Year 13 BTEC trip to Northwick Park Hospital

X Rays

BTEC students also need to know about the production and control of X-rays.

X-ray radiation is a form of electromagnetic radiation (photons). Most X-rays have a wavelength ranging from 0.01 to 10 nanometres, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3 × E16 Hz to 3 × E19 Hz) and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after Wilhelm Röntgen, who is usually credited as its discoverer, and who had named it X-radiation to signify an unknown type of radiation.

X-rays are part of the electromagnetic spectrum, with wavelengths shorter than visible light. Different applications use different parts of the X-ray spectrum.


X-rays are produced when a target of heavy metal is struck by electrons travelling at high speed. Only about 1 % of the electrons produce an X-ray photon; the rest is lost in heating up the target. X-rays are produced by:

Slowing the electron down, called bremsestrahlung (German for “braking radiation”)

By removing an inner electron; as electrons replace the inner electron, photons are emitted as the electrons undergo transitions from energy level to energy level.

Characteristic x-rays are emitted from heavy elements when their electrons make transitions between the lower atomic energy levels. The characteristic x-ray emission which is shown as two sharp peaks in the illustration below occur when vacancies are produced in the n = 1 or K-shell of the atom (the atom has become ionised) and electrons drop down from above to fill the gap. The photons have a particular energy.


The continuous distribution of x-rays which forms the base for the two sharp peaks at left is called “bremsstrahlung” radiation.

“Bremsstrahlung” means “braking radiation” and is retained from the original German to describe the radiation which is emitted when electrons are decelerated or “braked” when they are fired at a metal target. An inner electron is removed from the target and as electrons replace the inner electron, photons are emitted as the electrons undergo transitions from energy level to energy level. The accelerated charges result in electromagnetic radiation being given off, and when the energy of the bombarding electrons is high enough, that radiation is in the x-ray region of the electromagnetic spectrum.


X-rays with sharply defined frequencies are associated with the difference between the atomic energy levels of the target atoms.

The maximum energy that can be obtained is when all the energy from the electron is converted into the energy of the photon. So we can say:

Kinetic energy = photon energy

Kinetic energy = charge of electron × voltage



Linking this with the wave equation


we get:


Halving the current gives half the intensity as this is halving the number of electrons hitting the target. Therefore half the number of photons will be emitted.

The K-lines are not affected, nor is the maximum energy of the photons. This is because of the quantum nature of photon emission.

Increasing the voltage may well give more characteristic lines.

If the target material is changed, keeping the voltage the same, there will be completely different characteristic. As the proton number is increased:

E max remains constant, as it’s a function of the voltage;

The total intensity (the area under the graph) will change because there is a greater probability of a collision between the incoming electron and the electron shells. The more protons, the more electrons;

The characteristic lines are shifted to higher photon energies.

Low energy X-ray photons are called “soft X-rays”, while high energy photons are called “hard X-rays”.

A filter can be made of a sheet of material that will selectively absorb lower energy photons, so that the beam consists of harder X-rays. The beam is more penetrating.


The first radiograph – Mrs Röntgen’s hand

Radiography is an imaging technique that uses electromagnetic radiation other than visible light, especially X-rays, to view the internal structure of a non-uniformly composed and opaque object (i.e. a non-transparent object of varying density and composition) such as the human body. To create the image, a heterogeneous beam of X-rays is produced by an X-ray generator and is projected toward the object. A certain amount of X-ray is absorbed by the object, which is dependent on the particular density and composition of that object. The X-rays that pass through the object are captured behind the object by a detector (either photographic film or a digital detector). The detector can then provide a superimposed 2D representation of all the object’s internal structures.

An X-ray generator is a device used to generate X-rays. It is commonly used by radiographers to acquire an x-ray image of the inside of an object (as in medicine or non-destructive testing) but they are also used in sterilization or fluorescence.

An X-ray tube is a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity. The high velocity electrons collide with a metal target, the anode, creating the X-rays.

The tube contains a cathode, which directs a stream of electrons into a vacuum, and an anode, which collects the electrons and is made of copper to evacuate the heat generated by the collision. When the electrons collide with the target, about 1% of the resulting energy is emitted as X-rays, with the remaining 99% released as heat. Due to the high energy of the electrons that reach relativistic speeds the target is usually made of tungsten even if other material can be used particularly in XRF applications.

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.


Important points about the above diagram:

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.

If the cooling system were to fail, the machine would be turned off automatically.

In medical X-ray tubes the target is usually tungsten or a more crack-resistant alloy of rhenium (5%) and tungsten (95%), but sometimes molybdenum for more specialized applications, such as when softer X-rays are needed as in mammography. In crystallography, a copper target is most common, with cobalt often being used when fluorescence from iron content in the sample might otherwise present a problem.

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 17 degrees.

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:


The range of photonic energies emitted by the system can be adjusted by changing the applied voltage, and installing aluminium filters of varying thicknesses. Aluminium filters are installed in the path of the X-ray beam to remove “soft” (non-penetrating) radiation. The number of emitted X-ray photons, or dose, is adjusted by controlling the current flow and exposure time.

Simply put, the high voltage controls X-ray penetration, and thus the contrast of the image. The tube current and exposure time affect the dose and therefore the darkness of the image.

A cooling system is necessary to cool the anode; many X-ray generators use water or oil recirculating systems.

