• MRI Information
• Dartmouth's Brain Imaging Center
• Brain Imaging Techniques
• Brain Imaging Comparison Chart

Perhaps the greatest challenge faced during the early teenage years is coping with the demand to evolve with the constantly changing expectations of the environment, interpersonal relationships, and the self. In examining healthy children and adolescents’ age-associated changes in brain structure and function, we gain the potential to increase our understanding of normal development. Additionally this knowledge is critical to the identification of neurobiological anomalies associated with behavioral pathology. Basically what this all means is that we believe that one of the best ways to understand changes in adolescence is to study brain structure and function during the teenage years.

Our laboratory currently performs functional magnetic resonance imaging (fMRI) studies to examine changes in adolescents’ brain structure and function. If you’d like to learn more about what MRI is or what it’s like to have an MRI, read on!

What is an MRI?

MRI (magnetic resonance imaging) allows us to see the inside of the body by using a large magnet, radio waves, and powerful computers. The highly detailed images of an MRI can help us to visualize body tissues in a different way. It is almost like taking cuts or slices of the body but in a “virtual” (yet highly accurate) way on the computer. By using recent advances in MR technology we are able to look at both structure and function within the developing brain.

What is it like to have an MRI?

MRI machines consist of a magnet that surrounds the tube in which a study participant reclines. Participants lie face-up on a sliding table, and are moved into the tunnel. An intercom in the tunnel ensures constant two-way communication between the technicians and participants. Cameras allow participants to be seen at all times. While the magnet is operating, it makes a loud banging noise. For this reason, participants wear headphones to help muffle this noise.

Each study may ask different questions about the brain, so participants are often asked to perform different tasks while in the MRI machine. Our laboratory generally asks participants to perform a few tasks by viewing pictures (or stimuli) and pressing certain buttons depending on their thoughts or feelings about the stimuli.

Here are a few examples of what you might be asked to do during an MRI at our lab:

• Study one: Participants view a number of human faces displaying different types of facial affects. They are asked to identity specific affective expressions.
• Study two: In this study, we ask participants either to remember or forget different pictures. Pictures are series of images, rated for their varying degrees of emotional intensity.
• Study three: Participants are presented with two pictures simultaneously. They are asked to make rapid subjective decisions (via button press) about which picture they like better, or think is “cooler”.

Are there any risks to me in getting an MRI?

MRI does not use any radiation. MRI poses no risks unless participants have any metal objects implanted in their bodies. To prevent accidents, all subjects are asked a number of questions prior to an MRI scan to ensure their safety. Participants are all required to wear protective headphones to muffle the noise generated by the scanner.

What is Diffusion Tensor Imaging (DTI)?

Diffusion tensor imaging (DTI) is a relatively new magnetic resonance technique that holds great promise with regard to the exploration and growth of connections in the developing brain. DTI is based upon the observation that bipolar magnetic field gradient pulses cause the three-dimensional displacement of the water molecules within a given area. The unrestricted movement of molecules is known as isotropic diffusion. Diffusion tensor imaging examines anisotropic diffusion (either planar or linear), where molecular movement is restricted in one or more directions. Differences in diffusion anisotropy are thought to reflect unique structures within, and resultant organization of, the examined tissue. DTI technology is particularly useful for examining organized regions of the brain, especially white matter tracts. The myelin of parallel-running white matter axonal fiber bundles restricts diffusion in certain directions; diffusion in the direction of the fibers is faster than in the perpendicular direction. Investigators have recently used this observation to map the orientation of white matter tracks in the brain, assuming that the direction of the fastest diffusion would indicate the overall orientation of the fibers. In isolation, this technique will help us visualize neural pathways—or the lack thereof—in our subjects; thereby enabling an examination of how specific brain connections may grow during adolescence. Diffusion Tensor MRI (DTI), combined with blood oxygen level dependant (BOLD) fMRI, allows us to more directly investigate how both the structure and function of the brain influence our ability to perform certain tasks.

The Brain Imaging Center at Dartmouth recently received a make over!

This summer, a new 3.0T Philips Achieva fMRI machine is being installed in Dartmouth College’s Center for Cognitive Neuroscience in the Psychological & Brain Sciences Department. The magnet in this machine is twice as powerful as our previous 1.5T fMRI! To learn more about the Brain Imaging Center, visit: http://dbic.dartmouth.edu.

