Transcript M -AGNETIC R -ESONANCE I -MAGING

M -AGNETIC
R -ESONANCE
I -MAGING
CONTENT
• PHYSICS
• SAFETY
• APPLICATIONS
• QUESTIONS
PHYSICS
TYPES OF ATOMIC
MOTION
1. The electron orbits
the nucleus
2. The electron spins
on its own axis
3. ***The nucleus
spins on its own
axis***
MRI USES THE HYDROGEN ATOM
•1 electron orbits the nucleus
•The nucleus contains no neutrons but contains 1
proton
THE HYDROGEN NUCLEUS HAS A NET
POSITIVE CHARGE
•Hydrogen nucleus is a spinning, positively charged
particle
LAW OF ELECTROMAGNETISM
•A charged particle in motion will create a
magnetic field
•The postitively charged, spinning hydrogen
nucleus generates a magnetic field
WHY HYDROGEN?
•Very abundant in the human body-H20
•Has a large magnetic moment
MAGNETIC MOMENT
The tendency of an MR active nuclei to align its
axis of rotation to an applied magnetic field
MR ACTIVE NUCLEI
odd # protons
or
odd # neutrons
or
BOTH
e.g. Hydrogen1, Carbon13, Nitrogen15, Oxygen17,
Fluorine19, Sodium23, Phosphorus31
STABLE ATOMS
# protons = # electrons
IONS
# protons  # electrons
When a body is placed into the bore of the scanner, the strong magnetic
field will cause the individual hydrogen nuclei to either:
A) ALIGN ANTI-PARALLEL TO THE MAIN MAGNETIC FIELD
(B0)
OR
B) ALIGN PARALLEL TO THE MAIN MAGNETIC FIELD (B0)
Anti-parallel
high energy
B0
NMV
Parallel
low
energy
NET MAGNETIZATION
VECTOR
• An excess of hydrogen nuclei will line up
parallel to B0 and create the NMV of the
patient
N
N
S
S
size
direction
The magnetic
vector
THE NUCLEI WILL ALSO
PRECESS…
PRECESSION
• Due to the influence of
B0, the hydrogen
nucleus “wobbles” or
precesses (like a
spinning top as it
comes to rest)
• The axis of the
B0
nucleus forms a path
around B0 known as
the “precessional
path”
Precessional
path
Hydrogen
nucleus
PRECESSION
• The speed at which hydrogen precesses depends
on the strength of B0 and is termed the
“precessional frequency”
• The precessional frequency of hydrogen in a 1.5
Tesla magnetic field is 63.86 MHz
• The precessional paths of the individual hydrogen
nucleus’ is random, or “out of phase”
WE NEED THEM TO BE “INPHASE” OR TO RESONATE…
RESONANCE
Occurs when an object is exposed to an oscillating
perturbation that has a frequency close to its own
natural frequency of oscillation
•Ella Fitzgerald
•Tacoma Narrows bridge failure
RESONANCE con’t
• Frequency of the hydrogen proton in a 1.5T
magnetic field can be found in the RF band
of energy in the electromagnetic spectrum
RADIOFREQUENCY
ENERGY
• Follows the Law of Electromagnetism
(charged particles in motion will generate a
magnetic field)
• Magnetic field known in MR as B1
• Applied as a “pulse” during MR sequences
• The RF pulse is applied so that B1 is 90 to
B0
DURING RESONANCE…
1) The hydrogen atoms begin to precess “in phase”
1)
2) The hydrogen atoms align with the RF’s magnetic field
(B1) and they flip!!
RF
B0
B0
NMV
NMV flips!
PULSE
B1
B1
AS THE NUCLEI PRECESS IN-PHASE IN THE B1
PLANE, A CHANGING MAGNETIC FIELD IS
CREATED
IF YOU PLACE A RECEIVER COIL (ANTENNA) IN
THE PATH OF THE CHANGING MAGNETIC
FIELD, A CURRENT WILL BE INDUCED
THIS IS FARADAY’S LAW OF
INDUCTION
FARADAY’S LAW OF INDUCTION
A changing magnetic field will induce an
electrical current in any conducting medium
COILS
Used to:
•transmit pulses of radiofrequency energy
•receive induced voltage - MR SIGNAL
•increase image quality by tuning in to one body
part at a time
RELAXATION
When the RF pulse is turned “off”, the NMV “relaxes”
back to B0 (away from B1)
NMV
B0
B1
•RF pulses are applied very quickly in succession - RF
PULSE SEQUENCE
•3 minute sequence (20 slices, axial brain) - 60 RF pulses
may be applied
MR SIGNAL
• Collected by a coil
• Encoded through a series of complex
techniques and calculations (magic?)
• Stored as data
• Mapped onto an image matrix
TR - REPETITION TIME
Time from the application of one RF pulse to
another RF pulse
TE - ECHO TIME
Time from the application of the RF pulse to
the peak of the signal induced in the coil
T1 WEIGHTING
•A short TR and short TE will result in a T1
weighted image
•Excellent for demonstrating anatomy
T2 WEIGHTING
•A long TR and long TE will result in a T2
weighted image
•Excellent for demonstrating pathology
MANY OTHER DIFFERENT TYPES OF
IMAGES THAT COMBINE ABOVE AND
INCLUDE OTHER PARAMETERS
T1 WEIGHTED IMAGE
T2 WEIGHTED IMAGE
SAFETY
THE
MAGNET IS
ON ALL THE
TIME!!!
