Contrast Media in Medical Imaging

Contrast Media

• Most contrast media are iodine-containing compounds and are water-soluble.
• Iodine is a heavy element with an atomic mass of 127.
• Different iodine compounds behave differently in the body.
• Iodine compounds are used to study the urinary system, gall bladder, and bile ducts.
• Other compounds are used to study arteries, veins, the heart, and cavities in the brain.

Image Display

• CT images are displayed on a TV monitor and recorded on film for storage.
• The display matrix has on average 512x512 pixels, but can have up to 1024x1024 pixels.
• The display has only 256 shades of gray.
• The pixel size is on average 0.1 mm, with a scan field view of 40 cm.
• The computer assigns one shade of gray to each of the 128 CT numbers below and 128 CT numbers above the baseline.
• The center CT number is the 'window level', and the range of CT numbers above and below are called the 'window width'.

Windowing

• Windowing is also known as grey-level mapping, contrast stretching, histogram modification, or contrast enhancement.
• Windowing is used to adjust the display of CT images.
• The values are written as width and level (W:x L:y) in Hounsfield units (HU).

Head and Neck CT Scan

• Brain: W:80 L:40
• Subdural: W:130-300 L:50-100
• Stroke: W:8 L:32 or W:40 L:40
• Temporal bones: W:2800 L:600
• Soft tissues: W:350-400 L:20-60

Chest CT Scan

• Lungs: W:1500 L:-600
• Mediastinum: W:350 L:50

Abdomen CT Scan

• Soft tissues: W:400 L:50
• Liver: W:150 L:30

Spine CT Scan

• Soft tissues: W:250 L:50
• Bone: W:1800 L:400

2D Image Reconstruction in CT

• A single projection λφ(x') is defined as -ln[I0(x')/Ig(x')].
• The attenuation coefficient μ[x, y] is calculated using the filtered projection.
• The CT number of tissue is the fractional difference of tissue relative to water.
• CT numbers are rescaled in terms of CT numbers.

CT Number

• CT number = (μtissue - μwater) / μwater × 1000
• CT numbers are used to differentiate between tissues.
• CT numbers are measured in Hounsfield units (HU).

2D Image Display

• The display matrix has on average 512x512 pixels.
• The display has only 256 shades of gray.
• Windowing is used to adjust the display of CT images.

CT Imaging Orientation

• Computed tomography (CT) uses ionizing radiation to create a cross-sectional image.
• CT imaging allows visualization of a greater variety of tissue structures beyond the four basic densities (air, bone, soft tissue, and fat).
• Each individual picture of a CT study is referred to as a section or an axial "slice".

CT Imaging Scanner

• A CT scanner consists of an X-ray rotating source and a diametrically opposite detector unit.
• There are ~30,000 scanners worldwide, and 60 million CT scans are performed annually in the USA.

Basic Principle of CT Imaging

• Conventional CT: a series of 1D projections at different angles is acquired continuously by synchronously rotating the X-ray source and detectors through one complete revolution around the patient.
• Spiral/helical CT: 2D slices are acquired as in conventional CT, and multiple adjacent slices are acquired by moving the patient's couch along the direction perpendicular to the slices' plane.

Instrumentation for Helical CT

• X-ray source and couch moved at the same time → X-ray path = helical
• Conventional CT set-up modified as follows:
  • Power supply and signal transmission cables are substituted by multiple slip-rings.
  • X-ray tube: specially designed to withstand very high temperatures in anode.
  • X-rays produced (almost) continuously → no cooling period → anode reaches very high temperatures = higher than in conventional CT.