CT

Quality Control for CT

Scanners

Quality Control in CT

A good idea? YesRequired for accreditation? SometimesImproves image quality? Sometimes

Depends on starting pointReduces downtime? Sometimes

Depends on starting pointPossible recognition of problems before

clinical manifestationstotal failure

may reduce unscheduled downtime

High Contrast ResolutionBaseline established when scanner in good

operating conditionSubsequent test compared to baseline

CT Quality ControlStandard protocol required

same kVp, mAs, scan diameter, slice width, algorithm, focal spot, filter, etc

When performedAt regular intervalsAfter major service

tube change

CT # of WaterWhat do we check?

Average (ideally 0 by definition)Standard Deviation (ideally 0)Flatness

ROI’s at various locations in phantom

Results depend upon protocolkVp, mA, scan timeslice widthphantom sizeROI position

Problem causesMis-calibrationNoise / drift in detector system

CT # of WaterCan repeat for various

Slice thicknessesReconstruction algorithmsCentering positionsPhantom diametersFields of view

CT Number AccuracyPhantom with several materials of known CT #’sDo ROI’s at each and check CT # accuracy

High Contrast (or Limiting) Resolution

Phantomrows 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 seensmaller holes = better resolution

Visual scoring

Low Contrast Resolution Phantom

holes get smaller from column to columnHoles have less contrast from row to rowVisual scoring

Low Contrast ResolutionNoise 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 ResolutionPossible sources of failure: anything that

can increase noisedecreased dose to detectorsreduced tube outputelectronic noise

detectors amplifiers A - D convertor

Distance-Measuring AccuracyPhantom object with

precisely known object distances

Use distance-measuring software

Check vertical, horizontal & diagonal

Check monitor & hard copy

Monitor Performance / Hard Copy OutputStandard gray scale

image neededcomputer generated

(SMPTE pattern)Check monitor &

hard copy5% patchesContrast patchesResolution bars

Hard copyMeasure optical

density of steps with densitometer

Slice Localization / Thickness Accuracy

Phantom withslanted wire

Measure lengthRegularly spaced (in z

direction) objects count

Align phantom slice with laserMake single scan with known

slice thickness

Slice Localization / Thickness Accuracy

Table IndexingX-ray cassette on tableLoad table with weightSelect

multiple scanssmall beam widthlarge mm table increment

Table IndexingImage should show series of

exposed bandsMeasure distance between

bandsPossible causes of failure

slippage of table drive mechanism

table mis-calibration

Table BacklashDoes table return to same

position from both directions?Test

Apply masking tape to tableLoad table with weightNote numerical positionMark tape at starting position

using laser positionMove table away from starting

position & then back to same numerical location

Check laser vs. mark on tape

Laser AccuracyAlign phantom on

table matching outer laser to external marks on phantom

“Zero” tableScan phantom

“0” z-location should show correct phantom position on image

Repeat for internal laser

CT Noise CharacteristicsWater phantomMultiple scans changing only mAsMeasure standard deviation of CT #’s

using identical ROI’sNoise proportional to standard deviation

CT Noise Characteristics

Excessive noise can be caused bydetector problemselectronic noise in detector amplifier circuitsreduced output per mAs