Mri basic principles

Principles Of MRI – Neuro Radiology
By: Dr.Siddharth Sahu
DNB Neurosurgery Academic Registrar
MBBS, MS General Surgery

• Four basic steps are involved in getting an MR image-
1. Placing the patient in the magnet
2. Sending radiofrequency (RF) pulse by coil
3. Receiving signals from the patient again by coil
4. Signals are sent to computers for complex processing to get image.
• Imaging is based on proton imaging (positively charged particle in the
nucleus of every atom)

How does this proton help in MR imaging?
• Protons are positively charged - rotatory movement “spin” -
generates current - has a small magnetic field around it.
• When external magnetic field is applied, i.e. patient is placed in the
magnet, these randomly moving protons align and spin in the
direction of external magnetic field.
• Some of them align parallel and some anti-parallel to the external
magnetic field.
• When protons align, rotate around themselves (called spin) but also
their axis of rotation moves such that it forms a ‘cone’.
• This movement of axis of rotation of proton is called as ”precession”

• No of precessions of proton/sec is
precession frequency in Hertz.
• Precession frequency is directly
proportional to strength of external
magnetic field.
• Stronger the external magnetic field,
higher is precession frequency.

MAGNETIZATION
• For the orientation in space consider X, Y, and Z axes system.
• External magnetic field is directed along Z-axis. Conventionally,
Z-axis is the long axis of the patient as well as bore of the
magnet.
• Protons align parallel and anti-parallel to external magnetic
field i.e. along positive and negative sides of Z-axis.
• Forces of protons on negative and positive side cancel each
other.
• There are always more protons spinning on the positive side or
parallel to Z-axis than negative side.
• So, after canceling each other few protons remain on positive
side, which are not cancelled.
• Forces of these protons add up together to form a magnetic
vector along Z-axis.
• This is LONGITUDINAL MAGNETIZATION.

Transverse Magnetization
• Longitudinal magnetization vector forms
along Z-axis and in the long axis of the patient.
• At this stage radiofrequency pulse is sent.
• Precessing protons pick up some energy - go
to higher energy level and start precessing
anti-parallel - reduction in the magnitude of
longitudinal magnetization.
• Forces of protons now add up to form a new
magnetic vector in transverse (X-Y) plane. This
is called as “TRANSVERSE MAGNETIZATION” .
• When RF pulse and protons have same
frequency protons can pick up some energy
from RF pulse. This phenomenon is called as
“resonance”— the R of MRI.

MR Signal
• Transverse magnetization -
produces electric current.
• The coils receive this current
as MR signal .
• Strength of the signal
depends on magnitude of the
transverse magnetization.
• MR signals are Fourior
Transformed into MR image by
computers.

• Relaxation - recovery of protons back towards equilibrium after been
disturbed by RF excitation.
• Contrast in an MR image is determined by
- Relaxation times of protons
- tissue-proton densities.
• RF pulse causes LM and TM to form.
• RF pulse is switched off – TM Transverse Relaxation
LM Longitudinal Relaxation
• Net Magnetization Vector (NMV) is the sum of two vectors in
different planes (like LM and TM vector) represented as single vector
in a plane midway between the two.

LONGITUDINAL RELAXATION
• RF pulse is switched off – protons
loose their energy - is given to
surrounding or lattice, hence this is
also called as ‘spin-lattice’ relaxation.
• Magnitude of LM increases.
• When curve is plotted of LM
magnitude against time, it is called as
T1 curve
• T1 is the time taken for LM to
recover after RF pulse is switched off,
to original value

TRANSVERSE RELAXATION
• Protons which were precessing in phase
because of RF pulse, start losing phase after
RF pulse is switched off.
• This going out of phase of protons results
into gradual decrease in the magnitude of
TM and is termed as Transverse relaxation.
• The time taken by TM to reduce to its
original value is transversal relaxation time
or T2.

T1
• T1 is the time taken by LM to recover after RF pulse is switched off, to original
value.
• This is not exact time, but it is a ‘constant’.
• T1 is the time when LM reaches back to 63% of its original value.
• depends upon tissue composition, structure, surroundings.
• If lattice (surrounding matter) has magnetic field, which fluctuates at Larmor
frequency, transfer of thermal energy to the lattice is easy and fast - shorter T1.
• In water - molecules move too rapidly - increases time taken to transfer energy -
long T1.
• Fatty acids have frequency near Larmor frequency so there is fast energy
transfer from protons to lattice. Hence fat has short T1.
• T1 is longer in stronger magnetic field

T2
• T2 is time taken by TM to disappear.
• Like T1 it is a constant and not exact time.
• It is the time taken by TM to reduce to 37% of its original value.
• Depends on inhomogeneity of external magnetic field and inhomogeneity of local
magnetic field within tissues.
• As water molecules move very fast, their magnetic fields fluctuate fast. These
fluctuating magnetic fields cancel each other.
• So there are no big differences in magnetic field strength inside a tissue. Because
of lack of much inhomogeneity protons stay in step for a long time resulting into
long T2 for water.
• If liquid is impure and has larger molecules - move at slower rate. This maintains
inhomogeneity of magnetic field. As a result protons go out of phase very fast.
Hence impure liquids, larger molecules have short T2, e.g. Fat has shorter T2.

TR and TE
• TR (Time to Repeat) is the time interval between start of one RF pulse
and start of next RF pulse.
• TE (Time to Echo) is the time interval between start of RF pulse and
reception of the echo (signal).
• Short TR and short TE gives T1-weighted images.
• Long TR and long TE gives T2-weighted images.
• Long TR and short TE gives proton density images

• Long TR is >1500 ms (millisecond)
• short TR is <500ms (no fixed range)
• Short TE is around 15-20 ms
• long is above 70-75 ms.
• TR is always higher than TE

TI
• Time of Inversion.
• Time between inverting 180 degree pulse and 90 degree pulse in
Inversion recovery (IR) sequence.
• TI determines contrast IR sequence.

