MAGNETIC RESONANCE MAGNETIC RESONANCE IMAGINGIMAGING

Dr.Shahzad Ahmad DaulaDr.Shahzad Ahmad DaulaMID,DMLT,DDC

The University of LahoreThe University of Lahore

MRI PRINCIPLEMRI PRINCIPLE MRI is based on the principle of nuclear magnetic resonance. MRI is based on the principle of nuclear magnetic resonance. Two basic principles of NMRTwo basic principles of NMR

1.1. Atoms with an odd number of protons or neutrons have spin Atoms with an odd number of protons or neutrons have spin 2.2. A moving electric charge, be it positive or negative, produces a magnetic fieldA moving electric charge, be it positive or negative, produces a magnetic field

Body has many such atoms that can act as good MR nuclei Body has many such atoms that can act as good MR nuclei ((11H, H, 1313C, C, 1919F, F, 2323Na) Na)

Hydrogen nuclei is not only positively charged, but also has Hydrogen nuclei is not only positively charged, but also has magnetic spinmagnetic spin

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

WHY HYDROGEN IONS ARE USED WHY HYDROGEN IONS ARE USED IN MRI? IN MRI?

an unpaired proton which is positively chargedEvery 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 sensitiveAbundant in the body in the form of water and fatMRI is hydrogen (proton) imaging

BODY IN AN EXTERNAL BODY IN AN EXTERNAL MAGNETIC FIELD (MAGNETIC FIELD (BB00))

•In our natural stateIn our natural state Hydrogen ions in body Hydrogen ions in body are spinning in a haphazard fashion, and are spinning in a haphazard fashion, and cancel all the magnetism.cancel all the magnetism.

•When an external magnetic field is applied When an external magnetic field is applied protons in the body align in one direction. protons in the body align in one direction.

NET MAGNETIZATION Half of the protons align along the magnetic field and rest Half of the protons align along the magnetic field and rest

are aligned opposit.are aligned opposit.

At room temperature, the At room temperature, the

population ratio of anti-population ratio of anti-

parallel versus parallel parallel versus parallel

protons is roughly 100,000protons is roughly 100,000

to 100,006 per Tesla of to 100,006 per Tesla of BB00

These extra protons produce net magnetization vector (M)These extra protons produce net magnetization vector (M)

Net magnetization depends on BNet magnetization depends on B0 0 and temperatureand temperature

MANIPULATING THE NET MANIPULATING THE NET MAGNETIZATIONMAGNETIZATION

Manipulated by changing the magnetic field Manipulated by changing the magnetic field environment (static, gradient, and RF fields)environment (static, gradient, and RF fields)

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

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

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

T1 AND T2 RELAXATION When RF pulse is stopped higher energy gained by When RF pulse is stopped higher energy gained by

proton is retransmitted and hydrogen nuclei relax by proton is retransmitted and hydrogen nuclei relax by two mechanismstwo mechanisms

T1 or spin lattice relaxation- by which original T1 or spin lattice relaxation- by which original magnetization begins to recover. magnetization begins to recover.

T2 relaxation or spin spin relaxation - by which T2 relaxation or spin spin relaxation - by which magnetization in X-Y plane decays towards zero in an magnetization in X-Y plane decays towards zero in an exponential fashion. It is due to incoherence of H nuclei. exponential fashion. It is due to incoherence of H nuclei.

T2 values of CNS tissues are shorter than T1 valuesT2 values of CNS tissues are shorter than T1 values

T1 RELAXATIONT1 RELAXATION

After protons are Excited with RF pulse They move out of Alignment with B0

But once the RF Pulseis stopped they Realign after some Time And this is called t1 relaxationT1 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 TE (echo time) : time interval in which signals are measured

after RF excitationafter RF excitation TR (repetition time) : the time between two excitations is TR (repetition time) : the time between two excitations is

called repetition timecalled repetition time By varying the TR and TE one can obtain T1WI and T2WIBy varying the TR and TE one can obtain T1WI and T2WI In general a short TR (<1000ms) and short TE (<45 ms) scan is In general a short TR (<1000ms) and short TE (<45 ms) scan is

