Normative Values of Thymus in Healthy Children; Stiffness by Shear Wave Elastography
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Diagn Interv Radiol 2020; 26:147–152 PEDIATRIC RADIOLOGY © Turkish Society of Radiology 2020 ORIGINAL ARTICLE Normative values of thymus in healthy children; stiffness by shear wave elastography Zuhal Bayramoğlu PURPOSE Mehmet Öztürk Thymus grows after birth, reaches maximal size after the first few years and involutes by puber- Emine Çalışkan ty. Because of the postnatal developmental and involutional duration, we aimed to investigate normal stiffness values of mediastinal thymus by shear wave elastography (SWE) in different age Hakan Ayyıldız groups of children and discuss imaging findings of thymus. İbrahim Adaletli METHODS We prospectively examined 146 children (90 girls, 56 boys) who underwent a thyroid or neck ultrasound examination. All subjects underwent ultrasound and SWE evaluation of mediastinal thymus by parasternal and suprasternal approach. We grouped the subjects based on age as 0 to 2 months, >2 to 6 months, >6 months to 2 years, >2 to 5 years, >5 to 8 years, and greater than 8 years old. We investigated differences of mean shear wave elasticity (kPa) and shear wave veloc- ity (m/s) values among age groups and the association of SWE values with age, body mass index (BMI), height, and weight of the patients. RESULTS Median and range of age, height, weight, and BMI were 24 months (2–84 months), 85 cm (55– 120 cm), 12 kg (4.55–22 kg), 15.37 kg/m2 (13.92–17.51 kg/m2), 11 cc (2.64–23.15 cc), respectively. Mean shear wave elasticity of thymus of all participants was 6.76±1.04 kPa. Differences of mean elasticity values among the age and gender groups were not statistically significant. Thymus elasticity and velocity values showed highly significant negative correlations with age (r= –0.3), height (r= –0.26), weight (r=–0.3) (P < 0.001). CONCLUSION Quantitative evaluation of the thymus by SWE provides normative stiffness values based on age and gender groups. The thymus elasticity decreases with increased age, height, and weight. hymus is a soft, roughly triangular shaped lymphoreticular organ presenting an age dependent appearance and content. Embryologically, the thymus develops bilater- T ally at approximately the 5th week of gestation from the third and fourth pharyngeal pouches and migrates caudally and medially along with thymopharyngeal duct (1). Ventral wing of the third pharyngeal pouch forms the thymic tissue. Along the descent pathway, thymic tissue can be observed anywhere from the angle of the mandible to the manubrium sterni. An abnormally positioned thymus could be determined either as aberrant if thymus From the Department of Pediatric Radiology is located along the normal descent pathway, or as ectopic if it is located at different loca- (Z.B [email protected], H.A., İ.A.), İstanbul tions such as the pharynx, trachea, posterior neck, or esophagus (2). University, İstanbul School of Medicine, İstanbul, Turkey; Departmant of Radiology, Division of Ultrasonographic (US) features of mediastinal thymus have been published in a recent study Pediatric Radiology (M.Ö.), Selçuk University School (3). Thymus is a homogeneous soft tissue hypoechogenic to thyroid gland and includes punc- of Medicine, Konya, Turkey; Clinic of Pediatric Radiology (E.Ç.), Dr. Lütfi Kırdar Education and tuate echogenicities. The echogenicity is equal to or lower than strap muscles especially in new- Research Hospital, İstanbul, Turkey. borns and increases with age. On unenhanced computed tomography (CT), thymus density is Received 04 July 2019; revision requested 23 July lower than thyroid gland due to the internal iodine content of thyroid gland and similar with 2019; last revision received 07 November 2019; mediastinal lymph nodes. Therefore, it would be easy to differentiate plunging goiter or ectopic accepted 10 December 2019. thyroid from thymus on CT due to radiodensity differences. However, it will be difficult to differ- DOI 10.5152/dir.2019.19344 entiate rebound thymic hyperplasia from mediastinal lymphoreticular malignancy recurrence You may cite this article as: Bayramoğlu Z, Öztürk M, Çalışkan E, Ayyıldız H, Adaletli İ. Normative values of thymus in healthy children; stiffness by shear wave elastography. Diagn Interv Radiol 2020; 26:147–152. 