X-ray subject contrast

[@chad969]

I've noticed that different sources say seemingly contradictory things about whether pneumothorax increases or decreases contrast and I would really appreciate it if you could clear this up for me. The ARRT prep site Boardvitals has a quiz question that says: _"A pneumothorax will result in which of the following?"_ The answer is: _"decrease in tissue density and decrease in contrast"_ Conversely, Clover learning has a quiz question that says: _"During radiographic imaging of the chest, subject contrast is most likely to be increased with which of these pathological conditions?"_ The answer is: _"pneumothorax"_ So clover learning says that pneumothorax increases contrast, whereas Boardvitals says that it decreases contrast. Is one of them wrong?

[@RockTheRegistry]

Pneumothorax is free air trapped in the pleural space, compressing lung tissue. The most common cause is penetrating chest trauma. On x-ray it appears as a strip of radiolucency without vascular lung markings. Boardvitals is only half-correct and their question is a bad question. They're correct that pneumothorax causes decreased tissue density. That's the radiolucency apparent on the x-ray image... It's a bad question because it's unfocused. Are they asking about tissue density or contrast? The registry won't make you split hairs like that.

[@chad969]

@@RockTheRegistry I see, thank you

[@pathumrathnayaka2840]

Is there a way to compute the thickness of the subject using x-ray doses entering and exiting the subject and distance information?

[@RockTheRegistry]

Interesting question! The answer is yes, but of course it's fairly complicated. The calculations must control for the different attenuation values of different tissue types within the patient. In a related way, we regularly make distance measurements of patient anatomy using ROIs on x-ray and CT images.

[@adry5355]

You are the best man!

[@SharkRockstar]

What're the effects of intensity and hardness of x-ray on contrast?

[@RockTheRegistry]

Intensity and hardness are two completely different things. Intensity (mAs) is unrelated to subject contrast. Only penetrability (kVp) influences subject contrast. As kVp increases, subject contrast decreases. Increasing filtration will increase beam hardness, but this is primarily done to decrease patient skin dose.

[@SharkRockstar]

@@RockTheRegistry thank you so much!! And I also wanted to ask that do electrons with higher speeds have a higher chance to ionize target atom? If so, we could say that hardness is proportional to characteristic x-rays produced 🤔

[@RockTheRegistry]

@@SharkRockstar No. Don't go down that road. Hardness is only used to describe the impact of filtration on the beam.

[@SharkRockstar]

@@RockTheRegistry okay thank you.. sometimes you need someone to pull you out of a road that circles back to where you started 😅💕

[@115Carssssssssss]

@@user-tz1sj1gs9w low subject contrast means low contrast