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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