The overall goal of this work is to develop quantitative biomarkers of lymphatic system structure and function by translating MRI methods that have established relevance for blood flow imaging to the lymphatic system. Lymphatic impairment is known to reduce quality of life in many disabling conditions, including obesity, lymphedema, and cancer. Very recently, lymphatic vessel presence in the central nervous system (CNS) has been reported [1], and these vessels may be centrally relevant to neurodegenerative disorders such as Alzheimer’s disease and multiple sclerosis. However, the lymphatics are not nearly as well-understood as other bodily systems, largely owing to a lack of sensitive imaging technologies that have been optimized for lymphatic evaluation. Here, we focus on breast cancer treatment-related lymphedema (BCRL) in which mechanical insufficiency of the remaining axillary lymph nodes effects adequate process of the lymphatic load and results in an accumulation of protein rich fluid in the dependent tissues leading to swelling of the involved arm and/or upper truncal quadrant. BCRL is one of the most common comorbidities in breast cancer survivors undergoing lymph node biopsy or dissection [2]. Manual lymphatic drainage (MLD) is a component of complete decongestive therapy commonly used to manage lymphedema; however, MLD has been shown to have variable impact on outcomes as quantified using standard limb volumetrics [3]. The hypothesis of this study is that quantitative T2 is elevated in patients relative to controls secondary to greater fluid content in the targeted region of interest, which reduces following MLD compared with standard volumetric limb measurements and readings from commercial devices for quantifying fluid levels, e.g. bioimpedance spectroscopy and tissue dielectric constant, respectively. The long-term goal is to demonstrate abilities to accurately record structural and functional observables of lymphatic dysfunction, derived from noninvasive MRI equipment available at most hospitals, which can serve as trial end points for future rigorous, randomized clinical trials of lymphedema management therapies. Healthy control (n=17;age=22-74yrs) and unilateral BCRL (n=12;Stage 0-2;age=33-77yrs) participants provided informed, written consent and were scanned at 3T (Philips) using dual-channel body coil transmission and a 16-channel SENSE torso coil reception. Volunteers underwent bilateral measures of upper limb (i) circumference (perometer), (ii) Delfin MoistureMeterD tissue dielectric constant, and (iii) LDEX bioimpedance spectroscopy. The MRI protocol consisted of custom DWIBS imaging for large axillary node identification (spatial resolution=3x3x5mm;b-value=800 s/mm2;TR/TE/TI=8037/49.8/260ms), T1-weighted mDIXON for tissue and node structure (3D-gradient-echo;TR=3.4ms; spatial resolution=1.0x1.3x3.0mm), high-spatial resolution fat-suppressed T2-weighted MRI for node structure (turbo-spin-echo;TR=3500 ms;TE=60ms; spatial resolution=0.4x0.5x5 mm), and multi-echo quantitative T2 MRI (turbo-spin-echo;echoes=16;range=9-189ms;increment=12ms; spatial resolution=2.5x3x5mm). Additional lymphatic inflow, CEST, and spin labeling data were acquired but were not relevant to the above hypothesis. Total scan duration was 50 minutes, after which patients underwent a 50-minute MLD session with a certified lymphedema physical therapist, all measurements were repeated immediately after therapy. A subset of healthy controls (n=5) were scanned twice to assess reproducibility. We focus on changes in T2 relaxation times, a well-known surrogate marker of tissue composition, which were quantified in deep tissue bilaterally and compared across volunteers and within patients (i.e., affected vs. unaffected arms) using unpaired and paired t-tests, respectively.Non-MRI measures did not differ significantly pre- and post-MLD. Figure 1 shows examples of structural imaging and slice planning procedure. Tissue-water T2 was elevated in the affected arms of patients (T2=37.1±0.3ms vs. controls (35.3±0.2ms) (p=0.03), consistent with increased edema (Figure 2). T2 increased in the unaffected arm post-MLD (p < 0.001) consistent with fluid being redirected to healthy axilla during MLD; affected arm T2 changed more variably and depended on location and BCRL stage. Internal MRI measurements of deep tissue composition provided evidence of tissue structural changes following MLD in BCRL not detectable using conventional measures of limb volume, or the more recently developed bioimpedance spectroscopy and dielectric constant measurements, indicating that MRI may provide a basis for more sensitive internal mechanistic investigations of MLD and its utilization in condition management. Additional data regarding interstitial protein accumulation (CEST), lymph node perfusion (arterial spin labeling), and lymphatic flow (short-TR inflow) were acquired, which should allow for mechanisms of lymphatic impairment and progression risk to be more thoroughly evaluated, and which is the topic of ongoing work.For the first time, we demonstrate abilities to detect changes consistent with intervention-elicited lymphatic dynamics by using internal measures of tissue composition from MRI. These findings suggest that MRI may be well-suited to evaluate lymphatic functioning and lymphedema treatment response, and may have relevance for informing personalized lymphedema risk and early detection following breast cancer therapies.