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56 Cards in this Set
- Front
- Back
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What does DWI tell us about
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the movement of water
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What does DTI tell us about
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fiber orientation of WM tracts
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What does spectroscopy tell us about
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metabolic information
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What two things can a contrast agent tell u about a region
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if it is vascular or if the permeability has changed (retention of contrast)
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What are 3 types of magnets used in MR imaging
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permanent
resistive superconducting |
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What are the characteristics that are important when discussing magnets
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field strength
stability homogeniety |
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What are advantages of permanent magnets
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require no cooling
cheaper |
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What are disadvantages of permanent magnets
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heavy
less field homogeniety can not be turned off in emergencies |
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What type of scanner uses a permanent magnet
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open scanner
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What are advantages of a resistive magnet
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none
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What are disadvantages of a resistive magnet
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they have poor homogeniety
weak magnet high electric cost |
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How is a resistive magnet designed
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by running an electical current through a wire to create a magnetic field
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If a resistive magnet is used what type of MRI scanner is it
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open
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what type of magnet do most modern MRI machines use
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superconducting magnets
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How does a superconducting magnet work
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Superconducting
magnets are based on the principle of cooling down (4°K) certain metal conductors so that there is little or no resistance; therefore, a high electric current can be used to generate high- strength magnetic fields with no major heat disposition |
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What is a disadvantage of a superconducting magnet
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expensive
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What is required to cool a superconducting magnet
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cyrogen (usually liquid helium)
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What does field stength tend to effect
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the signal to noise ratio of the image (particularly in T1)
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Longitudinal relaxation
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T1
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Are both the signal and contrast to noise ration effected by the magnet field strength
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yes
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What is the cause of the worse image characteristics of a weak magnetic field
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The time required
for complete relaxation differs for different field strengths; for example, the T1 is shorter at lower field strengths and tends to increase at higher field strengths. These changes affect both the signal- and contrast-to-noise ratios ofMR images |
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What are the units of magnetic field strenght
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tesla
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What does 1 T equal
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10000 gauss
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What has been the challange when developing new MR machines
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increasing the S;N ratio
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What is the best way to increase the signal to noise ratio
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increase the strength of the magnet
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What is a low strength magnet
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.1-.5
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What is a medium strength magnet
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.5-1
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What is a high strength magnet
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1.5
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What is an ultra high strength magnet
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>3
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What are shims used for
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to make sure there is good local homogeniety of the magnet
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What is the purpose of the gradient coil
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to create magnetic field variation in three different directions for localization of signal
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How do shim and gradient coils work
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Basically, a shim or gradient coil is a de-
vice that can generate a spatially localized mag- netic field within the main B0 field by using elec- tric current |
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Where are shims and gradients coils located
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hysically, the shim and gradient
coils are placed concentric to each other in the magnet and activated at specific times of the pulse sequence. |
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Why is a shim required
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The quality of the received signal
requires good field homogeneity and thus re- quires a shim of the local magnetic field, which is the B0 field along the z direction. |
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What is the cause of suseptibility effects
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this is caused by an object placed in the magnetic field and these are reason shims are required to correct the this local changes in the magnetic field
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What is another name for a shim
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correction coil
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What are 2 types of shim coils
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active and passive
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What are passive shim coils
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these are shims that are configured at the time of installation of the magnet by using a metal plates with in the bore or surface of a magnet
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What are active shim coils
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these require an electrical current through special coils
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Do most magnets have active and passive shim coils
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yes
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What are gradient coils used for
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Gradient coils are used for
localization of theMR signal in three directions (x, y, and z) |
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What are the 3 parts of the gradient coil
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the coils that create magnetic effect in the X, Y and Z directions. These lie concentric to eachother
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What are the important paramenters of the gradient coil
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amplitude
rise time slew rate |
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What is the amplitude of the gradient coil
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the gradient strength
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What are the units of amplitude (gradient strength) of a gradient coil
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tesla/meter
G/cm |
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What is the slew rate of the gradient coil
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The slew rate
of a gradient system (in tesla per meters per sec- ond) is defined as the ratio of gradient strength divided by the rise time |
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What are RF coils used for
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to transmit RF energy to tissue
To receive the RF signal from the tissue of interest |
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Can an RF coil just recieve or just send signal
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yes
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What are 4 types of RF coils
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usual designs are sur-
face, saddle, quadrature, or phased array (mul- tiple elements) |
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Are RF coils designed for specific body parts
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yes
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Once a signal is received by an RF coil what happens to it
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it is transmitted to an RF amplifier and reconstructed in a the main computer
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Is there multiple RF coils that are used in a single study
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yes
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What is another name of multiple coil MR imaging
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multiple "channel" (like CT)
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What has the use of a greater amount of RF coils led to
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advances in pulse sequence design and imaging methods
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What are parellel imaging methods
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simultaneous acquistion of spatial harmonics or sensititivity encoding.
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What has increasing the number of RF coils done to imaging time and SNR
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These methods have resulted in reduced imaging time but also in a decrease in SNR
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