Mri basic principle and sequences

PRESENTER: DR PAWAN KUMAR

MRI-BASIC

PRINCIPLE/TECHNIQUE/READIN

G

Nuclear magnetic resonance

►Dr Isidor Rabi (Nobel in 1944), discovered NMR (Nuclear Magnetic Resonance) in the late 1930s, but considered it to be an artefact of his apparatus

►CJ Gorter, coined the term ‘Nuclear Magnetic Resonance’ in 1942.

►Bloch and Purcell (Nobel Prize in 1952 )if Certain nuclei are placed in magnetic field ,absorb energy in electromagnetic spectrum and re emit energy when regain their original position.

MRI HARDWARE

Permanent magnets

Resistive magnets: In a resistive magnet, an electrical current is passed through a loop of wire and generates a magnetic field.

Superconducting magnets: Superconducting magnets are the ones most widely used in MR machines at the present time. They also make use of electricity, but they have a special current carrying conductor.

This is cooled down to superconducting temperature

(about 4° K or -269° C). At this temperature, the current conducting material loses its resistance for electricity.

In MRI radio frequency coils are necessary to

send in the RF pulse to excite the protons, and to

receive the resulting signal. The same or different

coils can be used for transmission of the RF

pulse and receiving the signal.

Necessary Equipment

Magnet Gradient Coil RF Coil

Source: Joe Gati, photos

RF Coil

3T magnet

gradient coil

(inside)

MRI principle

MRI is based on the principle of nuclear magnetic

resonance (NMR)

Two basic principles of NMR:

1.Atoms with an odd number of protons or neutrons

have spin

2.A moving electric charge, be it positive or negative,

produces a magnetic field

Protons Protons possess a positive charge.

Like the earth they are constantly turning around

an axis and have their own magnetic field.

Protons

Normally protons are aligned in a random

fashion. This, however, changes when they are

exposed to a strong external magnetic field.

Then they are aligned in only two ways, either

parallel or antiparallel to the external magnetic

field

Precession

Protons perform a wobbling type of motion in a

strong magnetic field ,called precession.

Precession Frequency

To calculate the precession frequency.

Larmor equation:

B0 = is the strength of the external magnetic field,

which is given in Tesla (T)

is the angular frequency, and ‘y’is the so-

called gyromagnetic ratio.

EFFECT OF MAGNETIC FIELD

The z-axis runs in the direction of the magnetic

field lines,

The five protons, which "point" down cancel out

the magnetic effects of the same number of

protons, which "point"up

Radio Frequency (RF)

A short burst of some electromagnetic wave, which is

called a radio frequency (RF) pulse.

The purpose of this RF pulse is to disturb the protons,

which are peacefully precessing in alignment with the

external magnetic field.

Not every RF pulse disturbs the alignment of the protons.

For this, we need a special RF pulse, one that can

exchange energy with the protons.

Only when the RF pulse and the protons have the same

frequency, can protons can pick up some energy from the

radio wave, a phenomenon called Resonance.

Longitudinal magnetization the vectors along the z-axis point in the same

direction, and thus add up to a new magnetic sum

vector pointing up. As this magnetization is in

direction along/longitudinal to the external

magnetic field, it is also called longitudinal

magnetization

EFFECT OF RF PULSE ON

MAGNETIZATION

The radiofrequency pulse exchanges energy with

the protons (a), and some of them are lifted to a

higher level of energy, pointing downward in the

illustration (b).

In effect the magnetization along the z-axis

decreases, as the protons which point down

"neutralize" the same number of protons pointing

up.

Transversal Magnetization

When the protons randomly point left/right, back/forth

and so on, they also cancel their magnetic forces in

these directions

The RF pulse synchronize them - "in phase". They

now point in the same direction at the same time, and

thus their magnetic vectors add up in this direction.

This results in a magnetic vector pointing to the side

to which the precessing protons point, and this is in a

transverse direction.

This is why it is called transversal magnetization.

As the transversal magnetic vector moves around

with the precessing protons, it comes towards the

antenna, goes away from it, comes towards it

again and so on, also with the precession

frequency.

The resulting MR signal therefore also has the

precession.

Longitudinal and transversal

relaxation

The newly established transverse magnetization

starts to disappear (a process called transversal

relaxation), and the longitudinal magnetization

grows back to its original size (a process called

longitudinal relaxation).

Longitudinal/Spin lattice

relaxation

After the RF pulse is switched off, protons go

back from their higher to the lower state of

energy, i.e. point up again.

