finalsmri 2

• SURFACE COILS  Simplest design of coil

   Applications: spines, shoulders, TMJs,

  small body parts
• Endocavitary Coil:

  o Used for imaging the rectum

  o Used for imaging the wall of the

  cavity
• Neurovascular Coil:

  o Used for imaging the brain and neck
• PAIRED SADDLE COIL

   Commonly used for knee imaging

   Used for X and Y gradient coils
• HELMHOLTZ PAIR COIL

   Consists of two circular coils parallel to

  each other

   Used for Z gradient coils

   Used as RF coils for pelvis and cervical

  imaging
• BIRD CAGE COIL

   Provides best RF homogeneity of all the RF

  coils

   Commonly used as transceiver coil for head

  imaging

   Occasionally used for extremity imaging
• EXAMINATION ROOM DESIGN

   Room must be shielded against

  o Radio interference

  o Fringe magnetic fields

   Only nonmagnetic materials are used for

  the structure and finish

   Must have electric filters - Rationale: to remove interfering

  frequencies
• Plumbing: should be PVC or copper
• Polyvinyl chloride (PVC): structural

  concrete slab or walls
• Lighting: should be direct current
• FARADAY CAGE/RF SHIELD

   A wire-mesh shield enclosing the MR

  imager

   Used to attenuate extraneous sources of RF
• FRINGE MAGNETIC FIELD

   Magnetic field outside the patient aperture

   Must be considered in the design of MRI

  facility
• Problems in fringe MF:

  o Can interfere with the proper

  operation of mechanical and

  electronic equipment

  o Any large mass of ferromagnetic

  material can distort the homogeneity

  of the imaging volume by interacting

  with fringe MF
• FRINGE MAGNETIC FIELD - Takenote: distortion of fringe MF by a

  ferromagnetic object will cause a

  compensating distortion of the imaging

  volume
• Increase slice thickness Low
• Decrease slice thickness High
• Increase data collection High
• Decrease data collection Low
• Increase MRI signal

  acquisition High
• Decrease MRI signal

  acquisition Low
• Increase MRI signal strength High
• Decrease MRI signal strength Low
• SLICE THICKNESS

   Important in visualization of pathology
• Large slice thickness:

  o Provide less grainy images

  o Can’t visualize small pathologic

  lesions
• Small slice thickness:

  o Provide more grainy images

  o Can visualize small pathologic lesion
• CONTRAST RESOLUTION

   Low contrast resolution structures

   Most tissues: <1% difference

   MR parameters and biologic tissues:

  >30% difference

   Gray and white matter: 30-40% difference

   Null regions: total loss of contrast

  o Rationale: improper RF pulse

  selection
• MRI MAGNETIC FIELDS

   Exhibit threshold dose-response relationship
• 1.) Strong static magnetic field (B 0 )

   Unit: Tesla (T)

   Affects:

  o Membrane permeability

  o Enzymes kinetics

  o Nerve conduction
• Time-varying gradient magnetic field (B 1 )

   Unit: Tesla/second (T/s)

   Affects:

  o Visual phosphenes

  o Bone healing

  o Cardiac fibrillation
• RF emission

   Unit: Watts/kilogram (W/kg)

   Causes tissue heating

   Heat: the principal result of the interaction

  between RF field and tissue

   Heating: expressed as specific absorption

  rate (SAR)

  o Unit: W/kg
• MAXIMUM PERMISSIBLE DOSE (MPD)

   Not correct for MRI
• STRONG MAGNETIC FIELDS

   Physical Precautions:

  o Avoid any objects made from metal

  or iron inside the examination room

  o Patient must be free from metallic

  objects even internally

  o Warnings signs are necessary
• QUENCHING

   The events that occur when the liquid

  cryogens that cool the magnet coils boil off

  rapidly
• CONTRAST MEDIA

   Gadolinium-containing compounds

   Gadolinium: a metal with paramagnetic

  effect

  o Lower toxicity

  o Few side effects
• Teslascan: a new manganese-based

  paramagnetic liver contrast agent

  o Used in detection, characterization,

  localization and evaluation of lesions

  in the liver
• Feridex: an iron oxide mixture and the only

  supramagnetic contrast agents available

  o Used to detect and diagnose liver

  lesions
• GATING

   Technique used to organize the signal

   Technique used for improving image quality

   Used to reduce motion artifacts from the

  beating heart

   The moving object is “frozen” at that phase

  of its motion, reducing image blurring
• IMAGE ARTIFACT

   A structure not normally present but visible

  as a result of a limitation or malfunction in

  the hardware or software of MRI device
• QUENCHING - Caused:

  o Activation of the magnet STOP

  button

  o A fault in the magnet itself
• QUENCHING Results

  o Helium escaping very rapidly from

  the cryogen bath

  o A loud bang or thundering or hissing

  or rushing sound with the cold gas

  expulsion

  o Patient asphyxiation and frostbite