T1 Relaxation, Spin-lattice Relaxation, Longitudinal Recovery | MRI Physics Course #5
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- čas přidán 13. 06. 2023
- High yield radiology physics past paper questions with video answers
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T1 relaxation occurs much slower than T2 relaxation. It signifies the regaining of the longitudinal magnetisation vector after the radiofrequency pulse has stopped. Here we will review the process of spin-lattice relaxation and highlight how time of repetition (TR) can be used to amplify T1 contrast within tissues.
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Not sure if the question banks are for you?
If you're here, you're likely studying for a radiology physics exam. I've spent the last few months collating past papers from multiple different countries selecting the most commonly asked questions. You'll be surprised how often questions repeat themselves!
The types of questions asked in FRCR, RANZCR AIT, ARRT, FC Rad Diag (SA), ABR qualifying Core Physics and MICR part 1 are surprisingly similar and the key concepts remain the same throughout. I've taken the most high-yield questions and answered them in video format so that I can take you through why certain answers are correct and others are not.
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Does the selective flip of the longitudinal component of the net magnetization vector apply to 180° rephasing during T2* measurement?
From indonesia here..., thank you so much for this amazing information and explanation that i already looking for long time...
Hi from South Africa! I'm so glad you're finding it useful!
Why does dephasing not affect longitudinal magnetization regaining? Are the protons in phase when they line up along B0?
Thank you for your videos, it is very insightful. I have a question about Muscle and fat tissue. why T2 in fat tissue is longer than T2 in muscle while T1 in fat is shorter than T1 in muscle? is the muscle tissue denser in fat? does it relate to lattice ability of tissue? thank you!
Hello doctor, why is T1relaxation goes unaffected by magnetic inhomogenesity
Thanks for your very comprehensive video. I was just wondering if you mean z-axis at 14:26, when you talk about the longitudinal relaxation vector of fat and CSF?
Same question 😩
DOES t1 relaxation also happen faster in deuterium?
When we apply the 90 deg flip angle while T1 relaxation is occurring, how can all the protons be flipped by 90 deg? Don't they have to be resonating at the same frequency for the RF pulse to work, like the same way we were able to flip them into the transverse direction for T2? (we had to line up all the spins along the longitudinal direction first)
Great question. When we apply the next 90 degree RF pulse, the only magnetic fields that are on are the main field and the slice selection gradient. Therefore, all the spins in the slice we are imaging will be resonating at the same frequency (they will have different degrees of T1 relaxation which will determine the transverse magnetisation vector magnitude once we’ve applied the RF pulse.) Remember each time we measure signal (at TE) we are not measuring T1 or T2 specifically. The contribution of T1 or T2 in the image will depend on the timing of TE and TR. Hope that makes sense, feel free to ask more if not 🫱🏼🫲🏽
14:00 you lost me here I have no idea what your are taking about😩
I thought you mentioned in earlier videos that we can only record the singal in the transverse plane (perpendicular to B0). So how do we measure the magnetization Mz along the longitudinal axis ?
Hes been saying theres another pulse (after allowing a certain amount of longitudinal recovery) to make the signal transverse again so it can be measured.
Exactly 🙂 the magnitude of longitudinal magnetisation prior to the next RF pulse (which will flip the net magnetisation vector into the transverse plane) will determine the magnitude of the transverse magnetisation vector after the RF pulse. If we repeat the RF pulse before the tissues have regained their longitudinal magnetisation the difference in transverse magnetisation will be a function of the difference in longitudinal relaxation rates (known as T1). If we don’t want these differences to be included in the transverse mag vector we need to wait a longer time before the next RF pulse to allow full longitudinal relaxation (long TR). This will mean the differences in transverse magnetisation over time will be due to T2 decay differences in tissues. Can be difficult to understand, but once it clicks you’ll be 💯
@@radiologytutorials sorry for such an unnecessary question. seems like i was sleeping during the part of the video where you explained this :)
@sanawarhussain not unnecessary at all! It’s the most commonly asked question in MRI. Appreciate you asking so other watchers can see the answer here in the comments 👌🏼
but why are we using a TE after the first RF signal here? Arent we supposed to use TE only after the second RF pulse to see T1 differeneces?
Good point! Technically the very first TE sample for a slice won’t have T1 weighting (and therefore could be ignored) By convention it is included in the pulse sequence. This sequence is repeated many times (often hundreds) for each slice in order to fill the entirety of k-space. Including the first TE will be inconsequential in the overall image 🙂
i thought spin directions can only take 2 discrete states. how do we get a flip angle if we only have flip states are possible?
In an isolated magnetic field we think of spins as existing in two states (spin up and spin down). In reality the spins exist in an infinite number of orientations within the field (due to thermal collisions and spin-spin interactions) but a slight majority align parallel to the field. The flip angle is causes by the radiofrequency pulse (RF pulse) which acts perpendicular to the main magnetic field, This 'flips' spins into the transverse plane and causes them to precess in phase - the combination creates the transverse magnetisation/flip angle. Hope that helps conceptually 🙂
@@radiologytutorialsah ok thanks. That's quite helpful. So the protons are still spin up or down but only the protons that participate in the net nuclear moment have their magnetic moment change orientation. I'm guessing we also lose some net longitudinal magnetisation because some of the spins take an antiparalellel direction due to the RF pulse but this loss of net magnetic moment vector is outweighed by the in phase component that contributes to the transverse magnetisation vector.
revise time stamp 14:03
Is the TE time the only instance when T1/ T2 signal is read by the machine (for spin echo sequences). Im trying to understand the role of TR in the T1 based signal. For eg: when exactly is the T1 info sampled (given T1 and T2 relaxations are simultaneously occurring
Excellent question. This is the crux of MRI imaging. You are exactly right. TE is the only time we measure signal. This is going to be difficult to type out in my phone. Hope this helps (and doesn’t confuse you more 😅) The timing of the TE and the TR will determine if contrast in the image is predominantly T1 or T2. If we use a very short TE (ie the spins have had no time to lose transverse magnetisation - no time to show T2 differences) and we use a fairly short TR (ie we don’t let the spins gain all their longitudinal magnetisation - the degree of T1 relaxation will be different in different tissues) because we’ve flipped the spins back into the transverse plain with a short TR these spins will have different transverse magnetisation based on their T1 differences. Sampling the transverse magnetisation vector quickly (short TE) won’t allow for T2 differences to occur, meaning the signal we measure is representative of T1 contrast in tissues.
The opposite is also true. At long TRs the spins have all gained longitudinal magnetisation. There is no difference in T1 values because we’ve allowed all the spins to fully regain all longitudinal magnetisation. When we flip these spins at TR and and wait for a slightly longer TE the spins will lose transverse magnetisation at different rates (T2 relaxation) now when we measure the signal, the differences in contrast/signal will be because of T2 differences. So it’s the timing of Tr and TE that determines if we are highlighting differences in longitudinal relaxation rates (T1) or transverse relaxation rates (T2)
@@radiologytutorials thank you for this great but simple explanation and thanks for taking your time! these videos are greatly appreciated. The anatomy series is super helpful as well for the part 1 exam. When will your MRI q bank be available?
So glad they’re helpful. Hoping to get the MRI question bank done in the next month. But just have to see how much time I have 😣
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