MAGNETIC RESONANCE IMAGING

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MAGNETIC RESONANCE IMAGING
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Magnetic Resonance Imaging

• Rooted in a chemical technique known as nuclear
  magnetic resonance spectroscopy
• Dropped the word nuclear
• 1st NMR image of a human in 1977
• 1980 Moore and Hinshaw produced first
  recognizable image of the brain

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Magnetic Resonance Imaging

• Capable of producing axial images
• Multiplanar capabilities
• Soft tissue contrast and spatial resolution
  exceeds CT
• Capable of obtaining physiologic information

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MRI Technical

• Superconductive magnets most frequently employed
• Temp near absolute zero by liquid cryogens
  (helium and nitrogen)
• Magnet strength in tesla (T) or gauss (G)
• 1 T 10,000 G
• Magnet strength of 1.5 T is 25,000 times gt the
  earths magnetic attraction

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MRI Technical

• Categories of magnetic field strengths
• Ultra high field gt 4 T
• High field 1.5 4.0 T
• Medium field 0.5 1.0 T
• Low field 0.1 - 0.5 T
• Ultra low field lt 0.1 T

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MRI Technical

• The larger the field strength of the magnet the
  greater the signal to noise ratio.
• SNR also influenced by
• Surface coils
• Software

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IMAGE PRODUCTION

• Proton Properties
• Moving electric charge? magnetic field
• Size of charge and speed affects field strength
• Nucleus of hydrogen (single proton) has a small
  positive electric charge and is spinning ? small
  measurable magnetic field
• Water is biggest source of protons followed by fat

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IMAGE PRODUCTION

• Number of spins in excess related to magnet field
  strength
• 0.5 T 3 protons out of 2,000,000
• 1.0 T 6 protons out of 2,000,000
• 1.5 T 9 protons out of 2,000,000

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IMAGE PRODUCTION

• Signal used to produce the MR image comes from
  the protons in excess
• SNR is higher in higher field scanners
• Vol of H20 2 x 2 x 5 mm3 ? 1.2 x 10(22) total
  protons
• 6.02 x 10 (15) in excess (6 million billion)

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IMAGE PRODUCTION

• Frequency of precession is directly proportional
  to the strength of the magnetic field
• Defined by the Larmor equation
• At magnet field strengths used in clinical MRI
  systems 0.5 2.0 T the resonance (Lamor,
  precessional) frequency of Hydrogen is 2.13MHz to
  85MHz

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IMAGE PRODUCTION

• RF pulse matching the Larmor frequency of the
  protons is applied at a 90 degree angle to the
  magnetic field. The protons resonate and absorb
  the energy of the RF pulse.
• Misaligns ? higher energy state
• Angle of misalignment controlled by RF pulse
• Flip angle

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IMAGE PRODUCTION

• When the RF pulse is turned off, the protons
  realign
• Energy is release, expressed as an RF signal
• Processed by the computer? image

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IMAGE PRODUCTION

• Spin echo sequences repeat the RF pulses many
  times first at 90 degrees followed by 180
  degrees.
• TR (repetition time) interval between the two 90
  degree pulses
• TE (echo time) time between the pulse and the
  detection of the re-emitted signal

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THE IMAGE

• Factors affecting the image
• Motion
• Signal and resolution
• Tissue contrast

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THE IMAGE

• Motion
• One of the m/c causes of image degradation
• Cardiac motion, peristalsis, respiration
• Body movement related to patient discomfort
• Prolonged examination
• Set imaging protocols are useful

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THE IMAGE

• Signal and Resolution
• Signal amount of information on an image
• Inc slice thickness or field of view or,
  alternatively decreasing the matrix will increase
  signal to noise ratio
• of signal acquisitions affects signal
• Proximity of the slices
• Interposed gap to reduce cross talk

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THE IMAGE

• Signal and Resolution
• Resolution ability to distinguish small objects
• Changes designed to improve resolution will
  negatively affect signal
• Use of coils (surface or body) help to minimize
  the trade off

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THE IMAGE

• Tissue Contrast
• Soft tissue contrast in MRI is related to
  differences in proton resonance
• Differences in tissue specific properties can be
  emphasized by changing the imaging parameters
• Known as weighting the image

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Pulse Sequences

• Sequence TR (Msec) TE (Msec) TI (Msec) Flip
  Ang
• T1 lt 1000 lt 30 N/A 90
• Proton gt 1000 lt 30 N/A 90
• T2 gt 2000 gt 60 N/A 90
• FSE T2 gt 2000 gt 60 N/A 90 ETL 2-16
• GRE T1 variable lt 30 N/A 70-110
• GRE T2 variable lt 30 N/A 5-20
• FSE STIR gt2000 gt 60 120-150 180?90
• ETL 2-16

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Relative MR Signal Intensities

• Feature T1 T2
• Calcium VLow VLow
• Cortical bone VLow VLow
• Tendon Low Low
• Ligament Low Low
• Fat High Isointense
• Red Marrow Low Isointense
• Yellow Marrow High Isointense
• Water Low High
• Muscle Isointense Isointense

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Relative MR Signal Intensities

• Feature T1 T2
• Bone bruise Low High
• Occult Fx Low High
• Stress Fx Sig void Sig void
• Low edem High edem
• Acute Infection Low High
• Osteonecrosis Low High
• Tumor (majority) Isointense High
• Benign (most) Homogen Homogen
• Malignant (most) Homogen Heterogen

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Gadolinium (Gd-DTPA)

• Indications
• Mass cyst vs. solid
• Mass viable tumor vs. necrosis
• Infection soft tissue edema vs abscess
• Spine disc herniation vs scar tissue
• Spine Evaluation of cord lesions (tumors, MS,
  nerve sheath tumors)

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MRI Safety and Contraindications

• Contraindications
• Pacemaker
• Intracranial aneurysm clips
• Cochlear implant
• Intraocular metal
• Pregnancy

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MRI Safety and Contraindications

• Items To Be Removed
• Dentures/Bridgework
• Excessive eye makeup
• Wigs
• Hearing aids
• Bobby pins/ hair clips
• Watches
• Keys
• Wallets
• Belts and other loose metal

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MRI Safety and Contraindications

• Items That May Degrade Image Quality
• Hip pins and Herrington rods
• Bone rods, pins or screws
• Braces
• Most implants
• Hip and knee prosthetics
• Abdominal and thoracic surgical clips
• Dural clips