QC of CT System

spatial resolution (high resolution)

Can be calculated from the analysis of the spread of information within the system using the MTF. 

line pair phantoms

SPATIAL RESOLUTION (HIGH CONTRAST): Measuring the spatial resolution directly, using a _____________, is a simpler method and is often performed by a technologist. 

visible line pairs

The spatial resolution is given as the maximum number of _______________ (lead strip and space) per millimeter. 

monthly

SPATIAL RESOLUTION (HIGH CONTRAST): This test is performed ________ in most quality assurance programs.

10 to 20 lp/cm

SPATIAL RESOLUTION (HIGH CONTRAST): The spatial resolution of current scanners when images are reconstructed in a high-resolution algorithm is in the range of _______

0.5%

CONTRAST RESOLUTION (LOW CONTRAST): At the minimum, contrast resolution should be such that with a density difference of ____ a 5-mm object can be displayed. 

high

______ contrast objects are easier to resolve

laser lights

___________ located both inside and outside the gantry are used extensively for patient positioning and alignment. 

Accurate laser light performance

__________________ is critical and can be measured, most often using a specific phantom designed for the purpose and provided by the scanner manufacturer. 

2 mm

LASERLIGHT ACCURACY: The light field should coincide with the radiation field to within ______

semiannually

LASERLIGHT ACCURACY: This test is usually performed ___________.

Noise

_______ is measured by obtaining the standard deviation (SD) of the CT numbers within a region of interest (ROI). 

Uniformity

__________ refers to the ability of the scanner to yield the same CT number regardless of the location of an ROI within a homogeneous object. 

water phantom

NOISE AND UNIFORMITY: measured using a ______________

weekly

NOISE AND UNIFORMITY: These tests should be performed on a ________ basis

10

NOISE: A standard deviation of an ROI in a water phantom should not exceed ___

2

UNIFORMITY: there should be no more than a ±__ HU variation from an ROI placed at the center of the water phantom to those placed at the periphery. 

LINEARITY

This refers to the relationship between CT numbers and the linear attenuation values of the scanned object at a designated kVp value.

daily calibrations

LINEARITY: _______________ help to avoid fluctuations in linearity by compensating for these tiny changes. 

scanner malfunction

LINEARITY: Any deviation from the straight line indicates ______________

semiannually

Linearity is typically measured ____________

radiation

These measurements are performed by a medical physicist who follows very detailed specifications to report parameters such as effective dose and dose length product. 

• Beam hardening

• Partial volume artifacts

• Aliasing

• Edge gradient effect

• Motion

• Metallic artifacts

• Ring artifacts

• Out-of-field artifacts

• Tube arcing artifacts

• Helical and cone beam artifacts

Image artifatcs

Beam hardening

Individual rays are hardened to differing degrees, and this variation cannot be adjusted for by the reconstruction algorithm

Beam hardening is also known as cupping artifacts

dense objects

BEAM HARDENING: The beam is hardened more by ___________ (e.g., more by bone and less by fat).

1. Filtration

2. Calibration correction

3. Beam hardening correction software

features to minimize beam hardening

Partial volume artifacts

occurs when more than one type of tissue is contained within a voxel.

Partial volume artifacts

occurs when a dense object lies to the edge of the FOV.

aliasing

Insufficient projection data

undersampling

aliasing is also known as

fine stripes

Undersampling causes inaccuracies related to reproducing sharp edges and small objects and results in an artifact known as aliasing, in which ____________ appear to be radiating from a dense structure.

slowing ; reducing

Aliasing can be combated by _______ gantry rotation speed (i.e., increasing scan time) or by ________ the helical pitch.

streak artifacts

Metal objects in the SFOV will create

Out-of-field artifacts

caused by anatomy that extends outside of the selected SFOV.

streaks and shading

Out-of-field artifacts appear as ____________ on the image 

third-generation scanners

Ring artifacts occur with ____________ and appear on the image as a ring or concentric rings centered on the rotational axis.

Ring artifacts

They are caused by imperfect detector elements—either faulty or simply out of calibration.

recalibrating

In some instances technologists may eliminate circular artifacts by _________ the scanner. 

Tube arcing artifacts

A common cause of equipment-induced artifact occurs when there is an undesired surge of electrical current (i.e., a short-circuit) within the x-ray tube. 


arcing

________ tends to occur whenever there is a large difference in electrical potential, such as the case between the anode and cathode in an x-ray tube. 

increases

As an x-ray tube ages, the tendency to arc often __________ owing to such factors as degradation of the vacuum within the tube, which results in increased gas pressure.

x-ray output

The arcing causes a momentary loss of __________, which contaminates the x-ray signal collected at the detectors, affecting proper image reconstruction and hence producing artifacts. 

Helical interpolation artifacts

result in subtle inaccuracies in CT number and can be easily misinterpreted as disease

low pitch

Helical interpolation artifatcs can best be avoided by using a _________ whenever possible

windmill artifacts

_____________ appear only on MDCT (Multi-detector CT) helical systems.

Cone beam artifacts

Appear as either streaks or as bright and dark shading near areas of large density differences (e.g., bone and muscle). 

Beam hardening

Cause: X-ray beams are composed of different energies

Aliasing effect

Cause: Too few samples

Edge gradient effect

Cause: Angle of x-ray beam varieties between two similar view

Metallic

Cause: Objects present that are beyond the dynamic range of the scanner

Ring

Cause: Detector problem

Tube arcing

Cause: Electrical surge within the x-ray tube

Spiral Interpolation artifacts

Cause: Images are created from view that are not all in the same plane

Cone beam effect

Cause: Only on MDCT, from the cone-shaped x-ray beam

recalibrate

Corrective step for ring artifact

lower pitch

corrective step for spiral interpolation artifacts

Edge Gradient effect

straight line radiating from high-contrast areas

Tube arcing

no specific pattern

Spiral interpolation artifacts

subtle inaccuracies in CT number

cone beam effect

lines appear in a windmill formation