Week 4 Flashcards
Define Faraday’s Law, a.k.a. Law of Electromagnetism
If you pass a current of electricity through a coiled wire, the result will be in magnetic field
Magnetic field direction is determined by the direction of the current. True or false?
True
The right hand rule is a visual representation that explains current direction versus magnetic field direction. What does each component represent?
- The fingers curl and the direction of the magnetic field
- The thumb points in the direction of the current
What are the two types of imaging magnets?
- Permanent.
- Electromagnets.
How are permanent magnets constructed and what is the orientation of the magnetic field?
Constructed with many smaller magnets surrounding a solid iron core; vertical magnetic field
How are electromagnets constructed and what is the orientation of their magnetic field?
Constructed with multiple loops of copper wire and a constant power source; horizontal magnetic field
What are three advantages of a permanent magnet?
- Less power consumption.
- Consistent and reliable magnetic field.
- Weak fringe field.
What are three disadvantages of permanent magnets?
- Unquenchable.
- Intense dismantling process, borderline unmovable
- A weak magnetic field (~0.4 T)
What are the three advantages of electromagnets?
- Strong magnetic field.
- High image quality.
- Quenchable.
What are two disadvantages of electromagnets?
- High power consumption.
- Costly maintenance.
What are the two types of electromagnets?
- Resistive.
- Super conducting.
What are three advantages of resistive electromagnets?
- Horizontal or vertical magnetic fields.
- Quick on/off switch
- Weaker fringe field than superconducting
What are three disadvantages of resistive electromagnets?
- High power cost.
- Strict water cooling parameters
- Weak magnetic field, compared to super conducting
what are three advantages of super conducting electromagnets?
- Lower power cost than resistive.
- Strongest magnetic field (1.5+ T)
- Consistent magnetic field
What are the two disadvantages of super conducting electromagnets?
- Their strong magnetic field also creates a strong fringe field
- High maintenance costs due to the internal cryogen storage.
What is the purpose of magnetic shielding?
To limit and regress the range of the fringe field
What are the two types of magnetic shielding and how are they applied?
- Passive: an iron shell encasement for the magnet
- Active: most effective, uses an electric current applied in the opposite direction
If B0 is the direction of the magnetic field along the length of the bore, then what is B1?
The direction of the RF pulse that the selected gradient transmits to vary the magnetic field.
The gradients create magnetic field variations with an RF pulse in order to achieve spatial localization. What do the gradients create this RF pulse with?
The transmit coil
Where is the transmit coil located?
Perpendicular to B0 and deep within the MRI structural layers
What are the biological effects of the RF pulses and how do technologists limit them?
Tissue heating and Burns; the use of padding
Once the gradient emits an RF pulse via the transmit coil, what is used to receive the echo that is admitted from the patient?
Receiver coil
Where is the receiver coil located?
The receiver coil is manually chosen and attached to the exam table by the technologist. Its location depends on the specific body part being imaged.
What is a transmit receiver coil (T/R coil)?
A coil that can both transmit an RF pulse and receive the resultant echo, manually chosen and attached by technologist.
What are two advantages of using a T/R coil?
- Allows for the selection and conjunction of multiple exams using one coil
- Reduces SAR and B1 RMS levels by only exciting a small area of the body with RF pulses.
What are the three types of dedicated coils?
- Phase array coil.
- Volumetric coil.
- Surface coil.
How are phase array coils constructed?
With two separate pieces joined together, placed anteriorly and posteriorly to the patient. A.k.a. the sandwich coil.
How do phase array coils affect SNR and exam time?
Phase array coils increase SNR and decrease exam time
What are two advantages of using a phase array coil?
- Allow for parallel imaging.
- Can achieve a large field of view at a reduced exam time
What are two disadvantages of phase array coils?
- Expensive
- Having minimum field of view of about 16 cm.
What coil is used predominantly in imaging of the torso?
Phase array coil
How are volumetric coils constructed?
Constructed to the shape of the body part
What are three advantages of using volumetric coils?
- Ideal signal due to close proximity to the area of interest.
- Ability to utilize motion and metal reduction techniques.
- Can image a small field of view at high resolutions
What are two disadvantages of volumetric coils?
- Limited numbers of studies per coil, as coil is made to the shape of specific body part.
- Expensive.
How are surface coils constructed?
Constructed with multiple elements that can be manually activated by the technologist. They are long and flat to cover large, linear field of views
What are two advantages of using a surface coil?
- The ability to scan multiple exams without stopping and re-landmarking the patient.
- Obtains a rectangular field of view.
What coil is ideal for spinal imaging?
Surface coil
What is a disadvantage of using a surface coil?
Risk of “Annefact”, which is a glaring artifact caused by coil segments accidentally left activated outside the imaged field of view
When must radio frequency tuning be done manually by the technologist?
When a patient has an exceptionally low body fat percentage
Any coil used in MRI will (usually) automatically adjust the bandwidth of RF pulses before the exam to match the part being an image and calculate the optimal signal that should be used. What is this process called?
Radio frequency tuning
What does TVMF stand for?
Time varying magnetic fields
The FDA does not specify a numerical threshold for TV MF. TV MF is reached when a patient begins experiencing nerve stimulation from being in the magnetic field for too long. What are some of these peripheral nerve stimulations that a patient may experience?
- Hearing disruptions.
- Stars in the eyes (magnetophophenes)
- Involuntary motions and twitches.
To set the stage, a gradient has been selected to produce an RF pulse that will vary the magnetic field and allow for spatial localization. In this scenario, what would gradient amplitude refer to?
The amount of electrical current that must be used to produce the RF pulse.
During the process of electrical current production, the action of the gradient amplitude increasing is referred to as what?
Ramp up
During the process of electrical current production, the action of the gradient amplitude decreasing is referred to as what?
Ramp down
During the ramp up process of the gradient amplitude, what does the rise time refer to?
The time it takes the amplitude to reach its peak
During the ramp down process of the gradient amplitude, what does the fall time refer to?
The time takes the amplitude to decrease back down to baseline
What two factors determine the power of the gradient amplitude?
- Slew rate.
- Duty cycle.
What is the slew rate?
How quickly a gradient amplitude takes to reach peak.
What’s the difference between rise time and slew rate?
Rise time is measured in seconds. Slew rate is a speed and measured in milliTesla per meter per second (mT/m/s)
Given that slew rate is measured in mT/m/s, how would you calculate it?
Slew rate = gradient amplitude x rise time
What is duty cycle?
The amount of time the gradient amplitude remains at its peak during one TR period.
How is duty cycle measured?
As a percentage
What is the magnetic field homogeneity measured in?
Parts per million
What is shimming in regard to MRI?
Process in which shim coils are activated before a scan to balance the magnetic field.
What is the purpose of balancing the magnetic field via shimming?
To reduce the risk of error caused by molecular metal in the exam room environment.
What are the two types of shimming?
- Passive
- Active.
What’s the difference between passive and active shimming?
Passive shimming is a physical structure that is always present and always on. Active shimming is an electromagnetic structure that is only activated by the technologist.
