MRI BRAIN MRI BRAIN INTERPRETATION(BASICS)INTERPRETATION(BASICS)

By:Dr.Chander Bafna2nd year PG ResidentDeptt. Of General MedicineRNT Medical college & Associated Hospitals,Udaipur

disclaimerdisclaimerThis presentation’s sole purpose is to

get an elementary idea to interpret mri brain images bedside without going into its intricate technical aspects & physics & keeping it very simple..

Not meant for radio diagnosis people who need to deal with its physics & other aspects extensively..

MRI principleMRI principle MRI is based on the principle of nuclear magnetic

resonance (NMR) Two basic principles of NMR1. 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 Why Hydrogen ions are used in MRI? MRI?

Hydrogen nucleus has an unpaired proton(paramagnetic) 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 Body in an external magnetic field (magnetic field (BB00))

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

•When an external magnetic field is applied protons When an external magnetic field is applied protons in the body align in one direction. (As the compass in the body align in one direction. (As the compass aligns in the presence of earth’s aligns in the presence of earth’s magnetic field)magnetic field)

Net magnetizationNet magnetization

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

.At room temperature, the population ratio of anti- 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 Manipulating the net magnetizationmagnetization

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

When a patient is placed in the magnet the hydrogen atoms in the water of their body tissues line up along the magnetic field.Radiofrequency pulses are sent in, causing the atoms to ‘flip’ into another plane and then ‘relax’ back when the pulse is turned off. This recovery process is known as relaxation. This relaxation time varies from one type of tissue to another.

Each tissue is characterized by two relaxations times: T1 (longitudinal relaxation time) and T2 (transverse relaxation time).

Most of the images are created by one of these two characteristics being the predominant source of contrast.  This means when an image is described as a T1-weighted image T1 is the main source of contrast.

T1 and T2 relaxationT1 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

TR and TETR 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.

Very imp points to Very imp points to remember for interpretationremember for interpretationIf relaxation time is more Image is T2

weightedIf relaxation time is less image is T1

Weighted

More water more protons which have increased cumulative relaxation times T2 weighted

Less water less protons decreased cumulative relaxation time T1 weighted

INTENSITY IN MRIINTENSITY IN MRIIn contrast to the Hounsfield unit scale

for CT there is no standard normalization of MR signal intensity that is in common use, and thus there is no absolute reference scale with which to quantify lesion intensity. Intensity is interpreted only through direct visual comparison with surrounding tissues.

Hypointense : less bright than the tissue of reference

Isointense:equally bright as the tissue of reference

Hyperintense:brighter than the tissue of reference

T2* weightingT2* takes into account local

magnetic field homogeneities and is like a T2-weighted sequence but with extra dephasing effects. It is specifically sensitive to the presence of iron in the brain parenchyma and is therefore good for estimating the age of blood products.

Tissues differ in their T1 values, and therefore T1-weighted sequences are good for assessing regional anatomy.

T2 is affected by pathophysiological changes more easily than T1, and is therefore a good sequence for assessing pathology

Types of MRI imagingsTypes of MRI imagings

T1WIT1WI T2WIT2WI FLAIRFLAIR STIRSTIR DWIDWI ADCADC GREGRE MRSMRS MTMT Post-Gd imagesPost-Gd images

MRAMRA MRVMRV

MRI image appearance

The easiest way to identify T1 weighted images is to look for fluid filled spaces in the body (e.g. Cerebrospinal fluid in the brain ventricles and spinal canal, free fluid in the abdomen, fluid in the gall bladder and common bile duct, synovial fluid in joints, fluid in the urinary tract and urinary bladder, oedema or any other pathological fluid collection in the body).

Fluids normally appear dark in a T1 weighted image.   

Tissues and their T1 appearance

Bone marrow : - equal to or higher than that of muscle (fatty marrow is usually bright)Muscles- grayMoving blood : - dark White matter : - whiter Gray matter : - gray Fluids : - dark Bone : - dark  Fat : - bright Air : - dark  

Pathological appearance

Pathological processes normally increase the water content in tissues.

Due to the added water component this results in a signal loss on T1 weighted images and signal increase on T2 weighted images.