This energy is just handed over to their

surroundings, the so called lattice. And this is why

this process is not only called longitudinal

relaxation, but also spin-lattice-relaxation

T1-Curve

If one plots the longitudinal magnetization vs.

time after the RF pulse was switched off, one gets

a so-called T1-curve.

SPIN SPIN RELAXATION

After the RF pulse is switched off, protons lose

phase coherence, they get out of step thus

transversal magnetization decreases.

Plot showing transversal magnetization vs. time after

the RF pulse is switched off, one gets a curve as

illustrated,which is called a T2-curve.

Characteristics of T1 and T2

T1 is about 300 to 2000 msec, and T2 is about 30

to 150 msec.

It is difficult to pinpoint the end of the longitudinal

and transversal relaxation exactly.

T1 =63% of the original longitudinal

magnetization is reached.

T2 =37% of the original value,T1 is longer than

T2

T1 varies with the magnetic field strength; it is

longer in stronger magnetic fields.

90°pulse

If after the RF pulse, the number of protons on

the higher energy level equals the number of

protons on the lower energy level, longitudinal

magnetization disappears, and there is only

transversal magnetization due to phase

coherence.

The magnetic vector seems to have been "tilted“

90° to the side. The corresponding RF pulse is

thus also called a 90° pulse

TE vs TR

• TE: the time between the 90° pulse and the

echo.

• TR: the time between two pulse sequences, i.e.

from one 90° pulse to the next.

HOW TR AFFECTS THE SIGNAL

INTENSITY OF TISSUE

A and B are two tissues with different relaxation

times.

Frame 0 shows the situation before, frame 1

immediately after a 90° pulse.

When we wait for a long time (TRlong) the

longitudinal magnetization of both tissues will

have totally recovered (frame 5).

A second 90° pulse after this time results in the

same amount of transversal magnetization

(frame 6) for both tissues, as was observed

after the first RF pulse (frame 1).

When we do not wait as long , but send in the

second RF pulse after a shorter time (TRShort),

longitudinal magnetization of tissue B, which has

the longer T1, has not recovered as much as that

of tissue A with the shorter T1.

The transversal magnetization of the two tissues

after the second RF pulse will then be different

(frame 5). Thus, by changing the time between

successive RF pulses, we can influence and

modify magnetization and the signal intensity of

tissues.

TR AND SIGNAL INTENSITY

Brain has a shorter longitudinal relaxation time

than CSF.

With a short TR the signal intensities of brain and

CSF differ more than after a long TR.

T2*/T2effect/spin echo sequence

After the RF pulse is switched off, the protons

dephase (a-c).

The 180°pulse causes them to precess in the

opposite direction and so they rephase again (d-f).

T2* curve The 180° pulse refocusses the dephasing protons which results in a

stronger signal, the spin echo after the time TE.

The protons then dephase again and can be refocussed another time

by a 180° pulse and so on. Thus it is possible to obtain more than

one signal, more than one spin echo.

The spin echoes, however, differ in intensity due to so-called T2-

effects.

A curve connecting the spin echo intensities is the T2 curve. If we did

not use the 180° pulse, the signal intensity would decay much faster.

A curve describing the signal intensity in that case is the T*2 (T2

star) curve, The type of pulse

sequence,

that we used in our

experiment, is called a

spin echo sequence,

consisting of a 90°

pulse and a 180°

pulse

T2-curves for two tissues with different

transversal relaxation times; tissue A has a

shorter T2 than tissue B, thus loses transversal

magnetization faster.

With a short TE (TEshort) the difference in signal

intensity is less pronounced than after a longer

TE (TElong).

Short vs Long TE/TR

A TR of less than 500 msec is considered to be

short, a TR of more than 1500 msec to be long

A short TE is one that is as short as possible, a

long TE also is about 3 times as long.

A TE of less than 30 msec is considered to be

short, a TE greater than 80 msec to be long.

spin echo pulse

sequence

1. (90° - TE/2 -180° - TE/2 -> record signal at TE)

after TR (time from the beginning of one 90°

pulse to the next 90° pulse) follows another pulse

cycle and signal b measurement:

2. (90° - TE/2 -180° - TE/2 -> record signal at TE)

EFFECT OF TR/TE ON IMAGE By combining T1- and T2-curves signal intensity of certain

tissues can be determined for a pulse sequence using TR and

TE as illustrated.

With a long TR, differences in T1, in longitudinal magnetization

time are not very important any more, as all tissues have

regained their full longitudinal magnetization.

When we only wait a very short TE then differences in signal

intensity due to differences in T2 have not yet had time to

become pronounced.

