د قاسم MRI pdf - قاسم تاج الدين

MRI

Magnetic resonance imaging

(MRI )

MRI

Magnetic resonance imaging (MRI)
is noninvasive method of mapping the internal

structure and certain aspects of function within
the body.

It uses nonionizing electromagnetic radiation and

appears to be without exposure-related hazard .

HISTORICAL PERSPECTIVE

Name of MRI previously is nuclear magnetic

resonance (NMR),first described by Bloch and
Purcell in 1946 .

NMR has been used extensively as a laboratory

method for studying the properties of matter at
the molecular level (NMR spectroscopy

MRI

In applications to medicine, it is now commonly

referred to as magnetic resonance (MR).

Application for human study between
1973-1977

MRI

MR describes the phenomenon whereby the
nuclei of certain atoms, when placed in a
magnetic field, absorb and emit energy of a
specific or resonant frequency.

Mean that the nuclei of certain elements align

with the magnetic force when placed in
astrong magnetic field .

…

At the field strengths currently used in medical
imaging ,hydrogen nuclei (protons ) in water
molecules and lipids are responsible for
producing anatomical imaging .

MRI

• The body is

largely composed of water

molecules

which each contain two

hydrogen

nuclei

protons

. When a person goes inside

the powerful

magnetic field

of the scanner,

the

magnetic moments

of these protons align

with the direction of the field.

BASIC PRINCIPLE OF MRI

• The hydrogen (1^H) atom inside body possess “spin”
• • In the absence of external magnetic field, the spin

directions of all

• atoms are random and cancel each other.
• • When placed in an external magnetic field, the spins

align with the

• external field.
• • By applying an rotating magnetic field in the

direction orthogonal to

• the static field, the spins can be pulled away from the

z-axis with an

• angle \alpha

MRI

• •

The rapidly rotating transverse magnetization

(M_xy)

• creates a radio frequency excitation within the

sample.

• • If we put a coil of wire outside the sample, the RF
• excitation will induce a voltage signal.
• • In MRI, we measure this voltage signal.
• • Voltage produced is (Faraday’s Law of Induction)

MRI

If radiofrequency of hydrogen is applied

,aproportion of the protons change alignment
,flipping through apreset angle , rotate in phase
with one another.fallowing this radiofrequency
pulse,the protons return (realign )to their original
postion .

MRI

Advantage :
Provide high resolution anatomic structure (as
with X-ray CT)
• Provide high contrast between different soft
tissues (X-ray CT cannot)
• No exposure to radiation and hence safe
• More complicated instrumentation •
Disadvantage :
Takes longer to acquire a scan than CT, more
susceptible to patient motion

MRI

The hydrogen (1

^H) atom inside body possess “spin”

• In the absence of external magnetic field, the spin
directions of all
atoms are random and cancel each other.
• When placed in an external magnetic field, the spins
align with the
external field.
• By applying an rotating magnetic field in the direction
orthogonal to
the static field, the spins can be pulled away from the z-
axis with an angle \alpha

As the protons realign(relax )they induce aradio

signal which ,although very weak,can be
detected and localize by coils placed around
the patient .

An image represent the distribution of the

hydrogen protons can be built up .

MRI

The strength of signal depends not only on

proton density but also on tow relaxation
times .T1 and T2

MRI

T1-depend on the time the protons takes to
return to the axis of magnetic fields.

T1 WEIGHTING

• • Short TR:

• – Maximizes T1 contrast due to different

degrees of saturation

• – If TR too long, tissues with different T1

all return equilibrium already

• • Short TE:

• – Minimizes T2 influence, maximizes

signal

SPIN DENSITY

• Signal at equilibrium proportional to

• • Long TR:

• – Minimizes effects of different degrees of

saturation (T1 contrast)

• – Maximizes signal (all return to

equilibrium)

• • Short TE:

• – Minimizes T2 contrast -Maximizes signal

T2 WEGHTING

• Long TR:

• – Minimizes influence of different T1

• • Long TE:

• – Maximizes T2 contrast
• – Relatively poor SNR

MRI

Mean transfer of energy from the
protons to the tissue .

MRI

T2- depend on the time the protons take to

dephase .

MRI

T1 of tissue is longer than T2 of
tissue

MRI

• T2 is called transverse relaxation time, which

is the time for M_xy

• to decrease by 1/e.
• • Also called spin-spin relaxation time
• • T2 is much smaller than T1

• – For tissue in body, T2: 25-250ms, T1: 250-2500 ms

MRI

-Body fluid T1 is long compared to soft while

those of fatty structures are short

-Fluids have long T2 values than of soft tissues
and fat.

MRI

AT1-weighted images is one in which the
contrast between tissues due mainly to their
T1 relaxation propereties ,while in a T2-
weighted image the contrast between tissues
due mainly to their T2 relaxation
propereties .

MRI

Most pathological processes show increase T1 and

T2 relaxation times and these processes
therefore appear lower in signal (blacker )on a T1
–weighted scan and higher in signal (whiter ) on a
T2 –weighted scan than the normal surrounding
tissue

MRI

the T1 and T2 weighting of an image can be

selected by approperiatly altering the timing and
sequences of radiofrequency pulses .

MRI

T1 weighted images used for anatomical details

(normal stracture )and T2 weighted images used
for pathological processes

MRI SCANNER

Atypical MRI scanner consist of large circular

magnet.Inside the magnet are the
radiofrequency transmitter and receiver coils as
well as gradient coils to allow spatial
localization of the MRI signal .