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MAGNETIC RESONANCEMAGNETIC RESONANCE
IMAGINGIMAGING
Dr.Shahzad Ahmad DaulaDr.Shahzad Ahmad Daula
MID,DMLT,DDC
The Universi...
MRIPRINCIPLEMRIPRINCIPLE
 MRI isbased on theprincipleof nuclear magnetic resonance.MRI isbased on theprincipleof nuclear ...
WHY HYDROGEN IONS ARE USEDWHY HYDROGEN IONS ARE USED
IN MRI?IN MRI?
an unpaired proton which ispositively charged
Every ...
BODY IN AN EXTERNALBODY IN AN EXTERNAL
MAGNETIC FIELD (MAGNETIC FIELD (BB00))
•In our natural stateIn our natural state Hy...
NETMAGNETIZATION
 Half of the protons align along the magnetic field and restHalf of the protons align along the magnetic...
MANIPULATING THENETMANIPULATING THENET
MAGNETIZATIONMAGNETIZATION
 Manipulated by changing themagnetic fieldManipulated b...
T1 ANDT2 RELAXATION
 When RFpulse is stopped higherenergy gained byWhen RFpulse is stopped higherenergy gained by
proton ...
T1 RELAXATIONT1 RELAXATION
After protonsare
Excited with RF pulse
They moveout of
Alignment with B0
But oncetheRF Pulse
is...
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 measuredTE (echo time) : time interval in which signals ar...
Different tissues have differentDifferent tissues have different
relaxation times. These relaxationrelaxation times. These...
TYPES OF MRI
IMAGINGS
 T1WIT1WI
 T2WIT2WI
 FLAIRFLAIR
 STIRSTIR
 DWIDWI
 ADCADC
 GREGRE
 MRAMRA
 MRVMRV
 MRSMRS
...
T1 &T2 W IMAGING
GRADATION OF INTENSITYGRADATION OF INTENSITY
IMAGING
CT SCAN CSF Edema White
Matter
Gray
Matter
Blood Bone
MRI T1 CSF Edem...
CT SCAN
MRI T1 Weighted
MRI T2 Weighted
MRI T2 Flair
DARK ON T1DARK ON T1
Edema,tumor,infection,inflammation,hemorrhageEdema,tumor,infection,inflammation,hemorrhage
Low prot...
BRIGHT ON T1BRIGHT ON T1
 Fat,subacute hemorrhage,melanin,protein rich fluid.
 Slowly flowing blood
 Paramagnetic
subst...
BRIGHT ON T2
 Edema,tumor,infection,inflammation,subduralEdema,tumor,infection,inflammation,subdural
collectioncollection...
DARK ON T2DARK ON T2
 Low proton density,calcification,fibrous tissueLow proton density,calcification,fibrous tissue
 Pa...
WHICHSCAN BESTDEFINES THEWHICHSCAN BESTDEFINES THE
ABNORMALITYABNORMALITY
T1 WImages:T1 WImages:
SubacuteHemorrhageSubacut...
FLAIR & STIRFLAIR & STIR
CONVENTIONALICNVERSION
RECOVERY
- 180° preparatory pulse180° preparatory pulseis applied to flip the net magnetizationis a...
Contd:Contd:
 At TI, thenet magnetization vector of water isvery weak, whereasthat
for body tissuesisstrong. When thenet ...
SHORT TI INVERSION-RECOVERY
(STIR)
 an inversion-recovery pulseisused to nullan inversion-recovery pulseisused to null th...
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 ...
 Most pathologic processesshow increased SI on T2-WI, and the
conspicuity of lesionsthat arelocated closeto interfacesb/w...
 In addition to T2- weightening, FLAIR possesses
considerableT1-weighting, becauseit largely dependson
longitudinal magne...
Clinical Applications:
 Used to evaluate diseases affecting the brain parenchyma neighboring
the CSF-containing spaces fo...
 Mesial temporal sclerosis: m/c pathology in patients with partial
complex seizures.Thin-section coronal FLAIR is the sta...
 Embolic infarcts- Improved visualization
 Chronic infarctions- typically dark with a rim of high signal. Bright periphe...
T2 W
FLAIR
Subarachnoid Hemorrhage (SAH):
 FLAIR imaging surpasseseven CT in thedetection of traumatic supratentorial SAH.
 It hasb...
FLAIR
FLAIR
DWI & ADC
DIFFUSION-WEIGHTEDMRI
 Diffusion-weighted MRI is a example of endogenous contrast, using
the motion of protons to produce...
