MRI Physics 2 Flashcards by Elad Nevo

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

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The faraday cage is usually made out of which substance?

a. niobium-titanium
b. steel
c. copper
d. silver

c. copper

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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

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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

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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

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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

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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

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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

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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

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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

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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

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A large TR leads to

a. higher SNR
b. lower SNR
c. chemical shift artifacts
d. aliasing

a. higher SNR

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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)

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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

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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

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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

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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

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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.

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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

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permanent magnets display which property

a. diamagnetism
b. paramagnetism
c. ferromagnetism
d. none of the above

c. ferromagnetism

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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

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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

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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

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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

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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 = n*I 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

how is power supplied to the low field, permanent magnets often used in open MRI systems a. system uses superconducting currents b. system uses resistive currents c. a and b d. none of the above

d. none of the above

which of the following magnet systems typically have horizontally directed static fields I. permanent magnet II. resistive magnet III. superconducting magnet a. I only b. I and II only c. II and III only d. III only

c. II and III only

in order to excite protons located only in a particular slice of anatomy, the transmitted RF pulse should have what kind of waveform in the time domain a. sine wave b. sinc wave c. rectangular wave d. none of the above

b. sinc wave

if in gradient field Gz z is applied, then the factor Gz is called the a. gradient amplitude b. gradient slope c. a and b d. none of the above

c. a and b

the transmit gain (TG) determined during prescan finds a. central Larmor frequency b. the optimal B1 field strength for a 90deg RF excitation pulse c. adjusts the received MRI signal to prevent distortion d. all of the above

b. The optimal B1 field strength for a 90deg RF excitation pulse.

Suppose one chooses the receiver bandwidth BW and the field of view along the readout direction FOVx to have some original values. if one now doubles both the receiver bandwidth and the field of view, what happens to the readout gradient amplitude Gx A. Gx stays the same as original value B. Gx is twice as large as original value C. Gx is four times as large as original value D. Gx is four times as small as original value

A. Gx stays the same as original value

``` Fill in the blank: if the pixel size X in a 2D MRI image is decreased, then overall extent (size) of k space acquired must A. be decreased B. be increased C. remain unchanged D. b or c ```

B. be increased

``` Gradient Coils remove power from what unit? A. gradient amplifier B. RF amplifier C. array processor D. none of the above ```

A. Gradient amplifier

``` 49. which coil would give the highest SNR when imaging the prostate gland A. body coil B. torso coil C. 3 inch surface coil D. endorectal coil ```

D. Endorectal Coil

which of the following is NOT a true statement about K space A. k space corresponds to the MR image B. k space corresponds to the spatial frequency domain C. k space has both frequency and phase axes D. k space stores information from MR signals

A. K space corresponds to the MR image

``` the mathematical theory used to convert MR signal data to an image is called A. Nyquist's Theorem B. Faraday's Law C. Fast Fourier Transform D. none of the above ```

C. Fast Fourier Transform

``` the outer lines of k space contain data with A. low spatial resolution B. high spatial resolution C. both a and b D. neither a nor b ```

B. high spatial resolution

For a 2D acquisition, if the transmit bandwith w is increased, then the slice thickness is A. increased B. decreased C. unchanged D. varies because thickness depends on the receiver bandwidth

A. increased

``` suppose the FOV is square. In order to maintain the same SNR while decreasing the FOV length to 1/2 its orifinal size requires increasing NEX (or NSA) by what factor A. 2 B. 4 C. 8 D. 16 ```

D. 16

``` reducing the receiver bandwidth A. increases the SNR B. decreases the SNR C. increases the chemical shift artifact D. a and c ```

D. a and c

``` the inherent signal to noise ratio of patient body areas would obey A. SNR Pelvis < SNR lungs B. SNR Pelvis > SNR lungs C. SNR Pelvis = SNR lungs D. depends on pulse sequence ```

b. SNR Pelvis > SNR lungs

``` High SNR can be achieved in MRI by increasing scan time. what is the major factor limiting the scan time A. patient motion B. RF heating C. gradient noise D. claustrophobia ```

A. Patient motion

reducing slice thickness has the effect of I. increasing spatial resolution II. increasing SNR III. decreasing partial voluming A. I only B. I and II only C. I and III only D. I II and III

C I and III only

