F10168Ch19.qxd 16/10/06 20:50 Page 223 223 Chapter 19 Alternative and specialized imaging modalities INTRODUCTION This chapter provides a summary of these modalities and their main applications in the head An array of medical imaging modalities has been and neck region. developed in recent years and these continue to be developed at a phenomenal rate. Totally new imaging techniques have been introduced, while CONTRAST STUDIES the resolution and image quality of existing systems are continually being refined and These investigations use contrast media, radiopaque improved. Research and development have substances that have been developed to alter focused on manipulating and altering all three of artificially the density of different parts of the the basic requirements for image production — patient, so altering subject contrast — the differ- the patient, the image-generating equipment (to ence in the X-ray beam transmitted through find alternatives to ionizing radiation) and the different parts of the patient’s tissues (see Ch. 18). image receptor. Digital image receptors (solid- Thus, by altering the patient, certain organs, state and photostimulable phosphor plates) are structures and tissues, invisible using conventional now used routinely (see Ch. 6). More and more means, can be seen (see Fig. 19.1). Contrast studies, sophisticated computer software is being and the tissues imaged, include: developed to manipulate the image itself, once it ● Sialography — salivary glands has been captured. Many of these imaging ● Arthrography — joints modalities are playing an increasingly important ● Angiography — blood vessels role in dentistry. As a result, clinicians need to be ● Lymphography — lymph nodes and vessels aware of them and their application in the head ● Urography — kidneys and neck region. The main specialized imaging ● Barium swallow, meal and enema — GI tract modalities include: ● Computed tomography — general enhancement ● Contrast studies (see later). ● Radioisotope imaging (nuclear medicine) ● Computed tomography (CT) Types of contrast media ● Cone beam CT (CBCT) The main types include: ● Ultrasound ● Barium sulphate suspensions for investigating ● Magnetic resonance (MR). the gastrointestinal tract F10168Ch19.qxd 16/10/06 20:50 Page 224 224 ESSENTIALS OF DENTAL RADIOGRAPHY AND RADIOLOGY A C B Fig. 19.1 Examples of different contrast studies. A A left submandibular gland sialograph. B A lateral skull angiograph showing contrast media in the branches of the right internal carotid artery (courtesy of Mrs J. E. Brown). C An abdominal radiograph following a barium meal showing contrast media in the stomach, duodenum and small bowel. F10168Ch19.qxd 16/10/06 20:50 Page 225 Alternative and specialized imaging modalities 225 ● Iodine-based aqueous solutions used for all Patients particularly at risk other investigations and divided into: ● The elderly and very young children – Ionic monomers, including: ● Patients with a history of allergy to contrast * iothalmate (e.g. Conray®) media * metrizoate (e.g. Isopaque®) ● Diabetics * diatrizoate (e.g. Urografin®) ● Patients suffering from: – Ionic dimers, including: – Cardiac failure * ioxaglate (e.g. Hexabrix®) – Renal failure – Non-ionic monomers, including: – Severe pulmonary disorders, including * iopamidol (e.g. Niopam®) asthma. * iohexol (e.g. Omnipaque®) * iopromide (e.g. Ultravist®) Causes of complications ● Iodine-based oil solutions such as Lipiodol® Complications are due mainly to: (iodized poppy seed oil) used for lymphography and sialography ● Allergy ● MR contrast agents (e.g. gadolinium) ● Chemotoxicity ● Osmolality (osmotic pressure of the solution) — Harmful effects of contrast media with ionic monomer contrast media, the osmolality is three times greater than that of other agents; Ideally, contrast media should have no harmful the risk of complications arising when using effects at all. However, there is a small risk these substances is therefore also greater associated with their use, especially with the iodine- ● Anxiety. based aqueous solutions (the so-called general contrast media) when they are introduced into the blood stream. Considering a single dose of Prophylactic measures to minimize complications ● contrast medium contains more than 2000 times as Use of low osmolality contrast agents ● much iodine as the body’s total physiological Skin pre-testing (the value of this is in doubt) ● content, adverse or residual effects are remarkably Prophylactic steroids ● rare. Prophylactic antihistamines ● Reassurance to reduce levels of anxiety Important point to note ● Ask specifically about previous history of Several of the newer imaging modalities now iodine allergy. being used more routinely in dentistry, as discussed later, rely heavily on the use of