Huda MRI Flashcards
(30 cards)
1. Which would not be useful for medical MR imaging? (A) 1H (B) 13C (C) 16O (D) 23Na (E) 31P
C. 16-O
16O is an example of an even-even nucleus
(eight neutrons and eight protons), which has no
magnetic moment and thus cannot be used for
MR.
2. The Larmor frequency is the frequency of: (A) Pulse repetition (B) Nuclear precession (C) Phase encoding (D) Spatial encoding (E) Gradient switching
B. Magnetic nuclei precess at the Larmor frequency
when placed into a magnetic field.
3. The resonance frequency for 'H in a 1.5 T magnetic field is: (A) 63 H3 (B) 63 kH3 (C) 63 MH3 (D) 63 GH3 (E) 63 TH3
C. 63 MHz.
- The maximum MR signal is obtained by using
a:
(A) 90 degree RF tip, short TE, and short TR
(B) 45 degree RF tip, short TE, and short TR
(C) 90 degree RF tip, short TE, and long TR
(D) 90 degree RF tip, long TE, and short TR
(E) 45 degree RF tip, long TE, and short TR
C. Maximum signal strength is obtained by using
a 90 degree pulse to maximize the magnetization
in the transverse plane, starting with the maximum
longitudinal magnetization (i.e., long TR)
and minimizing dephasing (i.e., short TE).
- For most tissues, which of the following is
false?
(A) T1 is of the order of a few seconds.
(B) T2 is of the order of tens of milliseconds.
(C) T2 is relatively independent of field strength.
(D) T1 increases as field strength increases.
(E) T1 and T2 often increase with malignancy
A. T1 is of the order of hundreds of milliseconds,
not a few seconds.
6. The small amount of bound water produces no detected MR signal because: (A) T1 is too short (B) T2 is too short (C) T2* is very long (D) T2 is longer than T1 (E) T2* is longer than T2
B. The T2 is too short (microseconds) to give
rise to a detected signal from all solids including
bone.
7. For most biological tissues, T2 is: (A) Less than T1 (B) More than T1 (C) Less than T2* (D) More than 100 milliseconds (E) Less than 5 milliseconds
A. T2 is generally of the order of tens of milliseconds,
whereas T1 is on the order of hundreds
of milliseconds.
8. MR "shimming" is used to: (A) Minimize noise in RF coils (B) Correct for magnetic-field inhomogeneities (C) Reduce the noise level in MR systems (D) Minimize the possibility of quenches (E) Increase signal phase
B. Shimming is used to reduce field inhomogeneities
to a few parts per million.
- The superconducting magnets used in MR
normally have:
(A) No magnetic-field inhomogeneities
(B) Water cooling to dissipate heat production
(C) Coils with alternating electric currents
(D) Magnetic fields perpendicular to the bore axis
(E) Liquid helium coolant
E. Superconductors normally require liquid helium
coolant.
10. Gradient fields in MR are used most commonly to: (A) Increase T2 (B) Shorten T1 values (C) Localize MR signal source (D) Increase signal in large patients (E) Reduce electronic noise
C. Gradients define the MR image plane and
are used for frequency and phase encoding to determine
the spatial origin of the detected signals.
11. Which of the following is generally acceptable for MR at 1.5 T? (A) Cochlear implants (B) Pacemakers (C) Ferromagnetic aneurysm clips (D) Claustrophobic patients (E) Pregnant patients
E. Pregnant patients can undergo MR scans
no ionizing radiation
12. Which line is an exclusion zone for persons with pacemakers? (A) 0.5 G (B) 5 G (C) 50 G (D) 500G (E) Over 500 G
B. Areas with magnetic fields greater than 5 G
(0.5 mT) should be restricted to individuals with
pacemaker implants.
- Safety concerns for 1.5 T MR include all of
the following except:
(A) Fringe magnetic fields
(B) Electrical stimulation neurons
(C) RF heating effects
(D) Ferromagnetic surgical slips in patients
(E) Flying metallic objects
B. For magnetic-field strengths below 2 T, the
effect of magnetic field-induced electrical potentials
in neurons has not been observed.
- The FDA limit on power deposition in patients
undergoing MR does not include:
(A) 3.2 W/kg averaged over the head
(B) 8 W/kg peak value
(C) 0.4 W/kg averaged over body
(D) A less than 3 °C temperature rise in the heart
(E) A less than 1 °C core body temperature rise
D. There are no specific temperature rise limits
set for the heart by the FDA in MR.
15. The most common reconstruction method for MR units is: (A) 2DFT (B) 3DFT (C) Algebraic reconstruction (D) Back projection (E) Filtered back projection
A. 2DFT is standard on virtually all commercial
MR units.
16. Which does not generally affect the total patient examination time in MR? (A) Read encode matrix size (B) Number of phase-encoding steps (C) Number of pulse sequences used (D) Acquisitions (Nex) (E) TR
A. The number of pixels in the read-direction
is related only to the number of samples in FID
acquired and will not affect image acquisition
time.
17. In spin-echo imaging, the echo signal normally is measured: (A) Immediately (t = 0) (B) After time TE (C) After time 4 x T1 (D) After time T2 (E) After time TR
B. A phase-refocusing 180 degree pulse is applied
at time TE/2, which results in an echo at time
TE.
- In IR sequences, the T1 value is the time:
(A) Of the complete scan
(B) To the interval echo
(C) Between successive 90 degree pulses
(D) Between successive 180 degree pulses
(E) Between an initial 180 degree and subsequent
90 degree pulse
E. An IR sequence starts with a 180 degree inversion
pulse followed by a 90 degree readout
pulse after time TI.
19. Increased signal intensity in MR cannot arise as a result of: (A) Short T1 (B) Long T2 (C) Flow effects (D) Spin density (E) Dephasing effects
E. Dephasing effects always reduce signal intensities
(they may increase contrast, but not signal
intensity per se).
20. MR SNR cannot be improved by increasing the: (A) Matrix size (B) Number of acquisitions (C) Static magnetic-field strength (D) Section thickness (E) RF coil sensitivity
A. Increasing the matrix size will not increase
SNR but will reduce the SNR per pixel.
- Chemical shift artifacts are caused by:
(A) Different resonant frequencies of 1H in water
and fat
(B) Foreign chemicals agents in the patient
(C) Magnetic-field gradients
(D) Contrast agents
(E) Spin-lattice interactions
A. Chemical shift artifacts arise because of
the slightly differing resonance frequencies of proton
in different molecules.
22. In MR, motion results in ghost images that appear in which direction? (A) Read encode (B) Phase encode (C) Slice selection axis (D) PA (E) Lateral
B. Motion usually appears as a series of ghost
images of reduced intensity displaced in the phase encoding direction.
23. All of the following are MR artifacts except: (A) Chemical shift (B ) Bounce point (C) Zipper (D) Susceptibility (E) Reverberation
E. Reverberation artifacts occur in ultrasound.
24. Superparamagnetic materials are not: (A) Small particles (smaller than 350 A) (B) Single domains (C) Strongly magnetic (D) Related to ferromagnetic materials (E) Superconductors
E. Super paramagnetism has no relation to superconductivity.
