AAPM Banner
  • Volume/Page
  • Keyword
  • DOI
  • Citation
  • Advanced
   
 
 
 
Medical Physics / Volume 39 / Issue 1 / RADIATION IMAGING PHYSICS

Med. Phys. 39, 153 (2012); http://dx.doi.org/10.1118/1.3666947 (7 pages)

Antiscatter grids in mobile C-arm cone-beam CT: Effect on image quality and dose

S. Schafer, J. W. Stayman, and W. Zbijewski

Department of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland 21202

C. Schmidgunst and G. Kleinszig

Siemens Healthcare XP Division, Erlangen, Bavaria 91052, Germany

J. H. Siewerdsen

Department of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland 21202 and Department of Computer Science, Johns Hopkins University, Baltimore, Maryland 21218

View MapView Map

(Received 6 May 2011; accepted 17 November 2011; revised 2 November 2011; published online 14 December 2011)

Full Text: Read Online (HTML) | Download PDF FREE | View Cart
Purpose: X-ray scatter is a major detriment to image quality in cone-beam CT (CBCT). Existing geometries exhibit strong differences in scatter susceptibility with more compact geometries, e.g., dental or musculoskeletal, benefiting from antiscatter grids, whereas in more extended geometries, e.g., IGRT, grid use carries tradeoffs in image quality per unit dose. This work assesses the tradeoffs in dose and image quality for grids applied in the context of low-dose CBCT on a mobile C-arm for image-guided surgery.Methods: Studies were performed on a mobile C-arm equipped with a flat-panel detector for high-quality CBCT. Antiscatter grids of grid ratio (GR) 6:1–12:1, 40 lp/cm, were tested in “body” surgery, i.e., spine, using protocols for bone and soft-tissue visibility in the thoracic and abdominal spine. Studies focused on grid orientation, CT number accuracy, image noise, and contrast-to-noise ratio (CNR) in quantitative phantoms at constant dose.Results: There was no effect of grid orientation on possible gridline artifacts, given accurate angle-dependent gain calibration. Incorrect calibration was found to result in gridline shadows in the projection data that imparted high-frequency artifacts in 3D reconstructions. Increasing GR reduced errors in CT number from 31%, thorax, and 37%, abdomen, for gridless operation to 2% and 10%, respectively, with a 12:1 grid, while image noise increased by up to 70%. The CNR of high-contrast objects was largely unaffected by grids, but low-contrast soft-tissues suffered reduction in CNR, 2%–65%, across the investigated GR at constant dose.Conclusions: While grids improved CT number accuracy, soft-tissue CNR was reduced due to attenuation of primary radiation. CNR could be restored by increasing dose by factors of ∼1.6–2.5 depending on GR, e.g., increase from 4.6 mGy for the thorax and 12.5 mGy for the abdomen without antiscatter grids to approximately 12 mGy and 30 mGy, respectively, with a high-GR grid. However, increasing the dose poses a significant impediment to repeat intraoperative CBCT and can cause the cumulative intraoperative dose to exceed that of a single diagnostic CT scan. This places the mobile C-arm in the category of extended CBCT geometries with sufficient air gap for which the tradeoffs between CNR and dose typically do not favor incorporation of an antiscatter grid.

© 2012 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

The authors gratefully acknowledge valuable discussion with Dr. Rainer Graumann (Siemens Healthcare, Erlangen, Germany) and the support of the Minimally Invasive Surgical Training Center at Johns Hopkins University (Dr. Michael Marohn, Ms. Sue Eller, Ms. Katherine Braid, and Mr. Nicholas Louloudis). This work was supported by research collaboration with Siemens Healthcare (Erlangen, Germany) and by National Institutes of Health R01 Grant No. CA-127444.

