MRI Made SimpleMRI Made Simple

HH SchildHH Schild

HistoryHistory

Felix Bloch & Edward PurcellFelix Bloch & Edward Purcell Discovered magnetic resonance Discovered magnetic resonance

phenomenon in 1946phenomenon in 1946 Received Nobel Prize in 1952Received Nobel Prize in 1952 Used for chemical and physical Used for chemical and physical

molecule analysismolecule analysis Raymond Damadian in 1971 showed Raymond Damadian in 1971 showed

magnetic relaxation of tumor and magnetic relaxation of tumor and normal tissue differednormal tissue differed

MRI MRI

Patient is put in a magnetPatient is put in a magnet RF signal is sentRF signal is sent RF signal is switched offRF signal is switched off Patient emits a signal which is Patient emits a signal which is

used for used for Reconstruction of imageReconstruction of image

Proton are little Proton are little magnetmagnet

Proton spin around its axis like a planet, has positive changer and electrical field around it, and moving electrical field has a magnetic field like a bar magnet

Proton alignmentProton alignment

Protons align to Protons align to external magnetic external magnetic field parallel or anti-field parallel or anti-parallel.parallel.

Both have different Both have different energy levelenergy level

Like walking on feet Like walking on feet require less energy require less energy than on hand, parallel than on hand, parallel state is preferred to state is preferred to anti-parallelanti-parallel

Difference is Difference is 10,000,000/10,000,0010,000,000/10,000,0077

Precession Precession

Proton moves in like spinning topIn two axis wobbling motion calledPrecession, depends on magnetic field strength

Precession frequencyPrecession frequency

Precession frequency is dependent on Precession frequency is dependent on strength of external magnet fieldstrength of external magnet field

It is determined by Larmor EquationIt is determined by Larmor EquationWo = yBoWo = yBo

– Wo is precession frequency in Hz or MHzWo is precession frequency in Hz or MHz– Bo in magnetic field strength in TeslaBo in magnetic field strength in Tesla– y is gyro-magnetic ratio, for proton is y is gyro-magnetic ratio, for proton is

42.5MHz/Tesla42.5MHz/Tesla– Stronger the external magnetic field Stronger the external magnetic field

higher the precession frequencyhigher the precession frequency

Coordinate systemCoordinate system

Representation of magnetic force in Z axis, Proton vector as red arrow

Net magnetic forceNet magnetic force

Proton pointing in opposite direction cancels each others magnetic effectin respective direction.9 proton align up and 5 down, resulting in 4 proton up force

Net magnetic forceNet magnetic force

As there are more protons aligned parallel to the external magnetic filed, there is a net magnetic movement aligned with or longitudinal to the external magnetic field

Human magnetic Human magnetic vectorvector

In a strong external magnetic field a new magnetic vector is induced in the patient, who becomes a magnet himself.

This new magnetic vector is aligned with the external magnetic field

Measuring Measuring magnetizationmagnetization

Magnetization along an external magnetic field cannot be measuredFor this a magnetization transverse to the external magnetic field is necessary

RF pulse frequencyRF pulse frequency

After protons aligned along the external magnetic force, RF pulse is send for energy exchange. Which is only possible if RF pulse has same frequency as protons precession frequency

Effect of RF pulseEffect of RF pulse

The RF pulse exchange energy with the protons (a), and The RF pulse exchange energy with the protons (a), and some of them are lifted to a higher level of energy, some of them are lifted to a higher level of energy, pointing down (b), in effect the magnetization along pointing down (b), in effect the magnetization along the z-axis decreases, as the protons which point down the z-axis decreases, as the protons which point down neutralize the same number of proton pointing upneutralize the same number of proton pointing up

Whip like action of RF Whip like action of RF pulsepulse

Effect of RF pulseEffect of RF pulse

It lifts some protons to higher energy It lifts some protons to higher energy ( they point down), and it also causes the ( they point down), and it also causes the protons to precess in step, in phase.protons to precess in step, in phase.

