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.

Learn about the properties and uses of iodine-containing compounds as contrast media in medical imaging, including their behavior in the body and excretion pathways. Understand how different iodine compounds are used to study specific systems, such as the urinary and digestive systems.