1.   What are the differences between the magnetic properties of oxygenated and deoxygenated hemoglobin?

                 -oxygenated hemoglobin is diamagnetic which means it has no unpaired elections and zero magnetic moment; deoxygenated hemoglobin is paramagnetic which means it has both unpaired electrons and a significant magnetic moment.

 

2.   What time constant is affected by the oxygenation level of blood, as demonstrated by Thulborn and colleagues?

                

         - Relaxation time or T2

 

3.   What do the terms “diamagnetic,” “paramagnetic,” and “ferromagnetic” mean?

 

         Diamagnetic: having the property of a weak repulsion from a magnetic field.   It has no unpaired electrons and zero magnetic moment.

 

         Paramagnetic: having the property of being attracted to a magnetic field, though with less concentration of magnetic flux than ferromagnetic objects. It has both unpaired electrons and a significant magnetic moment.

 

        Ferromagnetic: any material that could exhibit spontaneous magnetization: a net magnetic moment in the absence of an external magnetic field

 

4.   How does positron emission tomography (PET) imaging work? 

 5.        What are the advantages and disadvantages of PET as compared to MRI/fMRI?

            Advantages of PET: silent environment, large signal to noise ratio, more tolerant of subject movement, capacity for metabolism study, people with metal in their bodies can do it, claustrophobia is not a problem, there is no inhomogeneity artifacts of MRI. 

            Advantages of fMRI/MRI: no radioactivity involved, subjects can be scanned repeatedly, slightly better spatial resolution than PET, Faster imaging time than PET, Gives better single-subject data, functional and structural scans can be done in one session.

            The disadvantages are just the opposite of the advantages. Main disadvantage for PET is radiation exposure to human subjects and poor spatial and temporal resolution. Main disadvantage of MRI/fMRI is inhomogeneity artifacts and subject movement which decreases signal to noise ratio.

6.         What does the acronym “BOLD” represent?

                        blood-oxygenation-level dependent

 

 

7.      What causes BOLD contrast? Describe the mechanism for BOLD in as much detail as possible. BOLD contrast is due to the difference in signal on T2 weighted images as a function of the amount of deoxygenated hemoglobin.

1. Neuronal activity causes increased metabolic demands and thus, increased oxygen consumption. This increases the amount of deoxygenated hemoglobin, given a constant blood flow.

8. How did Ogawa and colleagues demonstrate the existence of BOLD contrast? Describe some of their early experiments.

a.        They scanned anesthetized rodents using high field (7 T and greater) MRI. When rodents were breathing 100% oxygen, gradient-echo images of their brains showed structural differences but few blood vessels. When the rodents breathed normal air (21% oxygen), thin dark lines became visible throughout the cerebral cortex. If the oxygen level was reduced to 0%, the visible lines became even more prominent. These thin lines represent magnetic susceptibility effects caused by the presence of paramagnetic deoxygenated hemoglobin in blood vessels. In other conditions, because the hemoglobin was bound to oxygen, it was diamagnetic and thus, had little effect on the surrounding magnetic field.

 

 

 

9.   Why do Malonek and Grinvald refer to the BOLD response as “watering the entire garden for the sake of one thirsty flower”?  because oxygenated blood is distributed to more of the brain than just the active parts

 

 

 

10. What potential role might astrocytes have in supporting neuronal signaling? What energy costs might this incur?

            Astrocytes are star-shaped. Their many processes envelope synapses made by neurons. They perform many functions, including biochemical support of endothelial cells which form the blood-brain barrier  the provision of nutrients to the nervous tissue, and a principal role in the repair and scarring process in the brain. The energy cost is 2 ATP per glutamate molecule. 

 

 

11. According to the model of Buxton, how would decreased blood transit time through capillaries affect oxygen extraction? The amount of oxygen extracted is proportional to transit time through the capillary bed. Thus, as blood velocity increases, the transit time decreases. 

 

 

12. Why must there be an uncoupling of oxygen supply and oxygen consumption for BOLD contrast to be useful for functional neuroimaging?

                         fMRI is based on the detection at the macroscopic level of changes in the microscopic magnetic fields surrounding red blood cells. Going from oxygenated to deoxygenated changes its magnetic properties and thus, is the reason we can see contrast.

 

13. What is the initial dip? Why has its existence been difficult to demonstrate conclusively? 

               Initial dip = initial increase in deoxyhemoglobin. The initial dip is hard to demonstrate conclusively because it isn’t frequently observed as it is rare to have high-field (>4T) MR scanners for functional studies. 

 

14. Why might the initial dip have better functional resolution than the traditional positive BOLD response? The initial dip shows better spatial specificity. 

 

 

15. Why might exogenous contrast agents be used for fMRI?

                        In the earliest fMRI studies, exogenous contrasts – chemicals injected into the bloodstream of the subject – were used to obtain contrast. This increased blood perfusion to be detected. Thus, this increases signal-to-noise ratio.    

 

 

16. What were some of the characteristics of the early fMRI studies? What did they demonstrate? The first use of BOLD fMRI for functional mapping of the human brain used long-interval blocked design consistent with PET studies of its time. These studies looked at visual cortex activity and activity in the primary motor cortex.   

 

 

 

17. What was different about the 1992 study by Blamire and colleagues, as compared to those from other groups? 

a.       While most earlier fMRI studies used long stimulus durations, Blamire and colleagues used not only long stimulus durations, but also short stimulus durations. The stimulus durations were as short as 2s. 

 

 

 

18. What is the basic shape of the fMRI BOLD hemodynamic response?

 

            Varies: increasing rate of neuronal firing activity would increase HDR amplitude while increasing the duration of neuronal activity would increase HDR width. [yes but still need to tell us what basic shape is]

 

19. How long is the delay between stimulus onset and the peak of the hemodynamic response for short stimulus durations? 1-2 seconds [INCORRECT - tony]

 

 

 

20. What causes the poststimulus undershoot?

a.       The poststimulus undershoot is the decrease in MR signal amplitude below baseline due to the combination of reduced blood flow and increased blood volume.