MRI - Intro to MRI

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swtjo3joe
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285477
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MRI - Intro to MRI
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2014-10-13 06:58:30
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history atom hydrogen precession rf t1 t2 relaxation pulse sequence image quality signa noise ratio contrast resolution scantime echotime gradientfield resonance slicethickness frequency phase magnetism permanentmagnet electromagnets resistivemagnet superconductive highfield midfield lowfield fringefield magnetshielding shimcoils gradientcoils rfcoils pacs
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introduction to MRI
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  1. what did Bloc and Purcel get the noble prize in 1952 for?
    discovery of magnetic properties of an atom
  2. what year was Nuclear Magnetic Resonance (NMR) spectrometers widely used?
    1960
  3. What year was MRI readily utilized?
    1980
  4. what are the advantages of MRI?
    • no harmful effects
    • no ionizing radiation
    • cross-sectional images
    • images in any plane
    • digital images
    • exquisite tissue differentiation
  5. what are indications for mri head?
    • detect tumors, infection, blockages in vessels, aneurysms, hemorrhage, multiple sclerosis, various types of brain damage including the tissue damage caused by stroke
    • problems of the eyes, optic nerves, auditory canals and auditory nerves
  6. what are the indications for MRI of the spine?
    • problems of the cervical, thoracic, and lumbar regions of the the spines as well as the sacrum
    • tumors, degenerative disc disease, cord impingement, and MS can be visualized
  7. what are the indications for MRI of the joints?
    • used to evaluate the knee, ankle, hip, wrist, elbow, shoulder, and TMJ
    • MRI excels in ability to visualize the cartilage, ligaments, tendons, muscles and is used to diagnose cartilage tears, worn-out cartilage, torn ligaments, the presence of fluid, infection and bone tumors
  8. what are the advantages of having hydrogen atoms for MRI use?
    • odd number of protons
    • generates the signal we use to construct images in clinical MRI
    • abundant in the body
  9. what is created whenever a charged particle moves?
    a magnetic field
  10. why do hydrogen protons within every tissue in the body have magnetic fields?
    because they spin around on their axes generating magnetic fields
  11. which direction(s) do protons align with an externally applied magnetic field ()?
    • parallel or anti parallel
    • more parallel than anti parallel
    • result is the net magnetization (NM)
  12. why are there more protons aligning parallel than anti-parallel to the externally applied magnetic field?
    • because it only takes a small amount of energy to align parallel to the magnetic field.
    • It takes a lot of energy to align anti-parallel. 
    • there are more protons with weak energy than strong energy
  13. what happens to the net magnetization when the magnet is stronger?
    increases
  14. what is precession?
    the motion of the net magnetization as it wobbles around the main magnetic field of the MR system
  15. what is the Larmor equation?
    • describes the influence of the magnetic field strength on the precessional frequency of the protons' net magnetization (NM)
    • F = k X B
    • F=precessional or Larmor frequency (in MHz)
    • K= 42.6 MHz/T (gyromagnetic ratio for hydrogen)
    • B= magnetic field (in Tesla)
  16. what is an RF pulse?
    • used to change the alignment of NM
    • RF is electromagnetic energy that is in the same band of frequencies used by commercial radio stations
    • energy applied to the protons through the principle called resonance
    • also known as excitation pulse
    • tilts the NM direction from longitudinal direction towards the transverse plane
  17. prior to applying the RF energy, which direction are the protons aligned in respect to the main magnetic field ()?
    • parallel
    • also known as longitudinal direction
  18. what happens to the direction of the NM when RF energy is transmitted to the patient?
    the NM is tilted away from the longitudinal direction toward a plane which is perpendicular to the longitudinal known as the transverse plane
  19. what is relaxation?
    just right after the RF energy is turned off, the return of magnetization of protons to a less energized state resulting in a release of energy which can be measured
  20. what properties of protons within each of the tissue allow us to differentiate the tissues on MR images?
    protons within each of the tissue relax at a unique rate after the RF energy is turned off
  21. what are the two processes through which the tissues relax?
    T1 and T2 relaxation
  22. what is T1 relaxation?
