W3 - Scan Planning and IQ

Scan Parameters

• mA

• scan time

• slice thickness

• FOV

• reconstruction algorithm

• pitch

• kV

Automatic tube current modulation (ATCM)

adjusts mAs during each gantry rotation to compensate for large variations in x-ray attenuation

upcoupling effect

when the operator increases the mAs beyond the threshold (peak) value, which only contributes to dose and not IQ

higher mA =

shorter scan time → reduced risk of image degradation

IQs of CT

• spatial/detail (high contrast) resolution

• contrast (low contrast) resolution

• temporal resolution

Spatial vs. Contrast Resolution

• spatial =  level of detail visible in the image

• contrast = the ability of the system to differentiate between objects with similar densities

What is detrimental to spatial resolution (SR)

volume averaging

Types of SR

• In-plane = spatial resolution in the xy direction 

• Longitudinal resolution = spatial resolution in the z direction

Influencing Factors of Spatial Resolution (SR)

• slice thickness

• pixel size

• reconstruction algorithm

• pitch

Slice Thickness - SR

volume averaging takes a greater impact on thicker slices, thereby affecting SR

Pixel Size - SR

larger pixels will reduce in-plane spatial resolution due to volume averaging

Reconstruction Algorithm - SR

only reconstructs the raw data; can reduce artifacts but also reduce SR; some will optimize SR but reduce contrast resolution

Increased pitch =

reduced resolution

What is detrimental to Contrast Resolution (LCR)

level of noise

Influencing Factors of LCR

• mAs (dose)

• pixel size

• slice thickness

• reconstruction algorithm

• patient size

mAs (Dose) - LCR

increasing mAs can improve contrast resolution if it was initially degraded due to noise

Pixel Size - LCR

• the number of detected x-ray photons per pixel will decrease as pixel size decreases → decrease in contrast resolution

• smaller pixel → reduced LCR

Slice Thickness - LCR

• thicker slices allow for more photons to reach the detector → less noise

• BUT comes at the cost of spatial resolution in the z axis

Reconstruction Algorithms

likewise to SR, different reconstruction algorithms can focus on different resolutions for interested anatomy

Types of Algorithms

• Bone algorithm = lower contrast resolution (but better spatial resolution)

• ST algorithm = improve contrast resolution at the expense of spatial resolution

Patient Size - LCR

larger patients attenuate more x-ray photons, leaving fewer to reach the detectors → more noise → reduced contrast resolution 

CT and Contrast Resolution

superior to all other modalities; objects with as little as 0.5% density variation can be distinguished

Types of FOV

• SFOV =  area within the gantry, where raw data are acquired

• DFOV = how much, and what section, of the collected raw data are used to create an image

FOV and pixel size

• directly proportional

• pixel size = DFOV/matrix size