1. Key sequences in MSK imaging
Benoit Hainaux
Clinique Paris V
Paris
Basics Of MRI:How I Do It AFIIM -ISRA 2015

2. What is MRI ?
MRI is imaging of hydrogen proton (H+), which is in our body contained in water molecule, H20.
About 60% of our bodies are made of water, and it is present in almost every tissue H O
Tissues with water
(bone marrow and muscles) = signal
Tissues with no water (cortical bone, air) = no signal H

3. What is MRI ? On earth, our protons look everywhere…
On earth subject to a weak magnetic field, our protons look everywhere,
We also call protons “spin” because they turn round. As they are positive electric charge animated with spin, they produce their own magnetic field, and they can be considered as little magnets
Earth magnetic field : 47 μT

4. What is MRI ? In a strong magnetic field, protons align on it B0 B0 M
500,002
63.9 MHz at 1.5 T
127.8 MHz at 3 T
Protons, which are little magnets, are aligned with strong magnetic field, some in the same axis (spin-up state), and others in the opposite axis (spin down state)
On a sample of one million protons in a 0,5 T magnetic field and body temperature, 5 hundred thousand and 2 protons are spin-up and 5 hundred thousand are spin down.
So there is a little more protons in spin-up state, and the sum of it gives us the net magnetization vector called M, aligned with B0. M will give us the magnetic resonance signal
M also turn around B0 with a frequency of precession of 63.9 MHz at 1.5T, 127.8MHz à 3T
500,000

5. What is MRI ? IB0I = 1.5 T 63.9 MHz x MHz
MRI is the combination of a strong magnetic field and radiofrequency waves. Remember, it was the answer of the quizz. How do they work together
At balance state, M is aligned on B0
We apply a radiofrequency wave with a coil, near the frequency of precession of M, to supply energy to the spins. This is resonance phenomenon
The absorption of energy tip M in a transversal plane
The balance state return comes with a signal caught by the coil

6. What is resonance phenomenon ?
Balance state O z y x Mz
Now in details, in an orthonormal basis (O,x,y,z)
At balance state M is aligned in Oz axis, which is also the axis of B0

7. What is resonance phenomenon ?
90° Radiofrequency wave, Mz decay, Mxy grows simultaneously z Mz y
90° RF
90° radiofrequency wave tip Mz in the tranversal plane xOy, and become Mxy
Mz decay while Mxy grows, simultaneously
This is the resonance phenomenon
Mxy O x

8. What is relaxation ?
Back to balance state, Mxy decay quickly and Mz grows slowly, still simultaneously z Mz T1 y
When radiofrequency wave stops, Mxy decay quickly and Mz increase slowly still stimultaneously
This is relaxation
T2 relaxation is the process by which Mxy decays, and T1 relaxation the process by which Mz grows. Each tissue has own T1 and T2 T2
Mxy O x

9. What is T1 ?
T1 is the time required for tissues to reach 63% of Mz maximum value (longitudinal magnetization) Mz
63%
T1 relaxations times at 1.5 T
CSF : 4200 ms
Gray matter : 920 ms
White matter : 780 ms
Fat : 260 ms
T1 relaxation is due to energy transfer from the spin system into nearby molecules
The easier the energy is transferred, the shorter is T1 (longer in fluid, shorter in solid, even shorter in fat)
T1 relaxation times are given for a field strength value because frequency of precession depends on it. Relaxation times are longer at 3T T1
Time

10. What is T2 ?
T2 is the time required for tissues to fall to 37% of Mxy initial value (transverse magnetization)
Mxz
T2 relaxations times at 1.5 T
CSF : 2100 ms
Gray matter : 100 ms
White matter : 90 ms
Fat : 60 ms
We can notice that T2 relaxation times are shorter than T1
37% T2
Time

11. What is T2 ? T2 relaxation is due to spin dephasing B0 Time Mxy
90° RF
Mxy
The spin are in phase at the moment of radiofrequency applying and they quickly dephase after, which causes Mxy decay
There is a easy way to understand this phenomenon
As I told you before, spins can be considered as little magnets.
If you put closer two same pole of two magnets, they repel each other.
We can have the same reasoning with the spins. They repel each other, in other words they dephase, and Mxy decay.
Time

12. What is T2* ? T2* = T2 + main magnetic field inhomogeneities Mxz T2
T2 is not what happen really, it is T2*
Spin phases are scattered by these inhomogeneities. To sum up, spin are dephased by two phenomenon : because spins dephase each others (which is T2) and because main magnetic field can have intrinsic defects, and dephase even faster spins
So, what solution we have to get real T2 ? It is spin echo T2
T2*
Time