T1-WEIGHTED IMAGE
• LM is one of the determinants of strength of MR signal.
• Stronger the LM more will be the magnitude of TM after 90 degree
pulse - results into stronger signal.
• If T1 is short, there is early or maximum regain of LM after RF pulse is
switched off.
• So, if next RF pulse is sent, TM will be stronger and resultant signal
will also be stronger.
• Hence material with short T1 have bright signal on T1 weighted
images.

How do we make images
T1-weighted?
• This is done by keeping TR short.
• If TR is long the tissues with long T1
will also regain maximum LM giving
stronger TM with next RF pulse and
stronger signal.
• This will result in no difference
between tissues with different T1.
• So in T1-W images differences in
signal intensity in tissues is due to
their difference in T1 (Figs 2.5A and
B).

T2-WEIGHTED IMAGE
• Tissues with long T2 - bright on T2-W images.
• Longer the T2 of any tissue, larger TM will remain for more time.
• This will lead to stronger signal, because stronger TM gives stronger
signal.

How do we make images T2-
weighted?
• With short TE - difference between tissues
will be less pronounced.
• With long TE - signal difference between
tissues with long (A) and short (B) T2 will be
more.
• So, image with long TE is T2- weighted since
signal difference between tissues (contrast)
is determined by T2 of tissues.
• However, shorter the TE, stronger is signal.
So it has to be trade off between signal
intensity and T2 weighting.

PROTON DENSITY (PD) IMAGE
• Contrast in the image is determined by density of protons in the
tissue.
• T1 effect is reduced by keeping long TR
• T2 effect is reduced by keeping TE short.
• Long TR and short TE gives PD-weighted image

Basic four components make MR system.
• 1. The magnet to produce external magnetic field
• 2. Gradients to localise the signal
• 3. Transmitter and receiver coils for RF pulses
• 4. Computer system.

MAGNETISM
• Magnetism - fundamental property of matter.
• All substances possess some form of magnetism.
• Degree of magnetism depends upon the magnetic susceptibility of the
atom, which make the substance.
• Magnetic susceptibility is the ability of the substance to get affected by
external magnetic field (related to electron configuration of the atom.)
• Depending on the magnetic susceptibility, i.e. substance’s response to
magnetic field, substances can be
- paramagnetic,
- diamagnetic
- ferromagnetic.

Paramagnetism
• unpaired electrons within the atom
• results into a small magnetic field around them called magnetic
moment.
• When external magnetic field applied, these moment add together
and align in the direction of external magnetic field.
• Affect external magnetic field in positive way by attraction towards
the field resulting in a local increase in magnetic field.
• Examples of paramagnetic substances are gadolinium, oxygen,
melanin.

Diamagnetism
• Diamagnetic substances react in opposite way when external
magnetic field is applied.
• They are repelled by the magnetic field.
• Negative magnetic susceptibility

Ferromagnetism
• In a magnetic field - get strongly attracted.
• Retain their magnetism even when external magnetic field is
removed.
• Used to make permanent magnet.
• Examples of ferromagnetic substances are Iron, Cobalt and Nickel.

MAGNETIC FIELD STRENGTH
• Magnetic field strength is expressed by notation ‘B’, the primary field
as B0 and the secondary field as B1. The units of magnetic field
strength are Gauss and Tesla. Tesla was Father of Alternating Current
and Gauss was German mathematician.
• 1Tesla = 10 kG = 10,000 Gauss
• Gauss is a measure of low magnetic field strength. Earth’s magnetic
field strength is approximately 0.6 G.

• Clinical purpose have strength ranging from 0.2 to 3 Tesla.
• higher than 3T are used for research purposes.
• As strength increases resolution increases.
• Advanced MR applications like Spectroscopy, functional MRI, cardiac
MR are possible only on higher field strengths like 1.5 T.

MAGNETS
• Three types of magnets are in use for clinical MRI machines.
• 1. Permanent magnet
• 2. Electromagnet
• 3. Superconducting magnet

Permanent Magnet
• Ferromagnetic substances.
• Usually MR magnets made up of alnico (alloy of aluminium, nickel &
cobalt.
• Permanent magnets - do not require power supply and are of low cost.
• Magnetic field - directed vertically
• Open MRI is possible with permanent magnet - in claustrophobic
patients.
• Magnetic field strength achievable with permanent magnet is low (0.2
to 0.5 Tesla)

Electromagnets
• These are based on principle of electromagnetism.
• Law of electromagnetism
- moving electric charge induces magnetic field around it.
- If a current is passed through a wire, a magnetic field is created around that wire.
The strength of the resultant magnetic field is proportional to the amount of
current moving through the wire.
- When a wire is looped like a spring (coil) and current is passed through it, the
magnetic field generated is directed along the long axis of the coil.
• All wires at normal temperature tend to resist the passage of current. As the
resistence increases, current decreases with resultant reduction in field strength.
• To get a homogenous field current must be steady and stable. The heat generated
during this process is removed by running cooled water through tubes passing over
the ends of the coil.

Superconducting Magnets
• Some metals like mercury or Niobium-Titanium alloy - lose their
electric resistance at very low temperature and become
superconductors.
• As current increases magnetic field strength increases.
• Higher field strength is achieved by completely eliminating resistance.
• There is no power loss and continuous power supply is not required
to maintain magnetic field.

Structure of Superconducting Magnet (Fig. 3.2)
Superconducting wires
These are made up of Nb/Ti alloy. This alloy becomes superconducting at
10K (kelvin) and produces magnetic field when current is passed through
it. A wire containing filaments of Nb/Ti alloy embedded in a copper matrix
is wound tightly and precisely on an insulated aluminium bore tube and
fixed in place with a viscous, high thermal conductivity epoxy binder.
There are thousands of turns of the wire, which may be 30 Kilometer
long. Since it is not possible to wound, the coil with a single continuos
strand, coil has several interconnecting joints.
Helium
The coil of superconducting material is cooled to 4K (-269 degree celcium)
by liquid helium, which surrounds coil all around. Because of smaller heat
leaks into the system, cryogens like helium steadily boil off. This boil off is
reduced by much cheap liquid nitrogen. However, helium should be
replenished on regular basis, usually every six months.
Liquid Nitrogen and Radiation Shield
The can of liquid helium is surrounded by cooled liquid nitrogen and
radiation shields. This prevents any heat exchange between helium and
the surrounding. Nitrogen boils at 80 degree K and much cheaper than
helium. The liquid nitrogen and radiation shields reduce evapouration of
liquid helium to 0.3 liter per hour.

