Resonator coils for magnetic resonance imaging at 6 MHz.

The Alderman and Grant design of the slotted tube resonator has been modified with a different lumped capacitor distribution to obviate the need for a balanced electrical feed at low frequencies (6 MHz). A saddle coil resonator and a strip line resonator have been tuned with similar lumped capacitor distributions to obtain electrically balanced resonance.

Deconvolution techniques for removing the effects of chemical shift in 19F nuclear magnetic resonance imaging of perfluorocarbon compounds.

Nuclear magnetic resonance (NMR) imaging of perfluorocarbon (PFC) emulsions and neat liquids has shown potential for in vivo oxygen imaging in blood and organ tissue. PFC compounds exhibit complicated NMR spectra caused by chemical shifts and spin-spin couplings which can lead to artifacts and degraded spatial resolution of resulting NMR images. To correct for the chemical shift artifacts, the technique of spectral deconvolution has been applied to NMR imaging of PFC compounds. The temporal filter for this process can be directly applied to raw free induction decay data in projection reconstruction or to spin-echo data in two-dimensional Fourier transform imaging techniques. The effect of chemical shift artifacts was demonstrated through the NMR imaging of two PFC compounds (F-tributylamine and F-decalin) in phantoms. Methods are presented and demonstrated which allow the chemical shift artifacts to be removed and true images of the spatial distribution of the PFC's to be recovered.

Three-dimensional imaging using a frequency-domain synthetic aperture focusing technique.

A frequency-domain method for implementing the synthetic aperture focusing technique is developed and demonstrated using computer simulation. As presented, the method is well suited to reconstructing ultrasonic reflectivity over a volumetric region of space using measurements made over an adjacent two-dimensional aperture. Extensive use is made of both one- and two-dimensional Fourier transformations to perform the temporal and spatial correlation required by the technique, making the method well suited to general-purpose computing hardware. Results are presented demonstrating both the lateral and axial resolution achieved by the method. The effect of limiting the reconstruction bandwidth is also demonstrated.

Acoustoelectric detection of ultrasound power with composite piezoelectric and semiconductor devices.

The acoustoelectric interaction between an ultrasound wave propagating in a thin piezoelectric plate and an adjacent semiconductor results in a current that is proportional to the ultrasound intensity. These devices, inherently insensitive to the phase of the acoustic wavefront, can be used for large-area total power meters as well as spot intensity meters. They have broadband response and are capable of following the envelope of typical diagnostic imaging pulses. A previously derived model describing the interaction is reviewed. Experimental results obtained with several detectors are presented, including sensitivity and frequency response.

Deconvolution of chemical shift spectra in two- or three-dimensional [19F] MR imaging.

The chemical shift spectra of 19F in perfluorinated compounds (PFCs) present a nontrivial impulse response function for magnetic resonance (MR) imaging. The 19F images of organs containing PFCs can be degraded by blurring and ghost image artifacts. Two methods (noise masked deconvolution and maximum entropy deconvolution) are presented that allow the chemical shift spectra of 19F in PFCs to be used to extract high quality MR images free of chemical shift artifact. Both techniques rely on postprocessing of either the raw data or the original image to produce images that are not degraded by the chemical shift spectra of the compound being imaged and that exhibit a signal-to-noise ratio equal to or better than that observed in the original image. The techniques are general in that they can be used with many PFC spectra. Using MR imaging data obtained from phantoms filled with cis/transperfluorodecalin and perfluorotributylamine (FC-43), the methods are compared in terms of their (a) ability to eliminate the chemical shift artifact associated with the PFC spectrum; (b) signal-to-noise performance; and (c) ability to preserve information related to the density and the longitudinal relaxation rate of the resonant nuclei. The utility of these techniques is demonstrated by a series of three-dimensional Fourier transform in vivo images of FC-43 emulsion in a mouse liver.

MR imaging of the lung using liquid perfluorocarbons.

Certain perfluorocarbon (PFC) compounds, commonly used as the oxygen transport components of "blood substitutes," may be breathed as neat liquids with survival because of their chemical inertness and their high solubility for oxygen and carbon dioxide. In addition, the paramagnetism of oxygen reduces the fluorine T1 value according to an inverse relationship allowing a potential method of monitoring PO2 gradients in vivo. This article presents the results of magnetic resonance (MR) imaging of the lungs of mice and rats following breathing of four PFC liquids (FC-43, FC-75, PFOB, APF-215). The images presented were obtained at two magnetic field strengths (0.66 and 0.14 T) under conditions of breathing either ambient air or pure oxygen. Spin-lattice relaxation times (T1) for the PFCs are measured both in vitro and in vivo (in the lungs) as a function of the state of oxygenation. A MR image signal strength enhancement of up to 90% is demonstrated in vivo under conditions of pure oxygen breathing.

Magnetic resonance imaging of congenital hydrocephalus in the rat.

Magnetic resonance imaging (MRI) is an increasingly available technique in clinical medicine for the noninvasive imaging of soft tissues. The purpose of the present study was to demonstrate the potential utility of MRI in experimental toxicology and teratology studies. The progression of severity of prenatally induced hydrocephalus was observed in rat pups from 1 to 4 weeks of age. Pregnant Sprague-Dawley rats were given 0, 15, 30, or 45 mg/kg ethylenethiourea (ETU), po, on Gestation Day 15. The two higher doses have been reported to induce a high incidence of hydrocephalus, which is mild at birth but becomes extensive by 4 weeks of age. The low dose was a no effect level for hydrocephalus. None of the doses of ETU altered birth weight or litter size. Pups from each dose group were imaged serially, on Postnatal Days (PD) 6, 13, 17, and 27, in order to determine the progression in the severity of hydrocephalus. Littermates were also imaged on each of these days, then killed immediately in order to compare the anatomy of the brain with its MR image. Hydrocephalus was detectable in the images from all animals of the 30 and 45 mg/kg dose groups on PD 6, the earliest observation day. At this time, the lateral ventricles were dilated less than 1 mm. Hydrocephalus became increasingly severe, and by 4 weeks of age all of the 45 mg/kg group and approximately half of the 30 mg/kg group had died. The brains of the surviving 30 mg/kg rats were severely hydrocephalic, with little cortex remaining. In all cases, the MR image corresponded precisely with the brain anatomy observed after termination. We have demonstrated that MRI is a useful technique for noninvasively imaging lesions in experimental animals. A number of other potential uses for MRI in toxicology are presented.