13. What is spin echo ?
We use spin echo to refocus the spin and measure real T2
90° pulse tilt spins in transversal plane
Spins are dephased by T2* relaxation
180° pulse refocus spins and generate an echo, this echo is the signal received by the coil
By GavinMorley (Gavin W Morley) [CC BY-SA 3.0 ( via Wikimedia Commons

14. What is spin echo ? How is built a SE sequence TE TR 180° 90° 90°
This drawing is the pulse sequence of spin echo
TE (echo time) is the time from 90° pulse to the center of echo. TE/2 is the time between 90° pulse and 180° pulse
TR (repetition time) is the length of time between 2 cycle of SE sequence
During a TR is filled one line of the image
I won’t talk about gradient echo or echo plannar imaging, which are less used in musculoskeletal imaging
90°
90°

15. What is SE T1 ? TR is the factor who gives the T1 weighting
Short TR, ideal is between 400 and 600 ms Mz
signal
Fat
Tissues
Short TR : high T1 weighting
How do we do a SE T1 weighted image ?
As we saw before, T1 relaxation is the process by which Mz grows. So we will get T1 weighting on Mz growing curves
To get a T1 weighted image, we’re looking for the maximum signal gap (contrast) between tissues. Short TR will gives us this contrast
Time
Short TR

16. What is SE T1 ? Short TR = T1 weighting
Shortest TE possible = less T2 weighting possible
signal
TE < 20 ms
T1 weighted image : Tissue with shortest T1 is white
400 ms < TR < 600 ms
And what about the TE ?
First, I have to explain a little bit these curves. As I said before, T1 and T2 relaxations are simultaneous.
In this drawing, there is both T1 and T2 relaxation curves. I drew them separately for a clearer drawing, but they are simultaneous.
This mean that you always have T1 and T2 in the same picture, and TR and TE will allow you to promote either.
With a short TR, you stop T1 relaxation at the best T1 weighting you can have, and with a short TE, you avoid T2 weighting in your picture. Ideal TE is 0, but it does not exist… Yet !
In the quizz it was asked the whitest signal between CSF, bone marrow, muscles and cortical bone. We all can see that bone marrow (and others structures like subcutaneous fat) is the whitest.
Fat
T1 weighted
contrast
Tissues
Time

17. What is SE T2 ? TE is the factor who gives the T2 weighting
Long TE, between 70 ms and 130 ms
Mxy
signal
Now what about T2 relaxation.
It is the process by which Mxy decays. So we will get T2 weighting on Mxy decay curves.
To get a T2 weighted image, we’re looking for the maximum signal gap (contrast) between tissues. Long TE, between 70 ms and 130 ms will gives us this contrast
Tissues
Fat
Long TE : high T2 weighting
Time
Long TE

18. What is SE T2 ? Long TE = T2 weighting
Long TR = less T1 weighting possible (TR for T2 = 4TR for T1)
signal
T2 weighted image : Tissue with longest T2 is white
70 ms < TE < 130 ms
TR = 2000 ms
Fat
We asked you previously the parameters for a T2 weighting : here they are.
Same curves as we saw to explain SE T1.
In this case, you wait a long TR, about 4 times the TR for T1 weighting, to avoid T1 weighting because no more gap between T1 relaxation curves, and with a long TE, you get an echo with a maximum T2 weighting
Long T2 structures are hypersignal, like CSF. Subcutaneous fat is white on this T2 weighted image ans should be hyposignal according to curves, because it’s not simple SE but FSE, and fat signal persists with this kind of sequence
Tissues
Tissues
T2 weighted
contrast
Fat
Time

19. What is SE PD ?
B and C has same proton density (but different relaxation speed)
A has higher proton density Mz
signal
Long TR : High PD weighting A B
There is a third weighting used in musculoskeletal imaging, it is the proton density.
On these T1 relaxation curves, we have B and C, two structures with same proton density and different relaxation speed, and A, a structure with a higher proton density
If you wait a long TR, signal differences will not be the purpose of relaxation time but proton density C
Time
Long TR

20. What is SE PD ? Long TR = PD weighting
Shortest TE possible = less T2 weighting possible
signal
PD weighted image : Tissue with highest PD is white
TE < 20 ms
TR = 2000 ms A
PD weighted
contrast
With a long TR, you get the PD weighting, and with a short TE, you avoid T2 weighting in your picture.
Ideal TE is 0, same as T1 B C
Time

