Diagnostic imaging in emergency patients

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Chapter 4 Diagnostic imaging in emergency patients

The aim of this chapter is to explain briefly the need and usefulness of diagnostic imaging services in emergency situations. Many of these emergencies arise ‘after hours’ and staff in most emergency departments have no immediate access to radiologists. The chapter also outlines the various diagnostic imaging modalities available, the basic principles involved in each modality and some clues to interpreting some of the most obvious lesions.

IMAGING MODALITIES

Ultrasound

Transducers used in this examination pass ultrasound into the body and also receive the echoes. The intensity of the echoes depends on the degree of absorption of sound waves by various tissues. On the image, echogenic areas appear white and sonolucent areas (that transmit sound, e.g. fluid) appear black.

Immediate and instant demonstration of organs by real-time ultrasound imaging and the fact that it is non-invasive and harmless (no radiation) have made this tool very popular. It is used in the following:

Computerised tomography (CT)

The same principles are applied in CT as in plain X-rays, but there are two main modifications:

The resulting images are transverse sections of the part examined. Using the stored information of consecutive thin transverse sections, the images can be reconstructed in sagittal, coronal and oblique planes. CT is now a proven diagnostic tool which delivers valuable information to help in the early diagnosis of many lesions and disease. Its use is greatly appreciated in many emergencies such as head injuries and some chest and abdominal emergencies. It is also useful in demonstrating fractures that are not shown by plain X-rays.

Magnetic resonance imaging (MRI)

In the past 20 years MRI has gradually become the technique of first choice in the investigation of many diseases. The physics involved in MRI is more complex than for any other radiological technique. However, the basic principles are indicated by the original terminology, nuclear magnetic resonance (NMR).

Resonance

This is a phenomenon whereby an object is exposed to an external oscillating disturbance that has a frequency similar to its own frequency of oscillation. Therefore, when a hydrogen proton is exposed to an external disturbance with a similar frequency to its own, the proton gains energy from the external disturbance. This is called resonance. This can happen only if the external disturbance is applied at 90° to the magnetic field of the proton. The oscillation frequency of the hydrogen proton in a static magnetic field of strength used in clinical MRI corresponds to the radiofrequency band (RF) in the electromagnetic spectrum.

Therefore, for resonance of hydrogen to take place, an RF pulse at the same frequency as the oscillation of the hydrogen proton must be applied at 90° to the magnetic field of the proton. The application of the RF pulse that causes resonance is called excitation, as it results in the nuclei gaining energy. This energy causes the magnetic field of the protons to change direction. Enough RF pulse energy is given to the proton to change the direction from the longitudinal to the transverse plane (flip angle of 90°). Now the protons are rotating in the transverse plane. According to the laws of electromagnetism, if a receiver coil is placed in the transverse plane, the transverse magnetisation produces a voltage in the coil. This voltage constitutes the MR signal.

When the RF pulse is turned off, the hydrogen protons return to their original orientations in the longitudinal plane. This is called relaxation. There are two main types of relaxation (T1 and T2). T1 is the return of net magnetisation to the longitudinal plane. T2 is the decay of magnetisation in the transverse plane. These two relaxations and their time variances are used to create imaging sequences. All relaxation times are based on fat and water. This is where most of the body’s hydrogen protons are.

T1 images are known for their anatomical details. In T1 images, fluid appears black and fat appears white.

T2 images are known for their contrast. In these images, fluid appears white and fat appears grey.

Proton density images are a combination of T1 and T2. These images specifically look at the concentration of hydrogen protons.

The above is a simple explanation of the basics, but more complex physics is involved in the formation of MR images which is beyond the scope of this chapter.

Advantages and uses of MRI

No ionising radiation.

Free of artefacts from adjacent bones and gas. Therefore it is excellent to demonstrate soft tissues adjacent to bones, e.g. base of brain and spinal cord.

Excellent resolution—even without contrast enhancement, MR is much more sensitive in detecting contrast differences between various tissues. This is due to the intrinsic differences in hydrogen proton density as well as T1 and T2 relaxation, magnetic susceptibility and motion in various tissues.

Uses:

Contrast study

The main limitation in the use of plain X-rays is the superimposition of the shadows of various organs. In many instances this can be overcome by introducing contrast:

Intravenous contrast reaction

Even though the incidence of fatality is much lower than in street accidents, it is a great worry to doctors. Some statistics show that about 1 in 80,000 patients developed severe or fatal reaction when ionic contrast was used. Incidence of mild reaction is probably about 5–15%, moderate reaction about 1–2% and severe reaction is probably about 0.2%. However, with the use of non-ionic contrast and taking good precautions, the incidence of reaction is said to have reduced to about one-third to one-quarter of the frequency.

Usually patients who develop severe reaction have some other aggravating disease as well.

The exact pathogenesis of the reaction is not very clear. However, the following are possible mechanisms:

Symptoms and signs

Most reactions occur within minutes of injection. However, delayed reactions have also been reported.

Mild reactions—hot flush, burning sensation, arm pain, dizziness, nausea, vomiting, headache and urticaria—are thought to be due to systemic effects as a result of histamine liberation. Usually reassurance and restoration of the patient’s confidence is all that is required, but sometimes oral antihistamine for urticaria, mild analgesics and sometimes tranquillisers for anxiety (5 mg benzodiazepam) may also be helpful.

Moderate reactions involve a slightly more serious manifestation of the above symptoms, with or without a moderate degree of hypotension and bronchospasm. They usually respond to reassurance and antihistamine (IM or IV), benzodiazepam 5 mg, salbutamol inhalation for bronchospasm, hydrocortisone (100–500 mg IM or IV) and occasionally adrenaline 0.3–1 mL of 1/1000 IM. Oxygen by mask is administered.

Severe reaction can be life-threatening and involve a severe form of the above reactions plus convulsion, unconsciousness, laryngeal oedema, bronchospasm, pulmonary oedema, arrhythmia, hypotension, cardiac arrest, anaphylatic shock. Severe reactions require urgent treatment (see treatment of anaphylaxis in Chapter 40, ‘Dermatological presentations to emergency’).

Pre-deposing factors—in the presence of these, the incidence of reaction can be about 2–10 times as severe:

IMAGING OF THE HEAD

Common emergencies are: trauma, severe headaches, collapse, syncope, seizures and stroke.

Trauma

Plain X-rays of skull

Plain X-rays of the face

Most facial injuries can be evaluated clinically. However, X-rays are performed for:

In some cases facial fractures are associated with other serious emergencies, such as intracranial, neck or chest injuries, which may require emergency management such as maintenance of airway. In these patients, X-rays of the face can be postponed to the latter part of the management and may even be deferred for a few days.

Classification

For convenience, facial fractures can be divided into three types: upper third, middle third and lower third.

2. Middle third. This includes the nose, zygoma, orbit (floor, lateral and medial walls) and maxillae (mid-facial bones):