T1WIT1WI Long TR (>2000ms) and long TE (>45ms) scan is T2WILong TR (>2000ms) and long TE (>45ms) scan is T2WI Long TR (>2000ms) and short TE (<45ms) scan is proton Long TR (>2000ms) and short TE (<45ms) scan is proton

density imagedensity image

Different tissues have different Different tissues have different relaxation times. These relaxation relaxation times. These relaxation time differences is used to generate time differences is used to generate image contrast.image contrast.

TYPES OF MRI IMAGINGS

T1WIT1WI T2WIT2WI FLAIRFLAIR STIRSTIR DWIDWI ADCADC GREGRE

MRAMRA MRVMRV MRSMRS MTMT Post-Gd imagesPost-Gd images

T1 & T2 W IMAGING

GRADATION OF INTENSITY GRADATION OF INTENSITY IMAGING

CT SCAN CSF Edema White Matter

Gray Matter

Blood Bone

MRI T1 CSF Edema Gray Matter

White Matter

Cartilage Fat

MRI T2 Cartilage Fat White Matter

Gray Matter

Edema CSF

MRI T2 Flair

CSF Cartilage Fat White Matter

Gray Matter

Edema

CT SCAN

MRI T1 Weighted

MRI T2 Weighted

MRI T2 Flair

DARK ON T1DARK ON T1

Edema,tumor,infection,inflammation,hemorrhage Edema,tumor,infection,inflammation,hemorrhage Low proton density ,calcificationLow proton density ,calcificationFlow voidFlow void

BRIGHT ON T1BRIGHT 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 Edema,tumor,infection,inflammation,subdural collectioncollection

Methemoglobin in late subacute hemorrhageMethemoglobin in late subacute hemorrhage

DARK ON T2DARK ON T2

Low proton density,calcification,fibrous tissueLow proton density,calcification,fibrous tissue Paramagnetic substances (deoxy hemoglobin, Paramagnetic substances (deoxy hemoglobin,

methemoglobin,ferritin,hemosiderin,melanin.methemoglobin,ferritin,hemosiderin,melanin. Protein rich fluidProtein rich fluid Flow voidFlow void

WHICH SCAN BEST DEFINES THE WHICH SCAN BEST DEFINES THE ABNORMALITYABNORMALITY

T1 W Images:T1 W Images:Subacute HemorrhageSubacute HemorrhageFat-containing structuresFat-containing structuresAnatomical Details Anatomical Details

T2 W Images:T2 W Images:EdemaEdemaDemyelinationDemyelinationInfarctionInfarctionChronic HemorrhageChronic Hemorrhage

FLAIR Images:FLAIR Images:Edema, Edema, Demyelination Demyelination Infarction esp. in Periventricular locationInfarction esp. in Periventricular location

FLAIR & STIRFLAIR & STIR

CONVENTIONAL ICNVERSION RECOVERY

- 180° preparatory pulse180° preparatory pulse is applied to flip the net magnetization is applied to flip the net magnetization

vector 180° andvector 180° and null the signal from a particular entity null the signal from a particular entity

(eg, water in tissue).(eg, water in tissue).

- When the RF pulse ceases, the spinning nuclei begin to relax.When the RF pulse ceases, the spinning nuclei begin to relax.

When the net magnetization vector for water passes the When the net magnetization vector for water passes the

transversetransverse plane (the null point for that tissue), the plane (the null point for that tissue), the

conventional 90°conventional 90° pulse is applied, and the SE sequence then pulse is applied, and the SE sequence then

continues as beforecontinues as before ..

- The interval between the 180° pulse and the 90°- The interval between the 180° pulse and the 90° pulse is the pulse is the

TI ( Inversion Time).TI ( Inversion Time).

Contd: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)

an inversion-recovery pulse is used to nullan inversion-recovery pulse is used to null the signal from fat (180° RF the signal from fat (180° RF Pulse).Pulse).

