Currently, the most versatile technique for lymphatic imaging is DCMRL [13,14,15]. It allows for visualization of the central lymphatic system with high spatial and time resolution and therefore, enables the assessment of a wide spectrum of lymphatic conditions from simple lymphatic leakages to complex lymphatic flow abnormalities and malformations [1, 7, 12, 16,17,18,19,20,21,22]. As contrast-agent application for DCMRL is performed through lymph node accesses after ultrasound-guided cannulation, a correct and stable needle position within a lymph node is of paramount importance to ensure adequate enhancement of the lymphatic system. This poses a technical and logistical challenge, especially in children and infants in whom lymph nodes usually are small and DCMRL often has to be performed under sedation or general anesthesia [7,8,9] limiting possible transfers in and out of the MR-scanner for ultrasound-guided needle position revision. The possible number of transfers is limited in these cases, as the required anesthesia unit and accompanying tubing must be transported as well, avoiding any material dislocation thereby increasing logistical challenges and time consumption for every transfer.

As the initial step for successful MRL, groin lymph nodes for needle placement have to be identified on ultrasound. This can be performed as a separate ultrasound examination in the days or weeks prior to a scheduled DCMRL examination or during the actual DCMRL examination [9, 12]. However, there is, so far, no general consensus on a minimum lymph node size suitable for puncture. In a recent small series in pediatric patients [10], a minimum short lymph node diameter of 2 mm was used while the lowest lymph node diameter in a previous work of Wagenpfeil and colleagues was 1 mm [9]. In the present cohort, no minimum diameter was defined and therefore the mean diameter was as low as 2.5 mm with over 25% of patients presenting with only 1 mm lymph nodes. The technical and clinical success of DCMRL was still nearly perfect despite these very small access lymph nodes. As our experience demonstrates that DCMRL is successful in the vast majority of patients even without a predetermined minimum node diameter, sonographic evaluation in advance of the actual DCMRL is not strictly necessary, making logistics easier. Furthermore, over 25% of patients in the present study would otherwise have been excluded from DCMRL when employing the minimum lymph node diameter cut-off suggested by Fung et al. It needs to be noted, however, that our institution is a tertiary reference center for lymphatic imaging and interventions and that the interventionalist has over 13 years of experience with these technique.

Apart from the anatomical conditions in the groins of the patients, the technique of confirming accurate needle placement has recently been a matter of some discussion [11, 23]. Initial reports described confirmation by injection of water-soluble X-ray contrast agent under fluoroscopy [1, 3]. However, as this technique requires a combined XMR-suite, its application is limited. Nadolski and colleagues subsequently published their initial experiences with needle position validation by CEUS in adults, obviating the need for an XMR suite [8].

Fung et al demonstrated the feasibility of this CEUS approach also in pediatric patients [10]. They performed needle placement in groin lymph nodes in seven children with bilateral inguinal nodal access (14 punctured nodes) using a comparable puncture technique as in the present study with the patients also on a detachable MR-table [8, 10]. Included patients had a median age of 13 years (interquartile range 3.5–7.5 years) with attempted punctures of lymph nodes larger than 2 mm in diameter [10]. An ultrasound contrast agent was used to visualize efferent lymphatics as a sign of adequate needle positioning. After ultrasound contrast injection needle repositioning was necessary in 2/14 groins. A cannulation success rate of 12/14 nodes (85.7%) was reported. The subsequent MR contrast injection, was successful in all of these 12 cases [10]. However, although CEUS seems to be a viable tool for needle position confirmation for DCMRL, the question arises as to whether this additional off-label ultrasound contrast application is really necessary—especiallywhen examining children. Although very rare, side effects associated with ultrasound contrast agents, as well as the additional costs for the contrast agent also have to be considered [24].

The largest published series to date on the technical success of nodal DCMRL including 171 punctured lymph nodes in a primarily adult patient cohort demonstrated that the use of only saline solution instead of an ultrasound contrast agent is sufficient for needle position verification [9]. Overall technical success was observed in 169/171 lymph node punctures (98.8%). Primarily venous run-off was observed in only 6/171 lymph nodes (3.5%) on DCMRL and was resolved by minimal needle retraction in the scanner.

In the present study, we have investigated whether ultrasound-guided injection of physiologic saline provides a reliable assessment of correct needle placement in a large pediatric population. The main finding of this study was that lymph node distension is a reliable sign of correct needle placement. In 228/230 cases with lymph node expansion on saline-solution, adequate lymphatic run-off of MRcontrast-agent was demonstrated. In one additional case (1/230), only minimal retraction of the needle was required to achieve adequate lymphatic enhancement on DCMRL.

