Misregistration Artifact due to Respiratory Motion on Spiral CT of the Liver

PURPOSE: To determine the frequency and patterns of respiratory-induced misregistration artifact seen on spiral CT of the liver. MATERIALS AND METHODS: Two hundred patients with hepatic mass underwent spiral CT, and arterial phase images were compared with those of the portal phase in all cases and or of the delayed phase in 138. The patterns of misregistration artifact were divided into two groups: skipping, where at least two slices in the craniocaudal length of the mass were missed, and the partial volume veraging artifact thus excluded; and overlapping, where the same or reversed images were seen in succeeding sequences. We reviewed the location and size of the masses, and the presence or absence, and patterns of the misregistration artifact. RESULTS: Fourteen (7%) of 200 spiral CT scans demonstrated the misregistration artifact; in five of these there was skipping (involving a hepatic mass larger than 2 cm in two cases, and one smaller than 2 cm in three cases), and in nine there was overlapping (six masses larger than 2 cm, and three smaller than this). A lipiodol-laden mass measuring 5 mm was completely missed during the arterial phase. and in one case the spleen sequence was reversed. Thirteen (93%) of fourteen masses were located in the right lobe. CONCLUSION: Two patterns of misregistration artifact, skipping and overlapping, were observed, and their combined frequency was 7%. So as not to miss small hepatic masses or overestimate their size, careful respiratory control is therefore needed.

Motion Artifact Simulating Dissection of Ascending Aorta on CT

PURPOSE: The purpose of this study was to investigate the frequency, site and characteristics of motion artifact of ascending aorta mimicking dissection. MATERIALS AND METHODS: The authors evaluated postcontrast CT scans of 60 cases in 60 patients without symptoms of aortic dissection or aortic disease. A Toshiba 900S scanner was used, with 1 cm slice thickness and 1 sec scan time. Streak artifacts, and those relating to extra-aortic vascular structure or pericardial effusion were excluded from this study. RESULTS: Crescent-shaped motion artifacts were seen in 54 cases (90%), and occurred from 1 cm to 4 cm above the level of the aortic valve; between men and women, there was no significant difference in frequency. In each case, the artifact was seen at 1 to 5 (mean 2.9) levels. Its pattern was symmetric in 31 of 60 cases (51%), and at 15 other sites, symmetric artifacts were seen between the SVC and ascending aorta. At the margin of the aortic circumference, the direction of the artifact was left anterior-right posterior in 23.9% of cases, and anterior median-posterior median in 20.8%. CONCLUSION: On CT, motion artifact of ascending aorta occurs frequently. Findings relating to location, symmetric pattern and characteristic direction of artifact may be helpful in the differential diagnosis of aortic dissection and aortic motion artifact.

Artifacts on CT Angiography

PURPOSE: To demonstrate various artifacts on CT angiography (CTA) and by analysing their frequency and causeto improve diagnostic accuracy and CTA image. MATERIALS AND METHODS: CTA was performed with spiral CT in 21 patients with suspected cerebrovascular diseases. Original spiral CT images were obtained with 120ml nonionic contrast material, 3.0 ml/sec injection rate, 20-23 sec. Scan delay, 1mm collimation, 2mm/sec table feeds and 1mm reconstruction interval. MIP and SSD images of CTA were reconstructed with editing by using a standard processing algorithm of volume rendering technique and 3D techniques including Grow, ROI and Paint techniques. Artifacts onCTA were analyzed, and compared with original spiral CT images, conventional angiography and operative results. RESULTS: Artifacts on CTA were classified as follows : arising from CTA reconstruction, spiral CT scanning, physical properties of CT and patient. During CTA reconstruction, background inhomogeneity developed through slabediting(n=21), blurring along the longitudinal axis(n=21), contour artifact(n=21), tapering-off(n=18) and saccularfusion(n=1) developing because of improper threshold and window level, break-off or blurring through missing some source images(n=13) and incorrect scaling due to computer error(n=2). During spiral CT scanning, stair-step artifacts(n=21) developed through the effect of by rotation and aliasing, and inhomogeneity(n=21) through profile decrease. Artifacts related to physical property included those related by the partial volume effect(n=1) and thebeam hardening effect(n=1). Patient-related artifacts included metal artifacts(n=2), caused by a surgical clip, and motion artifacts(n=4). CONCLUSION: An analysis of the variety, frequency and cause of artifacts on CTA, canlead to better imaging and provide basic information for more exact diagnosis.