Chapter 2

 

1. What are the three main components of an MRI scanner?

 

A) The Static Magnetic Field – magnetic field creation

B) Radiofrequency Coils – the signaling mechanism

C) Gradient Coils – the framing tool

 

 

2. How does an MRI scanner generate the main magnetic field? What two criteria are important for the main magnetic field?

 

The fMRI creates the main magnetic field by passing current through very tightly coiled wire (electromagnetism.) The two criteria that are important are homogeneity and strength.A homogeneous magnetic field is one that has the same strength throughout a wide region near the center of the scanner bore. If there was a non uniform magnetic field, then location in the scanner would dictate the image results.

 

 

3. Why are superconducting electromagnets necessary for MRI?

 

Superconducting electromagnets are necessary because with zero resistance (due to extreme cooling) a very strong electric current can be passed through. This helps keep electric cost down.

 

 

4. What is the difference between a surface coil and a volume coil?

 

A surface coil is a single loop that consists of a inductor (L) – capacitor (c) circuit. This allows radiowaves to be sent to and received from the target molecules. The volume coil implements the exact same mechanism except it occurs over the whole volume of the head area. This allows for greater surface coverage albeit at lower resolution due to the fact that radiofrequency cables do have resistance and therefore exclude excess energy as heat.Following from their strengths and weaknesses, surface coils would be better employed when looking at one particular area of interest, while volume coils are better equipped to look at brain activity as a whole because the images it collects are more homogeneous. The optimal set up is to use volume coils to excite and surface coils to collect.

 

 

5. Why are gradients necessary for image generation? What sorts of coils are used to generate these gradients?

 

A gradient is necessary in order for the MR signal to become spatially controlled. Therefore, different molecules in the bore will contribute different signals depending on where they are located on the subject. The two types of coils are Maxwell and Golay pair. The basic distinction is the direction in which the gradient is created. In Maxwell, the gradient is created in the direction (parallel to) of the main magnetic field. In the Golay pair, however, the gradient is created perpendicular to the main magnetic field.

 

 

6. What is shimming, and why is it important?

 

Shimming is the process of correcting for inhomogeneities in the main magnetic field. These shimming coils are able to create first, second, and third order fields depending on the need. Each subject needs calibration as they will distort the field in different ways.There will naturally be incongruities in the field and shims are electromagnetic coils that compensate for these flaws in the static magnetic field.

 

 

7. Why might researchers want to monitor physiological changes like cardiac and respiratory rate during an fMRI experiment?

 

For two reasons: to improve quality of data and to deduce other cognitive functions. Sweating, breathing, etc. can all distort the data if not compensated for. Also, physiological changes like pupil dilation can help the experimenter deduce simple things such as arousal and novelty. Usually health concerns are not considered in fMRI as unhealthy participants are filtered. 

 

 

8. Describe the procedures of a typical fMRI experiment, beginning with recruitment of the subject.

 

A) Subject calls and is informed about the purpose of the experiment

B) An initial screening can be done just to check for aneurysm clips, tattoos, pacemaker

C) At the hospital subject is screened for metal again then put in scanner and told about emergency joystick, and a volume coil is placed over her head.

D) The first scans are simply structural, only the following ones are for the experiment.

E) After the experiment the subject can discuss the results with the conductor

 

 

9. What sorts of conditions/problems would prevent someone from being a subject in an fMRI experiment?

 

In short, if somebody has anything metal in or on their body they should not be allowed in the scanner. Also, if somebody shows previous, significant health problems they should probably be not allowed in the scanner as it could trigger a relapse. Some claustrophobic subjects are let go before the experiment begins. Also people that have internal ferromagnetic structures/devices, such as metal plates and aneurism clips are not used in experiments.

 

 

10. What effects do very strong static magnetic fields have upon human tissue?

 

Very little effects. Although innumerable studies have set out to show some ‘causation’ between static magnetic fields and health problems there have not been any solid conclusions. The only real risk involves very sudden movements of the head which might cause torque on hair in the ears and possibly the rods and cones in the retina. Some reported effects are vertigo, phosphenes, metallic taste sensations, sensitivity in teeth fillings, nausea, and headaches though these all occur when somebody move their head quickly in a static field. One cause could be the magnetohydrodynamic phenomena.

 

 

11. Forty five percent of subjects reported unusual sensations when entering the bore of a 4-T scanner. What was most important about this result?

 

Essentially that the scanner has a huge placebo effect. People reported ‘unusual sensations’ when in reality the magnet was down for repair.

 

 

12. What happens to metal brought within the static magnetic field? Consider both large external objects (e.g., oxygen canisters) and small internal devices (e.g., aneurysm clips).

 

Large objects will generally just fly towards the fMRI machine. This is known as ‘projectile effect.’ Other, small objects that simply have iron in them will rotate in order to align with the magnetic field. This process is known as torsion. Obviously, if an aneurysm clip tries to rotate out of position the patient will most likely bleed to death.

 

 

13. What effects do the changing gradient fields have upon the human body? How can these effects be minimized?

 

Since the human body is a conductor rapid changes in the magnetic field have the potential to cause some tingling sensations in the body. These are due primarily to peripheral nerve or muscle stimulation. The change in magnetic field over time, dB/dT, is therefore kept at sub-threshold values to make sure patients do not have this feeling. The other primary concern is with patients who have medical devices such as pacemakers. An induced current has the potential to interfere with the voltage of such devices. Anyone with a medical device in their body should be excluded from the experiment.

 

Human blood is an electrically conductive fluid. When these kinds of fluids are moved through a changing magnetic field, a magnetic field is created in the opposite direction. Currents induced in the body can cause peripheral nerve or muscle stimulation. In high magnetic fields this may become painful for the subjects. However, these results can be minimized by slowly moving the subject into and out of the scanner. The subjects are also instructed not to clasp their hands or cross their legs during scanning because these actions create conductive loops that may increase the effect of the magnetic gradient.

 

 

14. What is SAR? Why is it important for fMRI?

 

“A quantity that describes how much electromagnetic energy is absorbed by the body over time.” It is important in fMRI because when the machine directs radiofrequencies at the subject his/her body will absorb some of them and his/her body temperature will increase. Looped objects can increase absorption so clearly necklaces cannot be worn. The other way to reduce SAR involves determining the strength of both the static magnetic fields and the radiofrequencies.

 

 

15. Why is it important to avoid looping wires or necklaces near the head coil?

 

As stated above, loops are really good at focusing electromagnetic energy from the coil and therefore can cause ‘local burning’ on the subject. This is because wires and necklaces, even if they are not ferromagnetic, heat up more than the surrounding tissue.

 

 

16. What is the most common health consequence for MRI studies? How can it be minimized?

 

Claustrophobia is the biggest concern. This can be reduced by first screening any subjects who say they are claustrophobic. In order to reduce anxiety in general the experimenter should explain some of the noises to the subject in the scanner and maintain frequent communication. Also the subject should be informed that he/she can get out at anytime. If the subject feels he/she has ‘some’ control over the experiment then he/she will be that much more relaxed. Also, the experimenter could walk into the room to try to calm the subject and should always pay attention to telltale anxiety signs such as the participant asking “how much longer.” If at any time the subject declares that they are anxious, then they have to be removed from the scanner immediately. A few methods that researchers have employed to reduce anxiety includes: talking with the participants before the experiment to see if they are claustrophobic; talking with the subjects while they are in the scanner; directing air-flow in the scanner to reduce the fear of suffocation; providing a panic button for the subject.