STUDYGUIDES Flashcards
Calculate the resonant frequency of a 2.5T system
106.675 MHz/T or just 106.5
The strength of the net magnetization vector (NMV) is dependent upon two factors. One is the ambient temperature, name the second
The strength of the static magnetic field
Which element is imaged when performing clinical MR studies?
Hydrogen
Name the 3 energy fields necessary to create the MR signal
- Main magnetic field (B0)
- Radiofrequency field (B1)
- Gradients
Which scientists introduced the use of gradient magnetic fields (time varying magnetic fields) to perform the task of spatial localization of the MR signal?
Paul Lauterbur, Ph.D.
What interaction do radiofrequency photons that are transmitted at the resonant frequency undergo with the patient’s tissues?
They are absorbed
What interaction do radiofrequency photons that are transmitted at a frequency different from the resonant frequency undergo with the patient’s tissues?
There is no interaction. They are transmitted.
What did Orstead discover was the result of moving electrical charges?
A magnetic field
Which characteristic displayed by an electrical circuit at room temperature will be absent when a material is cooled below its critical temperature?
Resistance
Two things occur when the excitation pulse is transmitted at the Larmor (or resonant) frequency. First the spins begin to precess coherently, or in phase. Name the second.
They get excited and “flip” to precess within the transverse plane
What is the name of the effect that causes elevation of the T wave in cardiac gating?
Magneto-hemodynamic effect
What velocity does a paper clip achieve when released into a 1.5 T magnetic field?
40 MPH
What is the SAR limit in Watts/kilogram for the head in the USA?
3
Name three effects time-varying gradient magnetic fields may have on patients being scanned using EPI sequences.
- Peripheral nerve stimulation
- magnetophosphenes
- acoustic noise
What is the name of the most up-to-date MRI safety website?
www.mrisafety.com
What is the FDA limit for the strength of B0 in clinical MR when scanning an adult patient?
8 Tesla
What must everyone who remains in the system room during scanner operation be provided?
Hearing protection
What are the transient biologic effects produced by the radiofrequency fields (B1) encountered by patients in the MR environment?
- Tissue heating
- RF antennae effects
- thermal injuries (burns)
How much insulation must be placed between the patient and the bore of the scanner or any known conductive materials within the bore to ensure patient safety?
1 cm or 0.25”
Within what timeframe must all adverse events, MR safety incidents, or near misses be reported to the MR site medical director should they occur?
24 hours
Define Coulomb’s Law
The force between two charges is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them.
What is the distinction between electrification of an object through contact or friction from electrification via induction?
Electrification through induction does not require the objects to touch to become electrified
Name the 5 fundamental forces
Electric force, magnetic force, gravitational force, weak force, strong force
Which fundamental force is responsible for the formation of molecules?
Weak force
Which fundamental force is responsible for locking the protons and neutrons into the nucleus?
Strong force
If an object has a curved surface where will the highest concentration of charges be located?
Where the curvature is the greatest
What are 3 characteristics are present in a live electrical circuit at ambient temperature?
- Potential difference
- current
- resistance
State the formula to calculate power
P = IV
State the unit of measurement of power.
Watts
State the units of measurement for current, potential difference, and resistance
Current = Ampere
Potential difference = Volt
Resistance = Ohm
How many kilo-Gauss are in 1 Tesla?
10 kG
What is a magnetic moment?
A magnetic moment is a vector that denotes the direction of the north/south axis of a magnet and the amplitude of the magnetic field
What is meant by the term thermal equilibrium as it pertains to MRI?
Thermal equilibrium is the condition whereby the net number of transitions of hydrogen nuclei evenly align parallel and anti-parallel to B0
What is the term used to describe the magnetic vector produced as a result of the alignment of excess hydrogen nuclei with the external magnetic field?
Net Magnetization Vector
Name the three axes of the Cartesian coordinate system.
x, y, z
Which axis of the Cartesian coordinate system is commonly called the longitudinal axis?
the z-axis
Which axes of the Cartesian coordinate system are commonly referred to as the transverse plane?
x, y
In which direction do the magnetic lines of force flow outside of a magnet?
from north to south
In which direction do the magnetic lines of force flow within a magnet?
from south to north
What are the three components necessary to obey Faraday’s Law of Electromagnetic Induction?
a magnet, a conductor, movement between them
A material possesses high permeability and low retentivity. What type of magnetic susceptibility does this describe?
paramagnetic
Name the two classifications of scientific theories used to describe the basic principles of MRI.
