11. MRI & MRS Flashcards
How is a CT contrast formed?
CT contrast is due to tissue density dependent attenuation of x-rays
What is Hounsfield number?
Hounsfield number is a measure of how much x-rays have been attenuated in passing through a material ~ small range in the middle which covers water
What is MR image contrast?
The relative signal intensities between different tissue types and pathologies – depends on physical properties of the tissue such as water and fat content, cellular structure, cell density
What are the different types of weighted images in MRI?
- T1 = anatomical structure is detected.
- T2 = water is detected
How is a T2 weighted image formed?
- Every proton in the body will align to the magnetic field and.
- You can interact with the magnetisation using radiofrequency pathways
- You can insert a radiofrequency impulse into the head and this will then knock the magnetism to 90-degrees ~ gives off MR signal. And how quickly this realigns depends on where the proton is (fat or water)
- MR signal is then picked up and stored into a computer, and then turned into an image.
- You need many radiofrequency impulses to generate an image.
- The signal comes from water molecules
What is nuclear magnetism and resonance?
- The positive charge of a spinning proton produces a magnetic moment mu
- In a magnetic field B0 the magnetic moment of a proton precesses at the Larmor frequency nL
~ vL = 2pi-gamma-B0
How is a T1-weighted image generated?
- The strong magnetic field creates magnetisation in all the tissue
- This magnetisation is from the protons in water and fat in the tissue
- The magnetisation can be manipulated by radiofrequency pulses to produce an MRI signal to create an image
- The intensity in the image depends on water content, tissue structure, blood flow, perfusion, diffusion, paramagnetics etc
~ This is more sensitive to blood, haemorrhage and tissue-type. Highlights where the core of the tumour is.
What determines how strong a signal is?
There are two relaxation times that determine how strong the signal is. These are MR parameters that vary with tissue type e.g. grey or white matter, and with disease. Hence they allow images to be made that demonstrate anatomy and pathology.
- Mxy decays according to T2 which affects how long the MR signal lasts. ~ This depends on the properties of the tissue.
- Mz recovers according to T1 which affects how much M there is available to be excited to give the next signal.
What is a pulse sequence?
- The time you put in the radiofrequency pulse is known as the pulse sequence.
- An MR image is built up from a series of signal acquisitions
- This acquisition is repeated several hundred times to obtain the data to create a single image. TE and TR determine the image “contrast”
- The repetition time is the time between the different radiofrequency pulses
- But changing the echo times and the TR times on the scanner, you can change whether it is a T1 weighted image or a T2 weighted image
- Oblique – any slice orientation is possible
- Multislice – acquisitions on many different slices within the TR
Expand on T2 relaxation time.
• The T2 of tissue determines how quickly the MRI signal decays away after the radiofrequency pulse
• T2 is very dependent on how mobile the water is in the tissue and increases with
- Oedema, an increase in water content
- Demyelination, a loss of brain tissue structure
• T2 is reduced by the presence of paramagnetic ions
- Fe from blood breakdown products
- Gd from contrast agents
What is the significance of fat in a T1 weighted MRI?
- Fat has the shortest T1, so it comes out very bright on the image. This is because it recovers alone the z axis quickly.
- So, we can use T1 weighted MRI to visualise pathological changes due to neurodegenerative processes (where you get changes in brain volume, grey matter in particular).
What happens when the repetition time (TR) between pulses is much shorter than T1?
- When the repetition time (TR) between pulses is much shorter than T1 the magnetisation that can produce the MRI signal is reduced (“saturated”)
- The MR signal is then T1-weighted.
- Tissue with long T1 produces a smaller signal than tissue with short T1.
Describe T1 relaxation time.
- T1 is lower in white matter than grey matter because of myelinated neurones
- T1 is also dependent on how mobile the water is in the tissue and T1 increases slightly with oedema
- T1 is very dependent on the presence of paramagnetic ions which reduce T1
- Fe from blood breakdown products
- Gd from contrast agents
What are contrast agents? And what happens when water is in the vicinity of contrast agents?
- Paramagnetic (unpaired electrons) or superparamagnetic (ferrites)
- Chelated to reduce toxicity
- Water in the vicinity of the contrast agent experiences strong fluctuating magnetic fields hence T1 and T2 are reduced.
What are MRI characteristics of multiple sclerosis lesions?
- Inflammation
- Demyelination
- BBB leakage
