2. 1. What is an MRI?
• Magnetic resonance imaging
• Uses very strong magnetic field, RF wave, and computer to generate
the images.
• Resonance must occur in order to get the images.
3. 2. What happen when a patient is put in an
MRI system?
• Protons will aligned parallel and anti-parallel to main static magnetic
field longitudinal magnetization.
• Precess at Larmour frequency.
• RF pulse is applied to flip the protons.
• Transverse magnetization formed, RF pulse switched off.
• Transverse mag decay, and longitudinal mag reformed.
• Image is produced based on signal intensity.
4. 3. What is resonance?
• RF pulse is applied at same frequency with the Lamour/precession
frequency.
• Transfer of energy from RF pulse to the spin happen during
resonance.
5. 4. Why does the RF pulse have to be applied
exactly at the Larmor frequency for resonance to
occur?
• RF pulse is applied at same frequency with the Lamour/precession
frequency. [only same frequency, the resonance can occur.]
• Transfer of energy from RF pulse to the spin happen during
resonance.
• Proton absorb energy and flip to transverse plane.
** can elaborate with Q2
6. 5. What is T1 relaxation?
• Time for longitudinal mag to recover to its 63% of its max/initial
value.
• Spin lattice relaxation.
7. 6. What is T2 relaxation?
• Time for transverse mag to decay to 37% of its max value.
• Spin-spin relaxation
8. 7. Why T1 is longer than T2?
• Related to the composition of the
composition of the subject.
• T1 – spin lattice
• T2 – spin spin
• Transfer of energy is faster in spin spin
relaxation.
9. 8. How do you get T1-weighted image?
• Difference in T1 relaxation times are used to form T1 weighted-
image
• Short TR is used to maximize the difference in signal of tissue od
different T1 property
• Short TE is used to null the different in T2 property of the different
tissues.
10. 9. What is the difference between T2 and
T2*?
T2 T2*
• True T2
• Longer decay time
• Inhomogeneity is eliminated.
• More signal.
• Observed T2
• Faster
• Inhomogeneity of magnetic field in
tissues
• Less signal.
11. 10. Why fat appears bright on T1-weighted
image?
• Fat is large molecule and slow moving. T1 is shorter.
• Higher signal intensity compared to water.
12. 11. Long T1 materials are dark on T1-weighted
images, but long T2 materials are bright on T2-
weighted images?
• Long T1 material has higher signal intensity on T1w images.
• Long T2 material (water) has higher signal intensity on T2w images.
13. 12. How is a spin echo generated?
• Required 90 and followed by 180 RF pulse.
14. 13. In spin echo sequence, why 180° RF pulse
is applied?
• To rephase the dephasing spin.
• Eliminate the magnetic field inhomogeneity.
• Flip the protons to the opposite direction and change the direction of
spin.
15. 14. What are TE and TR?
• TE: time interval between the start of the RF pulse and the reception
the signal ( controls the T2)
• TR: time interval between the start of a RF pulse and the start of the
second RF pulse ( controls T1).
16. 15. What is the inversion recovery pulse
sequence?
• Is a spin echo pulse sequence preceeded with 180 degrees RF pulse,
then 90.
• Used to selectively null signal for tissues (fat/fluid) and generate
heavily T1 weighted images.
• Time between 180 and 90 degree pulse is termed time to invertion
(TI)
• Can selectively null the signal:
17. 16. What are the advantages of inversion
recovery sequence?
• Able to null the signal. (allow suppress signal of a specific tissue.)
• To make prominent T1w images
• Increase contrast between fat and water.
• Increase signal from tissue taking up the contrast.
• To reduce MR artifacts
18. 17. What are the fast imaging sequence used
in MRI?
• GRE based sequences:
• Reduces scanning time by reducing TR, achieved with using <90 degree RF pulse.
• Eg – Spoiled gradient recalled acquisition (SPGR), Turbo field echo (TFE), Fast low angle shot
(FLASH), Steady state free precession (SSFP)
• Echo Planar Imaging (EPI):
• An inverse recovery sequence usually applied to GRE.
• Used for functional imaging, real time cardiac imaging, perfusion/diffusion imaging.
• Fast/Turbo Spin Echo Sequence (FSE/TSE)
• Spin echo sequence with 90 degree tipping pulse followed by multiple 180 degree rephasing
pulses.
• Modification – Single-shot FSE Sequence (SSFSE) = FSE with only half the K-space filled. Eg
Half Fourier acquisition single-shot
19. 18. How does the scanner know the locations
of all the MR signals?
• Apply the gradient field, which is superimposed with the main
magnetic field along X, Y, Z axis for tissue localization.
• From gradient:
• Slice selection gradient (Z axis)
• Phase encoding gradient (Y axis)
• Frequency encoding gradient (X axis)
20. 19. How do you choose slice thickness?
• Referred back to Q18
• Using slice selection gradient.
• Simultaneous with RF pulse.
21. 20. Why are most MR contrast agents based
on the element gadolinium?
• Paramagnetic effect of gado – shorten the T1 and T2.
22. 21. What is MR image signal intensity
influenced by?
• PD
• T1 and T2 properties
• Slice thickness
• Tip angle value (flip angle)
• Distance of receiver coil to the patient.
• Type of sequence
23. 22. What is MR image contrast influenced by?
• T1 and T2
• PD
24. 23. What is MR imaging time influenced by?
• TR
• Number of excitation/signal averaging
• Matrix (smaller matrix, longer time)
25. 24. What are the common artefacts appear in
MR images?
• Motion
• Aliasing
• Chemical shift
• Magnetix suscepbility
** how to correct each artifacts, read urself.
26. 25. How are chemical shifts measured?
• When protons have different resonance frequency.
• Measure PPM (parts per million).
27. 26. What is magnetic susceptibility?
• Elements which have tendency to become magnetized when put into
a magnetic field.
• Alter the main magnetic field slightly.