Consequently pathological processes are usually bright on T2 weighted images and dark on T1 weighted images.

bright

T2 image characteristics

When an MRI sequence is set to produce a T2-weighted image, it is the tissues with long T2 values that produces the highest magnetization and appear brightest on the image.

A T2-weighted sequence produces T2 contrast mainly by de-emphasizing the T1 contributions. This is normally achieved by using a long repetition time TR (2000-6000ms) and a long echo time TE (100-150ms).

MRI image appearance

The easiest way to identify T2 weighted images is to look for fluid filled spaces in the body (e.g. Cerebrospinal fluid in the brain ventricles and spinal canal, free fluid in the abdomen, fluid in the gall bladder and common bile duct, synovial fluid in joints, fluid in the urinary tract and urinary bladder, oedema or any other pathological fluid collection in the body).

Fluids normally appear bright on T2 weighted images.  

Bone marrow: - equal to or higher than that of muscle (fatty marrow is usually bright)

Muscles- gray (darker than the muscle signal on T1 images)

Fat – bright (darker than the fat signal on T1 images)

White matter - darker than gray Moving blood- dark Gray matter - gray Fluids – bright  Bone - dark  Air - dark  

MRI image appearance

The easiest way to identify T1 weighted post gadolinium images is to look for blood vessels in the body (e.g arteries and veins in the brain, neck, chest, abdomen, upper limbs and lower limbs).

Blood vessels and pathologies with high vascularity appear bright on T1 weighted post gadolinium images.

Pathological appearance

Pathologies with hypervascularization will appear bright on T1 weighted post gadolinium images (e.g. tumours like hemangioma, Lymphangioma, hemangioendothelioma, Kaposi sarcoma, angiosarcoma, hemangioblastoma etc. and inflammatory processes like discitis, meningitis, synovitis, arthritis, osteomyelitis etc.)

Pathological processes with no vascularity will remain unenhanced (appear as dark on a T1 weighted post gadolinium image).

This sequence is commonly used in the brain and spinal cord where the lesions are normally covered by bright cerebrospinal fluid (CSF) signals.

A long inversion time suppresses the high CSF signal and improve the visualization of small periventricular and spinal cord lesions.

MRI image appearance

The easiest way to identify FLAIR images is to look for CSF filled spaces and lesions or other pathological processes in the brain or spinal cord.

Fluids normally appear dark and lesions or other pathological processes appear bright on image.

Images normally appear as a fluid suppressed T2 image

FLAIR axial sequence used in brain imaging

Diffusion-weighted magnetic resonance imaging (DW MRI) provides image contrast that is dependent on the random microscopic motion of water protons, which may be substantially altered by different pathological process.

Diffusion-weighted magnetic resonance imaging uses a T2-weighted pulse sequence with two extra gradient pulses which is equal in magnitude and opposite in direction.

As a result, imaging is more sensitive to water molecular motion in the direction of the additional applied gradients.

Higher water diffusion will result in greater signal attenuation and appears darker on the image.

For example, in brain scans regions of ischemic infarction are characterized as reduced water diffusion areas due to reduced blood flow.

Therefore, those areas appear hyperintense (bright) on diffusion-weighted images.

The apparent diffusion coefficient (ADC) measures themagnitude of diffusion, and as such it confirms what isseen on DWI – if the ‘ischaemia’ demonstrated on DWI isnot secondary to an artefact known as T2 shine-through.

Thus, ADC and DWI work as positive and negative picturesof each other.

These techniques derive contrast between stationary tissues and flowing blood by manipulating the magnitude of the magnetization.

The magnitude of magnetization from the moving spins is very large as compared to the magnetization from the stationary spins which are relatively small.

MRI image appearance

The easiest way to identify TOF images is to look for blood vessels in the body (circle of willis and carotids).

Blood vessels usually appear bright on TOF image.

White matter - dark grayMoving blood- bright  Gray matter - dark grayFluids – dark Bone - dark Air - dark

Sagittal TOF sequence used in MRV brain imaging

These techniques derive contrast between stationary tissues and flowing blood by manipulating the phase of the magnetization.

