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74 Cards in this Set

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•There are many ______ available to the operator when setting up a protocol.
parameters
• A ______ is defined as a “set of rules”
protocol
in MRI these _____ are a variety of parameters that are selected by the operator.
PROTOCOLS
•The four main considerations determining image quality are:
- Signal to noise ratio (SNR)
- Contrast to noise ratio (CNR)
- Spatial resolution
- Scan time
•The signal-to-noise ratio is defined as
the ratio of the amplitude of the MR signal to the amplitude of the background noise.
•The MR signal is
the voltage induced in the receiver coil by the precession of the NMV in the transverse plane. It occurs at specific frequencies and time intervals (TE).
•Noise is the undesired signal resulting from the MR system, the environment and the patient. T OR F?
TRUE
What occurs at all frequencies and randomly in time. ?
Noise
to increase the SNR usually requires ;
increasing the signal relative to the noise.
•The 8 Parameters that affect SNR are ;
field strength,
proton density,
coil type and position,
TR,
TE,
flip angle,
number of signal averages
receive bandwidth.
•The signal-to-noise ratio is affected by the strength of the magnet? Tor F
true
• The signal-to-noise ratio is affected by the strength of the magnet. At higher field strengths there are more spin up than spin down nuclei as fewer have enough energy to oppose the increased field. As a result;
As a result the NMV increases thereby increasing the available signal.
it is likely that measures to boost the SNR will be required WHEN?
When scanning areas with a low proton density
•The voxel is defined as 1.___ ___ __ _____ __ ___ ____ _____. Therefore the factors that affect the voxel volume are:2. a._____ _____
b.____ c.___ ______.
1.the pixel area multiplied by the slice thickness
2..• slice thickness
• FOV
• the matrix.
•Voxel volume is the product of ___ ___ __ ___ ____ ____ and the slice thickness.
the two inplane pixel dimensions
•The size of the voxel determines
how much signal each voxel contains.
Large voxels have a higher signal than small ones because
there are more spins in a large voxel to contribute to the signal
any setting of FOV, matrix size or slice thickness that results in large voxels leads to a
leads to a higher SNR per voxel.
• Small voxels improve resolution as they increase the likelihood of two points, close together in the patient, being in separate voxels and therefore distinguishable from each other. T or F?
True
Changing any dimension of the voxel changes the resolution and also affects ?
SNR.
•The size of the voxel determines
how much signal each voxel contains.
Large voxels have a higher signal than small ones because
there are more spins in a large voxel to contribute to the signal
any setting of FOV, matrix size or slice thickness that results in large voxels leads to a
leads to a higher SNR per voxel.
• Small voxels improve resolution as they increase the likelihood of two points, close together in the patient, being in separate voxels and therefore distinguishable from each other. T or F?
True
Changing any dimension of the voxel changes the resolution and also affects ?
SNR.
•Coarse matrix – is
•Coarse matrix – is one with low number of frequency encoding and/or phase encoding and results in a low number of pixels in the FOV – low resolution
•Fine matrix – is
•Fine matrix – is one with high number of frequency encoding and/or phase encoding and results in high number of pixels in the FOV – high resolution
•Halving the matrix doubles the dimension of each pixel along
the phase encoding axis.
•Halving the matrix doubles the dimension of each pixel along the phase encoding axis. There are less pixels to map over a given FOV dimension in the phase direction, therefore each pixel is larger along this axis. The SNR of each voxel ?
doubles
•When increasing the FOV, pixel dimensions along each axis increase because the same number of pixels needs to cover a larger FOV,( e.g. from a 12cm FOV to a 24cm FOV). The SNR of each voxel increases by a factor of ____ because the size of each pixel has doubled along each axis.
four
•Increasing the slice thickness increases the voxel volume along the dimension of the slice. Thick slices cover more of the patient’s body tissue and therefore have more spinning protons within them. SNR therefore
increases in proportion to increase in slice thickness.
•The TR determines how much _____ _______ is recovered between excitation pulses and how much is available to be flipped into the transverse plane.
longitudinal magnetization
•At very short TRs, very little longitudinal magnetization recovers so only a small amount of ______ _______ __ _____and therefore results in an image with ____ ___ .
1. transverse magnetization is created
2. poor SNR.
•Increasing the TR improves the SNR as more longitudinal magnetization, and therefore more transverse magnetization, is created. 1. T or F ?
Although short TRs are required for T1 weighting, reducing this parameter too much may severely compromise the SNR. 2. T or F ?
1. TRUE
2. TRUE
•The TE determines how much dephasing of transverse magnetization occurs between the ______ _____ ___ ___ _____.
excitation pulse and the echo.
At short TEs, as very little transverse magnetization has dephased, the 1. _____ _____ and therefore the SNR of the image is 2.____.
1.signal amplitude
2. high.
With a large flip angle all available longitudinal magnetization is converted into transverse magnetization and hence maximum signal amplitude True or False ?
TRUE 90degree fkip angle = max trans. magnetization and hence max signal amplitude.
the range of frequencies sampled during readout is ;
the receive bandwidth
Reducing the receive bandwidth reduces What?
reduces the proportion of noise sampled relative to signal.( Reducing the receive bandwidth is a very effective way of boosting the SNR )
•Halving the bandwidth increases the SNR by about 40%, but also increases -
the sampling time.
•The type of the coil used affects the amount of ____ _____ and therefore SNR
signal received
The contrast-to-noise ratio or CNRis defined as
the difference in SNR between two adjacent areas – ability to distinguish areas of high signal from areas of low signal.

