Magnetism in MR Flashcards
What type of magnet is used with most MRI systems currently manufactured?
Superconducting
What element in the body is the principal nucleus utilized in clinical MR imaging
Hydrogen
The precessional frequency of hydrogen, according to the Larmor equation, at 3Tesla is:
127.71 MHz
A system operating at 30,000 Gauss has a field strength of:
3 Tesla
The protons that align themselves with the direction of the static magnetic field (B0) exist in a ___________energy state than those anti-parallel to the magnetic field.
: Lower
___________ substances have strong magnetic properties and can be pulled into the magnetic field
Ferromagnetic
All of the following are ferromagnetic substances EXCEPT
A. Stainless Steel
B. Gold
C. Iron
D. Nickel
Gold
The first documented human MR image was obtained on a 15 MHz RF system, what field strength did it operate at?
0.352 Tesla
All of the following are TRUE except:
A. Excitation is the process of moving the net magnetization away from the Z axis
B. Magnetic field gradients are static
C. The RF field oscillates at the Faraday frequency
D. The resonance frequency in a 1.5 Tesla magnet is 63.86 Mhz
The RF field oscillates at the Faraday frequency
What is defined as the excess number of hydrogen protons aligned with the static magnetic field direction (B0)?
Longitudinal magnetization
The energy used to form MRI images is ____________ the patient’s tissues
Emitted from
All of the following are non-ferrous substances EXCEPT
A. Plastic
B. Titanium
C. Iron
D. Copper
iron
The extent to which a material or tissue becomes magnetized in an external magnetic field is called
Magnetic susceptibility
: If the RF energy transmitted is __ MHz, the slice through the patient would be in a magnetic field of .995T.
42.36
_________ magnets can be “shut off” the quickest in case of an emergency.
Resistive
The _________ is defined as the resonant frequency, and is equal to the product of the magnetic field and the gyromagnetic ratio
Precessional frequency
All of the following are true regarding RF EXCEPT
A. The RF pulse stimulates the protons into parallel or anti-parallel alignment
B. Relaxation begins after the RF is turned off
C. RF stimulates the protons to be in phase
D. RF converts FID into an echo
RF converts FID into an echo
The force that an object can be attracted to the magnetic field is reliant upon
A. The field strength (Tesla) of the MR system
B. The mass of the projectile
C. The specific ferromagnetic nature/properties of the material
D. All of the above
All of the above
: ______________ is the term used to describe the degree of magnetization of an object
Susceptibility
Force on an object in an magnetic environment depends on:
I. Mass of the object
II. Strength of the magnetic field
III. Strength of the RF field
IV. Ferromagnetic properties of the object
I, II, and IV
A magnetic vector possesses attributes of both __________ and _____________.
Magnitude; direction
Hydrogen is used in MR imaging because
Of its abundance in the body
: Calculate the Larmor frequency for H at 0.5Tesla
21.28 Mhz
Hydrogen protons in the anti-parallel state are referred to as:
A. Spin down
B. Low energy spins
C. High energy spins
D. Both A and B
E. Both A and C
Both A and C
Immediately following the application of the 90° RF pulse, the precessing protons:
A. Begin to precess in phase
B. Tip into the transverse plane
C. All move into a high energy state
D. Both A and B
Both A and B
: Immediately following the application of the 90° RF pulse, the transverse magnetization is:
Non-zero
In one T2 relaxation time:
63% of the transverse magnetization has decayed
In one T1 relaxation time:
63% of the longitudinal magnetization has recovered
A Free Induction Decay (FID) is produced from a:
RF pulse
When thermal equilibrium is reached
There are more hydrogen protons in the low energy state
At 1.5 Tesla, the chemical shift between fat and water is
220 Hz
Image 59 is an example of a:
Free induction decay (FID)
Which of the following most accurately defines what Image 59 is?
A. Metabolites detected in the VOI sampled
B. Ferromagnetic implant detection
C. The frequency of precession of the nuclear magnetic moment
D. The decaying, oscillating signal induced after 90º pulse
The decaying, oscillating signal induced after 90º pulse
Image 60 represents the:
Larmor equation
: Image 61 represents
T1 recovery
Image 62 represents:
T2 decay
Image 82 was acquired at what field strength
1.5 Tesla
The center frequency (Resonance freq) at 1.5 Tesla is ~63.86 mHz
Aside from the center frequency, what other indicator in Image 82 signifies it was acquired at 1.5 Tesla?
The chemical shift between fat and water is 220 Hz
Image 91 was acquired at what field strength
1.5 Tesla
The center frequency (Resonance freq) at 1.5 Tesla is ~63.86 mHz.
Image 92 was acquired at what field strength?
1.0 Tesla
The net magnetization vector (NMV), when aligned with the magnetic field direction, is aligned __________.
Along the longitudinal axis
The fringe magnetic field:
Can cause nearby medical devices to malfunction
: ____________ states that a changing magnetic field will induce a voltage
Faraday’s law of induction
What has a higher precessional frequency?
Water
A FID is produced from:
An RF pulse
How many radiofrequency pulses produce a FID?
1
How well a material attracts the imaginary lines of the magnetic field is defined as:
Magnetic permeability
_____________ occurs after initial RF application, causing phase differences to appear between precessing spins, resulting in decay of transverse magnetization.
Dephasing
The difference in chemical shift is approximately ______ parts-per-million
3.5
At 1 Tesla the frequency difference between that of fat and water is approximately:
147 Hz