OHM’S LAW OF
RESISTANCE
V = IR
V = voltage I = current R = resistance
R depends on the material, the length, the
cross-sectional area, and the temperature of
the loops of wire through which the current
flows
**Decreasing the temperature of the wire
will decrease resistance to the flow of
electricity
SUPERCONDUCTING
MAGNET
• No resistance to flow of electricity
• Coils of wire surrounded by cryogen bath
(Helium) at -273 C
• No external source of energy required
• Magnetic field present ALL THE
TIME!!!
Gauss - measure of magnetic field
strength
refrigerator magnet - 150-250 G
10,000 Gauss = 1T
MRI - 0.2T - 1.5T
 100x stronger that fridge magnet
THE STRONG MAGNETIC FIELD OF
THE MAGNET CAN TURN THE
FOLLOWING INTO DANGEROUS
PROJECTILES:
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wheelchairs
oxygen tanks
I.V. poles
I.D. tags
keys
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coins
scissors
trauma boards
sandbags
safety pins
THE CHANGING MAGNETIC
FIELDS CAN DO DAMAGE TO:
•Monitoring equipment
•Infusion pumps
•Credit cards
•Cellular telephones
•Any electronic device
THE FOLLOWING ARE
(USUALLY*) OKAY:
•Gold
•Silver
•Digital watches
•Eyeglass frames
•Snaps/zippers fastened to clothing
•Dental work
APPLICATIONS
ADVANTAGES
• Superior soft tissue contrast resolution excellent pathological discrimination
• No ionizing radiation
• Direct multi-planar imaging (transverse,
coronal, sagittal, any oblique)
• Non-invasive - vascular studies can be
performed without contrast
KNEE
ANGIOGRAPHIC
TECHNIQUES
• Circle of Willis
angiograms without
any contrast
ANGIOGRAPHIC
TECHNIQUES
•Studies using
contrast can also
be performed
RENAL MRA
GADOLINIUM
USEFUL FOR DETECTION OF:
• Tumours pre- and post-operative
• Infection
• Inflammation
• Post-traumatic lesions
• Post-operative changes
• MRA’s
DISADVANTAGES OF MRI
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Expensive
Long scan times
Audible noise (65-115dB)
Isolation of patient (claustrophobia,
monitoring of ill patients)
• Exclusion of patients with pacemakers and
certain implants
BRAIN
• Hemorrhage (stages of)
• Demyelinating disorders (M.S.)
• Infectious processes (encephalitis,
meningitis)
• Abscesses
• Neoplasms
• Neurofibromatosis
• Trauma
• Vascular disorders (AVM’s, infarcts,
aneurysms)
BRAIN (cont’d)
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Metastasis
Internal auditory canal pathology
Pituitary pathology
Hydrocephalus
Child abuse
Cranial nerve pathology
Congenital anomalies (for anatomical
review)
• Epilepsy (seizures in general)
AXIAL T2 BRAIN
SPINE
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Radiculopathy
Tumours
Trauma/contusion
Syringomyelia
Metastasis
Vascular disorders
Cord edema
M.S. plaques
SPINE (cont’d)
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Cauda equina syndrome
Tethered cord
Arachnoiditis
Marrow-replacing processes
Degenerative disc disease
Discitis
Congenital anomalies
SPINE
MUSCULOSKELTAL
(shoulder, knee, ankle, wrist, elbow, TMJ)
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Meniscal pathology
Ligament/tendon injury
Muscle/nerve impingement
Avascular necrosis
Labral tears (shoulder, hip)
Chondromalacia
Inflammation (osteomyelitis)
Primary bone tumours
Soft tissue tumours
SHOULDER
ABDOMINAL/PELVIC
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Liver pathology
Kidney pathology
Renal artery MRA
Fetal abnormalities
ABDOMINAL IMAGING
• Breath-hold scans to overcome motion
artifact problem
• MRCP’s - images of the biliary and
pancreatic ductal systems performed noninvasively (no contrast or endoscope!)
within seconds
• Fetal imaging very diagnostic
MRCP
FETAL BREATH-HOLD IMAGE
FETAL ENCEPHALOCELE
CARDIAC
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Co-arctation
RV dysplasia
Cinematic studies
Measure cardiac output, stroke volume,
ejection fraction
MR Spectroscopy (MRS)
• Information obtained is in the form of a spectrum
which provides the biochemical information
contained within a selected voxel of tissue
• Used to detect the absence or presence of a certain
compound
• Assists in differential diagnosis when standard
clinical radiological tests fail or are too invasive
Spectrum
MRS Current
Applications
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Multiple Sclerosis
Leigh’s
Huntington’s
Parkinson’s
Alzheimer’s
Epilepsy
other dementias
metabolic disorders
• Stroke
• Asphyxiation or
ischemic injury
• Tumours and
intracranial lesions
• Prostate cancer
• Encephalopathies
• Leukodystrophies
Functional MRI (fMRI)
•
research topic
• Detects changes in blood flow or
metabolism associated with specific motor
or sensory functions or stimuli
• Performed by scanning specific areas of the
brain/spine while: a) the subject performs
a certain motor task or b) exposing the
subject to certain external/internal stimuli
fMRI cont’d
• Subjects are scanned at rest and then during
exercise or exposure to various stimuli
• The two conditions are subtracted to reveal
areas of brain activation
• Areas of activation will have increased
levels of blood flow and are therefore
detectable
fMRI cont’d
• Mapping of the brain’s motor and sensory
areas
• Delineating primary cortical areas prior to
surgery on patients with tumours (to avoid
paralysis when operating on tumours in
dangerous locations)
• Assessment of brain function following
injury
MANY OTHER WORKS IN
PROGRESS…
QUESTIONS?????