Starting the Magnet
• Superconducting coil is cooled to -269 degree Celsius by helium and
liquid nitrogen.
• Magnet is energised by delivering current from external power source to
superconducting wire (coil). This process is called ramping.
• Once desirable level of current is achieved, power supply is cut off.
• The current continues to circulate through the coil.
• The current and the magnetic field produced by it remains constant.

Quench
• Quench is discharge or loss of magnetic field of superconducting magnet.
• Is because of increased resistance in the superconducting coil, which results
in heat formation - causes cryogens to evaporate.
• Vicious cycle - results in increased temperature, increased resistance,
evaporation of all cryogens and complete loss of magnetic field.
• All MR systems should have vent to pass helium to outside environment in
case quench occurs. Helium released inside the scan room can replace the
oxygen completely and can cause asphyxia. It also produces increased
pressure in the scan room, which may prevent opening of the door. Every
scan room should have oxygen monitor that will alarm if oxygen level falls
below critical level.
• To start the magnet after quench, cryogens are filled and wires are cooled.
Then ramping is done till desirable level of magnetic field is achieved.

Magnetic Field Homogeneity
- Magnetic field should be uniform
- The process of making the magnetic field homogenous is called as
“shimming”.
- This process is necessary because of the difficulty of winding a perfect
coil.

RADIOFREQUENCY COILS
• A loop of wire is coil.
• Radiofrequency (RF) coils - transmit RF pulse and to receive the
signals from the patient.
• Can be transmitters, receivers or both transceiver.
• Energy is transmitted in the form of short intense bursts of
radiofrequencies known as radiofrequency pulses.

Various types of RF coils are:
1. Body coil
2. Head coil
3. Surface or local coil
4. Phased array coil
5. Solenoid coil
6. Helmholtz coil

• Head and body coils
- are volume coils and are transceivers.
- They cover larger area and give uniform SNR
- SNR is lower than other types of coils.
- also act as transmitters for surface or local coils.
• Surface coils
- improve SNR significantly - used to image structures near the surface of the patient.
- Area covered is sensitive volume covered by the coil.
- Signal drops - distance of the structure increased from the coil.
- designed as per requirement for the particular parts
- All these coils give images with high SNR and high resolution.
• Phased Array coils
- combine advantages of surface coils (increased SNR and resolution) and volume coils (increased
coverage).
- consist of multiple small coils
- signal input of each coil is separately received and processed and then combined to form single
larger FOV.

• K-space - an imaginary space which represents raw data matrix.
• After acquisition all signals are stored in k-space.
• This raw data from k-space is then used to reconstruct image by Fourier Transformation.
• Signal is filled in k-space as horizontal line - The number of lines of k-space that are filled, is determined by the
number of different phase encoding steps.
• If 128 different phase encoding steps are selected then 128 lines of k-space are filled to complete the scan.

Sequences
1. FLASH
2. FLAIR
3. MEDIUM TI INVERSION RECOVERY SEQUENCE
4. CISS - SS Constructive Interference at Steady State.
5. MPRAGE - Magnetization Prepared Rapid Acquisition Gradient Echo
6. STIR - Short TI Inversion Recovery

1. FLASH
• Fast Low Angle Shot.
• Type: Gradient Echo
Uses:
1. Brain - high resolution T1-weighted 3D acquisition can be done pre and
post contrast.
2. T2-weighted FLASH - show acute bleed as dark signal
3. Flow and angiographic studies can be performed with FLASH.
4. In functional brain mapping by BOLD (blood oxygen level dependent)
imaging.

2. FLAIR - Fluid Attenuated Inversion Recovery
• Type: Inversion Recovery.
• Long TI (1500-2500 ms). CSF is effectively suppressed.
• Heavily T2-weighted images can be obtained without problems from CSF
partial volume effects and artifacts.

Perilesional Edema Brain infarctions are well seen –
Multiple chronic infarcts in left
periventricular region
Bright lesions of multiple sclerosis
better seen on FLAIR - Multiple
plaques are seen running
perpendicular to the callosal margin
(arrow) called as ‘Dowson’s fingers’.
Mesial temporal sclerosis
is better appreciated on
FLAIR Fast FLAIR shows subarachnoid hemorrhage

3. MEDIUM TI INVERSION RECOVERY
SEQUENCE
• Type: Medium TI (200-800 ms) IR and Multi SE Combined
• Use:
- Very good gray-white matter differentiation.
- shows cortical dysplasias well.
- forms a part of temporal lobe epilepsy protocol

4. CISS - SS Constructive Interference at Steady
State.
• Type: Gradient echo. T2-weighted. 3D
• CISS combines two true FISP images acquired
separately with some modifications.
• Uses:
- 3D acquisition of posterior cranial fossa gives
high resolution images showing cranial nerves
dark against background of bright CSF.
- routinely performed for suspected internal
auditory canal and cerebello-pontine angle
cistern pathologies.
- CISS can also be used to visualise spinal nerve
roots and optic nerve.

5. MPRAGE - Magnetization Prepared Rapid
Acquisition Gradient Echo
• Type: combination of medium TI inversion
recovery and gradient echo
• Uses:
- Thin slice 3D of the brain can be obtained.
- Shows good gray-white contrast and can be
used instead of routine T2-weighted
sequence.
- The sequence can also be used in post
contrast imaging .

6. STIR - Short TI Inversion Recovery
• Type: Short inversion time (TI)—80-150 ms
both T1 and T2 weighting to images.
• Uses:
- Pathology stands out - very easy to pick up
lesions.
1. Shows marrow edema very well, useful
detecting multiple lesions in bones, bone
metastases screening.
2. Orbital imaging specially for optic nerves.

