CT (Computed Tomography) Scans - A Level Physics

A basic description of the mechanism of CT (computed tomography) scans for medical use in remote sensing. Part of the A Level Physics revision series.

[@symoi1569]

2021 and still the best video out there about CT calculation. Respect!

[@tudorpopescu2061]

This explanation is simply the best one i have seen. you're doing a great job in explaining all these things. you are truly a life-saver

[@sittoris]

I have the same washing machine as you bro, according to the subtle background noise haha

[@Chiko-sc1gz]

Lmao

[@Tim1Awesome]

After 11 years, 2023, this still helps people, thanks mate.

[@akosuafrimpong4465]

I honestly don’t know how to thank you enough. Your videos have been so easy to follow and understand. God bless you!!!

[@stevenlachowski1371]

To those of you who say the algorithm doesn't work for anything more than a 2x2 array: For this size, only 4 total scans are necessary. A larger array will require more scans at differing angles to get a better picture of the object being scanned.

[@Photosounder]

But you also have to replace subtracting by 16 with proper ramp filtering.

[@laxminepal8801]

णत़ो

[@borntobereal2004]

our teacher gave us a problem for 4x4 array and l don't have any idea how to solve that. He didn't tell us anything, just gave us this video and now we have to solve it.

[@DrPhysicsA]

Indeed there is. Well spotted. On other videos you can also hear my dog bark and the kitchen clock which has a cow instead of a cuckoo.

[@gateaspirant2383]

Does contrast between the tissues depends on the difference in average atomic number? (For CT)

[@saraahmed632]

I'm learning this in my third year physics degree and you have explained it way better than my professor

[@ayeshashaikh4433]

I wanna cry, I was so stressed about this (cause my physics prof sucks) but you made it so easy! Thank you so much!!

[@DrPhysicsA]

Hi. Thanks for comments. Have you seen my 2 videos on Electromagnetism?

[@apayappirathapann]

Our college teacher made us to watch this video. I became big fan of Dr.Physics. you're making me love physics. Thanks indeed for your help sir. Could you do more A2 past paper questions on your channel please. Thanks again. And I would much appriciate you if you could do a video for course work preparation.

[@spottymaldoon]

What a very clever presentation - so often they are fudged and nobody, maybe including the presenter, comes out understanding. Thank you for this - and well done!

[@ravindranathhospital1362]

An awesome explanation. I could not find many videos and I am glad I spent my time on this video. You just made it so simple.

[@Photosounder]

Doing an offset (subtracting 16) at the end is actually here a way of doing a high pass filter. Since there are only two possible frequencies for each axis, then subtracting lowers DC (frequency 0) which makes the other frequency (nyquist) relatively higher. Only works in this particular case.

[@ironrace2004]

I love the way you broke this down. Very easy to understand the basic principles.

[@csy_-oi5tk]

I finally understand why my physics lecturer is a pure rubbish after watching this 👍

[@russellazure9316]

My teacher literally just plays this video in class as if it was 'his' lecture

[@thanidunawanjana8457]

After all these years, it helped me to clear my doubts. Thank you!

[@kingmetsoo5234]

Education never gets old 11yrs ago I was in grade 3 but today you helped sir in my A Level thank you

[@Guilherme-d5d]

Perfect and clear explanation. By far the best one! Thank you so much for sharing.

[@rajeswaribandyopadhyay8155]

The best explanation I have ever seen kudos to your dedication

[@tatujoy9921]

A Vary simple easy to understand video about the basic tenets of CT scanning, Very very good

[@Rosehillcottage9]

Thank you so much for making this video, it wasn't completely clear to me before but now I fully understand!

[@raguaviva]

fantastic, I never thought it was so simple, in fact I always thought it was magic!

[@anaarias6418]

This is such a good explanation. This is exactly what I was looking for. Thanks a lot!