    • the rate of regrowth of the net magnetization along the longitudinal direction
    • also know as recovery and spin lattice relaxation
    • T1 relaxation happens at different rates for different tissues
  23. what is T2 relaxation?
    • in addition to the T1 dependent regrowth of the NM along the direction of the main magnetic field, a second relaxation process, known as T2 relaxation occurs simultaneously, but independent of T1 relaxation
    • T2 relaxation is the rate of NM decaying in the transverse plane
    • also known as spin-spin relaxation
    • T2 relaxation happens at different rates for different tissues
  24. what is an echo?
    the measurable signal we get from the patient
  25. what is proton density?
    the number of protons which generate the MR echo in any given tissue
  26. what is a pulse sequence?
    a sequence of evens performed to collect a series of echoes from the protons in a patient
  27. what is echo time (TE)?
    the time from the excitation pulse to the measurement of the echo
  28. what is repetition time (TR)?
    the time from the excitation pulse until the cycle is ready to be repeated with the next excitation pulse
  29. what are some of the basic pulse sequences in MR imaging?
    • spin echo
    • fast spin echo
    • inversion recovery- stir, flair
    • gradient echo
  30. what are some examples of modified spin echos?
    • conventional spin echo
    • fast or turbo spin echo
    • inversion recovery
  31. what are some characteristics of spin echo pulse sequence?
    • also called conventional spin echo
    • starts with a 90 deg pulse followed by one or more 180 deg rephrasing pulses to generate a spin echo
    • T1 weighting- short TR (300-700ms), short TE (10-30ms)
    • T2 weighting- long TR (>2000ms), long TE (>80ms)
    • Proton density- long TR (>2000ms), short TE (<30ms)
  32. what are the advantages and disadvantages of a spin echo?
    • adv- good image quality (good SNR), very versatile, get a true T2 weighting
    • disadv- scan times relatively long
  33. what are some characteristics of fast spin echo pulse sequence?
    • also called turbo spin echo
    • same as SE pulse sequence but scan times much faster. this is achieved with multiple 180 degree rephrasing pulses, multiple echoes to fill more lines of space per pulse sequence.
    • the number of 180 deg rephrasing pulses is called the turbo factor or the echo train length
    • the higher the turbo factor the shorter the scan time. you get shorter times because more lines of space are being filled per sequence
  34. having a short turbo factor on a fast spin echo causes what to the weighting, scan time and image blurring?
    increased T1 weighting, longer scan times, reduced image blurring
  35. having a long turbo factor on a fast spin echo causes what to the weighting, scan time and image blurring
    increased T2 weighting, reduced scan time, increased image blurring
  36. what are the advantages and disadvantages of a fast or turbo spin echo?
    • adv- reduced scan times, can use higher resolution matrixes, image quality improved, and increased T2 information
    • disadv- more sensitive to motion, fat bright on T2 weighted images, increased image blurring
  37. what is an inversion recovery pulse sequence?
    starts with a 180 deg inverting pulse, then Net magnitization vector (NMV) recovers back to  then a 90 deg pulse is then applied at a time from 180 deg inverting pulse. So the time from the initial 180 deg pulse to the 90 deg pulse is called time to inversion or Tau time
  38. what type of image does an inversion recovery produce?
    heavily T1 weighted images to demonstrate anatomy
  39. what are the advantages and disadvantages of an inversion recovery pulse sequence?
    • adv- excellent T1 contrast, very good SNR as the TR is long
    • disadv- long scan times unless used in conjunction with Fast spin echo
  40. how do you calculate scan time on a fast spin echo pulse sequence?
  41. what is a STIR pulse sequence?
    • Short tau inversion recovery
    • uses T1 time to match the time that it takes fat to recover from full inversion to transverse plane so there is no longitudinal magnetization corresponding to fat
    • null fat signal
  42. STIR pulse sequence is most commonly used for what type of exams?
    musculoskeletal imaging as normal bone, which contains fatty marrow, is suppressed and lesions within bone such as bone bruising and tumors are seen more clearly
  43. what is a FLAIR pulse sequence?
