In vivo lymph node CEST-Dixon MRI in breast cancer patients with metastatic lymph node involvement

PURPOSE

Axillary lymph nodes (LNs) often present a reservoir for metastatic breast cancer, yet metastatic LN involvement cannot be discerned definitively using diagnostic imaging. This study investigated whether in vivo CEST may discriminate LNs with versus without metastatic involvement.

METHODS

3T MRI was performed in patients with breast cancer before clinically-indicated mastectomy or lumpectomy with LN removal, after which LN metastasic involvement was determined using histological evaluation. Non-contrast anatomical imaging, as well as B0 and B1 field maps, were acquired in sequence with three-point CEST-Dixon (3D turbo-gradient-echo; factor = 25; TR/TE1/ΔTE = 851/1.35/1.1 ms; spatial-resolution = 2.5 × 2.5 × 6 mm; slices = 10; four sinc-gauss pulses with duty-cycle = 0.5, total saturation duration = 701.7 ms; B1  = 1.5 μT; saturation offsets = -5.5 to +5.5 ppm; stepsize = 0.2 ppm; scan duration = 6 min 30 s). The mean z-spectrum from LNs with (n = 20) versus without (n = 22) metastatic involvement were analyzed and a Wilcoxon rank-sum test (significance: p < 0.05) was applied to evaluate differences in B0, B1 , and magnetization transfer ratio (MTR) in differing spectral regions of known proton exchange (nuclear Overhauser effect [NOE], amide, amine, and hydroxyl) between cohorts.

RESULTS

No difference in axillary B1 (p = 0.634) or B0 (p = 0.689) was observed between cohorts. Elevated MTR was observed for the NOE (-1.7 ppm; MTR = 0.285 ± 0.075 vs. 0.248 ± 0.039; p = 0.048), amine (+2.5 ppm; MTR = 0.284 ± 0.067 vs. 0.234 ± 0.31; p = 0.005), and hydroxyl (+1 ppm; MTR = 0.394 ± 0.075 vs. 0.329 ± 0.055; p = 0.002) protons in LNs from participants with versus without metastatic involvement.

CONCLUSIONS

Findings are consistent with a unique metastatic LN microenvironment detectable by CEST-Dixon and suggest that CEST MRI may have potential for mapping LN metastasis non-invasively in vivo.

Semiautomated segmentation of lower extremity MRI reveals distinctive subcutaneous adipose tissue in lipedema: a pilot study

Purpose

Lipedema is a painful subcutaneous adipose tissue (SAT) disease involving disproportionate SAT accumulation in the lower extremities that is frequently misdiagnosed as obesity. We developed a semiautomatic segmentation pipeline to quantify the unique lower-extremity SAT quantity in lipedema from multislice chemical-shift-encoded (CSE) magnetic resonance imaging (MRI).

Approach

Patients with lipedema (n = 15 ) and controls (n = 13 ) matched for age and body mass index (BMI) underwent CSE-MRI acquired from the thighs to ankles. Images were segmented to partition SAT and skeletal muscle with a semiautomated algorithm incorporating classical image processing techniques (thresholding, active contours, Boolean operations, and morphological operations). The Dice similarity coefficient (DSC) was computed for SAT and muscle automated versus ground truth segmentations in the calf and thigh. SAT and muscle volumes and the SAT-to-muscle volume ratio were calculated across slices for decades containing 10% of total slices per participant. The effect size was calculated, and Mann-Whitney U test applied to compare metrics in each decade between groups (significance: two-sided P < 0.05 ).

Results

Mean DSC for SAT segmentations was 0.96 in the calf and 0.98 in the thigh, and for muscle was 0.97 in the calf and 0.97 in the thigh. In all decades, mean SAT volume was significantly elevated in participants with versus without lipedema (P < 0.01 ), whereas muscle volume did not differ. Mean SAT-to-muscle volume ratio was significantly elevated (P < 0.001 ) in all decades, where the greatest effect size for distinguishing lipedema was in the seventh decade approximately midthigh (r = 0.76 ).

Conclusions

The semiautomated segmentation of lower-extremity SAT and muscle from CSE-MRI could enable fast multislice analysis of SAT deposition throughout the legs relevant to distinguishing patients with lipedema from females with similar BMI but without SAT disease.

