Bias Field Correction in Endorectal Diffusion Imaging

Prostate cancer is the most common type of cancer affecting males in North America today. Diffusion-weighted MRI (DWI) has been useful in prognosis because of its improved visualization of dense cancerous tissue due to restricted diffusion of water in these regions. An additional endorectal coil has been used to improve the signal-to-noise ratio (SNR) in the prostate gland region however introduces an undesirable bias field which shows increased intensity nearest the coil which diminishes with distance.

Previous approaches have adjusted the MRI acquisition process to compensate for the bias field however this does not account for bias fields caused by the anatomy. Other approaches are post-processing methods that use filtering, surface fitting or segmentation.

A bias field correction method was developed here at VIP that automatically estimates the bias field and corrects for it. The resulting correction allows for improved visualization of the prostate gland (Fig. 1) and of tumors in high b-value computations (Fig. 2).

Figure 1 showing images with uncorrected and corrected bias fields

Fig. 1. DWI of the prostate with b = 0 sec/mm² with an endorectal coil with and without the bias field. The proposed approach removes the bias field in the peripheral zone and brightens the prostate gland for improved delineation from the background

Figure 2 - Comparison of corrected and uncorrected images

Fig. 2. Computed high b-value b = 1500 sec/mm² with tumor: High b-value images display tumors with brighter intensities. Red arrows indicate tumors and the blue arrow indicates a bright region that could be mistaken for a tumor due to the bias field. The corrected image removes the bias field effectively to remove this incorrectly identified region