CAMRT MRI Practice Chapter 7
Questions and Answers Latest Verified
100%
In MRI what is the main source of noise? - CORRECT ANSWER-Thermal motion from
the patient, it is also generated by background electrical noise of the system.
What factors affect SNR (9) - CORRECT ANSWER-Magnetic field strength
proton density
coil type and placement
TR
TE
flip angle
NSA
receive bandwidth
voxel volume
B0 increases SNR ___________ - CORRECT ANSWER-increases
Proton density increases, SNR _____________ - CORRECT ANSWER-Increases
is it better for SNR to use a large or small coil? - CORRECT ANSWER-the smallest coil
possible for the body part should be used as noise is collected from the entire coil.
Should the coil be placed parallel or perpendicular to B0? - CORRECT ANSWER-
perpendicular, in the transverse plane.
As the TR increases SNR __________ - CORRECT ANSWER-Increases (more mag is
available to be flipped into the transverse plane).
If the flip angle is increases SNR ______________ - CORRECT ANSWER-increases
as TE increases, SNR _____________ - CORRECT ANSWER-decreases. This is
because a long TE allows considerable decay. SNR is lower in T2 images, higher in T1
and PD.
Doubling the NEX increases the SNR by _____________. To double the SNR you must
do what to the NEX? - CORRECT ANSWER-root 2, or 1.4, 40%. To double SNR
increase NEX by 4 times. This also increases the scan time by 4x.
Reducing the receive bandwidth results in less ___________ sampled relative to
___________. - CORRECT ANSWER-noise, signal. So SNR increases.
, Halving the receive bandwidth _________ SNR by _________% - CORRECT
ANSWER-increases, 40%
reducing the receive bandwidth increases SNR but what is the trade off? - CORRECT
ANSWER-The sampling window must increase which increases the minimum TE.
Chemical shift also increases. Reducing the receive bandwidth is a useful way to
improve SNR in T2 weighted images in conjunction with chemical sat techniques that
remove signal from fat or water.
Voxel volume is determined by? - CORRECT ANSWER-pixel area and slice thickness
Halving slice thickness does what to the SNR? - CORRECT ANSWER-Halves it
If voxel volume is halved, SNR ? - CORRECT ANSWER-Is halved
If FOV is halved what happens to SNR? - CORRECT ANSWER-It is decreased by a
factor of 4 because halving the FOV decreases the pixels in both axis.
What image weighting produces the best CNR? - CORRECT ANSWER-T2. Even
though T1 has the highest SNR, CNR in T2 weighted images is highest due to the
contrast difference between pathology and normal tissue.
IS fat or water sat techniques more efficient on low or high field systems? Why? -
CORRECT ANSWER-High. Because the processional frequency must be large enough
that they can be distinguished from each other. The higher the magnetic field the higher
the difference in precessional frequency.
What is one reason a fat sat might not be uniform? - CORRECT ANSWER-If there are
areas of dense fat and areas of sparse fat then the fat sat will not be uniform. Also
gradients used for spatial encoding vary the frequency across each slice which causes
the presaturation to appear non-uniform.
Does optimal saturation occur at the center of a slice/center of the imaging volume or
periphery of the slice/periphery of the imaging volume? - CORRECT ANSWER-Center
Why do presat pulses reduce the number of available slices for a given TR? -
CORRECT ANSWER-Because they deliver extra RF pulses which increases the SAR
What is the sat TR? - CORRECT ANSWER-The interval between the presaturation
pulses
What is the sat TR equal to? - CORRECT ANSWER-the can TR divided by the number
of slices.
Questions and Answers Latest Verified
100%
In MRI what is the main source of noise? - CORRECT ANSWER-Thermal motion from
the patient, it is also generated by background electrical noise of the system.
What factors affect SNR (9) - CORRECT ANSWER-Magnetic field strength
proton density
coil type and placement
TR
TE
flip angle
NSA
receive bandwidth
voxel volume
B0 increases SNR ___________ - CORRECT ANSWER-increases
Proton density increases, SNR _____________ - CORRECT ANSWER-Increases
is it better for SNR to use a large or small coil? - CORRECT ANSWER-the smallest coil
possible for the body part should be used as noise is collected from the entire coil.
Should the coil be placed parallel or perpendicular to B0? - CORRECT ANSWER-
perpendicular, in the transverse plane.
As the TR increases SNR __________ - CORRECT ANSWER-Increases (more mag is
available to be flipped into the transverse plane).
If the flip angle is increases SNR ______________ - CORRECT ANSWER-increases
as TE increases, SNR _____________ - CORRECT ANSWER-decreases. This is
because a long TE allows considerable decay. SNR is lower in T2 images, higher in T1
and PD.
Doubling the NEX increases the SNR by _____________. To double the SNR you must
do what to the NEX? - CORRECT ANSWER-root 2, or 1.4, 40%. To double SNR
increase NEX by 4 times. This also increases the scan time by 4x.
Reducing the receive bandwidth results in less ___________ sampled relative to
___________. - CORRECT ANSWER-noise, signal. So SNR increases.
, Halving the receive bandwidth _________ SNR by _________% - CORRECT
ANSWER-increases, 40%
reducing the receive bandwidth increases SNR but what is the trade off? - CORRECT
ANSWER-The sampling window must increase which increases the minimum TE.
Chemical shift also increases. Reducing the receive bandwidth is a useful way to
improve SNR in T2 weighted images in conjunction with chemical sat techniques that
remove signal from fat or water.
Voxel volume is determined by? - CORRECT ANSWER-pixel area and slice thickness
Halving slice thickness does what to the SNR? - CORRECT ANSWER-Halves it
If voxel volume is halved, SNR ? - CORRECT ANSWER-Is halved
If FOV is halved what happens to SNR? - CORRECT ANSWER-It is decreased by a
factor of 4 because halving the FOV decreases the pixels in both axis.
What image weighting produces the best CNR? - CORRECT ANSWER-T2. Even
though T1 has the highest SNR, CNR in T2 weighted images is highest due to the
contrast difference between pathology and normal tissue.
IS fat or water sat techniques more efficient on low or high field systems? Why? -
CORRECT ANSWER-High. Because the processional frequency must be large enough
that they can be distinguished from each other. The higher the magnetic field the higher
the difference in precessional frequency.
What is one reason a fat sat might not be uniform? - CORRECT ANSWER-If there are
areas of dense fat and areas of sparse fat then the fat sat will not be uniform. Also
gradients used for spatial encoding vary the frequency across each slice which causes
the presaturation to appear non-uniform.
Does optimal saturation occur at the center of a slice/center of the imaging volume or
periphery of the slice/periphery of the imaging volume? - CORRECT ANSWER-Center
Why do presat pulses reduce the number of available slices for a given TR? -
CORRECT ANSWER-Because they deliver extra RF pulses which increases the SAR
What is the sat TR? - CORRECT ANSWER-The interval between the presaturation
pulses
What is the sat TR equal to? - CORRECT ANSWER-the can TR divided by the number
of slices.