It is controlled by the same factors that affect SNR.
most important image quality factor is ?
The CNR is probably the most important image quality factor, as the objective of any examination is to produce an image where pathology is clearly seen relative to normal anatomy.
The CNR is increased by the administration of these 4 things..
1.______
2.______
3.______
4._______
a contrast agent,
magnetization transfer contrast,
chemical suppression techniques
T2 weighting
•1/ Why does T2 have much lower SNR than T1 ?
2/ what else is T2 used for?
1/ T2 has much lower SNR than T1 (due to longer TE) .
2/ • Better pathology visualization
•The purpose of administering contrast agents is
to increase the CNR between pathology and normal anatomy
•By saturating out normal anatomy, pathology is often seen more clearly
•These can be used to suppress signal from either fat or water ; Name these techniques.
chemical pre-saturation
Chemical pre- saturation is done using ?
• Partial saturation
• Full saturation
.Small molecules =
•Medium molecule =
•Large molecule =
water
fat
proteins
•Magnetization transfer contrast (MTC) is applied :
•MTC is applied before the excitation pulse
Magnetization transfer contrast (MTC) is;
•By selectively saturating the bound protons
•These pulses are either applied at a frequency away from the Larmor frequency where they are known as off resonant
•These protons, mainly bound to large proteins, membranes, and other macromolecules are called bound protons
•Spatial resolution is defined as
as the ability to distinguish between two points that are close together in the patient. It is entirely controlled by the size of the voxel.
•The imaging volume is divided into 1. ______.
•Each slice displays an area of anatomy defined as the 2. ___ __ _____.
1.slices
2. Field Of View (FOV).
•The FOV is divided into 1.________ . There are usually either 2.___ or ___ pixels along the frequency axis of the FOV and either 3. ___ ___ ___ ___ or ___ pixels along the phase axis of the FOV.
1. into pixels, the size of which is controlled by the matrix.
2. 256 or 512
3. 128, 192, 256, 384 or 512
•The phase matrix determines -
how many lines of K space are filled during data acquisition and therefore the scan time
•The voxel is defined as 1. _____________ .
Therefore the factors that affect the voxel volume are 2.a _____
b _____
c _____
1. the pixel area multiplied by the slice thickness.

2. • slice thickness
• FOV
• the matrix.
•1. Square pixels always provide better spatial resolution than rectangular pixels T OR F ?
•2. If phase number is less than frequency number, the pixels are longer in frequency direction – spatial resolution is therefore reduced along the phase axis. T OR F ?
•3. Some systems automatically keep the pixels square T OR F ?
•4. FOV should always cover the required anatomy along the phase axis T OR F ?
•5. Some systems have an option called : rectangular FOV to maintain spatial resolution with an uneven matrix T OR F ?
All True.
Rectangular FOV does these two things.?
•Te dimension of the FOV in the phase direction is reduced
•Reduces the scan time
•The scan time is the time to complete data acquisition or the time to fill K space
•The factors that affect scan time are:
- TR
- Phase matrix
- NEX
• Ideally an image Has :
Has high SNR, good spatial resolution and is acquired in a very short time
• Chart
Volume imaging
•The entire volume of tissue is excited and volume contains no gap
•SNR is superior and fewer NEX can be used
•Ideally an image has:
•Ideally an image has ;
High SNR, good spatial resolution and is acquired in a very short time
Volume imaging is ;
•The entire volume of tissue is excited and volume contains no gap
•The entire volume of tissue is excited and volume contains no gap
•SNR is superior and fewer NEX can be used
This is what type of imaging ?
Volume imaging
•Volume imaging is
the acquisition of magnetic resonance data from a volume rather than a single tomographic slice.
when No slice gap is employed, what type of imaging protocol is being used?
Volume imaging
•In conventional imaging the slice thickness affects the SNR, in volume imaging the entire volume of tissue is excited and the SNR is 1. ______ and so 2. ___________________.
1. superior
2. fewer NEX can be used.
What are the DISADVANTAGES OF VOLUME IMAGING ?
•The disadvantage of volume imaging are that in general, the scan times associated with them are very long
•VOLUME IMAGING FORMULA FOR FINDING THE SCAN TIME IS;
•Scan time= TR * NEX * PE * number of slices
•To obtain equal resolution in every plane,
each voxel should be symmetrical (isotropic) – have equal dimensions in every plane.
•Non symmetrical voxels are named:
anisotropic
•To obtain equal resolution in every plane, each voxel should be symmetrical (isotropic) – have equal dimensions in every plane. - If this is not true, what is the result?
poorer resolution.
•Non symmetrical voxels are named ?
: anisotropic
volume imaging is used for scanning images of ?
- Imaging of joints
- Brain
- ligaments