Synthetic magnetic resonance imaging (MRI) is a technique that can produce images with various contrast weighting by virtually changing repetition time (TR), echo time (TE), and inversion time (TI) after quantification of longitudinal T1 and transverse T2 relaxation times and proton density (PD)^1,2(Fig.1). The purpose of this study was to assess whether contrast-enhanced synthetic MRI is suitable for detecting brain metastases by comparing the lesion-to-white matter contrast, contrast-to-noise ratio, and number of brain metastases detected in synthetic and conventional magnetic resonance images.A 3.0-T MRI system (Discovery MR750w, GE Healthcare, Milwaukee, USA) was used for all imaging of ten patients (6 men and 4 women; mean age, 62.3 years; age range, 44–75 years). Quantitative MRI and conventional T1-weighted inversion-recovery fast spin-echo (conventional T1IR) MRI() before and after administration of a gadolinium-based contrast agent were performed on all the patients with a combined total of 167 brain metastases. Quantitative MRI was performed by using the two-dimensional QRAPMASTER pulse sequence³. Synthetic T1IR and synthetic T1-weighted (synthetic T1W) images were produced on off-line Windows PC. Lesion-to-white matter contrast and contrast-to-noise ratio were calculated for each image by a single investigator. The number of visible lesions in each image was assessed by two neuroradiologists independently. Statistical computing was performed with the free software R Ver. 3.2.1 (R Foundation for Statistical Computing, Vienna, Austria.). Statistical analysis was performed by the Friedman test and post-hoc pairwise tests with Holm correction. A two-sided P value of less than 0.05 was considered significant.The mean (± standard deviation [SD]) lesion-to-white matter contrast and mean contrast-to-noise ratio of the synthetic T1IR images (0.51 ± 0.47 and 20.98 ± 18.65, respectively) were significantly higher than those of the synthetic T1W (0.34 ± 0.38 and 14.20 ± 14.98, respectively) and conventional T1IR images (0.47 ± 0.47 and 18.65 ± 18.00, respectively). Totals of 130 and 124 metastases were detected in the synthetic T1IR images by the first and second radiologists, respectively; the corresponding numbers were 91 and 85 in the synthetic T1W images and 119 and 119 in the conventional T1IR images. The total metastases detected were comparable among the synthetic T1IR, synthetic T1W, and conventional T1IR images; however, more metastases were detected in the synthetic T1IR images (Fig.2).In the present study, contrast-enhanced synthetic T1IR produced the best quality contrast for detecting brain metastases. Although the numbers of lesions detected did not differ statistically, contrast-enhanced synthetic T1IR enabled the detection of more metastases than did contrast-enhanced conventional T1IR and contrast-enhanced synthetic T1W. In the qualitative analysis, several metastases, including metastases hidden by flow-related artifacts from the sigmoid sinus, were detected only in the synthetic T1IR and synthetic T1W images but not in the conventional T1IR images. These results demonstrate the potential advantages of synthetic MRI compared with conventional MRI. A potential advantage of synthetic MRI over conventional MRI is the creation of any contrast weighting after image acquisition. Although in the present study we used identical parameters for all 10 patients, it would also have been possible to synthesize images with parameters tailored to each individual to maximize the potential to detect brain metastases. Although the scan time for the quantitative MRI was longer than that of the conventional T1IR sequence (about 7 minutes and about 2 minutes, respectively) after administration of the contrast agent, the ability to synthesize images optimized for each individual and the potential to avoid the multiple scans needed for conventional T1W, T1IR, or FLAIR imaging are clear advantages of synthetic MRI. There are some limitations to the present study. First, the number of patients was small. Second, since MRI quantification was performed after conventional T1IR imaging after the administration of contrast agent, differences in the timing of image acquisition may have affected the degree of lesion enhancement.Synthetic T1IR images had superior contrast compared with synthetic T1W or conventional T1IR images. The ability to detect brain metastases was comparable among synthetic T1IR, synthetic T1W, and conventional T1IR imaging. Although further optimization of contrast weighting is needed, synthetic MRI has potential to maximize the detection of brain metastases.