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
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 = 37.91 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.
2 cm
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.
Diagnosis:
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.
Discussion
•
Retrospectively ECG-gated CT angiogram requires image reconstructed at the proper
cardiac phase and at the proper z location. To minimize the cardiac motion, only data
acquired at the same cardiac phases are used. Also, the acquisition window is limited
to a small fraction of the cardiac cycle.
•
As illustrated in the figure below, reconstructions (yellow boxes) start at 55% of the RR interval and last for 𝑇𝑟𝑜𝑡 = 0.33𝑠. The maximal z-direction coverage is determined by
the total collimator width: 𝑇𝑊 = 32 × 0.6 = 19.2 𝑚𝑚 .
•
With helical pitch 0.3, the patient table is moving at 𝑉𝑧 =
= 17.5 𝑚𝑚 𝑠. After a
𝑇
𝑟𝑜𝑡
60 𝑠
complete cardiac cycle 𝑇𝑅𝑅 =
= 1.33 𝑠, the patient is move by 𝑍𝑝 = 𝑉𝑧 × 𝑇𝑅𝑅 =
𝑇𝑊×𝑝𝑖𝑡𝑐ℎ
45 𝑏𝑝𝑚
23.3 𝑚𝑚.
•
Because 𝑍𝑝 > 𝑇𝑊, there are periodic
gaps in z-direction where data were
not acquired. Although small gaps can
be
interpolated
by
adjacent
acquisitions, larger gaps lead to
discontinuities and banding artifacts in
the reformatted images.
•
For 𝑍 ≤ 𝑇𝑊, the pitch is bounded by:
𝑇𝑟𝑜𝑡
𝑃𝑖𝑡𝑐ℎ ≤
𝑇𝑅𝑅
Discussion
•
In practice, the helical pitch is selected to be smaller than
𝑇𝑟𝑜𝑡
𝑇𝑅𝑅
, to compensate for 1) the
heart rate variation during the scan, and 2) decreased volume coverage due to helical
motion and cone-beam geometry(1).
•
(a), stair-step artifact associated with inclined contour can be seen in the reformatted
image for helical scan, is incorrect. When the high contrast object is sloped along zdirection, the contour positions are different for reconstruction that takes measurement
from one detector row to that which takes interpolations from two detector rows. This
position difference is periodic, causing groove on the object contour along z-direction.
However, this is not the case here, because the magnitude of this stair-step artifact is
directly proportional to the table increment (TI) for low and moderate pitches(2). For our
study here with TI about 0.5 cm, this artifact is almost invisible.
•
(b) is incorrect because although cardiac or respiratory motion can cause phase misregistration that appears as a banding artifact(3), it’s usually more severe at the heart
than that at the chest wall.
•
(c) is incorrect. The misalignment between x-ray focal spot, detector center channel and
the center of rotation causes the measured locations of projection rays to deviate from
the ray positions assumed by the filtered back-projection, which results in image
smearing. Combined with asymmetric helical weighting scheme, the smearing direction
is periodic and can be seen as a zig-zag pattern on the high contrast object contour in
the reformatted image(4). The bandings here are across the entire image rather than
restricted to the anatomical structure boundaries.
References
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): 185-196.
3) Nakanishi, T., Y. Kayashima, et al. (2005). "Pitfalls in 16–Detector Row CT of the
Coronary Arteries." Radiographics 25(2): 425-438.
4) Hsieh, J. (1999). “Investigation of 3D image artifact caused by projection weighting
and misalignment.” Image Processing, 1999. ICIP 99. Proceedings.