Radioisotopes, as defined in Chapter 2, are isotopes with unstable nuclei that undergo radioactive disintegration. This disintegration is often accompanied by the emission of radioactive particles or radiation. The important emissions include:

• Alpha particles

• Beta⁻ (electron) and beta⁺ (positron) particles

• Gamma radiation.

The main properties and characteristics of these emissions are summarized in Table 18.1.

Table 18.1

Summary of the main properties and characteristics of radioactive emissions

Property	Alpha particles	Beta^– particles	Gamma rays	Beta⁺ particles
Nature	Particulate – two protons and two neutrons	Particulate – electrons	Electromagnetic radiation – identical to X-rays	Particulate – positron, interacts very rapidly with a negative electron to produce 2 gamma rays – annihilation radiation – properties as shown in adjacent column
Size	Large	Small	Nil
Charge	Positive	Negative	Nil
Speed	Slow	Fast	Very fast
Range in tissue	1–2 mm	1–2 cm	As with X-rays
Energy range carried	4–8 MeV	100 keV–6 MeV	1.24 keV–12.4 MeV
Damage caused	Extensive ionization	Ionization	Ionization – similar damage to X-rays
Use in nuclear	Banned	Very limited	Main emission used	PET

Radioisotopes used in conventional nuclear medicine

Several radioisotopes are used in conventional nuclear medicine, depending on the organ or tissue under investigation. Typical examples together with their target tissues or target diseases include:

• Technetium (^99mTc) – salivary glands, thyroid, bone, blood, liver, lung and heart

• Gallium (⁶⁷Ga) – tumours and inflammation

• Iodine (¹²³I) – thyroid

• Krypton (⁸¹K) – lung.

^99mTc is the most commonly used radioisotope. Its main properties include:

• Single 141 keV gamma emissions which are ideal for imaging purposes

• A short half-life of hours which ensures a minimal radiation dose

• It is readily attached to a variety of different substances that are concentrated in different organs, e.g.:

– Tc⁺ MPD (methylene diphosphonate) in bone

– Tc⁺ red blood cells in blood

– Tc⁺ sulphur colloid in the liver and spleen

• It can be used on its own in its ionic form (pertechnetate ^99mTcO^4–), since this is taken up selectively by the thyroid and salivary glands

• It is easily produced, as and when required, on site.

Main indications for conventional isotope imaging in the head and neck

• Tumour staging – the assessment of the sites and extent of bone metastases

• Investigation of salivary gland function, particularly in Sjögren’s syndrome (see Ch. 32)

• Evaluation of bone grafts

• Assessment of continued growth in condylar hyperplasia

• Investigation of the thyroid

• Brain scans and assessment of a breakdown of the blood–brain barrier.

Advantages over conventional radiography

• Target tissue function is investigated.

• Computer analysis and enhancement of results are available.

Disadvantages

• There is poor image resolution – often only minimal information is obtained on target tissue anatomy.

• The radiation dose to the whole body can be relatively high.

• Images are not usually disease-specific.

• Difficult to localize exact anatomical site of source of emission.

• Some investigations take several hours.

• Facilities are not widely available.

Further developments in radioisotope imaging techniques include:

• Single photon emission computed tomography (SPECT), where the photons (gamma rays) are emitted from the patient and detected by a gamma camera rotating around the patient and the distribution of radioactivity is displayed as a cross-sectional image or SPECT scan enabling the exact anatomical site of the source of the emissions to be determined (see Fig. 18.3A).

Fig. 18.3A An axial SPECT scan showing increased activity (*hot spot*) in the right condyle (arrowed). (Kindly provided by Dr J Rees.)

• Positron emission tomography (PET). As shown in Table 18.1, some radioactive isotopes decay by the emission of a positively charged electron (positron) from the nucleus. This positron usually travels a very short distance (1–2 mm) before colliding with a free electron. In the ensuing reaction, the mass of the two particles is annihilated with the emission of two (photons) gamma rays of high energy (511 keV) at almost exactly 180° to each other. These emissions, known as annihilation radiation, can then be detected simultaneously (in coincidence) by opposite radiation detectors which are arranged in a ring around the patient. The exact site of origin of each signal is recorded and a cross-sectional slice is displayed as a PET scan (see Fig. 18.3B). The major advantages of PET as a functional imaging technique are due to this unique detection method and the variety of new radioisotopes which can now be used clinically. These include:

Fig. 18.3B An axial co-registered PET and CT scan showing accumulation of ¹⁸FDG in the left tongue base (arrowed). (Kindly provided by Dr J Rees.)