SIGNAL INTENSITY
Depends on density of
• protons (hydrogen ions) in that structure, longitudinal
• relaxation time (T1), transverse relaxation time (T2) and
• flow and diffusion effects.
• Intense signals are received from tissues with short T1 and long T2
and high proton concentration.
• Lowest signal intensity is seen in tissues with long T1, short T2 and
low proton concentration.

• Water has long T1 and long T2
- T1-weighted - dark, T2-weighted - bright.
• Fat has short T1 and short T2 and appears bright on T1-weighted and less
bright on T2- weighted images.
- Inspite short T2, fat does not turn dark on T2-weighted images because of
its high proton content.
• Air - always dark/black all sequences - very low hydrogen proton
concentration.
• Cortical bone - dark on both T1 and T2-weighted images - very low mobile
protons.
• Medullary bone - depends on the degree of fat replacement.
• Circulating blood in the vessels will be seen as
- flow void (dark) in all spin echo sequences
- bright on gradient echo sequences.

• Calcifications - dark on both T1 and T2- weighted images with some
exceptions.
• Lesions having high content of protenaceous material,
methemoglobin (subacute hemorrhage) and cholesterol debries -
bright on T1-weighted images.
• White matter (WM)
- bright on T1-weighted images as compared to gray matter (GM)
because of myelin (lipid) content of WM.
- T2-weighted images GM, because of its more water content, has high
signal intensity than WM.

• Posterior pituitary
- bright signal on T1-weighted images because of neurosecretary
granules - related to the functional status of the
hypothalamoneurohypophyseal axis.
- In adult clivus should be seen as homogeneous high signal intensity
on T1-w images because of its fatty marrow content.

SEQUENCE SELECTION
1. T2-w axial
2. Diffusion
3. T1-w sagittal
4. gradient Hemo
5. Flair- axial
6. T2-w coronal
• T1 - to see anatomy and
• T2 - pathologies.
• Pathologies have increased T1 and T2 relaxation times with edema hence
appear bright on T2-w images.

STROKE IMAGING
• Start with diffusion - show acute infarct.
• Gradient- hemo sequence - shows acute bleed
• Fast FLAIRE - shows subarachnoid haemorrhage, - done next.
• MR angiography can be done - status of the vessels.
• If infarcts - peripheral and hemorrhagic - phase contrast MR
venograms to rule out venous sinus thrombosis (Figs 9.2A to C).
• In case of posterior circulation stroke T1-w fat saturated axial sections
of the neck are done to see for vertebral pathologies like dissection or
thrombosis (Figs 9.3A to C).

A. Diffusion weighted axial image of the brain shows haemorrhagic infarct (arrow)
in left temporal region
B. T1-w axial image of the brain shows hyperintense left transverse sinus (arrows) suggestive of thrombosis
C. TOF venogram coronal view shows absent Transverse (T), sigmoid sinus (S) and Jugular vein (J)

A. DWI shows in acute infarct (arrows) left PCA territory
B. T1-w axial image of the neck shows vertebral artery dissection on left side (arrow)
C. Four vessel MR Angiogram shows dissected left vertebral artery (L)

Tumors
• MR best modality
• excellent soft tissue contrast and multiplanar capability.
• Intravenous gadolinium must - evaluation of brain tumors.
• Tumor enhancement - break in the blood-brain barrier and do not represent
tumor vascularity (Figs 9.4A to D).
• Tumor vascularity is evaluated with MR perfusion.
• MR perfusion and spectroscopy can be helpful in
1. differentiating neoplastic from non-neoplastic lesions,
2. grading of tumors
3. guidance for the biopsy.
4. Spinal cord screening - ependymoma, medulloblastoma, hemangioblastoma,
choroids plexus tumors to rule out ‘drop metastases’.

A. T1-w post contrast axial - enhancing
tumor in the right occipital lobe
B. Perfusion image CCBV map – lesion
is hypovascular. Red area
correspond with high perfusion
while blue or dark is suggestive of
least perfusion
C. T1-w post-contrast axial
predominantly non-enhancing
tumor in the right cerebral
hemisphere causing mass effect
and midline shift
D. Perfusion image CCBV map shows
the lesion to be hypervascular.

Infection
• Intravenous gadolinium injection is must
• Contrast enhanced MR is superior to contrast
enhanced CT - shows dural enhancement and
thickening better
• MR is inferior to CT in chronic and congenital
infectious processes with calcifications

CP Angle Lesions
• Presenting with tinnitus, hearing loss and
vertigo
• highly T2-w sequence called CISS that shows
dark cranial nerves in bright CSF.
• Include MR angiogram to rule out any
vascular loop as a cause of tinnitus.
• Intravenous gadolinium is injected - rule out
labrynthitis and any small enhancing acoustic
neuroma in IAC (Fig. 9.7).

Demyelinating Lesions
• T2-w images are mainstay.
• FLAIR show lesion near ventricular margin better by
suppressing CSF.
• FLAIR sagittal images best show callaso-septal
lesions and ‘Dowson’s finger’ running perpendicular
to the ventricular margin in multiple sclerosis.
• Spinal cord - screened in demyelinating lesions -
rule cord involvement.
• Contrast is given in multiple sclerosis to see the
activity of the lesion - Enhancing lesions are usually
active.

Trauma
• CT preferred over MRI for head injury.
- Convenient
- less time consuming
- available everywhere.
- shows acute bleed and bony fractures easily.
- Gradient hemo and T1-weighted images are important in showing
acute bleed.
• MR - useful in diffuse axonal injury and sequelae of head injury
- when CT in indeterminate in posterior fossa lesions.

MRI in Pediatric Brain
• Stages of normal myelination is important
- Myelination progresses caudocranially, dorsoventrally and
center to periphery.
- completed by two years of age.
- Myelinated white matter appears bright on T1-w and dark on
T2-w images (Figs 9.10A and B).
- modality of choice in pediatric brain tumors, congenital
anomalies and hyoxic-ischemic injury (HIE) (Figs 9.11A and B).
- Correlation with perinatal and birth history is required as
hypoxic-ischemic injury affects periventricular white matter in
premature infants while gray matter and water-shed areas are
affected in term infants.
- Diffusion weighted images are useful in showing acute HIE.
- Metabolic diseases can be evaluated with MR spectroscopy.

