CT Exam Review_Image Production

Which of the following reduces the scatter radiation reaching the detectors?

• Post-patient collimation

• Pre-patient collimation

• Pre-detector collimation

Answer explanation

Pre-detector collimation and post-patient collimation are synonymous. Both terms describe a device designed to remove scatter radiation before it reaches the detector.

Which of the following is the primary interaction between x-ray photons and tissue during computed tomography?

• Compton effect

• Bremsstrahlung

• Coherent scatter

• Characteristic

Answer explanation

Owing to the high-energy beam used in CT, the Compton effect is the predominant interaction between x-ray and matter. CT scanners use between 120 and 140 kVp, yielding x-ray energies with averages between 70 and 80 keV.

Tags

NGSS.HS-PS4-4

The process by which electrons are produced at the cathode of a CT x-ray tube is known as:

• rectification.

• thermionic emission.

• isotropic emission.

• anode heel effect.

Answer explanation

When the cathode of an x-ray tube is heated, electrons are released through the process of thermionic emission. It is these electrons that are then accelerated toward the anode for x-ray production.

The assignment of different generations to CT scanners is based on the configuration of the:

• patient and gantry.

• tube and collimators.

• anode and cathode.

• tube and detectors.

Answer explanation

CT scanners are often placed into one of several different "generations." The differences in generations are based on the relationship of the tube and detectors and the position of each during data acquisition.

When one is using a third-generation CT scanner, it is important that reference detectors positioned at the peripheral portions of the detector array be exposed to:

• homogeneous radiation.

• unattenuated radiation.

• monochromatic radiation.

• remnant radiation.

Answer explanation

Reference detectors are used to measure incident radiation intensity. This information is used by the computer during the calculation of the linear attenuation coefficient. If the patient is placed incorrectly within the gantry, the reference detectors may be partially blocked, causing an out-of-field artifact.

In 1979, the scientists __________ and __________ shared the Nobel Prize for their research in computed tomography.

• Godfrey Hounsfield and Allan McLeod Cormack
• David Wilson and Laura Thompson
• Michael Brown and Sarah White
• John Smith and Emily Johnson

Answer explanation

Dr. Godfrey Hounsfield and Dr. Allan Cormack shared the Nobel Prize for their work on the development of computed tomography in 1979.

The type of compensating filter used at the x-ray tube of a CT scanner is called a:

• scatter filter
• bowtie filter
• shielding filter
• beam filter

Answer explanation

The bow-tie filter used at the x-ray tube of a CT scanner absorbs a larger amount of radiation at the periphery of the patient, where the part thickness is reduced. The center of the patient is placed at the center of the filter, where the largest amount of radiation is allowed to pass through. The use of this type of filter attempts to compensate for the differences in thickness of the often oval-shaped patient.

The interaction between x-ray and matter that is responsible for the production of the scatter radiation absorbed by the patient and detectors is:

• Photoelectric effect
• Thompson scattering
• Pair production
• Compton effect

Answer explanation

In the Compton interaction, an x-ray photon ejects an outer-shell electron of an atom. The photon loses some of its energy in the collision and then continues on in a different, scattered direction. This interaction is the major source of the scatter radiation involved in the formation of the CT image.

Tags

NGSS.HS-PS4-5

NGSS.HS-PS4-3

The types of detectors used in computed tomography are:
1. Gas ionization
2. Stimulable phosphor
3. Scintillation

• 2 only

• 1 only

• 1 & 3

• 1 & 2

Answer explanation

The types of detectors used in computed tomography are scintillation and gas ionization detectors. Both operate by measuring the amount of transmitted radiation passed through the patient and transmitting this information to the computer for image reconstruction. Newer MDCT systems utilize solid-state scintillation detectors exclusively.

Tags

NGSS.HS-ETS1-3

Which of the following is used in gas ionization CT detectors?

• Neon gas
• Xenon gas
• Krypton gas
• Argon gas

Answer explanation

Gases with high atomic numbers, such as xenon, are used in ionization-type CT detector systems. The high atomic number of the xenon gas (Z = 54) increases the incidence of interaction with x-ray photons, thus improving the efficiency of the detector.