Hounsfield Units (Where do CT numbers come from?)

Hounsfield Units (a.k.a. CT numbers) express the values of all Computed Tomography (CT) images. This is a normalized scale of the X-ray attenuation values calculated during the CT scan.
This was produced by Brian Nett, PhD at howradiologyworks.com . See our site for more information on x-ray and CT physics.
Hounsfield Units are defines as:
HU= (mu_material - mu_water) / mu_water
This leads to HU values such as:
HU value of air: -1000
HU value of lung: -900 to -500
HU value of soft tissue/fat: -80 to 80
HU value of trabecular bone: 100 to 300
HU value of cortical bone: 300 to 1200
This topic may be referred to by these terms:
Hounsfield Units,
Hounsfield Units CT,
CT numbers,
Computed Tomography scale,
CT scale,
CT units,
attenuation values CT,
CT attenuation,
hounsfield unit in ct,
hounsfield scale,
ct numbers star wars (this one is just for fun),
computed tomography,
computed tomography registry review,
computed tomography brain,
computed tomography technologist,
radiologic technologist

[@vigneshvicky6720]

excellent man

[@HowRadiologyWorks]

Thanks for the comment

[@mkomakingu9073]

Excellent video Brian!

[@HowRadiologyWorks]

Thanks mkoma

[@tanish_mavi]

I just found your channel today only by searching the topic of CT Detector's (because i have my exam) and i am telling how u explains the concept of Radiology in very visual and eassy way there is no other channel on KZbin that can do that. Thank you so much for making these Wonderfull videos i try to finish all your videos on CT Scan and X-ray in 3hr's only . Keep it up the work i know one day u will cross the 1 million subscribes.

[@HowRadiologyWorks]

Tarnish I really appreciate your time also to leave this heartfelt message. Thanks again and please share with your colleagues

[@tanish_mavi]

@@HowRadiologyWorks Just Completed all questions because your videos 🤝. Thank you again

[@fifa97abdo]

Thank you so much ❤️🫂

[@HowRadiologyWorks]

You’re welcome 😊

[@peterboafo31]

That’s great

[@HowRadiologyWorks]

Thanks for the comment SBT 👍

[@joyharrison9473]

awesome video! I have a quick question, if you wanted to make the step size even smaller (which i believe would make it more contrasted) couldn't you multiply by an even bigger number than 1000? And if so, why is it not the norm to do that when trying to increase the contrast? Thanks!

[@HowRadiologyWorks]

Thanks Joy. The HU is the unit of storage for the CT images and then the gray level is determined by the window width and level. (See our video on that too) The window width and level determine the range of gray scale values displayed and also the contrast. Changing the definition of HU would not actually increase the contrast if the window width and level were changed also.

[@reemibrahim1426]

Very informative video but I wish there was no background music cuz it’s distracting

[@HowRadiologyWorks]

Thanks we have gone to no music for our new videos

[@schokimausi123]

Hello, I have a Question: so we have a HU scale, but what exactly is the Hounsfield Lookup Table (HLUT) ? Is it the same? And how does the HLUT work?

[@HowRadiologyWorks]

HULT usually means that the HU is used as the input to a lookup table to empirically estimate another quantity such as the water equivalent path length for radiation treatment planning.

[@schokimausi123]

@@HowRadiologyWorks Thank you very much!

[@HowRadiologyWorks]

@@schokimausi123 your welcome

[@aaronfricker5911]

good video thanks! But i wish the music was quieter... in comparison with your voice, it was rather distracting. The balance is a bit off. imo music is not even necessary

[@TheNettforce]

Thanks for the advice in my more recent videos I have removed the background music, sorry the settings were too loud here

[@zi6002]

can I know HU of haemorrhage? i will be grateful because i have an exam after two days

[@HowRadiologyWorks]

It depends on how long it has been, here is a nice diagram that describes the evolution of CT number over time radiopaedia.org/cases/evolution-of-ct-density-of-intracranial-haemorrhage-diagram?lang=us

[@zi6002]

okay sir , so can i say like its between 65-90 ? because i understood this from the graph and i don't know if its true or not

[@ranjit12345ful]

Hello dear brian ....can u please explain the effect of photon energy on attenuation values ... I learnt that the with increase in photon energy the attenuation values decrease ....yes ofcourse more energy means less attenuation and more x rays pass through the given tissue but then water is given zero at all photon energies right so if the attenuation values of other tissues are given relative to water why does the other values change ....looks simple but some where it is confusing me....kindly if u can explain....thank you ....God bless u 😊

[@TheNettforce]

Maybe I should do another video about this one. That will be more clear than in text. Thanks Ranjit

[@ranjit12345ful]

Yes please ...really grateful to u 🥰

[@HowRadiologyWorks]

I've added a new video to the channel based on your question, see kzbin.info/www/bejne/hKCtZJWIYrBnldE

[@ranjit12345ful]

@@HowRadiologyWorks ❤️❤️❤️ thanks a lot dear brian

[@TheNettforce]

@@ranjit12345ful you are welcome, thanks for the question. I’m sure others will learn from it too