The former result in decreasing the The former result in decreasing the magnetization along the Z-axis, the magnetization along the Z-axis, the longitudinal magnetizationlongitudinal magnetization

The later establishes a new The later establishes a new magnetization in the x-y-phase (->), a magnetization in the x-y-phase (->), a new transversal magnetization, which new transversal magnetization, which moves around with the precessing moves around with the precessing protonsprotons

A ship, passengers being distributed randomly A ship, passengers being distributed randomly all over the deck, the ship in a normal positionall over the deck, the ship in a normal position

When all passengers walk in equal step around When all passengers walk in equal step around the railingthe railing

The ship is leaning towards the side where the The ship is leaning towards the side where the people are, a new force is established, and people are, a new force is established, and become visiblebecome visible

RF pulse on protonRF pulse on proton

a.a. In a strong external magnetic field a new magnetic vector along the external field is establishedIn a strong external magnetic field a new magnetic vector along the external field is establishedb.b. Sending in an RF pulse causes a new transveral magnetization while longitudinal magnetization decreasesSending in an RF pulse causes a new transveral magnetization while longitudinal magnetization decreasesc.c. Depending on the RF pulse , longitudinal magnetization may even totally disappearDepending on the RF pulse , longitudinal magnetization may even totally disappear

Result of transverse Result of transverse magnetizationmagnetization

The new transversal The new transversal magnetization moves around magnetization moves around with the precessing protonswith the precessing protons

Thus for an external observer, Thus for an external observer, transversal magnetization transversal magnetization constantly changes its constantly changes its direction, and can induce a direction, and can induce a signal in an antennasignal in an antenna

Longitudinal relaxationLongitudinal relaxation

After the RF pulse is switched off, protons go back from their higher to the lower state of After the RF pulse is switched off, protons go back from their higher to the lower state of energy.energy.

Longitudinal magnetization increases and grows back to its original valueLongitudinal magnetization increases and grows back to its original value Energy of RF pulse is handed over to the surrounding latticeEnergy of RF pulse is handed over to the surrounding lattice This process is call longitudinal relaxation or spin lattice relaxationThis process is call longitudinal relaxation or spin lattice relaxation

T1 CurveT1 Curve

Recovery of longitudinal relaxation vs. time curve

Transverse relaxationTransverse relaxation

After the RF pulse is switched off, protons lose phase coherence, they get out of step.

T2 CurveT2 Curve

Transverse magnetization vs. time after RF pulse is switched off

T1 > T2 T1 > T2

Coupling of a T1 and Coupling of a T1 and T2- curve resembles T2- curve resembles a mountain with a a mountain with a slopeslope

T1- 300-2000 msec.T1- 300-2000 msec. T2- 30-150 msec.T2- 30-150 msec. It takes longer to It takes longer to

climb a mountain climb a mountain than to slide or than to slide or jump down.jump down.

Relaxation timeRelaxation time

It is difficult to pinpoint the end of the longitudinal and It is difficult to pinpoint the end of the longitudinal and transverse relaxation exactly. Thus T1 and T2 were transverse relaxation exactly. Thus T1 and T2 were not defined as the time when relaxation is completed. not defined as the time when relaxation is completed.

Instead T1 was defined as the time when about 63% Instead T1 was defined as the time when about 63% of the original longitudinal magnetization is reached.of the original longitudinal magnetization is reached.

T2 is the time when transverse magnetization T2 is the time when transverse magnetization decreases to 37% of the original value. decreases to 37% of the original value.

These percentages are derived equation 1-1/3 = 63%, These percentages are derived equation 1-1/3 = 63%, 1/e = 37%1/e = 37%

1/T1 is also called longitudinal relaxation rate1/T1 is also called longitudinal relaxation rate 1/T2 transverse relaxation rate1/T2 transverse relaxation rate

Liquids have long T1 & Liquids have long T1 & T2T2

Fat has short T1 & T2Fat has short T1 & T2

What influences T1/T2?What influences T1/T2?

T1 is longer than T2T1 is longer than T2

T1 varies with the magnetic field strengthT1 varies with the magnetic field strength

It is longer in strong magnetic fieldIt is longer in strong magnetic field

Water has a long T1 and fat has a short T1Water has a long T1 and fat has a short T1

T2 of water is longer than the T2 of impure T2 of water is longer than the T2 of impure

liquids containing larger molecules.liquids containing larger molecules.

90 degree RF pulse90 degree RF pulse

If after the RF pulse, the number of protons on the higher energy level equals If after the RF pulse, the number of protons on the higher energy level equals the number of protons on the lower energy level, longitudinal magnetization has the number of protons on the lower energy level, longitudinal magnetization has disappeared, and there is only transverse magnetization due to phase disappeared, and there is only transverse magnetization due to phase coherence.coherence.

