MIDTERM REVIEW Flashcards
Name the factors that impact the amount of electromagnetic induction that occurs when Faraday’s law is expressed
1) Strength of magnetic field
2) Speed of motion between lines of force and conductor
3) Angle between lines of flux and conductor
4) Number of turns in conductor
State what the fingers & thumb represent in the right- and left-hand thumb rules and in the right and left hand rules (see the video links in Unit 3 – Fundamentals)
Right hand
- Thumb indicates direction of current
- Fingers indicate direction of magnetic field
Left hand
- Thumb indicates direction of conductor will move
- Index finger indicates direction of magnetic field
- Middle finger indicates direction of conventional current
Name the 3 methods of electrification
- Friction
- Contact
- Induction
Explain the direction of current and electron flow in a circuit
Electrons move from areas of high to low concentration
Name the factors that impact the amount or presence of resistance in an electrical circuit
- Material’s conductivity
- Length
- Diameter
- Temperature
Know the formulas for Ohm’s Law and how to calculate power, and the units of measurement (e.g. current is measured in amps, resistance is measure in ohms) for the associated characteristics. Again, you will NOT perform calculations, but do need to know the formulas
Ohm’s Law V = IR
V - Potential difference (Volts)
I - Current (Amperes)
R - Resistance (Ohms)
Power P = IV
P - Power (Watts)
I - Current (Amperes)
V - Potential difference (Volts)
Name the 3 energy fields used in creating MR images
- Main magnet
- RF
- Gradients
Name the intrinsic (uncontrolled) parameters of patients’ tissues
- T1 recovery
- T2 decay
- Proton density
- Flow
- Diffusion
- Perfusion
List the potential biological effects of the 3 energy fields used to produce MR images
- Burns
- Projectiles
- Hearing loss
List the 5 fundamental forces in nature & state that are known to obey the inverse square law
- Electric force
- magnetic force
- gravitational force
- weak force
- strong force
Explain the purpose and functions of the pulse control unit
- Responsible for sequencing of gradients and RF coils
- Controls switching of gradient amplifiers at appropriate times
- Controls transmission and amplification of RF
- Monitors RF transmission to keep within FDA guidelines
- Amplifies FID signal before sending it to array processor for FFT
Describe the purpose of shimming
Maintain evenness / homogeneity of the magnetic field
List the configuration of hardware in the MR system gantry
- Patient
- RF coils or probe
- Gradient coils
- Shim coils
- Primary or main magnet coils
RF shielding vs. magnetic shielding
RF shielding has 1 (NO ACTIVE SHIELD)
1) Passive (Copper) active shielding in cryostat
Magnetic Shielding has 2
1) Active – another set of electromagnets added to the gantry; keeps external RF out; active shielding in cryostat
(Faraday’s cage made out of copper)
2) Passive – shield the fringe by putting up steel in the walls of system room and zone 4 or scanner
List & describe the levels of magnetic susceptibility
Ferromagnetic - High permeability, high retentivity (IRON)
Paramagnetic - High permeability, low (NO) retentivity (CONTRAST AGENTS LIKE GADOLINIUM) WHAT WE INJECT INTO PT, WILL ACT LIKE A MAGNET WHEN ITS IN EXTERNAL FIELD, BUT WHEN YOU TAKE IT OUT, IT NO LONGER ACTS LIKE A MAGNET)
Diamagnetic - Negative permeability and nothing to retain; repel magnetic field (ALUMINUM)
Nonmagnetic - Not going to interact with external magnetic field (GLASS)
Define the terms magnetic moment, magnetic domain, magnetic dipole
Magnetic moment - denotes the direction of the north/south axis of a magnet and the amplitude of the magnetic field
Magnetic domain - If we get down to 1 individual dipole (1 electron or the nucleus of an atom) then that dipole is considered a magnetic domain; this means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction
Magnetic dipole - magnetism will exist in a dipole; north and south pole
State the functions of an iron yolk in permanent magnet system configurations
It serves to provide structure to the system and intensify the magnetic field because of the iron and it being ferromagnetic
Explain the various types of main magnet configurations
- Permanent magnets
- Resistive electromagnets
- Superconducting
Name the advantages and disadvantages of primary magnet designs
Advantages
- Higher field strength
- Better SNR
- Smaller FOV
- Faster scan times thus reduced motion artifact
- High field homogeneity - 1 PPM can be achieved with careful shimming
Disadvantages
- Very strong fringe fields
- Cryogens are expensive and require maintenance
Explain the cryogenic state and its importance in MR
- Cryogens are the liquids used to supercool the windings
- The cryostat is the portion of the gantry that contains the windings of the main magnet and cryogens
- Heat leads to the vaporization of the cryogens over tine
- This is combated by surrounding the cryostat with vacuum chambers and cycling the liquid helium through cryogenerators
- AKA cold head are a replenishment system that maintains the temp of cryogens
State the FDA limits for the energy fields used in MRI
- 4T for patients 1 month and younger
- 8T for patients older than 1 month
Name the units used to describe magnetic field strength
Tesla
Perform calculations using the Larmor equation
multiply T by 42.6
Define electrification, electric induction, magnetic induction, and electromagnetic induction
Electrification - the action or process of charging something with electricity
Electric induction -
Magnetic induction - non magnetized iron bar brought into external magnetic field; dipoles of iron bar align themselves with the lines of force
Electromagnetic induction - production of an electromotive force across an electrical conductor in a changing magnetic field; Michael Faraday; required magnetic field, conductor, relative motion
State the conditions required for a nucleus to be MR active
For an element to be MR active it must posses an odd mass number
Describe the considerations of MR in pregnant patients and occupational workers in the MR environment
- Possible for amniotic fluid can get hot, some places can say no scanning for 1st trimester
- For techs, no issues just don’t be in the room during scan
State the common characteristics of electromagnetic radiation
- Can be defined by 3 parameters
- Energy, frequency, WL
- Visible light usually behaves like a wave and is characterized by its WL
Explain the advantages of MRI when compared to various diagnostic imaging modalities
MR provides superior contrast resolution between structures in the body like grey/white matter differences and liver/spleen differentiation
(But x ray and CT are excellent for spatial resolution)