• The normal motion of water molecules within living tissues is
random (brownian motion).
• In acute stroke, there is an a...
descriptio
n
T1 T2 FLAIR DWI ADC
White
matter
high low intermediat
e
low low
Grey
matter
intermediat
e
intermediat
e
high ...
 DW images usually performed with echo-planar sequences which
markedly decrease imaging time, motion artifacts and increa...
 The increased sensitivity of diffusion-weighted MRI in
detecting acute ischemia is thought to be the result of the
water...
• Core of infarct = irreversible damage
• Surrounding ischemic area  may be salvaged
• DWI: open a window of opportunity ...
 Ischemic Stroke
 Extra axial masses: arachnoid cyst versus epidermoid tumor
 Intracranial Infections
Pyogenic infectio...
APPARENT DIFFUSIONAPPARENT DIFFUSION
COEFFICIENTCOEFFICIENT
 It is a measure of diffusion
 Calculated by acquiring two o...
 The ADC may be useful for estimating the lesion age and
distinguishing acute from subacute DWI lesions.
 Acute ischemic...
NONISCHEMIC CAUSES FOR
DECREASED ADC
 Abscess
 Lymphoma and other tumors
 Multiple sclerosis
 Seizures
 Metabolic (Ca...
65 year male- Rt ACA Infarct
EVALUATION OF ACUTE STROKE ONEVALUATION OF ACUTE STROKE ON
DWIDWI
 The DWI and ADC maps show changes in ischemic brain
wi...
DW MR imaging characteristics of Various Disease Entities
MR Signal Intensity
Disease DW Image ADC Image ADC Cause
Acute S...
CLINICAL USES OF DWI & ADCCLINICAL USES OF DWI & ADC
Stroke:
 Hyperacute Stage:- within one hour minimal hyperintensity s...
GRE
GRE
 In a GRE sequence, an RF pulse is applied that partly
flipsthe NMV into the transverse plane (variableflip
angle).
...
GRE Sequences contd:
 This feature of GRE sequences is exploited- in detection of
hemorrhage, as the iron in Hb becomesma...
GREFLAIR
Hemorrhage in right parietal lobe
GRE Sequences contd:
Magnetic susceptibility imaging-
 - Basis of cerebral perfusionstudies, in which the T2* effects (ie...
GRADIENT ECHO
Pros:
 fast technique
Cons:
 More sensitive to magnetic susceptibility artifacts
 Clinical use:
 eg. Hem...
Axial T1 (C), T2 (D), and GRE (E) images show corresponding T1-hyperintense and GRE-
hypointense foci with associated T2 h...
MRS & MT-MRI
MR SPECTROSCOPY
 Magnetic resonance spectroscopy (MRS) is a means of
noninvasive physiologic imaging of the brain that
me...
PRINCIPLES:
 The radiation produced by any substance is dependent on its atomic
composition.
 Spectroscopy is the determ...
 If energy is applied to the system in the form of a radiofrequency
(RF) pulse that exactly matches the energy between bo...
TECHNIQUE:
Single volume and Multivolume MRS.
1) Single volume:
 Stimulated echo acquisition mode (STEAM)
 Point-resolve...
OBSERVABLE METABOLITES
Metabolite Location
ppm
Normal function Increased
Lipids 0.9 & 1.3 Cell membrane
component
Hypoxia,...
PRINCIPLE METABOLITESMetabolite Location
ppm
Normal
function
Increased Decreased
NAA 2 Nonspecific
neuronal
marker
(Refere...
Metabolite Location
ppm
Normal
function
Increased Decreased
Choline 3.2 Marker of
cell memb
turnover
Neoplasia,
demyelinat...
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/Cr
Inc acetate,
succinate,
amino acid,
lactate
Neuodegene
rative
Alzheimer
Dec
NAA/Cr
Dec NAA/
Cho
Inc
Myo/NAA...
CLINICAL APPLICATIONS OF
MRS:
 Class A MRS Applications: Useful in Individual Patients
1) MRS of brain masses:
 Distingu...
CLASS B MRS APPLICATIONS: OCCASIONALLY
USEFUL IN INDIVIDUAL PATIENTS
1) Ischemia, Hypoxia, and Related Brain Injuries
 Is...
MAGNETIZATION TRANSFER (MT) MRI
 MT is a recently developed MR technique that alters contrast of tissue on
the basis of m...
Magnetization transfer (MT) contd:-
 Basis of the technique: that the state of magnetization of an atomic nucleus can be
...
