Concept of Image Contrast Flashcards
What does image contrast mean?
the intensity differences between tissues
T1 contrast
differentiates tissues based on their T1 time constants
T2 contrast
differentiates tissues based on their T2 time constants
Proton Density (PD)
differentiates tissues based on their different water concentrations (density)
What is TE?
Time of Echo- time of pulse that excites it to the top of the echo where you observe it
What is TR?
Time of repeat- how fast you are repeating the pulse sequence
To get the highest contrast based on T1 tissue differences:
- TR should be short (so T1 is not recovered)
- TE should be kept as short as possible (so there is no dephasing)
Keeping TR constant, as you raise TE, you lose T1 contrast
To achieve the best contrast based on T2 tissue differences:
- TR should be long (so that T1 is recovered)
- TE should be long and close to T2 of tissues we want to image
Keeping the TR long, as you increase TE, T2 weighting increases
To get the real proton density image where the concentration of hydrogen nuclei is depicted in all tissues, we must:
- Use long TR, to make sure all spin have relaxed back to original equilibrium (no T1 effect)
- Use very short TE, not to lose any signal and not have Tw effect showing in the image
What are the 4 main steps of a Spin Echo Sequence?
90 degree RF pulse -> FID -> 180 degree RF pulse -> spin echo
True or False. T1 & T2 are modified, while TE & TR cannot be changed.
False. T1 & T2 are tissue properties so they cannot be changed. TE & TR can be modified.
What is T2* effects?
T2 relaxation with dephasing consequences (signal decay); combo of susceptibility (alignment of magnetic field) and magnetization
What is T2* mainly used for?
bleeding
How can we combat the effects of T2*?
By applying a 180 degree RF refocusing pulse
What is the goal of contrast agents in MRI and why?
With paramagnetic contrast agents the relaxation times could be lowered, which strongly enhances the signal.