The intensity of an x-ray beam is reduced by interaction with the matter it encounters. This attenuation results from interactions of individual photons in the beam with atoms in the absorber (patient). The x-ray photons are either absorbed or scattered out of the beam. In scattering, photons are ejected out of the primary beam as a result of interactions with the orbital electrons of absorber atoms. In the case of a medical x-ray beam, three mechanisms exist where these interactions take place: (1) Coherent scattering (X-ray photons are reradiated as lower energy photons), (2) Compton scattering, and (3) photoelectric absorption. In addition, about 9% of the primary photons pass through the patient without interaction to produce the image.

As a medical x-ray beam travels through matter, individual photons (packets of electromagnetic spectrum energy) are removed, primarily through Photoelectric (when an electron gets ejected photons of visible light are emitted when the atom comes out of the excited state) and Compton interaction (both an electron and a lower energy X-ray photon are emitted).


The image above left shows scattering and the image above right shows the photoelectric effect


The image above left shows Compton scattering and the above right image shows pair production.

Pair production is where a very high energy photon interacts with the nucleus of an atom. An electron and a positron emerge, losing their energy by ionisation until the positron is annihilated by an electron, generating two identical photons.

The reduction of beam intensity is predictable because it depends on physical characteristics of the beam and absorber. A monochromatic beam (one wavelength only) of photons, a beam in which all the photons have the same energy provides a good example. When just the primary (not scattered) photons are considered, a constant fraction of the beam in attenuated as the beam moves through each unit thickness of an absorber. Therefore 1.5 cm of water may reduce the beam intensity by 50%, the next 1.5 cm by another 50% (to 25% of the original intensity), and so on. This HVL (half value layer) is a measure of beam energy describing the amount of an absorber that reduces the beam intensity by half; in the preceding example, the HVL is 1.5 cm. The absorption of the beam depends primarily on the thickness and mass of the absorber and the energy of the beam.

There is actually a mathematical relationship between the thickness of the absorber and the intensity of the x-rays, the attenuation of X rays follows the inverse square law.


Where It is the intensity of x-rays after passing through the absorber, I0 is the original intensity, m is the absorption coefficient and x is the thickness of the absorber. The negative sign shows that the intensity decreases on absorption.


This leads to a formula for the half value thickness


Radiographers use another term, the mass attenuation coefficient which is the attenuation per unit mass of material. The equation is:

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 high atomic number material that is opaque to X-rays. This makes some parts of the body show up better. 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.

A ‘barium meal’ is a liquid containing barium 56 drunk by the patient which makes the digestive tract show up clearly on x-rays, or the patient can have an injection of iodine (atomic number 53) which makes the blood vessels stand out (this is called angiography).


The image below left is an X-ray machine at Northwick Park Hospital which is used to screen for breast cancer. A mammogram is an x-ray picture of the breast. Screening mammograms are used to check for breast cancer in women who have no signs or symptoms of the disease. Diagnostic mammograms are used to check for breast cancer after a lump or other sign or symptom of the disease has been found.


The image above right is of the Interventional Radiology Suite in Northwick Park Hospital. London. The machine in the foreground is an x-ray machine that can change orientation so that any part of the body can be examined and keep the dose minimal to the parts of the body that don’t need to be examined.

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.


The above image is of Mrs Hare’s treated broken wrist

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.

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.

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.

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.

Any of the different types of 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 mitochondria.

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.


Fluoroscopy is an imaging technique that uses X-rays to obtain real-time moving images of the interior of an object. In its primary application of medical imaging, a fluoroscope allows a physician to see the internal structure and function of a patient, so that the pumping action of the heart or the motion of swallowing, for example, can be watched. This is useful for both diagnosis and therapy and occurs in general radiology, interventional radiology, and image-guided surgery. It comes with image analysis software and data storage and retrieval.

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.


The doses of all types of radiation (X-ray and radioactivity) used in medicine are small, and the benefit of being able to find out what is wrong with a patient and then treat them often outweighs the increased risk of possibly developing cancer later in life.

Everyone receives a dose of radiation from background sources such as radioactive rocks, radon gas and cosmic rays. This can be between 1.5 and 7.5 mSv per year on average, depending on where a person lives. Compare this to the dose from a dental x-ray, which is about 0.01 mSv, the equivalent of about 1.5 days background radiation.

A chest CT scan gives a radiation dose of about 8 mSv, which is about the same as 3.5 years background exposure, but a person would receive the same dose from a four hour flight, about the time it takes to fly to Greece from London, as they are higher up and have less atmospheric protection from cosmic rays. This dose increases the risk of developing cancer by one in 2500, though the risk without ever having had an x-ray is already 1 in 3.

There are many ways to reduce the dangers from radiation. The first is only to use it when necessary. Before people realised it was dangerous, shoe shops used to x-ray people’s feet to check that new shoes fitted properly. This no longer happens as the benefit did not justify the risk. However, the benefits of seeing where a bone is broken so it can be safely and properly mended are considered worth the small extra risk.

Every x-ray examination has strict controls about the maximum radiation dose a patient can be given, and the patient can be covered with lead-rubber shield to protect the parts of them not being examined from the radiation. This is especially used to protect reproductive organs so there is less risk of a mutation being passed on.

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, leave the room or stand behind a lead shield. The risk of developing problems due to radiation exposure increases with total dose.

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.

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