History: A 61-year-old man with suspected thoracic aneurysm underwent ECG-gated chest CT angiogram. He was instructed to hold his breath during the exam and his heart rate was ~ 45 bpm. Scan protocol: Siemens Sensation 64 scanner, 120 kV, effective mAs = 498, collimation = 64 x 0.6 mm ( 32 x 0.6 mm with z flying focal spot), pitch = 0.3, rotation time = 0.33 s, CTDIvol = mGy On the sagittal reformatted image with start cardiac phase at 55%, artifacts (arrows) were seen at chest wall, aorta and ribs at multiple locations. Same artifacts were seen across different sagittal slices. What is the most likely cause of this artifact ? a)Stair-step artifact associated with inclined contours of anatomical structures. b)Combined irregular heart rate and incomplete breath-holding. c)Mechanical misalignment between x-ray focal spot, detector center-channel and the center of gantry rotation. d)Pitch was too high and data were missing in certain areas during image reconstruction. Sunday Case of the Day - Physics Authors: Yi Zhang Ph.D., Lifeng Yu, Ph.D., Shuai Leng, Ph.D., Cynthia McCollough, Ph.D. Department of Radiology, Mayo Clinic, Rochester MN 2 cm

Correct answer is (d). Pitch was too high and data were missing in certain areas during image reconstruction. Retrospectively ECG-gated CT angiogram requires image reconstructed at the proper cardiac phase and at the proper z location. With high pitch and low heart rate, the table travels beyond the desired z-locations at the pre-determined cardiac phases, causing gaps between successive axial slices. Large gaps cannot be completely corrected by interpolation and cause discontinuities and banding artifacts in reformatted images. Diagnosis:

Discussion

1)Hsieh, J. (2009). “Computed tomography: principles, design, artifacts, and recent advances.” Bellingham, WA, SPIE. 2)Fleischmann, D., G. D. Rubin, et al. (2000). "Stair-Step Artifacts with Single versus Multiple Detector-Row Helical CT1." Radiology 216(1): )Nakanishi, T., Y. Kayashima, et al. (2005). "Pitfalls in 16–Detector Row CT of the Coronary Arteries." Radiographics 25(2): )Hsieh, J. (1999). “Investigation of 3D image artifact caused by projection weighting and misalignment.” Image Processing, ICIP 99. Proceedings. References