SU‐E‐J‐107: SEMAC MRI Metal Artifact Reduction Technique for Radiotherapy Planning Using CT/MRI Fusion

Abstract

Purpose: To assess the utility of SEMAC MRI metal artifact reduction technique for radiotherapy (RT) planning using CT/MRI fusion in patients with spine stabilisation devices. Methods: Conventional MRI spin‐echo (SE) sequences used for spinal cord delineation and radiotherapy planning in the spine were compared with a prototype implementation of Slice Encoding for Metal Artifact Correction (SEMAC) (WARP works‐in‐progress software package, Siemens Healthcare, Erlangen, Germany). Sequences were first optimised at 1.5T (Siemens MAGNETOM Aera) using a phantom constructed of a spine stabilisation device suspended in gelatin and then applied in patient studies (n=2). Both patient and phantom MRI series were co‐registered with spine CT data to assess artifact extent and geometrical image distortion. Registered data sets were assessed in terms of artifact size, distortions, image quality, visualisation of the spinal cord, spinal fluid, disease site and soft tissues adjacent to metal implants. Results: The optimal parameters were found as a compromise between artifact reduction and MRI acquisition time, while maintaining image quality comparable to conventional sequences: TR/TE 560/15ms, res: 0.8×0.8×4mm, GRAPPA=3, BW=587Hz/Px, 30 slices, TA=3:57min, SEMAC parameters: Slice encoding steps=4, Excitation & Refocusing RF duration=1500us, slice correction factor=1.2. The metal artifact volumes (signal voids and pile‐ups) were reduced using 2D SEMAC sequence on average by 43±16 % from 15.6±2.5 cm3 to 8.9±1.4 cm3. The difference in spinal cord, fluid and lesion volumes outlined using conventional and SEMAC sequences were 7±0.15, 2±0.42 and 35±3.11% respectively in the 6cm long spine sections affected by metallic artifacts. Conclusion: The work shows the importance of MRI metal artifact reduction techniques for reliable RT planning in the vicinity of metallic implants. The use of SEMAC sequence improves geometrical accuracy of outlined volumes in the presence of spine stabilization devices which degrade geometrical information from both CT and conventional SE MRI.

Radiotherapy planning using MRI

“…In some cases spinal metastases are stabilised with the use of metallic fixation devices, which degrade image quality of MR and CT examinations (FIGURE 11 spine+metal). For lesions located in the vicinity of metallic implants, it may be advantageous to employ MR techniques for metal artifact reduction (Panek et al, 2013). Although the degree of confidence in the CT-MR fusion increases as the artifacts are restricted to a smaller volume, further work is required to validate this approach and quantify its benefits.…”

Metal artifact reduction in MRI-based cervical cancer intracavitary brachytherapy

“…In contrast to diagnostic radiology, reports on the use of metal artifact reduction in radiotherapy planning are limited. Panek et al presented the preliminary results of SEMAC in the use of two radiotherapy patients with spine stabilization devices in 2013 (Panek et al 2013). Metal artifact volumes were reduced on average by 43%.…”

Slice Encoding for Metal Artefact Correction in magnetic resonance imaging examinations for radiotherapy planning

“…Initial experiments on test objects suggested that a number of 4 to 6 SEMAC slice-encoding steps [11] produced significant reduction in signal loss, in agreement with other reported studies [17] . However, we found necessary to reduce the number of averages and to increase the echo-train length in order to keep data acquisition within 5–6 min [18] . The reduction in averages is compensated by the gain in signal due to the SEMAC encoding, keeping the signal-to-noise ratio (SNR) of the image at an acceptable level.…”