MAGNETIC RESONANCE IMAGING

DEFINITION MRI
 Provides sectional images
MAGNETIC RESONANCE IMAGING  No superimposition of structures
 A computer-based cross-sectional imaging  Clearly demonstrates structures even
modality without contrast media
 The use of magnetic field and radio waves to  Can resolved relatively small contrast
obtain a mathematically reconstructed image differences among tissue
 Originally called: Nuclear Magnetic  Contrast depends on the interaction of
Resonance matter with electromagnetic forces

ADVANTAGES OF MRI CONVENTIONAL

 Best low contrast resolution  Provides “flat” image
o Main advantage  Structures are superimposed
o Rationale:  Contrast media is required to clearly
 X-ray attenuation coefficient distinguished one anatomic structure or
in soft tissue – differ by <1% organ from one another
(in same tissue)  Limited in its ability to distinguish types of
 Spin density & T1 – differ by tissue
20-30% (in same tissue)  Cannot detect small attenuation changes
 T2 – differ by 40% (in same  Can only distinguish air, fat, bone, soft
tissue) tissue, and metal
 No ionizing radiation
 Contrast depends on differences of x-ray
 Direct multiplanar imaging attenuation
 No bone or air artifact
 Direct flow measurements BRIEF HISTORY
 Totally noninvasive
 Contrast media not required 1940’s
 Felix Bloch and Edward Purcell first
MRI CONTRAINDICATION discovered the properties of magnetic
 Cardiac pacemaker resonance
 Aneurysm clips  MRI spectroscopy: technique they used for
 Claustrophobia analysis of complex molecular structures
 Metallic fragments in the eye and dynamic chemical processes
 Cochlear implants 1952
 Internal drug infusion pumps  Bloch and Purcell shared a Nobel Prize in
 Neurostimulators Physics
 Bone growth stimulators 1971
 Raymond Damadian showed that the
MRI VS CONVENTIONAL RADIOGRAPHY relaxation time of water in a tumor differed

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 MAGNETIC RESONANCE IMAGING

from the relaxation time of water in normal DIFFUSION

tissue  Spontaneous random motion of molecules in
 Investigated excised rat tissue a medium
 He reported that there were significant
differences between normal rat tissues and FAT SUPPRESSED IMAGES
tumors  Fat tissue in the image is made to be of a
1973 lower, darker signal intensity
 Paul Lauterbur published the first cross-
sectional image of objects (two water-filled FREE INDUCTION DECAY
capillary tubes) obtained with MRI  Signal emitted by tissue after RF excitation
technique
1975
 Damadian obtained the first animal images GATING
1970s  Technique used in reducing motion artifacts
 MRI was progressing rapidly
1978 GRADIENT ECHO
 The first human head scans were obtained  Fast pulse sequence
 Followed by first human body scans  Often used with 3D imaging to generate T2-
weighted images
INDOMITABLE
 Original name of the first MRI machine GRADIENT MAGNETIC FIELD
 A change in the intensity of a magnetic field
DEFINITION OF TERMS in space
ANTENNA  Unit: mT/cm
 Device for transmitting or receiving radio
wave GYROMAGNETIC RATIO
ARTIFACTS  A constant, specific ratio for each nucleus
 Spurious finding in or distortion of an image  Half-life in MRI
ATTENUATION  Unit: MHz/T
 Reduction in energy or amount of a beam of  Hydrogen: 43 MHz/T
radiation when it passes through tissue or o Most abundant element in the body
other substances (60%)
COIL
 Single or multiple loops of wire designed to INVERSE RECOVERY
produce a magnetic field from current  Standard pulse sequence available in most
flowing through the wire MRI imagers
CLAUSTROPHOBIA  Used for T1-weighted images
 The fear of having no escape and being in
closed or small spaces or rooms LARMOR FREQUENCY
CRYOGENIC
 Relating to extremely low temperature
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 MAGNETIC RESONANCE IMAGING

 The frequency at which a nucleus precesses  Returning to equilibrium

in the magnetic field
 Unit: MHz RELAXATION TIME
 The time required for return
MAGNETIC MOMENT
 A force created when magnetic dipole is in a RESONANCE
magnetic field  Transfer of vibrating energy from one
 The inherent magnetism system to another
 Unit: T

