Flip Angle and Ernst Angle in Gradient Echo MRI | MRI Physics Course #17

High yield radiology physics past paper questions with video answers
Perfect for testing yourself prior to your radiology physics exam 👇
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Gradient echo MRI sequences require a short TR in order for faster acquisition times. The shorter TR would usually prevent T2 weighted images from being acquired. In this talk we will see the usefulness of using smaller flip angles to reduce TR and allow for T2 contrast despite short TRs. We will also review how smaller flip angles can in fact produce higher signal intensity (the angle which produces the highest signal for a given TR and T1 is known as the Ernst angle).
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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.
Happy studying,
Michael
#radiology #radres #FOAMrad #FOAMed

[@c.darlington2496]

Thank you for your time. You are a great teacher

[@radiologytutorials]

Thank you!

[@user-ew2cr5nk8z]

I have been going through all your videos and they are super super useful for my understanding. Thank you so much!!

[@radiologytutorials]

That's great! So glad they've been helpful 🥳

[@jonathanmcintire6818]

Question on longitudinal recovery and the description of the "90 degree" RF pulse: Is the 90 degree pulse defined as a literal 90 degrees, that by imparting the Larmor frequency for a specific time it will excite the magnetic vector for 90 degrees of change regardless of magnetic vector orientation? So that magnetization that is not fully recovered in the longitudinal plane could be driven past 90 degrees? It's my understanding this is how diffusion imaging works. So it would make sense that the "unrecovered" longitudinal magnetization would actually be driven PASSED 90 degrees, which is what leads to the lowered available transverse magnetization levels with shorter TR times. I would think that since we can drive the magnetization vector to 180 degrees, that we can drive past 90. As an example, in a short TR, fast T2 time spins recover to the full longitudinal plane, or zero, while long TR spins only recover to, let's call it, 45 degrees in that TR time period. That would mean that the next 90 degree pulse would drive the fully-recovered zero spins to 90 degrees, while the partially-recovered 45 degree spins would be driven to (45 + 90) 135 degrees, and thus passed the measured transverse plain. Is this simplified geometric explanation valid? Or are there other factors in play that keep the non-recovered spins from experiencing the 90 degree pulse, such as not responding to the Larmor frequency? Thanks!!!

[@radiologytutorials]

What an excellent question. You are completely right! Impressed with how clearly you asked this. At short TRs we do in fact drive partially recovered spins past 90 degrees. And this is what is responsible for the transverse magnetisation after the second TR to be reduced (equal to recovered longitudinal magnetisation). These spins driven past 90 degrees in fact create an echo (only at short TRs like you've described). This echo is not a proper rephasing echo like the 180 degrees pulse we saw in spin echo. The echo is called a stimulated echo which we will cover in the next talk when we look at coherent GRE sequences. Hope that makes some sense!

[@jonathanmcintire6818]

@@radiologytutorials OK awesome! Makes me feel like I'm getting a true grasp of the mechanism of action for creating pulse sequences. I think I did make an error though in my question in conflating long TRs with T2 recovery. As a technologist, I think of long TRs as T2 due to image WEIGHTING. However, the long TR I was referencing for recovery to zero transverse magnetization is actually referencing T1 recovery, correct? T2 dephasing times are actually very short, but we WEIGHT the images to T2 by allowing T1 recovery to fully take place so that both can get driven to 90 degrees fully, and we can then detect the short time spin dephasing differences between fat and water? It's kind of hard training my mind to think of the physics opposite of weighting! That would also mean that T1 WEIGHTED images actually take advantage of the differences in T2 dephasing in the short time frame, we just use a short TR because we can then drive the fat back to 90 and the unrecovered water past 90?

[@anumfarooq9431]

Excellent description

[@VahidChangizi-oq8bj]

Amazing. Thanks ❤

[@shagunsaini4567]

Thankyou for doing such a great lecture series. It is very much resourceful and well explained. Love your efforts. And also please do make a video on MRI contrast agents. Would be really helpful. 😊

[@radiologytutorials]

Ah yes! Will do a talk on gadolinium as well at time of flight MRI. Great suggestion

[@sanawarhussain]

thank you for the amazing video and explanation. It really helped me understood these phenomenons in-depth.

[@radiologytutorials]

Yay! Glad it was helpful! 🙂

[@shastriramroop4815]

Loving having these videos.

[@radiologytutorials]

I hope they are helpful!

[@shastriramroop4815]

@@radiologytutorials they sure are

[@shanishtabiharie8487]

Question. Where does the max signal come in de k space? in the centre?

[@sitinuramalina8068]

thank you for a great explanation!

[@stephenphilip8052]

Question about signal. If a long TR allows for full recovery of longitudinal signal, why does low flip angle still have a lower signal at long TR? Shouldn't they all be the same high signal since a long TR allows everything to recover fully?

[@djsprenkeler]

I am confused: in the end of the video you say that you can get T2 weighted images with gradient echo. But without that 180 rephasing pulse like in spin echo, you could only get T2*, right? or am I missing something?

[@radiologytutorials]

Technically yes, one should say T2* weighted. But by convention, when we talk about the weighting of an image we generally use T1 and T2 to describe what is contributing to the contrast most in the specific pulse sequence. If you think about it, even spin echo only reaches true T2 for a brief moment and the majority of signal is

[@ramjoshi4019]

your videos are very good, but volume is low, can u increase volume a bit plz...

[@user-gh9qr5oi9q]

Thankyou sir

[@sohailkohri7269]

Thanks

[@user-xv5zi5ds8n]

Thankyou🎉

[@ariwasonemeka9142]

Nice video

[@jushunoob]

Hello sir , I am a 1st yr radiology resident in India just 1 week into residency. Can you please help me with the books and essentials for the residency

[@abi077]

Hi sir..❤

[@user-re4cz8gr9u]

❤❤

[@thelabexperts]

I never understand why your video is very difficult to understand , You should teach point to point , use some animation ,use normal English ,use low voice so everyone can understand Thankyou

[@radiologytutorials]

Sorry to disappoint..

[@Thejungleismasif7]

@thelabexperts You find it difficult to understand because you’re not particularly intelligent. Sir Michael has done a stellar job once again

[@adamhounat6032]

Brother, this comment is a disgrace. Ustadh Michael has made 10s of brilliant, comprehensive and easily digestible videos covering the majority of Rad physics ALL for FREE, and you have the nerve to complain? This level of ungratefulness is fundamentally un-islamic.

[@rubyamir5245]

Brother! U must be right from your perspective… but the point is that these concepts are in fact very difficult… it’s not the teachers fault… 😢… I would suggest that u watch these videos multiple times in succession… Hope u will benefit… as these are a real treasure for radiology trainees!!

[@user-hc3fe7zr1e]

Thankyou sir

[@user-qt3ku6yi3q]

Thankyou sir