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CT
(ROENTGEN -RAY COMPUTED TOMOGRAPHY)
A beam of x-rays passes through the brain and is detected
according to the density of the tissue encountered. Detectors
positioned around the circumference of the scanner collect
attenuation readings from multiple angles. A computerized
algorithm reconstructs an image of each slice. |
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MRI
(MAGNETIC RESONANCE IMAGING)
When protons are placed in a
magnetic field, they become capable of receiving and then
transmitting electromagnetic energy. The strength of the transmitted
energy is proportional to the number of protons in the tissue.
Signal strength is modified by properties of each proton's
microenvironment, such as its mobility and the local homogeneity
of the magnetic field. MR signal can be "weighted"
to accentuate some properties and not others. When
an additional magnetic field is superimposed, one which
is carefully varied in strength at different points in space,
each point in space has a unique radio frequency at which
the signal is received and transmitted. This makes constructing
an image possible. It represents the spatial encoding of
frequency, just like a piano.
Read more
about MRI physics > |
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SPECT/PET
(SINGLE PHOTON/ POSITRON EMISSION COMPUTED TOMOGRAPHY)
When radiolabeled compounds
are injected in tracer amounts, their photon emissions can
be detected much like x-rays in CT. The images made represent
the accumulation of the labeled compound. The compound may
reflect, for example, blood flow, oxygen or glucose metabolism,
or dopamine transporter concentration. Often these images
are shown with a color scale. |
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| SIMPLIFIED TABULATION
OF TISSUE IMAGE CHARACTERISTICS: |
| NORMAL
TISSUE: |
|
MR-T1 |
MR-T2 |
Xray-CT2 |
| Dense bone |
dark |
dark |
bright |
| Air |
dark |
dark |
dark |
| Fat |
bright |
bright |
dark |
| Water |
dark |
bright |
dark |
| Brain |
"anatomic"3 |
intermediate |
intermediate |
|
1. Bright means
high signal intensity, dark means low, and interm. means intermediate.
2. Bright means high density/high attenuation of x-rays, dark means
low.
3. Grey matter appears grey, white matter white.
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| ABNORMAL
TISSUE: |
| |
MR-T1 |
MR-T2 |
Xray-CT2 |
Enhancement1 |
| Infarct |
dark |
bright |
 dark |
sub-acute |
| Bleed |
bright2 |
bright2 |
bright |
no |
| Tumor |
dark |
bright |
dark3 |
yes |
| MS plaque |
dark |
bright |
dark4 |
acute |
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1. Blood brain
barrier leak. For MR, gadolinium; for CT, iodinated contrast material.
2. Unless very fresh or very old.
3. Unless calcified.
4. Often isodense. |
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