these Main contrast studies used in the head and contrast-enhancing agents and clinicians should neck therefore be aware of the risks involved. These include: Complications ● Sialography — (see Ch. 33) The main complications associated with contrast ● Arthrography — (see Ch. 29) media can be divided into: ● Computed tomography — to provide general enhancement (see later) ● Mild, e.g. headache, nausea, warmth and/or ● Angiography — this involves the introduction pain, flushing, sneezing and constipation (GI of aqueous iodine-based contrast media into investigations) selected blood vessels. In the head and neck ● Moderate, e.g. vomiting, bronchospasm, urticaria region, this involves usually the carotids and hypotension (common, internal or external) or the vertebral ● Severe, e.g. cardiac arrhythmias, cardiac arrest, arteries. convulsions, anaphylactic shock and pulmonary oedema The procedure usually entails introducing a ● Fatal. catheter into a femoral artery followed by F10168Ch19.qxd 16/10/06 20:50 Page 226 226 ESSENTIALS OF DENTAL RADIOGRAPHY AND RADIOLOGY selective catheterization of the carotid or vertebral stationary gamma camera (see Fig. 19.2). This arteries, as required, using fluoroscopic control. investigation allows the function and/or the Once the catheter is sited correctly, the contrast structure of the target tissue to be examined under medium is injected and radiographs of the both static and dynamic conditions. appropriate area taken (see Fig. 19.1B). Radioisotopes and radioactivity Main indications for angiography in the head and Radioisotopes, as defined in Chapter 2, are isotopes neck with unstable nuclei which undergo radioactive ● To show the vascular anatomy and feeder disintegration. This disintegration is often vessels associated with haemangiomas. accompanied by the emission of radioactive ● To show the vascular anatomy of arteriovenous particles or radiation. The important emissions malformations. include: ● Investigation of suspected subarachnoid haem- orrhage resulting from an aneurysm in the ● Alpha particles Circle of Willis. ● Beta– (electron) and beta+ (positron) particles ● Investigation of transient ischaemic attacks ● Gamma radiation. possibly caused by emboli from atheromatous The main properties and characteristics of these plaques in the carotid arteries. emissions are summarized in Table 19.1. Radioisotopes used in conventional nuclear RADIOISOTOPE IMAGING medicine Radioisotope imaging relies upon altering the Several radioisotopes are used in conventional patient by making the tissues radioactive and the nuclear medicine, depending on the organ or tissue patient becoming the source of ionizing radiation. under investigation. Typical examples together This is done by injecting certain radioactive with their target tissues or target diseases include: compounds into the patient that have an affinity ● Technetium (99mTc) — salivary glands, thyroid, for particular tissues — so-called target tissues. The bone, blood, liver, lung and heart radioactive compounds become concentrated in ● Gallium (67Ga) — tumours and inflammation the target tissue and their radiation emissions are ● Iodine (123I) — thyroid then detected and imaged, usually using a ● Krypton (81K) — lung. A B Fig. 19.2 Technetium bone scan. A Left lateral head and neck image. B Anterior chest image. F10168Ch19.qxd 16/10/06 20:50 Page 227 Alternative and specialized imaging modalities 227 99mTc is the most commonly used radioisotope. Its Advantages over conventional radiography main properties include: ● Target tissue function is investigated. ● Single 141 keV gamma emissions which are ● All similar target tissues can be examined during ideal for imaging purposes one investigation, e.g. the whole skeleton can be ● 1 A short half-life of 6–2 hours which ensures a imaged during one bone scan. minimal radiation dose ● Computer analysis and enhancement of results ● It is readily attached to a variety of different are available. substances that are concentrated in different organs, e.g.: Disadvantages – Tc + MPD (methylene diphosphonate) in bone ● – Tc + red blood cells in blood Poor image resolution — often only minimal – Tc + sulphur colloid in the liver and spleen information is obtained on target tissue anatomy. ● ● It can be used on its own in its ionic form The radiation dose to the whole body can be (pertechnetate 99mTcO 4–), since this is taken up relatively high. ● selectively by the thyroid and salivary glands Images are not usually disease-specific. ● ● It is easily produced, as and when required, on Difficult to localize exact anatomical site of site. source of emission. ● Some investigations take several hours. ● Facilities are not widely available. Main indications for conventional isotope Further developments