Tissue Signal Characteristics
Signal in MR
images is high or low (bright or dark), depending on the pulse sequence used, and
the type of tissue in
the image region of interest. The following is a general guide to how tissue appears
on T1- or T2- weighted images.
Dark on T1-weighted image:
- Increased water, as in edema, tumor, infarction, inflammation, infection, hemorrhage
- (hyperacute or chronic)
- Low proton density, calcification
- Flow void
Bright on T1-weighted image:
- Fat
- Subacute hemorrhage
- Melanin
- Protein-rich fluid
- Slowly flowing blood
- Paramagnetic substances: gadolinium, manganese, copper
- Calcification (rarely)
- Laminar necrosis of cerebral infarction
Bright on T2-weighted image:
- Increased water, as in edema, tumor, infarction, inflammation, infection, subdural collection
- Methemoglobin (extracellular) in subacute hemorrhage
Dark on T2-weighted image:
·
- Low proton density, calcification, fibrous tissue
- Paramagnetic substances: deoxyhemoglobin, methemoglobin (intracellular), iron, ferritin,
- hemosiderin, melanin
- Protein-rich fluid
- Flow void
MR signal sources
When protons are placed in a magnetic field, they oscillate.The
frequency at which they oscillate depends on the strength of the magnetic
field. Protons are capable of absorbing energy if exposed to electromagnetic
energy at the frequency of oscillation.
After they absorb energy, the nuclei release or reradiate this energy so that
they return to their initial state of equilibrium. This reradiation or transmission
of energy by the nuclei as they return to their initial state is what is
observed as the MRI signal.
The return of the nuclei to their equilibrium state does not
take place instantaneously, but rather takes place over some time.
The return of the nuclei to their initial state is governed
by two physical processes:
o The relaxation back to equilibrium of the
component of the nuclear magnetization which is parallel to the magnetic field,
and
o The relaxation back to equilibrium of the
component of the nuclear magnetization which is perpendicular to the magnetic
field.
The time that it takes for these two relaxation processes to
take place is roughly equal to:
o Time T1 for the first process, and
o Time T2 for the second process.
The strength of the MRI signal depends primarily on three
parameters.
o Density of protons in a tissue: The greater
the density of protons, the larger the signal will be.
o T1
o T2
The contrast between brain tissues is dependent upon how
these 3 parameters differ between tissues. For most "soft" tissues in
the body, the proton density is very homogeneous and therefore does not
contribute in a major way to signal differences seen in an image.
However, T1 and T2 can be dramatically different for
different soft tissues, and these parameters are responsible for the major
contrast between soft tissues.
T1 and T2 are strongly influenced by the viscosity or
rigidity of a tissue. Generally speaking, the greater the viscosity and
rigidity, the smaller the value for T1 and T2.
It is possible to manipulate the MR signal by changing the
way in which the nuclei are initially subjected to electromagnetic energy. This
manipulation can change the dependence of the observed signal on the three
parameters: proton density, T1 and T2.
Hence, one has a number of different MR imaging techniques
("weightings") to choose from, which accentuate some properties and
not others.
Neuroimaging
A
simplified tabulation of tissue image characteristics:
Normal Tissue
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.
Abnormal Tissue
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.
