Optimizing CT Image Quality: Image Quality

November 2011 PART 2: IMAGE QUALITY DIAGNOSTIC TASK Diagnostic Task Contrast Noise Contrast/Noise (CNR) Anatomy Function Which Image Quality is Best? 15 month old infant shunt follow-up  ventricle size 1. Good CTDIvol CTDIvol 24 mGy 4.8 mGy 2. So-So 3. Poor Image Quality Always Task Dependent CONTRAST Optimization  Task To Be Defined Part 2 (Image Quality) Huda 1 November 2011 High Contrast Low Contrast  Photon Energy  Contrast  Tube Voltage (kV)  Photon Energy  Photon Energy  Dose Contrast 15 20 25 30 110 kV 10 35 40 70 kV Photon Energy (keV) Lower kV  Higher Contrast Yu et al., (11 year old boy) Radiographics 31 (2011) 835-848 80 kV 140 kV 120 kV CTDIvol Soft 5.2 mGy Tissue CT Enterography 100 kV Iodine CTDIvol (Vessel) 4 mGy mural stratification Part 2 (Image Quality) Huda 2 November 2011 Change in Image Contrast With kV NOISE Is Tissue Dependent! Small Changes In Soft Tissue Contrast Large Changes In Iodine Contrast Noise Also Known As Mottle Low Noise ~104/mm2 High mAs ~108/mm2 ~10/mm2 8047 . 3 8047 .5 High kV  Photons  Noise High Noise Low mAs Low kV Original dose Part 2 (Image Quality) Huda 3 November 2011 Low Contrast Phantom 25% dose Ka 2000 Noise Affects Visibility of “Low Contrast Lesions” Average 100 photons/pixel Average 10,000 photons/pixel 101 106 123 98 10,087 10,114 9,900 10,284 84 111 107 104 9,937 9,986 10,123 10,104 111 91 98 93 9,841 10,206 10,042 9,881 108 100 97 102 10,040 9,972 9,906 10,062 Pixel Value Fluctuations Pixel Value Fluctuations ±10% ±1%  mAs x 100 CONTRAST TO NOISE RATIO (CNR)  Noise x 10 Noise mAs “controls noise” Lesion Contrast Image Noise C >> N CNR Part 2 (Image Quality) Huda 4 November 2011 CNR & kV CNR & mAs 20 mAs 110 10 mAs 90 70 50 NUMBER OF X RAYS PER UNIT ENERG # of X-Ray Photons 0 10 20 30 40 50 60 70 80 90 100 110 X-Ray Photon Energy (keV) X-RAY ENERGY (keVI mAs only affects noise kV affects noise and contrast  mAs offers “any CNR” CNR adjustable at “any kV” 10 HGy Radiation Intensity 100 HGy N  Contrast  Lesion- < - Lesion- Noise CNR  level Noise Noise level Image Image 5-MAR-83 RTERIAL HEAD 15-MAR-83 C  Contrast  < Noise CNR  W 030 W 148 L 128 L 130 RIGHT 06. 174 012 03. 688 007 Part 2 (Image Quality) Huda 5 November 2011 IMAGE QUALITY REVIEW Image Quality Always Task Dependent Rembrandt 1669 Energy (kV)  Contrast  # X-ray Photons  Noise kV will always  Noise  mAs will always  Noise 85 AMERICOUSE Contrast to Noise Ratio  “Lesion Visibility” Next CARE Dose4D (kV + mAs)  CNR CNR adjustable at “any kV” Part 2 (Image Quality) Huda 6