Image Quality

CT Basic training : I.Q Image Quality GOLD LITE PTE LTD CT Basic training : I.Q Image Quality Image Quality Image Quality • Image quality is subjective • We can measure the quality of a system and express it in a technical way Image Quality GOLD LITE PTE LTD Image Quality CT Basic training : I.Q Image Quality Description High contrast resolution • The ability to see the smallest detail with more than 100H contrast differences • “x” mm = 10 / (2 x “y” lp/cm) (convert from ‘mm’ to ‘lp/cm’ or v.v.) • Influences on High contrast: – – – – L/R alignment vibration reconstruction filter focus size GOLD LITE PTE LTD Visual High Contrast CT Basic training : I.Q Image Quality Description Low contrast resolution • The ability to see the smallest contrast differences • “mm” x “%” = “low contrast factor” (on a contrast scale of 1000 H) 0.3% 0.1% 0.5% • Influences on Low contrast: – – – – – F/B alignment tube output (mA, kV, slice thickness) vibration reconstruction filter calibration GOLD LITE PTE LTD 1.0% Visual Low Contrast CT Basic training : I.Q Image Quality Description • • • • GOLD LITE PTE LTD Uniformity (Cupping / Capping) Linearity Slice thickness Partial Volume Effect CT Basic training : I.Q Uniformity (Cupping / Capping) • Measure all H-values on a profile in a water phantom Good: all values identical (+/- 2H) • CUPPING: Values on outside higher than in the middle • CAPPING: Values on outside lower than in the middle GOLD LITE PTE LTD CT Basic training : I.Q Linearity • Measure different materials with a known H-value • Plot the known and measured H-value Teflon Delrin Water LDPE Air OK GOLD LITE PTE LTD NOT OK = 970 H = 363 H = 0H = -103 H = -1000 H CT Basic training : I.Q Slice-thickness metal wire x-ray beam α front view side view side view (enlarged) reconstruction (enlarged) reconstruction • Make a scan through a phantom with a tilted metal wire • Measuring the length of the reconstructed wire • Angle α is known, the slice-thickness can be calculated GOLD LITE PTE LTD CT Basic training : I.Q Partial Volume Effect (1/2) pixel image = 2D voxel but the slice-thickness is the 3rd dimension • The H-value we see is the average of the contents of the ‘voxel’ GOLD LITE PTE LTD CT Basic training : I.Q Partial Volume Effect (2/2) If the voxel is homogenous : NO PROBLEM If the voxel is NOT homogenous : The displayed H-value is an average of more tissues Example: A small object cannot been seen Conclusion: If you want to see small objects: use thin slices GOLD LITE PTE LTD CT Basic training : I.Q Multiple Ring artifact (2/2) GOLD LITE PTE LTD CT Basic training : I.Q Streak artifact Caused by a sudden change in the data of channel - by defective detector - bad contact -? t k ac bl 900 GOLD LITE PTE LTD 896 RAW DATA PLOT channels views CT Basic training : I.Q Multiple Streak artifact (1/2) Caused by a many sudden changes in the data of channel - by defective detector - bad contact -? 900 GOLD LITE PTE LTD channels 896 RAW DATA PLOT views CT Basic training : I.Q Multiple Streak artifact (2/2) If the streaks can be connected by a circle, the streaks are caused by channel (Many changes during scan) GOLD LITE PTE LTD CT Basic training : I.Q Fan Streak artifact (1/2) This is a multiple streak artifact where all streaks originate from one point This appears when a complete view is damaged (lost) 900 GOLD LITE PTE LTD channels 896 RAW DATA PLOT views CT Basic training : I.Q Fan Streak artifact (2/2) GOLD LITE PTE LTD CT Basic training : I.Q Moiré pattern artifact This type of artifact can be caused by many fan-streak artifacts -or- interference from outside -or- ?? GOLD LITE PTE LTD CT Basic training : I.Q Multiple vibration artifact This artifact was caused by a spacer Making contact between DAS-assy and gantry-frame GOLD LITE PTE LTD CT Basic training : I.Q Reconstruction artifact GOLD LITE PTE LTD • All raw data is after reconstruction NOT OK • RTRU defect CT Basic training : I.Q Reconstruction artifact GOLD LITE PTE LTD • Bad contact • RTRU out + in = OK CT Basic training : I.Q Center Dot artifact GOLD LITE PTE LTD • Check with same parameters on a SS-phantom • Lower the “a-parameter” and perform air calibration CT Basic training : I.Q Mechanical artifact GOLD LITE PTE LTD • vibration artifacts • solved by adjusting the rotation belt tension CT Basic training : I.Q Black streak (1/2) GOLD LITE PTE LTD • What is the problem ? • black streak artifact (from where?) • from outside the head ! • how to follow-up ? • check scano ! (see next sheet) CT Basic training : I.Q Black streak (2/2) • • • GOLD LITE PTE LTD check scano ! slice is through the band ! inform the user of this problem !! CT Basic training : I.Q Finally • Some artifacts are ‘normal’ - Metal objects - Bone/soft-tissue , bone/air transitions can give problems during reconstruction and cause streak artifacts • This list is far from complete Please send images (+ description) on floppy or via email to us if you encounter interesting artifacts in the field GOLD LITE PTE LTD ... I.Q Image Quality Image Quality Image Quality • Image quality is subjective • We can measure the quality of a system and express it in a technical way Image Quality GOLD LITE PTE LTD Image Quality. .. LITE PTE LTD CT Basic training : I.Q Every step influences the ? ?image quality? ?? and can cause distortions From the HV Generator to the image printer GOLD LITE PTE LTD CT Basic training : I.Q Artifact... reconstruction filter focus size GOLD LITE PTE LTD Visual High Contrast CT Basic training : I.Q Image Quality Description Low contrast resolution • The ability to see the smallest contrast differences