Proton SpinProton Spin In absence of a magnetic In absence of a magnetic

field, protons spin at field, protons spin at randomrandom

A magnetic field is used to A magnetic field is used to align them (Balign them (B00))

Current MRI fields run at Current MRI fields run at 0.5 and 3.0 Teslas0.5 and 3.0 Teslas

1 Tesla is equal to 10,000 1 Tesla is equal to 10,000 Gauss Gauss

Earth's magnetic field Earth's magnetic field varies from between 0.3 - varies from between 0.3 - 0.7 Gauss. 0.7 Gauss.

Refrigerator magnet Refrigerator magnet ~10 -100 Gauss~10 -100 Gauss

MR visible nucleiMR visible nuclei

Protons must be unbound to allow them to Protons must be unbound to allow them to spin and change spinsspin and change spins

Bound protons are not MR visible as they Bound protons are not MR visible as they don’t emit RF from energy changesdon’t emit RF from energy changes

Cortical bone contains bound H and thus Cortical bone contains bound H and thus always appears black on MRalways appears black on MR

Fat and Water contain lots of unbound H Fat and Water contain lots of unbound H and make soft tissue imaging so sensitive and make soft tissue imaging so sensitive to MRto MR

PrecessionPrecession In a magnetic field the In a magnetic field the

proton precesses at a proton precesses at a given frequency based on given frequency based on the Larmor equationthe Larmor equation

This frequency is based on This frequency is based on field strength and the field strength and the specific nucleus – each has specific nucleus – each has a different frequencya different frequency

In MRI the precession In MRI the precession frequencies occur in the RF frequencies occur in the RF range and are detected by range and are detected by an antennaan antenna

Pulse sequencePulse sequence

Tipping the spin into a different plane Tipping the spin into a different plane from Bfrom B00

BB1 1 is the field used to tip the spin is the field used to tip the spin measured in degrees from Bmeasured in degrees from B00

9090°° RF pulse tips it into the RF pulse tips it into the transverse plane or x’transverse plane or x’

180180°° RF pulse inverts it to a –z RF pulse inverts it to a –z directiondirection

Pulse SequencePulse Sequence

Tipping the spin into another plane puts Tipping the spin into another plane puts them into a high energy state once the them into a high energy state once the new field is removednew field is removed

Decay happens rapidly back to the original Decay happens rapidly back to the original state and the energy given off by the state and the energy given off by the decay is in the radiofrequency rangedecay is in the radiofrequency range

This energy is called the FID (Free This energy is called the FID (Free Induction Decay)Induction Decay)

This is received by an antennaThis is received by an antenna

Signal to Noise RatioSignal to Noise Ratio

Signal to Noise RatioSignal to Noise Ratio• The amount of useful signal compared The amount of useful signal compared

to useless noise from RFto useless noise from RF Coherent signal is additive while Coherent signal is additive while

incoherent noise tends to cancel outincoherent noise tends to cancel out SNR is proportional to the square SNR is proportional to the square

root of the number of averages.root of the number of averages.• Doubling the # of slices doubles time Doubling the # of slices doubles time

but improves SNR by 1.4but improves SNR by 1.4

T2T2

Having been tipped Having been tipped into the transverse into the transverse plane, the net plane, the net magnetisation magnetisation begins to dephase begins to dephase (T2*)(T2*)

Rapid decay of Rapid decay of unified spin based unified spin based on tissue and on tissue and magnetic fieldmagnetic field

T1T1

Once fully Once fully dephased the spins dephased the spins return to return to equilibrium (T1)equilibrium (T1)

Takes longer than Takes longer than T2 to reach T2 to reach equilibriumequilibrium

TE and TRTE and TR

TE = Echo TimeTE = Echo Time• Time from 90Time from 90° pulse to the time after ° pulse to the time after

the 180° that the echo from the tissue is the 180° that the echo from the tissue is detecteddetected

TR = Repetition time TR = Repetition time • Time between pulses (ie ms spacing Time between pulses (ie ms spacing

between 90between 90°°-180-180°°-180-180° pulses° pulses))

TE governs T2TE governs T2

Long TE allows for differences in T2 Long TE allows for differences in T2 times to emerge. Water, with a long times to emerge. Water, with a long T2 time takes longer to decay and T2 time takes longer to decay and thus gives a stronger signal. T2 thus gives a stronger signal. T2 weighted = H20 brightweighted = H20 bright

Using a long TE interval, tissues with Using a long TE interval, tissues with long TE becomes brighter. A short TE long TE becomes brighter. A short TE would make long TE tissues darkwould make long TE tissues dark

