MRI & MRS Flashcards
CT contrast is due to what
CT contrast is due to tissue density dependent
attenuation of x-rays
MR image contrast - define
MR image contrast – i.e. 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
How is a magnetic moment produced
The positive charge of a spinning proton produces a magnetic moment μ
Describe precess of magnetic moment
In a magnetic field Bo the magnetic moment of a proton precesses at the Larmor frequency νL
Describe formation of MR imaging
MR Imaging is formed using a radiofrequency pulse to generate an MR signal from a slice of tissue
Describe use of magnetic field gradients
Magnetic field gradients are used to encode the signal in space so that the computer can generate an image
MRI safety issues
No ferromagnetic objects in the exam room
Scissors, stethoscopes, wheel chairs, gas cylinders
Hearing aids, watches, spectacles, (dentures – image quality)
MRI contraindications
Pacemakers
Infusion pumps
1st trimester pregnancy
Aneurysm clips (refer to manufacturers specifications
Metallic foreign bodies (orbit x-ray, shrapnel)
Describe the magnetisation in MRI - source, use
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
MRI image - intensity dependent on what
The intensity in the image depends on water content, tissue structure, blood flow, perfusion, diffusion, paramagnetics etc
The image signal intensity depends on T1 and T2 and provides contrast between tissue in an MR image.
T2 signal decay description
Mxy decays according to T2 which affects how long the MR signal lasts
T1 signal decay description
Mz recovers according to T1 which affects how much M there is available to be excited to give the next signal
T1/T2 relaxation times difference due to what
T1 and T2 relaxation times vary between different tissues and pathology
MR image built up from what
An MR image is built up from a series of signal acquisitions
T2 of tissue determines what
The T2 of tissue determines how quickly the MRI signal decays away after the radiofrequency pulse
T2 dependent on what
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 reduced by what
T2 is reduced by the presence of paramagnetic ions
Fe from blood breakdown products
Gd from contrast agents
Describe signal acquisitions in MRI
Radiofequency pulse -
MRI signal from tissue
Signals from tissue and csf are reduced depending on T1
Mz recovery with time after the RF pulse depends on the T1 relaxation time
CSF vs Mz T1
Tissue has a shorter T1 than csf so its Mz recovers more quickly
CSF has a very long T1
Formation of saturated MRI signal
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 sizes - effect on T1
Tissue with long T1 produces a smaller signal than tissue with short T1.
Define relaxation time
The time constant which determines the rate at which excited protons return to equilibrium
How is T1 effected in grey/white matter
T1 is lower in white matter than grey matter because of myelinated neurones
How is T1 effected in water
T1 is also dependent on how mobile the water is in the tissue and T1 increases slightly with oedema
T1 dependent on what
T1 is very dependent on the presence of paramagnetic ions which reduce T1
Fe from blood breakdown products
Gd from contrast agents
Contrast agent purpose
Used to increase the contrast of structures or fluids within the body in medical imaging. absorb or alter external electromagnetism or ultrasound.
Water in the vicinity of the contrast agent experiences strong fluctuating magnetic fields hence T1 and T2 are reduced.
Contrast agent definition
Paramagnetic (unpaired electrons) or superparamagnetic (ferrites)
Chelated to reduce toxicity
Define chelate
A compound containing a ligand (typically organic) bonded to a central metal atom at two or more points
Effect of shielded nucleus
A stronger shielded nucleus has a lower resonant frequency
Define MRS and it’s purpose
magnetic resonance spectroscopy = non-invasive, ionizing-radiation-free analytical technique to study metabolic changes in brain tumors, strokes, seizure disorders, Alzheimer’s disease, depression, and other diseases affecting the brain.
MRS process
Acquire signal from hydrogen protons (other endogenous nuclei also used),
MRS very frequently only acquires signal from a single localized region, referred to as a “voxel”.
What does MRS determine
MRS can be used to determine the relative concentrations and physical properties of a variety of biochemicals frequently referred to as “metabolites” due to their role in metabolism
Describe where NAA is found by MRS
(N-acetyl aspartate) found predominantly in neurons, marker for viable neurons, reduced in pathology
Describe where tCr is found by MRS
(Cr and PCr)
cell energy metabolism, marker of viable cell density in some tissue
Describe where tCho is found by MRS
(Cho, GPC, PC)
cell membrane metabolism (growth and degradation), elevated in tumors and gliosis
Describe where Glx is found by MRS
(glutamate & glutamine) amino acids
Describe where ml is found by MRS
(myo-Inositol) osmolyte found in glial cells, marker for gliosis inflammation, elevated cell membrane synthesis
Describe where Lac is found by MRS
(lactate) (not visible in normal brain spectra)
the end product of anaerobic metabolism, high in tumors & stroke etc.
Describe where Ala is found by MRS
Ala (alanine) (not visible in normal brain spectra) amino acid, marker for meningiomas
Describe where lipids are found by MRS
Lipids (not visible in normal brain spectra) membrane breakdown products, macrophages