Advances in the prevention and treatment of breast cancer-related lymphedema

PURPOSE

Breast cancer-related lymphedema (BCRL) represents a lifelong risk for breast cancer survivors and once acquired becomes a lifelong burden. This review summarizes current BCRL prevention and treatment strategies.

FINDINGS

Risk factors for BCRL have been extensively studied and their identification has affected breast cancer treatment practice, with sentinel lymph node removal now standard of care for patients with early stage breast cancer without sentinel lymph node metastases. Early surveillance and timely management aim to reduce BCRL incidence and progression, and are further facilitated by patient education, which many breast cancer survivors report not having adequately received. Surgical approaches to BCRL prevention include axillary reverse mapping, lymphatic microsurgical preventative healing (LYMPHA) and Simplified LYMPHA (SLYMPHA). Complete decongestive therapy (CDT) remains the standard of care for patients with BCRL. Among CDT components, facilitating manual lymphatic drainage (MLD) using indocyanine green fluorescence lymphography has been proposed. Intermittent pneumatic compression, nonpneumatic active compression devices, and low-level laser therapy appear promising in lymphedema management. Reconstructive microsurgical techniques such as lymphovenous anastomosis and vascular lymph node transfer are growing surgical considerations for patients as well as liposuction-based procedures for addressing fatty fibrosis formation from chronic lymphedema. Long-term self-management adherence remains problematic, and lack of diagnosis and measurement consensus precludes a comparison of outcomes. Currently, no pharmacological approaches have proven successful.

CONCLUSION

Progress in prevention and treatment of BCRL continues, requiring advances in early diagnosis, patient education, expert consensus and novel treatments designed for lymphatic rehabilitation following insults.

Indications of Peripheral Pain, Dermal Hypersensitivity, and Neurogenic Inflammation in Patients with Lipedema

Lipedema is a disease with abnormally increased adipose tissue deposition and distribution. Pain sensations have been described in the clinical evaluation of lipedema, but its etiology remains poorly understood. We hypothesized that pain sensitivity measurements and ex vivo quantitation of neuronal cell body distribution in the skin would be lipedema stage-dependent, and could, thus, serve to objectively characterize neuropathic pain in lipedema. The pain was assessed by questionnaire and peripheral cutaneous mechanical sensitization (von-Frey) in lipedema (n = 27) and control (n = 23) consenting female volunteers. Dermal biopsies from (n = 11) Stages 1-3 lipedema and control (n = 10) participants were characterized for neuronal cell body and nociceptive neuropeptide calcitonin gene-related peptide (CGRP) and nerve growth factor (NGF) distribution. Stage 2 or 3 lipedema participants responded positively to von Frey sensitization in the calf and thigh, and Stage 3 participants also responded in the arm. Lipedema abdominal skin displayed reduced Tuj-1+ neuronal cell body density, compared to healthy controls, while CGRP and NGF was significantly elevated in Stage 3 lipedema tissues. Together, dermal neuronal cell body loss is consistent with hyper-sensitization in patients with lipedema. Further study of neuropathic pain in lipedema may elucidate underlying disease mechanisms and inform lipedema clinical management and treatment impact.

Physical Therapy in Women with Early Stage Lipedema: Potential Impact of Multimodal Manual Therapy, Compression, Exercise, and Education Interventions

Background: Lipedema is a distinct adipose disorder from obesity necessitating awareness as well as different management approaches to address pain and optimize quality of life (QoL). The purpose of this proof-of-principle study is to evaluate the therapeutic potential of physical therapy interventions in women with lipedema. Methods and Results: Participants with Stage 1-2 lipedema and early Stage 0-1 lymphedema (n = 5, age = 38.4 ± 13.4 years, body mass index = 27.2 ± 4.3 kg/m²) underwent nine visits of physical therapy in 6 weeks for management of symptoms impacting functional mobility and QoL. Pre- and post-therapy, participants were scanned with 3 Tesla sodium and water magnetic resonance imaging (MRI), underwent biophysical measurements, and completed questionnaires measuring function and QoL (patient-specific functional scale, PSFS, and RAND-36). Pain was measured at each visit using the 0-10 visual analog scale (VAS). Treatment effect was calculated for all study variables. The primary symptomatology measures of pain and function revealed clinically significant post-treatment improvements and large treatment effects (Cohen's d for pain VAS = -2.5 and PSFS = 4.4). The primary sodium MRI measures, leg skin sodium, and subcutaneous adipose tissue (SAT) sodium, reduced following treatment and revealed large treatment effects (Cohen's d for skin sodium = -1.2 and SAT sodium = -0.9). Conclusions: This proof-of-principle study provides support that persons with lipedema can benefit from physical therapy to manage characteristic symptoms of leg pain and improve QoL. Objective MRI measurement of reduced tissue sodium in the skin and SAT regions indicates reduced inflammation in the treated limbs. Further research is warranted to optimize the conservative therapy approach in lipedema, a condition for which curative and disease-modifying treatments are unavailable.

