Applicator visualization using ultrashort echo time MRI for high-dose-rate endorectal brachytherapy

Brachytherapy, vol. 19, pp. 618–623

Abstract

Purpose The individual channels in an endorectal applicator for high-dose-rate endorectal brachytherapy are not visible on standard MRI sequences. The aim of this study was to test whether an ultrashort echo time (UTE) MRI sequence could be used to visualize the individual channels to enable MR-only treatment planning for rectal cancer. Methods and Materials We used a radial three-dimensional (3D) UTE pulse sequence and acquired images of phantoms and two patients with rectal cancer. We rigidly registered a UTE image and CT scan of an applicator phantom, based on the outline of the applicator. One observer compared channel positions on the UTE image and CT scan in five slices spaced 25 mm apart. To quantify geometric distortions, we scanned a commercial 3D geometric quality assurance phantom and calculated the difference between detected marker positions on the UTE image and corresponding marker positions on two 3D T 1 -weighted images with opposing readout directions. Results On the UTE images, there is sufficient contrast to discern the individual channels. The difference in channel positions on the UTE image compared with the CT was on average −0.1 ± 0.1 mm (left-right) and 0.1 ± 0.3 mm (anteroposterior). After rigid registration to the 3D T 1 -weighted sequences, the residual 95th percentile of the geometric distortion inside a 550-mm-diameter sphere was 1.0 mm (left-right), 0.9 mm (anteroposterior), and 0.9 mm (craniocaudal). Conclusions With a UTE sequence, the endorectal applicator and individual channels can be adequately visualized in both phantom and patients. The geometrical fidelity is within an acceptable range.

Authors 6

1. Roy P.J. van den Ende corresponding Aachen Department of Radiation Oncology

   Leiden University Medical Center

   Affiliation as printed

   Department of Radiation Oncology, Leiden University Medical Center, Leiden, the Netherlands. Electronic address: r.p.j.van_den_ende@lumc.nl

   Department of Radiation Oncology, Leiden University Medical Center, Leiden, the Netherlands

2. Ece Ercan Aachen C.J. Gorter Center for High Field MRI Department of Radiology, Leiden University Medical Center

   Leiden University Medical Center

   Affiliation as printed

   C.J. Gorter Center for High Field MRI, Department of Radiology, Leiden University Medical Center, Leiden, the Netherlands

3. Rick Keesman

   The Netherlands Cancer Institute

   Affiliation as printed

   Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands

4. Ellen M. Kerkhof Aachen Department of Radiation Oncology

   Leiden University Medical Center

   Affiliation as printed

   Department of Radiation Oncology, Leiden University Medical Center, Leiden, the Netherlands

5. Corrie A.M. Marijnen Aachen Department of Radiation Oncology

   Leiden University Medical Center · The Netherlands Cancer Institute

   Affiliation as printed

   Department of Radiation Oncology, Leiden University Medical Center, Leiden, the Netherlands; Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands

   Department of Radiation Oncology, Leiden University Medical Center, Leiden, the Netherlands

   Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands

6. Uulke A. van der Heide Aachen Department of Radiation Oncology

   Leiden University Medical Center · The Netherlands Cancer Institute

   Affiliation as printed

   Department of Radiation Oncology, Leiden University Medical Center, Leiden, the Netherlands; Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands

   Department of Radiation Oncology, Leiden University Medical Center, Leiden, the Netherlands

   Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands