Magnetic Resonance Imaging (MRI)

Principle of MRI

1. What kinds of nuclei can be used for MRI?

• Use Hydrogen atoms: sensitive to magnetic resonance and most abundant in the body (the majority of Hydrogen in Water)
• Atoms consist of Nucleus and Electrons
• Nuclei are made of protons and neutrons
• Nucleus have 2 properties: Spin and Charge --> magnetic moment
• Pairs of spins tend to cancel, so only atoms with an odd number of protons have spin
  • even number of protons, the Net Spin(magnetiztion vector sum) will be 0

2. How do protons interact with a magnetic field?

• Moving(Spinning) charged particle generates its own little magnetic field.
• Particles will tend to line up with external magnetic field(B0) lines and they will revolve around external field axis. This movement is called Precession
• Resonant (Larmor) frequency is the frequency at which the magnetization vector is precessing about an axis
• External magnetic field strength and resonant frequency have linear relationship
• Spinning particles with mass have angular momentum. Angular momentum resists attempts to change the spin orientation

3. what happen when RF pulse is applied to magnetic field?

• RF Excitation is the effect of a radiofrequency pulse (resonance) at the Larmor frequency
• If apply RF pulse which match with Larmor frequency, there will be resonance. Proton absorbs energy and flip from parallel (low energey state) to anti-parallell (hight energy state)
• If remove RF pulse, preton will release energy and go back to parallel(low energy state) --> T1 relaxation
• 외부의 에너지가 주어져서 자기공명현상으로 횡자화된 상태(세차운동의 축이 옆으로 다 누운 상태)에서, 전자기파를 갑자기 끊어주면, 주변으로 에너지를 방출하고, 세차운동의 축은 다시 z축에 가까워져서 저에너지상태인 종자화가 된다.

T1 Relaxation, T1 weighted MRI

• Different Tissue has different recovery time.
• The percentage of recovery translate to the signal intensity
• when shorter in T1 relaxation time, it will be brighter in T1 wieghted MR image
• Fat will recovery soon than CSF(cerebrospinal fluid), so fat has much shorter T1 time than CSF
• 대부분의 병변은 물을 많이 함유하고 있어(염증으로 인한 부종, 혈류가 풍부한 종양 등) T1에서 저신호강도(까맣게)로 보인다(물과 병변은 까맣게, 지방은 하얗게)

T2 Relaxation, T2 weighted MRI

• The transverse magnetiation decays to zero
• 종자화가 회복 (T1 relaxation) 되는 동시에 횡자화는 사라질 것이다. 횡자화가 사라지면서 37%까지 줄어드는데 걸리는 시간이 T2 relaxation time이며, 이를 통애 얻은 영상이 T2 weighted MR image이다
• 물이 많을수록 신호강도가 높다(밝다)

 

reference

https://blog.daum.net/ironmanjws/15861221

https://www.youtube.com/watch?v=Yubg0cmXSQg