INTRODUCTION
• Lack of contrast injection, radiation and invasiveness makes MR
angiography an attractive option for vascular assessment.
• MR angiography is based on flow information rather than
morphological imaging in conventional angiography.
• Hence it gives anatomical as well as hemodynamic information.

BLACK BLOOD IMAGING
• In this blood appears black and sequence used is spin-echo sequence
• In spin-echo sequence nuclei that receive both 90° and 180° pulses
will produce signal.
• Flowing blood - do not receive either 90 or 180 degree pulse - signal
is not produced - flowing blood appears dark.
• Slow flow, clot, occlusion - show signal - it will receive both 90 degree
excitation pulse and 180 degree rephasing pulse.

BRIGHT BLOOD IMAGING
• Blood appears bright - by gradient echo sequence and intravascular
contrast media.
• Excitation pulse is slice selective.
• Rephasing, which is done by gradients rather than 180 degree pulse .
• A flowing nucleus that receives an excitation pulse is rephased
regardless of its slice position and produces a signal.
• Short TR is used in GRE sequences, which results in repeated RF
pulses saturating stationary tissues.
• Increases contrast between flowing blood and stationary tissue thus
makes GRE more flow sensitive.

TYPES OF MR ANGIOGRAPHY
• Basic two types of MR angiography are used in routine practice.
1. Time of flight MRA (TOF-MRA)
2. Phase contrast MRA (PC-MRA)

Time of Flight MRA (TOF-MRA)
Time of Flight Phenomenon
• To produce a signal, a nucleus must receive both excitation pulse and
rephasing pulse. Stationary nuclei always receive both these pulses but
flowing nuclei present in excited slice may have exited the slice before
rephasing pulse hits them - is called time of flight phenomenon.
• Effect of phenomenon will be different in spin-echo and GRE sequences.
• In SE sequence TOF will result in signal void.
• Flow enhancement with bright signal from flowing blood in GRE (see bright
blood imaging).

• Gradient moment rephasing:
- Nuclei flowing along a gradient - change their phase because
magnetic field strength is altered along a gradient.
- If phase of the flowing nuclei is not maintained then signal will be
altered from flowing spins.
- To prevent this, gradients are adjusted in such a way that spins will
not lose their phase and gain phase. This is called as gradient moment
repahsing or nulling (GMR).
- For evaluation of arteries, spins or flowing nuclei in the veins should
be nulled or saturated - done by applying saturation pulses in the
direction of venous flow.

• Problems with TOF MRA are
- flow saturation- . Spins may get saturated /beaten down as they pass
down the stream in the imaging volume because they receive
multiple RF pulses.
- T1 sensitivity - Blood components with short T1 recovery time such as
methhemoglobin also appear bright on TOF-MRA and can make
differentiation of subacute hemorrhage difficult from flowing blood.

TOF-MRA can be of three types:
1. 2D TOF
2. 3D TOF
3. MOTSA (Multiple Overlapping Thin Slab Acquisitions)

• 2D TOF - acquisition is slice by slice.
- Sensitive to slow flow and gives large area of coverage - is used for slower
velocity vessels like peripheral arteries and for venography.
- Resolution of 2D TOF is lower than that of 3D TOF.
• 3D TOF - acquisition is from whole volume of the tissue
- good resolution and smaller vessels are better visualised.
- usually used for high velocity flow.
• MOTSA combines advantages of 2D (larger area of coverage) and 3D (high
resolution).
- volume is divided into multiple thin overlapping slabs during acquisition -
combined to single volume of data

• Data from TOF MRA - reformatted by technique called MIP (Maximum
Intensity Projection) to get angiogram.
- pixels with maximum intensity are selected - rest pixels are
suppressed - only vessels are visualised because they have pixels with
maximum intensity.
- Disadvantage of this techniques overestimation of stenosis of vessels.
- Hence, viewing of source images (axial images) recommended before
commenting on stenosis.

Phase Contrast MRA
• Uses changes in the phase of TM of the flowing blood to produce image contrast in
flowing blood.
• Phase shift is selectively introduced for moving spins (with use of magnetic field
gradients)
• After RF excitation pulse, spins are in phase.
- Gradients - applied to both stationary and flowing spins.
- Phase shift occurs in both stationary and flowing spins at different rates.
- A second gradient pulse of same amplitude and duration but of opposite polarity is
applied.
- In stationary nuclei reversal of phase shift occurs of exact amount, canceling the effect of
original phase shift and resulting in no net phase shift.
- However, since flowing nuclei have changed their position, the phase shift will not be
corrected.
- This phase shift is directly proportional to the change in location or distance the nuclei
traveled between applications of first and second gradient.
- These phase shifts are used by PC-MRA to create angiographic images and measure flow
velocities.

• provides information about flow direction.
• flow from head appears bright whereas flow from feet appears black.
• PC-MRA can be in 2D or 3D acquisition.

CONTRAST ENHANCED MRA (CEMRA)
• IV gadolinium.
• Sequence used is T1-weighted 3D spoiled gradient refocused GRE
sequence.
• Approximate T1 times of blood, muscle and enhanced blood are
1200 ms, 600 ms and 100 ms respectively.
• Minimum 0.2 mmol/kg of gadolinium - make T1 of blood shorter
than that of fat and muscle, so that it will appear brighter than fat.

APPLICATIONS OF MRA
• Cardiovascular and neurovascular diseases.
• TOF-MRA of carotid arteries and circle of Willis in patients with stroke
- saves patient from radiation and contrast medium and risks of
invasive procedures.
• In cerebral venous sinus thrombosis, phase contrast MR venography
(Fig. 11.5) gives accurate diagnosis and the extent of the disease apart
from parenchymal changes associated with it.
• MR angiographies performed in body and the peripheral vessels are
usually contrast enhanced.