As in conventional nuclear medicine, these radioisotopes can be used on their own or incorporated into diverse and biologically important compounds (e.g. glucose, amino acids and ammonia) and then administered in trace amounts to study:

– Tissue perfusion

– Substrate metabolism, often using ¹⁸F-fluorodeoxyglucose (¹⁸FDG)

– Cell receptors

– Neurotransmitters

– Cell division

– Drug pharmacokinetics.

PET can therefore be used to investigate disease at a molecular level, even in the absence of anatomical abnormalities apparent on CT or MRI (see later). It is also possible to superimpose a PET scan on a CT scan, by a technique known as co-registration, to determine a lesion’s exact anatomical position. Clinically it has been used in the management of patients with epilepsy, cerebrovascular and cardiovascular disease, dementia and malignant tumours.

Computed tomography (CT)

CT scanners use X-rays to produce computer generated tomographs – sectional or slice images (see Ch. 15). In CT the radiographic film is replaced by very sensitive crystal or gas detectors. The detectors measure the intensity of the X-ray beam emerging from the patient and convert this into digital data which are stored and can be manipulated by a computer, as described in Chapter 7. This numerical information is converted into a grey scale representing different tissue densities, thus allowing a visual image to be generated (see Fig. 18.4).

Fig. 18.4 An axial CT image of the head.

Equipment and theory

The CT scanner is essentially a large square piece of equipment (the gantry) with a central circular hole, as shown in Fig. 18.5A. The patient lies down with the part of the body to be examined within this circular hole. The gantry houses the X-ray tubehead and the detectors. The mechanical geometry of scanners varies. In so-called third-generation scanners, both the X-ray tubehead and the detector array revolve around the patient, as shown in Fig. 18.5. In so-called fourth-generation scanners, there is a fixed circular array of detectors (as many as 1000) and only the X-ray tubehead rotates, as shown in Fig. 18.5B. Whatever the mechanical geometry, each set of detectors produces an attenuation or penetration profile of the slice of the body being examined. The patient is then moved further into the gantry and the next sequential adjacent slice is imaged. The patient is then moved again, and so on until the part of the body under investigation has been completed. This stop–start movement means the investigation takes several minutes to complete and the radiation dose to the patient is high.

Fig. 18.5 Diagrams showing the principles of A a *third-generation* CT scanner – both the X-ray tubehead and the detector array rotate around the patient, B a *fourth-generation* CT scanner – the X-ray tubehead rotates within a stationary ring of detectors, and C *spiral CT* – the tubehead and detectors move in a continuous spiral motion around the patient as the patient moves continuously into the gantry in the direction of the solid arrows. D The Philips MX 8000 multislice spiral CT scanner.

As a result, spiral CT has been developed in recent years. Acquiring spiral CT data requires a continuously rotating X-ray tubehead and detector system in the case of third-generation scanners or, for fourth-generation systems, a continuously rotating X-ray tubehead. This movement is achieved by slip-ring technology. The patient is now advanced continuously into the gantry while the equipment rotates, in a spiral movement, around the patient, as shown in Fig. 18.5C. The investigation time has been shortened to only a few seconds with a radiation dose reduction of up to 75%.

Whatever type of scanner is used, the level, plane and thicknesses (usually between 1.5 mm and 6 mm) of the slices to be imaged are selected and the X-ray tubehead rotates around the patient, scanning the desired part of the body and producing the required number of slices. These are usually in the axial plane, as was shown in Fig. 18.4.

The sequence of events in image generation can be summarized as follows:

• As the tubehead rotates around the patient, the detectors produce the attenuation or penetration profile of the slice of the body being examined.

• The computer calculates the absorption at points on a grid or matrix formed by the intersection of all the generation profiles for that slice.

• Each point on the matrix is called a pixel and typical matrix sizes comprise either 512 × 512 or 1024 × 1024 pixels. The smaller the individual pixel the greater the resolution of the final image (as described for digital imaging in Chs 4 and 5).

• The area being imaged by each pixel has a definite volume, depending on the thickness of the tomographic slice, and is referred to as a voxel (see Fig. 18.6).

