Water Appearance on CT Scan
Water does NOT appear white on CT—it appears dark gray to black, with an attenuation value of 0 Hounsfield Units (HU), which is the reference standard for CT calibration. 1
Understanding CT Attenuation and Hounsfield Units
Pure water is calibrated to 0 HU (±4 HU) on properly calibrated CT systems, serving as the baseline reference point for all tissue density measurements 1
Water appears as a neutral gray density on CT images, distinctly different from:
- Bone and calcifications: appear bright white (high positive HU values, typically +400 to +1000 HU)
- Soft tissues: appear light gray (positive HU values, typically +20 to +70 HU)
- Fat/adipose tissue: appears dark gray to black (negative HU values, typically -190 to -10 HU) 1
- Air: appears black (approximately -1000 HU)
Clinical Context: Water as Contrast Agent
Water functions as a "neutral" or "negative" contrast agent in CT imaging, meaning it provides contrast by being darker than surrounding tissues rather than brighter 2, 3, 4
Water-soluble contrast agents (like Gastrografin) contain iodine and appear bright white on CT due to high attenuation values 1
Plain water used as oral contrast maintains its near-zero HU value and appears dark, which actually improves visualization of bowel wall enhancement and mucosal detail when IV contrast is administered 2, 3, 4
Studies demonstrate that water as oral contrast provides comparable or superior image quality compared to positive contrast agents for most abdominal structures, with better delineation of the duodenal wall specifically 2, 4, 5
Common Pitfall to Avoid
Do not confuse water-soluble iodinated contrast (which appears white) with plain water (which appears dark). The term "water-soluble contrast" refers to iodinated contrast agents that dissolve in water but contain high-density iodine molecules that make them appear bright white on CT 1. Plain water itself remains dark gray/black at 0 HU 1.