Physics: MR (Brief Overview and Emerging Applications) Flash Cards

Study Save to My Flashcards Offline Study Print Export to file

Play Memory | Create Card File | Append to Card File

Card Set Properties

Title:	Physics: MR (Brief Overview and Emerging Applications)
Description:	physics
Number of Cards:	56
Save Count:	0
Author:	loumuscarella11
Created:	2012-01-26
Tags:	physics
Private	No

Save Count represents the number of people who have saved this card set to their flashcard list. Consider this an endorsement!

- Question
- Answer
- Side 3

- What does DWI tell us about
- the movement of water
- What does DTI tell us about
- fiber orientation of WM tracts
- What does spectroscopy tell us about
- metabolic information
- What two things can a contrast agent tell u about a region
- if it is vascular or if the permeability has changed (retention of contrast)
- What are 3 types of magnets used in MR imaging
- permanent
  resistive
  superconducting
- What are the characteristics that are important when discussing magnets
- field strength
  stability
  homogeniety
- What are advantages of permanent magnets
- require no cooling
  cheaper
- What are disadvantages of permanent magnets
- heavy
  less field homogeniety
  can not be turned off in emergencies
- What type of scanner uses a permanent magnet
- open scanner
- What are advantages of a resistive magnet
- none
- What are disadvantages of a resistive magnet
- they have poor homogeniety
  weak magnet
  high electric cost
- How is a resistive magnet designed
- by running an electical current through a wire to create a magnetic field
- If a resistive magnet is used what type of MRI scanner is it
- open
- what type of magnet do most modern MRI machines use
- superconducting magnets
- How does a superconducting magnet work
- Superconducting
  magnets are based on the principle of cooling
  down (4°K) certain metal conductors so that
  there is little or no resistance; therefore, a high electric current can be used to generate high-
  strength magnetic ﬁelds with no major heat disposition
- What is a disadvantage of a superconducting magnet
- expensive
- What is required to cool a superconducting magnet
- cyrogen (usually liquid helium)
- What does field stength tend to effect
- the signal to noise ratio of the image (particularly in T1)
- Longitudinal relaxation
- T1
- Are both the signal and contrast to noise ration effected by the magnet field strength
- yes
- What is the cause of the worse image characteristics of a weak magnetic field
- The time required
  for complete relaxation differs for different ﬁeld
  strengths; for example, the T1 is shorter at lower
  ﬁeld strengths and tends to increase at higher ﬁeld
  strengths. These changes affect both the
  signal- and contrast-to-noise ratios ofMR images
- What are the units of magnetic field strenght
- tesla
- What does 1 T equal
- 10000 gauss
- What has been the challange when developing new MR machines
- increasing the S;N ratio
- What is the best way to increase the signal to noise ratio
- increase the strength of the magnet
- What is a low strength magnet
- .1-.5
- What is a medium strength magnet
- .5-1
- What is a high strength magnet
- 1.5
- What is an ultra high strength magnet
- >3
- What are shims used for
- to make sure there is good local homogeniety of the magnet
- What is the purpose of the gradient coil
- to create magnetic field variation in three different directions for localization of signal
- How do shim and gradient coils work
- Basically, a shim or gradient coil is a de-
  vice that can generate a spatially localized mag-
  netic ﬁeld within the main B0 ﬁeld by using elec-
  tric current
- Where are shims and gradients coils located
- hysically, the shim and gradient
  coils are placed concentric to each other in the
  magnet and activated at speciﬁc times of the pulse
  sequence.
- Why is a shim required
- The quality of the received signal
  requires good ﬁeld homogeneity and thus re-
  quires a shim of the local magnetic ﬁeld, which is
  the B0 ﬁeld along the z direction.
- What is the cause of suseptibility effects
- this is caused by an object placed in the magnetic field and these are reason shims are required to correct the this local changes in the magnetic field
- What is another name for a shim
- correction coil
- What are 2 types of shim coils
- active and passive
- What are passive shim coils
- these are shims that are configured at the time of installation of the magnet by using a metal plates with in the bore or surface of a magnet
- What are active shim coils
- these require an electrical current through special coils
- Do most magnets have active and passive shim coils
- yes
- What are gradient coils used for
- Gradient coils are used for
  localization of theMR signal in three directions
  (x, y, and z)
- What are the 3 parts of the gradient coil
- the coils that create magnetic effect in the X, Y and Z directions. These lie concentric to eachother
- What are the important paramenters of the gradient coil
  3
- amplitude
  rise time
  slew rate
- What is the amplitude of the gradient coil
- the gradient strength
- What are the units of amplitude (gradient strength) of a gradient coil
- tesla/meter
  G/cm
- What is the slew rate of the gradient coil
- The slew rate
  of a gradient system (in tesla per meters per sec-
  ond) is deﬁned as the ratio of gradient strength
  divided by the rise time
- What are RF coils used for
- to transmit RF energy to tissue
  
  To receive the RF signal from the tissue of interest
- Can an RF coil just recieve or just send signal
- yes
- What are 4 types of RF coils
- usual designs are sur-
  face, saddle, quadrature, or phased array (mul-
  tiple elements)
- Are RF coils designed for specific body parts
- yes
- Once a signal is received by an RF coil what happens to it
- it is transmitted to an RF amplifier and reconstructed in a the main computer
- Is there multiple RF coils that are used in a single study
- yes
- What is another name of multiple coil MR imaging
- multiple "channel" (like CT)
- What has the use of a greater amount of RF coils led to
- advances in pulse sequence design and imaging methods
- What are parellel imaging methods
- simultaneous acquistion of spatial harmonics or sensititivity encoding.
- What has increasing the number of RF coils done to imaging time and SNR
- These methods have resulted in reduced imaging time but also in a decrease in SNR