1.2.1 Diagnostic radiography

In diagnostic radiography, an image of structures within the patient’s body is produced on an image receptor or a monitor screen. Normally, of course, we cannot see inside each other’s bodies because light photons, to which our eyes are sensitive, are absorbed and reflected very close to the surface of body tissues.

Figure 1.1 shows a radiographer preparing the equipment for a common X-ray diagnostic examination.

Figure 1.1 A radiographer preparing to examine a patient’s chest.

As light is a form of electromagnetic radiation (see Ch. 17) and it seems logical to suggest that if we can take photons of electromagnetic radiation which have higher energies than light photons, then these may have sufficient energy to penetrate body tissues and allow us to visualize internal organs. X-rays are in this part of the electromagnetic spectrum and so will penetrate body tissues and allow us to image internal organs. Unfortunately the retina of the eye cannot detect X-rays and so we cannot see an image of a structure just by directing an X-ray beam on it. This means that the X-rays, which have passed through the body, must be made to strike an image receptor that will produce a visible image, for example an imaging plate.

The principal interactions involved in the basic requirements for the formation of a radiographic image are shown in Figure 1.2.

Figure 1.2 The principal interactions involved in the production of a radiographic image. A represents absorbed photons, S represents scattered photons and T represents transmitted photons.

X-rays are produced in the X-ray tube by accelerating electrons and causing these to collide with the target of the X-ray tube. To understand how this works we need to know about energy (see Ch. 4) and electricity (see Chs 7 and 11) as well as the construction of the X-ray transformer and X-ray tube (see Chs 14 and 30). The X-rays so produced are over a wide band of energies. Most of the photons that have insufficient energy to be of diagnostic value are removed from the beam using filtration (see Ch. 22) and the area of the patient irradiated is restricted using a diaphragm (see Ch. 25). The beam now interacts with the patient and a number of things may happen to the X-ray photons. These may be:

Different tissues will absorb different amounts of radiation (see Ch. 25) and so a differentiated radiation pattern leaves the patient. The scattered radiation is not helpful to the image, so when the scatter production is significant, as much as possible of this is removed using a secondary radiation grid (see Ch. 25). Finally, the radiation interacts with the image receptor, where it is ‘captured’ and passed into a digital imaging system (see Ch. 34).

Radiographers employ other radiations in the course of their clinical work. Magnetic resonance imaging (MRI; see Ch. 39) makes use of powerful magnetic fields as well as radiofrequency waves. Both of these are examples of non-ionizing radiations, which are not thought to cause cancers but can have other biological effects on the human body. Radiofrequency waves are a form of electromagnetic radiation, just like X-rays, but have a much lower frequency. As a result they are much less energetic and fit lower down the electromagnetic spectrum. Ultrasound (see Ch. 42) is another example of a non-ionizing radiation used in radiography. But unlike X-rays and radiofrequency waves, ultrasound needs a medium like fluid or body tissue to pass through. This is because it consists of a series of vibrations passing through molecules and is not a form of electromagnetic radiation. Sound cannot be heard in a vacuum. But X-rays, radio waves and light, which are all electromagnetic, can pass freely through outer space. Sound waves travel at different speeds in different body tissues, whereas X-rays always travel at the same extremely high speed, which is termed the ‘speed of light’– about 300 000 kilometers per second!

1.2.2 Therapeutic radiography

In therapeutic radiography, we are not trying to produce images but are using the biological effects of radiation to kill tumour cells. At the same time, we try to cause as little damage as possible to the healthy cells in the body.

1.3.1 Teletherapy

Here an external source of radiation (normally X-rays or gamma rays) is directed at the tumour. The aim is to give the maximum dose to the tumour and the minimum dose to the healthy tissue. This is often achieved by treating the tumour with a number of fields (see Fig. 1.3). The areas in the patient that receive doses of equal value are joined by lines called isodose lines. The shape and the position of these lines may be altered by the use of absorbing wedges and compensators and also by altering the energy and shape of the treatment beam. Once the treatment plan has been produced, the positions of the treatment fields on the patient are checked in a simulator where the treatment angles can be set up and the fields marked. In areas of the patient around the head and the neck, accurate positioning and immobilization are achieved by placing this part in a specially prepared clear plastic shell.

Figure 1.3 Grossly simplified examples of radiotherapy treatment plans. (A) Three-field plan for a pelvic tumour. (B) Three-field plan for a mediastinal tumour (note how the fields minimize dose to the spinal cord (SC) on the plan).

1.3.2 Brachytherapy

When we use external radiation beams, the radiation must travel through healthy tissue to reach the tumour. There are some situations where tumours are relatively accessible from the body surface or are in body cavities where sealed radioactive sources (see Ch. 19) may be inserted. This technique is known as brachytherapy. The technique can often mean that a large dose can be delivered to the tumour while a much smaller dose strikes the surrounding tissues because of the effects of the inverse square law (see Ch. 26). If the sources were implanted directly by the radiotherapist, the dose to the hands of the operator could be quite large. This is overcome by inserting a number of guides into the correct position and mechanically inserting the sources over the guides for the required treatment. This technique is called afterloading and is often used in the treatment of pelvic cancers.

1.3.3 Nuclear medicine

A third possibility is that the radiation can be delivered to the tissue by allowing the tissue to absorb a certain radionuclide (see Ch. 19). This is probably best illustrated by considering the treatment of an overactive thyroid gland. The activity of the gland can be reduced by surgical removal of part of the gland, or some of its tissue may be destroyed using radiation. To allow the thyroid to produce the required hormones, it must absorb iodine. The patient may be given sodium iodide (where the iodine is in the form of ¹³¹I) as a capsule or in an oral solution. Some of this isotope (see Ch. 19) is taken up by the gland and the rest is secreted in the urine. ¹³¹I is a beta particle emitter (see Ch. 19) and this results in a radiation dose to the thyroid tissue, which reduces its metabolic rate to normal. Because of the limited range of the beta particles produced, there is less radiation dose to structures around the thyroid than there would be if we used an external radiation beam. ¹³¹I also has a relatively short half-life(8 days) (see Ch. 19) and so the radiation hazard posed by the patient to others can be minimized.