X-Ray Brain Imaging

• Imaging of live brains has led to advancements in clinical neurology/neurosurgery
• There is increased understanding of human mental functions due to images showing aspects of brain physiology
• All imaging techniques contrast different amounts of something (e.g., light)

Imaging using X-Ray Density

• X-Rays were the first method to measure contrast in the head
• Skull films are made by directing x-rays through a person's head
• Excellent spatial resolution is enabled by skull films
• The CNS information is limited in this method
• Location of Ca++ salts are primarily in the bone, accounting for most of the x-ray density within the head
• Minimal contrast between gray matter and white matter, and less density than bone
• Contrast is mainly between dense and less dense bone
• CNS contributes little x-ray attenuation
• Skull film records x-ray density flattened into a 2-D image
• By injecting an x-ray-dense contrast agent, a series of angiograms can demonstrate arteries and veins
• Displacement of vessels informs about processes in the brain (e.g., a tumor) due to Vessel relationships to CNS structures
• Replacing cerebrospinal fluid with air enables contrast between CSF spaces and CNS
• Pneumoencephalogram: visualization of the above replacement
• Air causes the brain to float or sag, tugging on the meninges, making it a painful procedure
• Tomographic techniques replaced pneumoencephalography

Computed Tomography

• Tomography: taking pictures of slices
• Optical tomography: creating cross-sectional images of bone (developed in the 1950s)
• X-ray source and sheet of x-ray film oscillate around a pivot point inside the bone in optical tomography
• Computers construct maps of x-ray densities in planes through the head
• called X-ray computed tomography (CT)
• CT has revolutionized neuroradiology
• The range of x-ray densities from dense bone to air in sinuses is about 2000-fold
• The human visual system can discriminate 200-300 shades of gray
• Two things that differ by less than about 0.5% in density (e.g., gray matter and white matter) appear as same shade of gray
• Bone window setting: CT image demonstrating lots of details in bony areas, but little detail in the CNS
• Soft tissue window: enables distinguishing of gray matter, white matter, and CSF from one another
• Calcified intracranial structures stand out, but bony detail is lost
• Contrast agents (like those for angiography) enables visualizing blood vessels and structures with no blood-brain barrier
• includes dural septa and choroid plexus

Magnetic Resonance Imaging

• Introduced in the 1980’s
• Tomography is a key component
• Atomic nuclei with odd # of protons/neutrons act like aligned spinning magnets can be aligned with a strong external magnetic field
• Nuclei absorb radiofrequency electromagnetic waves at a resonant frequency
• Absorbed energy is emitted while realigning with external magnetic field, measured to map locations and abundance of resonant nuclei
• Suitable nucleus for mapping
  • Hydrogen mostly in water, but also in hydrocarbons and molecules
• Chemical situation of a hydrogen nucleus affects the rate at which it emits absorbed energy
• gray matter, white matter, and CSF can easily be distinguished by choosing measurement times
• Bone lacks signal in MRI, due to the relative lack of protons
• Arteries/veins lack signal because blood keeps moving after absorbing radiofrequency energy
• T1: measure of rate at which nuclei realign with the external field
• T1 images are good for revealing details of anatomy
• T2: measure of rate at which nuclei become desynchronized with each other as they wobble
• T2 images are effective for detecting pathology
• Flowing blood emphasized by MRI measurements
• Lower resolution, noninvasive method
• Magnetic resonance angiography (MRA) has largely replaced traditional angiography

Functional MRI

• CT and MRI can map the brain
• Functional imaging provides information on what different parts of brain are doing
• Techniques are often combined with structural data from CT and MRI
• Positron emission tomography (PET) uses mathematical analyses to map locations of radioisotopes in reconstructed brain slices
• Emitters: isotopes decay by colliding with nearby electron
• Detectors: simultaneous hits registered by gamma ray
• Positron-emitting isotopes can be incorporated into
• Ligands that bind neurotransmitter receptors
• Metabolites taken up by active neurons
• Typically, into water molecules
• Intravenously-injected H215O can track blood flow increases
• PET can provide indirect measure of localization
• Positrons travel few millimeters before interacting, spatial resolution isn't very good
• Short half-lives cause resolution to be low
• Blood flow can be mapped with temporal and spatial resolution
• Hemoglobin-deoxyhemoglobin ratios: measuring blood volume with level relative to regional cerebral blood flow
• This is the basis for blood oxygen level-dependent (BOLD) signal
• Mapping blood flow comes with greater temporal and spatial resolution, without need for radioisotopes