Transcript Magnetic Resonance Imaging

Magnetic Resonance Imaging
Dr Mohamed El Safwany, MD.
Intended learning outcome
► The
student should learn at the end of this
lecture MRI basics .
► Oxygen,
hydrogen, carbon, nitrogen
elements constitute 96 % of human body
mass.
► Oxygen is 65 % of body mass; carbon is
18.5 %, hydrogen 9.5 %, nitrogen 3.2 %.
► Let us ignore all elements but Hydrogen.
► Simplest
element with atomic number of 1
and atomic weight of 1
► When in ionic state (H+), it is nothing but
a proton.
► Proton is not only positively charged, but
also has magnetic spin (wobble)!
► MRI utilizes this magnetic spin property of
protons of hydrogen to elicit images!!
► We are magnets!
► The
protons (i.e.
Hydrogen ions) in
body are spinning in a
hap hazard fashion,
and cancel all the
magnetism. That is our
natural state!
► We need to discipline
them first
► Magnetic
field strength: 0.3 – 7 T (2500
times more than earth’s magnetic field).
Average field strength – 1.5 T
► Open magnet – less field strength, less
claustrophobic
► Closed magnet – more field strength,
claustrophobic
Proton alignment
► Compass
aligns with
the earth
► In a similar fashion,
► Our body protons
(hydrogen) align with
this external magnetic
field.
► Now, we are
disciplined (spinning
in line with each
other!)
► Radio
frequency pulses in resonance push
the aligned protons (H+) to a higher energy
level.
Radio Frequency pulse
► Same
as Radio waves – high wavelength,
low energy electromagnetic waves
► Radiofrequency coils
 Act as transmitter and receiver
 Different types of coils
► The
higher energy gained by the protons is
retransmitted (NMR signal)
Images
► BOLD
► T1WI
► MRA
► T2WI
► MRV
► PDWI
► Post-Gd
► DWI
► ADC
► GE
► Perfusion
► fMRI
images
images
images
► Volumetric images
► MR arthrograms
► FLAIR
► STIR
► Etc etc etc
T1-weighted contrast
T2-weighted contrast
Tissue Appearance
WT
FAT
H2O
MUSC
LIG
BONE
T1
B
D
I
D
D
Proton I
I
I
D
D
T2
B
I
D
D
Density
I
sequence exercise
Which sequence is it? T2
Which sequence is it? T1
Which sequence is it? FLAIR
Which sequence is it? DWI
Which sequence is it? ADC
Which sequence is it? GE
Which sequence is it? PostGd
Which sequence is it? MRV
Which sequence is it?, 3D MRV
Which sequence is it? MRA
Which sequence is it? T2
Which sequence is it? T1
Which sequence is it? STIR
Which sequence is it? MR
arthrogram
Before entering tunnel, there is
a checklist!
► No
mobiles, no credit cards, please!
► Known potential safety concerns due to large
static magnetic field:





Internal cardiac pacemakers
Steel cerebral aneurysm clips (ferromagnetic)
Small steel slivers embedded in eye
Cochlear implants
Stents anywhere in the body
Clinical Aspects of MRI
Advantages of MRI
1.
2.
3.
4.
5.
No ionizing radiation & no short/long-term
effects demonstrated
Variable thickness, any plane
Better contrast resolution & tissue
discrimination
Various sequences to play with to
characterise the abnormal tissue
Many details without I.V contrast
Disadvantages of MRI
► Time
consuming
► Not easily available (long waiting list)
► MRI
has limitations:
 Bone
 Air
 Time consuming
MR
CT
Faster
► Less expensive
► Less sensitive to patient
movements
► Easier in claustrophobics
► Acute haemorrhage
► Calcification
► Bone details
► Foreign body
►
►
►
►
►
►
No ionising radiation
Greater details, hence
more sensitive and more
specific
Any plane scanning
Contrast less allergic
No beam hardening
artefact
Indications (almost everything)
► Anatomy
► Trauma
► Congenital
► Dementia
anomalies
► Hereditary and
metabolic diseases
► Infections
► Demyelination
► Vascular event
► Tumours
► Hydrocephalus
► Cranial
nerves
► Arteriograms
► Venograms
► Skull base
► Pituitary gland
Vascular MRI
► Peripheral
vascular arteriogram with or
without I.V contrast
► Aortogram
 Dissection
► Pulmonary
arteriogram
 When CT is contraindicated
Cardiac MRI
► Coming
in a big way
► Very useful in congenital heart diseases,
cardiomyopathies
► Evidence is emerging in the evaluation of
myocardial infarction
Breast MRI
► Problem
solving
 Breast implants
 Recurrence
 Multifocal disease
Foetal MRI
► Assessment
of congenital anomalies
► Placental abnormalities
► Twin assessment
Text Book
► David
Sutton’s Radiology
► Clark’s Radiographic positioning and
techniques
Assignment
► Two
students will be selected for
assignment.
Question
► Define
differences between CT and MRI?
Thank You
44