Dose related, comparative evaluation of a novel bone-subtraction algorithm in 64-row cervico-cranial CT angiography

Comparative evaluation of a low-dose scan protocol for a novel bone-subtraction (BS) algorithm, applicable to 64-row cervico-cranial (cc) CT angiography (MSCTA).

BS algorithm assessment was performed in cadaveric phantom studies by stepwise variation of tube current and head malrotation using a 64-row CT scanner. In order to define minimum dose requirements and the rotation correction capacity, a low dose BS MSCTA protocol was defined and evaluated in 12 patients in comparison to a common manual bone removal algorithm. Standard MIPs of both modalities were evaluated in a blinded manner by two neuroradiologists for image quality composed, of vessel contour sharpness and bony vessel superposition, by using a five-point score each. Effective Dose (E) and data post-processing times were defined.

In experimental studies prescan tube current could be cut down to one-sixth of post-contrast scan doses without compromise of bone-subtraction whereas incomplete subtraction appeared from four degrees head malrotation on. Prescan E amounted to additional 1.1mSv (+25%) in clinical studies. BS MSCTA performed significantly superior in terms of bony superposition for vascular segments C3-C7 (p<0.001), V1-V2, V3-V4 (p<0.05, p<0.001 respectively) and the ophthalmic artery (p<0.05), whereas vessel contour sharpness in BS MSCTA only proved superior for arterial segments V3-V4 (p<0.001) and C3-C7 (p<0.001). MBR MSCTA received higher ratings in vessel contour sharpness for C1-C2 (p<0.001), callosomarginal artery (p<0.001), M1, M2, M3 (p<0.001 each) and the basilar artery (p<0.001). Reconstruction times amounted to an average of 1.5 (BS MSCTA) and 3min (MBR MSCTA) respectively.

The novel BS algorithm provides superior skull base artery visualisation as compared to common manual bone removal algorithms, increasing the Effective Dose by one-fourth. Yet, inferior vessel contour sharpness was noted intracranially, thus limiting the BS algorithm use to patients with suspected vessel pathology at the skull base level.