The resulting picture is thus neither T1- nor T2-weighted, but

mostly determined by the proton density of the tissues (for this,

ideally TE should be zero).

T2-weighted picture

When we wait a long TR and a long TE, differences

in T2 have had time enough to become pronounced,

the resulting picture is T2-weighted

T1-weighted picture

When we wait a shorter time TR, differences in T1

influence tissue contrast to a larger extent, the picture is

T1-weighted, especially when we also wait a short TE

(when signal differences due to differing T2s have not

had time to become pronounced)

T2-curves of different tissues can intersect. The signal intensity of

the tissues is reversed choosing a TE beyond the crossing point

(TEC): before this crossing point (e.g. at TE1) tissue A has a higher

signal intensity than tissue B.

This means that image contrast is still determined by differences in

T1: the tissue A with the shorter T1 has the stronger signal intensity.

At TEC both tissues have the same signal intensity, and thus cannot

be differentiated.

After the crossing point (e.g. at TE2) the relative signal intensities are

reversed, and tissue B has the stronger signal

T1W/T2W IMAGES

Useful for: Evaluating

anatomic detail

CSF: Dark

White Matter: White

Gray Matter: Gray

Vessels: Dark

Useful for: Looking at areas of

edema & pathology

CSF: Bright

White Matter: Gray

Gray Matter: Lighter than white

matter

Vessels: Dark

T1 Recovery

Short TR T1 contrast

(T1 Weighted)

TR 300-600 ms

TE 10-30 ms

Bright on T1 Fat, subacute hemorrhage, melanin, protein rich fluid.

Slowly flowing blood

Paramagnetic substances(gadolinium,copper,manganese)

9

Dark on T1

Edema, tumor, infection, inflammation,

hemorrhage(hyperacute, chronic)

Low proton density, calcification

Flow void

T2 Decay

Long TE T2 contrast

(T2 Weighted)

TR 2000 ms

TE 70 ms

Bright on T2

Edema, tumor, Infection, inflammation, subdural

collection

Met hemoglobin in late sub acute hemorrhage

Dark on T2

Low proton density,calcification,fibrous tissue

Paramagnetic substances(deoxy

hemoglobin,methemoglobin(intracellular),ferritin,h

emosiderin,melanin.

Protein rich fluid

Flow void

Which scan best defines the

abnormality

T1 W Images:

Subacute Hemorrhage

Fat-containing structures

Anatomical Details

T2 W Images:

Edema

Demyelination

Infarction

Chronic Hemorrhage

FLAIR Images:

Edema,

Demyelination

Infarction esp. in Periventricular location

Infarct

Acute : T1W –Isointense hypo intense

T2W-Hyper intense

Sub acute: T1W-Low signal,increasedsignal in

peripheral region..hemorrhage(metHb)

T2W- High signal

Chronic:T1W-low signal

T2W-High siignal

Hemorrhage

FLOW VOID-HOW

Flow effects are responsible for the black

appearance of flowing blood, the signal void in blood

vessels

Paramagnetic substances

like Gadolinium shorten

the T1 and the T2 of their

surroundings.

The respective T1- and

T2-curves are shifted

towards the left.

In effect, that means that

for a certain TR there is

more, for a certain TE

there is less signal

EFFECT OF CONTRAST MATERIAL

When tissue A and B has less

contrast:

T1-curves for tissue A and B are

very close to each other,

resulting in only a small

difference in signal intensity

between the tissues at TR.

NOW- the T1-curve of tissue A

is shifted to the left, as contrast

agent entered tissue A but not

tissue B. At the same time TR

there now is a much greater

difference in signal intensity, i.e.

tissue contrast

The inversion recovery

sequence

The inversion recovery sequence uses a 180° pulse which

inverts the longitudinal magnetization, followed by a 90° pulse

after the time TI.

The 90° pulse "tilts“ the magnetization into the transversal (x-y-)

plane, so it can be measured/received.

The tissue in the bottom row goes back to its original longitudinal

magnetization faster, thus has the shorter T1.

For the time TI, which is illustrated, this results in less transversal

magnetization after the 90° pulse

Short TI inversion-recovery (STIR)

sequence

Longer TE used..both long T1 and T2

…bright

Substances having

both LONG T1 and

T2 will be bright.

T1 and T2 of most

pathologic lesion

are prolonged.

And substances

with short T1 will be

suppressed eg.

hemorrhage, gd-

enhancement.

