MRI Phillip W Patton, Ph.D.. RAPHEX General Question 2001 D43: In MRI, the RF frequency is dependent...

MRI

Phillip W Patton, Ph.D.

RAPHEX General Question 2001

D43: In MRI, the RF frequency is dependent on the:

A. Diameter of the body part being imaged.B. Magnetic field strength.C. Pulse Sequence.D. RF coil.E. Relaxation time.

RAPHEX Answer to 2001 Question D43

B. The frequency used in MRI depends on the product of the magnetic field strength and the gyromagnetic ratio of the element being imaged. Since most MRI is of hydrogen atoms, the frequency depends only on the magnetic field strength.


D46: Gradient fields in MRI are principally used to:

A. Maintain a uniform magnetic field in the field of view.

B. Eliminate perturbations in the magnetic field due to site location.C. Shorten T1 to reduce scan time.D. Provide spatial localization.E. Measure the spin coupling.


D. Gradient fields are used to modify the magnetic field so that

resonance will be slightly different at different points in the volume of interest. It is then possible to localize these points.


D51:A higher intensity MRI signal is produced by:

A. Long T1, long T2.

B. Long T1, short T2.

C. Short T1, long T2.

D. Short T1, short T2.


C. The MRI signal strength depends on the value of the magnetization vector along the constant field (z) axis, which is then tipped into the x-y plane to produce the MRI signal.

A short T1 implies a rapid return of the magnetization to the z axis following excitation by the RF field. A long T2 implies a long dephasing time for the individual nuclei involved in producing the magnetization. Both these effects will lead to a large value of magnetization and a large MRI signal.


D53:For hydrogen imaging in a 1.0 T MRI unit, the frequency of the RF

signal is about ________ .

A. 40 HzB. 40 kHzC. 40 MHzD. 400 MHzE. 4 GHz


C. The frequency for hydrogen is 42 MHz x magnetic field strength in tesla.

Raphex 2001 Diagnostic Questions

D43. In MRI, the RF frequency is dependent on the:

A. Diameter of the body part being imaged

B. Magnetic field strength

C. Pulse sequence

D. Relaxation time

E. RF coil


B. The frequency used in MRI depends on the product of the magnetic field strength and the gyromagnetic ratio of the element being imaged. Since MRI is mainly of hydrogen atoms, the frequency depends only on the magnetic field strength.


D46. Gradient fields in MRI are principally used to:

A. Eliminate perturbations in the magnetic field due to site location

B. Maintain a uniform magnetic field in the field of view

C. Measure the spin coupling

D. Provide spatial localization

E. Shorten T1 to reduce scan time


D. Gradient fields are used to modify the magnetic field so that resonance will be slightly different at different points in the volume of interest.


D48. In MRI images, motion during the scans results in ghost images which appear in the ______ direction.

A. Amplitude

B. Frequency encoding

C. Phase encoding

D. Relaxation

E. Slice thickness


B. Motion usually appears as a series of ghost images of reduced intensity displaced in phase encoded direction.


D50. Which of the following does NOT generally affect the total exam time of an MRI study?

A. # of acquisitions

B. # of frequency encoding steps

C. # of phase encoding steps

D. # of pulse sequences in the study

E. TR


A. frequency encoding steps are determined only by the resolution setting of the ADC.


D54. In MRI the signal-to-noise ratio can be increased by all of the following except:

A. Switching from a volume to a surface coil

B. Increasing the number of acquisitions

C. Increasing the static magnetic field strength

D. Decreasing the slice thickness

E. Increasing TR


D.


D45. Superconducting magnets, compared to resistive magnets:

A. Are less expensive

B. Are more easily turned off

C. Do not require liquid helium

D. Have higher field strength


D. Superconducting magnets have field strengths of 0.3 – 10 T, while resistive magnets have strengths below 0.2 T.


D57-D59. Match the following MRI terms. (Answers may be used more than once.)

A. Gradient fields

B. RF

C. Shim coils

D. T1

E. T2

D57. Used to adjust magnetic field uniformity

D58. Used to localize MR signal

D59. Used to tip the net magnetization of spins

Raphex 2003 Diagnostic Answers

D57-D59. Match the following MRI terms. (Answers may be used more than once.)

A. Gradient fields

B. RF

C. Shim coils

D. T1

E. T2

D57. Used to adjust magnetic field uniformity C

D58. Used to localize MR signal A

D59. Used to tip the net magnetization of spins B


D49. Patients who have MRI scans should be screened to eliminate those who have:

A. Internal steel fragments

B. Metallic prostheses

C. Pacemakers

D. Surgical clips

E. All of the above


E. Pacemakers can be affected an cause the pulse rate to change dramatically. Some surgical clips are magnetic and can move causing bleeding. Metallic prostheses can heat due to the RF pulses distorting the images. Steel workers with small fragments in their eyes have lost vision during an MRI scan.


D57. All of the following are MRI artifacts except:

A. Chemical shift

B. Ring

C. Susceptibility

D. Wrap-around

E. Zipper


B. Bounce point artifact refers to a zero signal that can occur with certain inversion recovery pulse sequences. Chemical shift is a displacement difference between lipids and water. Zipper is a series of “dropouts” due to RF problems. Wrap-around represents aliasing due to the FOV being smaller than the object size.

MRI Phillip W Patton, Ph.D.. RAPHEX General Question 2001 D43: In MRI, the RF frequency is dependent...

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