat lungs were obtained in 30 min. It is conjectured that comparable human lung images would take much less time to make, possibly by the ratio of body weights, a factor of several hundred. 2) The success of the lung imaging suggested other porous media as candidates for thermally polarized gas NMR. Caprihan and coworkers obtained excellent images from partially sintered ceramics and Vycor glass. Since then, Beyea has developed the technique of spatially resolved BET curves for ceramics and other nanoporous solids. In this way, surface area, pore size, and porosity, averaged over an image voxel, can be spatially resolved. This greatly aids in the characterization of such materials, especially with regards to spatial heterogeneities. 3) Finally, we describe Codds propagator experiments on propane gas flowing through a packed bed of 300 microm beads. In order to increase signal-to-noise ratio, the flowing gas was pressurized to 170 kPa. Excellent quality propagators showing the discrete nature of the bead pack were obtained. This type of information is not available in comparable liquid studies because most spins will not diffuse far enough to sample the walls in the time available.

Velocity of mist droplets and suspending gas imaged separately

Nuclear Magnetic Resonance Images (MRIs) of the velocity of water droplets and velocity of the suspending gas, hexafluoroethane, are presented for a vertical and horizontal mist pipe flow. In the vertical flow, the upward velocity of the droplets is clearly slower than the upward velocity of the gas. The average droplet size calculated from the average falling velocity in the upward flow is larger than the average droplet size of mist drawn from the top of the pipe measured with a multi-stage aerosol impactor. Vertical flow concentrates larger particles because they have a longer transit time through the pipe. In the horizontal flow there is a gravity-driven circulation with high-velocity mist in the lower portion of the pipe and low-velocity gas in the upper portion. MRI has the advantages that it can image both phases and that it is unperturbed by optical opacity. A drawback is that the droplet phase of mist is difficult to image because of low average spin density and because the signal from water coalesced on the pipe walls is high. To our knowledge these are the first NMR images of mist.

Non-ferromagnetic retinal tacks are a tolerable risk in magnetic resonance imaging.

Should patients with cobalt alloy (ASTM F563) retinal tacks (Grieshaber cat. #611.95) in their eyes be subjected to the magnetic fields used in magnetic resonance imaging? Although the tacks are not ferromagnetic, they will experience a retarding torque when they are moved at the high angular velocities of human eye motion. Because retinal tacks are small (2.85 mm x 0.9 mm), the torque is difficult to measure. Rather, we measured the torque on a model 25.4 times larger and used a scaling law derived from Maxwells equations to calculate the force on the tack. The scaling law states that the torque varies with the cube of the objects length. To mimic the motion, models of retinal tacks were attached to Plexiglas rods and the assemblies were swung as pendulums. The pendulums were oriented in the magnetic field of a 1.5 T imager to experience the greatest retardation. Retarding torques were estimated from the rate of decrease of the pendulum amplitude, both inside and outside the magnet. Even if the retinal tacks were as conductive as 6061T6 aluminum alloy (25 MS/m) and the velocity of the surface of the eye were 24 cm/s (angular vel. of 1130 deg/s), the retarding torque would be only 1.6 times the weight of the tack acting with a lever arm as long as the distance from its tip to its center of gravity. The maximum retarding torque on an implanted retinal tack in a 1.5 T magnet is similar to the torque produced by gravity alone acting on the tack and is a tolerable risk.(ABSTRACT TRUNCATED AT 250 WORDS)

Research results on biomagnetic imaging of the lung tumors

Recent results on the development and implementation of a novel technology for lung tumor detection and imaging is presented. This technology offers high-sensitivity imaging of magnetic nanoparticles to provide specific diagnostic images of early lung tumors and potential distant metastases. Recent developments in giant magnetostrictive (GMS) or magnetic shape memory (MSM) materials have led to the possibility of developing small, low-cost, room-temperature, portable, high-sensitivity, fiber-optic sensors capable of robustly detecting magnetic nanoparticles, without direct contact with the skin. Magnetic nanoparticles are conjugated with antibodies, which target them to lung tumors. A prototype fiber-optic biomagnetic sensor, based on giant magnetostrictive or magnetic shape memory materials, with the requisite sensitivity to image the magnetic signals generated by antibody-labeled magnetic nanoparticles in lung tumors has been built and calibrated. The uniqueness of the biomagnetic sensor lies in the fact that it offers high sensitivity at room temperature, and is not a SQUID-based system. The results obtained during the process of choosing the right magnetostrictive materials are presented. Then, for the construction of an accurate image of the lung tumor, the optimum spatial distribution of one-channel sensors and nanoparticle polarization has been analyzed.