Fluid-attenuated inversion recovery

(FLAIR)

First described in 1992 and has become one of

the corner stones of brain MR imaging protocols

An IR sequence with a long TR and TE and an

inversion time (TI) that is tailored to null the signal

from CSF

Water bound to complex molecule with in plaque

has relatively shorter T1 than free water with in

ventricle.

Long inversion time effectively suppress free

water eg./csf.

Lesion that contain complex, partially bound

water (less mobile)…shorter T1 than free water

appears bright.

Effective in high lightening lesion eg

demyelination, stroke, Ischemic gliosis and

tumor.

Normal partially myelinated white matter tract and

Useful in diffrentiating acute infarct from cystic encephalomalacia

Useful in SAH …removes CSF signal

GRE

GRE

In a GRE sequence, an RF pulse is applied that

partly flips the NMV into the transverse plane

(variable flip angle).

Gradients, as opposed to RF pulses, are used to

dephase (negative gradient) and rephase

(positive gradients) transverse magnetization.

GRE Sequences contd:

This feature of GRE sequences is exploited- in

detection of hemorrhage, as the iron in Hb becomes

magnetized locally (produces its own local magnetic

field) and thus dephases the spinning nuclei.

The technique is particularly helpful for diagnosing

hemorrhagic contusions such as those in the brain .

GREFLAIR

Hemorrhage in right parietal lobe

DWI & ADC

Diffusion-weighted MRI

DWI images is obtained by applying pairs of opposing and balanced magnetic field gradients (but of differing durations and amplitudes) around a spin-echo refocusing pulse of a T2 weighted sequence.

Stationary water molecules are unaffected by the paired gradients, and thus retain their signal.

Non stationary water molecules acquire phase information from the first gradient, but are not re phased by the second gradient, leading to an overall loss of the MR signal

The normal motion of water molecules within living tissues is random (brownian motion).

In acute stroke, there is an alteration of homeostasis

Acute stroke causes excess intracellular water accumulation, or cytotoxic edema, with an overall decreased rate of water molecular diffusion within the affected tissue.

Tissues with a higher rate of diffusion undergo a greater loss of signal in a given period of time than do tissues with a lower diffusion rate.

Therefore, areas of cytotoxic edema, in which the motion of water molecules is restricted, appear brighter on diffusion-weighted images because of lesser signal losses

Apparent Diffusion Coefficient

Calculated by acquiring two or more images with

a different gradient duration and amplitude .

To differentiate T2 shine through effects or

artifacts from real ischemic lesions.

ADC

useful for estimating the lesion age and

distinguishing acute from subacute DWI lesions.

Acute ischemic lesions can be divided into

1- hyperacute lesions (low ADC and DWI-

positive)

2-subacute lesions (normalized ADC).

3-Chronic lesions can be differentiated from

acute lesions by normalization of ADC and DWI.

Nonischemic causes for decreased

ADC Abscess

Lymphoma and other tumors

Multiple sclerosis

Seizures

Metabolic (Canavans )

Evaluation of acute stroke on DWI

The DWI and ADC maps show changes in

ischemic brain within minutes to few hours

The signal intensity of acute stroke on DW

images increase during the first week after

symptom onset and decrease thereafter, but

signal remains hyper intense for a long period

(up to 72 days in the study by Lausberg et al)

The ADC values decline rapidly after the onset

of ischemia and subsequently increase from

dark to bright 7-10 days later .

This property may be used to differentiate the

lesion older than 10 days from more acute ones .

Chronic infarcts are characterized by elevated

diffusion and appear hypo, iso or hyper intense

on DW images and hyperintense on ADC maps

DW MR imaging characteristics of Various Disease Entities

MR Signal Intensity

Disease DW Image ADC Image ADC Cause

Acute Stroke High Low Restricted Cytotoxic edema

Chronic Strokes Variable High Elevated Gliosis

Hypertensive

encephalopathy

Variable High Elevated Vasogenic edema

Arachnoid cyst Low High Elevated Free water

Epidermoid mass High Low Restricted Cellular tumor

Herpes encephalitis High Low Restricted Cytotoxic edema

CJD High Low Restricted Cytotoxic edema

MS acute lesions Variable High Elevated Vasogenic edema

Chronic lesions Variable High Elevated Gliosis

Slice selection and thickness

There are two ways to determine slice

thickness.

(a). By using certain bandwidth of RF pulse

eg. If frequencies between 64 and 65 mHz,

protons in slice 1 will be influenced by the

RF pulse.

When the RF pulse only contains

frequencies between 64 mHz and 64.5

mHz, thus has a smaller bandwidth, slice 2,

which is half as thick as slice 1 will be

imaged.