Fast moving spins give rise to a larger signal and spins moving in one direction are assigned a bright signal and appear white in the scan , whereas spins moving in the opposite direction are assigned a dark signal and appear black on the scan.

Different velocity encoding values can be used in different scans to highlight different vessels.

High velocity encoding for arteries (40-70 cm/sec) due to arterial flow is fast.

Low velocity encoding for veins (10-20 cm/sec) due to venous flow is slow.

Phase contrast scans can be used for 2D or 3D imaging.

3D  Phase contrast( PC) sequence used in MRV brain  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

T2WT2W

T1WT1W

FLAIR SEQUENCEFLAIR SEQUENCE

Axial T2-weighted axial image demonstrating infarction dueto branch vessel occlusion in the left middle cerebral artery territory.

T2* image showing foci of haemorrhagic transformation

DWI sequence reveals restricted diffusion in the area of infarct.

cytotoxic oedema and neuronal cell death

Computed tomography (CT) retains a primary role in the evaluation of patients with acute stroke, but MRI, particularly DWI, is more sensitive and specific than CT for the detection of acute cerebral ischaemia.

ISCHAEMIC STROKE

Key sequences• T2-weighted and fluid-attenuated inversion recovery(FLAIR) axial• T2*-weighted gradient echo – to assess for haemorrhage• DWI• MRA (contrast-enhanced or TOF) of intra- and extracranialcirculation

Imaging findingsThere is a hyperintense signal in white matter on T2- weighted imaging and FLAIR within the first few hours.

Grey–white matter differentiation is lost and there is sulcaleffacement and a mass effect.

There is loss of arterial flow void, with major vessel occlusion (intravascular thrombus).

An area of abnormal blooming is seen with acute haemorrhage on T2*-weighted images.

Ischaemic tissue appears high signal on DWI, with corresponding low signal on the apparent diffusion coefficient (ADC) map image.

Time MRI Finding Etiology 2-3 min DWI - Reduced ADC Decreased motion of

protons2-3 min PWI - Reduced CBF,

CBV, MTTDecreased CBF

0-2 h T2-WI - Absent flow void signal

Slow flow or occlusion

0-2 h T1-WI - Arterial enhancement

Slow flow

2-4 h T1-WI - Subtle sulcal effacement

Cytotoxic edema

2-4 h T1-WI - Parenchymal enhancement

Incomplete infarction

8 h T2-WI - Hyperintense signal

Vasogenic and cytotoxic edema

16-24 h T1-WI - Hypointense signal

Vasogenic and cytotoxic edema

5-7 d Parenchymal enhancement

Complete infarction

MRI Findings in Transient MRI Findings in Transient Ischemic AttackIschemic Attack

A third to a half of the patients presenting with a TIA have lesions on DWI. A significant proportion of these patients may not reveal a corresponding lesion on T2-WI. PWI may be more sensitive but has not been adequately tested in patients with TIA.

Although TIAs have been traditionally defined as transient (< 24 h) neurologic deficits of vascular origin, the advent of MRI has led to reconsideration of the definition. Recent tissue definitions of stroke emphasize that TIA with persistent ischemia on diffusion-weighted imaging is classified as stroke rather than a TIA. TIA, on the other hand, requires either a normal diffusion-weighted image without evidence of ischemia or reversible changes on DWI suggestive of ischemia rather than infarction

63 year old with two episodes of visual field loss within four weeks

T2*-GRE image showing two haematomas of differentage in the left occipital lobe (arrow: late subacute; arrowhead: early subacute).

Key sequences• T2*-weighted-GRE axial as a screening sequence.• T1- and T2-weighted sequences to determine haemorrhageage.

INTRACRANIAL HEMORRHAGE(ICH)

Gradient EchoGradient Echo

Pros:fast technique

Cons: More sensitive to magnetic susceptibility

artifacts Clinical use:eg. Hemorrhage , calcification

GREFLAIR

Hemorrhage in right parietal lobe

T2*-GRE sequences feature similar signal changes as T2-weighted images but are more sensitive to susceptibility effects and require much shorter acquisition times.

