Bob Bury

Looking Through You...How do X-Rays Work ?

What is an X Ray ?
First of all, how do x-rays work? Well, they don't, of course, any more than light 'works'. X-rays describe radiation which is part of the spectrum which includes visible light, gamma rays and cosmic radiation.

X-rays were discovered by Wilhelm Roentgen, almost by

accident, in 1895, and within a year the new technology was already

in general use for imaging the human body. You can imagine how much

excitement this development caused at the turn of the 20th century,

not just for the medical profession, but also for the general public.

It wasn't only respectable x-ray clinics that were being set up

and advertised, there were also ads for demonstrations of the 'magic

rays' at soirees, as a form of entertainment.

We

consist largely of water

The point about x-rays is that, unlike visible light, they pass

straight through stuff, including the stuff that people are made

of. So, if you shine a beam of x-rays at someone, and put a piece

of film on the other side of them, you produce a shadow of the inside

of their body. This only works at all because the different tissues

that make up our bodies absorb x-rays to different extents. The

problem is, though, that we consist largely of water, so most of

our soft bits look much the same as far as x-rays are concerned.

In fact, there are really only three tissues that are sufficiently

different from each other in terms of x-ray absorption to show up

on an x-ray film: bone, air and soft tissue. So, on a chest x-ray,

for example (see picture, right, click

to enlarge), the bones (high absorption) are white, the air

in the lungs (very low absorption) is black, and all the soft tissues

(muscle, heart and blood vessels, skin) are a similar shade of grey.

But even with that limitation, radiology developed into a major

specialty during the 20th century.

Barium

and other contrast agents

The problem of all soft tissues looking the same was solved to some

extent by the introduction of artificial contrast material of various

types. One of the first substances to be used in this way was barium

sulphate. This is a salt of a heavy metal, and as such it absorbs

x-rays very well. It can be made into a thick suspension and swallowed

(the barium swallow or meal) or introduced into the colon via the

rectum (the barium enema). The barium coats the lining of the stomach

or colon, and when air is introduced to distend the gut, you get

really detailed pictures of the bowel lining, covered in a thin

layer of barium (the picture on the left (click

to enlarge) shows a barium enema - the barium is white, and

the air distending the colon is black). Small tumours and ulcers

can be diagnosed in this way.

Other

Contrast Media

There are other contrast media, usually containing iodine, which

are water soluble, and can be introduced into the body using other

routes. For example, if you attach iodine to a chemical which is

rapidly excreted in urine and then inject it into a vein, a series

of films over the next twenty minutes or so will show the contrast

material outlining the kidneys, ureters and bladder (the intravenous

urogram, or IVU). A similar substance injected directly into an

artery will demonstrate the vessel, and any organ (or tumour) which

it supplies. This is known as angiography, and it is possible to

pass a catheter (a fine plastic tube) from the femoral artery at

the groin into almost any vessel in the body. The picture on the

right shows the contrast material (black in this image) outlining

the aorta, the main blood vessel carrying blood down through the

abdomen to the pelvis and legs, and the renal artery supplying the

right kidney. It was from this type of investigation that the sub-specialty

of interventional radiology developed.

Interventional Radiology

Interventional techniques have transformed radiology from a purely

diagnostic specialty to one where radiologists can also treat patients.

For example; an angiogram of the vessels supplying the leg may show

a stenosis (narrowing) which is restricting the flow of blood and

preventing the patient from walking more than a few yards. Twenty

years ago, the demonstration of the stenosis would have marked the

end of the radiologist's involvement, and the patient would have

gone to a vascular surgeon to have an operation to fix the problem.

These days, most radiologists, having shown the narrowing, will

pass a special catheter across the stenosis, and then blow up a

balloon to stretch the vessel and restore the flow, a procedure

known as angioplasty. The same can be done for narrowings in the

coronary arteries which supply the heart muscle. Coronary angioplasty,

usually carried out by cardiologists, can relieve the angina suffered

by patients with coronary artery disease, and prevent the occurrence

of a full-blown hear attack, where a section of the heart muscle

loses its blood supply completely and dies. There are many other

areas where radiological techniques, usually performed under local

anaesthetic, with or without light sedation, have replaced fairly

major surgical procedures requiring a general anaesthetic. For example;

biopsy (removal of cells for microscopic examination) of tumours

or drainage of abscesses are easily carried out under imaging control,

thereby avoiding the need for surgery.

What's coming next ?

These fairly straightforward methods for using x-rays have served

us well throughout the 20th century, but from the 1970s on, we have

seen the advent not only of a revolutionary (literally) new way

of using x-rays, but also of several completely different imaging

methods which don't use x-rays at all. As a result, you will fail

to find an 'x-ray department' in many of our hospitals. Instead,

there will be a 'medical imaging' department, the name change reflecting

the wide range of technology now at our disposal. I'll talk about

these new imaging methods next time.

- January 2005

About the Author

Consultant Radiologist