Variation in aorta attenuation in contrast-enhanced CT and its implications for calcification thresholds

Abstract

Background CT contrast media improves vessel visualization but can also confound calcification measurements. We evaluated variance in aorta attenuation from varied contrast-enhancement scans, and quantified expected plaque detection errors when thresholding for calcification. Methods We measured aorta attenuation (AoHU) in central vessel regions from 10K abdominal CT scans and report AoHU relationships to contrast phase (non-contrast, arterial, venous, delayed), demographic variables (age, sex, weight), body location, and scan slice thickness. We also report expected plaque segmentation false-negative errors (plaque pixels misidentified as non-plaque pixels) and false-positive errors (vessel pixels falsely identified as plaque), comparing a uniform thresholding approach and a dynamic approach based on local mean/SD aorta attenuation. Results Females had higher AoHU than males in contrast-enhanced scans by 65/22/20 HU for arterial/venous/delayed phases (p < 0.001) but not in non-contrast scans (p > 0.05). Weight was negatively correlated with AoHU by 2.3HU/10kg but other predictors explained only small portions of intra-cohort variance (R2 < 0.1 in contrast-enhanced scans). Average AoHU differed by contrast phase, but considerable overlap was seen between distributions. Increasing uniform plaque thresholds from 130HU to 200HU/300HU/400HU produces respective false-negative plaque content losses of 35%/60%/75% from all scans with corresponding false-positive errors in arterial-phase scans of 95%/60%/15%. Dynamic segmentation at 3SD above mean AoHU reduces false-positive errors to 0.13% and false-negative errors to 8%, 25%, and 70% in delayed, venous, and arterial scans, respectively. Conclusion CT contrast produces heterogeneous aortic enhancements not readily determined by demographic or scan protocol factors. Uniform CT thresholds for calcified plaques incur high rates of pixel classification errors in contrast-enhanced scans which can be minimized using dynamic thresholds based on local aorta attenuation. Care should be taken to address these errors and sex-based biases in baseline attenuation when designing automatic calcification detection algorithms intended for broad use in contrast-enhanced CTs.

Reference distributions of aortic calcification and association with Framingham risk score

“…Our previous research has demonstrated strong relationships between coronary calcification scores and abdominal aortic calcification (MAC), specifically at the L3 and L4 levels 15 , 25 , 26 . Leveraging such evidence, we proposed the use of the MAC score to approximate coronary calcium measurement using computed tomography scans of the abdominal aorta 15 , 26 .…”

“…A central aortic lumen zone was identified on each relevant CT slice between L1 and L4 and its mean pixel value was computed for reference. Aortic calcification was determined by a dynamic threshold: calcification was identified as morphologic regions having pixel values five standard deviations above the reference 15 , 23 , 25 . Figure 4 displays a visualization of calcification region and resulting MAC scores for a higher (c.) and lower risk (d.) RAMP participant.…”

“…Region specific investigations of full aortic, thoracic aortic, and abdominal aortic calcium have been investigated 14 , 15 , 20 , 22 – 24 . Intravenous contrast is a major confounder of aortic calcification measurement, and as such, studies tend to stratify by use of contrast or phase 19 , 25 . Efforts to control for this confounding and create valid population-level estimates are ongoing 25 , 26 .…”

“…Intravenous contrast is a major confounder of aortic calcification measurement, and as such, studies tend to stratify by use of contrast or phase 19 , 25 . Efforts to control for this confounding and create valid population-level estimates are ongoing 25 , 26 .…”

Validation of a Length-Adjusted Abdominal Arterial Calcium Score Method for Contrast-Enhanced CT Scans

“…In patients with atherosclerotic cardiovascular diseases (ASCVDs) such as peripheral arterial occlusive disease (PAOD), abdominal aortic aneurysm (AAA), and carotid arterial stenosis, only contrast-enhanced CTs are commonly available. The standard threshold of 130 HU cannot be used in contrast scans because the blood contrast attenuation exceeds this threshold [7]. Therefore, a method to calculate calcium scores on contrast-enhanced CT is needed for patients with ASCVDs [8][9][10][11].…”