Transcript Physics of MRI

Basic Fundamentals of MRI
Acquisition of MRI images - PHYSICS
 Basic Sequences & Advanced

applications

Characterisation of Lesions.
MRI Physics
MRI Physics
Magnetic
Resonance
Imaging
MRI Physics



MAGNETIC
Tissues are made of Protons (H+)
Positive electrical charge spinning
around its own axis.
Moving Electrical charge produces
its own magnetic field.
Protons are analogous to small
BAR MAGNETS.
MRI Physics


Presence of a strong
magnetic field
This leads to alignment
of protons in the tissue
acc to the magnetic
field- LEVEL 1
MAGNETIC
MRI Physics

RESONANCE
Apart from spinning motion
these protons are also
vibrating at a frequency.

A specific radiofrequency wave passed in a
specific manner & specific plane.

Radiofrequency waves are NOT Radiation.
MRI Physics
RESONANCE
MRI Physics
RESONANCE


Raises further raise
the energy level of
protons – Level 2
Level of excitation
is different for
different tissues
hence signal is
different –
CONTRAST
IMAGING



Radiofrequncy
switched off &
excited protons
come back to
Level1
Energy / Signal
is released..
Signal picked up by
the Coil &
processed by a
computer.
MRI Physics




IN NUTSHELL
Patient is placed in a strong magnetic
field.
Radiowave is sent in for a short time &
switched off.
Patient emits signal.
Recived & used for reconstruction of
image.
TESLA FACTOR !!

Tesla is the unit of magnetic field strength.
TYPES OF MAGNETS :
• Resistive Magnets – Low
& Ultralow field strenghts.
• Permanent Magnets –
Low & Med field strength
(upto 0.3 T).
• Superconducting
Magnets 0.5 - 14T
TESLA FACTOR !!
FIELD SRENGTH
GRADIENTS
TYPE OF MAGNETIC FIELD
VERTICAL/ HORIZONTAL
IMAGE
QUALITY
S/N Ratio
Contrast
TYPE OF COILS
TYPE OF COMPUTER
IMAGE PROCESSING & SEQUENCES
CAMERA
TESLA FACTOR !!
As Magnetic field strength
increases SNR increases –
true for analyitical NMR But
NOT for whole-body MRI.

In higher field strengths
body produces an inherent
noise.

Most important factor in
Medical imaging is tissue
contrast – best in medium &
low field strngths.

TESLA FACTOR !!
SEQUENCES
BASIC SEQUENCES :




T1W.
T2W.
FAT SUPP T2W/ STIR.
FLUID SUPP T2W/ FLAIR.
ADVANCED APPLICATIONS :




MR ANGIOGRAPHY.
MR CHOLANGIOPANCREATOGRAPHY &
MR UROGRAPHY.
DIFFUSION.
JOINT MOTION STUDY – KINEMATIC MRI.
BASIC SEQUENCES



T1 Weighted –
ANATOMY.
FAT- WHITE
FLUID- BLACK
BASIC SEQUENCES



T2 Weighted –
PATHOLOGY.
FLUID- WHITE
FAT- NOT SO
WHITE
BASIC SEQUENCES

T2 Weighted
FAT SUPP/ STIR.
- PATHOLOGY FOR
MOST EXTRACRANIAL
REGIONS.


FLUID- WHITE
FAT- BLACK
BASIC SEQUENCES
T2 W
T2 W FAT SUPP
BASIC SEQUENCES



T2 Weighted
WATER SUPP/ FLAIR.
- BRAIN PATHOLOGY
CSF/ CLEAR FLUID BLACK
PATHOLOGY/
PATHOLOGICAL
FLUID- WHITE
BASIC SEQUENCES
FLAIR- WATER SUPP
T2 W
ADVANCED APPLICATION




DIFFUSION
Very sensitive to motion - picks up
microscopic motion of protons in edema.
Can differentiate between Cytotoxic
Edema (infarct) & Vasogenic Edema
(tumor/infection etc).
Special sequence, picks up infarct within
hours (2-3 hrs).
To diffrentiate between Old & Fresh
infarct.
ADVANCED APPLICATION
FLAIR
DIFFUSION
DIFFUSION
ADVANCED APPLICATION
FLAIR
DIFFUSION
DIFFUSION
H/O dense Left Hemiplegia – 3 hrs back.
FLAIR
DIFFUSION
ADVANCED APPLICATION