Article Outline

  1. INTRODUCTION
  2. METHODS AND MATERIALS
    1. Experimental setup
      1. Mobile C-arm
      2. Antiscatter grids
      3. Imaging phantoms
    2. Image quality evaluation
      1. Grid orientation
      2. Noise and CT number accuracy
      3. Contrast-to-noise ratio
  3. RESULTS
    1. Grid orientation
    2. Noise and CT number accuracy
    3. Contrast-to-noise ratio
  4. DISCUSSION
  5. CONCLUSION
Digital Object Identifier
http://dx.doi.org/10.1118/1.3666947

KEYWORDS and PACS

Keywords

bone, calibration, computerised tomography, dosimetry, image reconstruction, medical image processing, orthopaedics, phantoms, surgery, mobile C-arm, image-guided surgery, flat-panel detector, cone-beam CT, x-ray scatter, antiscatter grid, spine surgery, minimally invasive surgery, image quality

PACS

PUBLICATION DATA

ISSN

0094-2405 (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physicists in Medicine

  1. S. Schafer, S. Nithiananthan, D. J. Mirota, A. Uneri, J. W. Stayman, W. Zbijewski, C. Schmidgunst, G. Kleinszig, A. J. Khanna, and J. H. Siewerdsen, “Mobile C-arm cone-beam CT for guidance of spine surgery: Image quality, radiation dose, and integration with interventional guidance,” Med. Phys. 38 (8), 4563–4574 (2011)MPHYA6000038000008004563000001.
  2. J. H. Siewerdsen, D. J. Moseley, S. Burch, S. K. Bisland, A. Bogaards, B. C. Wilson, and D. A. Jaffray, “Volume CT with a flat-panel detector on a mobile, isocentric C-arm: Pre-clinical investigation in guidance of minimally invasive surgery,” Med. Phys. 32 (1), 241–254 (2005)MPHYA6000032000001000241000001. [ISI] [MEDLINE]
  3. J. H. Siewerdsen and D. A. Jaffray, “Cone-beam computed tomography with a flat-panel imager: Magnitude and effects of x-ray scatter,” Med. Phys. 28 (2), 220–231 (2001)MPHYA6000028000002000220000001. [MEDLINE]
  4. J. H. Siewerdsen, D. J. Moseley, B. Bakhtiar, S. Richard, and D. A. Jaffray, “The influence of antiscatter grids on soft-tissue detectability in cone-beam computed tomography with flat-panel detectors,” Med. Phys. 31 (12), 3506–3520 (2004)MPHYA6000031000012003506000001. [MEDLINE]
  5. L. Zhu, Y. Xie, J. Wang, and L. Xing, “Scatter correction for cone-beam CT in radiation therapy,” Med. Phys. 36 (6), 2258–2268 (2009)MPHYA6000036000006002258000001. [MEDLINE]
  6. W. Zbijewski, P. D. Jean, P. Prakash, Y. Ding, J. W. Stayman, N. Packard, R. Senn, D. Yang, J. Yorkston, A. Machado, J. A. Carrino, and J. H. Siewerdsen, “A dedicated cone-beam CT system for musculoskeletal extremities imaging: Design, optimization, and initial performance characterization,” Med. Phys. 38 (8), 4700–4713 (2011).
  7. C. Schmidgunst, D. Ritter, and E. Lang, “Calibration model of a dual gain flat panel detector for 2D and 3D x-ray imaging,” Med. Phys. 34 (9), 3649–3664 (2007)MPHYA6000034000009003649000001. [MEDLINE]
  8. M. J. Daly, J. H. Siewerdsen, D. J. Moseley, D. A. Jaffray, and J. C. Irish, “Intraoperative cone-beam CT for guidance of head and neck surgery: Assessment of dose and image quality using a C-arm prototype,” Med. Phys. 33 (10), 3767–3780 (2006)MPHYA6000033000010003767000001. [MEDLINE]
  9. R. Fahrig, R. Dixon, T. Payne, R. L. Morin, A. Ganguly, and N. Strobel, “Dose and image quality for a cone-beam C-arm CT system,” Med. Phys. 33 (12), 4541–4550 (2006)MPHYA6000033000012004541000001. [MEDLINE]
  10. S. Kim, S. Yoo, F.-F. Yin, E. Samei, and T. Yoshizumi, “Kilovoltage cone-beam CT: Comparative dose and image quality evaluations in partial and full-angle scan protocols,” Med. Phys. 37 (7), 3648–3659 (2010)MPHYA6000037000007003648000001. [MEDLINE]
  11. Y. Watanabe, “Derivation of linear attenuation coefficients from CT numbers for low-energy photons,” Phys. Med. Biol. 44 (9), 2201 (1999). [ISI] [MEDLINE]
  12. J. H. Siewerdsen, A. M. Waese, D. J. Moseley, S. Richard, and D. A. Jaffray, “Spektr: A computational tool for x-ray spectral analysis and imaging system optimization,” Med. Phys. 31 (11), 3057–3067 (2004)MPHYA6000031000011003057000001. [ISI] [MEDLINE]
  13. U. Neitzel, “Grids or air gaps for scatter reduction in digital radiography: A model calculation,” Med. Phys. 19 (2), 475–481 (1992)MPHYA6000019000002000475000001. [MEDLINE]
  14. J. H. Siewerdsen and D. A. Jaffray, “Optimization of x-ray imaging geometry (with specific application to flat-panel cone-beam computed tomography),” Med. Phys. 27 (8), 1903–1914 (2000)MPHYA6000027000008001903000001. [MEDLINE]