A varying degree of venous enhancement was observed in 5/230 punctures originating from the injection site, but necessitated needle repositioning in only 1/230 of cases. A possible explanation for venous enhancement may be lymph-venous shunts at the level of the lymph node as described by Kariya et al [25]. However, marked venous enhancement without lymphatic enhancement may also result from a central needle position in the lymph node hilum. Therefore, positioning the needle tip at the corticomedullary junction of the lymph node has been recommended [2, 4]. In contrast to saline injection, an ultrasound contrast agent used for needle position validation may be advantageous in showing primary venous run-off already before MR contrast-agent injection [7, 23]. This was demonstrated by Nadolski et al in 1/28 cases with enhancement of the femoral vein on ultrasound contrast injection [8]. However, primary venous run-off (venous run-off initially occurred before the enhancement of the lymphatic system) was only visible in 1/230 cases in our cohort and could be resolved in this case by simple needle retraction inside the scanner without the need for repeated ultrasound.

Interestingly, the initial report by Nadolski and colleagues on CEUS demonstrated necessary needle repositioning in 6/28 cases (21.4%), pointing to the overall usefulness of the technique as in these cases needle repositioning seems to have been necessary for a successful DCMRL [8]. However, rates of necessary needle repositioning in the report by Wagenpfeil et al in adults, as well as in the present report in pediatric patients both employing only saline injection were considerably lower (4.6% and 0.8%, respectively). A possible explanation may be that CEUS is able to detect smaller leakages of contrast-agent around the lymph node, as well as venous runoff with a higher sensitivity than plain saline injection. This subsequently may prompt repositioning of the needles. However, these repositioning procedures may ultimately not have been necessary for a successful DCMRL as smaller leakages and/or shunts not seen on saline-solution injection do not seem to play a clinically relevant role.

Finally, it is important to note that the application of nodal ultrasound contrast injection may be an interesting tool to assess efferent lymphatic flow originating from the lymph node or to even enable assessment of the lymph-venous junction [10, 23]. However, in our experience, the application of ultrasound contrast medium purely for sonographic evaluation of a proper needle position for DCMRL is not necessary.

This study has several limitations. First, it is important to note that this was a retrospective study on clinical DCMRL examinations with inherent methodological limitations, e.g., partially incomplete clinical documentation of patients’ prior history in the clinical database or availability of only some saved still ultrasound images which prevented the measurement of distended lymph node diameter. Furthermore, as the examinations were conducted in a clinical setting with varying lymph node sizes, no precise definition of acceptable distension of the punctured lymph node on saline injection was employed. Any visible increase in the size of the node on injection without leakage into the surrounding tissue was accepted to demonstrate an intranodal position of the needle tip. Second, the research was conducted at a single medical center, and all lymph node punctures were performed by the same experienced interventional radiologist, somewhat limiting the generalizability of the described success rates. Further investigations into technical and clinical success rates at different centers are certainly warranted. Third, the primary focus of this study was on ensuring correct needle placement and did not address the technique for securely transferring the patient from ultrasound back into the MR-scanner without the risk of needle dislocation. All DCMRLs were conducted using an MR scanner equipped with a detachable table, which allowed for minimal patient movement during the transfer in and out of the scanner. Different approaches might be necessary if the examination table cannot be moved in this manner. Additionally, it is important to acknowledge that lymph node diameter measurements could only be taken from the documented sonographic images, potentially leading to variations in the lymph node axis between different cases. As no additional imaging techniques demonstrate lymphatic run-off from the punctured lymph node other than the MRL itself when employed in this cohort of patients, the observed 2/230 primary technical failures of MR-contrast injection despite visible lymph node distension on ultrasound ultimately remain unclear. In one of these two cases, secondary needle movement was the most likely cause for initial technical failure as slight retraction of the needle led to technical success. In the other case also repeated needle positioning did not lead to lymphatic enhancement, making a lack of lymphatic run-off from the punctured lymph node the most likely cause of failure. Further investigation into lymphatic run-off rates from the punctured lymph node without confounders such as patient movement into an MR-scanner would be interesting in this respect.

In conclusion, using plain saline solution to confirm sonographically placed intranodal needle position for DCMRL is a very reliable and safe technique with a very high success rate of lymphatic enhancement in pediatric patients. The additional off-label application of ultrasound contrast-agent may not be necessary in a clinical setting but further comparative study is needed.