Chapter 25: Computed Tomography

Detailed Chapter Overview

Chapter 25 provides a foundational and comprehensive exploration of Computed Tomography (CT), a revolutionary imaging modality that creates detailed cross-sectional images of the body. This chapter is essential for any imaging professional, as it bridges the gap between traditional planar radiography and advanced three-dimensional visualization. The central theme of the chapter is the process of data acquisition and image reconstruction. It begins with a historical overview, tracing the evolution of CT from the first-generation "translate-rotate" scanners to today's incredibly fast multi-detector helical systems. The chapter meticulously details the primary components of a CT scanner—the gantry, x-ray tube, detector array, and computer system—and explains their synergistic function. A significant portion is dedicated to the fundamental concepts that define CT imaging, including the creation of voxels, the assignment of Hounsfield Units (HU) to represent tissue density, and the manipulation of window width and window level to optimize image contrast for different anatomical structures. The chapter thoroughly explains the principles of helical scanning and the concept of "pitch," which are central to modern CT. It also covers the critical role of intravenous and oral contrast agents in enhancing the visibility of vascular structures and organs. For every aspect of CT, from patient positioning and protocol selection to radiation dose considerations, the text provides a detailed and clinically relevant explanation, equipping the radiographer with the knowledge needed to operate this powerful diagnostic tool safely and effectively.

In-Depth Study Guide

Fundamental Principles of Computed Tomography

Computed Tomography creates cross-sectional images by acquiring and mathematically reconstructing x-ray transmission data from multiple angles around the patient.

1. Data Acquisition and the Gantry

• The Process: An x-ray tube rotates around the patient within a circular structure called the **gantry**. Opposite the tube is a bank of detectors. As the tube rotates, it emits a thin, fan-shaped x-ray beam that passes through the patient. The detectors measure the amount of radiation that exits the patient at thousands of different points.
• The Gantry: This is the main frame of the CT scanner. It houses the x-ray tube, the detector array, and the high-voltage generator. The central opening through which the patient passes is called the **aperture**.

2. From Voxel to Pixel: Image Creation

• Voxel (Volume Element): The CT computer divides the patient's body into a three-dimensional grid of tiny cubes called voxels. Each voxel represents a specific volume of tissue.
• CT Numbers (Hounsfield Units - HU): The computer calculates the degree to which each voxel attenuated the x-ray beam and assigns it a numerical value. This value is called a CT number or a Hounsfield Unit (HU). The scale is based on the density of water.
  • **Water** is assigned a value of **0 HU**.
  • **Dense substances** like bone, which attenuate the beam significantly, have high positive values (e.g., +1000 HU).
  • **Low-density substances** like air, which barely attenuate the beam, have low negative values (e.g., -1000 HU).
  • Fat has a negative HU value (approx. -100 HU), while soft tissues have small positive values.
• Pixel (Picture Element): The computer then represents each voxel as a two-dimensional square on the image, called a pixel. The brightness of each pixel is determined by the HU of the corresponding voxel, creating the grayscale image we see.

3. Windowing: Manipulating Image Contrast

The human eye can only distinguish a limited number of gray shades. The CT computer can measure thousands of different HU values. Windowing is the process of adjusting the grayscale mapping to optimize the visibility of specific tissues.

• Window Width (WW): This controls the **contrast** of the image. It determines the total range of HU values that will be displayed as shades of gray.
  • A **narrow window width** (e.g., 150 HU) results in a high-contrast image where there are few shades of gray. This is ideal for visualizing tissues with similar densities, such as the different gray and white matter structures in the brain.
  • A **wide window width** (e.g., 2000 HU) results in a low-contrast image with many shades of gray. This is ideal for visualizing tissues with very different densities, such as the lung parenchyma and bony structures of the thorax.
• Window Level (WL): This controls the **brightness** of the image. It sets the center or midpoint of the range of Hounsfield Units being displayed. To visualize bone, you would use a high window level. To visualize lung tissue, you would use a low (negative) window level.

Evolution of CT Technology

• First and Second Generation: "Translate-rotate" systems with a single pencil beam. Incredibly slow, taking many minutes per slice.
• Third Generation: The modern standard. Both the x-ray tube and a curved detector array rotate continuously around the patient. This design allows for much faster scanning.
• Helical (Spiral) CT: A major breakthrough where the patient is moved continuously through the gantry while the tube and detectors are continuously rotating. This acquires data in a "helix" or "spiral" pattern, allowing for the scanning of large volumes of the body in a single breath-hold.
• Multi-detector CT (MDCT): The current technology. Instead of a single row of detectors, there are multiple parallel rows (from 16 to 320 or more). This allows for the acquisition of many thin slices in a single rotation, dramatically increasing speed and image resolution.

Modern CT Procedures and Protocols

The Concept of Pitch

• Definition: In helical CT, pitch is the ratio of the distance the patient table moves during one 360-degree tube rotation to the width of the x-ray beam.
• Significance:
  • A **pitch of 1** means the table moves exactly the same distance as the beam width, resulting in continuous, seamless coverage.
  • A **pitch greater than 1** means the table moves faster, stretching the "helix." This reduces the radiation dose to the patient but can slightly decrease image quality.
  • A **pitch less than 1** means the table moves slower, resulting in overlapping data acquisition. This increases image quality but also increases the radiation dose.

The Role of Intravenous Contrast

• Purpose: IV contrast is essential for most CT studies of the chest, abdomen, and pelvis. The iodinated contrast makes blood vessels and highly vascular organs (like the liver and kidneys) appear bright white, allowing for their differentiation from surrounding structures and the detection of abnormalities like tumors or abscesses.
• Phases of Contrast Enhancement: The timing of the scan after the IV contrast injection is critical.
  • Arterial Phase: A very early scan (e.g., 20-30 seconds post-injection) when the contrast is primarily in the arteries. Essential for CT Angiography (CTA).
  • Portal Venous Phase: A later scan (e.g., 60-70 seconds post-injection) when the contrast has circulated through the arteries and is now prominently enhancing the portal vein and solid organs like the liver and spleen. This is the workhorse phase for most routine abdominal CTs.
  • Delayed Phase: A scan taken several minutes after injection, used to evaluate for contrast washout from lesions or to visualize the excretory phase of the urinary system.

Common CT Examinations

• Head CT: Often the first line of imaging for stroke, head trauma, and severe headaches.
• Chest CT: Used to evaluate for lung nodules, pulmonary embolism (requires a CTA protocol), pneumonia, and aortic dissection.
• Abdomen/Pelvis CT: A workhorse exam for diagnosing a vast array of conditions, including appendicitis, diverticulitis, kidney stones, abscesses, and cancer staging.