Lecture 10 - Brain Imaging Flashcards
CT is essentially a
Rotating x-ray beam where the brain is imaged from several directions
For CT what happens to the beam as it passes through brain tissue?
It attenuates (ie weakens)
In CT, what is the significance of beam attenuation as it passes through tissue?
Detectors on opposite side of source of beam pick up rate of attenuation (ie., rate at which x-ray beam weakens as it passes through tissue)
Rate of attenuation in CT
varies by tissue composition
Radiodensity information in CT
(ie., attenuation rate) is detected
How might CT allow for reconstruction of a 3D image?
If patient is moved through the scanner slowly, allows us to acquire information at multiple levels of the brain, and therefore reconstruct 3-D image
CT enhances visualization of what?
1) bony anatomy 2) acute hemorrhagte or stroke 3) elements with high atomic numbers show up better (calcium, iron, iodine, barium, lead)
For CT what are the advantages?
It is faster and less expensive than MRI, can be used as an initial screening and assessment tool (bone fragments, etc)
What are CT disadvantages compared to MRI?
1) Use of X-ray; 2) less contrast differences between soft tissues 3) lower spatial resolution, several millimeters in CT scan VS one millimeterin MRI scan
hypodense (dark) structures in CT can indicate
edema and infarction
hyperdense (bright) structures in CT
indicate calcifications and hemorrhage and bone trauma can be seen as disjunction in bone windows.
CT can detect tumors by
Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema
Infarct in CT VS MRI
1) CT: loss of gray / white differentiation in CT scan 2) Infarct in MRI: greater resolution to see the difference
Anatomic MRI is based on
principles of nuclear magnetic resonance
Anatomic MRI produces what?
High resolution images of the brain and spine
Advantage of MRI
no radiation; radio frequency waves instead of x-rays
Clinical applications of MRI
1) high resolution and detailed visualization of soft tissue 2) visualizes anatomy (gray and white matter; CSF) 3) identifies a wide range of pathological processes
What the body is comprised of that the MRI takes advantage of?
63% hydrogen atoms
What property of H atoms in the body does the MRI use?
protons in H atoms have a spin, like a top that produces a small magnetic field which aligns with or against the large externally applied magnetic field resulting in a precess or wobble at a frequency propotional to the magnetic field
What is the effect of H atoms lining up with the external magnetic field?
Net magnetization of the tissue
In MRI, how do you detect the net magnetization of the tissue?
By applying a radiofrequency pulse which tips the protons away from the direction of the magnetization
In MRI, what happens after you turn the RF pulse off?
The protons realign with the external magnetic field resulting in the decay of the energy that each spinning proton absorbed from the RF pulse
In MRI, what happens as the energy decays?
RF signal is emitted which is picked up with an antennae and decoded into images with Fourier transform algorithms
In MRI, explain the digital image construction
1) spin of proton decays 2) emits RF signals at different rates 3) Pixel = RF signal strength on a grey scale
What does the rate of RF emission depend on in MRI?
The composition of the tissue in which they are located
In MRI, what is the significance of pulse sequences?
1) vary timing of the RF pulse which attenuates the tissue of interest
Give an example of pulse sequences in MRI.
Rapid repetitions of the RF pulse enhances grey – white contrast
What is the best image strategy for the MRI of lesions?
Infrequent repetitions of RF pulse enhances signal from water which is usually increased in pathologic conditions
What are the diagnostic applications of anatomic MRI?
1) inflammatory disease (multiple sclerosis) 2) neoplastic disease (tumors) 3) epilepsy 4) cerebrovascular disease (stroke)
What is MRS?
1) Permits us to study chemical structure of the brain 2) separates out chemical mixtures in the brain 3) can derive concentrations and ratios of chemicals (metabolites)
How does MRS work?
1) an RF pulse is applied, after which each chemical component (metabolite) emits a specific frequency
What happens to the signals of MRS?
They are analyzed with Fourier transforms to generate NMR spectra which consists of multiple peaks
For MRS, what does the size of the peak correspond to?
The concentration of each chemical component
What are the MRS diagnostic markers?
1) NAA 2) Choline 3) Creatine 4) Lactate
MRS diagnostic marker NAA
located in cell bodies and dendrites; considered a neuronal marker
MRS diagnostic marker choline
cell membrane synthesis and degradation: marker for demyelination
MRS diagnostic marker creatine
glial marker