Automated bone removal in CT angiography: Comparison of methods based on single energy and dual energy scans

Purpose: To evaluate dual energy based methods for bone removal in computed tomography angiography (CTA) images and compare these with single energy based methods that use an additional, nonenhanced, CT scan. Methods: Four different bone removal methods were applied to CT scans of an anthropomorphic thorax phantom, acquired with a second generation dual source CT scanner. The methods differed by the way information on the presence of bone was obtained (either by using an additional, nonenhanced scan or by scanning with two tube voltages at the same time) and by the way the bone was removed from the CTA images (either by masking or subtracting the bone). The phantom contained parts which mimic vessels of various diameters in direct contact with bone. Both a quantitative and qualitative analysis of image quality after bone removal was performed. Image quality was quantified by the contrast-to-noise ratio (CNR) normalized to the square root of the dose (CNRD). At locations where vessels touch bone, the quality of the bone removal and the vessel preservation were visually assessed. The dual energy based methods were assessed with and without the addition of a 0.4 mm tin filter to the high voltage x-ray tube filtration. For each bone removal method, the dose required to obtain a certain CNR after bone removal was compared with the dose of a reference scan with the same CNR but without automated bone removal. The CNRD value of the reference scan was maximized by choosing the lowest tube voltage available. Results: All methods removed the bone completely. CNRD values were higher for the masking based methods than for the subtraction based methods. Single energy based methods had a higher CNRD value than the corresponding dual energy based methods. For the subtraction based dual energy method, tin filtration improved the CNRD value with approximately 50. For the masking based dual energy method, it was easier to differentiate between iodine and bone when tin filtration was applied. The CNRD value decreased only with 4% in that case. Compared to the dual scan based methods, the dual energy based methods had the advantage that only a single scan was made without the need of image registration. This might be easier to implement in clinical practice. Vessel preservation was better with bone subtraction than with bone masking. Smaller vessels were completely occluded by the bone mask. None of the bone removal methods was dose neutral. Conclusions: In general, dual scan based methods that use the lowest tube voltage available, have a higher CNR than the dual energy based approaches at the same dose level. Tin filtration improves the ability to differentiate between iodine and bone for the dual energy based masking method. In clinical practice, the advantages of the dual energy masking method might outweigh its disadvantage of a slightly higher dose penalty compared to the conventional dual scan masking method.

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