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

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

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

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

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

FSE STIR

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 sub-arachnoid space disease)

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 WFLAIR

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

FLAIR

DWI & ADC

DIFFUSION-WEIGHTED MRI Diffusion-weighted MRI is a example of endogenous contrast, using

the motion of protons to produce signal changes

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. Nonstationary water molecules acquire phase information from the first gradient, but are not rephased 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.

• 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 diffusion-weighted images because of lesser signal losses

Restriction of DWI is not specific for stroke

description

T1 T2 FLAIR DWI ADC

White matter

high low intermediate

low low

Grey matter

intermediate

intermediate

high intermediate

intermediate

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 (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).

Ischemic Stroke Extra axial masses: arachnoid cyst versus epidermoid tumor Intracranial Infections

Pyogenic infection

Herpes encephalitis

Creutzfeldt-Jakob disease Trauma Demyelination

APPARENT DIFFUSION APPARENT DIFFUSION COEFFICIENTCOEFFICIENT

It is a measure of diffusion

Calculated by acquiring two or more images with a different gradient

duration and amplitude (b-values)

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

Abscess

Lymphoma and other tumors

Multiple sclerosis

Seizures

Metabolic (Canavans )

65 year male- Rt ACA Infarct

EVALUATION OF ACUTE STROKE ON EVALUATION OF ACUTE STROKE ON DWIDWI

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 & ADCCLINICAL USES OF DWI & ADCStroke:

Hyperacute Stage:- within one hour minimal hyperintensity seen in DWI

and ADC value decrease 30% or more below normal (Usually <50X10-4

mm2/sec)

Acute Stage:- Hyperintensity in DWI and ADC value low but after 5-

7days of ictus ADC values increase and return to normal value

(Pseudonormalization)

Subacute to Chronic Stage:- ADC value are increased (Vasogenic edema)

but hyperintensity still seen on DWI (T2 shine effect)

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.

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.

GREFLAIR

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.

GRADIENT ECHO

Pros: fast technique

Cons: More sensitive to magnetic susceptibility artifacts Clinical use: eg. Hemorrhage , calcification

Axial T1 (C), T2 (D), and GRE (E) images show corresponding T1-hyperintense and GRE-hypointense foci with associated T2 hyperintensity (arrows).

MRS & MT-MRI

MR SPECTROSCOPY

Magnetic resonance spectroscopy (MRS) is a means of

noninvasive physiologic imaging of the brain that

measures relative levels of various tissue metabolites

Purcell and Bloch (1952) first detected NMR signals from

magnetic dipoles of nuclei when placed in an external

magnetic field.

Initial in vivo brain spectroscopy studies were done in the

early 1980s.

Today MRS-in particular, IH MRS-has become a valuable

physiologic imaging tool with wide clinical applicability.

PRINCIPLES:

The radiation produced by any substance is dependent on its atomic

composition.

Spectroscopy is the determination of this chemical composition of a

substance by observing the spectrum of electromagnetic energy emerging

from or through it.

NMR is based on the principle that some nuclei have associated magnetic

spin properties that allow them to behave like small magnet.

In the presence of an externally applied magnetic field, the

magnetic nuclei interact with that field and distribute themselves to

different energy levels.

These energy states correspond to the proton nuclear spins, either

aligned in the direction of (low-energy spin state) or against the applied

magnetic field (high-energy spin state).

If energy is applied to the system in the form of a radiofrequency

(RF) pulse that exactly matches the energy between both states. a

condition of resonance occurs.

Chemical elements having different atomic numbers such as

hydrogen ('H) and phosphorus (31P) resonate at different

Larmor RFs.

Small change in the local magnetic field, the nucleus of the atom

resonates at a shifted Larmor RF.

This phenomenon is called the chemical shift.

TECHNIQUE:

Single volume and Multivolume MRS.

