MRI angio, safety and extras Flashcards
How does gradient echo affect blood vessels in angiography?
-Gradient echo exposes tissues to multiple short TRs
-Stationary tissues loose transverse magnetisation but fresh blood will have a lot more transverse magnetisation and produce a strong bright signal.
How does spin echo affect blood vessels in angiography?
-Produces a flow void if the material moves faster than TE/2.
-The signal from spin echo depends on the tissue receiving both a 90° and 180° RF pulse to generate the echo. If miss either of these b/c flowing too fast then appears dark.
How does turbulent flow show up on a SE sequence?
-Turbulent flow produces loss of coherence and results in a low signal area.
In which direction is pulsatile movement artefact most obvious?
-Movement artefact is more apparent in the PEG direction as each PEG step is separated by time TR compared to TE.
Signal to noise ratio is increased by
- increasing field of view
- decreasing matrix size
- reducing the bandwidth
- increasing slice thickness
- Increasing the magnetic strength
Resolution is determined by the FOV and the data matrix size. It is increased by:
- reducing the FOV
- reducing the slice thickness
- increasing the matrix size
- using stronger gradients
- using more phase encoding steps.
Give some CI to gadolinium
- GFR <50/dialysis patients as defined by the
WHO. - Pregnancy
- Haemolytic anaemia
- Renal dysfunction
Describe MOA of gadolinium - is the effect greater on T1 or T2?
-It is a positive paramagnetic contrast agent and increases local field strength. It shortens both T1 and T2 of hydrogen nuclei.
-The effect is greater on T1 than on T2, hence the area of uptake is made brighter on a T1 weighted image.
-T1 weighting is usually used.
Can you use gadolinium for STIR sequences?
No - STIR sequences rely on T1 suppression to negate the signal from fat. Gadolinium contrast would serve to do the opposite and is hence not used on STIR imaging.
How does gadolinium affect MR angiography on T1 and on T2?
-Gadolinium agent greatly shortens the T1 of blood –> makes it appear bright on a T1 acquisition.
*Imaging after gadolinium is almost aways using T1 weighting and areas of uptake appear bright on T1.
-Effect of gadolinium of T2 –> causes a darker area on T2. Not useful.
How to postive, negative and ferromagnetic contrast agents affect T1 and T2?
-Positive contrast agents in MR reduce T1 more than T2 and negative agent contrasts reduce T2 more than T1
-Ferromagnetic agents: decrease the MR signal to provide negative contrast
Is paramagnetism caused by paired or unpaired electrons?
caused by the presence of UNPAIRED atomic electrons. most MR agents are paramagnetic. Paramagnetic agents create their own alternating magnetic field which encourages relaxation in neighbouring protons (shortening T1 and T2).
What is NSF? How is this related to gadolonium?
-NSF is a chronic fibrotic disease causing the deposition of collagen within the skin, leading to skin induration, joint contractures and disability.
-Gadolinium is excreted mainly by the renal pathway (80%), and in the vast majority of cases NSF develops in the first six months following treatment.
What patients are at risk of NSF?
- with end stage renal failure (GFR <20 ml/min)
- on dialysis
- who have received a liver transplant.
Which type of sequence as a higher SAR?
Fast spin echo sequences have higher SAR than gradient echo as they use 180° RF pulse.