``` Now, suppose that one desired to achieve better spatial reolution by decreasing the voxel linear dimensions x,y and z each by a factor of 2. how many NEX would now have to be performed to maintain the original scan SNR A. 2 B. 6 C. 8 D. 64 ```

D. 64

``` which pulse sequence generally gives the best SNR A. SE B. GRE C. FSPGR D. EPI ```

A. SE

``` the duration of the frequency encoding gradient is called A. tau time B. the period C. the sampling time D. the relaxation time ```

C. the sampling time

``` the scan time for a non multiplanar (scan performed one slice at a time) GRE sequence is A. scan time = (TR)(Npe)(NEX) B. scan time = (TR)(Npe)(NEX)(#slices) C. scan time = (TR)(Npe)(NEX)/ ETL D. none of the above ```

B. scan time = (TR)(Npe)(NEX)(#slices)

``` the range of frequencies sampled during the readout gradient is called the A. sampling time B. Transmit bandwidth C. receiver Bandwidth D. nyquist theorem ```

C. receiver bandwidth

``` If the FOV is doubled, then the pixel size in K space will A. increase by a factor of 2 B. increase by a factor of 4 C. decrease by a factor of 2 D. decrease by a factor of 4 ```

C. decrease by a factor of 2

``` which of the following would be used for an axial magnet field gradient coil A. helmholtz pair B. maxwell pair C. golay pair D. none of the above ```

B. maxwell pair

``` Which is used to minimize the effect of fringe fields? A. SE pulse sequences B. GRE Pulse sequences C. active shielding D. contrast enhancement ```

C. active shielding

``` the temperature of liquid He is closest to ___ and the critical temperature at which niobium-titanium becomes superconducting is closest to ____ A. 4 C, ,8 C B. 8 C, 4 C C. 4 K, 8 K D. 8K , 4 K ```

C. 4 K, 8 K

``` the location of the liquid He bath surrounding the coil windings in a superconducting magnet is known by what name A. cryostat B. ALNICO C. thermostat D. none of the above ```

A. Cryostat

``` a typical value for a gradient rise time TR on a high field MRI scanner could be A. .6ms B. 60ms C. .6s D. 60s ```

A. .6ms

``` where is the dB/dt the largest during gradient switching A. isocenter B. outside the scanner C. furthest edge of linear gradient D. 5 gauss line ```

C. furthest edge of linear gradient

``` for a CSE sequence how does reducing the receiver bandwidth affect the minimum TE A. increases minimum TE B. decreases minimum TE C. does not affect minimum TE D. depends on the gradient amplitude ```

A. increases the minimum TE

``` the acquisition time for a 3D GRE sequence with TR = 50ms, matrix = 256x128x64, and NEX = 1 is closest to A. 6.4s B. 51 s C. 410 s D. none of the above ```

C. 410 sec

In comparing 2D and 3D imaging sequences, in Nz is the slice encode matrix then the SNR(3D) is ___ than the SNR(2D) by the factor ____ a. less than, Nz b. less than, (Nz)^1/2 c. greater than, Nz d. greater than, (Nz)^1/2

d. greater than, (Nz)^1/2

using the MRI direction conventions typical for a superconducting magner, the directions (slice selection, frequency encoding, phase encoding) for a sagittal scan would be a. X,Y,Z b. X, Z, Y c. Y,X,Z d. Y, Z, X

b. X, Z, Y

``` if the receiver bandwidth BW is increased, what happens to the total scan time A. increased B. decreased C. unchanged D. either a or b ```