TR governs T1TR governs T1

Tissue with a long TR (like water) Tissue with a long TR (like water) takes a long time to recover to takes a long time to recover to equilibriumequilibrium

Using a short TR interval, tissues with Using a short TR interval, tissues with short TR (like fat) have given off short TR (like fat) have given off more signal (brighter) than a slower more signal (brighter) than a slower decaying tissuedecaying tissue

PD weightingPD weighting

Proton Density weightingProton Density weighting• Density of spins or number of protons in Density of spins or number of protons in

a tissuea tissue Short TE – decreases water signalShort TE – decreases water signal Long TR – decreases fat signalLong TR – decreases fat signal Suppression of fat and waterSuppression of fat and water Better anatomic detailBetter anatomic detail

MRI ContrastMRI Contrast

T2-weighting requires long TE, long T2-weighting requires long TE, long TRTR

T1-weighting requires short TE, short T1-weighting requires short TE, short TRTR

PD-weighting requires short TE, long PD-weighting requires short TE, long TRTR

Four MRI images: Same sliceFour MRI images: Same slice

Figure 1.Figure 1. Representative axial brain images of the eight patients with ACA infarctions. Patient numbers appear in the  Representative axial brain images of the eight patients with ACA infarctions. Patient numbers appear in the upper left corner of each panel. Patient 1, T1-weighted axial MRI; patient 2, CT scan; patient 3, T2-weighted MRI; patient upper left corner of each panel. Patient 1, T1-weighted axial MRI; patient 2, CT scan; patient 3, T2-weighted MRI; patient 4, T1-weighted MRI; patient 5, proton density MRI; patient 6, CT scan; patient 7, T2-weighted MRI; and patient 8, T2-4, T1-weighted MRI; patient 5, proton density MRI; patient 6, CT scan; patient 7, T2-weighted MRI; and patient 8, T2-weighted MRI. weighted MRI.

MRAMRA

Specific gradients are applied to Specific gradients are applied to phase and dephase the free H rapidlyphase and dephase the free H rapidly

H that is flowing into an area that H that is flowing into an area that was just dephased has not yet been was just dephased has not yet been dephased, thus producing high signaldephased, thus producing high signal

This is repeated rapidly and flow can This is repeated rapidly and flow can be monitoredbe monitored

Spin-Echo MRISpin-Echo MRI This “family” includes This “family” includes T1, T2 & Proton DensityT1, T2 & Proton Density RF pulses excite the protons to flip 90 degrees from the RF pulses excite the protons to flip 90 degrees from the

main magnetic field, then 180 degrees (to refocus the main magnetic field, then 180 degrees (to refocus the phase of precession). The first “echo” (RF from the phase of precession). The first “echo” (RF from the tissue) is obtained. This first echo provides the data for tissue) is obtained. This first echo provides the data for either T1 or proton density contrast. To obtain the data either T1 or proton density contrast. To obtain the data for T2 contrast, a second 180 degree pulse is then for T2 contrast, a second 180 degree pulse is then applied to obtain a second “echo” applied to obtain a second “echo”

This is termed the This is termed the 90-180-18090-180-180 pulse train. pulse train. The times between the RF excitations (The times between the RF excitations (TR & TETR & TE) vary ) vary

depending on the desired contrast. This produces various depending on the desired contrast. This produces various tissue signal intensity relationships which are referred to tissue signal intensity relationships which are referred to as “as “weightingweighting” characteristics” characteristics

Spin EchoSpin Echo

(Gp) phase encoding gradient (y plane)(Gp) phase encoding gradient (y plane) (Gf) frequency encoding during signal detection(Gf) frequency encoding during signal detection (Gs) slice-selection gradient during RF pulse to (Gs) slice-selection gradient during RF pulse to

spin up and refocus only the slice of interestspin up and refocus only the slice of interest

Inversion Recovery/ IRInversion Recovery/ IR

In this family the pulse In this family the pulse train is train is 180-90-180180-90-180

the time of inversion / the time of inversion / TITI (“tee eye”) = the (“tee eye”) = the time between the first time between the first 180 and the 90 pulse180 and the 90 pulse

TI times can be tailored TI times can be tailored by suppressing signal by suppressing signal from specific types of from specific types of tissue. Greater contrasttissue. Greater contrast

STIR/STIR/Short TI Inversion Recovery Short TI Inversion Recovery

A short TI value A short TI value suppresses the suppresses the signal from fat signal from fat and causes and causes abnormal water abnormal water destiny to “light destiny to “light up” brightly. up” brightly.

one of the best one of the best ways to image ways to image bone marrow bone marrow and spinal cord and spinal cord pathology. (T2 pathology. (T2 with fat with fat saturation also saturation also works well.)works well.)