When there is more difference in magnetic

field strength between the level of the feet

and the head, i.e. The magnetic gradient is

steeper, the resulting slice will be thinner,

even though the RF pulse bandwidth is the

same.

HOW TO READ

THE IMAGING PLANES

- Axial plane: Transverse images represent

"slices" of the body

- Sagittal plane: Images taken

perpendicular to the axial plane which

separate the left and right sides (lateral

view)

- Coronal plane: Images taken

perpendicular to the sagittal plane which

separate the front from the back. (frontal

view)

How do you describe abnormalities

on MR?

Hyperintense (more intense): If an abnormality is bright (white) on MR,

we describe it as hyperintense.

Isointense (the same intensity): If an abnormality is the same intensity

to a reference structure, we describe it as isointense.

Hypointense (less intense): If an abnormality is dark on MR .

IT IS CT OR MRI?CT: Computed Tomography MRI: Magnetic Resonance Imaging

Imaging Plane: in the axial

plane. The axial data set can then be

used to reconstruct images in other

planes, sagittal and coronal are the

most common.

Windows: "brain window" AND ‘bone

window ‘

White and Black:air within the

sinuses is black, the brain

parenchyma has a gray appearance

and the skull is bright white

Imaging Plane: in any plane, not just

axial

Sequences: different type of image

is referred to a sequence.

White and Black:same structure

may be bright or dark depending on

the type of sequence; CSF for

example is bright on T2, but dark on

T1. The tissue and imaging

characteristics are a lot more

complicated than CT.

Types of MR Images

Useful for: Evaluating anatomic

detail

CSF: Dark

White Matter: White

Gray Matter: Gray

Vessels: Dark

Useful for: Evaluating for BBB breakdown

in the setting of tumor, infection, MS etc.

CSF: Dark

White Matter: White

Gray Matter: Gray

Vessels: Brigh

Useful for: Looking at areas of edema &

pathology

CSF: Bright

White Matter: Gray

Gray Matter: Lighter than white matter

Vessels: Dark

Useful for: Evaluating areas of edema with

CSF subtraction. Edema stands out

because is CSF dark

CSF: Dark

White Matter: Gray

Gray Matter: Lighter than white matter

Vessels: Dark

Useful for: stroke imaging, abscess,

cellular tumors

CSF: Dark

White Matter: Gray

Gray Matter: Lighter than white matter

Fuzzier image than FLAIR

T2* (T2-star, or SWI)

Form of T2-weighted image which is

susceptible to iron or calcium

Blood, bone, calcium appear dark

Area of blood often appears much larger

than reality(“blooming”)

Useful for: Identification of early

hemorrhage

Look for: DARK only

Recognition:

o Like T2 except

o Cranium, scalp are dark or absent

o Dark areas near frontal and temporal

bones

o Hemorrhage is darker than brain

Apparent Diffusion Coefficient (ADC

Map)

Contains actual data relevant to diffusion

image

Areas of restricted diffusion are dark

Useful for:

o Excluding T2-shine through

o Real restricted diffusion is bright on DWI,

dark

on ADC

Look for: DARK only

Recognition

o Images marked ADC

o Grainy dark images

MR ARTIFACTSArtifacts in MR images refer to pixels that do not faithfully represent the

anatomy being studied.

1 Motion Artifacts- Motion artifacts occur as a result of movement of

tissue during the data acquisition period.

2.Sequence/Protocol-Related Artifacts- results from the specific

measurement process

used to acquire the image.

subtypes

2a. Aliasing

2b. Chemical Shift Artifacts

2c. Phase Cancellation Artifact

2d. Truncation Artifacts

2e. Coherence Artifacts

2f. Magnetic-Susceptibility Difference Artifacts

3. External Artifacts. External artifacts are generated from sources

other than patient tissue.

3a. Magnetic Field Distortions

3b. Measurement Hardware

3c. Noise

refrences

1.MRI: Basic Principles and Applications, Third Edition, by M. A. Brown

and R. C. Semelka.ISBN 0-471-43310-1 © 2003 John Wiley & Sons,

Inc.

2.Schering: MRI made easy,by Hans H. Schild Nationales Druckhaus

Berlin.ISBN 3-921817-41-

3. Bradley’s text book of neurology-6th edition

4.Robert A Ziemarman’s Neuroimaging clinical and physical principle.

5. https://sites.google.com/a/wisc.edu/neuroradiology/image-

acquisition/the-basics

THANK YOU