COMPUTED TOMOGRAPHY

September 11, 2024
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TOPIC: COMPUTED TOMOGRAPHY

🌟 IMPORTANT POINTS TO REMEMBER

🔹Tomography: The word tomography is derived from the Greek word ‘tomos’ meaning ‘a technique of X-ray photography by which a single plane is photographed with the outline of structures in other plane eliminated’.

🔹History: Allen McLeod Cormack proposed the theoretical idea of CT in 1950s and the practical device was developed in the late 1960s by Godfrey Newbold Hounsfield.

🔹X-ray source: To produce more photons, large kilovoltage ranging from 200 to 800 mAs and milliamperage settings as high as 120–140 kVp are required.

🔹Filters: They can be mathematical filters (these are the computer algorithms) and inherent tube filtration (filters made of aluminium and Teflon).

🔹Collimators: There can be prepatient collimators and postpatient collimators.

🔹Detector array: Multiple scintillation detectors with photomultiplier tubes – In the past, early scanners used sodium iodide crystals coupled to photomultiplier tubes. 

  • Multiple scintillation detectors with photodiodes – Scintillation material used with photodiodes is cadmium tungstate and ceramic material made of high-purity rare earth oxides,
  • Single multichamber inert gas detectors – Gas ionisation detectors consist of gas chambers separated by tungsten plates.

🔹Data acquisition system: Image reconstruction. (Image reconstruction from the raw data sets finally yields the image data sets. It starts with the selection of the desired fov.)

🔹Image quality: The image quality of a CT scan depends on spatial resolution, contrast resolution and image noise.

🔹Principle of CT scanning: CT is based on the simple X-ray principle that when X-rays are passed through the human body, X-ray photons are absorbed or attenuated by the body at different levels according to the different densities of the body structures. The resultant photons emitted from the body when captured by a detector produce an image of the structures being exposed.

🔹Positioning of patient: There are two reference lines which are used for CT scanning. Orbitomeatal line – It is the standard reference line for various scans of skull. This line extends from centre of the orbit to the external acoustic meatus. Reid base line – It also serves as reference line along with the orbitomeatal line. It extends from lower border of orbits to the superior border of external auditory meatus.

🔹Types of CT imaging: Different types of CT imaging are contrast CT, spiral or helical CT, multislice CT, 3D CT, multidetector CT, CBCT and EBCT.

🔹Artifacts in CT: Patient motion artifacts, metal artifacts, beam hardening artifacts, partial volume artifacts and stair-step artifacts/ring artifacts.

🔹Principles in CT interpretations: CT uses the differences in attenuation of the X-ray beam by different tissues to form an image. The lowest attenuation occurs in air and the highest attenuation occurs in bone, dentine, enamel or metal.

📌 MULTIPLE CHOICE QUESTIONS (MCQs)

💡Which of the following is not the disadvantage of conventional radiography technique?

  1. Two-dimensional picturisation
  2. Overlapping of shadows in image
  3. More shades of grey
  4. Less resolution, difficulty in specifying exact nature of tissue (air, fluid, etc.)

Answer : 3

💡The first computed axial tomography scanners of 1972 were introduced for intracranial imaging by

  1. Godfrey Hounsfield
  2. Frank Harrison
  3. Felix Bloch
  4. Jonathon Hutchison

Answer: 1

💡Generations in CT scanner are based on which of the following features?

  1. Type of detectors
  2. Number of detectors
  3. Motion of gantry frame
  4. All of the above

Answer: 4

💡Materials used in detectors in CT are

  1. Sodium iodide
  2. Calcium fluoride
  3. Bismuth germinate
  4. All of the above

Answer: 4

💡Which of the following is a type of console?

  1. Operating console
  2. Reporting console
  3. Independent console
  4. All of the above

Answer: 4

💡Reference lines used in CT are

  1. Radiographic base line – line drawn from outer canthus of eye to midpoint of external auditory meatus
  2. Reid base line – line drawn from lower border of the orbit to the superior border of external auditory meatus
  3. Both of the above
  4. None of the above

Answer : 3

💡Depending on the amount of absorption within a block of tissue, the CT number which is assigned to each pixel is between

  1. +1000 and −1000
  2. +100 and 0
  3. +100 and −100
  4. +1000 and 0

Answer: 1

💡Term used for the method used to vary the density and the contrast in CT images is

  1. Windowing
  2. Shadowing
  3. Formatting
  4. Attenuation

Answer: 1

💡When the window level is constant (low)

  1. Bone appears white, all soft tissues appear white or light grey and air appears black
  2. Bone appears white, air appears grey and soft tissue appears black
  3. Bone appears black, air appears black and soft tissue appears grey
  4. None of the above

Answer: 1

💡CT involving the simultaneous movement of the patient’s table and the X-ray tube, which results in volume acquisition of the data from which individual tomographic images can be reconstructed, is known as

  1. Spiral CT
  2. Multiplanar reformatted imaging
  3. Contrast CT
  4. 3D CT

Answer: 1

💡CT numbers of various biological tissues are

  1. Air: −1000
  2. Water: 0
  3. Bone, cementum, enamel: 1000–3000
  4. All of the above

Answer: 4

💡Which of the following is not a disadvantage of CT?

  1. It is sophisticated, costly and expensive to maintain.
  2. It allows reconstruction of cross-sectional images of entire maxilla or mandible or both from single imaging procedure.
  3. Very high-density materials produce severe artifacts.
  4. Very thin contagious or overlapping slice may result in a high radiation dose.

Answer: 2

💡Contrast materials used for CT are

  1. 76% urografin (sodium diatrizoate), 60% urografin (in children)
  2. Trivideo–400 (sodium iodothalamate), trivideo 280 (in children)
  3. Meglumine salt iodothalamate
  4. All of the above

Answer: 4

Oral Medicine and Radiology