CT Image Quality Indices

Noise

The CT numbers vary randomly, and the image is mottled or grainy.

?? Noise is reduced by :

Using a higher mA, longer rotation time, higher kVp , a larger slice thickness and large pixel.

CT spatial resolution (high contrast resolution)

The ability of the image to differentiate between structures with very different attenuation characteristics (CT numbers) when they are very close together.

• A high level of spatial resolution can be achieved by having small pixels and thin slices.

For two objects to be seen as separate the detectors must be able to identify a gap between them.

Utilizing a bone, sharp, high frequency or high pass algorithm during reconstruction can improve the spatial resolution.

In addition,

• Shortened scan times(s), reduce motion, and may help improve spatial resolution
• Decreasing the pitch increase resolution

CT contrast resolution (low contrast resolution)

• The ability to discriminate between structures with very similar attenuation characteristics (CT numbers).
• The ability to differentiate a structure that varies only slightly in density from its surrounding
• Object Size is directly related to contrast resolution, larger objects are easily visualized.
• Object attenuation is directly related to contrast resolution, high attenuation objects are easily seen.
• Small, low attenuating objects are the most difficult to image.

The limiting factor for contrast resolution is noise.

Any factor that reduces the amount or visibility of noise improves contrast resolution, choice of mA, kVp , slice thickness, matrix size, FOV, and reconstruction filter affects contrast resolution.

Keeping all other scan parameters, the same, as pixel size decreases, the number of detected x-ray photons per pixel will decrease. (The noise will increase)

Thicker slices allow more photons to reach the detectors, they have a better SNR and appear less noisy.

However, this improvement comes at the cost of spatial resolution.

Patient size also effect the contrast resolution.

• For the same x-ray technique, larger patients attenuate more x-rays photons, leaving fewer to reach the detectors.
• This reduces SNR, increases noise, and results in lower contrast resolution.

Temporal Resolution

• Temporal resolution is still the major limitation of coronary CT and the main cause of non-diagnostic images, and therefore all possible measures must be taken to improve this parameter.
• CT requires at least 180° plus fan angle of projection data to perform image reconstruction.

• This implies that the intrinsic temporal resolution of a standard CT scan is in the order of gantry rotation time /2.
• The temporal resolution is most dependent on the speed of the gantry rotation.

Single-source 64-slice CT with half-scan reconstruction resulting in temporal resolution of 165 ms? and dual-source 64-slice CT with half-scan reconstruction resulting in temporal resolution of 83 ms .

In conclusion

• Image quality relates to how well the image represents the object scanned.
• In CT, image quality is directly related to its usefulness in providing an accurate diagnosis.
• Manipulating scan parameters may improve one aspect of image quality while decreasing another aspect.
• Many factors influence how well a CT image represents the actual object scanned.
• In addition, image quality is closely linked to radiation dose.