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1 All authors: Department of Radiology, Division of Body Imaging, Boston Medical Center, 88 E Newton St., Atrium 2, Boston, MA 02118.
Received February 2, 2004;
accepted after revision April 20, 2004.
Address correspondence to B. C. Lucey.
Abstract
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MATERIALS AND METHODS. We examined the CT scans of 132 consecutive patients (84 men and 36 women; age range, 12-90 years; mean age, 43 years) who presented to the emergency department after experiencing blunt abdominal trauma. Twelve patients were excluded because they had disease processes known to be associated with lymphadenopathy. All imaging was performed using 3.2-mm collimation on MDCT scanners with IV contrast material. Two radiologists evaluated the images by consensus and recorded the presence of mesenteric lymph nodes greater than 3 mm in the short axis. Lymph node size, number, and location (central, peripheral, or right lower quadrant) were documented. All studies were reviewed on a PACS workstation.
RESULTS. Of the 120 patients with otherwise normal CT scans, 47 had mesenteric lymph nodes greater than 3 mm. Of these 47 patients, 22 (47%) had five or more lymph nodes detected. Twenty-five (53%) of the 47 patients had four or fewer nodes. The mean size of the largest nodes was 4.8 mm (range, 3-9 mm), and the mean size of the nodes found per patient was 3.6 mm (range, 3-6 mm). These nodes were identified only at the mesenteric root in 32 patients (68%), only in the mesenteric periphery in eight patients (17%), and only in the right lower quadrant in five patients (11%). Nodes were identified in more than one location in two patients (4%).
CONCLUSION. Incidental finding of mesenteric lymph nodes is common, reflecting more widespread use of thin-collimation MDCT and PACS workstations. In general, these nodes are small, measuring less than 5 mm. Such nodes when found in an otherwise healthy population are clinically insignificant and require no further imaging.
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All CT examinations were performed using a 4-MDCT scanner (MX8000, Philips Medical Systems) with a slice collimation of 3.2 mm, a reconstruction interval of 3 mm, kVp of 140, and mA of 200-250. All imaging was performed after the IV administration of 100 mL of nonionic iodinated contrast material, iohexol 320 mg I/mL (Optiray, Mallinckrodt Imaging) injected at a rate of 3 mL/sec using a power injector via a 20-gauge catheter ideally sited in the antecubital vein. Imaging was performed during the portal venous phase with a delay of 60 sec used in all cases. In accordance with our departmental protocol for imaging blunt abdominal trauma cases, the patients were not given oral contrast material. Two radiologists in consensus using PACS computer workstations reviewed all MDCT scans. The presence of any mesenteric lymph nodes greater than 3 mm as measured electronically in the short-axis diameter was recorded. We chose 3 mm as the cutoff size because we believe that lymph nodes of this size or larger can be identified with certainty and that lymph nodes smaller than 3 mm may not be reliably defined.
Lymph nodes were identified as well-defined round or oval lesions of soft-tissue density that were clearly distinct structures, separate from vessels and bowel. By scrolling through the images, one could identify vessels as tubular structures and trace them back to larger vessels. Loops of small bowel also could easily be distinguished from the lymph nodes by scrolling through the images and connecting the loops both proximally and distally. We recorded the number of the lymph nodes and noted their location as being one of three sites—the mesenteric root, the peripheral mesentery, and the mesentery of the right lower quadrant. In cases in which lymph nodes were detected in more than one location, their presence was documented in all detectable locations. This division of locations was arbitrary, and its use in this study was based on our experience in identifying nodes, which tend to cluster at the mesenteric root or in the right lower quadrant. On occasion, lymph nodes appear to be scattered throughout the periphery of the mesentery. The nodes in these locations were therefore recorded separately. We defined the mesenteric root as the area of the mesentery closest to the superior mesenteric artery and its first-order branches and the mesenteric periphery as the mesentery supplied by the distal branches of the superior mesenteric artery. The mesentery of the right lower quadrant was defined as that area adjacent to the terminal ileum, cecum, and appendix. The short-axis diameter of the largest node in each location was measured electronically and recorded. If multiple nodes were detected at one location, the mean short-axis measurement of the lymph nodes was also obtained electronically and recorded. All measurements were recorded in a computer database.
