Computed tomography

• Fundamentals of Computed Tomography (CT)
  • Passing a rotating fan beam of x-rays through the patient
    • Basically many x-rays at different angles around the patient
  • Computer can generate many images from different angles from a single examination
    • Can even generate a 3D image
  • 10 – 100x more radiation than x-ray
  • The appearance of tissues on CT
    • Densities are generally the same as for x-rays
    • Bright on CT = high density
    • Dark on CT = low density
    • The actual radiodensity of a tissue can be measured
      • The radiodensity of a tissue is expressed in Hounsfield units (HU) or CT numbers
        • HU is the reduction coefficient of the tissue relative to water
      • The HU of air is -1000
      • The HU of fat is -80
      • The HU of water is 0
      • The HU of blood is +50
  • Higher sensitivity than x-ray
  • Types
    • High-resolution computed tomography (HRCT)
      • For lung and skull
    • Dual energy CT (DECT)
  • CT scans are presented as a series of slices of tissue
    • Slices are always viewed as if from below the patient
      • I.e. structures on the patient’s right side is on the left of the image
      • Also true for MRi
    • The slices can be thick or thin
      • Thin slices (high-resolution CT):
        • Slower scanning
        • Increased dose (more pictures are taken)
        • Higher image detail
      • Thick slices:
        • Faster scanning
        • Lower dose
        • Worse image detail
  • Contrast agents
    • Used in 75% of all CT studies (contrast CT)
  • Windowing
    • Windowing is a process where the CT image is manipulated by a computer (the “window” is adjusted)
      • Adjusting the “window width” adjusts the range of CT numbers which are visible (changes contrast)
        • Example: A wide window shows tissues with CT numbers between 400 and 2000
          • Good for differentiating tissues with different radiodensity
          • Like air and soft tissue
        • A narrow window shows tissues with CT numbers between 50 and 350
          • Good for differentiating tissues with similar radiodensity
          • Like different soft tissues
      • Adjusting the “window length” or “window center” changes the brightness of the CT image
    • This manipulation changes the appearence of the picture to highlight certain structures
  • Bolus tracking
    • A technique used to optimising the timing of contrast CT
    • The technician takes a picture before the bolus is injected
    • Draws a region of interest (ROI) on the pulmonary trunk
    • The machine will then continously take low resolution CT images through the pulmonary trunk
    • The contrast bolus is injected
    • When the Hounsfield unit of the ROI exceeds a certain threshold -> the contrast bolus has reached the pulmonary trunk -> the machine will start to take high resolution CT images of the whole lung‎