Magnetization transfer (MT) contd:-
 These off-resonance pulses, which may be added to standard MR pulse
sequences, reduc...
Magnetization transfer (MT) contd:-
 The MT effect is superimposed on the intrinsic contrast of the baseline image
 Amou...
MT Effect
CLINICAL APPLICATION
• Useful diagnostic tool in characterization of a variety of CNS infection
• In detection and diagnos...
T1 T2
MT
PC
MT
NEUROCYSTICERCOSIS
Findings vary with the stage of disease
 T1-W MT images are also important in demonstrating perilesion...
Magnetization transfer (MT) contd:-
Qualitative applications:
 MR angiography,
 postcontrast studies
 spine imaging
 M...
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 & ot...
MTR Quantitative applications contd:
 May be used to differentiate between progressive multifocal leukoencephalopathy
and...
Thank you
Magnetic resonance imaging
Magnetic resonance imaging
Magnetic resonance imaging
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Magnetic resonance imaging

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Magnetic resonance imaging

  1. 1. MAGNETIC RESONANCEMAGNETIC RESONANCE IMAGINGIMAGING Dr.Shahzad Ahmad DaulaDr.Shahzad Ahmad Daula MID,DMLT,DDC The University of LahoreThe University of Lahore
  2. 2. MRIPRINCIPLEMRIPRINCIPLE  MRI isbased on theprincipleof nuclear magnetic resonance.MRI isbased on theprincipleof nuclear magnetic resonance.  Two basic principlesof NMRTwo basic principlesof NMR 1.1. Atomswith an odd number of protonsor neutronshavespinAtomswith an odd number of protonsor neutronshavespin 2.2. A moving electric charge, beit positiveor negative, producesamagnetic fieldA moving electric charge, beit positiveor negative, producesamagnetic field  Body hasmany such atomsthat can act asgood MR nucleiBody hasmany such atomsthat can act asgood MR nuclei ((11 H,H, 1313 C,C, 1919 F,F, 2323 Na)Na)  Hydrogen nuclei isnot only positively charged, but also hasHydrogen nuclei isnot only positively charged, but also has magnetic spinmagnetic spin  MRI utilizesthismagnetic spin property of protonsof hydrogen toMRI utilizesthismagnetic spin property of protonsof hydrogen to elicit imageselicit images
  3. 3. WHY HYDROGEN IONS ARE USEDWHY HYDROGEN IONS ARE USED IN MRI?IN MRI? an unpaired proton which ispositively charged Every hydrogen nucleusisatiny magnet which producessmall but noticeablemagnetic field. Hydrogen atom istheonly major speciesin the body that isMR sensitive Abundant in thebody in theform of water and fat MRI ishydrogen (proton) imaging
  4. 4. BODY IN AN EXTERNALBODY IN AN EXTERNAL MAGNETIC FIELD (MAGNETIC FIELD (BB00)) •In our natural stateIn our natural state Hydrogen ions in bodyHydrogen ions in body are spinning in a haphazard fashion, andare spinning in a haphazard fashion, and cancel all the magnetism.cancel all the magnetism. •When an external magnetic field is appliedWhen an external magnetic field is applied protons in the body align in one direction.protons in the body align in one direction.