SPECIFIC ABSORPTION RATE (SAR)

MAGNETIZATION  The power absorbed during RF irradiation
 The large-scale macroscopic magnetic  Unit: W/kg
moment resulting from many nuclear
magnetic moments SPIN DENSITY (SD)
 Concentration of hydrogen nuclei in tissue
NET MAGNETIZATION  Principal determinants of MRI signal
 Magnetic moments of individual hydrogen
nuclei aligned in the external magnetic field T1 RELAXATION TIME
 Spin-lattice relaxation time
PERFUSION  Longitudinal relaxation time
 Flow of blood through vessels of an organ or  Decrease in signal: 63% of maximum value
anatomic structure
T2 RELAXATION TIME
PRECESSION  Spin-spin relaxation time
 The wobble of the rotational axis of a  Transverse relaxation time
spinning body about a stationary axis  Decrease in signal: 37% of maximum value
 Describes a cone
 Rate of precision: increases as MF strength TESLA
increase  SI unit of magnetic field strength
 1T = 10,000 G
RADIO FREQUENCY (RF)
 Electromagnetic radiation having a PHYSICAL PRINCIPLES
frequencies from 0.3kHz to 300 GHz
 RF range in MRI: 1-100 MHz SIGNAL PRODUCTION
 MRI depends on the properties of the
RAW DATA nucleus
 Information obtained by radio reception of  Hydrogen nuclei: element used in most
the MRI signal as stored by a computer MRI
o Rationale:
RELAXATION
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 MAGNETIC RESONANCE IMAGING

 Strongest nuclear magnets FAST SPIN-ECHO

 Create the strongest MRI  Known as rapid acquisition recalled echo
signal
 Most common element in the FLUID ATTENUATED INVERSION
body RECOVER (FLAIR)
 Suppresses signal from cerebrospinal fluid
PULSE SEQUENCES (CSF)
 Most applicable in the brain
SPIN ECHO SEQUENCE o For seizures disorders
 Classic imaging sequences o For spinal cord injuries
 Most widely use pulse suquence SHORT TAU INVERSION RECOVERY
 Used with timing parameters to yield T1-  Suppresses signal from fat
weighted images
 Also provide pro ton-density weighted FAST SPIN-LATTICE RELAXATION RATE
images  Short T1
 Time consuming  Produces high MRI signal in T1-weighted
 Slow patient “throughput” or productivity images

INVERSE RECOVERY SLOW SPIN-SPIN RELAXATION RATE

 A sequence that accentuates T1 information  Long T2
 Time consuming  Produces high MRI signal in T2-
 Slow patient “throughput” or productivity weighted images

FAT SAT FUNDAMENTAL CONCEPTS

 Pulse sequence used to saturate fat
3 PRINCIPAL INDEPENDENT
FAT SUPPRESSED IMAGES PARAMETERS
 Used to minimize the high signal intensity  Spin density
from fat tissue overwhelming small signal  T1
intensity in the tissue of interest  T2

GRADIENT ECHO FUNDAMENTAL PARTICLES

 Oldest imaging sequence CHARACTERISTICS
 Most common type of faster imaging  Charge
sequence  Spin

FAST GRADIENT ECHO CLASSIC MECHANICS

 Fast gradient imaging technique  Describes the motion of large object
 Good wherever there is turbulent flow
QUANTUM MECHANICS

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 MAGNETIC RESONANCE IMAGING

 Describes the motion of atoms and their PROPERTIES THAT INLUENCES IMAGE
constituents APPEARANCE
 Nuclear/spin density
MAGNETIC DIPOLE  Relaxation times – T1 and T2
 Small magnet created by the electron orbit  Flow phenomena