INTRODUCTION
• relatively new technique
• integral part of brain imaging protocols.
What is diffusion?
• Diffusion is random movement of water protons - Brownian motion. It
is result of the dissipation of the thermal energy.
• In Isotrophic diffusion possibility of a water protons moving in any
one particular direction is equal to the probability that it will move in
any other direction.
• In Anisotrophic diffusion water diffusion has preferred direction.

How do we acquire diffusion weighted images?
• The basic sequence is called “Stesjkal-Tanner pulsed gradient spin
echo sequence”.
• It is a spin echo sequence with diffusion gradients applied before and
after 180 degree pulse .

b-value
• indicates the magnitude of diffusion weighting provided by diffusion
gradients.
• indicates sensitivity of the sequence to diffusion.
• expressed in sec/cm square.
• depends on amplitude, separation and duration of diffusion gradient.
• B-value increases with
- diffusion gradient strength,
- duration of their application and
- time between applications of two gradients.

ADC: Apparent Diffusion Coefficient
• is measure of diffusion.
• calculated from b-value zero and various higher b-value
images.
• Area with reduced ADC (restricted diffusion) will manifest
as bright area on diffusion weighted images (DWI) while
same area will turn dark on ADC map.
• three sets of images acquired.
- one with b-value zero (i.e. without diffusion gradients
applied),
- second with higher b-value usually b=1000 and
• third set of images is automatically calculated from these
two images, called as ADC map - bright areas on DWI turn
black suggesting true restricted diffusion

T2 Shine Through
• Signal intensity on DWI not only depends on ADC but also on tissue
T2.
• T2 can cause paradoxical decrease in signals of restricted diffusion or
when diffusion is normal can be mistaken for restricted diffusion on
DWI.
• To differentiate this T2 shine through from actual restricted diffusion,
ADC maps are used. ADC map will show reduced signal in actually
diffusion restricted area while there will be bright signal in case of T2
shine through on ADC map.

Diffusion “Trace”
• To average out anisotrophic white matter tract effects on diffusion of
water, image with higher b-value like b=1000 is taken in three
directions—X, Y, Z axes.
• Diffusion changes along all three axes then averaged to get image
called ‘trace’ diffusion image.

CLINICAL USES OF DIFFUSION
Stroke
• Failure of Na-K ATPase pump - results into influx of water into cells -
cytotoxic edema - reduction in diffusion of water in that area.
• As bright signal on DWI and dark signals on ADC map - diffusion imaging
can detect early ischaemic tissue as early as minutes to hours.
• DWI shows stroke - when all other images including T2-weighted images
are normal.
• CBF to drop below 15-20 ml/100 gm of brain tissue /minute to be
manifested as bright signal on DWI (reduced ADC).
• Vasogenic edema - increased fluid in extracellular space - show increased
diffusion (increased ADC).

A. DWI shows bright area in the right parietal region.
B. On ADC map image the area turns black suggestive of acute infarct.
C. On gradient Hemo image no evidence of any bleeding, making the infarct non-hemorrhagic
D. TOF MRA of Circle of Willis shows absent right ICA, MCA and ACA (arrow).
E. TOR MRA of carotid arteries shows complete occlusion of right ICA from its origin (arrow).

Chronic infarct is dark on DWI and bright on ADC

Hypoxic Ischemic Injury to Newborns
• Cytotoxic edema is easily appreciated on DWI in HIE in newborns.
Regions commonly affected - basal ganglion and water-shed areas.
Epidermoid versus Arachnoid Cyst
• Epidermoid is composed of keratin, debries and solid cholesterol -
provide barrier or hindrance to water proton diffusion.
• Epidermoid to be bright on DWI.
• Arachnoid cyst - clear CSF containing cyst, will be same as CSF signal
intensity.

Abscess versus Cystic Neoplasm
• Abscess contains thick fluid with hindrances to water
diffusion.
• It shows restricted diffusion (bright on DWI) in the
center.
• Neoplasm with central necrosis does not show
restricted diffusion in the center (Figs 12.5A to C).
Lymphoma versus Toxoplasma in HIV
• Ratio of ADC in the center of rim enhancing intra-
cranial lesion relative to normal white matter is
significantly higher in Toxoplasmosis.
• ADC ratio more than 1.6 is only seen in toxoplasmosis
• ADC ratio less than one is seen only in lymphoma. lymphoma in left cerebral hemisphere
which is bright on DWI
(A) and dark on ADC
(B) suggesting restricted diffusion of water
molecules due to high cellularity

INTRODUCTION
• Perfusion refers to the passage of blood from an arterial supply to
venous drainage through the microcirculation.
• Necessary for the nutritive supply to tissues and for clearance of
products of metabolism.
• Perfusion altered in pathologies - measuring changes in perfusion can
be helpful in diagnosis , monitoring and assessing treatment
response.
• MR perfusion can be performed with exogenous (injectable) contrast
agent like Gadolinium or by endogenous contrast agent.

MR Perfusion with Exogenous Contrast Agent
Technique
• 0.2 mmol/kg of Gd-DTPA IV at 5 ml/second and rapid T2*-weighted EPI
sequence is run for 60-90 seconds.
Mechanism
• Paramagnetic agents like Gd cause shortening of both T1 and T2 of the
tissue or region in which they go.
• Decrease in T1 relaxation time on T1-weighted images - brightening.
• Reduction in T2 relaxation time on T2 or T2*-weighted images – blackening
• In perfusion - gadolinium passes through the microvasculature there is
decrease in signal from magnetic susceptibility induced shortening of
T2*relaxation times - more the signal drop more will be the perfusion.
• More the number of small vessels - more will be signal drop -
microvascularity or relative perfusion of that region or tissue can be
determined.

Permiability/Leakiness
• Areas of severe blood-brain barrier break-down are frequently seen in
necrotic tumor and irradiated tumor beds - results in Gd in
extravascular space.
• T1- enhancing effects of this Gd may predominate to counteract the
T2 signal lowering effects of Gd, resulting in falsely low rCBV values.
• Measures to reduce the permiability induced effects on rCBV -
mathematical correction with calculation of permiability or K2 maps -
use of Dysprosium that has stronger T2* effects but negligible T1
effects, instead of Gd.

