Neuroscience/Objectives/Lecture 4
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Clinical imaging of the central nervous system
1. LEARN the various parts of the electromagnetic spectrum used for brain imaging.
Brain imaging uses x-rays, gamma rays, radio waves, and near infrared waves. The type of wave is determined by the frequency and wavelength of the photons used. Frequency and wavelength are related by
c = wavelength * f
which is related to energy by
E = hf
or
energy = hc / wavelength
where h is Planck's constant.
2. EXPLAIN the physical principles underlying radiography.
Radiography uses high energy (therefore low wavelength), ionizing radiation (x-rays), which is differentially absorbed by tissues. Calcium in bone readily absorbs x-rays, while soft tissue absorbs them minimally. Areas of high absorption are represented as white on film, whereas low absorption is represented as black.
3. EXPLAIN the physical principles underlying computed tomography.
CT is similar to radiography in that it uses ionizing x-rays. The difference is that in CT, a source and detector rotates around the subject to obtain radiographic images from many different angles. These images are processed computationally and the technique of backprojection is used to reconstruct the original image of the subject.
The technician can alter the intensity of radiation used; increasing intensity increases the energy of photons, reducing their ability to interact with tissues. Thus higher intensities can be used to gain better resolution of bones (eg, skull), while lower intensities are better for visualizing soft tissues. CT is well suited for visualizing intracranial blood since blood absorbs x-rays better than surrounding tissue. Changes in density of absorption can also be used to identify areas of infart in the brain. CT is also good for reconstructing a three-dimensional image of the skull.
4.. EXPLAIN the physical principles underlying positron emission tomography.
PET uses gamma rays emitted from within the body rather than applied exogenously. A tracer substance (eg, fluorodeoxyglucose) which emits positrons is administered. Emitted positrons travel (usually for 1-2 mm) until they encounter an electron, at which point they mutually anhilate, producing coincident photons (gamma rays). These gamma rays are detected and used to reconstruct an image of the subject.
When identifying regional functions, control scans are often produced along with scans for the experimental condition. Subtractive logic can be used to isolate the differences between the experimental and control conditions to yield regional functional differences.
Images are color-coded, with blue representing low gamma radiation and red indicating high radiation.
5. LIST the clinical uses of positron emission tomography in neuroimaging.
Flurodeoxyglucose is used to measure glucose metabolism. This is useful in oncology since metastatic tumors typically have higher glucose metabolisms than other tissues.
O-15-tagged water (which is inert) can be used to measure bloodflow. This is useful in identifying neurodegenerative diseases, since a lack of neuronal activity (such as would be found in a neurodegenerative disease) will result in reduced bloodflow to the affected areas.
6. EXPLAIN the physical principles underlying magnetic resonance spectroscopy.
Hydrogens in water have different local environments depending on their location within the body. Applying an external magnetic field will cause some of the protons to align their spins with the field. Subsequent applications of RF pulses causes the protons to come out of alignment and enter a high energy state. Protons differentially relax from this higher energy state into a lower one; as they relax, they emit photons in the RF range (called an echo). The characteristics of this echo (particularly the rate at which RF photons are released) are indicative of the local environment in which the protons reside.
In MR spectroscopy, these physical features can be used to noninvasively measure about 30 different chemicals.
Repitition time (TR; the time between RF pulses) and echo time (TE; the time between RF pulse and echo) are two controls used in MR.
7. EXPLAIN the physical principles underlying magnetic resonance imaging.
MR imaging and MR spectroscopy share the same physical principles.
8. LIST the major components in a clinical magnetic resonance imaging system.
- Gradients (apply magnetic gradients)
- Body coil
- Passive shims
- Patient coil
9. EXPLAIN the sources of image contrast in magnetic resonance imaging.
The realignment of nuclear spins with the applied magnetic gradient is longitudinal relaxation, and T1 is the time it takes to longitudinally realign. T2 is the time it takes to realign transversely. These parameters can be weighted to alter the image contrast.
Short TE, short TR gives T1 images which have low contrast; short TE, long TR gives proton density images; long TE, short TR gives poor quality images because of interference; long TE, long TR gives TR images which have high contrast.
10. DESCRIBE how magnetic resonance imaging is used to identify regions of brain pathology.
Subtle changes in image density can be used to identify ischemia. In addition, diffusion-weighted MRI can be used to identify autism: typically, developing white matter shows greater fractional anisotropy (which characterizes how water diffuses in the brain) than in white matter of a brain with autism.
11. DESCRIBE how magnetic resonance imaging can be used to image the cerebral vasculature.
Hemoglobin molecules have magnetic susceptibility, which can be used to image the cerebral vasculature.
12. EXPLAIN how magnetic resonance imaging can be used to
measure brain function.
Hemoglobin experiences different magnetic susceptibility depending on its oxygenation status, which can give insight into the oxygen delivery to the brain.
13. EXPLAIN how near infrared spectroscopy is used to image brain function.
Infrared spectroscopy uses optical tomography. Its physical underpinnings are similar to PET and fMRI, but it is even less invasive than those modalities.
14. DESCRIBE how functional magnetic resonance imaging is used in neurosurgical planning.
By using fMRI, a neurosurgeon can pinpoint areas of deficit or tumor that need to be excised. This can be used in concert with a CT as well.
