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T1 RELAXATIONAfter protons are Excited with RF pulse They move out of Alignment with B 0 But once the RF Pulse is stopped they Realign after some Time And this is called t1 relaxati

MRI SEQUENCES

Tushar Patil, MD

Senior Resident Department of Neurology

King George’s Medical University

Lucknow, India

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

 Body has many such atoms that can act as good MR nuclei (1H, 13C, 19F, 23Na)

 Hydrogen nuclei is one of them which is not only positively charged, but also has magnetic spin

 MRI utilizes this magnetic spin property of protons of hydrogen to elicit images

WHY HYDROGEN IONS ARE USED IN MRI?

 Hydrogen nucleus has an unpaired proton which is positively charged

 Every hydrogen nucleus is a tiny magnet which produces small but noticeable magnetic field

 Hydrogen atom is the only major species in the body that is MR sensitive

 Hydrogen is abundant in the body in the form of water and fat

 Essentially all MRI is hydrogen (proton) imaging

BODY IN AN EXTERNAL MAGNETIC

• In our natural state In our natural state Hydrogen ions in body are

spinning in a haphazard fashion, and cancel all

the magnetism.

• When an external magnetic field is applied protons

in the body align in one direction (As the compass

aligns in the presence of earth’s

magnetic field)

NET MAGNETIZATION

 Half of the protons align along the magnetic field and rest are aligned opposite

.

 At room temperature, the

population ratio of

parallel versus parallel

protons is roughly 100,000

to 100,006 per Tesla of B0

 These extra protons produce net magnetization vector (M)

 Net magnetization depends on B0 and temperature

MANIPULATING THE NET

MAGNETIZATION

 Magnetization can be manipulated by changing the magnetic field environment (static, gradient, and RF fields)

 RF waves are used to manipulate the magnetization of H nuclei

 Externally applied RF waves perturb magnetization into different axis (transverse axis) Only transverse magnetization produces

signal

 When perturbed nuclei return to their original state they emit RF signals which can be detected with the help of receiving coils

T1 AND T2 RELAXATION

 When RF pulse is stopped higher energy gained by proton is retransmitted and hydrogen nuclei relax by two mechanisms

 T1 or spin lattice relaxation- by which original magnetization (Mz) begins to recover

 T2 relaxation or spin spin relaxation - by which magnetization in X-Y plane decays towards zero in an exponential fashion It is

due to incoherence of H nuclei

 T2 values of CNS tissues are shorter than T1 values

T1 RELAXATION

After protons are

Excited with RF pulse

They move out of

Alignment with B 0

But once the RF Pulse

is stopped they Realign

after some Time And

this is called t1 relaxation

T1 is defined as the time it takes for the hydrogen nucleus to recover 63% of its longitudinal magnetization

T2 relaxation time is the time for 63% of the protons to become dephased

owing to interactions among nearby protons

TR AND TE

 TE (echo time) : time interval in which signals are measured after RF excitation

 TR (repetition time) : the time between two excitations is called repetition time

 By varying the TR and TE one can obtain T1WI and T2WI

 In general a short TR (<1000ms) and short TE (<45 ms) scan is T1WI

 Long TR (>2000ms) and long TE (>45ms) scan is T2WI

 Long TR (>2000ms) and short TE (<45ms) scan is proton density image

Different tissues have different relaxation times These relaxation time differences

is used to generate image contrast.

TYPES OF MRI IMAGINGS

T1 & T2 W IMAGING

Blood Bone

MRI T1 CSF Edema Gray

Matter White Matter Cartilage Fat

CT SCAN

MRI T1 Weighted

MRI T2 Weighted

MRI T2 Flair

DARK ON T1

 Edema,tumor,infection,inflammation,hemorrhage(hyperacute,chronic)

 Low proton density,calcification

 Flow void

BRIGHT ON T1

 Fat,subacute hemorrhage,melanin,protein rich fluid.

 Slowly flowing blood

 Paramagnetic substances(gadolinium,copper,manganese)

 9

BRIGHT ON T2

 Edema,tumor,infection,inflammation,subdural collection

 Methemoglobin in late subacute hemorrhage

DARK ON T2

 Low proton density,calcification,fibrous tissue

 Paramagnetic substances(deoxy hemoglobin,methemoglobin(intracellular),ferritin,hemosiderin,melanin.