classical physics and quantum physics
Define the term: atom
the smallest component of an element having the chemical properties of the element, consisting of a nucleus containing combinations of neutrons and protons and one or more electrons bound to thenucleus by electrical attraction
Define the term: molecule
any subatomic particle contained within the nucleus
Define the term: proton
a subatomic particle with a positive charge that is contained within the nucleus by the strong fundamental force of nature
Define the term: neutron
a subatomic particle with a neutral charge and slightly more mass than a proton that is contained within the nucleus
Define the term: electron
a subatomic particle with a negative charge that spins about its own axis while orbiting the nucleus of an atom
Define the term: isotope
any of two or more forms of a chemical element, having the same number of protons in the nucleus, or the same atomic number, but having different numbers of neutrons in the nucleus, or different atomicweights. There are 275 isotopes of the 81 stable elements, in addition to over 800 radioactive isotopes,and every element has known isotopic forms. Isotopes of a single element possess almost identicalproperties.
Define the term: ion
an electrically charged atom or group of atoms formed by the loss or gain of one or more electrons, as a cation (positive ion), which is created by electron loss and is attracted to the cathode in electrolysis, or as an anion (negative ion), which is created by an electron gain and is attracted to the anode. Thevalence of an ion is equal to the number of electrons lost or gained and is indicated by a plus sign forcations and a minus sign for anions, thus: Na+, Cl−, Ca++, S=.
What is the mass number and why is it important in MRI?
The mass number is the number of protons and neutrons contained in an atom’s nucleus. It is important because for an element to be MR active it must posses an odd mass number.
Which element is imaged when performing clinical magnetic resonance scans?
hydrogen
What does the size of a magnetic vector represent? What does the arrowhead of a magnetic vector represent?
The size of the vector represents the magnitude or strength of the magnetic field. The arrowhead represents the direction of the north pole.
What conditions are necessary for resonance
The scanner must transmit a radiofrequency pulse that matches the precessional frequency of the nuclei.
Describe the two results of resonance.
The spins precess in-phase or coherently, and their magnetic moments flip into the transverse (Mxy) plane.
Define T1 recovery
The time it takes for a tissue to recover 63% of its longitudinal magnetization.
Define T2 decay.
The time it takes for a tissue to lose 63% of its transverse magnetization.
Which pulse sequence timing parameter determines the time interval applied between one radiofrequency excitation pulse to the next?
TR or repetition time
Which pulse sequence timing parameter determines the time interval between the excitation pulse and the peak of the echo?
TE or echo time
Which set of coils is closest to the patient within the MR system gantry?
the RF probe or coil
What is the purpose of the shim coils within the MR system gantry?
to keep the B0 field even or homogenous
What alloy is the primary magnet winding composed of?
niobium titanium
What are the two types of magnetic shielding?
active and passive
What material is used to provide passive shielding to the MR suite?
steel
Where are the active shielding electromagnet windings housed within the MR system gantry?
within the cryostat
What is the purpose of the Faraday cage that is built into the walls of the MR system room?
to prevent external RF from entering the system room and contaminating the signal transmitted by the system, as well as to protect the signal picked up by the receiver coil
What type of material is the Faraday cage composed of?
copper
How many sets of gradient coils does a MR system have?
three: x, y, & z
What increments must the MR system table have the ability to position patients at?
milimeters
Which component of the MR system allows the user to perform image processing and post-processing tasks?
the console or operator interface
According to the FDA all patients must be monitored how?
visually & verbally
The need for a large storage capacity and the ability to perform multiple calculations per second requires that the MR system operate with what type of computer system?
a minicomputer
Which portion of the MR system hardware monitors the specific absorption rate of various pulse sequences?
the pulse control unit
What is the primary role of the pulse control unit?
the sequencing of the RF and gradient coils of the MR system
What calculation is performed by the MR system computer when sampling data points for temporary storage in the array processor?
the fast Fourier transformation (FFT)
How many shades of grey can each pixel in the image display?
one
What unit of measurement is used to describe gradient amplitude?
mT/m or G/cm
What phenomenon is responsible for producing the acoustic noise made by the gradients during the performance of a pulse sequence?
the Lorentizan force
What is the definition of gradient rise time?
the time it takes for the MR system’s gradient coils to reach maximum amplitude
Which type of coil allows for the high SNR of a small receiver coil and use of a large of FOV of a large receiver coil?
a phased array coil
What type of coil is required to perform pulse sequences with parallel imaging?
a multi-channel or multi-element coil
FID-
It is in the transverse plane and it is the first time they relax back is FID
The giving up of the energy or the relaxing back of the NMV
Our signal is decaying while it relaxes back to b0
FID-
It is in the transverse plane and it is the first time they relax back is FID
The giving up of the energy or the relaxing back of the NMV
Our signal is decaying while it relaxes back to b0