Elevated magnetic resonance imaging measures of adipose tissue deposition in women with breast cancer treatment-related lymphedema

PURPOSE

Breast cancer treatment-related lymphedema (BCRL) is a common co-morbidity of breast cancer therapies, yet factors that contribute to BCRL progression remain incompletely characterized. We investigated whether magnetic resonance imaging (MRI) measures of subcutaneous adipose tissue were uniquely elevated in women with BCRL.

METHODS

MRI at 3.0 T of upper extremity and torso anatomy, fat and muscle tissue composition, and T₂ relaxometry were applied in left and right axillae of healthy control (n = 24) and symptomatic BCRL (n = 22) participants to test the primary hypothesis that fat-to-muscle volume fraction is elevated in symptomatic BCRL relative to healthy participants, and the secondary hypothesis that fat-to-muscle volume fraction is correlated with MR relaxometry of affected tissues and BCRL stage (significance criterion: two-sided p < 0.05).

RESULTS

Fat-to-muscle volume fraction in healthy participants was symmetric in the right and left sides (p = 0.51); in BCRL participants matched for age, sex, and BMI, fat-to-muscle volume fraction was elevated on the affected side (fraction = 0.732 ± 0.184) versus right and left side in controls (fraction = 0.545 ± 0.221, p < 0.001). Fat-to-muscle volume fraction directly correlated with muscle T₂ (p = 0.046) and increased with increasing level of BCRL stage (p = 0.041).

CONCLUSION

Adiposity quantified by MRI is elevated in the affected upper extremity of women with BCRL and may provide a surrogate marker of condition onset or severity.

CLINICAL TRIAL

NCT02611557.

Magnetic resonance imaging and bioimpedance evaluation of lymphatic abnormalities in patients with breast cancer treatment-related lymphedema

PURPOSE

Breast cancer treatment-related lymphedema (BCRL) evaluation is frequently performed using portable measures of limb volume and bioimpedance asymmetry. Here quantitative magnetic resonance imaging (MRI) is applied to evaluate deep and superficial tissue impairment, in both surgical and contralateral quadrants, to test the hypothesis that BCRL impairment is frequently bilateral and extends beyond regions commonly evaluated with portable external devices.

METHODS

3-T MRI was applied to investigate BCRL topographical impairment. Female BCRL (n = 33; age = 54.1 ± 11.2 years; stage = 1.5 ± 0.8) and healthy (n = 33; age = 49.4 ± 11.0 years) participants underwent quantitative upper limb MRI relaxometry (T₂), bioimpedance asymmetry, arm volume asymmetry, and physical evaluation. Parametric tests were applied to evaluate study measurements (i) between BCRL and healthy participants, (ii) between surgical and contralateral limbs, and (iii) in relation to clinical indicators of disease severity. Two-sided p-value < 0.05 was required for significance.

RESULTS

Bioimpedance asymmetry was significantly correlated with MRI-measured water relaxation (T₂) in superficial tissue. Deep muscle (T₂ = 37.6 ± 3.5 ms) and superficial tissue (T₂ = 49.8 ± 13.2 ms) relaxation times were symmetric in healthy participants. In the surgical limbs of BCRL participants, deep muscle (T₂ = 40.5 ± 4.9 ms) and superficial tissue (T₂ = 56.0 ± 14.8 ms) relaxation times were elevated compared to healthy participants, consistent with an edematous micro-environment. This elevation was also observed in contralateral limbs of BCRL participants (deep muscle T₂ = 40.3 ± 5.7 ms; superficial T₂ = 56.6 ± 13.8 ms).

