Blood-oxygen-level-dependent imaging

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Blood-oxygen-level-dependent imaging, or BOLD-contrast imaging, is a method used in functional magnetic resonance imaging (fMRI) to observe different areas of the brain or other organs, which are found to be active at any given time. [1]

Contents

Theory

Neurons do not have internal reserves of energy in the form of sugar and oxygen, so their firing causes a need for more energy to be brought in quickly. Through a process called the haemodynamic response, blood releases oxygen to active neurons at a greater rate than to inactive neurons. This causes a change of the relative levels of oxyhemoglobin and deoxyhemoglobin (oxygenated or deoxygenated blood) that can be detected on the basis of their differential magnetic susceptibility.

In 1990, three papers published by Seiji Ogawa and colleagues showed that hemoglobin has different magnetic properties in its oxygenated and deoxygenated forms (deoxygenated hemoglobin is paramagnetic and oxygenated hemoglobin is diamagnetic), both of which could be detected using MRI. [2] This leads to magnetic signal variation which can be detected using an MRI scanner. Given many repetitions of a thought, action or experience, statistical methods can be used to determine the areas of the brain which reliably have more of this difference as a result, and therefore which areas of the brain are most active during that thought, action or experience.

Criticism and limitations

Although most fMRI research uses BOLD contrast imaging as a method to determine which parts of the brain are most active, because the signals are relative, and not individually quantitative, some question its rigor. Other methods which propose to measure neural activity directly have been attempted (for example, measurement of the Oxygen Extraction Fraction, or OEF, in regions of the brain, which measures how much of the oxyhemoglobin in the blood has been converted to deoxyhemoglobin [3] ), but because the electromagnetic fields created by an active or firing neuron are so weak, the signal-to-noise ratio is extremely low and statistical methods used to extract quantitative data have been largely unsuccessful so far.

The typical discarding of the low-frequency signals in BOLD-contrast imaging came into question in 1995, when it was observed that the "noise" in the area of the brain that controls right-hand movement fluctuated in unison with similar activity in the area on the opposite side of the brain associated with left-hand movement. [1] BOLD-contrast imaging is only sensitive to differences between two brain states, [4] so a new method was needed to analyse these correlated fluctuations called resting state fMRI.

History

Its proof of concept of blood-oxygen-level-dependent contrast imaging was provided by Seiji Ogawa and Colleagues in 1990, following an experiment which demonstrated that an in vivo change of blood oxygenation could be detected with MRI. [5] In Ogawa's experiments, blood-oxygen-level-dependent imaging of rodent brain slice contrast in different components of the air. At high magnetic fields, water proton magnetic resonance images of brains of live mice and rats under anesthetization have been measured by a gradient echo pulse sequence. Experiments shown that when the content of oxygen in the breathing gas changed gradually, the contrast of these images changed gradually. Ogawa proposed and proved that the oxyhemoglobin and deoxyhemoglobin is the major contribution of this difference. [6]

Other notable pioneers of BOLD fMRI include Kenneth Kwong and colleagues, who first used the technique in human participants in 1992. [7]

See also

Related Research Articles

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<span class="mw-page-title-main">Functional magnetic resonance imaging</span> MRI procedure that measures brain activity by detecting associated changes in blood flow

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<span class="mw-page-title-main">Haemodynamic response</span>

In haemodynamics, the body must respond to physical activities, external temperature, and other factors by homeostatically adjusting its blood flow to deliver nutrients such as oxygen and glucose to stressed tissues and allow them to function. Haemodynamic response (HR) allows the rapid delivery of blood to active neuronal tissues. The brain consumes large amounts of energy but does not have a reservoir of stored energy substrates. Since higher processes in the brain occur almost constantly, cerebral blood flow is essential for the maintenance of neurons, astrocytes, and other cells of the brain. This coupling between neuronal activity and blood flow is also referred to as neurovascular coupling.

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<span class="mw-page-title-main">Seiji Ogawa</span> Japanese researcher (born 1934)

Seiji Ogawa is a Japanese biophysicist and neuroscientist known for discovering the technique that underlies Functional Magnetic Resonance Imaging (fMRI). He is regarded as the father of modern functional brain imaging. He determined that the changes in blood oxygen levels cause its magnetic resonance imaging properties to change, allowing a map of blood, and hence, functional, activity in the brain to be created. This map reflected which neurons of the brain responded with electrochemical signals to mental processes. He was the first scientist who demonstrated that the functional brain imaging is dependent on the oxygenation status of the blood, the BOLD effect. The technique was therefore called blood oxygenation level-dependent or BOLD contrast. Functional MRI (fMRI) has been used to map the visual, auditory, and sensory regions and moving toward higher brain functions such as cognitive functions in the brain.

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References

  1. 1 2 E. Raichle, Marcus (2010). "The Brain's Dark Energy". Scientific American. 302 (3): 44–49. Bibcode:2010SciAm.302c..44R. doi:10.1038/scientificamerican0310-44. PMID   20184182. The fMRI signal is usually referred to as the blood-oxygen-level-dependent (BOLD) signal because the imaging method relies on changes in the level of oxygen in the human brain induced by alterations in blood flow.
  2. ↑ Chou, I-han. "Milestone 19: (1990) Functional MRI". Nature. Retrieved 9 August 2013.
  3. ↑ Yablonskiy, Dmitriy A.; Haacke, E. Mark (1994). "Theory of NMR signal behavior in magnetically inhomogeneous tissues: The static dephasing regime". Magnetic Resonance in Medicine. 32 (6): 749–763. doi:10.1002/mrm.1910320610. PMID   7869897.
  4. ↑ Langleben, Daniel D. (1 February 2008). "Detection of deception with fMRI: Are we there yet?". Legal and Criminological Psychology. 13 (1): 1–9. doi:10.1348/135532507X251641.
  5. ↑ Raichle, ME (3 February 1998). "Behind the scenes of functional brain imaging: a historical and physiological perspective". Proceedings of the National Academy of Sciences of the United States of America. 95 (3): 765–72. Bibcode:1998PNAS...95..765R. doi: 10.1073/pnas.95.3.765 . PMC   33796 . PMID   9448239. Ogawa et al. were able to demonstrate that in vivo changes blood oxygenation could be detected with MRI.
  6. ↑ OGAWA, SEIJI (1990). "Oxygenation-sensitive contrast in magnetic resonance image of rodent brain at high magnetic fields". Magnetic Resonance in Medicine. 14 (1): 68–78. doi:10.1002/mrm.1910140108. PMID   2161986. S2CID   12379024.
  7. ↑ Roche, Richard A.P.; Commins, Seán; Dockree, Paul M. (2009). "Cognitive neuroscience: introduction and historical perspective". In Roche, Richard A.P.; Commins, Seán (eds.). Pioneering studies in cognitive neuroscience. Maidenhead, Berkshire: McGraw Hill Open University Press. p. 11. ISBN   978-0335233564.
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