 Protein rich fluid

 Flow void

WHICH SCAN BEST DEFINES THE ABNORMALITY

FLAIR & STIR

CONVENTIONAL INVERSION

RECOVERY

-180° preparatory pulse is applied to flip the net magnetization vector 180° and null the signal from a particular entity (eg, water in tissue)

-When the RF pulse ceases, the spinning nuclei begin to relax When the net magnetization vector for water passes the transverse plane (the null point for that tissue), the conventional 90° pulse is applied, and the SE sequence then continues as before.

-The interval between the 180° pulse and the 90° pulse is the TI ( Inversion Time)

Conventional Inversion Recovery Contd:

Conventional Inversion Recovery Contd:

 At TI, the net magnetization vector of water is very weak, whereas that for body tissues is strong When the net magnetization vectors are flipped by the 90° pulse, there is little or no transverse magnetization in water, so no signal is generated (fluid appears dark), whereas signal intensity ranges from low to high in tissues with a stronger NMV.

 Two important clinical implementations of the inversion recovery concept are:

Short TI inversion-recovery (STIR) sequence

Fluid-attenuated inversion-recovery (FLAIR) sequence.

SHORT TI INVERSION-RECOVERY (STIR)

SEQUENCE

 In STIR sequences, an inversion-recovery pulse is used to null the signal from fat (180° RF Pulse).

 When NMV of fat passes its null point , 90° RF pulse is applied As little or no longitudinal magnetization is present and the transverse magnetization is insignificant

 It is transverse magnetization that induces an electric current in the receiver coil so no signal is generated from fat

 STIR sequences provide excellent depiction of bone marrow edema which may be the only indication of an occult fracture.

 Unlike conventional fat-saturation sequences STIR sequences are not affected by magnetic field inhomogeneities, so they are more efficient for nulling the signal from fat

Comparison of fast SE and STIR sequences for depiction of bone marrow edema

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

 In contrast to real image reconstruction, negative signals are recorded as positive signals of the same strength so that the nulled tissue remains dark and all other tissues have higher signal intensities.

 Most pathologic processes show increased SI on T2-WI, and the conspicuity of lesions that are located close to interfaces b/w brain parenchyma and CSF may be poor in conventional SE or FSE T2-WI sequences.

 FLAIR images are heavily T2-weighted with CSF signal suppression, highlights hyperintense lesions and improves their conspicuity and detection, especially when located adjacent to CSF containing spaces

 In addition to T2- weightening, FLAIR possesses considerable T1-weighting, because it largely depends on longitudinal magnetization

 As small differences in T1 characteristics are accentuated, mild T1-shortening becomes conspicuous

 This effect is prominent in the CSF-containing spaces, where increased protein content results in high SI (eg, associated with arachnoid space disease)

sub- High SI of hyperacute SAH is caused by T2 prolongation in addition to T1 shortening

Clinical Applications:

 Used to evaluate diseases affecting the brain parenchyma neighboring the CSF-containing spaces for eg: MS & other demyelinating disorders

 Unfortunately, less sensitive for lesions involving the brainstem & cerebellum, owing to CSF pulsation artifacts

 Helpful in evaluation of neonates with perinatal HIE.

 Useful in evaluation of gliomatosis cerebri owing to its superior delineation of neoplastic spread

 Useful for differentiating extra-axial masses eg epidermoid cysts from arachnoid cysts However, distinction is more easier & reliable with DWI.

 Mesial temporal sclerosis: m/c pathology in patients with partial complex seizures.Thin-section coronal FLAIR is the standard sequence in these patients & seen as a bright small hippocampus on dark background of suppressed CSF-containing spaces However, normally also mesial temporal lobes have mildly increased SI on FLAIR images.

 Focal cortical dysplasia of Taylor’s balloon cell type- markedly hyperintense funnel-shaped subcortical zone tapering toward the lateral ventricle is the characteristic FLAIR imaging finding

 In tuberous sclerosis- detection of hamartomatous lesions, is easier with FLAIR than with PD or T2-W sequences

 Embolic infarcts- Improved visualization

 Chronic infarctions- typically dark with a rim of high signal Bright peripheral zone corresponds to gliosis, which is well seen on FLAIR and may be used to distinguish old lacunar infarcts from dilated perivascular spaces.

