First principles of emergency imagingchapter 1

The discovery of X-rays by German physicist Wilhelm Conrad Roentgen in 1885 has profoundly influenced medical diagnosis and treatment. Radiographs are used in nearly 70% of diagnostic imaging examinations.

Radiographs are produced when X-rays pass through a part of the body and the resultant image is captured on a film or imaging plate. The image on the film is the result of the interaction of the X-rays with various tissues of the body part imaged. The amount the X-rays penetrate the body tissue is dependent on the density of the tissue. For example, bone, which has a higher density than soft tissue, allows fewer X-rays to pass through it and hence appears ‘white’, whereas soft tissue allows more X-rays to pass through and appears dark.

Permitting the passage of X-rays with little attenuation and hence almost entirely invisible on X-ray and under fluoroscopy.

The property of being relatively resistant to the passage of X-rays.

Ultrasound is an imaging technique that uses high-frequency sound waves and their echoes. The ultrasound machine transmits high-frequency (1 to 15 megahertz (MHz)) sound pulses into the body using a probe. Some of the sound waves reflect back to the probe, while some travel on further. The reflected waves are detected by the probe and relayed to the machine. The machine calculates the distance from the probe to the tissue using the speed of sound in tissue (1540 m/s) and the time of each echo's return. This is converted into two-dimensional grayscale images.

Doppler echoes are usually displayed with grayscale brightness corresponding to their intensity. In colour Doppler, echoes are displayed with colours corresponding to the direction of flow that their positive or negative Doppler shifts represent (towards or away from the transducer). The brightness of the colour represents the intensity of the echoes.

A structure that does not produce any internal echoes.

A relative term used to describe an area that has decreased brightness of echoes relative to an adjacent structure.

Also a relative term, used to describe a structure that has increased brightness of echoes relative to an adjacent structure.3

Fluoroscopy is a common technique used to obtain real-time images of moving body parts and internal structures of a patient compared to static radiographic examinations. In the context of emergency imaging, fluoroscopy is mostly used in paediatric patients to look for upper or lower gastrointestinal (GI) obstructions. It is also used for hydrostatic reduction of intussusception.

Computed tomography (CT) uses multiple thin beams of X-rays that pass through a desired volume (part of the body being scanned) from multiple angles (usually over 180 degrees). As the X-rays penetrate the body, the X-ray beams will be attenuated, depending on the type of tissue they have travelled through. On the opposite side from where the X-rays originate, there is an array of detectors measuring the amount of X-rays that have travelled through the volume. This allows determination of the attenuation of individual beams as they pass through the volume.4

Having a density similar to that of another or adjacent tissue, e.g. isodense subdural haematoma.

Appears less dense than the surrounding tissue, e.g. fat is hypodense when compared to bone on a CT image.

Appears denser than adjacent tissue, e.g. fresh blood is hyperdense.

An MRI scanner applies a strong magnetic field to an area of interest. Hydrogen atoms present in most tissue placed within this field will mostly align parallel to the external magnetic field. As they align they also spin like a spinning top (precess). Electromagnetic radiation or a radio-frequency pulse is then applied to the precessing nuclei. When the electromagnetic field is turned off, the nuclei will return to their original precession around the external magnetic field. This involves two processes: T1 and T2 relaxation. The information is then converted into an image by computers in the MRI scanner.

T1-weighted images convey the longitudinal relaxation time of tissues, whereas T2-weighted images depict the transverse relaxation time.

Brighter than adjacent tissue, e.g. water is hyperintense on T2 images and fat is hyperintense on T1 images.6

Darker than adjacent tissue, e.g. water is hypo-intense on T1 images.

Body structures with different densities (i.e. on a chest X-ray the difference between the high-density bone and low-density air provides a contrast) are well seen on plain radiographs and CT. However, it is not possible to distinguish structures with similar density or average atomic numbers. In such cases, a contrast medium is used to improve the contrast between different organs, either by changing the density of the organ (e.g. air in the large bowel in colonography) or increasing the attenuation and Hounsfield unit by injecting a substance such as an iodine-containing contrast medium.

When a power injector is used, a 22G or larger needle/cannula 1.25 – 1.5 inch length is preferred for IV contrast injection. It is advisable to obtain a good backflow of blood to test adequate positioning of the needle in the vein.7

Adequate positioning of the cannula in the vein is then checked again by flushing 10 ml of saline into the vein before delivering the injection of contrast.

Adverse reactions to iodinated contrast media are extremely low, but do occur. They are broadly classified as:8

Contrast media can result in acute renal failure, leading to morbidity and mortality. Contrast media are toxic to the renal tubular ceells as well as resulting in renal ischaemia. Therefore, prior to administration of contrast, patient risk factors such as hypertension, diabetes, and pre-existing renal insufficiency need to be determined.