Faculty Bibliography

This study describes the fabrication and characterization of a targeted polymer-based contrast agent for multi-modal ultrasound and magnetic resonance imaging, providing the potential for cell labeling and tracking in vivo. Ultrasound and MRI contrast enhancing particles (UMCEP) were fabricated using a biodegradable polymer, poly(lactic-co-glycolic) acid (PLGA) particles encapsulating octafluoropropane and monocrystalline iron-oxide nanoparticles (MION). Targeting was achieved using an avidin-palmitate stabilizer which provided a high density of sites for conjugation of biotinylated tigands. Particle size analysis of the UMCEPs revealed a heterogeneous distribution of particles sizes with a mean diameter close to 1 mu m (0.88 mu m +/- 0.03, s.e.m) and scanning electron micrographs (SEMI) showed a pronounced increase in surface roughness of avidin-coated UMCEPs compared to plain UMCEPs. UMCEPs caused a nine-fold increase in signal intensity compared to background at 40MHz and higher harmonics (80 MHz and 120 MHZ peaks) were observed in the frequency spectra at multi-cycle, medium pressure excitation regimens. MRI phantom studies with UMCEPs showed T2-weighted signal darkening, with a measured relaxivity of 7.4s(-1) at a concentration of 4mg/ml of particles. Cell culture experiments with UMCEPs conjugated to human transferrin showed a two-fold increase in uptake of targeted UMCEPs by TfR expressing (TfR+) cells (3.46 +/- 1.21 s.e.m particles/cell) compared to TfR negative (TfR-, control) (1.66 +/- 0.32 s.e.m particles/cell). Polymer particles are an attractive option for multi-modal contrast imaging from the point of view of particle size, targeting function and the ability to use them as non-viral drug and gene delivery vehicles.

Ultrasound biomicroscopy (UBM) has emerged as an important in vivo imaging approach for analyzing normal and genetically engineered mouse embryos. Current UBM systems use fixed-focus transducers, which are limited in depth-of-focus. Depending on the gestational age of the embryo, regions-of-interest in the image can extend well beyond the depth-of-focus for a fixed-focus transducer. This shortcoming makes it particularly problematic to analyze 3-D data sets and to generate accurate volumetric renderings of the mouse embryonic anatomy. To address this problem, we have developed a five-element, 40-MHz annular array transducer and a computer-controlled system to acquire and reconstruct fixed- and array-focused images of mouse embryos. Both qualitative and quantitative comparisons showed significant improvement with array-focusing, including an increase of 3 to 9 dB in signal-to-noise ratio and an increase of at least 2.5 mm in depth-of-focus. Volumetric-rendered images of brain ventricles demonstrated the clear superiority of array-focusing for 3-D analysis of mouse embryonic anatomy

Current high frequency ultrasound imaging,or ultrasound biomicroscopy (UBM) systems have the capability of imaging mouse embryos with a frame rate of up to 200 frames per second. 3D datasets can also be acquired with the appropriate hardware. Because of the limited depth of field (DOF), effective volumetric analysis is limited to approximately 2 mm about the passive focus for a fixed-focus transducer operating at 40 MHz. This shortcoming imposes a size limit which the current technology can effectively be used to segment out accurate volumetric anatomy of the embryonic mouse. Previously we reported the fabrication a five-element 40-MHz annular array and an array imaging system. Wire phantom measurements with this array reveals an increase in DOF from 1-2 mm (fixed-focus) to more than 10 mm with array focusing. When imaging attenuating media such as mouse embryos, it is expected that the figures of merit for the array images, such as the signal to noise ratio (SNR) and the depth of field (DOF) will be reduce. Images from mouse embryos from at gestational ages 11 (E11.5) and 13 (E13.5) were acquired with this annular array system, and show the superior image definition and quality over the fixed-focus images. Quantitative estimates of the SNR and DOF were calculated, and segmentation of the brain ventricles was accomplished for both gestational ages

The effect of a copper-clad polyimide (CCP) backing layer on piezopolymer transducer performance is evaluated. High-frequency, spherically curved polyvinylidene fluoride (PVDF) transducers with and without a CCP backing layer are electrically and acoustically tested. The results showed very similar operating characteristics. B-mode in vivo images of a mouse embryo also showed no qualitative differences, indicating that the CCP backing layer does not affect transducer performance

Recently there has been growing interest in the development and use of iron-based contrast agents for cellular imaging with MRI. In this study we investigated coexpression of the transferrin receptor and ferritin genes to induce cellular contrast in a biological system. Expression of transgenic human transferrin receptor and human ferritin H-subunit was induced in a stably transfected mouse neural stem cell line. When grown in iron-rich medium, the transgenic cells accumulated significantly more iron than control cells, with a trend toward an increase in reactive oxygen species, but no detrimental effects on cell viability. This cellular iron significantly increased the transverse relaxivities, R2 and R2*, at 1.5 T and 7 T. By comparing measurements in the same cell samples at 1.5 T and 7 T, we confirmed the expected increase in relaxivity with increasing field strength. Finally, supplemented transgenic cells transplanted into mouse brain demonstrated increased contrast with surrounding neural tissue on T2*-weighted MR brain images compared to controls. These results indicate that dual expression of proteins at different critical points in the iron metabolism pathway may improve cellular contrast without compromising cell viability

There are currently no noninvasive imaging methods available for auditory brain mapping in mice, despite the increasing use of genetically engineered mice to study auditory brain development and hearing loss. We developed a manganese-enhanced MRI (MEMRI) method to map regions of accumulated sound-evoked activity in awake, normally behaving mice. To demonstrate its utility for high-resolution (100-mum) brain mapping, we used MEMRI to show the tonotopic organization of the mouse inferior colliculus. To test its efficacy in an experimental setting, we acquired data from mice experiencing unilateral conductive hearing loss at different ages. Larger and persistent changes in auditory brainstem activity resulted when hearing loss occurred before the onset of hearing, showing that early hearing loss biases the response toward the functional ear. Thus, MEMRI provides a sensitive and effective method for mapping the mouse auditory brainstem and has great potential for a range of functional neuroimaging studies in normal and mutant mice

This paper investigates the feasibility of fabricating a five-ring, focused annular array transducer operating at 40 MHz. The active piezoelectric material of the transducer was a 9-microm thick polyvinylidene fluoride (PVDF) film. One side of the PVDF was metallized with gold and forms the ground plane of the transducer. The array pattern of the transducer and electrical traces to each annulus were formed on a copper-clad polyimide film. The PVDF and polyimide were bonded with a thin layer of epoxy, pressed into a spherically curved shape, then back filled with epoxy. A five-ring transducer with equal area elements and 100-microm kerfs between annuli was fabricated and tested. The transducer had a total aperture of 6 mm and a geometric focus of 12 mm. The pulse/echo response from a quartz plate located at the geometric focus, two-way insertion loss (IL), complex impedance, electrical crosstalk, and lateral beamwidth all were measured for each annulus. The complex impedance data from each element were used to perform electrical matching, and the measurements were repeated. After impedance matching; fc approximately equal to 36 MHz and -6-dB bandwidths ranged from 31 to 39%. The ILs for the matched annuli ranged from -28 to -38 dB