Physiology of Behavior

Research Methods: Introduction to Neuroimaging

Nathaniel W. Nelson, Ph.D., LP, ABPP University of St. Thomas, GSPP

Neuroimaging

• Structural Neuroimaging • Computerized Tomography (CT) • Magnetic Resonance Imaging (MRI) • Diffusion Tensor Imaging (DTI)

• Functional Neuroimaging • Positron Emission Tomography (PET) • Functional MRI (fMRI)

Computerized Tomography (CT)

• In modern era (circa 1970’s), CT first widely used approach to neuroimaging

• Entails passing narrow X-ray beams through the same object (e.g., brain) at many different angles, creating many different images of it, then combining images with use of computing and mathematics to create 3- dimensional image

• Absorption of X-ray radiation varies with tissue density • High-density tissue (e.g., bone) absorbs much radiation • Low-density tissue (e.g., ventricles) absorb little radiation • CT software translates absorption differences into a contrast-reversed brain

image

Copyright © Allyn & Bacon 2007

CT Scanner

Computerized Tomography

Within a CT scanner, a beam of X-rays is used to image progressive “slices” through tissues of the body, including the brain and skull. Differences in structure or tissue type, such as tumors or bleeding, can be seen in CT scans.

CT Brain Scans

The patient has a lesion in the right occipital-parietal area (scan 5). The lesion appears white because it was accompanied by bleeding; blood absorbs more radiation than the surrounding brain tissue. Rostral is up, caudal is down; left and right are reversed. Scan 1 shows a section through the eyes and the base of the brain

CT Series

Adapted from Kaufman (2001)

A. Acute stroke from occlusion of right MCA. Stroke is darker (more hypodense) than normal tissue because it is deprived of blood, which is normally radiodense

B. Stroke is outlined. Pie-shaped area includes lateral portion of right cerebral hemisphere, which contains origin of corticospinal tract for left face and arm

C. Cerebral hemorrhage originating in left thalamus, extending to lateral ventricle. Blood is denser than brain tissue and is thus white.

D. Left-sided parietal cerebral hemorrhage (arrows), obliterates occiptal horn of left lateral ventricle. 3 small white areas = normal choroid plexus calcification of third and lateral ventricles

Computerized Tomography (CT)

A. Horizontal CT scan of patient with Broca’s Aphasia

B. Schematic of horizontal section; black area is area of lesion (it is hypodense relative to normal brain because it is deprived of blood, which is usually radiodense)

C. Reconstruction of the brain, showing lateral view of left hemisphere with lesion in black

Adapted from Kolb & Whishaw (2003)

Magnetic Resonance Imaging (MRI)

• Unlike CT, MRI does not use X-rays • Patients are placed in strong magnet that forces

hydrogen protons (hydrogen is found in water, which is found in soft tissue) to spin in parallel with magnetic field

• Next, a radiofrequency (RF) pulse is applied to orient protons in same direction; after each RF, protons resume previous alignment (“relax”) within magnetic field and thereby emit energy

• This energy forms a detectable signal that is characteristic of the tissue

Copyright © Allyn & Bacon 2007

MRI scans

MRI Scans of Human Brain 5-29

MRI: Advantages in Detecting Brain Tumor

• MRI is more likely than CT to detect: • Small lesions • White matter lesions • Precise location, internal structure, and involvement of

surrounding brain regions • Lesions encased in bone

• Optic gliomas • Acoustic neuromas • Pituitary adenomas • Posterior fossa tumors

Advantages and Disadvantages of MRI Versus CT

MRI Advantages • Greater Imaging Ability

• Greater resolution • Detect smaller objects • Distinguish white/gray matter • 3 Planes: Coronal, Axial, Sagittal

• No Distortion from Bone • Can display posterior fossa, pituitary,

optic nerves, spinal cord • Does not utilize Ionizing Radiation • Applied to Intra- and Extra-cranial

arteries • Identify White Matter Plaques

• MS • PML • AVM • Mesial-temporal sclerosis

MRI Disadvantages • Expense: MRI Costs 2-3 times more

than CT • Duration: 40 minutes instead of <20

minutes for CT • Claustrophobia • CT more rapid detection of acute

hemorrhage, head trauma, subarachnoid hemorrhage

• Ferrous metal devices are precluded by MRI magnet

• Pacemaker • Aneurysm clips • Cochlear implant

Diffusion Tensor Imaging 5-30

This image shows a sagittal view of some of the axons that project from the thalamus to the cerebral cortex in the human brain, as revealed by diffusion tensor imaging.

Positron-emission Tomography (PET) • After CT, PET is next in neuroimaging development • PET camera is donut-shaped array of radiation detectors positioned to encircle the head • Water containing radioactive material injected into blood stream or gas with radioactive material is inhaled • As radioactive material breaks down, it releases particles that are detected by PET camera • Computer reconstructs variations in density of flow of particles from different locations to produce image in

each section of brain • Because materials are covered through bloodstream, variations in image = areas of higher and lower blood

flow, which are then depicted by varying color gradients • Color images made during different kinds of mental activity indicate which brain areas are active during

execution of particular cognitive functions • PET is based on unique behavior of positions

• Radioactive material releases positrons as it decays • Positrons emerge from nucleus of a radioactive atom as a function of instability • Released positrons lose kinetic energy after traveling just a few millimeters in brain tissue • At rest, they become attracted to electrons • Positron and electron are annihilated when they come together, which results in release of energy called a

gamma ray (photon of a certain frequency) • These photos exit the head at speed of light • Areas of greater blood flow will emit more photons • PET records these events

Positron-emission Tomography (PET)

Adapted from Kolb & Whishaw (2003)

PET Scans

The top row shows three horizontal scans from a person at rest. The bottom row shows three scans from the same person while he was clenching and unclenching his right fist. The scans show increased uptake of radioactive agent in regions of the brain that are devoted to the control of movement, which indicates increased metabolic rate in these areas. Different computer-generated colors indicate different rates of uptake of the agent, as shown in the scale at the bottom.

PET Scans of a Patient with Parkinson’s Disease–like Symptoms

The scans show uptake of radioactive L-DOPA in the basal ganglia of a patient with Parkinson’s disease–like symptoms induced by a toxic chemical before and after receiving a transplant of fetal dopaminergic neurons. (a) Preoperative scan. (b) Scan taken 13 months postoperatively. The increased uptake of L-DOPA indicates that the fetal transplant was secreting dopamine.

Neuroimaging: Comparisons of Techniques

Adapted from Kolb & Whishaw (2003)

Functional MRI (fMRI) • Modification of MRI that permits measure of regional metabolism in the

brain • fMRI permits detection of oxygen levels in brain’s blood vessels • Greater resolution than PET and studies completed much faster

Functional MRI Scans

These scans of human brains show localized average increases in neural activity of males (left) and females (right) while they were judging whether pairs of written words rhymed.