Magnetism - Magnetism in MRI Flashcards
What type of magnet is used with most MRI systems currently manufactured?
Superconducting
Superconducting magnet systems are the most common due to the high field strength and the associated imaging capabilities.
What element in the body is the principal nucleus utilized in clinical MR imaging?
Hydrogen
Hydrogen is the principal nucleus utilized in MR imaging, a primary element in both water and fat, two tissue contrasts in MRI.
The precessional frequency of hydrogen, according to the Larmor equation, at 3Tesla is:
127.71 MHz
The Larmor equation for precessional frequency: 42.57 MHz per Tesla
A system operating at 30,000 Gauss has a field strength of:
3 Tesla
10,000 Gauss = 1 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
Lower energy protons are parallel to magnetic field direction
___________ substances have strong magnetic properties and can be pulled into the magnetic field.
Ferromagnetic
Ferromagnetic substances are attracted to the magnetic field and can have a missile/torpedo like effect, placing patients, visitors and employees in potential danger.
All of the following are ferromagnetic substances EXCEPT:
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:
The RF field oscillates at the Faraday frequency
The RF field oscillates at the Larmor Frequency!
What is defined as the excess number of hydrogen protons aligned with the static magnetic field direction (B0)?
Longitudinal magnetization
Longitudinal magnetization (or Net Magnetization Vector) is defined as the excess number of hydrogen protons aligned with the static magnetic field.
The energy used to form MRI images is ____________ the patient’s tissues.
Emitted from
Unlike other imaging modalities, MRI obtains signal information via the energy emitted from the patient’s tissues following RF excitation pulses.
The extent to which a material or tissue becomes magnetized in an external magnetic field is called:
Magnetic susceptibility
Magnetic susceptibility is the extent to which a material or tissue becomes magnetized in an external magnetic field.
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
Resistive magnets can be “shut off” the quickest in case of an emergency.
The _________ is defined as the resonant frequency, and is equal to the product of the magnetic field and the gyromagnetic ratio.
Precessional frequency
The precessional frequency is defined as the resonant frequency, and is equal to the product of the magnetic field and the gyromagnetic ratio.
All of the following are true regarding RF EXCEPT:
RF converts FID into an echo
The force that an object can be attracted to the magnetic field is reliant upon:
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
Force on an object in an magnetic environment depends on the mass and the ferromagnetic properties of the object, as well as the strength of the magnetic field.
A magnetic vector possesses attributes of both __________ and _____________.
Magnitude; direction
A magnetic vector possesses attributes of both magnitude (strength) and direction.
Hydrogen is used in MR imaging because:
Of its abundance in the body
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:
Both A and C
Hydrogen protons in the anti-parallel state are referred to as spin down and/or high energy spins
Immediately following the application of the 90° RF pulse, the precessing protons:
Both A and B
Immediately following the application of the 90° RF pulse, the precessing protons begin to precess in phase and tip into the transverse plane.
Immediately following the application of the 90° RF pulse, the transverse magnetization is:
Non-zero
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
In one T1 relaxation time 63% of the longitudinal magnetization has recovered
A Free Induction Decay (FID) is produced from a:
RF pulse
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
The difference in chemical shift is approximately 3.5 parts-per-million (ppm) which at 1.5 Tesla corresponds to a frequency difference between that of fat and water of approximately 220 Hz
Image 59 is an example of a:
Free induction decay (FID)
The net magnetization vector (NMV), when aligned with the magnetic field direction, is aligned __________.
Along the longitudinal axis
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
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
Faraday’s law of induction states that a changing magnetic field will induce a voltage
What has a higher precessional frequency?
Water
A FID is produced from:
An RF pulse
A Free Induction Decay (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
Magnetic permeability – how well a material attracts the imaginary lines of the magnetic field
_____________ occurs after initial RF application, causing phase differences to appear between precessing spins, resulting in decay of transverse magnetization.
Dephasing
Dephasing occurs after initial RF application, causing phase differences to appear between precessing spins, resulting in decay of transverse magnetization
The difference in chemical shift is approximately ______ parts-per-million
3.5
The difference in chemical shift is approximately 3.5 parts-per-million
At 1 Tesla the frequency difference between that of fat and water is approximately:
147 Hz
At 1 Tesla the frequency difference between that of fat and water is approximately 147 Hz