  5. 5. NETMAGNETIZATION  Half of the protons align along the magnetic field and restHalf of the protons align along the magnetic field and rest are aligned opposit.are aligned opposit.  At room temperature, theAt room temperature, the population ratio of anti-population ratio of anti- parallel versus parallelparallel versus parallel protons is roughly 100,000protons is roughly 100,000 to 100,006 per Tesla ofto 100,006 per Tesla of BB00  These extra protons produce net magnetization vector (M)These extra protons produce net magnetization vector (M)
  6. 6. MANIPULATING THENETMANIPULATING THENET MAGNETIZATIONMAGNETIZATION  Manipulated by changing themagnetic fieldManipulated by changing themagnetic field environment (static, gradient, and RF fields)environment (static, gradient, and RF fields)  RF wavesareused to manipulatethemagnetization ofRF wavesareused to manipulatethemagnetization of H nucleiH nuclei  Externally applied RF wavesperturb magnetizationExternally applied RF wavesperturb magnetization into different axis(transverseaxis). Only transverseinto different axis(transverseaxis). Only transverse magnetization producessignal.magnetization producessignal.  When perturbed nuclei return to their original stateWhen perturbed nuclei return to their original state they emit RF signalswhich can bedetected with thethey emit RF signalswhich can bedetected with the help of receiving coilshelp of receiving coils
  7. 7. T1 ANDT2 RELAXATION  When RFpulse is stopped higherenergy gained byWhen RFpulse is stopped higherenergy gained by proton is retransmitted and hydrogen nuclei relax byproton is retransmitted and hydrogen nuclei relax by two mechanismstwo mechanisms  T1 orspin lattice relaxation- by which originalT1 orspin lattice relaxation- by which original magnetization begins to recover.magnetization begins to recover.  T2 relaxation orspin spin relaxation - by whichT2 relaxation orspin spin relaxation - by which magnetization in X-Y plane decays towards zero in anmagnetization in X-Y plane decays towards zero in an exponential fashion. It is due to incoherence of Hnuclei.exponential fashion. It is due to incoherence of Hnuclei.  T2 values of CNS tissues are shorterthan T1 valuesT2 values of CNS tissues are shorterthan T1 values
  8. 8. T1 RELAXATIONT1 RELAXATION After protonsare Excited with RF pulse They moveout of Alignment with B0 But oncetheRF Pulse isstopped they Realign after someTimeAnd thisiscalled t1 relaxation T1 is defined as the time it takes for the hydrogen nucleus to recover 63% of itslongitudinal magnetization
  9. 9. T2 relaxation time is the time for 63% of the protons to become dephased owing to interactions among nearby protons.
  10. 10. TR AND TE  TE (echo time) : time interval in which signals are measuredTE (echo time) : time interval in which signals are measured after RF excitationafter RF excitation  TR (repetition time) : the time between two excitations isTR (repetition time) : the time between two excitations is called repetition timecalled repetition time  By varying theTR and TE onecan obtain T1WI and T2WIBy varying theTR and TE onecan obtain T1WI and T2WI  In general ashort TR (<1000ms) and short TE (<45 ms) scan isIn general ashort TR (<1000ms) and short TE (<45 ms) scan is T1WIT1WI  Long TR (>2000ms) and long TE (>45ms) scan isT2WILong TR (>2000ms) and long TE (>45ms) scan isT2WI  Long TR (>2000ms) and short TE (<45ms) scan is protonLong TR (>2000ms) and short TE (<45ms) scan is proton density imagedensity image
  11. 11. Different tissues have differentDifferent tissues have different relaxation times. These relaxationrelaxation times. These relaxation time differences is used to generatetime differences is used to generate image contrast.image contrast.
  12. 12. TYPES OF MRI IMAGINGS  T1WIT1WI  T2WIT2WI  FLAIRFLAIR  STIRSTIR  DWIDWI  ADCADC  GREGRE  MRAMRA  MRVMRV  MRSMRS  MTMT  Post-Gd imagesPost-Gd images
  13. 13. T1 &T2 W IMAGING
  14. 14. GRADATION OF INTENSITYGRADATION 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
  15. 15. CT SCAN MRI T1 Weighted MRI T2 Weighted MRI T2 Flair
  16. 16. DARK ON T1DARK ON T1 Edema,tumor,infection,inflammation,hemorrhageEdema,tumor,infection,inflammation,hemorrhage Low proton density ,calcificationLow proton density ,calcification Flow voidFlow void
  17. 17. BRIGHT ON T1BRIGHT ON T1  Fat,subacute hemorrhage,melanin,protein rich fluid.  Slowly flowing blood  Paramagnetic substances(gadolinium,copper,manganese)  9
  18. 18. BRIGHT ON T2  Edema,tumor,infection,inflammation,subduralEdema,tumor,infection,inflammation,subdural collectioncollection  Methemoglobin in late subacute hemorrhageMethemoglobin in late subacute hemorrhage
  19. 19. 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
  20. 20. WHICHSCAN BESTDEFINES THEWHICHSCAN BESTDEFINES THE ABNORMALITYABNORMALITY T1 WImages:T1 WImages: SubacuteHemorrhageSubacuteHemorrhage Fat-containing structuresFat-containing structures Anatomical DetailsAnatomical Details T2 WImages:T2 WImages: EdemaEdema DemyelinationDemyelination InfarctionInfarction Chronic HemorrhageChronic Hemorrhage FLAIRImages:FLAIRImages: Edema,Edema, DemyelinationDemyelination Infarction esp. in Periventricular locationInfarction esp. in Periventricular location
  21. 21. FLAIR & STIRFLAIR & STIR
  22. 22. CONVENTIONALICNVERSION RECOVERY - 180° preparatory pulse180° preparatory pulseis applied to flip the net magnetizationis applied to flip the net magnetization vector 180° andvector 180° andnull the signal from a particular entitynull 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 theWhen the net magnetization vector for water passes the transversetransverseplane (the null point for that tissue), theplane (the null point for that tissue), the conventional 90°conventional 90°pulse is applied, and the SE sequence thenpulse 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 thepulse is the TI ( Inversion Time).TI ( Inversion Time).