CONCLUSIONS

Regional MRI measures substantiate a growing literature speculating that superficial and deep tissue, in surgical and contralateral quadrants, is affected in BCRL. The implications of these findings in the context of titrating treatment regimens and understanding malignancy recurrence are discussed.

CEST MRI quantification procedures for breast cancer treatment-related lymphedema therapy evaluation

PURPOSE

To quantify chemical exchange saturation transfer contrast in upper extremities of participants with lymphedema before and after standardized lymphatic mobilization therapy using correction procedures for B₀ and B₁ heterogeneity, and T₁ relaxation.

METHODS

Females with (n = 12) and without (n = 17) breast cancer treatment-related lymphedema (BCRL) matched for age and body mass index were scanned at 3.0T MRI. B₁ efficiency and T₁ were calculated in series with chemical exchange saturation transfer in bilateral axilla (B₁ amplitude = 2µT, Δω = ±5.5 ppm, slices = 9, spatial resolution = 1.8 × 1.47 × 5.5 mm³ ). B₁ dispersion measurements (B₁ = 1-3 µT; increment = 0.5 µT) were performed in controls (n = 6 arms in 3 subjects). BCRL participants were scanned pre- and post-manual lymphatic drainage (MLD) therapy. Chemical exchange saturation transfer amide proton transfer (APT) and nuclear Overhauser effect (NOE) metrics corrected for B₁ efficiency were calculated, including proton transfer ratio (PTR'), magnetization transfer ratio asymmetry ( M T R asymmetry ' ) , and apparent exchange-dependent relaxation (AREX'). Nonparametric tests were used to evaluate relationships between metrics in BCRL participants pre- versus post-MLD (two-sided P < 0.05 required for significance).

RESULTS

B₁ dispersion experiments showed nonlinear dependence of Z-values on B₁ efficiency in the upper extremities; PTR' showed < 1% mean fractional difference between subject-specific and group-level correction procedures. PTR'_APT significantly correlated with T₁ (Spearman's rho = 0.57, P < 0.001) and body mass index (Spearman's rho = -0.37, P = 0.029) in controls and with lymphedema stage (Spearman's rho = 0.48, P = 0.017) in BCRL participants. Following MLD therapy, PTR'_APT significantly increased in the affected arm of BCRL participants (pre- vs. post-MLD: 0.41 ± 0.05 vs. 0.43 ± 0.03, P = 0.02), consistent with treatment effects from mobilized lymphatic fluid.

CONCLUSION

Chemical exchange saturation transfer metrics, following appropriate correction procedures, respond to lymphatic mobilization therapies and may have potential for evaluating treatments in participants with secondary lymphedema.

Implementation of Single-Tab Electrodes for Bioimpedance Spectroscopy Measures

Background: Bioimpedance spectroscopy (BIS) demonstrates proficiency in early identification of breast cancer treatment-related lymphedema (BCRL) development. Dual-tab electrodes were designed for consistent and easy electrode placement, however, single-tab electrodes positioned to mimic dual-tab placement on the body may make BIS technology more accessible in community hospitals and outpatient settings. The purpose of this study is to evaluate use of single-tab electrodes for BIS measurements and assess whether single-tab electrodes provide consistent BIS measurements in controls and patients with BCRL. Methods and Results: Upper limb BIS ratios were obtained using ImpediMed L-Dex^® U400 in controls (n = 13; age = 23-75 years; 9 repeated measurements) using dual-tab and single-tab electrodes. BCRL patients (n = 17; Stage = 1.65 ± 0.49; number nodes removed = 16.3 ± 7.7; age = 50.9 ± 10.6 years; age range = 33-77 years) and healthy controls (n = 19) were evaluated to determine if single-tab electrodes provided discriminatory capacity for detecting BCRL. Intraclass correlation coefficients (ICC), linear mixed-effects models, Wilcoxon rank-sum tests, and linear regression with two-sided p-values <0.05 required for significance were applied. Single-tab electrodes were found to be statistically interchangeable with dual-tab electrodes (ICC = 0.966; 95% confidence interval = 0.937-0.982). No evidence of differences in single-tab versus dual-tab measurements were found for L-Dex ratios (p = 0.74) from the linear mixed-effects model. Repeated trials involving reuse of the same electrodes revealed a trend toward increases in L-Dex ratio for both styles of electrodes. Single-tab electrodes were significant (p < 0.0001) for discriminating between BCRL and control subjects. Conclusion: Findings expand upon the potential use of BIS in clinic and research settings and suggest that readily available single-tab electrodes provide similar results as dual-tab electrodes for BIS measurements.

