2. The fundamentals of CT and MRI imaging, examination methods.

From greek.doctor
  • 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


  • Magnetic resonance imaging
    • Magnetic fields applied to the body align hydrogen atoms in the body
      • When the fields are released, radio waves are released
      • These radio waves are detected, and the frequency of the waves depend on the environment of the atoms (the tissues)
    • Especially used for CNS, joints, heart, angiography (visualization of vessels)
    • Can’t be used for people with old pacemakers
      • (Can be used with new pacemakers)
    • MRi contrast agents
      • Molecular basis of them (I don’t understand what these mean)
        • Water content
        • Restricted water movement
        • Macromolecular motion
        • Lipid content
        • Paramagnetic ions
    • Types
      • T1 weighted
        • Fat is white
        • Water is dark
        • Brain, muscle is gray
        • Bone itself is dark BUT bone marrow (fat) is white
          • This makes bone look white
        • Gadolinium (contrast material) is white
        • More effective at visualizing normal anatomy
      • T2 weighted
        • Fat is dark
        • Blood, oedema, CSF is white
        • Brain, muscle is gray
        • More effective at visualizing inflammation
      • Diffusion weighted (DWI)
        • Measures how easy it is for water molecules to move around in a tissue
        • Ischaemic tissue is white
      • Proton density weighted (PD)
      • Flow sensitive
        • Time of flight (TOF)
        • Can measure flow of fluids without contrast
        • MR angiography
        • MR venography
        • CSF flow studies
      • Fat or water saturation
        • Fluid-attenuated inversion recovery (FLAIR)
      • Short tau inversion recovery (STIR)
      • Diffusion tensor (DTI)
      • Susceptibility-weighted (SWI)
    • Advantages
      • No radiation
    • Disadvantages
      • Motion causes artifacts
      • Expensive
      • Strong magnets