Optimizing CT Image Quality - Part 2 - Image Quality

Optimizing CT Image Quality Image Quality   Walter Huda, PhD Professor of Radiology Medical University of South Carolina   Diagnostic Task Contrast Noise Contrast to Noise Ratio (CNR) What do we mean by "image quality?" CTDIvol - 24 mGy ← CTDIvol - 4.8 mGy ← 1. Good? 2. So-so? 3. Poor? Image Quality is Always Task Dependent Optimization → Task To Be Defined  High Contrast  Low Contrast ↑ Photon Energy   - ↓ Contrast ↑ Photon Energy   - ↓ Dose Lower kV → Higher Contrast Higher kV → Lower Contrast ↑ Tube Voltage (kV)   ↓ Photon Energy Yu et al., (11 year old boy) Radiographics Volume 31 (2011) pgs 835-848 CT Enterography 120 kV CTDIvol - 5.2 mGy 100 kV CTDIvol - 4 mGy 100 kV CTDIvol - 4 mGy 100 kV CTDIvol - 4 mGy   80 kV Soft Tissue     Iodine (Vessel) 140 kV   Change in Image Contrast With kV Is Tissue Dependent! Small Changes In Soft Tissue Contrast Large Changes In Iodine Contrast Noise Also Known As Mottle ~108/mm2 ~104/mm2 ~10/mm2 ↑ Photons ↓ Noise Low Noise High Noise High mAs - High kV Low mAs - Low kV Original Dose 25% Dose Low Contrast Phantom Noise Affects Visibility of “Low Contrast Lesions” Average 100 photons/pixel Pixel Value Fluctuations ±10% Average 10,000 photons/pixel Pixel Value Fluctuations ±1% ↑ mAs x 100 ↓ Noise x 10 mAs “controls noise” Lesion Contrast Image Noise CNR Contrast > Noise CNR and kV kV affects noise and contrast CNR and mAs mAs only affects noise ↔ mAs offers “any CNR” CNR adjustable at “any kV” Contrast < Noise N ↓ ► CNR ↑ Contrast < Noise C ↑ ► CNR ↑ Diagnostic Task Image Quality is Always Task Dependent Contrast Energy (kV) ↓ Contrast ↑ Noise # X-ray Photons ↔ Noise ↑kV will always ↓ Noise ↑mAs will always ↓ Noise Contrast to Noise Ratio (kV + mAs) ↔ CNR CNR adjustable at “any kV”