T2 W FLAIR

Subarachnoid Hemorrhage (SAH):

 FLAIR imaging surpasses even CT in the detection of traumatic supratentorial SAH

 It has been proposed that MR imaging with FLAIR, gradient-echo T2*-weighted, and rapid high-spatial resolution MR angiography could be used to evaluate patients with suspected acute SAH, possibly obviating the need for CT and intra-arterial angiography

 With the availability of high-quality CT angiography, this approach may not be necessary.

FLAIR

DWI & ADC

• 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.



• Reduction of extracellular space

• 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 weighted images because of lesser signal losses

diffusion- Restriction of DWI is not specific for stroke

intermedia te

high intermedia

te

intermedia te

CSF low high low low high

 DW images usually performed with echo-planar sequences which markedly decrease imaging time, motion artifacts and increase sensitivity to signal changes due to molecular motion.

 The primary application of DW MR imaging has been in brain imaging, mainly because of its exquisite sensitivity to early detection of ischemic stroke

 The increased sensitivity of diffusion-weighted MRI in detecting acute ischemia is thought to be the result of the water shift intracellularly restricting motion of water protons (

water shift intracellularly restricting motion of water protons (cytotoxic edema cytotoxic edema ), whereas the conventional T2 weighted images show signal alteration mostly as a result of vasogenic edema

• Core of infarct = irreversible damage

• Surrounding ischemic area  may be salvaged

• DWI: open a window of opportunity during which Tt is beneficial

• Regions of high mobility “rapid diffusion”  dark

• Regions of low mobility “slow diffusion”  bright

• Difficulty: DWI is highly sensitive to all of types of motion (blood flow, pulsatility, patient motion)

APPARENT DIFFUSION COEFFICIENT

 It is a measure of diffusion

 Calculated by acquiring two or more images with a different gradient duration and amplitude values)

(b- To differentiate T2 shine through effects or artifacts from real ischemic lesions.

 The lower ADC measurements seen with early ischemia,

 An ADC map shows parametric images containing the apparent diffusion coefficients of diffusion weighted images Also called diffusion map

 The ADC may be useful for estimating the lesion age and distinguishing acute from subacute DWI lesions. 

 Acute ischemic lesions can be divided into hyperacute lesions (low ADC and DWI-positive) and subacute lesions (normalized ADC)

 Chronic lesions can be differentiated from acute lesions by normalization of ADC and DWI

 a tumour would exhibit more restricted apparent diffusion compared with a cyst because intact cellular membranes in a tumour would hinder the free movement of water molecules

NONISCHEMIC CAUSES FOR DECREASED ADC

65 year male- Rt ACA Infarct

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 (Fig 2).

 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

CLINICAL USES OF DWI &

value (Pseudonormalization) (Pseudonormalization)

 Subacute to Chronic Stage:- ADC value are increased (Vasogenic edema) but hyperintensity still seen on DWI :- ADC value are increased (Vasogenic edema) but hyperintensity still seen on DWI (T (T 2 shine effect)

GRE

 Because gradients do not refocus field inhomogeneities, GRE sequences with long TEs are T2* weighted (because

of magnetic susceptibility) rather than T2 weighted like SE sequences

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 and in pigmented villonodular synovitis.

 SE sequences, on the other hand- relatively immune from magnetic susceptibility artifacts, and also less sensitive in depicting hemorrhage and calcification

GRE FLAIR

Hemorrhage in right parietal lobe

GRE Sequences contd:

Magnetic susceptibility imaging-

 - Basis of cerebral perfusion studies, in which the T2* effects (ie, signal decrease) created by gadolinium (a metal injected intravenously as a chelated ion in aqueous solution, typically in the form of gadopentetate dimeglumine) are sensitively depicted by GRE sequences.

 - Also used in blood oxygenation level–dependent (BOLD) imaging, in which the relative amount of deoxyhemoglobin in the cerebral

vasculature is measured as a reflection of neuronal activity BOLD MR imaging is widely used for mapping of human brain function.