Computerized Tomography (CT) Scanning Ch 12 PP

Computerized Tomography

Overview

  • Also known as:

    • CT Scanning

    • CAT Scan

    • Computerized Axial Tomography

Historical Context

  • Invented: 1971

  • Technology:

    • Utilizes ionizing radiation for image acquisition

    • Acquires images in a manner similar to slicing bread

    • Incorporates a moving X-ray tube to generate multiple sequential images

  • Detectors:

    • Utilizes photodetectors which convert X-ray photons into light, then into electrical currents

  • Clinical Relevance:

    • Initially the primary modality for imaging the spine and brain before the advent of MRI

    • First imaging modality allowing surgeons to visualize the brain non-invasively

    • Continues to be an essential tool for trauma assessment and body imaging

Image Characteristics

  • Digital Radiographs:

    • Produce 2-dimensional images characterized by length and width

    • Composed of a matrix of pixels (picture elements)

  • CT Scans:

    • Generate 3-dimensional images defined by length, width, and depth

    • Constructed from a matrix of voxels (volume elements)

    • A wider matrix results in a pixelated image; smaller voxels provide a clearer image

Technical Components of CT Scanning

CT Scanning Mechanics

  • Gantry Composition: Includes:

    • X-ray tube

    • Collimators

    • Detectors

  • Operational Process:

    • The machine's gantry rotates 360 degrees around the patient's body

    • Detectors are positioned directly across from the X-ray tube, rotating in synchrony

    • X-ray beams penetrate the patient, where the attenuated X-ray beam is collected by the detectors

    • X-ray Attenuation Details:

    • Bone: Attenuates the most X-rays

    • Tissue: Moderate attenuation

    • Fat and Air: Attenuate similarly, but less than bone and tissue

  • Signal Conversion Process:

    • Detectors convert incoming photons into an electrical signal, which is sent to an analog-to-digital converter

    • While capturing images, the patient table moves in increments determined by the operator, taking numerous slices for a comprehensive exam

Hounsfield Units (HU)

  • Definition: Hounsfield unit (HU) provides a tissue density measurement for each voxel in CT imaging

  • Significance:

    • Displays consistent differences across various tissue densities

    • Influences the visual output of images regarding brightness and contrast

  • Window Level Specification:

    • The window of an image relates to the number of shades of gray—known as contrast—in the image

    • The level represents the midpoint of the gray scale range

Example of Hounsfield Units

  • Brain Imaging Specs:

    • Windowed at 80 HU and leveled at 38 HU

    • Indicates:

    • 1 = black, 80 = white

    • 78 shades of gray between 1 and 80

    • To calculate visible shades of gray:

    1. Determine midpoint (level) = 38 (which is half of 80)

    2. Calculate visible range:

      • (38−40=−2)(38 - 40 = -2)

      • (38+40=78)(38 + 40 = 78)

    • Interpretation: All shades of gray from -2 to 78 are visible: anything below -2 appears as black, above 78 appears as white

Hounsfield Unit Standards by Body Part

  • Fat: -50 HU

  • Blood: 80 HU

  • Bone: 160 HU

Field of View in CT Imaging

  • Scan Field of View (SFOV):

    • Defined as the number of detectors encompassed by the X-ray beam

    • Maximum SFOV is determined by the manufacturer specifications

    • Critically, if a body part lies outside this field, it cannot be reconstructed into an image

  • Display Field of View (DFOV):

    • Must be less than or equal to SFOV

    • A pivotal factor set by the technician for each study

    • It is essential for determining the resolution of CT images

    • Decreasing DFOV enhances resolution

    • Post-examination, raw data (SFOV) can be used to generate images with different desired DFOV

Technical Parameters in CT Scanning

  • Key Factors Used:

    • kVp (kilovolt peak) and mAs (milliampere-seconds)

    • Common kVp ranges: 80 to 140 kVp in CT scans

    • Common mAs ranges: 10 to 400 mAs

    • A reduction in mAs allows pronounced control over visible structures in the images

  • Scan Time:

    • Refers to the duration needed for the X-ray tube to complete a 360-degree rotation around the patient to capture an exposure

  • Image Quality:

    • Higher kVp and mAs result in better image quality, but adherence to the ALARA (As Low As Reasonably Achievable) principle is critical

  • Advancements in Slice Thickness:

    • Early CT scans featured slices of 10 mm thickness

    • Modern technology can achieve slices thinner than 1 mm

CT vs. MRI

  • Comparison:

    • Next section elaborates on differences and utilities of CT versus MRI technologies, exploring varying applications in medical imaging.