MRI Physics 2 Flashcards
Which of the following variables controls the MRI signal sampling rate?
a. frequency encoding matrix
b. phase encoding matrix
c. transmit bandwidth
d. receiver bandwidth
d. receiver bandwidth
The faraday cage is usually made out of which substance?
a. niobium-titanium
b. steel
c. copper
d. silver
c. copper
A 3.0T superconducting MR magnet has high performance gradients with a maximum amplitude of 100 mT/m. this is equivalent to a maximum gradient amplitude of
a. .1G/cm
b. 1G/cm
c. 10G/cm
d. 1000G/cm
c. 10G/cm
The center of K space contains data with
a. the highest signal amplitude along both phase and frequency directions
b. the lowest signal amplitude along both phase and frequency directions
c. the highest signal amplitude along the phase direction only.
d. the highest signal amplitude along the frequency direction only.
a. the highest signal amplitude along both phase and frequency directions
filling only half of k space along the frequency axis is called
a. partial echo
b. 1/2 NEX
c. a and b
d. none of the above
a. Partial echo
the time of the phase encoding gradient to achieve its proper amplitude is the
a. sample time
b. TE
c. rise time
d. tau time
c. rise time
Suppose that the FOV = 25cm on a side and the matrix = 512x512. which value below is the closest to the area of a single pixel
a. .25mm
b. .25 mm^2
c. .50 cm
d. .50 cm^2
b. .25mm^2
which value below is closest to the scan time fora CSE pulse sequence with 15 slices, TR = 500ms, FOV = 12cm, matrix of 256x256 and 1 NEX. Assuming only 10 slices can be acquired in one acquisition with the paramaters above
a. 2 min 8 sec
b. 21 min 20 sec
c. 4 min 16 sec
d. 42 min 40 sec
c. 4 min 16 sec
the direction of current through the gradient coil controls
a. the value of the static magnetic field
b. the polarity of the gradient
c. the gradient rise time
d. none of the above
b. the polarity of the gradient
if the FOV is 16cm, matrix is 256x256 and the slice thickness is 8mm with 2mm slice gaps then the total size of a single voxel is closest to
a. .625 mm^3
b. 3.13 mm^3
c. 3.91 mm^3
d. 6.25 mm^3
b. 3.13mm^3
the best strategy to increase both snr and spatial resolution is to
a. increase voxel volume size
b. decrease voxel volume size
c. increase NEX
d. decrease NEX
c. increase NEX
A large TR leads to
a. higher SNR
b. lower SNR
c. chemical shift artifacts
d. aliasing
a. higher SNR
if the readout gradient amplitude is Gx, then
a. FOVx is proportional to (Gx)
b. FOVx is proportional to (Gx^2)
c. FOVx is inversely proportional to (Gx)
d. FOVx is inversely proportional to (Gx^2)
c. FOVx is inversely proportional to (Gx)
consider how SNR depends on the number of phase encoding steps (NPE). first increasing NPE has the effect of increasing the total scan time. second, for a given fixed FOV, increasing NPE decreases the voxel volume. In this case, SNR is proportional to
a. NPE
b. 1/NPE
c. (NPE) ^1/2
d. 1/(NPE)^1/2
d. 1/NPE^1/2
during a CSE acquisition, the pase endoding gradient for any particular slice is
a. turned on before the frequency encoding gradient
b. turned on after the frequency encoding gradient
c. turned on both before and after the frequency encoding gradient
d. not turned on
a. turned on before the frequency encoding gradient
Image contrast setting on the console is related to _____ and image brightness is related to _____
a. window, level
b. level, window
c. resolution, SNR
d. SNR, resolution
a. window, level
an RF excitation pulse that is ____ in the time domain will be _____ in the frequency domain.
a. broad, narrow
b. narrow, broad
c. a and b
d. none of the above
c. a and b
cross talk or cross excitation is due to
a. static magnetic field inhomogeneities
b. non linear gradient magnetic field
c. imperfect RF waveform
d. none of the above
c. imperfect RF waveform.
