Anatomical location and distribution characteristics of retropharyngeal lymph nodes
The retropharyngeal space was first described by Rouviere [7] as a potential recess in front of the prevertebral muscle, which covers the prevertebral fascia and is divided into two layers. The front layer is the pterygoid fascia andthe buccal pharyngeal tendon is attached to the midline.The space between the buccopharyngeal fascia and the pterygoid fascia is the retropharyngeal space. The lymph nodes of the posteromedial group, lateral retropharyngeal lymph nodes, and adipose tissue are located in it. The lymph nodes of the posteromedial group are distributed along the medial border of the prevertebral muscle, often at the level of the C2 - 3 vertebral body.Llateral retropharyngeal lymph nodes are distributed along the lateral border of the prevertebral muscle, located in the carotid sheath, and often at the level of C1 - 2 at the base of the skull [8]. Therefore, the scope of this study was defined from the suprasellar cistern to the level of the superior border of the C3 vertebral body.
MR axial T1WI and T2WI sequences show that the nasopharyngeal mucosa, fascia, longus capitis muscle, and occipital clivus can be seen in the rear of the nasopharynx from front to back (Fig. 1). However, the fascia layer cannot be seen under MRI stratification. Still, MRI has the advantages of the high soft-tissue resolution, multi-parameter, and multi-directional imaging and can better display primary nasopharyngeal and lymph node lesions [9].
Lateral retropharyngeal lymph nodes is more common, especially in children and adolescents. Norma [10] found that children without obstructive sleep apnea(OSA) and children with OSA were 97.3% and 91.3%, respectively. In this study, the presence of lymph nodes in the retropharyngeal group was almost always seen in the random group of children and adolescents, consistent with previous studies. The average short-axis diameter of children and adolescents is 5.90 mm - 5.98 mm, and lymph nodes with a short-axis diameter ≥ 10 mm account for 0.5% and 0.7% of normal lymph nodes in males and females, respectively, which is much lower than the 90% of children's lymph nodes reported by Park [11]. The short-axis is greater than 10 mm, and the mean short diameter is also slightly lower than the mean value of retropharyngeal lymph nodes in children studied by Ozlugedik [8], which is 6.8 mm. King and Ma [1-2] found that the short-axis diameter of the retropharyngeal lymph nodes in normal populations did not exceed 4.5 mm, and the average short-axis diameter of the adult group in this study was 4.13 mm - 4.38 mm, which was consistent with their research. The relationship between lymph node size and age in the normal retropharyngeal group has not been mentioned in the previous literature. Still, for thoracic lymph nodes, the effect of age on lymph node size has been shown [12] and we will conduct further research in the future.
There are few studies on the clinical significance of the longest diameter of retropharyngeal lymph nodes. Studies have reported that the proportion of lymph node extracapsular invasion positively correlates with the maximum diameter [13]. Based on the observation of CT images, Chua used the longest diameter of lymph nodes ≥ 10 mm as the diagnostic criteria for retropharyngeal lymph node metastasis [14]. One study used 12.10 mm as the threshold [15], the sensitivity was 63.9%, and the specificity was 93.9%. However, when the Bayes function was used to evaluate the diagnostic value further, it was found that the discriminative ability of this variable was weak, suggesting that the clinical significance of the longest diameter of the lymph nodes in the retropharyngeal group needs to be confirmed in a larger number of cases. This study found that the average longest diameter of lateral retropharyngeal lymph nodes was 9.11 mm - 9.28 mm in children and adolescents, 5.95 - 6.49 mm in adults, and the average longest diameter was < 10 mm, which was consistent with the Chua study. Among them, the number of lymph nodes with the longest diameter ≥ 10 mm is the largest in children and adolescents. Statistics show that the longest diameter of lymph nodes in children and adolescents is larger than in adults and that in males is larger than in females. Luscieti [16] found that lymph node germinal centers were clearly present in infants and children, less in young adults, and usually absent in older adults, with a slight but gradual decrease in paracortical and medullary areas with age, which could explain the age-related changes in lymph node size found in this study. The author believes that region and ethnicity will also affect the value of lymph node size, and this study can provide a basis for future research.
Previous studies have assessed the number and distribution of normal mediastinal lymph nodes from clinical, radiological, and anatomical perspectives, with conflicting and inconsistent results [17]. This study found that there were 1559 lymph nodes on the left side of lateral retropharyngeal lymph nodes, accounting for 51.6%, and 1459 lymph nodes on the right side, accounting for 48.4%.
Although lymph node size is an essential criterion for evaluating lymph nodes with all imaging modalities, other factors such as shape, borders, internal structure, and enhancement features are also important.
Analysis of imaging features and DWI selection of lateral retropharyngeal lymph nodes
Compared with conventional SE sequence scans, diffusion-weighted imaging can provide much information that conventional MR cannot offer. Human physiological activities and pathological changes are of great significance [18]. However, the image quality of DWI images is affected by many factors. Neck soft tissue and adipose tissue have low signals in the DWI sequence, while benign and malignant lymph node lesions have high signs. Only by reasonably matching various scanning parameters to maximize the signal-to-noise ratio of the image can it be helpful for diagnosis. The diffusion sensitivity factor (b value) determines the intensity of signal attenuation caused by diffusion. The larger the b value, the more accurately it can reflect the diffusion image and the measured ADC value. A small b value is not sensitive to the diffusion movement of water molecules, and tissue signal attenuation is affected by other movements, especially vascular perfusion and microcirculation [19]. Still, it is also easier to produce susceptibility artifacts and reduce the image quality.
Using different b values (600 s/mm2, 800 s/mm2, and 1 000 s/mm2), Luo [20] showed that the larger the b value, the more significant the difference between the ADC values of benign and malignant lymph nodes. A high b value can reduce the T2 penetration effect. At the same time of imaging, the influence of intralesional microcirculation on DWI is reduced. LI [21] selected the b value of 1000 s/mm2 in 3.0T MRI for research, and took the ADC value of 0.89 × 10-3 mm2/s as the best diagnostic critical value to differentiate the benign and malignant retropharyngeal lymph nodes in the neck and the sensitivity, specificity and accuracy are 95.7%, 95.1% and 96.5%, respectively. Therefore, in this study, the b = 1 000 s/mm2 was used for image acquisition, and the ADC value of the lymph nodes in the retropharyngeal group was in the range of (0.789 ± 0.179) × 10-3 mm2/s to (0.819 ± 0.150) × 10-3 mm2/s. Statistical analysis showed that the ADC values of the lymph nodes in the posterolateral group of males in children, adolescents and adults were lower than those in females of the same age group. Compared with the same gender, the ADC values in the adult group were higher than those in the children and adolescents, but the difference was not statistically significant. In previous lymph node DWI studies, the effect of gender on ADC was not analyzed, and the ADC value varied by 14% between scanners and was more variable when measured in small volumes such as lymph nodes [22]. In the study of Ricardo Donners [23], no significant age-dependent ADC value of lymph nodes was found. Consistent with the results of this study.
We believe that different b values, locations and areas, patient age, and body temperature influence tissue diffusing capacity. Therefore, determining the value of normal health is also important for benchmarking. Under the study of large sample size, a unified ADC value was established as the best diagnostic cut-off point to help clinical judgment of lymph node properties.