21. Why rapid imaging ? Fast SE or Turbo SE
Using multiple 180° pulses to generate multiple echoes
Significantly reduce scanning time
In routine imaging, we don’t use classic spin echo, which is too long, we now use FSE

22. What is FSE – TSE ? Pulse sequence ETL 180° 180° 180° 180° 180° 90°
During a TR interval, multiple echoes are generated, and each echo fill one line of the image
Number of echo is echo train length or turbo factor, here it’s 5 (4 to 32 for routine imaging). Means that scanning time is 5 times reduced compared to a classic SE sequence
TE is replaced by effective TE, the time at which central lines of k-space are being filled. We can consider effective TE in FSE sequence the same as TE in SE sequence,
ETL is the most important parameter. It affects image quality and weighting (longer ETL result in more T2 weighting, because more late echoes and longer effective TE)
TEeff TR

23. What about fat signal ? Fat signal is low on SE T2 weighted images
Multiple echoes on FSE T2 weighted images render fat bright
SE T2
TR : 3000
TE : 80
FSE T2
TR : 3000
TEeff : 80
What about fat signal on FSE T2 weighting ? Here we have two images, one SE T2 weighted and one FSE T2 weighted, with the same parameters
Fat signal is low on SE T2 weighted images because fat T2 is short and the TE is long
Notice the difference of signal between these two T2 weighted picture. Both are T2 weighted, only fat on FSE sequence is bright.
Multiple echoes, especially the early ones, contributes in a T1 weighting for the fat
It brings us to the last point of this presentation, how to delete the fat signal

24. What is fat sat ? Fat saturation
Combination of RF pulse and spoiler gradient
Can be used with all sequences
FSE T1
FSE T1 fat sat
Gadolinium enhanced
FSE T2
FSE T2 fat sat
We can see on this FSE T1 fat sat weighted image the complete cancelation of fat signal
The hypersignal below is due to fat sat defaults, we will talk about it after
Same on FSE T2 weighed images, fat signal is deleted, the same hypersignal is observable

25. What is fat sat ?
Water and fat peak have different frequency of precession
Water
Signal
Fat
It is based on the difference of frequency of precession between water and fat
Frequency of precession
210 Hz at 1.5 T
420 Hz at 3 T

26. What is fat sat ? How does it works z Mz y Mfat O x
FS RF y
A RF pulse with a frequency set to stimulate only fat spins tilt fat magnetization in transversal plane, and a spoiler gradient dephase fat spins, leaving only water signal. After that, the standard sequence can starts, with no fat signal
Fat sat option makes sequences longer, because of the previous RF and spoiler gradient
There is an other way to delete fat, with the inversion-recovery sequence
Spoiler
Gradient
Mfat O x

27. What is IR ? Inversion recovery SE sequence preceded by a 180° pulsez Mz y
180° RF O x
The 180° pulse reverse Mz and become –Mz
When the radiofrequency waves stops, -Mz decay and Mz grows again
The STIR sequence is used in musculoskeletal imaging to delete fat signal, and example of sternum, ham-string and pectoral imaging
We will see after why we sometimes use FAT SAT, and sometimes STIR
-Mz
STIR delete fat signal

28. What is STIR ? Short TI (140ms) : delete fat signal Mz -Mz Fat TI
Tissues
180° RF
Now what’s happen if we look relaxation curve, 180° pulse reverse Mz, then they grows from –Mz to Mz, and have a zero value at a precise time, the TI or inversion time
If we use 90° pulse when fat net magnetization is null (at the end of TI), fat vector will not tip in transversal plane, like in a classic spin echo sequence, but all others vectors will.
Only fat will not give signal at the end. If you put a longer TI (about 2300 ms), you delete fluid signal, this is FLAIR sequence
-Mz

29. What is STIR ? STIR pulse sequence TI TE TR 180° 180° 180° 90°
STIR pulse sequence starts with the 180° inverting pulse and continues like a standard SE sequence
The weighting PD with short TE, and T2 if TE is longer
90°

30. Hazards Ferromagnetic objects « missile effect » Implantable materiel
Quench
Some basic safety rules. This is a strong magnet, always be careful and inform people, both patient and professionals (non radiologist physician, cleaning staff, etc…)
Missile effect transform simple scissors in a dangerous weapon
Always ask for materials who can be damaged by magnetic field and/or radiofrequencies, such as pacemakers, cochlear implants, recent stents, etc…
Quench can occur if there is less than 30% helium in the tank, the most important thing is to take away the patient if it happens. You can trigger a quench if a life is in danger (someboy between an attracted object and the magnet)

31. Thank you for your attention
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Musculoskeletal MRI techniques – Eric Lévêque (in french)