[@prathameshmandke5966]

Hey, Thanks for the clear and concise explanation. I have 2 queries. 1. Are the computed voxel values directly interpretated as image pixel intensities which is what we see as the CT image? 2. As per my understanding, the detector will record the intensities of the received beam whereas the numbers that you mention in the voxel's are attenuation coefficients. So, more the attenuation coefficient or a voxel lesser will be the intensity recorded. What exactly do the summed values 7, 9, etc. represent? Are they the attenuation coefficients or the intensities recorded by the detector? Thanks in advance!

[@aditimishra5853]

Realyyyy very veryyyyyy much thank you sir for this wonderful explanation thank you so much...... no one literally no one explained like you

[@saeedkamoon6466]

very good presentation it easy, simple and clear..... thank you

[@DrPhysicsA]

I'm not sure which part of the video you are referring to, but I suspect I mean the ability and extent of the Xray to penetrate the material through which it is passing.

[@yudhvirgupta9510]

DrPhysicsA somya

[@KanchanaAturupane]

thank you DrPhysicsA.... no one could've explained that any better

[@xiangyu711]

The algorithm is fantastic! Thanks for explaining this!

[@marcisrancans]

WAJEIH RAJPOOT, fanned out x-ray beam is used to gather full slice data at particular moment of exposure for greater resolution. First xray machines in fact used single focused beam to gather data, but as computer data processing capabilities rise, it was possible to use fan-type beam and use detector array insted of just one.

[@calvinlwz231294]

Sir, I don't understand why a deduction of 2X2 box with 7 7 9 9 is made? Is there a more complicated explanation to this?

[@rf4085]

Fantastic video. Thank you sir. Your students are very fortunate to have a great teacher. Happy new year.

[@ishratmaqbool303]

Really appreciable...thank you Sir could you please mention the name of this book in which this method is given please Sir ... Humble request...

[@antua38]

Awesome video, gonna use the example on my oral radiology test next week.

[@wajeihrajpoot1448]

awsum.... but i have a question why cat-scanner shows "fanned-out" array of x-ray beams....directed through patients?

[@srinivasan7892]

I still can't understand why you did the sum only for three projection(why did you left remaining projection) and dividing your fellow answer with three? CT scan doesn't stop with scanning three projection right?? it scans the whole body,it means it pixel value changes throughout the 360° right? so you can't divide it with three..will you divide it with 6??

[@NEILALEXANDERMADRIGAL]

What do you call this kind of CT reconstruction?? Because we have different kinds of CT reconstruction - Back projection, Iterative, and Analytic Method. What is this kind?

[@frankamoscatelli983]

Very nice description. But I think you need to emphasize that you solver the *forward* problem. That is, given the voxel absorption values, find the absorptions of the 4 beams. Actually going into hospital and getting a CT scan and reconstructing the voxel values from it is the *inverse* problem. Is the inverse solution unique? Often they are not.

[@Sigmamenhealth]

0:25 its Dota

[@confirmhandle]

What kind of computer of processor would a CT scanner use to add all this up? So much computation needed for just one voxel, insane !

[@Ihaveanamenowtaken]

Any processor that can do floating point operations will do. But if you want fast results, you need a decent processor. Any entry-level processor and some nice memory would work quite well.

[@surenthar6032]

Y it's 16 to subtract and 3 to divide??

[@yahiaaymen7876]

2020 and this video is still amazingly useful

[@patricktan7120]

Why is the background reading the sum of attenuation for one scan??? And why is everything divided by 3 in the final step?????

[@gateaspirant2383]

Does contrast between the tissues depends on the difference in average atomic number? (For CT)

[@ChillingMysteriesTV]

super clear cut explanation. thank you

[@shockingvideos5225]

respected sir there is a point in the syllabus that understand how the image of 8-voxel can be developed using CT scan You explained 4-voxel, please explain it

[@AlexLilley]

Thanks for this. My Year 13's and me are very grateful!!

[@rameshsuthar4]

Well.... If I take a 3by 3 box then from how many angles I need to project??? Plz reply... Is there any formula for that..?

[@phyzjqk]

Awesome vid. The first step is the Radon transform

[@mosab643]

Who came up with the formula?

