Anatomy Cross-Sectional Slices Measurement Quiz

Image Quality

• Insufficient mAs results in increased noise, which primarily affects contrast resolution.
• Spatial resolution can be evaluated using a line pairs phantom.
• Contrast resolution can be evaluated using a phantom that contains objects of varying sizes.
• Slice thickness accuracy can be measured using a phantom that includes a ramp, spiral, or step-wedge.
• Laser light accuracy is most often measured using a specific phantom designed for the purpose and provided by the scanner manufacturer.
• Noise and uniformity can be measured using a water phantom.
• Linearity is the relationship between CT numbers and the linear attenuation values of the scanned object at a designated kVp value.

Quality Assurance

• Quality Assurance (QA) methods are designed to ensure that the CT system is producing the best possible image quality using the minimal radiation dose to the patient.
• QA programs are shared between CT technologists and medical physicists.
• The tests that make up the QA program must be performed on a regular basis.
• The results from all tests must be recorded using a consistent format.
• Documentation should indicate whether the tested parameter is within specified guidelines.

QA Phantoms

• Many aspects of image quality can be evaluated using phantoms.
• Most phantoms are designed with many components so that a single phantom can be used to examine a broad range of scanner parameters.

Radiation Dose

• Technologists must be well acquainted with the factors that affect the radiation dose to the patient.
• Technologists must also provide safety instruction to anyone who must remain in the scan room.

3D Reformation

• Projection displays can be used to display 3D reformation.
• Maximum-intensity projection (MIP) selects voxels with the highest value to display.
• Minimum intensity projection (MinIP) selects voxels with the lowest value to display.

Multidetector Row Systems (MDCT)

• MDCT systems use many detector elements situated in a row.
• MDCT provides longer and faster z-axis coverage per gantry rotation.
• Slice thickness is determined by a combination of the x-ray beam width and the detector configuration.

Applications for Axial Scanning

• Axial scans are used in protocols in which the acquisition speed is not a major concern and optimal resolution is required.
• Axial scans are used when slices are gapped, or when exposure will be interrupted.

Helical Scanning

• Helical scanning was introduced in the late 1980s.
• Key aspects of helical scanning include continually rotating x-ray tube, constant x-ray output, and uninterrupted table movement.
• Helical scanning eliminates the interscan delay and has several advantages, including the ability to optimize iodinated contrast agent administration, reduces respiratory misregistration, and reduces motion artifacts from organs.

Pitch in SDCT and MDCT

• Increasing the pitch in SDCT will result in a scan covering more anatomy lengthwise for a given total acquisition time and will reduce the radiation dose to the patient.
• Decreasing the pitch in SDCT slows down the table speed and decreases the anatomy covered, and will increase the radiation dose to the patient.
• Pitch in MDCT is defined as table movement per rotation divided by beam width.

CT Scanners

• There are three main types of CT scanners in use today: Single Slice, Multi-slice, and Dual Source.
• Single Slice CT scanners have one x-ray tube and one row of detectors, providing 1 channel of spatial data.
• Multi-slice CT scanners have one x-ray tube and multiple rows of detectors, providing multiple channels of spatial data.
• Dual Source CT scanners have two x-ray tubes at 90° to each other, multiple rows of detectors x 2, providing multiple channels of spatial data.

CT Generations

• There are five generations of CT scanners today:
  • 1st generation: Parallel beam, translate rotate
  • 2nd generation: Fan beam, translate rotate
  • 3rd generation: Fan beam, rotate only
  • 4th generation: Fan beam, stationary circular detector
  • 5th generation: Electron Beam, all stationary