MRI and MRS Flashcards
Briefly, describe CT.
CT contrast is due to tissue density dependent attenuation of x-rays.
We can see some basic pathological changes.
It is quick, cheap, safe and simple, making it the first port of call when diagnosing a patient.
What does MR imaging show?
It showcases the relative signal intensities between different tissue types and pathologies – it depends on physical properties of the tissue such as water and fat content, cellular structure, cell density, etc.
What happens to the proton when the patient enters the MRI scanner?
It aligns with the magnetic field of the MRI scanner, and the proton’s magnetic moment precesses around the magnetic field at a certain frequency.
The frequency is 128 megahertz; this means that if you put in a radio frequency of 128 megahertz, then you can alter the direction of the magnetisation.
In MRI scanning, we knock it out of the magnetic field by 90 degrees, and how it returns back to the original magnetic orientation gives us the difference in contrast.
How do we generate an MR image?
MR imaging is formed using a radio-frequency pulse to generate an MR signal from a slice of tissue.
Magnetic field gradients are used to encode the signal in space so that the computer can generate an image
Describe the MRI signal source.
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 radio-frequency 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.
What is the difference between T1 and T2 relaxation times?
T2 is signal decay in the xy axis.
T1 is signal recovery along the z axis.
T1 and T2 relaxation times vary between different tissues and pathology.
The image signal intensity depends on T1 and T2 and provides contrast between tissue in an MR image.
Looking more closely at the MR signal we find 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.
Mz recovers according to T1 which affects how much M there is available to be excited to give the next signal.
Describe 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 (echo time) and TR (repetition time) determine the image “contrast”.
Oblique – any slice orientation is possible
Multislice – acquisitions on many different slices within the TR
Why don’t we get any signal from the cell membranes?
Because the signal decays nearly instantly, so it cannot be visualised.
Describe 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 is recovers along the z axis quickly.
Thus, 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).
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
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.
What are contrast agents?
They are paramagnetic (unpaired electrons) or superparamagnetic (ferrites).
Water in the vicinity of the contrast agent experiences strong fluctuating magnetic fields, hence T1 and T2 are reduced.
They’re chelated to reduce toxicity.
What are some safety issues during an MRI?
SAFETY: no ferromagnetic objects in the exam room:
- scissors, stethoscopes, wheel chairs, gas cylinders
- hearing aids, watches, spectacles, (dentures – image quality)
CONTRAINDICATIONS:
- pacemakers
- infusion pumps
- 1st trimester pregnancy
- aneurysm clips (refer to manufacturers specifications)
- metallic foreign bodies (orbit x-ray, shrapnel)
