Triple Physics Topic 1 - X-Rays

X-rays are very short wavelength electromagnetic waves.  The wavelength is about 1 × 10-10 m, about the size of an atom.  The waves are very energetic and can pass through materials that are opaque to light.

The use of X-rays for making shadow pictures of bones has been around for over a century.  The first X-ray machine was built by a German, Wilhelm Röntgen in the late nineteenth century.  Within a few months, the machines were becoming widespread in hospitals.  They were also used for amusement as there was little knowledge about the risks involved.  Nowadays X-rays are used for:

• looking at fractures in bones;

• looking at teeth to diagnose any decay;

• looking for the shadows caused by tumours and other disease in soft tissue;

• treatment of tumours by radiotherapy.

 X-rays have a wavelength of 1 × 10-10 m.  What is their frequency?

Generation of X-rays

The most common X-ray generator is the rotating anode tube.  It is an evacuated glass envelope, immersed in oil to cool it and surrounded by lead.

• Electrons are boiled off the hot filament which glows just like a light bulb.

• They are accelerated by the anode voltage.

• They hit the target, giving off energy mostly as heat, but 1 % is given off as X-rays.

• The target would rapidly melt, so it is turned by an AC induction motor.  The rotor is in the evacuated glass bulb, while the stator (the coils of wire) is on the outside. The cathode spins at 3000 rpm.

 The X-ray tube takes a current of 100 milliamps (0.1 A) at a potential difference of 200 000 volts.  What is the power taken by the tube?

 The X-ray tube has an efficiency of 1 %.    (a)  What is the power that is turned into X-rays? (b)  What is the power that is wasted? (c)  What happens to the wasted power?

The cooling system of an X-ray machine is vital to prevent overheating.  Actually if the cooling system were to fail, the machine would be turned off automatically.  The X-ray bulbs are precision pieces of kit and extremely expensive.  Even a jolt may well break the filament and lead to a massive bill.

Controlling the X-ray Beam

Unlike light or electron beams, X-rays cannot be focused.  So they can only make shadow images.  If you use a small point source of light, you get sharp shadows.  If it's a wide source of light, the shadows become fuzzy.  Obviously the doctor wants a sharp shadow.

There are various ways in which an X-ray source can be made into a point source:

• The beam is made narrow by the geometry of the anode to about 17o.

• The beam can be limited by using apertures.  This can be a simple diaphragm or a cone made from lead.

• Scattering in the tissues can make the picture fuzzy.  A grid made of strips of lead will absorb any scattered X-rays.

The diagram shows how the X-ray beam can be directed:

 Why is a lens not used?

Making X-Ray Images

X-rays themselves are very difficult to focus.  Therefore indirect means have to be used.

The commonest way of getting an image from the X-ray machine is a simple photographic film.  The films used vary in size according to the investigation.  For a dental X ray, the film would be about 3 x 4 cm; for a chest X-ray it would be 40 x 50 cm.  Unlike a film in a camera, these films are double sided, i.e. they have the emulsion on both sides.  The films are developed in the usual way in a photographic dark room.  The films produce a negative image, so that the shadows of bones appear light.  There is no reason, other than its being a waste of time and money, that the positive image could not be printed.  Doctors examine the developed films on light boxes.  With a broken bone, the problem is easy to see; looking for small cancers is not so easy.

Radiograph from Michael Müller-Hillebrand, Wikimedia Commons

To reduce the exposure of a patient, the film is placed in an image intensifier.  If you have had an X-ray in hospital, you will have seen these as the metal cases that contain the film.  The intensifier screen is a layer of zinc sulphide, a fluorescent material, that glows (fluoresces) when exposed to X-rays.  It absorbs the X-rays and retransmits them as visible light.  The light then deposits the silver grains on the film as well as the X-ray photons.  These devices can intensify the image by about 40 times, although the resolution is decreased a little.   The best resolution is about 0.1 mm.