[@hanaa_-_]

i just did this today at school.thank you soo much i understand it way better now! keep up the good work,you got another sub :)

[@ericthecyclist]

I'm puzzled by the apparent additive nature of the voxels rather than multiplicative. If I shine a light through a two filters that individually block out 50% of the light, I'd expect that combining the two would block out 1 - .5*.5 = 75% of the light, not 100%. Do XRays operate differently, or is the percentage of X rays blocked so small that using addition is an acceptable approximation for multiplication?

[@yurifranken]

these numbers are in a logarithmic scale, adding logarithms of numbers is the same as multiplying the numbers

[@ClassicContent]

Ah yes I've explored your page thank you very much. Understanding is the key, I think I'll have a greater chance of getting an A in the summer because of your helpful videos!

[@fifasyeda5329]

Did u get the A?

[@hbale18]

Thanks for the simple explanation. What is the reasoning behind subtracting the sum total of 7 and 9 from the table before the final result?

[@DrPhysicsA]

Can you remind me the time on the video at which this occurs please?

[@juanesarango]

hbale18 The idea is to substract the whole atenuation of the initial body (in this case the initial table). Which is 16 in every emission direction: 7+9=16, 4+11+1=16, 4+12=16, 3+5+8=16...

[@silverreyes7912]

at 10:07 do we just divide by a common factor; such as for the this example the common factor was 3?

[@oneinabillion654]

Hi, one of the question mentioned "background reading". May I know what that is?

[@shiromusk4236]

The sum of the densities

[@kennykobau]

Why do we have to rotate it in 4 directions? can it done in any other number of directions?

[@paulshallard6053]

this algorithm works fine for this particular 2x2 matrix but if you scale it up to 3x3 using the numbers 1-9 it fails resulting in negative intensities?

[@irq20xdfr]

Hello thank you! And how is the final results of the matrix transformed in an image like the black pages the laboratories give you as fault ? Thank you

[@marcisrancans]

Does not work with 4x4 grid. The sum of right side attenuation numbers is larger than 60% of final grids values. Therefore subtraction gives me negative atenuation values. But one thing is kind a working -> values which are the largest in my case are still positive after subtraction.

[@khurramsarfraz3489]

wow thanks alot sir. You even explain better than my college teacher! :D

[@almasmahir2091]

You are indeed a legend,boss

[@ctoumi2]

Excellent tutorial - Better than my Imperial College lecturers.

[@fredericzoni365]

Is that an simplification for the filtered back projection (FBR)?

[@messapatingy]

How does this scale from a 2 by 2 to an N by N grid?

[@jakeb358]

That's awesome it really helps you understand stuff like this better

[@kokitoz]

Thank you very much, I've laways searched for a physics explaination like this!

[@lananhnguyen3336]

how did you do it can you share with me , thank you

[@Chinese_Story132]

Thank you so much! such a clear explanation!

[@adrianobueno6984]

Nice explanation, thank you! One question: how to get from voxels to pixels to produce the image of that slice?

[@gateaspirant2383]

Does contrast between the tissues depends on the difference in average atomic number? (For CT)

[@bluespike1482]

Now I know way more than I need to. Thank you!

[@chaymm]

ima need this with a 3x3, have never been asked to do one below a 3x3.

[@shaniceleitch7670]

DrPhysicsA, CT means Computerized axial Tomography right?or does Computed Tomography mean the same?

[@shaniceleitch7670]

+Shanice Leitch (Ms. Curly) I am not too sure about the above post which is why I asked...

[@Kenneth_the_Philosopher]

Wait a minute! Is he saying roughly that the image is created using a matrix?

[@zatulmutalif3230]

sir what do you mean by when you said 'INTENSITY' of the x ray??

[@Roboticdock]

Thank you for sharing your knowledge. I appreciate it!

[@blub232324]

Is there a washing maschine running in the background?

[@deltaslayer1132]

Thx for the awesome vid, but I'm not sure what method this is for image reconstruction? Filtered Back Projection (FBP) or the iterative method? It's helps me a lot if you or someone knew! :)

[@juanesarango]

DeltaSlayer Hi, It's the Filtered Back Projection (FBP) method!