147 only by density-based CT evaluation. Thymic tribution in distinguishing thymic tissue by a years of SWE experience and a radiologist tissue is hyperintense than salivary glands radiation-free diagnostic modality. with a year of pediatric radiology and SWE and isointense to lymph nodes on T2-weight- experience, in consensus. The patients were ed magnetic resonance imaging (MRI). Along Methods in the supine position. Once the whole thy- the descent pathway, ectopic or aberrant thy- Subjects and study design mus gland was evaluated carefully in terms mic tissue should be kept in mind among the This prospective study included 146 chil- of absence of a local heterogeneity or con- differential diagnosis of anterior cervical and dren, 56 boys and 90 girls, between March tour lobulation (Fig. 1), the best acquisitions superior mediastinal lymph nodes depicted and June 2019. The local ethics committee demonstrating parallel propagation lines on T2-weighted MRI. Despite well-described of the institutional review board approved were obtained by consensus of the two ra- imaging features, many cases regarding ab- this study (File number: 2019/99). Before diologists. Propagation map along with SWE errant thymic tissue either located as cervical parasternal and suprasternal US and SWE window were evaluated when the lines were or intrathyroidal have been published as a examinations, informed consent was ob- parallel and smooth on the propagation diagnostic challenge (4, 5). In such cases an tained from the parents of participants. Due mode (arrival time contour). For SWE mea- ultrasound-based evaluation that is noninva- to lack of sufficient number of cases to con- surements, the entire linear probe surface sive and quantitative, such as SWE, is required stitute nomograms for each age, the sub- was covered with ultrasonic gel pad of 5 mm as an additional imaging modality. jects were divided into several age groups in in thickness. Any external pressure was not US is a noninvasive, radiation-free, repro- order to reveal changes in thymus stiffness. applied to the probe, and the operator was ducible, first-choice imaging modality in Subjects were analyzed in classes A, B and C, stationary. The long axis of the probe was po- pediatric patients with anterior mediastinal consisting of six, four, and two age groups, sitioned such that it was perpendicular to the enlargement, especially in the first few years respectively. The classifications include neo- tissue surface. When the tissue with parallel of life. US is indispensable for newborns with nate, infant, preschool age children, school propagation lines was obtained, five circular mediastinal widening, because of partial os- age group, and adolescent age group (Table regions of interest (ROI) of 3 mm in diameter sification of the sternal segments and costal 1). Demographic data were noted as gender, were selected on the acquisition by consen- cartilages and also homogeneous echo tex- height, weight, and body mass index (BMI). sus of the two radiologists (Fig. 2). The me- ture of an enlarged thymus gland providing We included patients who are proposed to dians of five shear wave elasticity and shear an acoustic window. As a descriptive quali- be healthy during SWE examination based wave velocity values were selected to reveal tative evaluation, contour lobulation and on physical examinations. Exclusion criteria closest value for each thymus gland. The heterogeneity in the thymus would lead to were rejecting participation in the study, SWE parameters were measured in kilopas- consideration of neoplasia or parenchymal hematological malignancy or chemoradio- cal (kPa) for shear wave elasticity (SWe) and diseases. In terms of quantitative evaluation, therapy history, prematurity for newborns, meters/second (m/s) for shear wave velocity the strain ratio of intrathyroidal ectopic thy- fever during assessment, weight loss, previ- (SWv). The elastographic scale was set to 0 to mic tissue compared to thyroid gland has ous mediastinal surgery, primary immuno- 40 kPa and 0 to 8 m/s with a real-time prop- been investigated by strain elastography re- deficiency, immunosuppressant medication agation map. Overall examination regarding vealing semiquantitative data (6). However, use, or focal intraglandular lesion. the scan of the gland with gray-scale US, ob- there is lack of quantitative criteria for diag- taining optimal propagation lines, selection nosing thymic tissue by US-based applica- Ultrasonographic evaluation of the ROIs by consensus took 5–6 minutes in tions such as shear wave elastography (SWE). All gray-scale and SWE evaluations were general, but the examination took 2–3 min- Thymus presents a postnatal growth peri- performed by using an AplioTM 500Platinum utes longer in newborns and infants. od followed by involution. In this study, we US device (Canon Medical Systems Co. Ltd.) aimed to investigate SWE values as diagnos- and a high-frequency linear probe that was Statistical analysis tic quantitative data according to age and set to small parts preset (frequency range, All data were processed in Microsoft Of- gender groups during developmental and 5–14 MHz). SWE evaluations were performed fice Excel and transferred to SPSS (version involutional processes. Being aware of nor- by a pediatric radiologist with more than 21.0, IBM Corp.) for statistical