30 Questions
What information is assigned to each pixel in a reconstructed CT image?
CT numbers or Hounsfield units
Who is the inventor of CT technology, after whom Hounsfield units are named?
Sir Godfrey Hounsfield
What is the mathematical relationship between linear attenuation coefficients and CT numbers?
CT number = μ + 1000
What is the unit of measurement for linear attenuation coefficients?
μ (mu)
What are CT numbers related to?
Linear attenuation coefficients of the tissues
What is the significance of Sir Godfrey Hounsfield in CT technology?
He invented CT technology
What is the purpose of assigning CT numbers or Hounsfield units to each pixel in a reconstructed image?
To display the linear attenuation coefficients of the tissues
What is the primary reason water is used as a reference material in CT scans?
Because of its uniform density and abundance in the body
What is the unit of measurement assigned to each pixel in a CT scan?
Hounsfield Unit (H.U.)
What is the linear attenuation coefficient of dense bone?
0.46
Which of the following tissues has a linear attenuation coefficient closest to water?
White matter
What is the significance of a CT value of zero?
It is assigned to water as a reference material
What happens to CT numbers when tissue density increases?
They become positive
Why do different references yield slightly varying linear attenuation coefficients?
Due to differences in calculation methods
What is the value of air on the scale where water has a value of 0?
-1000
What is the purpose of the scaling factor (K) in CT scans?
To establish a relative basis for CT numbers
What is the unit of measurement for CT numbers?
HU (Hounsfield Units)
What is the CT number for water?
0
What is the formula to calculate the CT number?
CT Number = (μtissue - μwater) / μwater
What is the range of CT numbers for soft tissue?
10-30
What is the attenuation coefficient of water?
0.19
What is the final step in the CT process?
Image display, storage, and documentation
What is the purpose of converting the digital CT image to a grayscale image?
To facilitate interpretation by the radiologist
What type of display device is usually used to display the CT image?
LCD (Liquid Crystal Display) monitor
What is the disadvantage of CRT monitors compared to LCD monitors?
They are heavier, larger, hotter, and less durable
What is the arrangement of numerical values called in a digital CT image?
Matrix
What represents a volume of patient data or slice thickness in a CT image?
Voxel
What is the result of assigning a level of gray to each pixel in a CT image?
The image becomes grayscale
What corresponds to the CT number or Hounsfield units for each pixel in a CT image?
Level of gray
What is the purpose of displaying the CT image on a television monitor?
To interpret the image by the radiologist
Study Notes
CT Images Display
- Each pixel in a reconstructed image is assigned a number related to the linear attenuation coefficients (μ) of tissues.
- These numbers are called CT numbers or Hounsfield units, named after the inventor of CT technology, Sir Godfrey Hounsfield.
Linear Attenuation Coefficients
- Linear attenuation coefficients (μ) for some tissues of the body:
- Air: 0.0003 cm⁻¹
- Fat: 0.162 cm⁻¹
- Water: 0.180 cm⁻¹
- White matter: 0.187 cm⁻¹
- Blood: 0.182 cm⁻¹
- Dense bone: 0.46 cm⁻¹
Hounsfield Units (HU)
- Water is used as a reference material and assigned a CT value of zero.
- Tissue materials with density (or attenuation) greater than water have +ve CT numbers.
- Those less dense than water have –ve CT numbers.
- A Hounsfield unit (HU) is a digital value assigned to each pixel, lying on a scale where:
- Water has a value of 0 HU
- Air has a value of –1000 HU
- Bone has a value of +1000 HU
CT Number Calculation
- CT number is calculated as: CT number = [(μtissue – μwater) / μwater] × K, where K is a constant factor.
- Value of K determines a contrast factor (scaling factor).
CT Numbers for Common Tissues
- CT numbers for common tissues:
- Air: 1000 – HU
- Lungs: –300 to 900 HU
- Fat: –80 to 120 HU
- Water: 0 HU
- Muscle: 10 to 30 HU
- Soft tissue: 50 to 100 HU
Image Display
- The final step in the CT process involves image display, storage, and documentation.
- The CT image is displayed on a television monitor, which is an essential component of the control or viewing console.
Displaying the Image
- The display device is usually an LCD (liquid crystal display), each pixel within the image is assigned a level of gray.
- The gray level assigned to each pixel corresponds to the CT number or Hounsfield units for that pixel.
Display Monitors
- CRT (cathode-ray tube) monitors are heavier, larger, hotter, and less durable than the newer LCD monitors.
- LCD monitors produce higher luminance and high spatial resolution.
CT Image Matrix
- The digital CT image is displayed on an arrangement of numerical values called a matrix.
- The matrix is made up of a series of picture elements, arranged in rows and columns, called pixels.
- A voxel (volume element) represents a volume of patient data or slice thickness, giving the pixel and added dimension.
This quiz covers the basics of CT imaging, including CT image display, Hounsfield units, and attenuation coefficients and CT numbers for common tissues.
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