    • fluid attenuated inversion recovery
    • uses T1 time corresponding to the time of recovery of CSF from 180 deg to the transverse plane
  44. what type of exams is a FLAIR pulse sequence used for?
    • brain and spine imaging to see periventricular and cord lesions, and abnormalities in the ventricles more clearly
    • nullifies CSF signal
    • used to visualize multiple sclerosis plaques, acute sub-arachnoid hemorrhage and meningitis
  45. what is a gradient echo pulse sequence?
    • uses a variable flip angle so that the TR and scan time can be reduced without producing saturation
    • flip angle less than 90 deg
    • used to acquire T2*, T1 and PD weighting images
    • Advantage: allow for a reduction in the scan time as TR is greatly reduced
  46. gradient echo pulse sequence enhances what?
    • fluid
    • shows water as bright
    • fake T2 or T2*
  47. what is a pixel?
    a single picture element formed by the intersection of a row and column of the image grid
  48. what is an image matrix?
    • grid of rows and columns of pixels which form the digital image
    • the image may be square or rectangular
    • pixels themselves may be square or rectangular
    • technologist controls these factors
  49. what is a gray scale?
    • a gray scale value is assigned to each pixel of the image
    • used to differentiate structures on a digital image
    • determined by T1 relaxation, T2 relaxation, and proton density
  50. what is spatial localization?
    • used to located in three dimensions, from where within a patient's body the MR echoes originated
    • every sequence uses three different magnetic field gradients in order to accurately map the signal coming from each anatomical location into its appropriate location on the image
  51. what are magnetic field gradients?
    • is a very small change in the magnetic field as you move along a particular axis through the patient's body
    • this is created by sending electrical current though a hardware component called a gradient coil located within the magnet of the MR system
    • is superimposed over the static magnetic field and turned on just before the RF pulse is generated
  52. patient coordinate systems: which parts of the body and the direction are the x, y and z coordinates located?
    • x-axis- axis which runs across the patient from left to right
    • y-axis- axis which runs along the anterior to posterior direction through the patient
    • z-axis- runs from the patient's head to the patient's feet
  53. magnetic field gradient applied along the z-axis of the patient (head-to-toe) is called what?
    z gradient
  54. magnetic field gradient applied along the x-axis of the patient (left to right) is called what?
    x gradient
  55. magnetic field gradient applied along the y-axis of the patient (anterior to posterior) is called what?
    y gradient
  56. what is the purpose of gradients?
    • changes the precessional frequency of the protons in the patients body along all three axis
    • we have to match the frequency at which we transmit the RF energy to the frequency at which the protons resonate
  57. what is the main job of gradients?
    • ensures that only the protons within a single slice at one time will respond to the RF energy
    • the gradient causes the net magnetization's precessional frequency to vary depending on the protons' position along the axis that the gradient is applied
  58. what happens during slice selection?
    • in MRI, the RF energy can not be physically focused only on the protons in a single slice. the entire body of the patient within the magnet is typically exposed to the RF energy
    • according to the resonance principle, energy transfer occurs only if the frequency of the transmitting system is matched to that of the receiving system
  59. what else happens during slice selection when a gradient is turned on?
    • by turning a gradient on, a spectrum of precessional frequencies is created along the gradient axis. we can then selectively disturb the magnetization at a specific position along that axis in the body. we do this by transmitting an RF pulse which is at the same frequency as the precessional frequency at the location of interest along the gradient axis. only at that location will protons absorb the RF energy
    • even though the entire pt is exposed to the RF energy, only those protons at the correct precessional frequency will absorb it
  60. slice selection determines what three important features of an MR image?
    • position of the slice
    • the orientation of the slice- sagittal (x-gradient), coronal (y-g), transverse( z-g) 
    • the thickness
  61. how can you vary slice thickness in an MR image?
    • changing the strength of the magnetic field gradient- using a stronger magnetic field for slice selection can give us thinner slices
    • or changing the range of frequencies contained within the RF pulse
    • a plane between any two orthogonal orientation, called an oblique slice, is selected when two slice selection gradients are turned on simultaneously
  62. how is multi-slice selection occur in MRI?
