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Computed tomography scanning in emergency medicine


Essentials

  • 1

    Computed tomography (CT) scans are a major diagnostic modality in emergency medicine.

  • 2

    Emergency physicians are ordering CTs more frequently than previously for a variety of reasons.

  • 3

    Artefacts are occasionally encountered in CT scans, and clinicians should be familiar with these artefacts.

  • 4

    Contrast media reactions and carcinogenic effects of radiation are recognized potential adverse effects of CT scanning.

Introduction

Computed tomography (CT) was developed in 1971 by Godfrey Hounsfield and Allan Cormack and adopted into medical practice. By the early 1980s, CT scanning was in general clinical use in the United States, and within a generation, most large emergency departments acquired their own dedicated scanners. Emergency medicine has embraced the utilization of CT scans, as it significantly aids in clinical diagnosis. It has revolutionized the approach to patients with traumatic injury, neurological emergencies, abdominal pain and chest pain. It is cost-effective and fast. It is, however, a modality that presents some risks to patients, and clinicians need to be prudent in their use. It is also an area of considerable development, with CT scanners becoming faster and more precise, and therefore increasing utility.

Development science

CT scan machines consist of a gantry around a patient, with an x-ray source on one side of the gantry and detectors on the opposite side, moving in synchrony. Early scanners imaged one slice at a time (‘step and shoot’), with the table stationary while a static image was acquired. This produced a series of parallel slice images (tomographic images) of a region of the body. The beam produced by the source can be adjusted, producing widths from 1 to 20 mm. Traditionally, images were produced which displayed the volume of data as axial slices (perpendicular to the long axis of the body), but current scanners are able to display the collected data as multiplanar slices which improves diagnostic yield ( Figs 23.2.1 to 23.2.3 ).

Fig. 23.2.1, Axial CT of head.

Fig. 23.2.2, Coronal CT of head.

Fig. 23.2.3, Sagittal CT head.

Helical (spiral) CT scanners move the patient rapidly and continuously through a circular gantry opening that is equipped with a source and multiple detectors, which are continuously rotating and provide volumetric acquisition. The source describes a helical trajectory relative to the patient.

Windows

The objects displayed in a scan can be differentiated from adjacent organs by their differential attenuation of the x-ray beam based on their individual density. The density of the tissues is measured in Hounsfield units (HU). Water has a density of 0, and tissues denser than water have values greater than 0, while less-dense tissues have negative values. The accepted convention is for high-density structures to be displayed in lighter shades and low-density structures to be displayed in darker shades. The denser a structure, the lighter the shade displayed. The scale extends up to about +4000 for very dense metals, with cancellous bone about +700 and dense bone about +3000. Blood is in the range of +35 to +45 and muscle about +40. At the other end of the scale, air is −1000, lung −700 and fat −84 ( Figs 23.2.4 and 23.2.5 ).

Fig. 23.2.4, Axial CT of lung using lung windows preset.

Fig. 23.2.5, Axial CT chest using preset for CT angiogram.

Humans can only perceive a limited number of grey shades, and so to highlight the tissues of interest, the full range of density values is not displayed. Instead, the display shows a narrow portion of the full range to allow clear differentiation of one tissue from another and pathological tissue from normal tissue.

For example, bone windows are a preset that will shift the grey scale displayed to center on the range of densities which are typical of bone and allow detection of subtle abnormalities, such as fractures. As a consequence of focusing the display on such high-density structures, there is a marked decline in the ability to assess soft tissues on bone windows.

Display convention

The accepted convention for displaying images is for the right side of the patient to be on the left side of the image.

Artefacts

There are some imaging artefacts that affect the quality of the images generated and hence the diagnostic quality of the scan. These artefacts are classified as physics-based artefacts, patient based, scanner based and multi-section based. Most emergency physicians (EPs) are familiar with patient movement ( Fig. 23.2.6 ) and metallic artefacts ( Figs 23.2.7 and 23.2.8 ).

Fig. 23.2.6, Pseudofracture of C6 from patient movement.

Fig. 23.2.7, Metallic artefacts from hip prostheses.

Fig. 23.2.8, Coronal CT abdomen with metallic artefact obscuring bladder.

Physics-based problems include beam hardening (resulting from the absorption of low-energy photons after passage through an object, leaving only high-energy photons and a higher energy beam), which can produce the streaks and dark bands. Undersampling is another physics-based problem, in which the distance between CT samples is large enough to create mis-registration of information about small objects or sharp edges. Partial volume averaging is when the densities in a single CT is averaged rather than displaying separate individual densities. This occurs because every CT slice displayed is a 2D representation of a finite 3D thickness of tissue. Ring artefacts, a scanner-based problem, occur due to mis-calibration or failure of one or more detector elements ( Fig. 23.2.9 ).

Fig. 23.2.9, Ring artefacts in a very bariatric patient.

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