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Principles of MRI magnetic resonance imaging Dr Mohammed Bader Hassan FIBMS,DMRD

Principles of MRI magnetic resonance imaging

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Objectives By the end of this lecture the student should be able to 1.calrify the basics of MRI physics 2.define major MRI sequences (T1&T2) 3.identify the MRI advantages and contraindications 4.identify the major MRI studies such is angiography, MRCP and myelography

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PowerPoint PresentationDr Mohammed Bader Hassan
FIBMS,DMRD
Objectives
By the end of this lecture the student should be able to
1.calrify the basics of MRI physics
2.define major MRI sequences (T1&T2)
3.identify the MRI advantages and contraindications
4.identify the major MRI studies such is angiography, MRCP and myelography
Basic facts
the human contain a predominant of hydrogen atoms (in water ,fat ,and other tissues)
The hydrogen atom contain a proton only
The proton is a particle with a positive charge
Each proton have a spin movement
The moving charge is electrical current which have magnetic field
So each proton have own magnetic field
P
What happen when the body inside the magnetic field
1.the proton (small magnets )organize the selves with the external magnets
2.a precession movement begin (just like when you hit the top of spin )it is a wobble movement
This precession have the specific frequency according to type of atom and magnetic field (so it is a well known frequency)
how can we get a picture from MRI
It is difficult to measured aligned magnets inside a river of magnetic field so we should shift this magnetic field
We send a radio wave in the same frequency of the precession of the proton (radio frequency )resonance!
Measuring Tissue Magnetization:
For an energy transfer to occur the RF Pulse must be at the same
frequency as the precessing Mo. ie The Resonance Frequency.
Much like a tuning fork experiment.
B
A
B
C
D
E
8
What happen after radiofrequency is sent
The radio frequency should be in the same frequency of the precession of proton so it can transfer energy to proton
The energy which is picked up by proton will make the proton in higher energy this will make the longitudinal magnetic vector of the body is zero
And it will synchronized the precession of the protons which will move in the same direction & will give moving transverse vector
Moving transverse vector
What happen after the radiofrequency is off
The energy will be loosed from the spinning proton and longitudinal vector appear again in a specific time according to tissue component.
The synchronization (the transvers vector will be loosed in specific time according to tissue component
T1 Relaxation
T1 Relaxation
T1 relaxation is regrowth of the longitudinal magnetization
T1=the time which is needed for most of the longitudinal vectored to re appear
T1 Relaxation
T1 Relaxation
T1 Relaxation
T1 Relaxation
T2 Decay
x'
y'
z'
-
-
-
+
+
+
The time which is needed to disappear of most of the transverse vectored is T2
Decay of transvers vectoreT2
Encoded through a series of complex techniques and calculations (magic?)
Stored as data
T1 Relaxation:
this would mean that the protons
with short T1 time would
contribute the highest signal
this would mean that the protons
with a long T2 time would
contribute the highest signal
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
Image formation
The image will depend on T1,T2, and amount of proton within the tissue (spin density ) .
The fatty tissues is loosing energy quickly so it will re appear its longitudinal vector rapidly ( high T1 signal)
Fatty tissue will loss synchronization quickly
Water loosing energy and synchronization slowly so it will appear bright at T2 and black on T1
Solid material will not give a signal on t1 and t2 (signal void )
air will appears signal void because lack of H .
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
Relative Relaxation Rates
T1-weighted Sagittal
T1-weighted Axial
T2-weighted Axial
90 deg
Fat & water
90 deg
The ability of imaging vessels without contrast (MR angiography )
Have a good soft tissue contrast
Contraindication of MRI
Patient with pacemaker
Patient with bullet injury or ferromagnetic F.B ,or surgical clip (because of heat and missile effect )
Pregnancy especially first trimester
Claustrophobia reported that between 1 % and 10 % of patients experience some degree of claustrophobia which in the extreme cases results in their refusal to proceed with the scan
bioeffect
No known or expected harmful effects on humans using field strengths up to 10 Tesla
Currently pregnant women are normally excluded from MRI scans during the first trimester although there is no direct evidence to support this restriction
The most invasive MR scans involve the injection of contrast agents (e.g. Gd-DTPA).
noise
Monitoring equipment
Infusion pumps
Credit cards
Cellular telephones
Gold
Silver
Special MRI studies
MR angiography :non-invasive imaging of the vascular tree , In normal circumstances flow effects cause unwanted artefacts, but in MRA these phenomemna are used advantageously ,
Direction of
Blood Flow.
into the transverse
plane and gives
a high signal.
This causes
high flip angles.
Eg: 33ms & 70deg.
into the transverse
plane and gives
a high signal.
in the same manner as stationary
tissue and contributes no signal.
61
No need for contrast
No need for endoscopy
MR mylogarphy
Heavily T2 weighted image ,used for imaging spinal canal and theca sac , with out the need for injection of contrast in the thecal sac ( non invasive method )
QUESTIONS?????