    • unlike CT, the pt table of an MR system does not move in order to align new slices with the energy source
    • mri is a multi-slice imaging modality which allows us to acquire many adjacent cross=sectional slices in order to thoroughly study the anatomy
    • we not only want to collect an echo from one slice, we need echoes from adjacent slices too
    • one echo is collected at each slice location for all of the slices before the TR period may be repeated
  63. what are the three gradients inside the MR system for?
    slice selection- determines the position, orientation and thickness of the slice

    frequency- indicates how fast the magnetization is precessing. it is used to divide the slice into rows and columns

    phase- indicates the direction the magnetization is pointing. it is used to divide the slice into rows or columns
  64. what is the sequence of the gradients? give the order
    • slice selection
    • phase encoding
    • frequency encoding
  65. which 2 of the 3 gradients can be switched out of order?
    phase and frequency
  66. along which direction do motion artifacts appear as a blurring of signal? which gradient
    phase direction
  67. chemical shift artifacts appear as a displacement of fatty structures along which direction? which gradient?
    frequency direction
  68. what is raw data?
    the echo information collected from the patient
  69. how is raw data transformed into an MR image?
    • Fourier transform
    • a complex mathematical calculation used to decode the frequency and phase information in order to reconstruct the image from the raw
    • the special computer that computes all raw data is called the array processor
  70. what are the key influences on image quality?
    • signal-to-noise ratio
    • image contrast
    • image resolution
    • scan time
  71. what are the controlling factors for image quality?
    • TR
    • TE
    • signal averages
    • FOV
    • matrix
    • slice thickness
    • flip angle
    • gap
    • echo train length
  72. what is repetition time parameter?
    • TR is the parameter that controls the contrast between tissues in an image due to T1 relaxation
    • directly affects scan time
    • directly affects SNR
  73. Increasing TR will do what to longitudinal relaxation, T1-weighted contrast, S/N, maximum allowable # of slices, and scan time?
    • increase longitudinal relaxation
    • decrease T1-weighted contrast
    • increase S/N
    • increase maximum allowable # of slices
    • increase scan time
  74. what is echo time parameter?
    • TE is the parameter that controls the contrast between tissues in an image due to T2 relaxation
    • Long TE times yield high T2 contrast, but reduced signal-to-noise-ratio
  75. Increasing TE will do what to transverse relaxation
    T2-weighted contrast
    S/N
    maximum allowable # slices
    gradient echo susceptibility
    • increase transverse relaxation
    • increase T2-weighted contrast
    • decrease S/N
    • decrease maximum allowable # slices
    • increase gradient echo susceptibility
  76. what are signal averages?
    • NSA, NEX, AVG
    • increasing the number of signal averages yields higher signal-to-noise images, but at the expense of scan time
  77. increasing the signal averages will do what to S/N, scan time and motion artifacts?
    • increase S/N
    • increase scan time
    • decrease motion artifacts
  78. increasing the FOV will do what to the amount of anatomy displayed, S/N, truncation (ringing) artifacts, and in-plane resolution?
    • increase anatomy displayed
    • increase S/N
    • increase truncation
    • decrease in-plane resolution
  79. for a given field of view, increasing the phase encoding steps will do what to
    S/N
    spatial resolution
    scan time
    • decrease S/N
    • increase spatial resolution
    • increase scan time
  80. for a given field of view, increasing the frequency steps will do what to
    S/N
    spatial resolution
    scan time
    • decrease S/N
    • increase spatial resolution
    • not affect scan time
  81. increasing slice thickness does what to
    anatomical coverage
    S/N
    partial volume artifacts
    spatial resolution
    • increase anatomical coverage
    • increase S/N linearity
    • increase partial volume artifacts
    • decrease spatial resolution
  82. increasing the rectangular field of view will do what to
    amount of anatomy displayed
    S/N
    scan time
    spatial resolution
    • increase the amount of anatomy displayed
    • increase S/N
    • increase scan time
    • not affect spatial resolution
  83. what happens when your flip angle is at the maximum without exceeding 90 deg?
    better S/N of the resulting images, provided that the TR is relatively long
  84. what is echo train length?