Fig. 18.6 Diagram of a typical CT *pixel* matrix and an individual *voxel*.

• Each voxel is given a CT number or Hounsfield unit between, for example, +1000 and −1000, depending on the amount of absorption within that block of tissue (see Table 18.2).

Table 18.2

Typical CT numbers for different tissues

Tissue	CT number	Colour
Air	–1000	Black
Fat	–100 to –60
Water	0
Soft tissue	+40 to +60
Blood	+55 to +75
Dense bone	+1000	White

• Each CT number is assigned a different degree of greyness, allowing a visual image to be constructed and displayed on the monitor.

• The patient moves through the gantry and sequential adjacent sections are imaged.

• The selected images are photographed subsequently to produce the hard copy pictures, with the rest of the images remaining on disc.

Image manipulation

The major benefits of computer-generated images are the facilities to manipulate or alter the image and to reconstruct new ones, without the patient having to be re-exposed to ionizing radiation (see also Ch. 7).

Window level and window width

These two variables enable the visual image to be altered by selecting the optimal range and level of densities to be displayed.

• Window level – this is the CT number selected for the centre of the range, depending on whether the lesion under investigation is in soft tissue or bone.

• Window width – the range selected to view on the screen for the various shades of grey, e.g. a narrow range allows subtle differences between very similar tissues to be detected.

Reconstructed images

The information obtained from the original axial scan can be manipulated by the computer to reconstruct tomographic sections in the coronal, sagittal or any other plane that is required, or to produce three-dimensional images, as shown in Fig. 18.7. However, to minimize the step effect evident in these reconstructed images, the original axial scans need to be very thin and contiguous or overlapping with a resultant relatively high dose of radiation to the patient.

Fig. 18.7 A reconstructed three-dimensional CT image of the skull and facial bones.

Main indications for CT in the head and neck

• Investigation of intracranial disease including tumours, haemorrhage and infarcts

• Investigation of suspected intracranial and spinal cord damage following trauma to the head and neck

• Assessment of fractures involving:

– The orbits and naso-ethmoidal complex

– The cranial base

– The odontoid peg

– The cervical spine

• Assessment of the site, size and extent of cysts, giant cell and other bone lesions (see Fig. 18.8).

Fig. 18.8 Part of an axial CT scan showing an expansive cystic lesion in the anterior palate (black arrows). Mucosal thickening within the antrum is also evident (white arrows). (Kindly provided by Dr J. Kabala and Dr J. Luker.)

• Assessment of disease within the paranasal air sinuses (see Ch. 31)

• Tumour staging – assessment of the site, size and extent of tumours, both benign and malignant, affecting:

– The maxillary antra

– The base of the skull

– The pterygoid region

– The pharynx

– The larynx

• Investigation of tumours and tumour-like discrete swellings both intrinsic and extrinsic to the salivary glands

• Investigation of osteomyelitis

• Investigation of the TMJ

• Preoperative assessment of maxillary and mandibular alveolar bone height and thickness before inserting implants (see Ch. 23).

Advantages over conventional film-based tomography

• Very small amounts, and differences, in X-ray absorption can be detected. This in turn enables:

– Detailed imaging of intracranial lesions

– Imaging of hard and soft tissues

– Excellent differentiation between different types of tissues, both normal and diseased.

• Images can be manipulated.

• Axial tomographic sections are obtainable.

• Reconstructed images can be obtained from information obtained in the axial plane.

• Images can be enhanced by the use of IV contrast media to delineate blood vessels.

Disadvantages

• The equipment is very expensive.

• Very thin contiguous or overlapping slices result in a generally high dose investigation (see Ch. 3).

• Metallic objects, such as fillings, may produce marked streak or star artefacts across the CT image.

• There are inherent risks associated with IV contrast agents (see earlier).

Ultrasound

Diagnostic ultrasound is now established as the first choice imaging modality for soft tissue investigations of the face and neck, particularly of the salivary glands (see Ch. 32). It is a non-invasive investigation that uses a very high frequency (7.5–20 MHz) pulsed ultrasound beam, rather than ionizing radiation, to produce high resolution images of more superficial structures. The use of colour power Doppler allows blood flow to be detected.

