Using Multiparametric MRI to Evaluate Tissue Changes After Laser Ablation
By: Dan Sperling, MD
MRI-guided focal laser ablation of prostate cancer tumors is a safe and minimally invasive way to destroy prostate cancer with minimal-to-no side effects in qualified patients. Because the treatment is done under multiparametric, or functional, MRI, it is possible to obtain high resolution, accurate localization of the zones to be ablated. In addition, MRI offers the advantage of monitoring the ablation process in the target area by means of MRI thermometry, a parameter that detects shifts in the proton resonance frequency of water molecules in stationary tissues as temperatures rise.[i]
There is growing interest in this treatment modality. However, little is known about the actual tissue changes that occur, and how multiparametric MRI can be used to confirm such changes after treatment. In addition to real-time monitoring of ablative temperatures, it is important to develop an image-based method for immediate evaluation of tissue change outcomes following ablation. Such a method would be able to identify mpMRI markers that would indicate favorable treatment response and longer term cancer control, and define which mpMRI markers are most sensitive to post-treatment changes in prostate tissue.
A multidisciplinary, multicenter team, including Dan Sperling MD, has now published a paper presenting “the first attempt at examining focal treatment-related changes on a per-voxel basis (high resolution) via quantitative evaluation of MR parameters” before and after laser ablation.[ii] The team retrospectively assembled pre- and post-laser ablation mpMRI scans (T2-weighted and diffusion-weighted functions, including DWI-generated Apparent Diffusion Co-efficient (ADC) maps. A registration system was devised that permitted image alignment of each pre- and post-ablation case. A uniform scale was created in order to standardize the pre- and post-ablation T2s intensity and ADC value, and these were quantified. Absolute differences between the normalized pre- and post-treatment parameters were then calculated. According to the authors, “Quantitatively combining the ADC and T2w MRI parameters enabled construction of an integrated MP-MRI difference map that was highly indicative of changes” specific to the ablation zone.
In comparing the changes in each parameter, the evidence suggests that T2w texture may be both sensitive and specific in identifying tissue changes after ablation. The same parameter, under visual evaluation, also seemed to indicate laser ablation effects related to treatment, such as temporary fluid build-up in the region. The authors concluded, “Our preliminary results thus indicate great potential for non-invasive MP-MRI imaging markers for determining focal treatment related changes, and hence long- and short-term patient outcome.”
[i] Rieke V and Pauly KB. MR thermometry. J Magn Reson Imaging. Feb 2008; 27(2): 376–390.
[ii]Viswanath S, Toth R, Rusu M, Sperling D, Lepor H, Futterer J, Madabhushi A. Quantitative evaluation of treatment related changes on multi-parametric MRI after laser interstitial thermal therapy of prostate cancer. Proc Soc Photo Opt Instrum Eng. 2013 Mar 15;8671:86711F.