    • obtains additional echoes during a TR period
    • reduces scan time
  85. increasing echo train length will do what to S/N and scan time?
    • decrease S/N
    • decrease scan time
  86. what are the 3 different qualifications of magnetism?
    • diamagnetism- causes magnetic flux to expand causing a void. negative effect
    • paramagnetism- causes magnetic flux to come together. positive effect. brighter
    • ferromagnetism- strong attraction to magnet
  87. what is RF shielding?
    • an RF-opaque barrier which confines the radio frequency energy.
    • made of sheets of copper.
    • embedded behind the walls, doors and windows
  88. what are the two reasons we need RF shielding?
    • contain RF energy generated by the MR system
    • restrict external radio signals so they are not detected by the RF coils
  89. what are the two ways for RF shielding?
    • passive- surround room with copper sheets (Faraday Cage)
    • active- superconducting coils at each end of the magnet
  90. what are two types of magnets used in MR?
    • permanent 
    • electromagnets
  91. what is shimming in MR?
    • makes the magnetic flux even
    • used to improve homogeneity
  92. what are the characteristics of a permanent magnet?
    • constructed from hundreds of permanently magnetized ceramic bricks
    • very heavy
    • low field, max at .35T
    • cannot be turned off in the event of an emergency
    • low cost to operate
    • no cryogens
    • provides an open design
    • has a vertical magnetic field
  93. what is an electromagnet?
    • made from coils of wire through which an electric current is passed
    • two types:
    • resistive
    • superconducting
  94. what is a resistive electromagnet?
    • composed of a core which contains either air or iron, wrapped with a coil of resistive wire
    • resistive wire dissipates some of the electrical current flowing through it in the form of heat
    • limited to 1.0 T
    • a benefit of resistive magnets is the ability to quickly shut down the magnetic field if necessary
    • magnetic field can be either vertical or horizontal
  95. what is a superconducting electromagnet?
    • most common
    • uses materials when cold that will have no resistance to the flow of electricity
    • can generate fields up to 3T
    • cost of operations is high due to the cryogens and materials in the magnet
    • cryogens is a mixture of liquid nitrogen and helium
    • when ramping down the magnet it should be down over a period of several minutes 
    • has a horizontal magnetic field
  96. what are the field strength equivalents?
    • high-field: 1T and greater
    • mid-field: .5T to .9T
    • low-field: less than .5T
  97. what is the fringe magnetic field?
    • field outside of the magnet
    • usually measured in Gauss
    • may pose a safety risk
    • magnetic shielding must constrain the fringe field to a smaller space
  98. how many gauss is considered the safe zone?
    beyond the 5 gauss line
  99. what are the two types of shielding used from the magnetic field?
    • passive- MR magnet encased in a box, made of magnetically attractive material such as iron. Expensive and inconvenient 
    • Active- built right into the magnet which incorporates a system of electrical coils which largely cancels the fringe magnetic field outside the magnet
  100. what is the purpose of magnetic shimming?
    imaging requires homogenous field to provide good geometric sharpness and to allow even spectral fat saturation
  101. what are the two ways of magnet shimming?
    • passive- metal shims placed on scanner during installation
    • active- performed by an electromagnetic coil. can be used to shim the system for each patient or each sequence within a protocol
  102. how can gradient performance be measured?
    • maximum gradient amplitude- the maximum variation in the magnetic field that can be attained. MR systems with high maximum gradient amplitudes can acquire thinner slices with better resolution than those with lower gradient amplitudes
    • slew rate is another measure. It is the rate at which the gradient coils can switch on and off from their maximum gradient amplitude. systems with high slew rates are capable of performing the most demanding imaging techniques such as echo planar imaging and diffusion weighted imaging
  103. what are RF coils?
    • loop of wire used to transmit and/or recieve radio frequency energy
    • faraday's law of induction states that any changing magnetic field which is near an electrically conductive material will cause a current to flow in that conductive material

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