C. unchanged

``` which kind of magnet can achieve teh highest magnetic field A. permanent B. resistive C. superconducting D. b and c ```

C. superconducting

if a liner gradient with amplitude 10mT/m is applied in the z direction, what is the additional magnetic field at a distance 20cm from the magnet isocenter a. 1 gauss b. 10 gauss c. 40 gauss d. 400 gauss

c. 40 gauss

3D data acquisition requires a readout gradient and a. 1 phase encoding gradient b. 2 phase encoding gradients c. 3 phase encoding gradients d. none of the above

b. 2 phase encoding gradients

the difference between signal to noise ratios between two adjacent areas is a. spatial resolution b. partial volume c. contrast d. contrast to noise ration

d. contrast to noise ration

roughly, what is the required homogeneity of the main magnetic field need for MRI and MRS applications a. MRI 1ppm, MRS 10ppm b. MRI 10ppm, MRS 1 ppm c. MRI 10ppm, MRS 100ppm d. MRI 100ppm, MRS 10ppm

b. MRI 10ppm, MRS 1 ppm

suppose one is performing a 2D SE MRI scan with 1 NEX. increasing which of the following will allow for increased scan coverage I. slice thickness II. TR III. TE a. I only b. I and II only c. II and III only d. I II and III

b. I and II only

if the FOV is square and the matrix is 512x256, with 512 the value of the matrix in the frequency direction, then the pixels are a. square b. rectangular with longer length in teh the frequency direction c. rectangular with longer length in the phase direction d. either b or c it depends on the control variables selected

c. rectangular with longer length in the phase direction

to achieve a thick slice thickness and a small FOV a. shallow slice select gradient slope, ,shallow frequency encode gradient slope b. shallow slice select gradient slope, steep frequency encode gradient slope c. steep slice select gradient slope, shallow frequency encode gradient slope d. steep slice select gradient slope, steep frequency encode gradient slope

b. shallow slice select gradient slope, steep frequency encode gradient slope

A patient is laying supine in the MR scanner receiving a 2D CSE brain scan with axial slice images. in which direction is the readout gradient usually places a. horizontal b. vertical c. longitudinal d. none of the above

b. vertical

if one chooses to decrease TE to increase SNR, the major trade off for this action is a. decreased spatial resolution b. increased scan time c. increased motion artifact d. decreased T2 weighting

d. decreased T2 weighting

in order to reformat a volume acquisition so taht the spatial resolution is equal in every plane requires a. high SNR b. low SNR c. isotropic voxels d. motion compensation

c. isotropic voxels

``` compared to a small surface coil, a large coil with increased area of received signal tends to have a. high SNR b. aliasing when the FOV is small c a and b d. none of the above ```

b. aliasing when the FOV is small

in SE pulse sequences the SS gradient is switched on during a. the 90deg excitation pulse b. the 180deg refocusing pulse c. the 90deg excitation pulse and the 180deg refocusing pulse d. neither the 90deg nor the 180deg refocusing pulse

c. the 90deg excitation pulse and the 180deg refocusing pulse

which of the following is not an advantage of volume acquisition a. continguous slices can be acquired b. thinner slices are achievable c. SNR is decreased because the whole volume is excited d. reformatting is any plane is possible

c. SNR is decreased because the whole volume is excited

3D data acquisition requires a. small slice gaps b. large slice gaps c. a or b d. none of the above

d. none of the above

if and RF coil is of the receive only type, which coil is typically used for transmission a. head coil b. body coil c. extremity coil d. surface coil

b. body coil

consider a voxel volume with FOV = 8cm Slice thickness = 4mm. with all other parameters fixed, doubling the size of the FOV while decreasing the slice thickness to 1/2 its original value does what to SNR a. SNR is increased by a factor of 2 b. SNR is decreased by a factor of 2 c. SNR is unchanged d. depends on matrix size

a. SNR is increased by a factor of 2

the top portion of k space is a mirror (conjugate) image of the a. right b. left c. botton d. none of the above

c. bottom

the scan time for a 3D acquistion is given by a. TR*NEX*(bandwidth)*(slice thickness) b. TR*NEX*(Npe)*(slice thickness) c. TR*NEX*(Npe)/ETL d. TR*NEX*(Npe)* (# slices)

d. TR*NEX*(Npe)* (# slices)

for a 2D CSE pulse sequence, which magnetic field gradient is usually used for encoding information along the long axis of the patient anatomy a. slice selection b. frequency encoding c. phase encoding d. none of the above

b. frequency encoding

a quadrature coil increases SNR compared to a single coil by what factor a. .707 b. 1.41 c. 2 d. does not increase SNR over single coil

b. 1.41

the idea that a signal must be sampled at least twice per cycle in order to reproduce the signal accurately is known as a. faraday's law b. nyquist theorum c. receiver bandwidth d. sampling rate

b. nyquist theorum

the MRI signal data in k space is translated into teh MRI image using what process a. MIP b. FFT c. ADC d. COB

b. FFT

aliasing in the frequency direction can be compensated by what process a. undersampling b. oversampling c. increasing NEX d. decreasing NEX

b. oversampling

spin warp refers to a. precessing magnetization b. rectilinear k space filling c. EPI d. the starship enterprise

b. rectilinear k space filling

MRI Physics 2 Flashcards

(100 cards)