1) Single volume:

Stimulated echo acquisition mode (STEAM)

Point-resolved spectroscopy (PRESS)

It gives a better signal-to noise ratio

2) Multivolume MRS:

chemical shift imaging (CSI) or spectroscopic imaging (SI)

much larger area can be covered, eliminating the sampling error to an extent

but significant weakening in the signal-to-noise ratio and a longer scan time.

Time of echo: 35 ms and 144ms.

Resonance frequencies on the x-axis and amplitude (concentration) on the y-

axis.

OBSERVABLE METABOLITES

Metabolite Locationppm

Normal function Increased

Lipids 0.9 & 1.3 Cell membrane component

Hypoxia, trauma, high grade neoplasia.

Lactate 1.3TE=272(upright)TE=136 (inverted)

Denotes anaerobic glycolysis

Hypoxia, stroke, necrosis, mitochondrial diseases,

neoplasia, seizure

Alanine 1.5 Amino acid Meningioma

Acetate 1.9 Anabolic precursor

Abscess ,Neoplasia,

PRINCIPLE METABOLITESMetabolite Location ppm

Normal function

Increased Decreased

NAA 2 Nonspecific neuronal marker

(Reference for chemical shift)

Canavan’s disease

Neuronal loss, stroke,

dementia, AD, hypoxia,

neoplasia, abscess

Glutamate , glutamine,

GABA

2.1- 2.4 Neurotransmit

ter

Hypoxia, HE Hyponatremia

Succinate 2.4 Part of TCA cycle

Brain abscess

Creatine 3.03 Cell energy marker

(Reference for metabolite

ratio)

Trauma, hyperosmolar

state

Stroke, hypoxia, neoplasia

Metabolite Location ppm

Normal function

Increased Decreased

Choline 3.2 Marker of cell memb turnover

Neoplasia, demyelination

(MS)

Hypomyelination

Myoinositol 3.5 & 4 Astrocyte marker

ADDemyelinatin

g diseases

METABOLITE RATIOS:

Normal abnormal

NAA/ Cr 2.0 <1.6

NAA/ Cho 1.6 <1.2

Cho/Cr 1.2 >1.5

Cho/NAA 0.8 >0.9

Myo/NAA 0.5 >0.8

MRS

Dec NAA/CrInc acetate, succinate,

amino acid, lactate

Neuodegenerative

Alzheimer

Dec NAA/Cr

Dec NAA/ ChoInc

Myo/NAA

Slightly inc Cho/ CrCho/NAA

Normal Myo/NAA± lipid/lactate

Inc Cho/CrMyo/NAACho/NAA

Dec NAA/Cr± lipid/lactate

MalignancyDemyelinatin

g disease Pyogenic abscess

CLINICAL APPLICATIONS OF MRS:

Class A MRS Applications: Useful in Individual Patients

1) MRS of brain masses:

Distinguish neoplastic from non neoplastic masses

Primary from metastatic masses.

Tumor recurrence vs radiation necrosis

Prognostication of the disease

Mark region for stereotactic biopsy.

Monitoring response to treatment.

Research tool

2) MRS of Inborn Errors of Metabolism

Include the leukodystrophies, mitochondrial disorders, and enzyme defects that

cause an absence or accumulation of metabolites

CLASS B MRS APPLICATIONS: OCCASIONALLY USEFUL IN INDIVIDUAL PATIENTS

1) Ischemia, Hypoxia, and Related Brain Injuries

Ischemic stroke

Hypoxic ischemic encephalopathy.

2)Epilepsy

Class C Applications: Useful Primarily in Groups of Patients (Research)

HIV disease and the brain

Neurodegenerative disorders

Amyotrophic lateral sclerosis

Multiple sclerosis

Hepatic encephalopathy

Psychiatric disorders

MAGNETIZATION TRANSFER (MT) MRI

MT is a recently developed MR technique that alters contrast of tissue on

the basis of macromolecular environments.

MTC is most useful in two basic area, improving image contrast and tissue

characterization.

