As time allows, I am going to post some pages with normal and abnormal radiographs, and explain what is going on so that you have some basic appreciation of what on earth your vet is talking about. This page contains a really basic introduction to what radiology is, and if you scroll to the bottom (or use the navigation bar on the left) you can go to other pages with lots of pictures of normal and abnormal radiographs.
So what the heck is a radiograph already!?
"Radiograph" is the proper term for what most lay-people call "x-rays". Sometimes we lazy medical folks call them "rads" just so we can confuse you even more.
Why the difference? Well, I will try to explain it without getting too deep into physics, which may be hugely interesting to physicists but is not very interesting to the person who just wants to know what Fido's vet meant when he said "the radiographs show a lesion".
An x-ray is actually a type of radiation. An x-ray machine generates (by almost magical means involving lots of physics) x-ray radiation, which is completely invisible. This radiation shoots from the x-ray tube (the thing with the pink label in the picture) in a fairly well-focused beam, down through relevant bits of the animal on the table, and onto a piece of special film. When the film is developed, the parts of it that were exposed to the most radiation are darkest, and those with the least radiation are lightest, leading to the typical black-and-white-and-grey picture that we slap up on the viewer for you.
This piece of developed film that has the picture on it is called the "radiograph". The radiation is the x-ray. So the next time your vet says, "Come in here and take a look at Fido's x-ray," you can very properly say, "Don't you mean his radiograph? X-rays are invisible." Your vet will be suitably impressed, or embarrassed, or something.
Density and stuff
Be not afraid. Physics shall only intrude slightly, in a way that will not be injurious to our delicate sensibilities.
Density is just the amount of "stuff" in a given volume. The denser something is, the more it will weigh in comparison to the same volume of something else. I think that convoluted sentence needs an example ....
Think of hamburger meat. Think about how much a cup of raw hamburger meat weighs. It's pretty hefty. You could injure someone if you decided that a burger-assault was called for. Now, think of chocolate mousse. Mmmmmmm, chocolate. Light, fluffy, airy, and delicate. Think about how much a cup of chocolate mousse weighs. Not much, right? In fact, should you be so foolish as to waste that chocolate by throwing it at someone, they would scarcely notice.
Raw hamburger meat is dense. Most of the soft tissues of the body are the same density as raw meat, which is just cow muscle. Mousse, on the other hand, is more like the density of the lungs. Lungs are full of air and have not a lot of "tissue" density.
Lungs and Water and Bones, Oh My!
So, what does this foray into the food sciences have to do with radiographs? Simple concept - "stuff" stops x-rays. The denser the stuff, the more x-rays are stopped from reaching the film. In a body, bones are very dense. They will stop a lot of x-rays dead in their tracks, so not much radiation hits the film. Lungs are not dense at all (being all mousse-y and full of air) and most x-rays zoom right through them and hit the film.
Remember back up there where I said that "When the film is developed, the parts of it that were exposed to the most radiation are darkest, and those with the least radiation are lightest"? Lots of x-rays penetrate the lungs and hit the film, so lungs look black on radiographs. Not many x-rays penetrate all the way through bone, so bones will look quite white on radiographs. X-rays penetrate muscles and watery tissues in a medium way, so they turn out various shades of grey.
This radiograph shows a few different densities. This is a cat's abdomen. His head is to the left in the picture; he is lying flat on his right side. You can see his spine at the top; you can see each separate vertebra, nice and white because bone is dense. In his abdomen you can see a lot of shades of grey, forming distinct shapes. These are all different structures like the liver, spleen, kidneys, and bladder. You can also see bits of black in various places - the bigger bits are pockets of air in his colon, fondly referred to as "potential flatulence". You can just see the back edge of his lungs - the very black area on the far left of the picture, underneath the delicately white curve of his little ribs.
Thorax - normal
Thorax - abnormal
Abdomen - normal
Abdomen - abnormal