Editorial for “Value of T2 Mapping MRI for Prostate Cancer Detection and Classification”

Editorial for “Value of T2 Mapping MRI for Prostate Cancer Detection and Classification” Prostate Imaging Reporting and Data System (Pi-RADS) category is used to diagnose prostate cancer, and the criterion for this judgment is based on the signal intensity (SI) of the lesion on the diffusion-weighted images (DWIs) and T2-weighted images (T2WIs). T2WI is an integral part of multiparametric magnetic resonance imaging protocol recommended in Pi-RADS. Prostate cancer shows low SI on T2WI compared to the surrounding tissue, which mainly consists of glandular tissues containing water that shows high SI on T2WI. However, unlike DWI, which has an objective ADC value, the diagnosis of cancer on T2WI has to be made subjectively by comparing the SI with that of the surrounding tissue. SI of the lesion on T2WI is a relative value that is changed by sequence parameters of MRI and also greatly affected by radiofrequency (RF) inhomogeneity of transmit and receive field. Many studies are being conducted to obtain objective image information for diagnosing prostate cancer, including perfusion study to see vascularity, MR spectroscopy study to evaluate biochemical changes of the lesion, and various MR diffusion studies and T2 mapping to check the microstructure of the lesion. T2 mapping is a quantitative measurement of the T2-relaxation time of tissues which is obtained by acquiring signals at multiple different echo times and fitting them into exponential functions with a variety of nonlinear least-squares algorithms. Many studies have used T2 mapping to evaluate brain and cardiac tissues as well as changes in hyaline cartilage. The commercialized T2 mapping sequence makes it possible to obtain and compare the T2 relaxation time from a variety of structures in various organs, and values obtained by T2 mapping sequence are considered to be relatively objective in differentiating diseased and normal regions. In the past, it took 15–20 minutes to perform T2 mapping, which gave 7– 10 different TE values. Nowadays, T2 mapping can be done within 5 minutes by using methods such as parallel imaging and model-based reconstruction method which gives more than 15 different TE values. In the MR fingerprinting technique, T1 mapping and T2 mapping can be obtained by postprocessing the data that had been acquired while changing the flip angle and echo time for about 5 minutes. In this issue of JMRI, the article “Value of T2 mapping for prostate cancer detection and classification” showed that prostate cancer and surrounding tissues were well distinguished in the T2 mapping image. The average T2 relaxation time of prostate cancer was 85 msec in the peripheral zone (PZ) and 75 msec in the transition zone (TZ), and authors suggested 92 msec as a cut-off value for diagnosing prostate cancer. These values are not significantly different from those of existing studies, which support that T2 relaxation time obtained from T2 mapping can prove to be an objective value. A systematic review on quantitative T2 mapping for diagnosing prostate cancer showed that T2 mapping can reliably differentiate cancer from normal prostate tissue in the PZ only. In the TZ, high prevalence of benign prostatic hyperplasia, hemorrhage, and calcification can decrease T2 relaxation times, mimicking malignancy. In addition, the fact that the incidence of cancer in the TZ is less than that in the PZ, makes it difficult to collect statistically useful data. The current study included a relatively large proportion of cancers that occurred in the TZ and also showed that there is a significant difference in T2 relaxation time between cancer and surrounding tissues in the TZ. In this study, the difference in T2 relaxation time according to the tumor grade did not show a statistically significant difference. Quantitative T2 values are linked to the cell density as is with the ADC values. T2 mapping also provides functional information in terms of different sizes of the water compartments resulting from different proportions of stromal and glandular tissue between normal and malignant prostatic tissue. It can be expected that the changes in T2 relaxation time according to tumor grade could be confirmed in the future study with larger sample size. Several previous studies have reported that the T2 mapping image was more helpful in diagnosing prostate cancer compared to the conventional T2WI, albeit without noticeable superiority. However, the quantification method is meaningful in that it is a method which gives an objective value. It is expected that this study will be helpful in setting a reliable threshold value and providing contents for future studies.