LARMOR EQUATION SPIN DENSITY

 Formula: ω = γBo  An indication of hydrogen concentration
o ω = Larmor frequency or frequency  Strength of signal is proportional to the
of precession (MHz) number of nuclei
o γ = gyromagnetic ration (MHz/T)
 constant value
o Bo = strength of external MF (T) T1 RELAXATION TIME
 Spins begin to precess at smaller and smaller
RESULTS OF RF ENERGY ABSORPTION BY angle (out of plane)
NUCLEI  The return of net magnetization along the Z
 Nuclei will turn upside down or flip axis to its normal equilibrium state
(energized or excited)  A characteristic of tissue itself
 Nuclei are caused to precess in phase
T2 RELAXATION TIME
EQUILIBRIUM MAGNETIZATION VECTOR  Spins begin to precess out of phase with
(M0) each other
 Amplitude of the net magnetization vector at  Exponential loss of signal caused by
equilibrium dephasing in the XY plane
 Factors that determine the amplitude:  Always less than or equal to T1 relaxation
o Spin density time
o Gyromagnetic ratio
o Strength of the external magnetic T1-WEIGHTED IMAGES (T1W)
field  Useful for showing anatomical detail
 Larger M0:
o More intense MRI signal T2-WEIGHTED IMAGES (T2W)
o Brighter MR image  Useful for showing pathology

FOURIER TRANSFORM FLOW PHENOMENA

 Used to generate the spectrum of free  Weak MRI signals: moving substances
induction decay (NMR spectrum) (e.g. blood)
 Essential to most imaging techniques  High MRI signals: stagnant (immobile)
blood (blood clot)
NMR PARAMETERS o Short T1 and long T2

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 MAGNETIC RESONANCE IMAGING

IMAGING PRINCIPLES o Magnetic tape or optical disk

 For permanent storage and retrieval
GRADIENT MAGNETIC FIELD
 A magnetic field that changes in strength in MRI GANTRY
a given direction  Contains magnets, shim coils, gradient coils
 The only difference between NMR and RF transceiver coils
spectrometer and MR imager  Power supply:
 Functions: o High current power for magnet
o Used to localized MRI signal o Precision power for secondary coils
o Used for slice selection
o Used for encoding the location of CT GANTRY
MRI signal  Contains x-ray tube and detectors
 Power supply: high voltage generator
GRADIENT COILS IMAGING MAGNETS
 Current-carrying coils designed to produce a
desired gradient magnetic field MAGNET
BACK PROJECTION RECONSTRUCTION  Major component of MRI system
 The earliest and easiest to understand
THREE GROUPS
2D FOURIER TRANSFORMATION  Permanent
 Recently used technique  Resistive
 Requires a rigorous mathematical  Superconducting
development
PERMANENT MAGNET
MR EQUIPMENT AND IMAGES  The simplest in design and least expensive
to operate
OPERATING CONSOLE  Material: bricklike ceramics
 Used to control the computer o Inexpensive
 Laser or multiimage camera: used to print o Easy to magnetize
the image o Light weight
 Advantages:
COMPUTER ROOM o Low capital cost
 Houses the electronics o Low operating cost
o For transmitting the radiowave pulse o Negligible fringe file
sequences  Disadvantages:
o For receiving and analyzing the o Limited field strength (0.3 T)
MRI signal o Fixed field strength
 Storage of raw data/computer- o Very heavy
constructed images:
o Computer disk RESISTIVE MAGNET (0.15 T)
 For temporary storage  Magnets have finite resistance
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 MAGNETIC RESONANCE IMAGING

o Rationale: conductor used in coils is SHIM COILS

not a perfect conductor  30 individual windings
 Can be turned off but requires continuous  For improving the homogeneity of the main
power and water cooling magnetic field
 Uses 4 large coils  Prevent degradation of image quality
 Advantages:  Shimming the magnet: the process
o Low capital cost
o Easy coil maintenance GRADIENT COILS
o Negligible fringe field  Produces gradient magnetic field
 Disadvantages:  3 coils:
o High power consumption o Z gradient coils – for transaxial slice
o Water cooling required o X gradient coils – for coronal slice
o Significant fringe field o Y gradient coils – for sagittal slice
SUPERCONDUCTING MAGNET  Frequency-encoding gradient: X gradient
 A magnet containing coils made from a  Phase-encoding gradient: Y gradient
superconducting metal alloy
 Requires no continuous power source but RF COILS/PROBE
more difficult to maintain  Transceiver coils
 No electrical resistance  Used for transmitting RF signals and/or
 Dewar: a double-walled flask of metal with receiving MR signal
vacuum between the walls  RF probe/Probe Assembly:
 Uses cryogens: o A rigid unit consisting of antenna col
o Cooling agents and support material (plastic, fiber
o Used to optimized superconductivity glass, insulators)
o Liquid nitrogen o Functions:
 97 K  Maintain the coil in its
 Filled in outermost chamber intended shape
o Liquid helium  Protect the coil from damage
 47 K o Location:
 Filled in innermost chamber  Inside the gradient coils
 Advantages:  Closest to the patient
o High field strength (2 T)
o High field homogeinity COILS
o Lower power consumption
 Disadvantages: RF COILS
o High capital cost  Antenna of the MRI system
o High cryogen cost
o Intense fringe field SURFACE COILS
 Simplest design of coil
SECONDARY COILS  Applications: spines, shoulders, TMJs,
small body parts
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 MAGNETIC RESONANCE IMAGING