  23. 23. Contd:Contd:  At TI, thenet magnetization vector of water isvery weak, whereasthat for body tissuesisstrong. When thenet magnetization vectorsare flipped by the90° pulse, thereislittleor no transversemagnetization in water, so no signal isgenerated (fluid appearsdark), whereassignal intensity rangesfrom low to high in tissueswith astronger NMV.  Two important clinical implementationsof theinversion recovery concept are:  Short TIinversion-recovery (STIR) sequence  Fluid-attenuated inversion-recovery (FLAIR) sequence.
  24. 24. SHORT TI INVERSION-RECOVERY (STIR)  an inversion-recovery pulseisused to nullan inversion-recovery pulseisused to null thesignal from fat (180° RFthesignal from fat (180° RF Pulse).Pulse).  When NMVWhen NMV of fat passesitsnull point , 90° RF pulseisapplied. Asof fat passesitsnull point , 90° RF pulseisapplied. As littleor no longitudinallittleor no longitudinal magnetization ispresent and thetransversemagnetization ispresent and thetransverse magnetizationmagnetizationisinsignificant.isinsignificant.  It istransversemagnetization thatIt istransversemagnetization that inducesan electric current in theinducesan electric current in the receiver coil so no signal isgenerated from fat.receiver coil so no signal isgenerated from fat.  STIRSTIRsequencesprovideexcellent depiction of bonemarrow edemasequencesprovideexcellent depiction of bonemarrow edema which may betheonly indication of an occult fracture.which may betheonly indication of an occult fracture.  UnlikeUnlikeconventional fat-saturation sequencesSTIRconventional fat-saturation sequencesSTIRsequencesarenotsequencesarenot affected by magnetic field inhomogeneities,affected by magnetic field inhomogeneities,so they aremoreefficientso they aremoreefficient for nulling thesignal from fatfor nulling thesignal from fat
  25. 25. Comparison of fast SE and STIR sequences for depiction of bone marrow edema FSE STIR
  26. 26. 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.
  27. 27.  Most pathologic processesshow increased SI on T2-WI, and the conspicuity of lesionsthat arelocated closeto interfacesb/w brain parenchymaand CSF may bepoor in conventional SE or FSE T2-WI sequences.  FLAIR imagesareheavily T2-weighted with CSF signal suppression, highlightshyperintenselesionsand improvestheir conspicuity and detection, especially when located adjacent to CSF containing spaces
  28. 28.  In addition to T2- weightening, FLAIR possesses considerableT1-weighting, becauseit largely dependson longitudinal magnetization  Assmall differencesin T1 characteristicsareaccentuated, mild T1-shortening becomesconspicuous.  Thiseffect isprominent in theCSF-containing spaces, whereincreased protein content resultsin high SI (eg, associated with sub-arachnoid spacedisease)  High SI of hyperacuteSAH iscaused by T2 prolongation in addition to T1 shortening
  29. 29. 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.
  30. 30.  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
  31. 31.  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.
  32. 32. T2 W FLAIR
  33. 33. Subarachnoid Hemorrhage (SAH):  FLAIR imaging surpasseseven CT in thedetection of traumatic supratentorial SAH.  It hasbeen proposed that MR imaging with FLAIR, gradient-echo T2*-weighted, and rapid high-spatial resolution MR angiography could beused to evaluatepatients with suspected acuteSAH, possibly obviating theneed for CT and intra-arterial angiography.  With theavailability of high-quality CT angiography, thisapproach may not be necessary.
  34. 34. FLAIR FLAIR
  35. 35. DWI & ADC
  36. 36. DIFFUSION-WEIGHTEDMRI  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
  37. 37. • 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
  38. 38. descriptio n T1 T2 FLAIR DWI ADC White matter high low intermediat e low low Grey matter intermediat e intermediat e high intermediat e intermediat e CSF low high low low high
  39. 39.  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
  40. 40.  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
  41. 41. • 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).
