Assessment of lymphatic impairment and interstitial protein accumulation in patients with breast cancer treatment-related lymphedema using CEST MRI

Abstract

Purpose: Lymphatic impairment is known to reduce quality of life in some of the most crippling diseases of the 21st century, including obesity, lymphedema, and cancer. However, the lymphatics are not nearly as well-understood as other bodily systems, largely owing to a lack of sensitive imaging technologies that can be applied using standard clinical equipment. Here, proton exchange-weighted MRI is translated to the lymphatics in patients with breast cancer treatment-related lymphedema (BCRL).

Methods: Healthy volunteers (N = 8) and BCRL patients (N = 7) were scanned at 3 Tesla using customized structural MRI and amide proton transfer (APT) chemical exchange saturation transfer (CEST) MRI in sequence with the hypothesis that APT effects would be elevated in lymphedematous tissue. APT contrast, lymphedema stage, symptomatology, and histology information were evaluated.

Results: No significant difference between proton-weighted APT contrast in the right and left arms of healthy controls was observed. An increase in APT contrast in the affected arms of patients was found (P = 0.025; Cohen's d = 2.4), and variability among patients was consistent with documented damage to lymphatics as quantified by lymphedema stage.

Conclusion: APT CEST MRI may have relevance for evaluating lymphatic impairment in patients with BCRL, and may extend to other pathologies where lymphatic compromise is evident.

Keywords: APT; CEST; MRI; breast cancer; lymphatic; lymphedema.

Figure 1. Spatial coverage and planning of lymphatic imaging

(A) Orthogonal representations of 3D T2-weighted mDIXON scans, which are used for identifying the approximate location of axillary lymph nodes and surrounding muscle and fat structure. (B) Multi-TR B1 maps for one control subjectshowing typical B1 consistency between right and left arms. In the bottom right panel, an example region-of-interest (ROI) used for amide proton transfer (APT) analysis is shown (white). (C) The 3D Diffusion Weighted Imaging with Background Suppression (DWIBS) scan (maximum intensity projection) demarcates the approximate location of axillary lymph nodes, and (E) the high spatial resolution fat-suppressed T2-weighted scans highlights structural characteristics of the lymph nodes. These images can be acquired non-invasively using standard MRI hardware in less than 10 min, and can be used for guiding assessment of functional changes in human lymphatics.

Figure 2. Amide proton transfer (APT) effects in healthy control volunteers

(A) Amide protons ([NH]~100 mM) which resonate at +3.5 ppm from the water resonance are in constant exchange (rate~40 Hz) with surrounding bulk water ([H₂O]~110M). By selectively irradiating at the amide resonance prior to water detection, NH proton magnetization is reduced, which attenuates the detectable water signal following exchange. This effect is commonly visualized in the CEST z-spectrum (B,C), in which water signal following different radiofrequency (RF) pre-pulse offsets is shown. The lineshape is influenced by broad nuclear overhauser (NOE), T1/T2, and RF spillover effects, with small, additional attenuation apparent at +3.5 ppm owing to proton exchange. Mean (N=8) control z-spectra for (B) right and (C) left arms, along with Lorentzian fits. The squares denote acquired data whereas the solid lines denote the Lorentzian fit. The Lorentzian fit reflects information of B₁, T₁, and T₂, however not exchange and therefore provides a reference for exchange effects. (D) Both the Lorentzian APT analysis (APT_L) and Asymmetry APT analysis (APT_A) provide similar information (P<0.05) in control volunteers, however the variation in the APT_L approach is much smaller owing to a reduction of sensitivity to NOE and magnetization transfer effects on the –ppm side of the z-spectrum. Right and left arms are plotted as separate data points (16 data points total). (E) APT_L maps from a representative control volunteer (central slices covering axilla shown), which demonstrate the range of consistency between APT_L measures in right and left axilla.

Figure 3. Amide proton transfer effects in healthy and lymphedematous arms

(A-B) The size of the APT effect is similar in right and left arms of healthy control subjects, but (C-D) increased in lymphedematous arms, on average, of patients with BCRL (P<0.05). Note the range of APT contrast in patients (red circles; Fig. C), however the consistent asymmetry observed between the lymphedematous arm and contralateral arm (D). (E) The per-subject asymmetry of the APT_L effect is near zero in control volunteers, but significantly (P<0.05) elevated in BCRL patients. Red circles denote individual subject data and bars denote one standard deviation.

Figure 4. Amide proton transfer (APT) effects in patients

APT effect size is observed to be elevated in lymphedemetaous arms relative to contralateral arms, most notably subdermally on the medial aspect of upper arm. Breast tissue has been masked out owing to differential contrast from reconstructive surgery. Corresponding z-spectra are shown to the right, which denote a reduction in signal intensity in the region of the APT effect (+3.75 to +3.25 ppm range). Patient information is discussed in detail in the text and also in Table 2.

Figure 5. Relationship between amide proton transfer (APT) effect size and clinical measures of impairment as defined by breast cancer treatment-related lymphedema (BCRL) stage (Table 1)

Data are taken from all controls in the control group (green), and limbs of patients with node removal (e.g., both arms from patient 1 and 6 are included owing to bilateral node resection). Data suggest that APT contrast increases with increasing lymphatic impairment, and differences between control arms and Stage 0 BCRL patients are suggested. Additional data in a larger volume of patients, also controlling for duration since node resection and other risk factors (e.g., radiation therapy, age, sarcoma, cellulitis and obesity) are likely necessary to fully understand the relationship between APT contrast and lymphatic compromise. Bars denote one standard deviation.