MRCP
Heavily T2W sequence – Only Fluid appears
bright.
2 types – (1) Single shot breath hold (3 secs)
(2) 3D MRCP.
No contrast / injection.
Non-invasive modality for deliniation of biliary
anotomy.
All non-theraputic indications of ERCP.
ADVANCED APPLICATION
MRCP
T2 W FSE
3D MRCP
ADVANCED APPLICATION
T2 W Axial
MRCP
Single Shot MRCP - 3 sec
ADVANCED APPLICATION




MRAngio
Flow sensitive sequences – picks up flow.
No contrast / injection.
Arterial study – Head & Neck vessels(2D/3D TOF)
Venous study – Intracranial venous sinuses
(2D TOF & Phase Contrast)
ADVANCED APPLICATION
MRAngio
INDICATIONS :
1.
2.
3.
4.
5.
6.
Stroke – infarct (Single vessel / Multiple vessel).
TIA.
Secondary Hemorrhage – AVM, Aneurysm.
VBI.
Trigeminal Neuralgia & Hemifacial spasm – aberrent
loops.
Venography - Venous sinus thrombosis .
Contrast Enhanced MRA :
1.
2.
Renal Vessels – RAS.
Limb vessels – Femoral, Popliteal , Subclavian &
Brachial.
ADVANCED APPLICATION
MRAngio
MRA – NECK VESSELS
MRA – CIRCLE OF WILLIS
ADVANCED APPLICATION
MRAngio
CONTRAST MRA – RENAL VESSELS.
ADVANCED APPLICATION
PCA ANEURYSM
MRAngio
AVM
ADVANCED APPLICATION KINEMATIC STUDY
EXTENSION
NEUTRAL
FLEXION
Pathology
inflammation, infection, infarct, tumor etc. have high water content
Hypintense on T1 & Hyperintense in T2
SOLID
+
T2W - +
T1W -
T2W - +++
HIGH PROTEIN FLUID FLAIR - SUPP
FAT – T1W +++
T1W - ++
T2W ++
T2W - ++
CALCIFICATION – T1W+++
BLOOD- METH Hb
T2W+++
T1W - ++
 FLOW – T1W +++
T2W - ++
T2W +++

FLUID
T1W - +++
LESION CHARACTERISATION
BRAIN EDEMA
CYTOTOXIC
VASOGENIC
Due to arterial infarct.
Due to tumor, infection,
venous infarct
Extends thru grey-white
matter junction (involves
grey matter).
Involves only white matter.
Along specific arterial
distribution.
NOT along specific arterial
distribution
Bright on diffusion.
Not bright on diffusion.
CONTRAST MRI INDICATED
LESION CHARACTERISATION
CYTOTOXIC EDEMA
BRAIN EDEMA
VASOGENIC EDEMA
T1W AXIAL
FLAIR COR
T2W AXIAL
T2W SAG
T1W +++
T2W +++
FLAIR NOT SUPP
POST CONT
ENHANCEMENT
LESION CHARACTERISATION
T1 W AXIAL
++
T2 W AXIAL
+++
H/O headache following vehicular accident.
FLAIR AXIAL
+++
LESION CHARACTERISATION
T1W
T2W
T1W
T
T2W
POST CONT T1
POST CONT T1
LESION CHARACTERISATION
T2W AXIAL.
T1W AXIAL
T2W SAG
T1W SAG
LESION CHARACTERISATION
T2W AXIAL.
T1W AXIAL.
T2W SAG
T1W SAG
LESION CHARACTERISATION
T1W
T2W
POST CONT T1
LESION CHARACTERISATION
T2W AXIAL.
T1W AXIAL.
T2W CORONAL
T1W CORONAL
T2 FAT SUPP
LESION CHARACTERISATION
T2W AXIAL.
T1W AXIAL.
T2W SAG
FAT SUPP COR
LESION CHARACTERISATION
T2W AXIAL
T1W AXIAL
T2 SAG
T1 SAG
MRI Basics
SEQUENCES
SEQUENCE
T1W
FAT
WHITE
FLUID
BLACK
T2W
NOT SO
WHITE
WHITE
T2 FAT
SUPP
STIR
T2 FLU
SUPP
FLAIR
BLACK
WHITE
CSF/CLEAR FLUID BLACK
PATH FLUID- WHITE
I've got so much to learn!