  42. 42.  Ischemic Stroke  Extra axial masses: arachnoid cyst versus epidermoid tumor  Intracranial Infections Pyogenic infection Herpes encephalitis Creutzfeldt-Jakob disease  Trauma  Demyelination
  43. 43. APPARENT DIFFUSIONAPPARENT 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
  44. 44.  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
  45. 45. NONISCHEMIC CAUSES FOR DECREASED ADC  Abscess  Lymphoma and other tumors  Multiple sclerosis  Seizures  Metabolic (Canavans )
  46. 46. 65 year male- Rt ACA Infarct
  47. 47. EVALUATION OF ACUTE STROKE ONEVALUATION 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
  48. 48. 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
  49. 49. CLINICAL USES OF DWI & ADCCLINICAL USES OF DWI & ADC Stroke:  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)
  50. 50. GRE
  51. 51. GRE  In a GRE sequence, an RF pulse is applied that partly flipsthe NMV into the transverse plane (variableflip angle).  Gradients, as opposed to RF pulses, are usedto dephase (negative gradient) and rephase (positive gradients) transverse magnetization.  Because gradients donot refocus field inhomogeneities, GRE sequences with long TEsare T2* weighted (because of magnetic susceptibility) ratherthan T2 weighted like SE sequences
  52. 52. GRE Sequences contd:  This feature of GRE sequences is exploited- in detection of hemorrhage, as the iron in Hb becomesmagnetized locally (produces its own local magnetic field) andthus dephases the spinning nuclei.  The technique is particularlyhelpful for diagnosing hemorrhagic contusions such as thosein the brain and in pigmented villonodular synovitis.  SE sequences, on the other hand- relativelyimmune from magnetic susceptibility artifacts, and also lesssensitive in depicting hemorrhage and calcification.
  53. 53. GREFLAIR Hemorrhage in right parietal lobe
  54. 54. GRE Sequences contd: Magnetic susceptibility imaging-  - Basis of cerebral perfusionstudies, in which the T2* effects (ie, signal decrease) createdby gadolinium (a metal injected intravenously as a chelatedion in aqueous solution, typically in the form of gadopentetatedimeglumine) are sensitively depicted by GRE sequences.  - Also used in blood oxygenationlevel–dependent (BOLD) imaging, in which the relativeamount of deoxyhemoglobin in the cerebral vasculature is measuredas a reflection of neuronal activity. BOLD MR imaging is widelyused for mapping of human brain function.
  55. 55. GRADIENT ECHO Pros:  fast technique Cons:  More sensitive to magnetic susceptibility artifacts  Clinical use:  eg. Hemorrhage , calcification
  56. 56. Axial T1 (C), T2 (D), and GRE (E) images show corresponding T1-hyperintense and GRE- hypointense foci with associated T2 hyperintensity (arrows).
  57. 57. MRS & MT-MRI
  58. 58. 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.
  59. 59. 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).
  60. 60.  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.
  61. 61. 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.
  62. 62. OBSERVABLE METABOLITES Metabolite Location ppm Normal function Increased Lipids 0.9 & 1.3 Cell membrane component Hypoxia, trauma, high grade neoplasia. Lactate 1.3 TE=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,
  63. 63. 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
  64. 64. Metabolite Location ppm Normal function Increased Decreased Choline 3.2 Marker of cell memb turnover Neoplasia, demyelination (MS) Hypomyelinat ion Myoinositol 3.5 & 4 Astrocyte marker AD Demyelinatin g diseases
  65. 65. 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
  66. 66. MRS Dec NAA/Cr Inc acetate, succinate, amino acid, lactate Neuodegene rative Alzheimer Dec NAA/Cr Dec NAA/ Cho Inc Myo/NAA Slightly inc Cho/ Cr Cho/NAA Normal Myo/NAA ± lipid/lactate Inc Cho/Cr Myo/NAA Cho/NAA Dec NAA/Cr ± lipid/lactate Malignancy Demyelinatin g disease Pyogenic abscess
  67. 67. 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
  68. 68. 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
  69. 69. 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.
  70. 70. 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.
  71. 71. 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.
  72. 72. 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)
  73. 73. MT Effect
  74. 74. 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
  75. 75. T1 T2 MT PC MT
  76. 76. NEUROCYSTICERCOSIS Findings 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)
  77. 77. 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.
  78. 78. GRE images of the cervical spine without (A) and with (B) MT show improved CSF–spinal cord contrast
  79. 79. 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
  80. 80. 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
  81. 81. Thank you

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