Figures (6) Tables (1)

Figures (click on thumbnails to view enlargements)

FIG.1
(a) Experimental setup for antiscatter grids applied to mobile C-arm CBCT. (1) Antiscatter grid mounted on the FPD. (2) Phantom setup (a head-equivalent extension was removed for better visualization). (3) X-ray tube shown at the start-scan position. (b) Thorax phantom; (c) reconstruction of tissue-equivalent inserts in the thorax phantom.

FIG.1 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.2
Zoomed and magnified projection image showing (a) an example of gridline aliasing stemming from incorrect angle-dependent gain calibration, and (b) the removal of such shadows/aliasing with correctly applied angle-dependent gain calibration. Grid line artifacts in projection images cause high frequency artifacts in the reconstructions (c), an effect that does not occur in correctly processed images (d).

FIG.2 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.3
Hounsfield unit inaccuracy, Δ (left axis), and voxel noise (right axis) as a function of grid ratio. Grids are seen to improve HU accuracy but impart an increase in voxel noise unless accompanied by an increase in exposure technique.

FIG.3 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.4
CNR as a function of grid ratio for (a) thorax phantom bone protocol, (b) thorax phantom soft-tissue protocol, (c) abdomen phantom bone protocol, and (d) abdomen phantom soft-tissue protocol. At fixed patient dose (see Table 1) grids impart a significant reduction in soft-tissue CNR.

FIG.4 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.5
Tissue-equivalent inserts in the thoracic and abdomen phantoms for the bone and soft-tissue protocols at various grid ratios. The images correspond to the CNR measurements of Fig. 4. A reduction in scatter artifacts extending vertically from the cortical bone insert toward the spine insert is visible. The horizontal high- and low-intensity streak artifacts around the cortical bone insert arise from the incomplete orbit of the C-arm (∼178°).

FIG.5 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.6
Axial and sagittal views of the anthropomorphic body phantom with implanted spine hardware (resolution 0.3 × 0.3 × 0.9 mm3). Images with a 10:1 grid in place (c, d) exhibit little or no improvement in overall image quality compared to the gridless images (a, b) and show a slight increase in image noise unless the dose to the patient is increased to counter the loss in primary fluence.

FIG.6 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

Tables

Table I. Technique settings and dosimetry for task-specific protocols in the thorax and abdomen. “Bone protocol” refers to a lower dose technique sufficient for high-contrast bone visualization. “Soft-tissue protocol” refers to a higher dose technique sufficient for soft-tissue visualization.

View Table


Close

Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
William R. Hendee, Editor
Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
One Physics Ellipse
College Park, Maryland 20740
301-209-3352

© 2012, American Association of Physicists in Medicine. Individual readers of this journal, and nonprofit libraries acting for them, are freely permited to make fair use of the material in it, such as to copy an article for use in teaching or research. (For other kinds of copying see "Copying Fees.") Permission is granted to quote from this journal in scientific works with the customary acknowledgment of the source. To reprint a figure, table, or other excerpt requires, in addition, AAPM may require that permission also be obtained from one of the authors. Address inquiries and notices to Penny Slattery, Journal Manager, Medical Physics Journal, AAPM, One Physics Ellipse, College Park, MD 20740-3846; email: journal@aapm.org.

close