Bone-subtraction CT angiography: evaluation of two different fully automated image-registration procedures for interscan motion compensation

Abstract

Background and purpose: Bone-subtraction techniques have been shown to enhance CT angiography (CTA) interpretation, but motion can lead to incomplete bone removal. The aim of this study was to evaluate 2 novel registration techniques to compensate for patient motion.

Materials and methods: Fifty-four patients underwent bone-subtraction CTA (BSCTA) for the evaluation of the neck vessels with 64-section CT. We tested 3 different registration procedures: pure rigid registration (BSCTA), slab-based registration (SB-BSCTA), and a partially rigid registration (PR-BSCTA) approach. Subtraction quality for the assessment of different vascular segments was evaluated by 2 examiners in a blinded fashion. The Cohen kappa test was applied for interobserver variability, and the Wilcoxon signed rank test, for differences between the procedures. Motion between the corresponding datasets was measured and plotted against image-quality scores.

Results: Algorithms with motion compensation revealed higher image-quality scores (SB-BSCTA, mean 4.31; PR-BSCTA, mean 4.43) than pure rigid registration (BSCTA, mean 3.88). PR-BSCTA was rated superior to SB-BSCTA for the evaluation of the cervical internal and external carotid arteries (P<.001), whereas there was no significant difference for the other vessels (P=.157-.655). Both algorithms were clearly superior to pure rigid registration for all vessels except the basilar and ophthalmic artery. Interobserver agreement was high (kappa=0.46-0.98).

Conclusion: Bone-subtraction algorithms with motion compensation provided higher image-quality scores than pure rigid registration methods, especially in cases with complex motion. PR-BSCTA was rated superior to SB-BSCTA in the visualization of the internal and external carotid arteries.

Fig 1.

Comparison of BSCTA with rigid registration BSCTA, SB-BSCTA, and PR-BSCTA: CTA (MIP) shows a patient with high-grade stenosis of left ICA. Rigid registration leads to insufficient removal of the lower cervical spine (A, Lateral. B, Frontal projection). The edges of the individual registration slabs from SB-BSCTA (C and D) can be identified by the bone remnants from the mandible and cervical spine (arrows). Weighting of total bone mass within each slab is performed in this algorithm, explaining the distribution of the bone remnants. PR-BSCTA (E and F) demonstrates almost complete elimination of the bone pixels from the mandible and cervical spine. The thyroid cartilage remains misregistered with both approaches.

Fig 2.

CTA (MIP, frontal [A and C] and lateral [B and D] views) shows a patient with atherosclerotic disease at the carotid bifurcation and siphon. Correct registration and successful removal of the calcified plaque at the carotid siphon is obtained with both registration techniques. Pulsation leads to incomplete removal at the left carotid bifurcation, more pronounced with SB-BSCTA (A and B), whereas removal of the calcifications at the right carotid bifurcation is almost perfect with both methods. Removal of the lower cervical spine is more complete with SB-BSCTA compared with PR-BSCTA (C and D). The OA (arrow) can be identified with both approaches. The right ECA is occluded.

Fig 3.

CTA (MIP, frontal [A and C] and lateral [B and D] views) shows a patient with occlusion of the right carotid artery and steno-occlusive disease at the left carotid bifurcation and right VA. Both registration techniques provide excellent image quality; more bone remnants can be depicted with SB-BSCTA (A and B) in comparison with PR-BSCTA (C and D) in this patient.

Fig 4.

Graphic representation of the individual motion amplitudes for each patient. Note that the patients are ranked by increasing complexity of motion. Trend lines indicate the displacement of different anatomic landmarks.

Fig 5.

Graphic representation (and trend lines) of the quality scores achieved with the different techniques for individual vessels (A, VA. B, Cervical ICA. C, PC-ICA). There is a trend toward decreased image quality with increasing complexity of motion, expressed by the SD of the motion amplitudes except for the PC-ICA.