Lis t of Tables
Background
One of the strengths of functional MRI in monkeys will be the integration of functional MR activation patterns with underlying neuronal firing. One of the goals of functional MRJ is to achieve resolution comparable to more invasive neuronal recording techniques. As described above, in bridging the gap between human functional MRI and monkey electrophysiology, one of the important comparisons to be made is between direct recordings of neuronal activity and functional MRI changes in the same.
One is to make simultaneous electrophysiology and fMRJ measurements while the animal is in the MR scanner. This is technologically very challenging as the material requirements for the two techniques are very different. Similarly, electrophysiology recordings are best achieved in an electrically and magnetically shielded environment, rather than in the presence of strong oscillating magnetic fields and eddy currents found inside an MR scanner.
C hapter 2 Magnetic Resonance Imaging in Macaque Cortex
Abstract
An MR-compatible design to restrain the monkey is also described together with a suitable EPI sequence for BOLD images, optimized for monkey T2, with voxel sizes of 29-65 ~Ll, and MPRAGE sequence for anatomical studies with 0.8 mm isotropic resolution, optimized for monkey T l.
Introduction
- Sti mulu s
- S urg ica l Techn iqu e and Expe rime nta l Se tup
The monkey was trained to lie supine with its head up (the “sphinx” position) in a model bore of a clinical MR scanner. The central post was placed with the monkey's head placed in a stereotaxic frame (Kopf instruments) with its long axis parallel to the axis of the head (cranio-caudal direction). The monkey's head was positioned in the center of the radio frequency (RF) coil for maximum SNR.
Results
The exact location of the activation as mapped on the anatomical MPRAGE image can be shifted by up to 1-2 pixels. Our paradigm was designed primarily to stimulate as much of the visual system as possible. Alternatively, the part of the image that captured the monkey's gaze (e.g., a face) may not have been the most potent VI stimulus, with V I receiving maximal stimulation from elsewhere in the image.
Conclusion
This paves the way for fMRI to be extended to include the study of the cortex of non-human primates. With a direct link to macaque electrophysiological studies now possible, we may also be able to gain further insight into the nature of the fMRl response. Optimization of parameter values for complex pulse sequences with simulated annealing: application to 3D MP-RAGE imaging of the brain.
ACknowledgment
Direct Comparison of Visual Cortex Activation in Human and Nonhuman Primates Using
- Introduction
- Stimulus Paradigm
- Discussion
- Conclus ion
The acrylic tube containing the monkey was then inserted into the M Rl machine (Figure 3.1), and the head cap was attached to the window in the receive/send coil. In the macaque, areas of activation have been manually subdivided into distinct retinotopic cortical regions from anatomical studiesl2,36. Areas of fMRI activation are clearly seen as discrete areas in the striated and extrastriated visual.
This technique provides high-resolution images of discrete areas of fMRl activation in the macaque monkey, which are comparable to visual fMRI studies in humans. The strong activation in the primary and secondary areas (VI, V2) of macaque retinotopic visual cortex (Figure 3.2) confirms that the cartoon movie stimulates visual pathways. The activation in the human visual cortex shows a similar pattern of retinotopic cortical activation, with ventral and dorsal stream activation.
There are asymmetries in the patterns of visual activation in both the macaque and human fMRI studies. Nevertheless, some asymmetry can be seen in the activation pattern in the macaque (e.g. dorsal VI in Figure 3.2d). However, this resolution turned out to be too coarse to define anatomical structures in the brains of small macaques.
The TI and T2 values of the macaque brain are similar to humans, and the functional distribution of activation in the visual pathways also appears to be homologous.
- Acknowledgment
Mapping Human Brain Function with Magnetoencephalography, Anatomical Magnetic Resonance Imaging, and Functional Magnetic Resonance Imaging. A direct demonstration of functional specialization within movement-related visual and auditory cortex in the human brain. Analysis of local and wide-field movements in the supratemporal visual areas of the macaque monkey.
Enhancing tMRI Contrast in Awake- behaving Primates Using Intravascular Magnetite
- Abstract
- Introduction
- Materials and Method
- Animal Subjects
- Stimulus Paradigm
- Results
- Discussion
- Conclusion
- References
- Acknowledgment
The time course of the percent change in MR1 signal in this volume was plotted for. There is a linear relationship between the transverse relaxation rate of the magnetite-dextran contrast medium in blood and the concentration of Fe. This model assumes that the effect of the decreased transverse relaxation rate due to BOLD is negligible compared to the increase due to CBY changes with the intravascular contrast agent.
