Distance-based assessment of spatial artifact extension in the prostate from fiducial markers in diffusion-weighted magnetic resonance imaging.

[BACKGROUND] Fiducial markers in image-guided prostate cancer radiotherapy reduce geometric uncertainty during daily patient setup and enable assessment of target position changes. Diffusion-weighted magnetic resonance imaging (MRI) for target delineation improves prostate cancer localization, beneficial for intraprostatic focal boost. Artifacts from fiducial markers on prostate diffusion-weighted MRI (DWI) need to be investigated, as they could be detrimental for target delineation. This study aims to determine the distances of artifact extensions caused by fiducial markers in DWI and in the apparent diffusion coefficient (ADC) maps and to assess how motion and signal-to-noise ratio (SNR) influence the artifact size in ADC maps.

[MATERIALS AND METHODS] Three phantoms were used: two homogeneous gel phantoms-one containing three cylindrical gold fiducial markers (GFM) and the other containing three spherical gold anchor (GA) markers-and a third heterogeneous phantom consisting of a piece of sirloin embedded with three GFM and three GA. Diffusion-weighted images were acquired on a 3T MRI system. The artifacts were analyzed along the phase-encoding (PE) and frequency-encoding (FE) directions. Motion was induced and simulated during acquisition, and SNR was varied. The impact of motion and SNR on the artifact extension was evaluated, and the artifact extensions in diffusion images from eight patients were also analyzed.

[RESULTS] The artifacts were smaller in the ADC maps compared to DWI. The largest artifact extension occurred along the PE-direction. Larger artifact extensions were observed in homogeneous phantom images compared to patient images. In homogeneous phantom images: 13.8  ±  0.4 mm / 9.1  ±  0.4 mm (PE/FE) in DWI with b = 0 s/mm and 11.6  ±  0.9 mm / 8.1  ±  0.4 mm (PE/FE) in the ADC map. In patient images: 10.7  ±  1.2 mm / 8.2  ±  1.3 mm (PE/FE) in DWI with b = 0 s/mm and 7.3  ±  1.6 mm / 6.8  ±  1.1 mm (PE/FE) in the ADC map. Motion caused larger artifact extensions compared to no motion. A motion of 2 mm increased the artifact from 11.6 ± 0.9 mm / 8.1 ± 0.4 mm (PE/FE) to 14.1 ± 0.8 mm / 9.7 ± 0.4 mm (PE/FE) in homogeneous phantom images and from 10.3 ± 0.8 mm / 8.1 ± 0.4 mm (PE/FE) to 13.1 ± 0.8 mm / 8.4 ± 0.8 mm (PE/FE) in heterogeneous phantom images. Lower SNR resulted in smaller visible artifact extensions.

[CONCLUSION] This study assessed the distances of artifact extensions in homogeneous phantoms, heterogeneous phantoms, and patient images caused by fiducial markers in DWI and ADC maps. ADC maps had smaller artifact extensions compared to DWI. The artifact extensions were largest in the homogeneous phantom, smaller in the heterogeneous phantom, and the smallest in the patient images. In patient images, the extensions were approximately 7-11 mm (PE) and 7-8 mm (FE). However, extensions reached up to ∼14 mm (PE) and ∼9 mm (FE) in homogeneous phantom images, suggesting that the true artifact extension may be partially obscured in patient images. Further, motion in images caused larger artifact extensions, and lower SNR caused smaller artifact extensions. The study underlines the need for precise marker placement to avoid obscuring critical anatomical structures, especially for delineation of small boost volumes, and distorting ADC values in quantitative analyses of tumors.