(click the green text links as you go through the explanations)
As I'm sure you know, acute blood is hyperdense, meaning it is BRIGHT compared to the grey tones of the brain.
So, we are looking for areas of hyperdensity. There was acute subarachnoid hemorrhage (SAH) in the cistern surrounding the brainstem here. There's also acute SAH in the right Sylvian fissure/temporal lobe here, and some more in the right temporal lobe here.
There is also a thin subdural hematoma that's "hidden." Did you see it? The thin LEFT frontal-temporal subdural hematoma is hard to see with the default brightness and contrast settings. This is the slice that has the subdural hematoma.
Start by TURNING THE BRIGHTNESS and CONTRAST DOWN.
Here is original scan with the default brightness/contrast the CT machine gave you.
Click the 2nd icon from the left on the toolbar (hovering over it reveals its name "Adjust window/level") and use that to turn the brightness & contrast down just a little by click-holding your trackpad and scrolling DOWN (reduce brightness) and to the RIGHT (reduce contrast) just a little.
If using a mouse, left-click-hold and slide the mouse DOWN and to the RIGHT.
Make the scan's brightness & contrast look like this.
Once you're done adjusting the brightness/contrast, click the 1st icon from the left on the toolbar (Stack Scroll), to be able to scroll through the CT slices again.
This is the slice that has the subdural hematoma with the default brightness and contrast settings. Look at that and then look at that same slice with the brightness & contrast turned down.
The acute subdural hematoma was previously hidden because it was just as white as the bone next to it. This can sometimes happen if the brightness/contrast is too high. Turning the brightness & contrast down a little will darken the thin acute subdural hematoma (while the bone remains white). This makes it so the subdural hematoma is not bright white and stands out against the white bone.
Remember, you are not done looking at the scan until you have turned the brightness & contrast down to appropriately search for subdural hematomas.
The 3rd icon from the left on the toolbar (the down arrow) opens a dropdown menu of CT presets. Clicking the "Brain (3)" preset achieves a similar reduction in brightness & contrast. You can also just hit the number 3 on your keyboard to change the preset.
Side note: There is a hyperdensity in the left temporal lobe and this might look like acute blood, but it's actually just the superior tip of the bone. If we look at the bone sequence and find that equivalent slice, we can see that same bright spot (arrows). Going down one slice inferiorly, you can see how that bright spot becomes the rest of the bone.
Now, in this patient it probably doesn't matter whether the EM physician finds or misses the thin subdural since there are other obvious patches of blood that will trigger the trauma/neurosurgery consult.
But imagine if the only intracranial bleeding was that thin subdural; you wouldn't want to miss it.
Let's look at some more cases.