• rCBV:
Relative
cerebral
blood
volume
• CBF :
Cerebral
blood flow
• TTP : Time
to peak
• MTT : Mean
transit time

Clinical Applications
• Stroke, brain tumors, dementia and psychiatric illnesses, migraine
headaches, trauma, epilepsy and multiple sclerosis.
MR Perfusion in Stroke
• Important to detect brain ischemia and salvageable tissue in early
window period of 3-6 hours.
• DWI and PWI together effective in detection of early ischaemia -
before infarction or detected on T2- weighted images.
• Mismatch between PW and DW represent potentially salvageable
tissue (penumbra) , indicator of clinical outcome.

• Small mismatch - good clinical outcome.
• Large mismatch - poor clinical outcome and larger vessel occlusion.
• PWI is more sensitive than DWI for detecting ischemia in early
period.

MR Perfusion in Brain Tumors
• Gliomas- useful in grading gliomas,
- guiding biopsies
-differentiating therapy
induced necrosis from recurrent/residual
tumor.
- Tumor area with highest rCBV
value yields good results and increases
diagnostic confidence.
• RCBV maps can differentiate between
therapy induced necrosis (decreased
rCBV/ complete loss of rCBV) from
recurrent/residual tumor (elevated rCBV)

• Metastasis—Perfusion may help in differentiating a solitary
metastasis from glioma based on differences in measurement of
peritumoral rCBV.
• In metastasis, there is no histological evidence of tumor beyond the
outer contrast enhancing margin of the tumor so rCBV will not be
raised. In high grade glioma, the peritumoral region represents a
variable combination of vesogenic edema and tumor cells infiltrating
along perivascular spaces hence show increased rCBV.

ARTERIAL SPIN LABELING (ASL)
• ASL is a non-invasive method to assess tissue perfusion
without exogenous contrast injection or radiation.
• Arterial blood flowing towards the region of interest is
tagged by magnetic inversion pulses (proton phase is
changed).
↓
• After a delay to allow for inflow of tagged blood, image is
acquired in slice of interest. This image is called ‘tag image’.
↓
• Second image of same slice of interest is againacquired
without in-flowing tagged blood. This image is called
‘control image’.
↓
• Tag image is subtracted from control image.
↓
• This results into perfusion image representing ‘tagged blood’
that flowed into the image slice. ASL involves T1- weighted
imaging.

INTRODUCTION
• Application of MR to access various metabolites or biochemical from
the body tissues non-invasively.
• Then used to diagnose
monitoring
assessing response to the treatment.
• Present clinical use are mainly 1H (Hydrogen) and 31P (Phosphorus)
spectroscopy.

PRINCIPLES
1. MR images are reconstructed from the entire proton signal from
the tissue dominated by water and fat proton signals.
- Protons from other metabolites do not contribute to imaging -
negligible concentration.
- Aim in MRS itself is to detect these small metabolites.
- Most metabolite signals of clinical interest resonate between
resonant frequencies of water and fat - to be able to detect small
metabolites signal from water protons need to be suppressed.

1. Patient positioning
2. Global shimming
• Optimisation of magnetic field homogeneity is done over the entire volume
detected by receiver coil.
3. Acquisition of MR images for localization
• Images are obtained in all three planes (coronal, axial and sagittal) for
placement of voxel.
• MR images already obtained during routine imaging can be used for the
localization purpose if patient is not moved (Fig. 14.2).
4. Selection of MRS measurement and parameters

5. Selection of VOI (volume of interest)
• SVS can be used for local or diffuse diseases.
• CSI is used in irregularly-shaped large pathology
is used and where other side comparison is
required (Fig. 14.4).
6. Localized shimming
• Optimisation of homogeneity over selected
volume of interest.
7. Water suppression
• Water peak is suppressed so that smaller
metabolite peaks are visible.
Placement of VOI. In Alzheimer’s
disease hippocampus is the VOI. See
the placement of VOI (white box)

8. MRS data collection
• SVS usually - 3-6 minutes
• CSI usually - upto 12 minutes for data acquisition.
9. Data processing and display
• Acquired data is processed to get spectrum and spectral maps.
• Zero point of spectrum is set in the software itself by frequency of a particular
compound called Tetramethylsilane (TMS).
10. Interpretation
• Area under the peak of any metabolite is directly proportional to no of spins contributing
to the peak.
• Absolute values for each metabolite - vary with age and population.
• Interpretation should always base on ratios of metabolites and comparison with normal
side.

NAA: N-Acetylaspartate
• Peak position: 2.02 ppm.
• Marker and any insult to brain causing neuronal loss or degeneration -
reduction of NAA.
• Absent in tissues with no neurons e.g. metastasis, meningioma.
• Reduced in: hypoxia, infarction, Alzheimer’s, Herpes encephalitis,
hydrocephalus, Alexander’s disease, Epilepsy, Neoplasms, stroke,
NPH, Closed head trauma (Diffuse Axonal Injury).
• NAA increased in: Canavan’s disease

Cr: Creatine
• Peak position: 3.0 ppm. Second peak at 3.94 ppm
• Serves as high energy phosphates and as a buffer in ATP/ADP
reservoir. It increases with age.
• Increased in hypometabolic states , trauma.
• Reduced in hypermetabolic states, hypoxia, stroke, tumor.

Cho: Choline
• Constituent of phospholipids of cell membrane.
• Precursor of acetyl choline and phosphatidyl choline.
• Is indicator of cell membrane integrity.
• Increases with increased cell membrane synthesis and increased cell
turnover.
• Increased in: Chronic hypoxia, epilespy, Alzheimer’s, gliomas and other
tumors, trauma, infarction, hyperosmolar states, diabetes mellitus.
• Reduced - hepatic encephalopathy, stroke.

mI: Myoinositol
• Peak position: 3.56 ppm. Second peak at 4.1 ppm.
• Marker of gliosis.
• Increased in: Alzheimer’s, frotal lobe dementias, diabetes,
hyperosmolar states.
• Decreased in: hepatic and hypoxic encephalopathy, stroke, tumor,
osmotic pontine myelinolysis, hyponatremia.
Lac: Lactate
• Not seen in normal brain spectrum.
• elevated in hypoxia, tumor, mitochondrial encphalopathy, IC
haemorrhage, stroke, hypoventilation, Canavan’s disease, Alexander,
hydrocephalus.

