CT Registry Study Flashcards

Beam Hardening

• Beam hardening occurs when lower energy photons are filtered out as X-rays pass through an object, resulting in a "harder" beam of higher energy photons.
• Two types of beam hardening artifacts:
  • Cupping: the center of a round object appears darker than the periphery; commonly seen in the head.
  • Streak: dark bands create in lines between two dense objects where nearly all photons are absorbed.
• Compensation methods for beam hardening include:
  • Filtration: Removing low energy photons before the beam reaches the object.
  • Calibration: Using phantoms to set compensated mA/kVp.
  • Correct software applications: Iterative reconstruction can mitigate issues.
  • Avoidance: Adjusting the gantry position or patient positioning to minimize effects.

Aorta Bifurcation

• The abdominal aorta bifurcates at the L4 level into the right and left common iliac arteries.

Contrast Media and Liver Scans

• Typical scan delay after injection of contrast media for liver studies is 30-45 seconds.
• Optimal window settings for tissue differentiation in the liver:
  • Window width: 140
  • Window level: 60

Window Settings

• Best bone window settings for pelvis:
  • Window width: 2000
  • Window level: 350
• Various window levels for different body parts include:
  • Brain: Window width = 80, Level = +40
  • Lung: Window width = 1500, Level = -400
  • Abdomen: Window width = 400, Level = +50
  • Bone: Window width = 1600, Level = +500

mAs Usage

• Typical mAs values for routine CT exams of the abdomen range from 200 to 300 mAs.

Anatomic Formations

• The floor of the orbit is constructed by the maxillary and zygomatic bones.
• The lateral wall of the orbit comprises the zygomatic and sphenoid bones.
• The medial wall is formed by the ethmoid and lacrimal bones.
• Common carotids bifurcate into internal and external carotid arteries at the C3-C4 level.

Effective mAs Calculation

• Effective mAs is calculated as mAs divided by the pitch.

Window Level and Width Definitions

• Window level is the midpoint in grayscale, while window width measures the range of grayscale values.
• Wider window widths correspond to lower contrast, allowing more grayscale values.

Voxel and Image Construction

• Voxel volume calculation involves slice thickness, matrix size, and field of view; example with 3 mm slice thickness, 512x512 matrix, and 25.6 cm FOV yields a voxel volume of 0.75 mm³.
• Increased voxel size can result from increased slice thickness.

Artifacts and Calibration

• Ring artifacts arise from calibration errors or defective detectors causing consistent erroneous readings across angular positions.
• The focal spot size is typically large (0.6-1.2 mm) to prevent overheating of the anode due to high mA.

CT Number Interpretation

• If a tissue with a CT number of +300 appears white, it likely corresponds to a window level of 0 and a window width of 500.

Prepare for the CT Registry with these essential flashcards that cover key concepts and procedures in CT imaging. This selection excludes anatomy but includes critical tips like beam hardening reduction and optimal scan delays. Ace your CT Registry exam with concise, focused study material!