The magnetic vector seems to have been tilted 90degree to the sideThe magnetic vector seems to have been tilted 90degree to the side The corresponding RF pulse is thus also called a 90degree pulseThe corresponding RF pulse is thus also called a 90degree pulse

90degree RF pulse90degree RF pulse

a.a. Before RF pulseBefore RF pulseb.b. After 90d RF pulse, longitudinal magnetization decreases, After 90d RF pulse, longitudinal magnetization decreases,

proton also precess in phase, causes new transversal proton also precess in phase, causes new transversal magnetization magnetization

c.c. After the RF pulse is switched off longitudinal magnetization After the RF pulse is switched off longitudinal magnetization increases, and transversal magnetization disappears, increases, and transversal magnetization disappears,

Magnetizing vectorMagnetizing vector

a.a. Before RF pulse there is only longitudinal Before RF pulse there is only longitudinal magnetizationmagnetization

b.b. After the 90dgr RF pulse there is only transversal After the 90dgr RF pulse there is only transversal magnetization and this is spinning aroundmagnetization and this is spinning around

c.c. With time after the removal of RF pulse the With time after the removal of RF pulse the transversal magnetization decreases and transversal magnetization decreases and longitudinal magnetization increases in spiral motionlongitudinal magnetization increases in spiral motion

Free induction decayFree induction decay

The sum magnetic vector constantly, its The sum magnetic vector constantly, its direction and magnitude, while it direction and magnitude, while it performs its spiraling motionperforms its spiraling motion

The sum vector induces an electrical The sum vector induces an electrical current in an antenna, the MR signal.current in an antenna, the MR signal.

This is greatest immediately after RF This is greatest immediately after RF signal is switched off and then decreasessignal is switched off and then decreases

The signal intensity reduces but The signal intensity reduces but frequency is constant frequency is constant

This is called free induction decay (FID) This is called free induction decay (FID) signalsignal

Long and Short TRLong and Short TR

a.a. A and B are two tissue with different relaxation times. Frame 0 shows the situation before, frame 1 A and B are two tissue with different relaxation times. Frame 0 shows the situation before, frame 1 immediately after a 90degree pulse. When we wait for a long time (TR long) the longitudinal immediately after a 90degree pulse. When we wait for a long time (TR long) the longitudinal magnetization of both tissue will have totally recovered (frame 5). A second 90d pulse after this magnetization of both tissue will have totally recovered (frame 5). A second 90d pulse after this time results in the same amount of transversal magnetization (frame 6) for both tissue.time results in the same amount of transversal magnetization (frame 6) for both tissue.

b.b. When we do not wait as long as in a, but send a second RF pulse after a shorter time (TR short), When we do not wait as long as in a, but send a second RF pulse after a shorter time (TR short), longitudinal magnetization of tissue B which has the longer T1, has not recovered as much as that longitudinal magnetization of tissue B which has the longer T1, has not recovered as much as that of tissue A with shorter T1. The transversal magnetization of the two tissue after the second RF of tissue A with shorter T1. The transversal magnetization of the two tissue after the second RF pulse will then be different (frame 5). Thus by changing the time between successive RF pulse we pulse will then be different (frame 5). Thus by changing the time between successive RF pulse we can influence and modify magnetization and the signal intensity of tissue.can influence and modify magnetization and the signal intensity of tissue.

Tissue signal with Tissue signal with short TRshort TR

Brain has a shorter longitudinal relaxation time than Brain has a shorter longitudinal relaxation time than CSF. With a short TR the signal intensities of brain CSF CSF. With a short TR the signal intensities of brain CSF differ more than after a long TRdiffer more than after a long TR

T2 - Weighted imageT2 - Weighted image

After the RF pulse is switched off, the protons After the RF pulse is switched off, the protons dephase (a-c). dephase (a-c).

The 180deg pulse causes them to precess in the The 180deg pulse causes them to precess in the opposite direction and so they rephase again (d -f)opposite direction and so they rephase again (d -f)

180 turn 180 turn

When a rabbit and a When a rabbit and a turtle run in one turtle run in one direction for a certain direction for a certain time, then turn time, then turn around and run in the around and run in the opposite direction opposite direction with the same speed with the same speed for the same time for the same time they will arrive at the they will arrive at the starting point at the starting point at the same time.same time.