Lipids
• Peak position: 0.9, 1.3, 1.5 ppm.
• Not seen in normal brain spectrum
• Seen in acute destruction of myelin.
• Increased in high grade tumors (reflects necrosis), stroke, multiple
sclerosis lesions and tuberculomas.
Amino Acids
• Alanine (at 1.3-1.4 ppm), Valine (at 0.9 ppm), leucine (at 3.6 ppm) are
usually multiplets visualised at short TE.
• They invert at TE of 135 ms.
• Alanine is seen in meningioma
• Valine and leucine are markers of abscess.

Amino Acids
• Alanine (at 1.3-1.4 ppm), Valine (at 0.9 ppm), leucine (at 3.6 ppm) are
usually multiplets visualised at short TE.
• They invert at TE of 135 ms.
• Alanine is seen in meningioma
• Valine and leucine are markers of abscess.

Clinical Uses of MRS
Brain Tumors
• In tumors there is increase in Cho, lactate and lipid. There is reduction in
NAA and Cr.
a) MRS in tumor evaluation:
- differentiate neoplasm from non-neoplastic lesions.
- Grade gliomas based on metabolite ratios (Figs 14.6A to C).
b)In treatment planning:
- Biopsy of higher choline area has shown higher success and increased
diagnostic confidence

In Treatment Monitoring
• Differentiation of radiation necrosis and gliosis
from residual or recurrent neoplasm.
• Radiation necrosis - reduced peaks of all
metabolites
• Recurrent/residual tumor characteristic spectrum
elevated choline
Neonatal Hypoxia
• Decrease in NAA, Cr, MI
• Increase in Cho, lactate/lipid peaks
• Progressive decrease in NAA, Cr and MI can be
used to monitor the condition.
• In neonatal hemorrhage MRS - used to determine
hypoxia, as hypoxia is one of the causes of
neonatal hemorrhage.

Metabolic Disorders and White Matter Diseases
• Elevation of lactate - in mitochondrial disorders like MELAS (Mitochondrial
Encephalopathy Lactic Acidosis and Stroke) (Fig. 14.9).
Stroke
• NAA and Cr are reduced
• Cho and lactate are elevated.
• Extent, severity of ischemia and peripheral penumbra can be defined
Epilepsy
• NAA/Cr is reduced in affected lobe
• Be used to localise intractable epilepsy.
Closed Head Trauma
• Diffuse axonal injury - decrease in NAA/Cr ratio and absolute concetration of
NAA.

Lymphoma
- Elevation of lactate, lipids and choline;
- reduction of NAA, Cr and MI
Abscess
• Difficult to differentiate from neoplasm.
• Visualisation of amino acid peaks at 0.9 ppm(valine, leucine and
isoleucine.)
• There may be peaks representing acetate, pyruvate, lactate succinate,
which are end products arising from microorganisms.

CSF FLOW STUDIES
• There is continuous to and fro movement of CSF with cardiac cycle.
• During systole, because of expansion of cerebral hemispheres, there is
craniocaudal movement of the CSF from lateral to third to fourth ventricle.
• During diastole CSF moves cuadocranially.
• On conventional MR imaging this manifests as flow void in the acqueduct.
• CSF flow studies are done by phase contrast method, which is used also for MR
angiography and venography.
• CSF in acqueduct
- bright during systole (craniocaudal flow)
- dark during diastole (caudocranial flow).
• Normal acqueductal stroke volume is 42 microliters.
• Stroke volume more than 42 microlitre is suggestive of hyperdynamic flow.

Clinical Applications of CSF Flow Studies
NPH—Normal Pressure Hydrocephalus
1. Elderly patients with clinical triad of
dementia,
gait disturbances
incontinence of urine.
• mean intraventricular pressure - normal (compensated hydrocephalus)
• pulse pressure is increased several times - pressure pounds against paracentral fibers—corona
radiata (‘waterhammer pulse’) causes compression of the cortex - symptoms.
• Conventional MR images show ventricular dilatation out of proportion to the sulcal widening.
• Hyperdynamic flow (increased to and fro motion) that manifests as increased flow void in
acqueduct on routine MR images.
• If this flow void is extensive, from third ventricle to fourth ventricle through the acqueduct,
then there is good response to ventricular shunting, which is treatment for NPH.
• Acqueductal stroke volume more than 42 microliter on CSF flow studies has good response to
ventricular shunting.
• Patient with stroke volume less than 42 microliter is less likely to benefit from shunting.
• Thus MR has diagnostic as well as prognostic value in NPH.

2. Shunt evaluation—
• Stop valve that allows unidirectional flow.
• Flow images of the patent shunt signal during systole-diastole will be
bright-gray-bright-gray.
• Blocked shunt - signal in the tube will be gray in both systole and diastole.
• After shunting flow through the acqueduct is reversed (caudocranial during
the systole) because of low pressure pathways through the shunt for the
CSF that is pushed upwards by cerebellum and choroid plexus in fourth
ventricle
• If the flow in acqueduct is normal (i.e. craniocaudal during systole), it is
suggestive of shunt block.
3. Differentiation of arachnoid cyst from mega cisterna magna.
Arachnoid cyst will show movement during systole and diastole and will
show different flow than surrounding CSF.

• ABSOLUTE CONTRAINDICATIONS
• 1. Internal cardiac pacemakers
• 2. Implantable cardiac defibrillators
• 3. Cochlear implants
• 4. Neurostimulators
• 5. Bone growth stimulators
• 6. Electrically programmed drug infusion pumps, vascular access ports
• 7. Intraocular foreign body 8. Aneurysm clips

Mri basic principles

More Related Content

What's hot (20)

Similar to Mri basic principles (20)

Recently uploaded (20)

Mri basic principles

Editor's Notes