[@harrymarquis9691]

Hi, I think more precisely it is the simple back projection method. The filtered method applies some form of filtering (such as convolution) on the projected profiles before it is back projected. Correct me if i'm wrong :) Cheers

[@christianengkeblle2328]

Great video. But can you please *FAST* explain if you got a 4x4 I have followed the first steps. But when I subtract the combined of the first "scanning" from the calculated values, some become negative? What am I doing wrong?

[@mandolinic]

Christian Engkebølle I appreciate this is way too late for you, but someone else might need to know. I suspect that you're still only doing 4 scans. To solve for more voxels you're going to need more information. I've not worked through the maths, but my intuition is that you need at least as many scans as you have voxels. I think that if you were to simulate doing 16 scans, all at different angles, then you'd be able to solve for 16 voxels.

[@adnanalkurdi4823]

is he washing ? or is it a blender?

[@lugar25031993]

definitely washing

[@AntiProtonBoy]

I never knew about the initial subtraction part after accumulating the values in the voxels. I assumed you just compute the average (ie div by 4).

[@kokitoz]

Thank you very much, I've laways searched for a physics explaination like this! is there an algoritm for more block greater than 2x2? thanks again!

[@hazifahn]

Thank you for explaining!

[@reychill91]

by the way, are you boiling water when you were doing the video?

[@tudorpopescu2061]

sounded like a washing machine

[@confirmhandle]

Sounds like a CT scanner gantry ramping up to speed and back down!

[@MikeAdorff]

First class explaination! Thank you!

[@shimshonwinograd]

Does this algorithm should work for tables bigger than 2X2? I couldn't make it work...

[@DrPhysicsA]

+Shimshon Winograd No. The algorithm gets much more complicated which is why computers are needed. If you do the 2x2 with a,b,c and d you will see why this particular algorithm works.

[@dynamicblood1]

Is this an iterative method of image reconstruction?

[@gretawilliams8799]

Yes sunny

[@turturicagabriel9435]

What is the name of this method ?

[@Hussainfhs]

Please explain Iterative reconstruction

[@ClassicContent]

Hello Doctor Physics, thank you for the fantastic videos. Is there any chance you would be able to make a video on Magnetic Fields? or recommend any links or other videos?

[@asvipoudel1979]

Machismo which method is this?

[@zahiddar3547]

very helpful Sir can u please make a video about iterative reconstruction

[@prasadanatekar]

Great way of explaining! Thank you.

[@limitless1692]

wow so ct is using x ray and it builds a 3d image that is so interesting thanks !

[@AhmedAdeljr]

I did not understant , why did you divide the last table by 3 ? thank u

[@DrPhysicsA]

+Ahmed Aljazzar See 10:07. Its a kind of averaging.

[@shimshonwinograd]

What kind of averaging exactly??

[@zaighamabbas2041]

+Shimshon Winograd the result after the subtraction has the culmination of the same values from different angles. If we dont divide then we wont get the actual attenuations but the cumulative attenuations from the different angles.

[@hansk9346]

suprisingly, I understand the dividing things. but why subtract with the sum of initial value? why subtracting with 16? please dont answer with, because 7+9 is 16

[@amianifineug1353]

because there are three direction of x ray

[@zakirullah4088]

Great! explained very well. Thanks

[@rhishavpoudel9766]

I don't understand why the algorithm is done? Thank you.

[@aditimishra5853]

Wonderfullllllllllll physicssss lovelyyy explanation sir

[@jesusalbertogonzalezcanas7415]

at 0:30 is that the dota 2 logo? hahaha

[@mosolymark]

hahaha :DDD

[@chifachelly402]

What is a voxel? I need a scientific definition plz.

[@turquoisecat761]

A voxel is to 3D imaging (such as CT, MRI etc) what a pixel is to 2D imaging (such as standard X-ray imaging)

[@lukc3613]

awesome!! who came with the mathematical model , its genius..

[@hakkooma]

The two persons who invented this method receives a nobel prize in Physiology or Medicine in 1979 , for this invention. They were : Sir Godfrey Hounsfield , and Allan MacLeod Cormack .