Lipedema and Dercum's Disease: A New Application of Bioimpedance

Background: Lipedema and Dercum's disease (DD) are incompletely characterized adipose tissue diseases, and objective measures of disease profiles are needed to aid in differential diagnosis. We hypothesized that fluid properties, quantified as tissue water bioimpedance in the upper and lower extremities, differ regionally between these conditions. Methods and Results: Women (cumulative n = 156) with lipedema (n = 110), DD (n = 25), or without an adipose disease matched for age and body mass index to early stage lipedema patients (i.e., controls n = 21) were enrolled. Bioimpedance spectroscopy (BIS) was applied to measure impedance values in the arms and legs, indicative of extracellular water levels. Impedance values were recorded for each limb, as well as the leg-to-arm impedance ratio. Regression models were applied to evaluate hypothesized relationships between impedance and clinical indicators of disease (significance criteria: two-sided p < 0.05). Higher extracellular water was indicated (i) in the legs of patients with higher compared with lower stages of lipedema (p = 0.03), (ii) in the leg-to-arm impedance ratio in patients with lipedema compared with patients with DD (p ≤ 0.001), and (iii) in the leg-to-arm impedance ratio in patients with stage 1 lipedema compared with controls (p ≤ 0.01). Conclusion: BIS is a noninvasive portable modality to assess tissue water, and this device is available in both specialized and nonspecialized centers. These findings support that regional bioimpedance measures may help to distinguish lipedema from DD, as well as to identify early stages of lipedema.

Bilateral Changes in Deep Tissue Environment After Manual Lymphatic Drainage in Patients with Breast Cancer Treatment-Related Lymphedema

BACKGROUND

Breast cancer treatment-related lymphedema (BCRL) arises from a mechanical insufficiency following cancer therapies. Early BCRL detection and personalized intervention require an improved understanding of the physiological processes that initiate lymphatic impairment. Here, internal magnetic resonance imaging (MRI) measures of the tissue microenvironment were paired with clinical measures of tissue structure to test fundamental hypotheses regarding structural tissue and muscle changes after the commonly used therapeutic intervention of manual lymphatic drainage (MLD).

METHODS AND RESULTS

Measurements to identify lymphatic dysfunction in healthy volunteers (n = 29) and patients with BCRL (n = 16) consisted of (1) limb volume, tissue dielectric constant, and bioelectrical impedance (i.e., non-MRI measures); (2) qualitative 3 Tesla diffusion-weighted, T₁-weighted and T₂-weighted MRI; and (3) quantitative multi-echo T₂ MRI of the axilla. Measurements were repeated in patients immediately following MLD. Normative control and BCRL T₂ values were quantified and a signed Wilcoxon Rank-Sum test was applied (significance: two-sided p < 0.05). Non-MRI measures yielded significant capacity for discriminating between arms with versus without clinical signs of BCRL, yet yielded no change in response to MLD. Alternatively, a significant increase in deep tissue T₂ on the involved (pre T₂ = 0.0371 ± 0.003 seconds; post T₂ = 0.0389 ± 0.003; p = 0.029) and contralateral (pre T₂ = 0.0365 ± 0.002; post T₂ = 0.0395 ± 0.002; p < 0.01) arms was observed. Trends for larger T₂ increases on the involved side after MLD in patients with stage 2 BCRL relative to earlier stages 0 and 1 BCRL were observed, consistent with tissue composition changes in later stages of BCRL manifesting as breakdown of fibrotic tissue after MLD in the involved arm. Contrast consistent with relocation of fluid to the contralateral quadrant was observed in all stages.

CONCLUSION

Quantitative deep tissue T₂ MRI values yielded significant changes following MLD treatment, whereas non-MRI measurements did not vary. These findings highlight that internal imaging measures of tissue composition may be useful for evaluating how current and emerging therapies impact tissue function.

Lymphedema evaluation using noninvasive 3T MR lymphangiography

PURPOSE

To exploit the long 3.0T relaxation times and low flow velocity of lymphatic fluid to develop a noninvasive 3.0T lymphangiography sequence and evaluate its relevance in patients with lymphedema.

