CT Quality Control for CT Scanners

Quality Control in CT A good idea? Yes Required for accreditation? Sometimes Improves image quality? Sometimes Depends on starting point Reduces downtime? Sometimes Depends on starting point Possible recognition of problems before clinical manifestations total failure may reduce unscheduled downtime

High Contrast Resolution Baseline established when scanner in good operating condition Subsequent test compared to baseline

CT Quality Control Standard protocol required same kVp, mAs, scan diameter, slice width, algorithm, focal spot, filter, etc When performed At regular intervals After major service tube change

CT # of Water What do we check? Average (ideally 0 by definition) Standard Deviation (ideally 0) Flatness ROI’s at various locations in phantom Results depend upon protocol kVp, mA, scan time slice width phantom size ROI position Problem causes Mis-calibration Noise / drift in detector system

CT # of Water Can repeat for various Slice thicknesses Reconstruction algorithms Centering positions Phantom diameters Fields of view

CT Number Accuracy Phantom with several materials of known CT #’s Do ROI’s at each and check CT # accuracy

High Contrast (or Limiting) Resolution Phantom rows of equally spaced high contrast objects holes get smaller from row to row Bars of decreasing size & separation Determine smallest row of holes where all holes can be clearly seen smaller holes = better resolution Visual scoring

Low Contrast Resolution Phantom holes get smaller from column to column Holes have less contrast from row to row Visual scoring

Low Contrast Resolution Noise limited less noise = more rows visualized Standardize mAs more mAs  less noise  better low contrast resolution reconstruction algorithm / filter smoothing reduces noise improves low contrast resolution

Low Contrast Resolution Possible sources of failure: anything that can increase noise decreased dose to detectors reduced tube output electronic noise detectors amplifiers A - D convertor

Distance-Measuring Accuracy Phantom object with precisely known object distances Use distance-measuring software Check vertical, horizontal & diagonal Check monitor & hard copy

Monitor Performance / Hard Copy Output Standard gray scale image needed computer generated (SMPTE pattern) Check monitor & hard copy 5% patches Contrast patches Resolution bars Hard copy Measure optical density of steps with densitometer

Slice Localization / Thickness Accuracy Phantom with slanted wire Measure length Regularly spaced (in z direction) objects count Align phantom slice with laser Make single scan with known slice thickness

Slice Localization / Thickness Accuracy

Table Indexing X-ray cassette on table Load table with weight Select multiple scans small beam width large mm table increment

Table Indexing Image should show series of exposed bands Measure distance between bands Possible causes of failure slippage of table drive mechanism table mis-calibration

Table Backlash Does table return to same position from both directions? Test Apply masking tape to table Load table with weight Note numerical position Mark tape at starting position using laser position Move table away from starting position & then back to same numerical location Check laser vs. mark on tape

Laser Accuracy Align phantom on table matching outer laser to external marks on phantom “Zero” table Scan phantom “0” z-location should show correct phantom position on image Repeat for internal laser

CT Noise Characteristics Water phantom Multiple scans changing only mAs Measure standard deviation of CT #’s using identical ROI’s Noise proportional to standard deviation

CT Noise Characteristics Excessive noise can be caused by detector problems electronic noise in detector amplifier circuits reduced output per mAs

The End