The use of a fluorescent screen (without a film) can allow doctors to view events in real time.  This diagnostic method is called fluoroscopy.  To get a decent image, though, you need quite a high intensity.  In the old days machines with fluorescent screens were available as an amusement in shops or fun-fairs.  Nobody knew or cared about the risks then.

 Why do doctors use intensifying methods when taking X-ray pictures?

Image intensifier tubes can be used to avoid an increased dose of X-rays.  The fluorescent screen is connected to a photocathode.  Electrons are accelerated onto a second zinc sulphide screen, intensifying the original image by a factor of 1000.

Light from the second zinc sulphide screen passes to a TV camera for recording or direct viewing.

A more modern method of detecting X-rays in real time is the use of charge-coupled devices (CCDs).  These pick up a digital image that is exported to a computer.

X-ray shadows are clearest where there is greatest difference in density of the tissues.  For example bones are quite opaque to X rays.  Soft tissues are slightly opaque while air is transparent.  Lungs full of air show readily. The contrast  can be increased by using a material that is opaque to X-rays.  Studies of the function of the gastro-intestinal tract are carried out in real time, using the X-ray opaque material barium in the form of a barium meal.  This shows up readily on X-rays.

Uses for X rays

X-rays are a common diagnostic tool.  It is non-invasive, but there are risks due to the energetic radiation.  As well as the normal shadow pictures, X-ray tomography makes images of cross sections of the whole body.  This can be useful if there are a number of diseased sites in the body.

Energetic X-rays are used to treat cancer in a process called  radiotherapy.  The tumour is exposed to high energy X-rays and killed.  However there can be side effects.  Also the dose has to be worked out carefully.  10 percent less dose can leave a tumour unaffected, while ten percent more can damage the patient.

People working with X-rays have to take care as they could accumulate a high dose as they work:

• They wear a film badge to check the amount of radiation they get;

• They wear lead aprons while the machine is turned on.

• The machine is in an enclosed room and the controls are in a separate room.

• Interlocks are arranged so that nobody can walk into the X-ray room while the machine is turned on.  If that were to happen, the machine would be turned off immediately.

 X-ray machines are most commonly found in hospitals.  Give one other place you would find an X-ray machine.  What is it used for?

Dangers of X-rays

X-rays can do immense damage to biological material:

• Water is ionised to form free radicals that are highly reactive.  Free radicals can combine to make hydrogen peroxide H2O2 which is a powerful oxidising agent and can damage the DNA of the chromosomes;

• At the molecular level, enzymes, RNA and DNA are damaged, and metabolic pathways are interfered with.

• At the sub-cellular level cell membranes are damaged, along with the nucleus, chromosomes, and mitochrondria.

• Cellular level, cell division is damaged.  Cells can die, or be transformed to malignant growth.

• Tissue and organ damage.  There can be disruption to the central nervous system, death of bone marrow and the lining to the gastro-intestinal system, leading to sickness and death.  Cancers may arise.

• Whole animal can die; or life is shortened;

• Populations: mutations can alter the genetic characteristics of populations.

X-ray doses are very carefully controlled and maximum limits are set to minimise the risks to patients.  These limits are well below the doses that would cause the least harm.  However the procedures, although very safe, always run a very slight risk of long term harm.  So does watching the TV all day.

Bones absorb X-rays which means that good shadow pictures are easy to get.  Soft tissue pictures are harder to obtain.  They tend to be fuzzy, but there are differences in the absorption by soft tissues.  A lung cancer can show up as a shadow on a chest X-ray.  The detection of diseased lung tissue is done by X-ray because it's impossible to do with ultrasound.

 State what precautions those working with X-rays should take.

 Summary X-rays are very short wavelength electromagnetic radiation; X-rays are produced using a rotating anode tube; X-rays cannot be focused by lenses. X-rays are used to produce shadow pictures. There are many ways in which X rays are used as a diagnostic tool. Also X-rays can be used to treat cancers