We read the recent article entitled “Diffusion-weighted MR Imaging of Focal Liver Lesions in the Left and Right Lobes: Is There a Difference in ADC Values?” by Schmid-Tannwald et al with great interest. They reported a comprehensive retrospective study to determine possible differences in apparent diffusion coefficient (ADC) values of benign and malignant focal liver lesions (FLLs) and normal liver parenchyma between left and right hepatic lobes. The authors concluded that ADC values of normal liver parenchyma and benign and malignant FLLs calculated from noncardiac-gated diffusion-weighted (DW) imaging are significantly higher in the left hepatic lobe compared with the right hepatic lobe, and this may be a limitation for characterization of FLLs based on ADC measurements . Because we have a special interest for DW imaging in the characterization of FLLs , we would like to make a contribution regarding possible practical methods for overcoming this potential limitation in DW imaging.
In a recent and comprehensive study that aimed to assess the value of visual assessment of signal intensities on b800 s/mm 2 DW images and ADC maps in differentiation of benign and malignant FLLs performed by Battal et al , benign and malignant FLLs could be differentiated by visual assessment more accurately than by ADC measurement. We think that visual assessment can function as a subjective ADC normalization method. In this technique, the signal intensities of the lesions are compared with the adjacent normal liver parenchyma on the same DW images and ADC maps. Schmid-Tannwald et al reported that measured ADC values of normal left liver lobe parenchyma and benign or malignant focal lesions in the left liver lobe are significantly higher than those located in the right lobe. As an example, in the visual assessment, a left liver lobe focal lesion that has a high ADC value according to that of the identical right lobe can be more accurately evaluated on the background of the adjacent left liver lobe, which has also higher ADC values from the normal right liver lobe and vice versa.
We speculate that another more objective method may be normalization of the measured absolute ADC of the lesion (ADC l ) by dividing a measured reference structure’s ADC (ADC r ) such as adjacent normal liver parenchyma, spinal cord, or paraspinal muscle on same image. The normalized ADC (ADC n ) can be calculated as follows: ADC n = ADC l /ADC r .
As also emphasized by authors, DW imaging and ADC measurement turned out to be a valuable diagnostic tool for the characterization of FLLs . However, reported ADC values in normal liver parenchyma, diffuse liver diseases, and FLLs, and cutoff values in differentiating malignant and benign lesions are highly variable because of the use of different scanners, b values, and diffusion sequences and techniques . We believe that visual assessment of DW images and ADC maps, and normalization of the measured ADC values, may be useful for overcoming these potential obstacles in DW imaging.
References
1. Schmid-Tannwald C., Jiang Y., Dahi F., et. al.: Diffusion-weighted MR imaging of focal liver lesions in the left and right lobes: is there a difference in ADC values?. Acad Radiol 2013; 20: pp. 440-445.
2. Battal B., Kocaoglu M., Akgun V., et. al.: Diffusion-weighted imaging in the characterization of focal liver lesions: efficacy of visual assessment. J Comput Assist Tomogr 2011; 35: pp. 326-331.
3. Battal B., Akgün V., Kocaoğlu M.: Diffusion-weighted MRI beyond the central nervous system in children. Diagn Interv Radiol 2012; 18: pp. 288-297.
4. Battal B., Kocaoglu M., Atay A.A., et. al.: Multifocal peliosis hepatis: MR and diffusion-weighted MR-imaging findings of an atypical case. Ups J Med Sci 2010; 115: pp. 153-156.