advanced imaging

• Computed tomography (CT)


  Advanced imaging technique that uses X-rays to create cross-sectional images of the body
• Main CT components


  1. CT gantry (tube, filters, detectors) and patient table

  2. Computer

  3. CT operating console and workstation

  PACS
• The CT scanning room

  • Gantry

  Patient table

  Shielding

  Room for stretchers, resuscitation team, emergency equipment, medical gases

  Injector for IV contrast

  Control room with computers, consoles and workstations
• CT table


  Motorised platform which patient lies on for CT examination

  Lightweight (carbon)

  Differs between diagnostic radiology (DR) and radiotherapy (RT) machines

  DR tables dipped for patient comfort, RT tables flat to ensure reproducibility
• CT gantry


  Houses the data acquisition system

  Rotating frame has X-ray tube mounted on one side and detectors on the other

  Controls to lift/move table

  Tilt range

  Laser lights for centring/localisation

  RT: wider bore
• Inside the CT gantry

  1. ray tube
  2. 4. Collimators

  5. Detectors
• CT scanner design


  • X-ray tube needs to withstand higher energies, currents and scan times compared to standard X-ray

  Bow-tie filter filters beam to reduce skin dose

  Collimators restrict X-ray beam to detector array and reduce scatter radiation

  Scintillation detectors efficiently convert X-ray photons to electrical signal
• Slip ring technology


  Allows continuous rotation of the gantry and table feed, enabling faster scanning
• Helical scanning


  Images acquired constantly as table moves slowly through gantry

  Faster scans, reduced movement artefacts

  But susceptible to cone beam artefact and requires z-interpolation
• Sequential/axial scanning


  Stop-start rotation of the gantry, slower scans
• Multi-slice CT


  Multiple detector rows in the z-axis, enabling multiple slices per rotation

  Faster acquisition, reduced movement artefacts, longer scan volumes, good for CT angiography
• Benefits of multi-slice CT

  • Allows sub-millimeter scanning, multiplanar reformats, 3D reconstructions, high resolution CT

  Ability to choose thicker or thinner slice thickness for different purposes
• Anisotropic resolution


  Voxel dimensions in x, y and z planes are not similar, leading to 'stair-step' artefacts
• Isotropic resolution


  Voxel dimensions in x, y and z planes are similar, providing greater spatial and contrast resolution
• CT scanning indications
• Patient preparation


  Pregnancy check, contraindications to contrast agent, changed into gown

  Intravenous contrast to visualise blood vessels and organ vasculature

  Oral contrast to visualise GI tract
• Patient positioning


  Patient must be centred into the isocentre of the gantry using laser lights

  Centring in isocentre reduces dose, optimises image quality and reduces artefacts
• Scout scan


  Allows planning of slices and field of view, checks for artefacts, allows calculation of mA modulation
• mA modulation


  Varies the tube current along the patient to optimise image quality
• Pitch


  Ratio of table movement per rotation to slice thickness

  Increasing pitch decreases scan time but reduces scan quality, decreasing pitch increases scan time but improves quality
• Field of view (FOV)

  Radiotherapy - wide FOV to see full outline of patient

  Diagnostics - variable FOV depending on area of interest

  Scan FOV vs display FOV
• Intravenous contrast agent


  Injected by pump, scan activated at specific time intervals to visualise arterial or portal venous phases
• Bolus tracking


  Used when looking at arterial system, scan triggered when contrast reaches a certain density in the vessel of interest
• During the CT scan

  No one except patient in room, watch patient, check for contrast reaction, explain waiting for image reconstruction
• Viewing CT images


  Check images immediately post-acquisition, look for artefacts, pathology outside field of view, incidental findings
• Common CT artefacts


  • Beam hardening

  Motion

  Ring artefact

  Partial volume artefact
• CT dose


  CTDIvol - absorbed dose per unit mass of irradiated volume

  DLP - dose length product, indicative of total radiation to patient

  National diagnostic reference levels (DRLs) to standardise doses
• Image post-processing


  Reformats, magnification, window adjustments, multiplanar reconstructions, maximum intensity projections, volume rendering
• Methods to optimise dose and improve image quality

  • Optimisation of protocols

  Limit scanning to only necessary area

  Staff training

  Daily QA and air calibration of scanner

  Dose reduction software (mA modulation, iterative reconstruction)

  Use appropriate protocol (e.g. low dose, paediatric)