In:
Medical Physics, Wiley, Vol. 42, No. 5 ( 2015-05), p. 2638-2647
Abstract:
In radiation therapy of pancreatic cancer, tumor alignment prior to each treatment fraction is improved when intratumoral gold fiducial markers (from here onwards: markers), which are visible on computed tomography (CT) and cone beam CT, are used. Visibility of these markers on magnetic resonance imaging (MRI) might improve image registration between CT and magnetic resonance (MR) images for tumor delineation purposes. However, concomitant image artifacts induced by markers are undesirable. The extent of visibility and artifact size depend on MRI‐sequence parameters. The authors’ goal was to determine for various markers their potential to be visible and to generate artifacts, using measures that are independent of the MRI‐sequence parameters. Methods: The authors selected ten different markers suitable for endoscopic placement in the pancreas and placed them into a phantom. The markers varied in diameter (0.28–0.6 mm), shape, and iron content (0%–0.5%). For each marker, the authors calculated T 2 ∗ ‐maps and Δ B 0 ‐maps using MRI measurements. A decrease in relaxation time T 2 ∗ can cause signal voids, associated with visibility, while a change in the magnetic field B 0 can cause signal shifts, which are associated with artifacts. These shifts inhibit accurate tumor delineation. As a measure for potential visibility, the authors used the volume of low T 2 ∗ , i.e., the volume for which T 2 ∗ differed from the background by 〉 15 ms. As a measure for potential artifacts, the authors used the volume for which |Δ B 0 | 〉 9.4 × 10 −8 T (4 Hz). To test whether there is a correlation between visibility and artifact size, the authors calculated the Spearman's correlation coefficient ( R s ) between the volume of low T 2 ∗ and the volume of high |Δ B 0 |. The authors compared the maps with images obtained using a clinical MR‐sequence. Finally, for the best visible marker as well as the marker that showed the smallest artifact, the authors compared the phantom data with in vivo MR‐images in four pancreatic cancer patients. Results: The authors found a strong correlation ( R s = 1.00, p 〈 0.01) between the volume of low T 2 ∗ and the volume with high |Δ B 0 |. Visibility in clinical MR‐images increased with lower T 2 ∗ . Signal shift artifacts became worse for markers with high |Δ B 0 |. The marker that was best visible in the phantom, a folded marker with 0.5% iron content, was also visible in vivo , but showed artifacts on diffusion weighted images. The marker with the smallest artifact in the phantom, a small, stretched, ironless marker, was indiscernible on in vivo MR‐images. Conclusions: Changes in T 2 ∗ and Δ B 0 are sequence‐independent measures for potential visibility and artifact size, respectively. Improved visibility of markers correlates strongly to signal shift artifacts; therefore, marker choice will depend on the clinical purpose. When visibility of the markers is most important, markers that contain iron are optimal, preferably in a folded configuration. For artifact sensitive imaging, small ironless markers are best, preferably in a stretched configuration.
Type of Medium:
Online Resource
ISSN:
0094-2405
,
2473-4209
Language:
English
Publisher:
Wiley
Publication Date:
2015
detail.hit.zdb_id:
1466421-5
