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Diagnostic X-ray imaging is very common in modern medicine. For example, in the UK, over 40 million radiological examinations are performed each year. The great majority of these are conventional medical and dental X-rays but increasing numbers of more sophisticated tests, such as CT scans, are being undertaken. They may be carried out as part of the investigation of symptoms in an individual patient or as part of a general screening process. In the latter case, the tests are known as asymptomatic because they are not undertaken in response to reported symptoms but as part of a programme for early detection of certain medical conditions. An important example of this is mammography as part of a breast screening programme, which is made available to women in the 50–70 year old age group in order to detect early signs of breast cancer.

Most diagnostic radiographic examinations are performed using X-ray sets in combination with some type of digital image acquisition system. As a result of recent technological developments, almost all ‘film–screen’ systems have been replaced by digital radiography (DR) or computed radiography (CR) systems. These use solid-state detection systems and have the advantage that the resulting image is stored in electronic format and can be accessed remotely by medical staff. Digital data files allow for image manipulation, processing and analysis, which were not possible with film-based detectors. This can lead to improved diagnosis, and provides opportunities for patient dose reduction.

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Whichever technique is used, careful selection of the X-ray beam quality (voltage) and the exposure enables good-quality radiographs to be obtained with quite small doses to the patient. For example, using the latest technology and the best techniques available, a chest X-ray can deliver as little as 150 μGy entrance surface dose to the chest of the patient (which gives an effective dose of only 10 μSv). With properly designed and operated systems, and with the minimum practicable beam size, the dose to other parts of the body will be much less than this. In some cases it may be necessary to take more than one radiograph, but clearly the number of 'shots' should be kept to the absolute minimum.

The dose to the radiographer is minimized by good design of the facility, for example by the provision of a shielded cubicle in which the radiographer must stand to operate the set. Occasionally, difficulties arise; for example, young children may need to be held in the correct position. If a harness cannot be used it is better for the parent rather than the radiographer to hold the child, as the parent is unlikely to be exposed frequently in this way. A similar problem sometimes occurs in dental radiography when it is not possible to clamp the film in position in the mouth. In this case, the patient should hold the film rather than the dentist or other members of the staff.

In addition to the hazard from the primary beam, X-rays are scattered from the patient or nearby materials, so constituting a further hazard. This scatter needs to be considered when deciding how to protect the staff or members of the public who might be involved.

An important point to bear in mind in medical X-ray work is that a significant reduction in dose can be obtained with quite thin shielding because of the relatively low X-ray energy used (often less than 100 kVp). For example, lead-impregnated materials are available, which can be made into aprons and gloves and are equivalent in shielding ability to approximately 0.25–0.35 mm of lead, which is suitable to protect staff from scattered radiation. Similarly, the lead-glass of control room windows is equivalent to 2 mm of lead and is sufficient to protect radiographers from the primary X-ray beam.

14.3.2 Diagnostic fluoroscopy

In fluoroscopy, the detection system is a fluorescent screen coupled to an image intensifier.

Rather than the single short-duration pulse used in radiography, the X-ray tube remains on (or is continuously pulsed) during the examination. The screen fluoresces under irradiation and therefore gives a live picture. The principle of this technique is illustrated in Figure 14.1.

X-ray tube

Figure 14.1 Principle of fluoroscopic examination.

The output from the image intensifier can be fed to a video system allowing the medical staff to view the moving images on a television monitor, which is often outside the controlled radiation area. In ‘digital’ fluoroscopic systems the analogue video system can be digitized with an analogue-to-digital converter. Alternatively, digitization may be accomplished with a digital video camera (charge-coupled device) or via direct capture of X-rays with a flat panel detector (FPD) similar to the type used in modern radiographic systems.

In some types of examination, much higher-quality images can be obtained, often with reduced dose, by injection of contrast media. These are chemical solutions that absorb X-rays more effectively than the body organs or fluids and so give enhanced images. This technique is commonly used in angiography, which is concerned with investigations of blood vessels.