MATERIALS AND METHODS

A 3.0T turbo-spin-echo (TSE) pulse train with long echo time (TE_effective  = 600 msec; shot-duration = 13.2 msec) and TSE-factor (TSE-factor = 90) was developed and signal evolution simulated. The method was evaluated in healthy adults (n = 11) and patients with unilateral breast cancer treatment-related lymphedema (BCRL; n = 25), with a subgroup (n = 5) of BCRL participants scanned before and after manual lymphatic drainage (MLD) therapy. Maximal lymphatic vessel cross-sectional area, signal-to-noise-ratio (SNR), and results from a five-point categorical scoring system were recorded. Nonparametric tests were applied to evaluate study parameter differences between controls and patients, as well as between affected and contralateral sides in patients (significance criteria: two-sided P < 0.05).

RESULTS

Patient volunteers demonstrated larger lymphatic cross-sectional areas in the affected (arm = 12.9 ± 6.3 mm² ; torso = 17.2 ± 15.6 mm² ) vs. contralateral (arm = 9.4 ± 3.9 mm² ; torso = 9.1 ± 4.6 mm² ) side; this difference was significant both for the arm (P = 0.014) and torso (P = 0.025). Affected (arm: P = 0.010; torso: P = 0.016) but not contralateral (arm: P = 0.42; torso: P = 0.71) vessel areas were significantly elevated compared with control values. Lymphatic cross-sectional areas reduced following MLD on the affected side (pre-MLD: arm = 8.8 ± 1.8 mm² ; torso = 31.4 ± 26.0 mm² ; post-MLD: arm = 6.6 ± 1.8 mm² ; torso = 23.1 ± 24.3 mm² ). This change was significant in the torso (P = 0.036). The categorical scoring was found to be less specific for detecting lateralizing disease compared to lymphatic-vessel areas.

CONCLUSION

A 3.0T lymphangiography sequence is proposed, which allows for upper extremity lymph stasis to be detected in ∼10 minutes without exogenous contrast agents.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2017;46:1349-1360.

Clinical feasibility of noninvasive visualization of lymphatic flow with principles of spin labeling MR imaging: implications for lymphedema assessment

PURPOSE

To extend a commonly used noninvasive arterial spin labeling magnetic resonance (MR) imaging method for measuring blood flow to evaluate lymphatic flow.

MATERIALS AND METHODS

All volunteers (n = 12) provided informed consent in accordance with institutional review board and HIPAA regulations. Quantitative relaxation time (T1 and T2) measurements were made in extracted human lymphatic fluid at 3.0 T. Guided by these parameters, an arterial spin labeling MR imaging approach was adapted to measure lymphatic flow (flow-alternating inversion-recovery lymphatic water labeling, 3 × 3 × 5 mm) in healthy subjects (n = 6; mean age, 30 years ± 1 [standard deviation]; recruitment duration, 2 months). Lymphatic flow velocity was quantified by performing spin labeling measurements as a function of postlabeling delay time and by measuring time to peak signal intensity in axillary lymph nodes. Clinical feasibility was evaluated in patients with stage II lymphedema (three women; age range, 43-64 years) and in control subjects with unilateral cuff-induced lymphatic stenosis (one woman, two men; age range, 31-35 years).

RESULTS

Mean T1 and T2 relaxation times of lymphatic fluid at 3.0 T were 3100 msec ± 160 (range, 2930-3210 msec; median, 3200 msec) and 610 msec ± 12 (range, 598-618 msec; median, 610 msec), respectively. Healthy lymphatic flow (afferent vessel to axillary node) velocity was 0.61 cm/min ± 0.13 (n = 6). A reduction (P < .005) in lymphatic flow velocity in the affected arms of patients and the affected arms of healthy subjects with manipulated cuff-induced flow reduction was observed. The ratio of unaffected to affected axilla lymphatic velocity (1.24 ± 0.18) was significantly (P < .005) higher than the left-to-right ratio in healthy subjects (0.91 ± 0.18).

CONCLUSION

This work provides a foundation for clinical investigations whereby lymphedema etiogenesis and therapies may be interrogated without exogenous agents and with clinically available imaging equipment. Online supplemental material is available for this article.