MT is accepted as an additional way to generate unique contrast in MRI

that can be used to our advantage in a variety of clinical applications.

Magnetization transfer (MT) contd:-

Basis of the technique: that the state of magnetization of an atomic nucleus can be transferred to a like nucleus in an adjacent molecule with different relaxation characteristics.

Acc. to this theory- H1 proton spins in water molecules can exchange magnetization with H1 protons of much larger molecules, such as proteins and cell membranes.

Consequence is that the observed relaxation times may reflect not only the properties of water protons but also, indirectly, the characteristics of the macromolecular solidlike environment

MT occurs when RF saturation pulses are placed far from the resonant frequency of

water into a component of the broad macromolecular pool.

Magnetization transfer (MT) contd:-

These off-resonance pulses, which may be added to standard MR pulse sequences, reduce the longitudinal magnetization of the restricted protons to zero without directly affecting the free water protons.

The initial MT occurs between the macromolecular protons and the transiently bound hydration layer protons on the surface of large molecules’

Saturated bound hydration layer protons then diffuse and mix with the free water proton pool

Saturation is transferred to the mobile water protons, reducing their longitudinal magnetization, which results in decreased signal intensity and less brightness on MR images.

Magnetization transfer (MT) contd:-

The MT effect is superimposed on the intrinsic contrast of the baseline image

Amount of signal loss on MT images correlates with the amount of macromolecules in a given tissue and the efficiency of the magnetization exchange

MT characteristically:

Reduces the SI of some solid like tissues, such as most of the brain and spinal cord

Does not influence liquid like tissues significantly, such as the cerebrospinal fluid (CSF)

MT Effect

CLINICAL APPLICATION

• Useful diagnostic tool in characterization of a variety of CNS infection

• In detection and diagnosis of meningitis , encephalitis, CNS tuberculosis ,

neurocysticercosis and brain abscess.

TUBERCULOMA

• Pre-contrast T1-W MT imaging helps to better assess the disease load in CNS

tuberculosis by improving the detectability of the lesions, with more number

of tuberculomas detected on pre-contrast MT images compared to routine SE

images

• It may also be possible to differentiate T2 hypo intense tuberculoma from T2

hypo intense cysticerus granuloma with the use of MTR, as cysticercus

granulomas show significantly higher MT ratio compared to tuberculomas

T1 T2

MTPCMT

NEUROCYSTICERCOSISFindings vary with the stage of disease

T1-W MT images are also important in demonstrating perilesional gliosis in

treated neurocysticercus lesions

Gliotic areas show low MTR compared to the gray matter and white matter.

So appear as hyperintense

BRAIN ABSCESS

Lower MTR from tubercular abscess wall in comparison to wall of

pyogenic abscess(~20 vs. ~26)

Magnetization transfer (MT) contd:-

Qualitative applications: MR angiography, postcontrast studies spine imaging MT pulses have a greater influence on brain tissue (d/t high conc.

of structured macromolecules such as cholesterol and lipid) than

on stationary blood. By reducing the background signal vessel-to-brain contrast is

accentuated, Not helpful when MR angiography is used for the detection and

characterization of cerebral aneurysms.

GRE images of the cervical spine without (A) and with (B) MT show improved CSF–spinal cord contrast

Magnetization transfer (MT) contd:- Quantitative applications:

Multiple sclerosis: discriminates multiple sclerosis & other demyelinating disorders, provides measure of total lesion load,

assess the spinal cord lesion burden and to monitor the response to different treatments of multiple sclerosis systemic lupus erythematosus, CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leuko encephalopathy), Multiple system atrophy, Amyotrophic lateral sclerosis, Schizophrenia Alzheimer’s disease

MTR Quantitative applications contd: May be used to differentiate between progressive multifocal leukoencephalopathy

and HIV encephalitis To detect axonal injury in normal appearing splenium of corpus callosum after head

trauma In chronic liver failure, diffuse MTR abnormalities have been found in normal

appearing brain, which return to normal following liver transplantation

Thank you