Fluoroscopy is also used during interventional procedures so that, for example, a surgeon can view procedures being undertaken inside the body of the patient. A modern fluoroscopic facility is shown in Figure 14.2. From a staff protection viewpoint, an important consideration is that the surgeon’s hands may be close to the X-ray beam for appreciable periods of time and the resulting ‘extremity’ doses need to be monitored and controlled. In addition, a significant amount of radiation is back-scattered from the patient, and medical staff within approximately 2 metres of the patient must wear lead aprons and, if required, lead thyroid shields. It might also be necessary for them to make use of lead-glass spectacles or a lead-glass screen to protect their eyes.

Lead flaps Lead apron Display

monitor with ‘last image hold’

Thyroid shield worn on neck

Over-couch image intensifier

Figure 14.2 The use of fluoroscopy equipment. Reproduced from Huntingdon Daily.com

The annual occupational effective dose to interventional radiologists and cardiologists is typically 2 mSv, although this can be significantly higher depending on their workload and the radiation protection control measures they employ. In some cases, the radiation exposure of this group may exceed three-tenths of the annual dose limit and make it necessary for them to be designated in the UK as classified persons for occupational dose monitoring purposes.

14.3.3 Computed tomography

Computed tomography (CT) uses an X-ray tube and an array of detectors arranged in a supporting framework to rotate around the patient. A continuously rotating collimated X-ray beam passes through the body, and the output from the detectors is analysed by a computer, which produces pictures of cross-sections, or slices, of the body. As in the case

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of fluoroscopy, the quality of the images can be greatly increased by injection of contrast media. The principle of operation is illustrated in Figure 14.3, which shows a CT system in which the source and detector system are rotated around the patient as he or she is traversed through the system. A typical modern installation is shown in Figure 14.4. CT is used for many types of radiological examination and is particularly useful for the diagnosis and follow-up of malignant tumours. Modern CT equipment is capable of multi-slice helical scanning where the patient table, and therefore the patient, moves through the X-ray fan beam while the tube is rotating. Rather than just a slice, or set of slices, a ‘volume’

of the patient is irradiated. This leads to a much faster acquisition of imaging data from a greater section of the patient but, if not controlled properly, may lead to increases in patient effective dose.

X-ray tube

Detector array Figure 14.3 Schematic illustration of transmission computed tomography.

Figure 14.4 A modern computed tomography installation (courtesy of Philips Healthcare).

Computed tomography scanning achieves high-contrast resolution by using a high X-ray tube output, which in turn leads to relatively high patient doses. The actual dose received by the patient depends on the type and extent of the examination, but typical effective doses are between 1 mSv and 10 mSv. Although CT is a low-frequency technique representing only 7 per cent of all diagnostic X-ray examinations carried out annually in the UK, its contributes nearly 50 per cent of the total UK collective dose from diagnostic X-ray procedures.

The equipment is normally located in a shielded room with the radiographer located in an adjacent control room with a lead-glass viewing window. In the event that the patient needs attention, the X-ray beam would automatically switch off when the door is opened.

Another type of tomographic technique used in diagnosis is positron emission tomography (PET), which involves injection of radioisotopes into the body. This is discussed later in section 14.5.

14.4 RADIOTHERAPY

It has been noted that the main application of radiotherapy is in the treatment of cancer.

The aim is to deliver as high a dose as possible to the malignant tissue without causing excessive injury to surrounding healthy tissue. Typically, absorbed doses of a few tens of grays are required and they are usually delivered as a series of smaller doses, for example 20 doses of 2 Gy at intervals of 2 or 3 days. This fractionation is necessary to reduce deterministic side-effects.

In teletherapy, or external beam therapy, the radiation is administered by a machine positioned some distance from the patient. The most common method of treatment uses equipment such as linear accelerators to deliver high-energy electron beams of 6–20 MeV or high-energy X-rays of 6 MVp. However, in some countries, collimated beams of g radiation from large Co-60 sources are still used. For treatment of superficial tissue, X-rays of about 200 kVp are often used.

