Korean J Radiol.  2009 Jun;10(3):260-268. 10.3348/kjr.2009.10.3.260.

Optimization of the Scan Protocol for the Reduction of Diaphragmatic Motion Artifacts Depicted on CT Angiography: a Phantom Study Simulating Pediatric Patients with Free Breathing

• Baek JH
• Lee W
• Chang KH
• Chung JW
• Park JH

• KMID: 1779453
• DOI: http://doi.org/10.3348/kjr.2009.10.3.260

Abstract

OBJECTIVE
This study was designed to optimize the scan protocol of CT angiography to reduce diaphragmatic motion artifacts in pediatric patients with free-breathing. MATERIALS AND METHODS: A phantom with twelve tubes with different diameters was constructed. To simulate free-breathing, the phantom was connected to a motor, and the phantom moved along the axis of scan. Scans were performed under several conditions: different pitch (1, 1.5) and gantry rotation time (0.37 and 0.75 sec), and different movement range (1 cm, 3 cm) and rates (20/min, 40/min). For CT scanning, a 16-channel CT scanner was used and fixed factors of the CT protocol were as follows: 100 effective mAs, 80 kVp, reconstruction with a soft-algorithm, beam collimation 16x75 mm, reconstruction thickness of 1 mm, and an interval of 0.5 mm. CT scans were repeated five times. Each tube was evaluated with the use of a grading system (0 for images where tubes were not discriminable and 2 for images where tubes were clearly discriminable). RESULTS: A higher pitch and shorter gantry rotation time produced images with a higher grade. Average grades for the higher pitch (1.5) and faster gantry rotation time (0.37 sec) for each combination of movement were as follows: 1.94 (range 1 cm and rate 20/min), 1.42 (range 1 cm and rate 40/min), 0.86 (range 3 cm and rate 20/min) and 0.52 (range 3 cm and rate 40/min). Average grades for the lower pitch (1) and slower gantry rotation time (0.75 sec) for each combination of movement were 1.08, 0.56, 0.32 and 0.08, respectively. CONCLUSION: The scanning speed and especially the pitch are important parameters for CT scans to overcome a respiratory motion artifact.

Keyword

Computed tomography; Infants and children; Cardiovascular system

MeSH Terms

*Artifacts
Coronary Angiography/*methods
Diaphragm
Feasibility Studies
Pediatrics/*methods
*Phantoms, Imaging
*Respiration
Tomography, X-Ray Computed/*methods

Figure

• 

  Fig. 1 Image of phantom tube obtained with use of Faxitron X-ray system. For accurate measurement of diameter of tubes, we used four types of round 'reference metal bearings (*)' of variable size that were already measured. In descending order of size, diameters of bearings are 1/4 inch, 3/16 inch, 1/8 inch and 5/32 inch (1 inch = 25.4 mm).

• 

  Fig. 2 Phantom model placed on CT and corresponding maximum intensity projection image. A. Phantom is connected to small ventilator and placed on CT table. B. Example of maximum intensity projection image. Maximum intensity projection image was reconstructed using commercially available software from 1 mm-thickness axial images with scan condition of pitch 1.5-gantry rotation 0.37 sec and no movement of phantom.

• 

  Fig. 3 Representative phantom tubes for each score of non-stenotic tubes (#7-10 in Fig. 1) under different scan conditions and phantom movement conditions. A. Maximum intensity projection image of tubes with no movement of phantom shows clear margins of all tubes. B. Maximum intensity projection image of tubes for condition of pitch 1.5-gantry rotation 0.37 sec and amplitude 1 cm-20 RPM. All of tubes show clear margins, with score of 2. C. Maximum intensity projection image of tubes for condition of pitch 1-gantry rotation 0.37 sec and amplitude 1 cm-20 RPM. All of tubes are discernable, but margins of upper three tubes are not as clear as described in (B) and are scored 1. D. Maximum intensity projection image of tubes for condition of pitch 1-gantry rotation 0.75 sec and amplitude 3 cm-40 RPM. Margins of tubes are not clear and tubes cannot be separated from each other, and are scored 0.

• 

  Fig. 4 Velocity curves for phantom movement with different scanning speeds (different pitches and gantry rotation times) for condition amplitude 3 cm-20 RPM, with ground as a reference point. P = Pitch, GR = gantry rotation time

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  Fig. 5 Velocity curves for phantom movement with pitch 1 and gantry rotation time 0.37 sec. With increase of amplitude and rate of phantom movement, velocity curves show higher amplitude and frequency.

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  Fig. 6 Graphs of average scores of image quality of tubes for each scan condition are shown. In general, average score increases as scanning speed increases. For phantom movement of amplitude 3 cm-20 RPM and 40 RPM, scores with scanning speed of 3.2 cm/sec (pitch 1-gantry rotation 0.37 sec) are lower than scores for scanning speed of 2.4 cm/sec (pitch 1.5-gantry rotation 0.75 sec).

• 

  Fig. 7 Averages and standard deviations of measured diameter of #7 tube without stenosis for condition of 1 cm-20 RPM are presented. Dotted line marks true value of tube diameter, 5.82 mm, measured from high resolution plain radiograph of phantom. As scanning speed increases, average values of measured diameter approach true value. Standard deviation shows minimum value at scanning speed of 4.8 cm/sec.

• 

  Fig. 8 Representative samples of motion artifact. A. Image without distortion for condition of 4.8 cm/sec scanning speed (pitch 1.5 and gantry rotation 0.37 sec) and phantom movement of amplitude 1 cm-20 RPM. All of tubes show clear margins. B. For condition of 2.4 cm/sec scanning speed (pitch 1.5 and gantry rotation 0.75 sec) and phantom movement of amplitude 1 cm-20 RPM, uppermost tube shows decreased diameter (white arrow) as compared to tube with same location in A. This is example of misregistration when phantom tubes moves faster than CT table in same direction. As velocity changed constantly, tubes with similar diameter showed different width on CT images. C. For condition of 4.8 cm/sec scanning speed (pitch 1.5 and gantry rotation 0.37 sec) and phantom movement of amplitude 1 cm-40 RPM, seventh tube (white arrow) shows increased diameter as compared to tube with same location in A. This is example of misregistration when object moves slowly in opposite direction to that of CT table.

• 

  Fig. 9 Differences of gantry rotation angle per unit distance of 1.8 cm for each scan condition. A. For condition of 1.5 pitch and 0.75 sec rotation time, gantry rotates 360° for 1.8 cm. B. For condition of 1 pitch and 0.37 sec rotation time, gantry rotates 540° for 1.8 cm.

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