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
Computerised tomography (CT)
The same principles are applied in CT as in plain X-rays, but there are two main modifications:
Helical CT scanning
Helical or spiral CT scanning is an improvement that allows very quick scanning of a patient (shorter scanning time than conventional CT) with increased accuracy of lesion detection resulting from volumetric data acquisition. In conventional CT, X-ray exposure and patient movement through the gantry alternate whereas, in helical CT, X-ray exposure and patient movement take place simultaneously giving a ‘spiral impression’.
Advantages
Multi-slice CT scans
Advantages and uses
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
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.
T2 images are known for their contrast. In these images, fluid appears white and fat appears grey.
Summary
Advantages and uses of MRI
Contrast enhanced MRI
The enhanced contrast shows subtle parenchymal as well as leptomengingeal lesions not otherwise visible, e.g. very small metastatic lesions, acoustic neuromas of 2–3 mm, pituitary microadenomas, differentiation of the actual size of tumour from the surrounding oedema and differentiation of more malignant areas from less malignant areas. These are helpful for the purpose of treatment and to select the exact site for biopsy.
Contrast study
Interventional radiology
Intravenous contrast reaction
Usually patients who develop severe reaction have some other aggravating disease as well.
Symptoms and signs
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’).
Prevention and precautions
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: