Characterizing blood volume fraction (BVF) in a VX2 tumor.

OBJECTIVES: Neovascular proliferation of a tumor's blood supply is an important precursor of malignant growth. Evaluation of blood volume may provide useful information for the characterization, prognosis and response of tumors to therapy. The purpose of this study was to determine and compare the blood volume of tumor tissue measured noninvasively by MRI and microbubble contrast ultrasound imaging. MATERIALS AND METHODS: Twenty-two rabbits injected with VX2 tumors were studied. The blood volume fraction in tumor and muscle tissue was obtained from MRI T(1)-weighted images using a blood-pool agent, Clariscan, and by ultrasound using Definity and pulse inversion imaging. RESULTS AND CONCLUSIONS: Similar results were obtained from MRI and ultrasound. Estimation of the blood volume in tissue in the rim of a VX2 tumor 1.5 to 5.0 cm in diameter relative to that in the surrounding muscle was (mean+/-S.D.) 3.31+/-1.43 by MRI and 2.99+/-1.83 by ultrasound. The blood volume in the tissue relative to the total tissue volume (relative blood volume fraction) measured by MRI was 13+/-4.1% in tumor versus 4+/-1.4% in muscle (P<.01). Our data also suggested that, compared to the distribution volume of an extracellular contrast agent, Gd-DTPA, Clariscan as an intravascular agent demonstrated high-quality depictions of vascular structure of the tumor.

Detecting microcirculatory changes in blood oxygen state with steady-state free precession imaging.

Recently, it has been demonstrated that oxygen-weighted images of whole blood can be obtained with steady-state methods. In this article, based on computational and experimental models, we investigate the potential for employing this technique to monitor oxygen changes in microcirculation. Results show that oxygen-sensitive images of rabbit kidney and muscle may be obtained at high signal-to-noise ratio within a few seconds. The results also show that in steady-state free precession imaging, in addition to the exchange mechanism that generates oxygen contrast in blood, there are additional mechanisms that provide oxygen-sensitive contrast in microcirculation.

Oxygen-sensitive contrast in blood for steady-state free precession imaging.

Steady-state free precession (SSFP) methods have gained widespread recognition for their ability to provide fast scans at high signal-to-noise ratio. This paper demonstrates that such methods are also capable of reflecting functional information, particularly blood oxygenation state. It is well known that SSFP signals show substantial sensitivity to small off-resonance frequency variations. However, that mechanism cannot explain the oxygen-sensitive contrast in blood that was observed with steady-state methods using phase-cycled radiofrequency pulses. From theoretical and experimental models it is demonstrated that the mechanism responsible for such contrast originates from the motion of spins through local field inhomogeneities in and around deoxygenated red blood cells. In addition, this work shows that it is critical to choose the scan parameters carefully for robust oxygen-sensitive contrast. Finally, it is demonstrated that it is possible to build a quantitative model that incorporates the Luz-Meiboom model, which had been used in the past to estimate quantitative measures of vascular blood oxygen levels. It is envisioned that this method could be instrumental in real-time imaging focused on detecting diseases where the oxygen state of blood is impaired.

The response of vestibulo-ocular reflex pathways to electrical stimulation after canal plugging.

The vestibulo-ocular reflex (VOR) allows clear vision during head movements by generating compensatory eye movements. Its response to horizontal rotation is reduced after one horizontal semicircular canal is plugged, but recovers partially over time. The majority of VOR interneurons contribute to the shortest VOR pathway, the so-called three-neuron arc, which includes only two synapses in the brainstem. After a semicircular canal is plugged, transmission of signals by the three-neuron arc originating from the undamaged side may be altered during recovery. We measured the oculomotor response to single current pulses delivered to the vestibular labyrinth of alert cats between 9 h and 1 month after plugging the contralateral horizontal canal. The same response was also measured after motor learning induced by continuously-worn telescopes (optically induced motor learning). Optically induced learning did not change the peak velocity of the evoked eye movement (PEEV) significantly but, after a canal plug, the PEEV increased significantly, reaching a maximum during the first few post-plug days and then decreasing. VOR gain also showed transient changes during recovery. Because the PEEV occurred early in the eye movement evoked by a current pulse, we think the observed increase in PEEV represented changes in transmission by the three-neuron arc. Sham surgery did not result in significant changes in the response to electrical stimulation or in VOR gain. Our data suggest that different pathways and processes may underlie optically induced motor learning and recovery from plugging of the semicircular canals.