Colorectal cancer is the third most commonly diagnosed cancer in the United States, and the second leading cause of cancer death(1). Accurate pre-operative staging is necessary to guide therapy (2,3) and it includes assessment of tumor infiltration into the mesorectum, distance from pelvic side walls, and regional lymph node involvement (4). Treatment response has been conventionally measured after 6-8 weeks of neoadjuvant chemoradiotherapy using morphologic criteria such as size and signal intensity of tumor (5). These MRI criteria are applied after completion of treatment, and patients are therefore committed to a full course of toxic therapy that may not be effective. Hence, there is a clinical opportunity to develop quantitative imaging techniques that can help define tumor surgical margins and provide early assessment of treatment response. Quantitative relaxometry (particularly T1 mapping) has been shown to be an early predictor of treatment response, but is not routinely used because the requisite acquisition times are long. Magnetic Resonance Fingerprinting (MRF) provides simultaneous mapping of multiple MR properties (T1 and T2 relaxation times) from target tissue in a single acquisition of approximately 50 second (6-8). In this study, we aim to evaluate the feasibility of using MRF to assess T1 and T2 relaxation times for colorectal cancers and standardize the MRF sequence for evaluation of rectal tumors. In this institutional review board approved study, 8 patients (5 male, age range: 45-78 years) presenting with newly diagnosed, biopsy-proven primary rectal cancer were scanned on either the Verio or Skyra (3T Siemens Systems). MRF slices were acquired during clinical scanning, before contrast injection and were planned using a T2-weighted sequence acquired in a true transverse plane. The MRF data were acquired as multiple 2D slices using MRF-FISP (8) with the following parameters: FOV 400 mm, in-plane spatial resolution 1.0 x 1.0mm², slice thickness 5mm, variable TRs ranging from 11.2 to 15.5 ms, variable flip angles ranging from 5 to 55° and ~39 seconds per slice acquisition. The raw data were reconstructed to generate T1 and T2 maps off-line using MATLAB 2014a (MathWorks, Natick, Massachusetts). Regions of interest (ROIs) were drawn for measurement of average T1 and T2 relaxation times for the tumor, mesorectum and normal rectal wall. Two-tailed Student’s t-test was used to compare the mean T1 and T2 relaxation time obtained from the tumor and normal tissues. 16 ROIs were drawn on 8 lesions with 2 slices evaluated per patient (Figure 1,2). The mean T1 value for rectal tumors was 1647 ms ±186 ms, the mean T2 value was 84 ms±42 ms. The mean T1 value for normal mesorectal fat was 537 ms ± 134 ms, the mean T2 value was 136ms ±62 ms. There was a statistically significant difference between T1 relaxation time of tumor and mesorectum (p<0.01*10-10). The differences in T2 relaxation times between tumor and mesorectum did not meet the significance criterion (p=0.058). It was not possible to draw ROIs in the rectal wall with the current resolution of MRF. T stage of a tumor, including mesorectal invasion guide neoadjuvant treatment strategies (2-4). Tumor-induced desmoplastic reaction produces T2 hypointense fibrotic tissue which mimics the tumor infiltrating mesorectum leading to overestimation of tumor extent and staging failure (9). On post treatment MRI images, it can be difficult to distinguish residual tumor from fibrotic tissue and hence predict complete response. Because T1 and T2 relaxation times are a reflection of tissue properties and microarchitecture, it is intuitive to hypothesize that the relaxation time of tumors should be different from normal tissues. This is the first in vivo study to report T1 and T2 relaxation time of rectal tumors and normal mesorectal fat. The T2 values for tumor in our study are in agreement with the values reported in a previous in vitro study in which relaxation time was measured in excised tumor specimens using the spin echo technique (10). Mesorectum is primarily composed of fat and the T1 and T2 relaxation time of mesorectum obtained in our study is higher than the values previously reported for fat (10, 11). Difference in the composition of mesorectal fat compared to subcutaneous fat11 and complex relaxation characteristics of fat may be responsible for these differences. The major limitation of this study is the small number of patients. This study lays the foundation for future studies that use relaxometry to define tumor margins and also represent a potential method for early response assessment that could lead to alterations in therapy. Magnetic resonance fingerprinting allows rapid measurement of T1 and T2 relaxation time of rectal tumors and mesorectum.