Learning Objectives

By the end of this section, the learner should be able to:

• Explain the basic physical principles behind CT image formation.
• Describe how tissue density relates to attenuation and Hounsfield units.
• Interpret CT images with correct orientation and density assessment.
• Identify common clinical indications for CT, particularly in emergency settings.
• Compare CT with other imaging modalities, including X-ray, MRI, and ultrasound.

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Overview

Computed tomography (CT) is one of the most widely used imaging modalities in modern medicine. Due to rapid acquisition times, high spatial resolution, and the ability to generate cross-sectional and three-dimensional images, CT has replaced many traditional radiographic investigations, particularly in emergency and trauma care.

This section reviews the basic science of CT scanning, principles of image interpretation, and advantages and limitations compared with other imaging techniques.

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Basic Principles of CT Imaging

CT imaging is based on the use of X-rays, a form of ionizing electromagnetic radiation. Unlike conventional radiography, which produces a single two-dimensional image, CT acquires multiple X-ray projections from different angles around the patient.

Attenuation

• As X-rays pass through the body, they are attenuated (absorbed or scattered) to varying degrees.
• The degree of attenuation depends on tissue density and composition.
• Detectors measure the difference between emitted and transmitted X-rays.

This attenuation data is processed using complex mathematical reconstruction algorithms to generate cross-sectional images, which can be stacked to form three-dimensional representations of anatomy.

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Hounsfield Units (CT Numbers)

Each voxel in a CT image is assigned a Hounsfield Unit (HU), which quantifies tissue density relative to water.

• High-density tissues (e.g., bone) attenuate more X-rays and appear white
• Low-density tissues (e.g., air-filled lungs) attenuate less and appear black

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Contrast-Enhanced CT Imaging

CT scans may be performed with or without contrast, depending on the clinical question.

Intravenous (IV) Contrast

Iodinated contrast agents enhance vascular and soft tissue visualization and are used to:

• Assess the cardiovascular system (aneurysms, dissections, atherosclerosis)
• Differentiate tumors and characterize malignancy
• Perform CT urography, as contrast is excreted via the kidneys into the urinary tract

Oral Contrast

Used to evaluate the gastrointestinal tract, including:

• Crohn disease
• Bowel obstruction
• Diverticulitis
• Appendicitis

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Interpreting a CT Scan

Image Orientation

• CT images are most commonly displayed in the axial (transverse) plane
• Images are viewed as if looking up from the patient’s feet
• Orientation can be remembered using RALP:
  • Right
  • Anterior
  • Left
  • Posterior

Images may also be reconstructed in the coronal and sagittal planes to aid diagnosis.

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Image Appearance

• Tissues with high attenuation appear bright (white)
• Tissues with low attenuation appear dark (black)
• Windowing (e.g., brain, lung, bone windows) optimizes visualization of specific tissues

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Clinical Relevance: Intracranial Hemorrhage

CT is the first-line imaging modality for suspected intracranial bleeding due to speed, availability, and sensitivity for acute blood.

Types of Intracranial Hemorrhage

• Extradural (Epidural):
  • Lentiform (biconvex) shape
  • Arterial origin
  • Often associated with a skull fracture
• Subdural:
  • Crescent-shaped
  • Venous (bridging veins)
  • Common in the elderly
• Subarachnoid:
  • Blood in subarachnoid cisterns
  • Often due to ruptured aneurysm
• Intracerebral:
  • Localized parenchymal bleed
  • Associated with hypertension, diabetes, or trauma

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Comparison with Other Imaging Modalities

CT is particularly valuable in emergency and trauma settings, where rapid diagnosis is essential (e.g., stroke, hemorrhage, vascular dissection, renal stones).

Advantages

• Rapid image acquisition
• Excellent spatial resolution
• Superior bone and acute hemorrhage detection
• 3D and multiplanar reconstruction

Limitations

• Uses ionizing radiation
• Higher radiation dose compared to a plain X-ray
• Caution is required in children and pregnant patients

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Summary of Imaging Modalities