 Endocavitary Coil:  Polyvinyl chloride (PVC): structural

o Used for imaging the rectum concrete slab or walls
o Used for imaging the wall of the  Lighting: should be direct current
cavity
 Neurovascular Coil: FARADAY CAGE/RF SHIELD
o Used for imaging the brain and neck  A wire-mesh shield enclosing the MR
imager
PAIRED SADDLE COIL  Used to attenuate extraneous sources of RF
 Commonly used for knee imaging
 Used for X and Y gradient coils FRINGE MAGNETIC FIELD
 Magnetic field outside the patient aperture
 Must be considered in the design of MRI
facility
HELMHOLTZ PAIR COIL  Problems in fringe MF:
 Consists of two circular coils parallel to o Can interfere with the proper
each other operation of mechanical and
 Used for Z gradient coils electronic equipment
 Used as RF coils for pelvis and cervical o Any large mass of ferromagnetic
imaging material can distort the homogeneity
of the imaging volume by interacting
BIRD CAGE COIL with fringe MF
 Provides best RF homogeneity of all the RF  Takenote: distortion of fringe MF by a
coils ferromagnetic object will cause a
 Commonly used as transceiver coil for head compensating distortion of the imaging
imaging volume
 Occasionally used for extremity imaging
MR IMAGES
FACILITY DESIGN
FACTORS INFLUENCING SPATIAL
EXAMINATION ROOM DESIGN RESOLUTION
 Room must be shielded against Effect in Spatial
Factors
Resolution
o Radio interference
Increase slice thickness Low
o Fringe magnetic fields
Decrease slice thickness High
 Only nonmagnetic materials are used for Increase data collection High
the structure and finish Decrease data collection Low
 Must have electric filters Increase MRI signal
High
o Rationale: to remove interfering acquisition
frequencies Decrease MRI signal
Low
 Plumbing: should be PVC or copper acquisition
Increase MRI signal
High
strength
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 MAGNETIC RESONANCE IMAGING

Decrease MRI signal o Rationale: to minimize the

Low
strength unsharpness caused by involuntary
and voluntary motion
SLICE THICKNESS
 Important in visualization of pathology BIOLOGICAL HAZARDS
 Large slice thickness:
o Provide less grainy images MRI MAGNETIC FIELDS
o Can’t visualize small pathologic  Exhibit threshold dose-response relationship
lesions
 Small slice thickness: THREE PHYSICAL FIELDS THAT MIGHT
o Provide more grainy images SUSPECT OF PRODUCING BIOLOGICAL
o Can visualize small pathologic lesion RESPONSE
1.) Strong static magnetic field (B0)
 Unit: Tesla (T)
CONTRAST RESOLUTION  Affects:
 Low contrast resolution structures o Membrane permeability
 Most tissues: <1% difference o Enzymes kinetics
 MR parameters and biologic tissues: o Nerve conduction
>30% difference 2.) Time-varying gradient magnetic field (B1)
 Gray and white matter: 30-40% difference  Unit: Tesla/second (T/s)
 Null regions: total loss of contrast  Affects:
o Rationale: improper RF pulse o Visual phosphenes
selection o Bone healing
o Cardiac fibrillation
APPEARANCE OF NORMAL TISSUES 3.) RF emission
SPIN  Unit: Watts/kilogram (W/kg)
TISSUES T1 T2
DENSITY  Causes tissue heating
Fat and
High/white Short/white Long/white  Heat: the principal result of the interaction
skin between RF field and tissue
Very Very
Bone Low/black
long/black long/black  Heating: expressed as specific absorption
White rate (SAR)
High/white Short/white Long/gray
matter o Unit: W/kg
Gray
High/white Long/gray Long/gray
matter MAXIMUM PERMISSIBLE DOSE (MPD)
Very Very Very
CSF  Not correct for MRI
high/white long/gray long/black

