Design and initial evaluation of a low‐cost 3‐Tesla research system for combined optical and functional MR imaging with interventional capability

Abstract

A 3-Tesla research system has been developed for functional and interventional magnetic resonance imaging (MRI) procedures on animal models based on a low field niche spectrometer. Use of two stages of fourth harmonic frequency multiplication has allowed us to produce a highfrequency spectrometer with good frequency stability based on a low-frequency direct digital synthesizer. The system has been designed with the ability to introduce interventional tools such as biopsy needles, radiofrequency (RF) electrodes, and fiber optics for optical spectroscopy and thermal ablation as well as drug infusions to allow function to be studied in the presence of external challenges. Full MR-compatible physiologic support capability allows animals to be maintained in a stable condition over extended periods of study. Functional MR images have been acquired by using gradient echoes (TR/TE ‫؍‬ 40/12 msec) from the rat whisker barrel cortex using electrical stimulation (5-V, 1.5-mA, 1-msec pulses at 5 Hz via two needle electrodes inserted into the rat whisker pad). Initial results using respiratory gas challenges of 100% N 2 , 100% O 2 , and 10% CO 2 have shown excellent agreement between single wavelength (633 nm) optical and functional MR time series with subsecond time resolution. The 1-mm copper electrodes for interventional radiofrequency ablation procedures were easily visualized in the superior colliculus by using gradient echo sequences. This novel, lowcost, high field system appears to be a useful research tool for functional and interventional studies of rat brain and allows concurrent optical spectroscopy. J. Magn. Reson. Imaging 2001;13:87-92.MOST INTERVENTIONAL STUDIES to date have focused on use of low-field systems because of availability of open magnets, which provide good access to patients. However, many new techniques that could be of advantage for monitoring the effects of interventional MR require high-field systems and fast imaging techniques. Interventional MRI research at high field (Ն3 Tesla) in humans should really be preceded by careful animal experimentation in, for example, the rodent model.Research into interventional and functional MRI (fMRI) in rodents is more challenging than that in humans because of the much smaller volume of the rodent brain (1-2 mL compared with 1500 mL for the human brain), requiring use of very small voxels and hence high field strengths to achieve adequate signal-to-noise ratio.We have previously developed low field-dedicated MR systems for extremity and neonatal research, which have proved to be both cost effective and clinically useful (1). The objective of this project was to adapt this dedicated low-frequency MR technology for use with high-field magnets and high-performance gradients. This simple modular add-on design may be of use for converting older low-frequency spectrometers for use with higher field magnets as interest grows in applications of fMRI. Our overall aim is to compare high temporal and spatial resolution optical and fMRI and spectroscopic studies of the...

Intraoperative Human Functional Brain Mapping Using Optical Intrinsic Signal Imaging

“…For example, combined OISI-fMRI studies have also contributed to our knowledge of the etiology of these signals [101,[130][131]. The recent development of a system that allows simultaneous fMRI and OIS spectroscopy [132] promises to further this endeavor. This combination has also been used to test the clinical utility of intraoperative human OISI for neurosurgical guidance by comparing intraoperative OISI maps with pre-surgical fMRI maps [101,[130][131].…”

Method for rapid MRI needle tracking

“…5) Find the needle shift parameters (X 0 , Y 0 ) that minimize the phase cost function (Eq. [5]). 6) Display the needle image or tip (the edge closer to the object center) as a graphic overlay on the reference image.…”

“…Such procedures usually involve the insertion of a needle into a target organ. Needle insertion may be used for biopsy (2), fine-needle aspiration (FNA) (3), and thermal treatment or ablation (4,5). It is essential to monitor the needle in real time in such cases, and thus rapid imaging techniques are implemented.…”