Practicalities Psychophysiological Laboratory in the Magnet Safety and Costs

The physical properties ofMRI, as well as the financial costs, place a number of practical constraints on the design and execution of fMRI-based studies and thereby impact experimental design.

As indicated previously, the experiments take place in the bore of a large, powerful magnet. The presence of this large, static magnetic field precludes the participation of subjects who would be adversely affected by it. For example, subjects with pacemakers or other forms of implanted metalic devices that would be subjected to strong forces by the magnet are clearly ineligible. The fact that subjects must lie in a relatively confined space rules out volunteers who suffer from claustrophobia. The physical position and limitations of the subject's movement also constrain various experimental procedures that would be simple in an ordinary behavioral laboratory (Fig. 2).

In addition to the main, static magnetic field, there are strong varying electromagnetic fields from the gradient magnets (for generating images) and from the RF oscillator (for flipping the protons to obtain the NMR signal). Each of these fields has associated safety considerations. For the most part, this is a minor issue at 1.5 T. It can be more of an issue for some pulse sequences at higher fields. The manufacturers of MRI scanners are required to build in various safety measures to protect subjects (such as calculating the heating effects of the RF pulses for a person of a given size and weight). Nonetheless, each imaging facility normally includes a screening form (and sometimes a metal detector) to prevent the inadvertent harming of subjects from a collection of (sometimes not so obvious) potential dangers.

In addition to these safety considerations, the physics of MRI has other practical consequences. All of the devices used to present stimulation (visual, auditory, etc.) and to obtain behavioral and physiological response measures (button pushes, breathing and heart rate, etc.) must be constructed in an MR-compatible manner. As fMRI has become a more widespread enterprise, various companies have made it a business to supply such equipment. However, custom design of devices for specialized experiments is still common.

The single most vexing problem in the practical application of fMRI is head movement. Although pulse sequences have been developed to collect an entire slice of brain data in less than 50 msec, and multiple slices (for entire brain coverage) can be collected in 2-3 sec, the amount of information in each such image is limited. That is, the amount of functional contrast in the images—the differences in the signals between two experimental states—is small. To make up for this, many images are collected over extended periods of time—at least minutes and sometimes hours. During these time periods it is important that the subject's head move as little as possible.

A variety of techniques are used to encourage subjects to keep their heads as motionless as possible, but none is perfect. For well-motivated healthy adult subjects, this is usually not an insurmountable problem. For children and older patient populations, it can be the main reason that data are discarded. Although there are data analytic procedures for transforming images of moving heads back to a fixed position, these procedures are limited. Indeed, because the moving head actually distorts the main magnetic field in different ways, no motion correction algorithms can fix the problem perfectly. There are many extra coils in an MRI scanner that are used to make the main magnetic field as uniform as possible, despite the irregularities introduced by the presence of a human head in the bore of the magnet. These "shimming" coils are supplied with electric current designed to minimize the magnetic field distortions introduced by the head at the beginning of a scanning session. However, they are not modified on the fly, during the session, so any subject head movement results in more than just a displacement of the image; it also causes distortions that are much more difficult to correct.

Figure 2 Subject in the magnet. Functional MRI is conducted in the environment of a MRI suite. There are many ways to present stimuli and obtain responses from the subject in such a suite, but all must be compatible with the difficult and hostile electromagnetic environment of MRI. In the example shown, visual stimuli are projected onto a rear-projection screen and are viewed by the subject via a mirror. Auditory stimuli can be presented via MR-compatible headphones or by the speaker systems typically included in MRI scanners. Finally, subject responses can be collected using an MR-compatible button box. A variety of commercial and custom approaches to the problems of presenting stimuli and recording responses in fMRI have been developed.

Figure 2 Subject in the magnet. Functional MRI is conducted in the environment of a MRI suite. There are many ways to present stimuli and obtain responses from the subject in such a suite, but all must be compatible with the difficult and hostile electromagnetic environment of MRI. In the example shown, visual stimuli are projected onto a rear-projection screen and are viewed by the subject via a mirror. Auditory stimuli can be presented via MR-compatible headphones or by the speaker systems typically included in MRI scanners. Finally, subject responses can be collected using an MR-compatible button box. A variety of commercial and custom approaches to the problems of presenting stimuli and recording responses in fMRI have been developed.

Finally, it should be noted that MRI time is not cheap. Charges for an hour of clinical imaging are in the hundreds of dollars. Therefore, when designing a study, the total number of imaging minutes is one of the parameters that must also be considered in the trade-offs.

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