PHYSICAL HAZARDS
EXAMINATION TIME
 Keep as short as possible
STRONG MAGNETIC FIELDS
 Physical Precautions:

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 MAGNETIC RESONANCE IMAGING

o Avoid any objects made from metal

or iron inside the examination room GATING
o Patient must be free from metallic  Technique used to organize the signal
objects even internally  Technique used for improving image quality
o Warnings signs are necessary  Used to reduce motion artifacts from the
beating heart
QUENCHING  The moving object is “frozen” at that phase
 The events that occur when the liquid of its motion, reducing image blurring
cryogens that cool the magnet coils boil off
rapidly ARTIFACTS
 Caused:
o Activation of the magnet STOP IMAGE ARTIFACT
button  A structure not normally present but visible
o A fault in the magnet itself as a result of a limitation or malfunction in
 Results the hardware or software of MRI device
o Helium escaping very rapidly from
the cryogen bath
o A loud bang or thundering or hissing MAGIC ANGLE ARTIFACT
or rushing sound with the cold gas Increase of the T2 time;
Description
expulsion bright signal in tendons
o Patient asphyxiation and frostbite Angle about 55o to the
Cause
 If quench does occur: main magnetic field
o RT should evacuate immediately Solution Angle not about 55o
o It takes 72 hrs to ramp magnet up to
full magnetic potential PARTIAL VOLUME ARTIFACT
Loss of contrast between
Description
two adjacent tissues
CONTRAST MEDIA Insufficient resolution
Cause
 Gadolinium-containing compounds Solution Thinner slices
 Gadolinium: a metal with paramagnetic
effect METAL ARTIFACT
o Lower toxicity Magnetic susceptibility
Same as
o Few side effects and clip artifact
 Teslascan: a new manganese-based Description
Signal dropouts; bright
paramagnetic liver contrast agent spots; spatial distortion
o Used in detection, characterization, Cause Field inhomogeneity
Remove the metal; don’t
localization and evaluation of lesions
take a gradient echo
in the liver Solution
sequence; take a short
 Feridex: an iron oxide mixture and the only echo time
supramagnetic contrast agents available
o Used to detect and diagnose liver MOTION ARTIFACT
lesions Same as Phase encoded motion;
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 MAGNETIC RESONANCE IMAGING

phase effect; instability;

smearing artifacts ALIASING ARTIFACT
Description Blurring and ghosting Backfolding, phase
Movement of the Same as wrapping & wrap
Cause
imaged object around artifacts
Compensation Description Image wrap around
Solution techniques; more Anatomy extends out of
averages; antispasmodic Cause the FOV within the
plane of the image
HERRINGBONE ARTIFACT Large FOV;
RF noise; Interference; Solution oversampling; foldover
Same as suppression
crisscross
Description Static on the image
Electromagnetic ZEBRA STRIPES ARTIFACT
Cause
emissions Same as Zero fill artifact
Shielding; eliminate the Zebra stripes or other
Solution Description
factor of disturbance anomalies
Cause Signal changes
RF OVERFLOW/DATA CLIPPING Surface coil; change the
Solution
ARTIFACT sequence
Description Image non-uniform
Cause Signal too intense ZIPPER ARTIFACT
Manually decrease of Same as Star artifact
Solution
the receiver gain Bands through image
Description
center
SHADY ARTIFACT Hardware or software
Cause
Localized problems
Description inhomogeneous Large FOV;
Solution
brightness oversampling
Cause Various causes
Check correct
Solution positioning; call the -THE END-
service “There are no secrets to success. It is the result of
preparation, hard work learning from failure”
SLICE OVERLAP ARTIFACT 06/18/14
Description Slice overlap
Cause Loss of signal
Solution Saturation

STAIRCASE ARTIFACT
Description Staircase like nuance
Slices too thick; no
Cause
overlapping
Solution Overlapping slices

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