In addition to selection of the appropriate energy, the dose to healthy tissue is minimized by varying the direction of the beam through the body. This is done either by using a different orientation for each treatment or by continuously rotating the source around the tumour during the treatment. The principle is illustrated in Figure 14.5, which shows treatment of a brain tumour using a rotating teletherapy unit containing a linear accelerator. Although the tumour is being irradiated continuously, the surrounding regions are exposed for only a small fraction of the time. It is essential to use a well-defined beam, and this is achieved by means of collimators. Modern systems have collimators that incorporate many overlapping and independently movable ‘leaves’, called multi-leaf collimators (MLCs). Intensity-modulated radiation therapy (IMRT) is an advanced external beam technique used to minimize the amount of normal tissue being irradiated. The radiation beam intensity modulation is achieved by moving the leaves in the MLC during the course of treatment, thereby delivering a radiation field with a non-uniform (i.e. modulated) intensity.

Except in some low-voltage (< 100 kVp) superficial X-ray therapy, the problem of providing local shielding is such that the treatment must be performed in a shielded room with interlocks arranged to shut down the equipment should the door be opened. In so-called ‘mega-voltage’ therapy, such as that carried out with linear accelerators (or ‘linacs’), the equipment is housed in a room with no windows and concrete walls that are over a metre thick. Entry into the room is often via a shielded maze, which is designed to reduce scattered

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Patient with head tumour

Gamma beam

Linear Accelerator

Rotates around patient

Figure 14.5 Treatment of a brain tumour using a linear accelerator teletherapy unit. (Image courtesy of Varian Medical Systems of Palo Alto, California. Copyright (2012), Varian Medical Systems. All rights reserved.)

radiation to outside areas. The radiotherapist remains outside the room, observing the patient via closed-circuit television (CCTV) and communicating via an intercom system.

All radiotherapy is carried out within an overall system of quality assurance. This requires a detailed treatment plan for the patient and the application of suitable quality controls at all stages. Usually, this involves a simulation system that includes a CT scan in order to define precisely the region to be irradiated. Often, a mould is produced for a patient and this serves as both a patient immobilizer and a means of defining beam direction. The timing of exposures is under automatic control to ensure the correct dose to the target area.

Regular testing and calibration of equipment (often daily) is a key aspect of quality assurance.

In addition to gammas or electrons, beams of protons can be used to treat cancer.

Protons with energies ranging typically from 70 to 250 MeV can be produced from particle accelerators such as cyclotrons or synchrotrons. The advantage of proton therapy is the ability to localize the radiation dosage more precisely when compared with other types of external beam radiotherapy.

As noted earlier, radiotherapy can also be effected by brachytherapy, which involves the application of small, sealed sources into the tumour (interstitial brachytherapy) or adjacent to a tumour (intracavitory or contact brachytherapy). The sources are normally either applied by surface applicators or inserted into body cavities or organs by specially designed delivery systems called remote afterloaders. In these cases, the exposure is fractionated, with individual exposures lasting from a few minutes to a few hours. In some cases, for example for treatment of prostate cancer, small radioactive pellets or ‘seeds’

are surgically implanted and remain in the body delivering a dose at a relatively low rate until the required dose has been delivered. The most common types of source used in brachytherapy are iridium-192, caesium-137 and cobalt-60.

The source delivery systems are designed to minimize radiation exposure of staff.

Where seeds are implanted into patients, special attention has to be given to the control of exposure of nursing and medical staff and controls need to be placed on visitors. Adequate protection can be achieved by sensible application of the principles of time, distance and shielding. In the case of sources that are reused, regular leakage testing is required

and written emergency procedures should specify the actions to be taken in the event of damage to or loss of a source.

It is possible, in some cases, for patients containing sources to be discharged. This is decided on a case-by-case basis, taking account of the radionuclide, the half-life and the dose rate, which together define the risk to other persons.

Also, in radioisotope therapy, cancers can be treated using unsealed radioactive materials. For example, many gigabequerels of iodine-131 can be administered orally to patients in the radiotherapy of thyroid cancer. The use of unsealed radioisotopes in medical diagnosis and therapy is dealt with in section 14.5.

Finally, it must be re-emphasized that when external beams of radiation are used, or sealed sources or radioactivity injected into the body, the aim in radiotherapy is always to deliver a precisely predetermined dose to the target region while minimizing as far as possible the dose to adjacent healthy tissue.