Lange Physics and Instruments

Which of the following is NOT considered a degradation of the reconstruction image due to an artifact?

• degrading the image quality

• segmenting the voxel data

• affecting the perceptibility of detain

• leading to misdiagnosis

Answer explanation

CT artifacts seriously degrade the quality of a CT image, since they are described as inconsistencies between the reconstructed CT numbers in the image and the true attenuation coefficients of the object. Artifacts can affect the image to the point of undiagnostic use; therefore, it is imperative for technologists to understand why artifacts occur and how they can be prevented or suppressed.

What are the two principal demands placed on an x-ray tube for CT imaging?

• heat dissipation and beam filtration

• high x-ray intensity and heat dissipation

• high x-ray intensity and rotation speed

• line focus principle and rotating anode

Answer explanation

With the introduction of spiral/helical CT scanning new technical advancements are needed to be made in order to deal with the continually rotating tube for longer periods of time. The construction of a larger anode (200 mm) allows for the use of higher tube currents. The heat capacity with these disks is also increased with an improvement in heat dissipation.

Which of the following photon interactions produce a photon that is deflected from its original site of interaction?

• characteristic radiation

• Compton interaction

• Photoelectric interaction

• deterministic effect

Answer explanation

Compton interaction occurs between moderate energy x-rays (above 75 keV) and outer shell electrons resulting in the ionization of the target atom. A proton of the energy is absorbed and a photon is produced with reduced energy. When that photon ejects from the site of interaction, it is deflected from that of the original photon. Due to its change indirection, this form of interaction is termed a scattering process. Since CT filters the x-ray photons are they leave the x-ray tube, short energy wavelengths are produced making the beam harder and higher chance of the photon to penetrate the body to produce a signal at the detectors.

In the early days of CT scanning, reconstruction algorithms were very antiquated. What type of reconstruction algorithm was performed by Hounsfield during his first experiments?

• back projection

• filtered back projection

• algebraic reconstruction

• iterative reconstruction

Answer explanation

The simplest form of reconstruction was the algebraic method of reconstruction performed by Hounsfield. This method used a linear system of equations to solve a problem. The mathematical steps were sequential since the first image matrices were 80 x 80, only 6,400 calculations needed to be performed.

Which is defined as the z-axis segmentation?

• voxel

• pixel

• matrix

• FOV

Answer explanation

A voxel is synonymous with volume element for a 2D CT image in the z-axis direction. The voxel is expressed by the width of the sides of the pixels (area) and the width of the slice.

A digital image is composed of multiple pixels. What is considered a characteristic of a pixel?

• cube-shaped volume of tissue that

• unit of linear attenuation

• two-dimensional square of data

• unit of x-ray exposure in air

Answer explanation

A pixel is a two-dimensional picture element that represents the smallest discrete block in a digital image display field. Multiple pixels compose the image matrix with each pixel representing a brightness level in the reconstructed image based on the atomic number and mass density of the tissue.

What is considered an electronic system for archiving, transmitting, viewing, and manipulating images in the radiology department?

• DICOM

• PACS

• HIS

• RIS

Answer explanation

Picture Archiving and Communication System (PACS) is one of the two key elements that form the radiology department's information infrastructure. The term PACS encompasses a broad range of technologies necessary for the storage, retrieval, distribution, manipulation, and display of images.

What is one of the advantages to using high kVp during CT imaging?

• reduction of imaging time

• reduction of reconstruction time

• reduction of radiation intensity at the image receptor

• reduction of radiation dose

Answer explanation

One of the advantages of utilizing a high-frequency generator is the ability to use high kVp to minimize the photoelectric absorption which in turn reduces radiation dose. Higher kVp means that the photons have higher energies and can penetrate thicker object. The radiation dose is proportional to the square of the kVp.

A CTA study was performed on a patient suspected of having an abdominal aneurysm. The 3D technologist requested the voxels to be isotropic to eliminate loss of detail. Which of the following constitutes an isotropic voxel?

• one in which all sides are equal

• one in which the slice thickness is larger than the pixel area

• one in the image needs to be reconstructed on a smaller matrix

• one in the pixel size is smaller than the slice thickness

Answer explanation

The term isotropic is described as uniformity in all orientations. For a CT image this means the x, y, and x planes must have the same dimensions.

If one wants to increase the contrast resolution of an image, which of the following would be the best factor to change?

• increase mAs

• Increase kVp

• increase beam filtration

• decrease of kV

Answer explanation

Photon flux is primarily dependent on the combination of kVp, mAs, and beam filtration with mAs being the most straightforward technical factor to change when contrast resolution is required. mAs is directly proportional to the number of photons produced and the patient dose received.