Magnetic Resonance Imaging IV

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Magnetic Resonance Imaging IV

• Angiography
• Instrumentation
• Quality Assurance

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Magnetic Resonance Angiography

• Signal from blood dependent on relative
  saturation of surrounding tissues and incoming
  blood flow in the vasculature
• In multislice volume, repeated excitation of
  tissues and blood causes partial saturation of
  the spins, depending on the T1 characteristics
  and the TR of the pulse sequence
• Blood outside of imaged volume does not interact
  with RF excitations
• Unsaturated spins enter imaged volume produce
  large signal relative to blood within volume
  (flow-related enhancement)

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Flow presaturation

• In some cases, flow-related enhancement is
  undesirable and may be eliminated using
  presaturation pulses applied to volumes just
  above and below the imaging volume
• Same presaturation pulses helpful in reducing
  motion artifacts caused by adjacent tissues
  outside the imaging volume

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Flow-related signal loss

• Flow-related signal loss occurs when rapidly
  flowing blood moves through the excited slab of
  tissue, but does not experience the full
  refocusing 180-degree pulse, resulting in a flow
  void
• Blood appears black in the image

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Time-of-flight angiography

• Technique relies on tagging of blood in one
  region of the body and detecting it in another
• Differentiates moving blood from surrounding
  stationary tissues
• Tagging accomplished by spin saturation,
  inversion, or relaxation to change the
  longitudinal magnetization of moving blood
• Penetration of tagged blood into volume depends
  on T1, velocity, and direction of blood

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Time-of-flight MRA (cont.)

• Detectable range limited by eventual saturation
  of tagged blood
• Long vessels difficult to visualize
  simultaneously in a 3D volume
• 2D stack of slices typically acquired, where even
  slowly moving blood can penetrate region of RF
  excitation in thin slices

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Time-of flight MRA (cont.)

• Each slice acquired separately
• Blood moving in one direction can be selected by
  delivering presaturation pulse on adjacent slab
  superior or inferior to slab of data acquisition
• Thin slices helpful in preserving resolution of
  flow pattern

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MR angiography maximum intensity projection of
carotid arteries
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Phase contrast angiography

• Relies on phase change that occurs in moving
  protons such as blood
• One method uses a bipolar gradient
• One gradient with positive polarity followed by
  second gradient with negative polarity, separated
  by a delay time ?T
• Second acquisition of same view of data (same
  PEG) reverses polarity of bipolar gradients
• Moving spins encoded with negative phase
• Stationary spins exhibit no phase change

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Phase contrast MRA (cont.)

• Subtracting second excitation from first cancels
  magnetization due to stationary spins but
  enhances magnetization due to moving spins
• Degree of phase shift directly related to
  velocity encoding time, ?T, and the velocity of
  the spins in the excited volume
• Intensity variations are dependent on amount of
  phase shift
• Image is inherently quantitative and can provide
  estimate of mean blood flow velocity and direction

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Magnitude and phase images provide contrast of
flowing blood. Magnitude images are sensitive to
flow, but not to direction phase images provide
direction and velocity information.
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Magnet

• Heart of the MR system
• Performance criteria include field strength,
  temporal stability, and field homogeneity
• Parameters affected by magnet design
• Air core magnets
• Wire-wrapped cylinders main field parallel to
  long axis of cylinder - horizontal
• Solid core magnets
• Permanent magnet or electromagnet main field
  runs between poles of magnet - vertical

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Air core magnet (A) and solid core magnet (B)
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Resistive magnet

• Most of solid core design
• Use continuous electric power to produce magnetic
  field
• Field strengths from 0.1 T to about 0.3 T
• Turn off field in emergency
• High electricity costs relatively poor
  uniformity/homogeneity of field
• Open design enables claustrophobic patients to
  tolerate examination
• Fringe fields of main magnet better confined

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Superconductive magnet

• Typically use air core electromagnet
  configuration
• High field strengths (0.3 T to 3.0 T clinical
  systems 4.0 T to 7.0 T research systems)
• High field uniformity
• High initial capital and siting costs, cryogen
  costs, difficulty in turning off main magnet in
  emergency, extensive fringe fields
• Uncontrolled quenching can cause explosive
  boiling of liquid He and severe damage to magnet
  windings

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Cross-sectional view of a superconducting magnet
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Ancillary equipment

• Shim coils (adjust main magnetic field improve
  homogeneity in central volume)
• Gradient coils
• RF coils (create B1 field detect transverse
  magnetization)
• Control interfaces, RF source, detector, and
  amplifier, A-to-D converter, pulse programmer,
  computer, gradient power supplies, image display

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Proximity head coil (A) and surface coil detector
(B)
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Head coil with built-in mirrors
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Surface coil in use for extremity imaging
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Block diagram of a typical MRI system
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Magnet siting

• Patients with pacemakers or ferromagnetic
  aneurysm clips must avoid fringe fields above 0.5
  mT image intensifiers, gamma cameras, and color
  TVs severely impacted by fringe fields lt 0.3 mT
• Administrative control for magnetic fringe fields
  is 0.5 mT (5 G), requiring controlled access to
  areas that exceed this level
• Disruption of fringe fields can reduce
  homogeneity of active imaging volume

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Quality control

• Components of MR system that must be periodically
  checked
• Magnetic field strength
• Magnetic field homogeneity
• System field shimming
• Gradient linearity
• System RF tuning
• Receiver coil optimization
• Environmental noise sources
• Power supplies
• Peripheral equipment
• Control systems

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A quality assurance phantom for MRI
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Another quality assurance phantom for MRI
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