T2 effectT2 effect

The 180The 1800 0 pulse refocuses pulse refocuses the dephasing protons the dephasing protons which results in a stronger which results in a stronger signal, the spin echo after signal, the spin echo after the time TE. The protons the time TE. The protons then dephase again and then dephase again and can be refocussed another can be refocussed another time by a 180time by a 1800 0 pulse and so pulse and so on. Thus it is possible to on. Thus it is possible to obtain more than one obtain more than one signal, more than one spin signal, more than one spin echo. The spin echo, echo. The spin echo, however differ in intensity however differ in intensity due to so-called Tdue to so-called T2 2 – effect.– effect.

A curve connecting the spin A curve connecting the spin echo intensities is the Techo intensities is the T2 2

curve. If we did not use the curve. If we did not use the 1801800 0 pulse, the signal pulse, the signal intensity would decay much intensity would decay much faster. A curve describing faster. A curve describing the signal intensity in that the signal intensity in that case the Tcase the T**

2 2 (T(T2 2 star) curve.star) curve.

T2-effectT2-effect

If we record the signal If we record the signal of two buses as the of two buses as the buses drives away. The buses drives away. The signal vanish due to signal vanish due to extrinsic (bus speed) extrinsic (bus speed) and intrinsic (exhaustion and intrinsic (exhaustion of the passengers) of the passengers)

Without having the bus Without having the bus come back (i.e. a 180come back (i.e. a 1800 0

pulse), it is impossible to pulse), it is impossible to say whether a decrease say whether a decrease in signal intensity is due in signal intensity is due to inherent tissue to inherent tissue properties , or due to properties , or due to external influence, i.e. external influence, i.e. different bus speed.different bus speed.

TT22- weighted sequence- weighted sequence

T2-curve of the two T2-curve of the two tissue with different tissue with different transversal transversal relaxation timesrelaxation times

Tissue A has a Tissue A has a shorter T2 than shorter T2 than tissue B, thus looses tissue B, thus looses transversal transversal magnetization faster.magnetization faster.

With short TE the With short TE the difference in signal difference in signal intensity is less intensity is less pronounced than pronounced than after a long TEafter a long TE

Spin Echo sequenceSpin Echo sequence

The spin echo sequence consists of a 90The spin echo sequence consists of a 9000 and a 180 and a 18000 pulse pulse After the 90After the 9000 pulse protons are dephasing due to external and pulse protons are dephasing due to external and

internal magnetic fields inhomogeneitiesinternal magnetic fields inhomogeneities The 180The 18000 pulse rephase the dephasing protons, and stronger signal , pulse rephase the dephasing protons, and stronger signal ,

the spin echo results.the spin echo results. The 180The 18000 pulse serve to “neutralize” the external magnetic field pulse serve to “neutralize” the external magnetic field

inhomogeneitiesinhomogeneities Signal decrease from one echo to the next, when using multiple 180Signal decrease from one echo to the next, when using multiple 18000

pulse is due to internal T2-effectspulse is due to internal T2-effects By choosing different TEs this signal can be T2 weighted in varying By choosing different TEs this signal can be T2 weighted in varying

degrees with very short TEs, T2-effects have not yet had time to degrees with very short TEs, T2-effects have not yet had time to really show upreally show up

With longer TEs, the signal intensity difference between tissue will be With longer TEs, the signal intensity difference between tissue will be depending very much on their T2s , their transversal relaxation timesdepending very much on their T2s , their transversal relaxation times

With very long TEs, there should be even more T2-weighing, however With very long TEs, there should be even more T2-weighing, however , signal intensity as such would be so small, that at best it can just , signal intensity as such would be so small, that at best it can just barely be distinguished from the background noisebarely be distinguished from the background noise

Spin Echo sequenceSpin Echo sequence

1.1. (90(9000 – TE/2 – 180 – TE/2 – 18000 – TE/2 -> record signal at TE) after – TE/2 -> record signal at TE) after TR (time from the beginning of one 90TR (time from the beginning of one 9000 pulse) follows pulse) follows another pulse cycle and signal measurement.another pulse cycle and signal measurement.

2.2. (90(9000 – TE/2 – 180 – TE/2 – 18000 – TE/2 -> record signal at TE) – TE/2 -> record signal at TE)