Previous studies have shown that the rate of change of transverse relaxation rate with blood iron concentration is proportional to cerebral blood volume (i.e. cerebral blood volume CBY(t) can be calculated from the slope of the plot of R2* change with [Fe ]blood”), as is described in Equation 6. This can be written in terms of the ratio of the MR signal during photostimulation to that at rest from Equation I. This R2*(t) value was interpolated to the actual time point of the light stimulation measurements, R2*(r), to allow for changes in R2* due to hepatic Fe excretion (see Eq. 2).MR signal change, Set) after photostimulation was fitted to a gamma variable function described by Eq. 5 (Figure 4.2).
The duration of a positive LARGE change (measured between rising above 10% maximum signal change to falling below 10% maximum change) was 22 seconds (1-23 seconds after stimulus onset). The elimination half-life of Feridex (198 minutes) is of the same order of magnitude as the duration of many primate physiological studies. The variability of the MR signal change in Figure 4.2 is greater with magnetite-enhanced tMRl than with BOLD.
This is due to the reduction of the resting MR signal by the addition of magnetite-dextran.
- Abstract
- Introduction
- Experimental
- MRM Instrumentation
- Characterizing BOLD Contrast by Modulation of Inspired Oxygen Tension
- fMRI
- Results
- BOLD Contrast by Modulation of Oxygen Tension
- fMRI
- Discussion
- Relaxation Times
- Conclusions
First, we describe the results of in vivo measurements of the key magnetic resonance parameters T I, T 2 and T 2' in the mouse brain. Next, we describe experiments showing that the magnitude of BOLD contrast in the mouse brain can reach dramatic levels at 11.7 T; these experiments involved artificial modulation of the inspired oxygen tension. From these data a map of the relative sensitivity of various brain regions to controlled changes in blood oxygenation was derived.
The approximate location of the image slice shown in Figure 5.3 is indicated by a shaded line in Figure 5.4b. In addition, the need to work with anesthetized animals and the vertical orientation of the animals in the magnetic bore may affect the results. Part of the SNR loss can be compensated for by working at a higher magnetic field strength.
Two additional experimental considerations that may affect our results are the physiological effects of anesthesia and the vertical orientation of the mouse in the magnet bore. The second potential complication stems from the orientation of the animal in a vertical position in the magnet bore. In addition, new tools must be developed and refined for precise control of the mouse's physiology within the limits of the magnetic bore.
High-resolution magnetic resonance imaging of the brain in dy/dy mice with merosin-deficient congenital muscular dystrophy.
- Acknowledgment
Dynamic mapping at the laminar level of odor-evoked responses in the olfactory bulb of rats by functional MRI. Functional MRI of somatosensory activation in rats: effect of hypercapnic upregulation on perfusion and BOLD imaging. Physiological basis for BOLD MR signal changes due to hypoxia/hyperoxia: blood volume separation and magnetic susceptibility effects.
4 Tesla gradient recalled echo characteristics of photic stimulation-induced signal changes in the human primary visual cortex. Quantitative assessment of blood flow, blood volume, and blood oxygenation effects in functional magnetic resonance imaging. Magnetic resonance imaging (MRI) detection of murre brain responses to light: temporal differentiation and negative functional MRI changes.
The structural organization of layer IV in the somatosensory area (SI) of the mouse cerebral cortex. Vascular imprints of neuronal activity: relationships between cortical blood flow dynamics, oxygenation, and volume changes after sensory stimulation. The effect of bulk sensitivity on snapshot imaging of mice at 7.0 T: a comparison of snapshot imaging techniques.
Evidence for exchange of arterial spin-labeled water with tissue water in rat brain from diffusion-sensitive perfusion measurements.
Appendix A Improving Behavioral Control in Monkey fMRI Studies
- Behavioral Training I Reward
- References
To minimize these artifacts, the magnetic susceptibility of the material must be similar to the sample under investigation (in this case brain). The main disadvantage of this design is the time required to individually machine the bottom surface to the curvature of the monkey's head. To achieve the high level of immobilization and cooperation required for functional MRI studies. the monkey still requires a lot of training.
For training purposes, a simulator was built that incorporated many of the elements of the MRI scanner. The inset graph shows the time course of the MR signal for l) voxels in the normal cortex. Processing of the video signal was performed using a commercially available digital signal processing board (lscan, Cambridge, MA).
The inset (top panel) is a detail of one side of the LED·tiher intert'tee and (bottom panel) the final tiber bundle. The initial design used optical delivery of light to the eye via telecommunication infrared light emitting diodes (LEDs). The semi-silvered mirror was invisible to the subject, and the illuminated visible light of the stimulus reflected the infrared light of the illuminated eye (Figure A.12).
The CCD camera was placed on the head of the scanner and the visual stimulus on the leg (Figure A.II).
Appendix B Towards Combined Functional Magnetic Resonance Imaging and Electrophysiology
A procedure for using proton magnetic resonance imaging to determine stereotaxic coordinates of the monkey brain. Stereotaxic lesions of the hippocampus in monkeys: determination of surgical coordinates and analysis of lesions using magnetic resonance imaging.
Appendix C List of Abbreviations
