MR Physics 3: Parameters for Image Contrast Flashcards
Why are hydrogen nuclei important in MRI?
Used for MRI because of their magnetic susceptibility and their vast amount in the human body.
An intrinsic property of the hydrogen nuclei is their rotation (spin) which makes them magnetic along the rotational axis.
What is proton density?
Proton density (PD) =most basic MRI measure, representing the concentration of water protons (mobile hydrogen atoms) in each voxel.
How is MRI contrast developed?
Developed due to differences in:
T1, T2, T2* and proton density between different tissue types
What does the recovery time of T1 and T2 depend on?
Molecular motion of each spin near the Larmor frequency causes local magnetic field fluctuations with both T1 and T2 relaxation
What is relaxation?
The process in which spins release the energy received from a radiofrequency pulse
What are the two types of relaxation?
T1 - spin-lattice or longitudinal relaxation
T2 - spin-spin or transverse relaxation
How can MRI contrast be changed?
By changing TR and TE
-By altering the time at which the signal is measured after excitation
-By altering the time allowed to recover between pulses
What is TR?
Time to repetition
i.e. how quickly the entire pulse signal is repeated
What is TE?
Time to echo
i.e. the time between signal excitation and maximum signal
Can TR and TE be manipulated by the researcher?
Yes, they are both under operator control
How does proton density affect T1 contrast?
Controls the image appearance by altering the TR
Differences in PD and T1 between tissues in each image voxel provide image contrast
What does a long TR produce?
Proton density weighted image
What does a short TR produce?
T1 weighted image
What type of image does a spin echo sequence generate?
T2 weighted
What type of image does a gradient echo sequence generate?
T2* weighted
What is a spin echo sequence?
The simplest form of the spin-echo (SE) pulse sequence consists of 90°-pulse, a 180°-pulse, and then an echo.
How is a gradient echo sequence different to spin echo sequence?
Gradient echo uses magnetic gradients to generate a signal, instead of using 180 degrees radiofrequency pulse like spin echo = faster image acquisition time
The gradient echo formation results from applying a dephasing gradient before the frequency-encoding or readout gradient.
What causes the difference between T2 and T2*?
The type of echo sequence they use
What does increasing echo time do?
Increases T2 weighting
What does does decreasing echo time do?
Increases PD weighting
Summarise TR and TEs effect on images.
A long TR and short TE sequence is usually called Proton density-weighted
A short TR and short TE sequence is usually called T1-weighted
A long TR and long TE sequence is usually called T2-weighted
A short TR and long TE is TOO NOISY - dark image not much signal
What is signal to noise ratio?
The Signal to Noise Ratio (SNR) is a measure of the image signal in an area of tissue with respect to the background tissue.
What is contrast to noise ratio?
The Contrast to Noise Ratio (CNR) in an MRI image is the contrast between the average image values in a tissue of interest relative to the background (i.e. the surrounding tissue).
What do we do with SNR and CNR?
We have to compromise between SNR and CNR to capture the optimal signal
What is isointensity?
When the amount of decay of SNR and amount of decay od CNR hit the same point together
= creates an isointense point which will have the same brightness even if the T2 values are different
What has a long T1 and long T2?
Water/CSF
What has a long T2 and short T1?
Extracellular methemoglobin
What has an intermediate T2 and intermediate T1?
Muscle
Grey matter
White matter
What has a short T2 and a long T1?
Air
Cortical bone
Deoxy Haemoglobin
Tendons
Fibrosis
What has a short T2 and short T1?
Fat
Paramagnetic agents
Proteinaceous solution
What 3 pulse sequences change image contrasts?
Spin echo sequence
Gradient echo sequence
Susceptibility weighted imaging
What are three contrast agents?
Gandolinium
Chemical Enhanced Saturation Transfer (CEST)
Hyperpolarisation
What is a pulse sequence?
A series of events we use to control the magnetisation
it contains RF pulses and magnetic gradients
How magnetisation (signal) is prepared changes the contrast
What are the pros of gradient echo imaging?
Faster imaging
Shorter TE
What are the cons of gradient echo imaging?
Poor magnetic field (e.g. air pockets)- homogeneity distorts the image
What are the pros of spin echo imaging?
Less susceptible to magnetic field homogeneity
What are the cons of spin echo imaging?
Ususally longer TE
Slower imaging that gradient echo imaging
What is magnetic susceptibility?
How the magnetic is induced in a substance when exposed external magnetic field
Substance can be diamagnetic, paramagnetic or ferromagnetic
What are diamagnetic substances?
Diamagnetic materials are the most unique as they repel both poles of magnets
Not attracted to external magnetic fields
What are paramagnetic substances?
Paramagnetic materials are weakly attracted to a single pole
Attracted to external magnetic fields
What are ferromagnetic substances?
Ferromagnetic materials are strongly attracted to both poles of magnets
Highly attracted to external magnetic fields
What are gradient echo T2* sequences sensitive to?
Tissue susceptibility
What can susceptibility weighted imaging (T2*) do?
Enhance detection of calcifications and haemorrhage/blood.
What are 3D FatSat FLASH images?
Images that exploit a chemical shift difference between water and fat for image editing
A chemical shift selective pulse before the acquisition saturates the fat signal
Very thin slice acquisitions (<= 1mm) allow cartilage to be highlighted
How does the 3D Fat/Water separation work?
Chemical shift difference between water and fat can be used to produce in-phase and out-phase images
For a particualr TE the fat magnetisation will be 180 degrees out of phase
Aquire in-phase (water + fat) and out-phase (water- fat) images
How is the Dixon technique linked to 3D FatSat FLASH images?
It consists of the acquisition of in-phase and out-of-phase images, from which water-only (WO) and fat-only (FO) images are reconstructed
This produces four sets of images (or more) per acquisition
How do we use contrast agents?
Inject these substances that change the T1/T2 of tissue
What is gandolinium-chelate?
Paramagnetic substance (weakly magnetic)
It shortens T1
Increases signal on T1 weighted image
Produces dynamic contrast enhanced MRI
What is gandolinium-chelate useful for detecting?
Lesions and tumours
What is CEST contrast?
CEST measures the transfer of magnetization from molecular protons to the solvent water protons, an effect that becomes apparent as an MRI signal loss (“saturation”)
This allows molecular information to be accessed with the enhanced sensitivity of MRI
How does CEST work?
Irradating tissue with long RF pulse saturates the signal macromolecule proteins
Protein protons exchange with mobile protons, reducing the water signal
CHanges the image contrast dependent on protein concentration
How does hyperpolarisation contrast work?
Creates substances with dramatically increased nuclear spin polarization (and thus, greatly enhanced MR signal intensity) to enhance the information and contrast available on an MR image.
What does EPI stand for?
Echo-planar imaging
What is EPI?
Very fast gradient echo imaging sequence «1s
Whole of k-space is acquired in a single TR period
But its susceptible to image distortion
How does EPI work?
EPI = fast-imaging technique whereby a 2-D image can be obtained in less than a second, by quickly switching the magnetic gradients back and forth after a single excitation pulse
