M01.12.002 Computed Tomography (CT) Scanning

Computed tomography (CT) scanning is an extremely common imaging modality in modern medicine, rapidly replacing many diagnostic radiographic procedures due to technological advancements. This article outlines the basic science behind CT scans, describes the principles of interpretation, and highlights their advantages and drawbacks compared to other imaging techniques.

Basic Principles of CT Scans

Creation and Detection

CT scans are created using a series of X-rays, a form of electromagnetic radiation. The CT scanner emits X-rays towards the patient from various angles, and detectors in the scanner measure the difference between the X-rays absorbed by the body and those transmitted through the body. This process is called attenuation.

• High-Density Tissue (e.g., Bone): Absorbs more radiation, resulting in less detected signal.
• Low-Density Tissue (e.g., Lungs): Absorbs less radiation, resulting in more detected signal.

Hounsfield Units

The amount of attenuation is determined by the density of the imaged tissue, assigned a Hounsfield Unit (HU) or CT Number. The higher the density, the higher the HU.

Reconstruction

Unlike conventional X-rays, which provide a two-dimensional image, CT scans use advanced mathematical algorithms to create three-dimensional representations of the body. This involves acquiring projections from different angles and reconstructing the data into a 3D model viewable on a 2D monitor.

Contrast Imaging

CT scans can be performed with or without contrast, depending on the diagnostic requirement.

• Intravenous Contrast: Used to visualize the cardiovascular system, identify malignant tumors, and perform procedures like CT Urogram.
• Oral Contrast: Used for imaging the digestive system, such as in cases of Crohn’s disease, bowel obstruction, diverticulitis, or appendicitis.

Interpreting a CT Scan

Orientation

CT images are commonly presented in the transverse plane, oriented as if looking up from the patient’s toes. The acronym RALP (Right, Anterior, Left, Posterior) can help determine the orientation.

Image Density

The brightness and contrast of the imaged tissues are affected by their density, quantified by the Hounsfield Scale of radiodensity:

Clinical Relevance: Intracranial Bleeds

Intracranial bleeds, potentially life-threatening conditions, are commonly investigated using CT scans. Types of intracranial bleeds include:

1. Extradural Hematoma: Arterial bleeding outside the dura mater, creating a lemon-shaped lesion.
2. Subdural Hematoma: Venous bleed between the dura and arachnoid layers, creating crescenteric lesions.
3. Subarachnoid Hemorrhage: Aneurysm rupture in the subarachnoid space, tinting the CSF with blood.
4. Intracerebral Hemorrhage: Localized lesions due to hypertension, diabetes, or trauma, with surrounding edema.

Comparison to Other Imaging Techniques

CT scanning is ideal for emergency cases due to its quick scan times and ability to provide immediate diagnoses. However, it uses radiation, which can be harmful, especially to younger patients. Despite this, the benefits often outweigh the risks.

Advancements in CT Technology

• Virtual Colonoscopy: Replacing traditional barium enema studies.
• Cardiac Gating: Allows detailed studies of cardiac arteries and ejection fraction measurements.
• Advanced 3D Applications: Improve visualization of certain pathologies.

Summary of Imaging Modalities

CT scans have become indispensable in modern diagnostic imaging, offering rapid and detailed views of internal structures, and aiding in the diagnosis and management of various medical conditions.