Brain Imaging Techniques

• X-ray
• Nuclear (PET, SPECT)
• Magnetic Resonance (MRI, fMRI, DTI)
• Surface electrical activity (EEG, MEG, ERP)
• Near-infrared (NIRS)
• Computed Tomography (CT)
• Ultrasound

In the 1970s, researchers began using computers to process information from x-rays passed through the brain to form a single whole-brain image. This technology provided the first direct pictures of normal brain anatomy in live humans.

PET: Positron Emission Tomography

PET measures emissions from radioactively labeled chemicals that have been injected into the bloodstream, and uses the data to produce two- or three-dimensional images of the distribution of the chemicals throughout the brain. For more information, click here.

 

SPECT: Single Photon Emission Computed Tomography

Similar to PET, this imaging procedure also uses radioactive tracers and a scanner to record data that a computer uses to construct two- or three-dimensional images of active brain regions. For more information, click here.

MRI: Magnetic Resonance Imaging

MRI uses magnetic fields and radio waves to produce high-quality two- or three-dimensional images of brain structures without injecting radioactive tracers. For more information, click here.

fMRI: Functional Magnetic Resonance Imaging

Functional MRI (fMRI) relies on the magnetic properties of blood to enable scientists to see images of blood flow in the brain as it is occurring. Thus researchers can make “movies” of changes in brain activity as patients perform various tasks or are exposed to various stimuli. For more information, click here.

DTI: Diffusion Tensor Imaging

for information on DTI, click here

Surface Electrical Activity Monitoring

EEG: Electroencephalography

Electroencephalography uses electrodes placed on the scalp to detect and measure patterns of electrical activity emanating from the brain. For more information, click here.

MEG: Magneto-encephalography

Magnetoencephalography (MEG) is the measurement of the magnetic fields produced by electrical activity in the brain, usually conducted externally, using extremely sensitive devices. For more information, click here.

ERP: Event-Related Potential

An event-related potential (ERP) in the brain is used to investigate the electrophysiological responses measured by electroencephalography (EEG) as a response to a certain event. This event is usually the exposition of a stimulus. As the EEG reflects thousands of simultaneously ongoing brain processes, the brain response to a certain stimulus or event of interest is usually not visible in the EEG. For this reason, the ERP is calculated by averaging the EEG data of dozens to thousands of stimulus expositions. Certain conditions manipulated by the independent variables of the stimulus set are compared, e.g., the presentation of a word and a non-word compared can reflect the temporal characteristics of semantic processing in the brain. For more information, click here.

Near-infrared Spectroscopy (NIRS)

Near infrared spectroscopy (NIRS) is a noninvasive technique that uses specific, calibrated wavelengths of near infrared light to illuminate tissue and activity below the skin (in our case, inside the brain). These wavelengths of light scatter in the tissue and are absorbed differently dependent on the amount of oxygen attached to hemoglobin in the arterioles, venules, and capillaries. Unabsorbed light returns as an optical signal and is analyzed to produce a ratio of oxygenated hemoglobin to total hemoglobin, expressed as %StO₂.

In practice, near infrared light penetrates tissues such as skin, bone, muscle and soft tissue where it is absorbed by chromophores (hemoglobin and myoglobin) that have absorption wavelengths in the near infrared region (approximately 700-1000nm). These chromophores vary in their absorbance of NIRS light, depending on changes in oxygenation. Complex algorithms differentiate the absorbance contribution of the individual chromophores.

Computed Tomography (CT)

Computed tomography (CT), sometimes called CAT scan, uses special x-ray equipment to obtain many images from different angles, and then join them together to show a cross-section of body tissues and organs. CT scanning provides more detailed information on head injuries, stroke, brain tumors, and other brain diseases than do regular radiographs (plain x-ray films). It also can show bone, soft tissues and blood vessels in the same images. CT of the head and brain is a patient-friendly exam that involves radiation exposure. For more information, click here.

Ultrasound

Ultrasonography is another procedure for viewing areas inside the body. High-frequency sound waves that cannot be heard by humans enter the body and bounce back. Their echoes produce a picture called a sonogram. While the use of ultrasound technologies in brain scanning and brain surgery has only begun to be developed in the past few years, there are exciting possibilities for applying these technologies to brain imaging.