T2W fast-spin echo with fat-sat T2W fast-spin echo with fat-sat (saturation>suppression)(saturation>suppression)

shows normal meniscus, shows normal meniscus, but increased signal at but increased signal at the posterior of the distal the posterior of the distal femoral metaphysis in femoral metaphysis in teenager with a cortical teenager with a cortical desmoid desmoid

FLAIR= Fluid Attenuated Inversion FLAIR= Fluid Attenuated Inversion RecoveryRecovery

Long TI inversion Long TI inversion recovery produces recovery produces bright signal from bright signal from brain pathology but brain pathology but CSF signal is CSF signal is suppressedsuppressed

This allows better This allows better contrast of CNS contrast of CNS lesions when they lesions when they are located at CSF are located at CSF interfaces (common interfaces (common in MS)in MS)

True Inversion Recovery True Inversion Recovery aka T1 FLAIRaka T1 FLAIR

New pulse New pulse sequence on sequence on 3T systems3T systems

Produces true Produces true anatomic anatomic contrast contrast between grey between grey vs. white brain vs. white brain mattermatter

Gradient Echo or Field EchoGradient Echo or Field Echo

In this technique, the protons are In this technique, the protons are excited to excited to flip angles of less than 90 flip angles of less than 90 degreesdegrees (which is faster than SE) (which is faster than SE)

instead of using a 180-degree instead of using a 180-degree refocusing RF pulse the precessions refocusing RF pulse the precessions are “refocused” by are “refocused” by reversing the reversing the polarity of the magnetic field polarity of the magnetic field gradientgradient (The higher magnetic field (The higher magnetic field side becomes the lower magnetic side becomes the lower magnetic field side and vice versa.). field side and vice versa.).

This can be used to rapidly produce This can be used to rapidly produce extremely extremely bright water signal in bright water signal in the CSF the CSF (the Rapid Myelographic (the Rapid Myelographic technique) technique) or in synovial fluidor in synovial fluid. .

T2* (tee 2 star) gradient echo/GE T2* (tee 2 star) gradient echo/GE with fat saturationwith fat saturation

a variation of GE and FSE that a variation of GE and FSE that suppresses the signal from fat while suppresses the signal from fat while imaging fluids brightly but is imaging fluids brightly but is extremely fast. extremely fast.

another fine technique for imaginganother fine technique for imaging bone marrow pathology and bone marrow pathology and pathology of articular cartilage.pathology of articular cartilage.

Gradient Echo or Field EchoGradient Echo or Field Echo

Rapid imaging Rapid imaging produces produces myelographic myelographic effect effect

Sensitive to Sensitive to magnetic magnetic susceptibility susceptibility (Hgb)(Hgb)

3D Imaging3D Imaging

This modification of Gradient/ Field This modification of Gradient/ Field Echo, and Fast Spin Echo/FSE, Echo, and Fast Spin Echo/FSE, acquires data for imaging as a block acquires data for imaging as a block or cube rather than as slices. The or cube rather than as slices. The data can later be “cut up” into slices data can later be “cut up” into slices of any thickness in any desired of any thickness in any desired plane. plane.

Allows for Allows for ultra thin slice images ultra thin slice images (1mm or less) (1mm or less) when necessary.when necessary.

69 yr old patient presented with 69 yr old patient presented with loss of consciousness. loss of consciousness.

A) CT demonstrates hypodensity A) CT demonstrates hypodensity consistent with infarction of consistent with infarction of basilar artery distribution in the basilar artery distribution in the left cerebellar lobe,vermis and left cerebellar lobe,vermis and medial portion of the right medial portion of the right cerebellar lobe.cerebellar lobe.

B,C) MRI FLAIR and MRI T2WI B,C) MRI FLAIR and MRI T2WI demonstrate vasogenic edema in demonstrate vasogenic edema in the same distribution.the same distribution.

D) ADC map (b=1000) D) ADC map (b=1000) demonstrates diffusion restriction demonstrates diffusion restriction due to cytotoxic edema in the due to cytotoxic edema in the same distribution.same distribution.

E,F) DWI (b=500, b=1000) also E,F) DWI (b=500, b=1000) also demonstrates diffusion restriction demonstrates diffusion restriction due to cytotoxic edema.due to cytotoxic edema.

This case demonstrates acute This case demonstrates acute infarction in the left and right infarction in the left and right cerebellar lobes and the vermis in cerebellar lobes and the vermis in a stage of intermixed vasogenic a stage of intermixed vasogenic and cytotoxic edema. and cytotoxic edema.

What’s wrong here?

MADDISON P et al. J Neurol Neurosurg Psychiatry 2000;69:563-564

©2000 by BMJ Publishing Group Ltd