RTR.6 Analyzing IQ and Responding

Spatial Resolution

• ability to image small objects that have high subject contrast

• affected by focal spot size and detector material, smaller the better

Contrast Resolution

• ability to image small objects that have high subject contrast

• improves with low kVp

Subject Factors that Affect IQ

• subject contrast

• patient thickness → thicker = more absorption

• tissue mass density → denser = more absorption

• atomic number → higher = more scatter

• object shape

Window Width

the number of Hounsfield units represented on a specific image

Wide WW

• Best for imaging tissues with high contrast  

• Reduces image contrast, but also suppresses noise, thus often used for obese pts or for metallic objects 

Narrow WW

best for low contrast tissues eg. brain

Window Level

• The centre value of the WW thus selects which Hounsfield values are displayed as shades of gray

• Should be set at a point that is roughly the same value as the average attenuation number of the tissue of interest

Factors that Increase Scatter Radiation

• high kVp and also high mA

• large field size

• increased patient thickness

Lossless Image Compression

can be reconstructed to be the same as the original image, 10:1 or 2:1

Lossy Image Compression

can provide compression factors of up to 100:1 or greater; not acceptable for archiving any images as it represents a lack of data

XR Preprocessing Components

• interpolation = for dead pixels

• flatfielding = equalizes the response of each pixel to a uniform X-ray beam, eg., anode heel effect

CT Artifact - Aliasing

• Insufficient projection data (aka undersampling), due to extended helical pitch

• Consist of evenly spaced lines, easy to distinguish from anatomic structures, thus rarely render an image undiagnostic

CT Artifact - Beam Hardening

• Can produce either cupping artifacts (the periphery of the image is lighter) or streak artifacts (of dark bands or streaks between dense objects)

• Filtration, bow tie filter, increase kVp, select appropriate SFOV

CT Artifact - Partia Volume

• When a dense object is lying at the edge of FOV, it may only show up in a small number of views

• Use thinner slices

CT Artifact - Edge Gradient

• Due to irregularly shaped objects that have a pronounced difference in density from surrounding structures

• Unavoidable, but can be reduced by thinner slices

CT Artifact - Ring

• Caused by imperfect detector elements—either faulty or simply out of calibration

• Appear as a ring or concentric rings centred on the rotational axis

What type of Artifact is this?

Motion Artifact

What type of Artifact is this?

Metallic Artifact

What type of Artifact is this?

Out of Field Artifact

• Caused by anatomy beyond the SFOV hardening the beam, but ignored in the reconstruction process eg. arms being by the side

QC - Exposure Reproducibility

ensuring that an X-ray generator always produces the same intensity each time with the same set of technical factors; maximum variability is +-5%

QC - Beam Quantity

X-ray generators should emit a specific amount of radiation per unit of X-ray tube current and time; maximum variability is +-10%

QC - Kilovolt Accuracy

the kV stated on the control panel should produce an X-ray beam with a comparable and consistent amount of energy; maximum variability is +-5%

QC - Timer Accuracy

to ensure accurate exposure timer; +-20% for exposure times less than 10 ms

QC - mA and Exposure time Linearity and Reciprocity

• to ensure constant radiation output with various mA and exposure time combinations

• eg. g. 50 mA at 1sec and 100 mA at 0.5sec, both with the same kVp, should yield the same amount of radiation output

• +-10%

QC - Linearity

sequential increase in mAs should produce the same sequential increase in the exposure measured, +-10%

QC for DSA

needs a weekly QC to evaluate DSA function with a DSA phantom

QC for Fluoro

Quarterly QC to evaluate table angluation accuracy and smooth motion

QC - Electronic Display Device Performance

Yearly, SMPTE and/or TG-18 to assess for any

• Distortion

• Reflection

• Luminance

• Resolution

• Noise

• Glare

• Chromacity (quality of colour)

Bolus (arterial) Phase

• Immediately follows injection

• HU difference of 30 or more between the aorta and IVC

Non-equilibrium (venous) Phase

• 1min post injection

• HU difference between 10-30

Equilibrium (delayed) Phase

• 2min post injection

• HU difference less than 10

Wide Histogram =

• peaks on the far ends of the graph = lots of white and black shades

• high contrast image

Narrow Historgram

peak is at the center of the graph = similar shades of gray, low contrast