Therefore, they are very useful in the context of emergency diagnosis of hyperacute ICH as well as chronic haemorrhages and microbleedings.

Post-contrast T1-weighted image showing a low-signal area in the right temporal lobe

T2-weighted axial image confirming extensive high-signalchange in the same territory.

HERPES ENCEPHALITIS

produces superficial grey matter disease, and pathological specimens show petechial haemorrhages and inflammation in affected areas

MRI is the most sensitive imaging modality, but findings may not be apparent in the early stages of infection.

Key sequences• T2- and T2*-weighted and FLAIR axial• T1-weighted post-contrast• DWI

Imaging findings

There is high T2 and FLAIR signal, which most often begins in the medial temporal and frontal lobes.

The lentiform nucleus is characteristically spared.

Progression to bilateral disease and parietal lobe involvementis typical.

Contrast enhancement in a cortical gyral pattern, particularlyaround the sylvian fissure, is characteristic.

There is marked hyperintensity on DWI

The absence of enhancement does not excludeencephalitis.

The differential diagnosis between acute ischemic strokeand herpes encephalitis can be problematic.

Clinical presentation(acute onset in ischaemic stroke, more progressivein herpes encephalitis) and biological tests (polymerasechain reaction for herpes simplex virus) are especiallyuseful to distinguish these conditions

T2-weighted magnetic resonance image of a biopsy-proven, right parietal tuberculoma. Note the low–signal-intensity rim of the lesion and the surrounding hyperintense vasogenic edema.

T1-weighted gadolinium-enhanced magnetic resonance image in a patient with multiple enhancing tuberculomas in both cerebellar hemispheres.

MRI is more sensitive than CT scanning in determining the extent of meningeal and parenchymal involvement.

In tuberculosis meningitis (TBM), gadolinium-enhanced T1-weighted images demonstrate prominent leptomeningeal and basal cistern enhancement

With ependymitis, linear periventricular enhancement is present. Ventricular dilatation due to hydrocephalus is usually seen. Deep gray-matter nuclei, deep white matter, and pontine infarctions resulting from vasculitis are hyperintense on T2-weighted images.

Diffusion-weighted MRI is especially sensitive in depicting early ischemic lesions when findings on the T2-weighted MRIs are norma

Noncaseating granulomas are homogeneously enhancing lesions. Caseating granulomas are rim enhancing.

Parenchymal tuberculomas

typically hypointense on T2-weighted images

Tuberculomas, like bacterial cerebral abscesses, have hypointense walls or rims on T2-weighted MRIs.

The differential diagnoses include fungal infections, bacterial infections, neurocysticercosis, and cerebral metastases.

DWI is a useful adjunct to help narrow the differentialdiagnosis. This shows an abscess as having markedly highDWI signal with corresponding low signal on the apparentdiffusion coefficient (ADC) map image.

Only acute ischaemic stroke classically has the same DWIand ADC signal characteristics as an abscess, but ringenhancement in acute stroke is unusual.

The differential diagnosis of a ring-enhancing intracranialmass lesion is wide and includes primary brain tumours(e.g. glioblastoma), metastases, resolving haematoma, infarctionand radiation necrosis.

Ependymal and meningeal enhancement per se can be anon-specific sign and is encountered in other conditionssuch as tumoursChemotherapyshunt placement and Post lumbar puncture.

Distinguishing features between glioma and abscess arethat an abscess rim is usually thin (2–7 mm) and smoothon its outer and inner aspects and an abscess contains fluidthat shows restricted diffusion on appropriate imaging.Clinical signs such as fever are also useful in helping distinguishthem.

Additional differential diagnoses include solitary metastasis– sometime differentiation is very difficult withoutbiopsy. Metastases have greater vasogenic oedema fortheir size, and of course may be multiple.

Which scan best defines the abnormalityWhich scan best defines the abnormality

T1 W Images:Subacute HemorrhageFat-containing structuresAnatomical Details

T2 W Images:EdemaDemyelinationInfarctionChronic Hemorrhage

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

Evaluation of acute stroke on DWIEvaluation 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

Dr. Chander Bafna