MRI Flashcards
(15 cards)
what is the major advantage of MRI over other imaging modalities
Excllent soft tissue contrast, High resolution, Can acquire images at any desired orientation throughout the body, Non-ionising radiation.
However it is slow and noisy
Generally how are the images acquired
Magnetic fields from the scanner interact with the hydrogen atoms, and allow their location to be detected
By adding up all the individual magnetic moments of each nucleus we obtain the net magnetisation vector, M.
In its equilibrium position M is aligned along the direction of the external magnetic field (B) and is denoted in this location by M.
MRI involves the manipulation of what
net magnetisation vector, M.
How is the magnetisation vector manipulated
A short burst of radiofrequency energy (RF pulse) applied perpendicular to Bo and denoted as B1.
What happens to, M, when it is exposed to a 90o RF pulse
M is flipped into the transverse plane. Relaxation then occurs as the protons lose energy by emitting it at frequency w. M returns to its equilibrium position
How is the MR signal generated
Rotating Magnetic field induces a voltage in the receiver coil that oscillates at w. MR signal is a decaying signal that is called the Free Induction Decay (FID) The magnitude of FID signal depends on the amount of net magnetisation in the equilibrium position immediately before the application of 90o pulse.
What are the four main components in a pulse sequence
RF pulses, Slice select gradient, Frequency encoding gradient, phase encoding gradient
Describe briefly what happens during spin-spin & spin-lattice relaxation
a few hundred milliseconds, Protons rapidly dephasing in the transverse plane.
Several seconds, Net magnetisation factor slowly returning to the z axis
What are the three ways contrast agents are introduced into the body and what are they used for imaging
Oral (Bowel), Aqueous gel (Dynamically access the pelvis e.g. rectal & vaginal prolapse) or Intravenous contrast agents
What are intravenous contrast agents commonly used for imaging
Used to enhance contrast between tumour and normal tissue, (as tumour may not have significant differences in T1,T1 and proton density compared to normal tissue) Can also be used to enhance the differences between blood vessels & other tissues
Gadlinium is often used as a contrast agent, what are some of the properties and how is it treated
Becomes strongly magnetised in a magnetic field, very toxic so is chelated with DTPA. When it enters the body it is rapidly passed from vasculature to perfused tissues
How does Gadolinium enhance images.
Because it is a large molecule it takes longer to cross the blood-brain barrier, when it does it accumulates in lesions where the barrier has been disrupted. excreted via the kidneys or bile and has a half life of 90 minutes
How does Gadolinium chemically alter tissues
Decreases both T1 & T2 but more dominantly shortens T1 of a tissue. Therefore we indirectly detect the presense of agents as those tissue will have a faster recovery time.
Why is the timing of the scan so important with post contrast acquisition
too early - Not enough contrast agents in the blood stream
Too late - Contrast agent will be in the veins
Timed by watching real time dynamic scan, starting the scan when the contrast leaves the heart through the aorta