Demodulation refers to
a. a gray scale of an MRI image
b. shifting central RF frequency to zero
c. adding a gradient magnetic field to the static field
d. spatially encoding the MRI signal
b. shifting central RF frequency to zero
permanent magnets display which property
a. diamagnetism
b. paramagnetism
c. ferromagnetism
d. none of the above
c. ferromagnetism
when the readout gradient is turned on, a direct relationship is established between spatial position and ____
a. larmor frequency
b. precession phase
c. a and b
d. none of the above.
a. larmor frequency
for a 2D CSE acquisition with 1 NEX the number of phase encode steps performed is ____ the phase encode matrix
a. less than
b. greater than
c. the same as
d. either a or b
c. the same as
for a 256 square matrix and a FOV of 12cm, the pixel length is closest to
a. .25mm
b. .50 mm
c. .25 mm^2
d. .50 mm^2
b. .50mm
increasing the receiver bandwidth ____ the minimum TE and ____ the SNR
a. increases, increases
b. increases,decreases
c. decreases,increases
d. decreases, decreases
d. decreases,decreases
suppose that TR = repetition time, TE = echo time, Ts = sample time, T0 = overhead, then the # slices achievable in a period is
a. TE/(TR + Ts/2 + T0)
b. (TR + Ts/2 + T0)/TE
c. TR/(TE + Ts/2 + T0)
d. (TE + Ts/2 + T0)/TR
c. TR/(TE + Ts/2 + T0)
motion artifacts in 2D MRI scans are most likely to show up in which direction
a. slice select
b. frequency encode
c. phase encode
d. all of the above
c. phase encode
a typical value for the receiver bandwidth for a 1.5T superconducting scanner is
a. 16 Hz
b. 16 kHz
c. 16 MHz
d. none of the above
b. 16 kHz
the recognized units for SAR are
a. J/lb
b. J/kg
c. W/lb
d. W/kg
d. W/kg
for a 2D acquistion, the weakest phase encoding gradient amplitude (Gy = 0) has the ___ MRI signal and is positioned so that it occupies the ___ part of K space
a. lowest, central
b. lowest, peripheral
c. highest, central
d. highest, peripheral
c. highest, central
valid units for the spatial frequency k include I. Hz II. 1/seconds III. cm IV. 1/meter
a. I only
b. I and II only
c. III only
d. IV only
d. IV only
the approximate reception region of a circular surface coil with radius R extends to what depth
a. R
b. 2R
c. PiR^2
d. 2PiR
a. R
the wires used in most superconducting MRI magnets to maintain the large static field are made of ____ and are cooled by ____ cryogens
a. copper, liquid nitrogen
b. copper, liquid helium
c. niobium/titanium, liquid nitrogen
d. niobium/titanium, liquid helium
d. niobium/titanium, liquid helium
lining the walls of the MR scan room with steel is a method of _____ while using additional soleniod coils with opposite polarity is _____
a. active shimming, passive shimming
b. passive shimming, active shimming
c. active shielding, passive shielding
d. passive shielding, active shielding
d. passive shielding, active shielding
which coils are responsible for the acoustic noise heard in MRI scans
a. RF
b. supersonducting
c. shim
d. gradient
d. gradient
positioning small pieces of metal around the magnet bore is a method of _____, while placing additional current carrying wire around the bore is _____.
a. active shimming, passive shimming
b. passive shimming, active shimming
c. active shielding, passive shielding
d. passive shielding, active shielding
b. passive shimming, active shimming
the electric current I through a wire with resistance R due to an applied voltage V can be calculated from Ohm’s law. which equation below is Ohm’s Law
a. I = VR
b. I = R/V
c. I = V/R
d. none of the above
c. I = V/R
phased array coils are composed of multiple
a. volume coils
b. surface coils
c. a and b.
d. none of the above
b. surface coils
consider a solenoidal electromagnet with n = # of windings per unit length, I = electric current. then the magnetic field inside the solenoid is
a. B = n/I
b. B = nI
c. B = n I^2
d. none of the above
b. B = n * I
which of the following coils are typically transmit/receive RF coils
I. head coil
II. Body coil
III. extremity coil
a. I only
b. II only
c. I and III only
d. I, II and III
d. I, II and III
Which of the following can be associated with volume RF coils
I. transmit/receive
II. high SNR compared to surface coil
III. uniform excitation
a. I only
b. I and II only
c. I and III only
d. II and III only
c. I and III only