Equipment and theory

Several machines are available – an example is shown in Fig. 18.9. The high frequency ultrasound beam is directed into the body from a transducer placed in contact with the skin. Jelly is placed between the transducer and the skin to avoid an air interface, as shown in Fig. 18.10A. As the ultrasound travels through the body, some of it is reflected back by tissue interfaces to produce echoes, which are picked up by the same transducer and converted into an electrical signal and then into a real-time black, white and grey visual echo picture, which is displaced on a computer screen. Examples are shown in Fig. 18.10.

Fig. 18.9 The Philips iU 22 diagnostic ultrasound machine.

Fig. 18.10 A A patient undergoing an ultrasound investigation of the left submandibular salivary gland. The transducer is placed against the skin overlying the gland. Note the jelly between the transducer and the skin. B Ultrasound image of the submental region showing the use of Doppler colour flow imaging to detect blood flow in the hilar vessels of a small submental lymph node (arrow heads). **C (i)** Ultrasound image of a pleomorphic adenoma in the parotid gland showing a hypoechoic (dark) mass with a well-defined, lobulated outline; **(ii)** ultrasound image of the same patient with colour power Doppler showing blood flow in the fine vessels within the benign tumour. D Ultrasound image of the submental region demonstrating the value of colour Doppler imaging in differentiating vessels from other structures; **(i)** shows a submental vessel (arrowed) while **(ii)** illustrates blood flow within the vessel. (Figures B, C and D kindly provided by Mrs J.E. Brown.)

The ultrasound image is also a sectional image or tomograph, but it represents a topographical map of the depth of tissue interfaces, just like a sonar picture of the seabed. The thickness of the section is determined by the width of the ultrasound beam. Areas of different density in the black/white echo picture are described as hypoechoic (dark) or hyperechoic (light).

Utilization of the Doppler effect – a change in the frequency of sound reflected from a moving source – allows the detection of arterial and/or venous blood flow, as shown in Fig. 18.10. As can be seen, the computer adds the appropriate colour, red or blue, to the vascular structures in the visual echo picture image, making differentiation between structures straightforward.

The ultrasound wave must be able to travel through the tissue to return to the transducer. If it is blocked by the tissue, no image will result. Since air, bone and other calcified materials block nearly all the low frequency ultrasound beam its diagnostic use is limited. However, the newer high frequency machines enable penetration of more superficial structures to provide high resolution images.

Main indications for ultrasound in the head and neck

• Evaluation of swellings of the neck, particularly those involving the thyroid, cervical lymph nodes or the major salivary glands – ultrasound is now regarded as the investigation of choice for detecting solid and cystic soft tissue masses

• Detection of salivary gland and duct calculi (see Ch. 32)

• Determination of the relationship of vascular structures and vascularity of masses with the addition of colour flow Doppler imaging

• Assessment of blood flow in the carotids and carotid body tumours

• Assessment of the ventricular system in babies by imaging through the open fontanelles

• Therapeutically, in conjunction with the sialolithotripter, to break up salivary calculi into approximately 2-mm fragments which can then pass out of the ductal system so avoiding major surgery

• Ultrasound-guided fine-needle aspiration (FNA) biopsy.

Advantages over conventional X-ray imaging

• Sound waves are NOT ionizing radiation.

• There are no known harmful effects on any tissues at the energies and doses currently used in diagnostic ultrasound.

• Images show good differentiation between different soft tissues and are very sensitive for detecting focal disease in the salivary glands.

• The technique is widely available and relatively inexpensive.

Disadvantages

• Ultrasound has limited use in the head and neck region because sound waves are blocked by bone. Its use is therefore restricted to the superficial structures.

• The technique is operator-dependent.

• Images can be difficult to interpret for inexperienced operators.

• Real-time imaging means that the radiologist must be present during the investigation.

Magnetic resonance (MR)

Magnetic resonance is another specialized imaging modality that does not involve the use of ionizing radiation. It is now widely available and is becoming increasingly sophisticated and important in imaging intracranial and soft tissue lesions.

Essentially it involves the behaviour of protons (positively charged nuclear particles, see Ch. 2) in a magnetic field. The simplest atom is hydrogen, consisting of one proton in the nucleus and one orbiting electron, and it is the hydrogen protons that are used to create the MR image. The image itself is another example of a tomograph or sectional image that at first glance resembles a CT image.