Chapter 1: An Introduction to fMRI

 

Study Questions

 

 

1. Describe the goals and methods of phrenology. What concept did the phrenologists introduce?

 

Phrenologists tried to map brain functions in accordance with the amount of brain tissue devoted to a cognitive function that influenced behavior. They assumed that increases in brain size would translate into measurable bumps on the skull. Phrenologists, despite collapsing on scientific grounds, were important because they introduced the idea of localization. Localization is the idea that the brain may have distinct regions that support particular mental processes.

 

 

2. What is functional magnetic resonance imaging?

 

fMRI is a neuroimaging technique that uses standard MRI scanners to investigate changes in brain function over time. From this, one can create images of the functional organization of the brain.

 

 

3. What are the limitations of lesion studies of the brain? How can functional neuroimaging help overcome these limitations?

 

A well-appreciated problem results from the network structure of the brain: The fact that damage to area X impairs behavior Y indicates that X is necessary for Y, but not that X is sufficient for Y. A given brain region may support more than one function, and each function may be supported by multiple brain regions. Another problem is finding patients with isolated brain damage. Many patients have diffused damage from strokes or trauma. In addition, a lesion's effects can diminish over time through healing or compensation by other brain regions. Trying to find the context in human lesion studies is difficult. However, by using fMRI in concert with TMS, a researcher can create temporary, localized lesions and avoid some of the above limitations.

 

 

 

4. What are the limitations of drug studies of the brain? How can functional neuroimaging help overcome these limitations?

 

A main disadvantage of drug studies is the difficulty in identifying functions of specific brain regions following systemic application of a drug. Also, many drug manipulations have relatively slow time courses, with functional changes that can take place over weeks, so inferences about short-term cognitive processes become challenging. Using animal and other techniques to identify localization and manipulation can help centralize the identifying functions of the regions.

 

 

 

5. What are the limitations of electrophysiological studies of the brain? How can functional neuroimaging help overcome these limitations?

 

Electrophysiological methods suffer from a trade-off between localization accuracy and invasiveness. Single-unit studies allow very precise localization of a activity to a specific cell in a specific brain region, but require the insertion of electrodes directly into the brain and are thus restricted to animal studies. While extra-cranial EEG and MEG studies do not damage the brain, it is mathematically impossible to uniquely identify the locations of the neural sources that cause a given pattern of activity on the skull (the inverse problem). Animal usage and complimentary techniques, such as combining fMRI and EEGin one study, can help overcome this limitation.

 

 

 

6. To what aspect of imaging does “contrast” refer? How could a single image be high-contrast in one sense and low-contrast in another?

 

Contrast is the intensity difference between different quantities being measured by an imaging system. It also refers to the physical quantity being measured. Contrast is expressed with respect to variation in contrast due to noise and to discuss results in terms of the magnitude of the intensity different quantities divided by the variability in their measurements (contrast-to-noise ratio). Images are created that distinguish between active and non-active areas of the brain. Functional contrast provides information about a physiological correlate of brain function (deoxygenated blood). An image may have high contrast-to-noise despite small absolute intensity differences if there is very little variability within each property being measured.

 

 

 

7. What is the difference between structural contrast and functional contrast?

 

In structural contrast, intensity difference is based on the physical brain structure. Whereas in functional contrast, intensity difference is based on the physiological brain function (e.g. blood oxygenation change).

 

 

 

8. What are voxels?

 

Voxels are the basic sampling units of MRI, also known as a three-dimensional volume element.

 

 

9. What is functional resolution, and how is it different from spatial and temporal resolution?

 

Functional resolution is the ability to map measured physiological variation to underlying mental processes. It differs from spatial resolution, as it extends beyond spatial locations and from temporal resolution, as it is over periods of time. However, spatial and temporal resolution, along with other properties, are included in functional resolution.

 

 

10. What is resonance?

 

Resonance is tendency of a system to oscillate at maximum amplitude at a certain frequency. In brain imaging, magnetic resonance is the absorption of energy from a magnetic field that oscillates at a particular frequency. Resonant frequency is the oscillation that provides maximum energy transfer to the system.

 

 

11. Why did physicists use oscillating magnetic fields to study magnetic resonance effects?

 

If the frequency of the oscillating magnetic field matches the spin frequency of the atomic nucleus, then the nucleus would absorb energy from the field. The resonant frequency needed for the oscillating field depends upon the strength of the static magnetic field. Thus, the frequency of the oscillating field constant and the strength of the static field is changed by adjusting the current in the magnet.

 

 

 

12. Describe the experimental apparatus used by Felix Bloch and his colleagues to measure nuclear magnetic resonance effects.

 

Bloch places a sample of water in a brass box between the poles of a strong magnet, whose field strength they could manipulate. An adjacent transmitter coil sent electromagnetic energy into the sample, while a second detector coil was used to measure changes in the energy absorbed by the water (as emitted back to the environment). The sample was presaturated for 24 hours in the magnetic field to ensure that relaxation would occur. Bloch labeled the effects nuclear induction, and nuclear magnetic resonance (NMR).

 

 

13. How did Damadian and Lauterbur each contribute to the development of MRI?

 

Damadian hypothesized that similar differences might be observed in the water molecules between cancerous and noncancerous cells during NMR – thus, making NMR a cancer detector. It was the first clear biological application for NMR. Moreover, it created images providing information of how the quantities varied over space. Lauterbur recognized that NMR had considerable potential, if an image method could be developed. By measuring how much energy was emitted at different frequencies, one could identify how much of that object was present at each spatial location. The idea of inducting spatial gradients in the magnetic field proved to be the fundamental insight that led to the creation of MR images. He also realized that a single gradient could only provide information about one spatial dimension, thus the need for multiple gradients.

 

 

 

14. What is an image? What advance was most critical to the development of techniques for image formation in MRI?

 

An image is a visual description of how one or more quantities vary over space. Mansfield’s echo-planar imaging (EPI) allowed collection of an entire two-dimensional image slice by changing spatial gradients rapidly following a single electromagnetic pulse from a transmitter coil.

 

 

 

15. What were the two reasons for the change in terminology from “nuclear magnetic resonance” to “magnetic resonance imaging”?

 

The term NMR was abandoned in large part due to the negative health connotations of the word nuclear, which was justified because NMR does not use ionizing radiation. Moreover, the change can be attributed to the desire of hospital officials to separate MR scanning from nuclear medicine departments.

 

 

 

16. Why was there a boom in MRI use in the 1980s? How did this growth set the stage for fMRI?

 

MRI scanners were approved in 1985 by the FDA for clinical use. This allowed MRI scans to be prescribed by physicians and billed to insurance companies and Medicare. Rather than having to subsidize the enormous cost of a scanner that was used for research, hospitals now saw them as a source of profit. This made structural MRI one of the most common diagnostic imaging procedures. Over the next decade, thousands of MRI scanners were in hospitals. This proved invaluable to the growth of interest in fMRI.