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Only two (4%) of the 47 patients had lymph nodes detected at more than one site. One patient had nodes detected at both the mesenteric root and the right lower quadrant. The second patient had nodes detected at the mesenteric root and scattered throughout the peripheral mesentery.
Twenty-two (47%) of the 47 patients had five or more lymph nodes detected at one or more locations in the mesentery. The remaining 25 (53%) had four or fewer lymph nodes detected. All five patients with right lower quadrant nodes had four or fewer nodes detected. Six (75%) of the eight patients with nodes scattered through the peripheral mesentery had four or fewer nodes detected. Fourteen (44%) of the 32 patients with lymph nodes at the mesenteric root had four or fewer nodes detected (Table 1).
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The diameter of the largest single lymph node present in each patient ranged from 3 to 9 mm, with a mean diameter of 4.6 mm. The mean diameter of the largest node in patients with mesenteric root nodes only was 4.8 mm. The mean diameter of the largest node in patients with peripheral mesenteric nodes only was 3.75 mm. The mean largest diameter of right lower quadrant nodes was 4.0 mm (Table 2).
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The mean diameter of all the nodes detected in each patient ranged from 3 to 6 mm, with a mean diameter of 3.6 mm. The mean diameter of the lymph nodes at the mesenteric root was 3.75 mm. The mean diameter of the peripheral mesenteric nodes was 3.0 mm. The mean diameter of the right lower quadrant nodes was 3.4 mm. The number of patients with lymph nodes identified is detailed in Tables 3 and 4. Table 3 provides the short-axis diameter of the largest lymph node identified in each patient. Table 4 provides the mean short-axis diameter of all the mesenteric lymph nodes identified.
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The most obvious reason that we see these nodes in current clinical practice is the use of MDCT scanners. The thin collimation possible with MDCT allows improved spatial resolution for detecting and discriminating between small objects. Therefore, less volume averaging occurs between the small lymph nodes and other structures within the mesentery, particularly the small bowel and vessels. Among the other factors responsible for the more frequent visualization of mesenteric lymph nodes is routine use of PACS computer workstations for reviewing CT studies by scrolling through images. By scrolling, one may more clearly distinguish lymph nodes from vessels, because vessels can be traced as tubular structures extending over many images. Lymph nodes stand out against the mesenteric fat as tiny, often rounded, soft-tissue densities that appear and disappear over one or two images. Faster scanning times and techniques such as bolus tracking for administering IV contrast material also help to better opacify the mesenteric vessels, allowing easier detection of lymph nodes.
Most reports defining the size criteria for normal lymph nodes were written in the era before MDCT and PACS [1-7]. Many of these reports made no reference to lymph nodes at the mesenteric root or to nodes scattered throughout the mesentery. Indeed, one of the earlier reports of CT of the normal mesentery stated that normal lymph nodes were not routinely identified in the mesentery [8]. Therefore, unlike many areas of radiology, no standardized reference point has been determined for the size of normal mesenteric nodes.
More recent reports using MDCT have described the presence of lymph nodes within the mesentery seen with coexistent inflammatory, infectious, or malignant processes [9-11]. The size of these nodes has been reported to range from 5 to 20 mm. It is clear that smaller lymph nodes are detectable using MDCT.
Tumors involving lymph nodes as small as 5 mm in the short-axis diameter have been detected on MDCT in patients with proven malignancy. This is alarming, given that many studies defining the size criteria for normal abdominal lymph nodes have suggested that normal nodes may be as large as 9 mm in the upper and 11 mm in the lower paraaortic regions [5]. Because cross-sectional imaging is now performed with increasing frequency, mesenteric lymph nodes that are smaller than these cutoff points are routinely identified.
Having detected mesenteric lymph nodes, we must decide what to do with them. Some radiologists suggest obtaining a follow-up MDCT scan to ensure that these nodes are not the earliest manifestation of lymphoma or metastatic disease from an occult primary neoplasm. Even if these lymph nodes are thought to result from an infectious or inflammatory process, obtaining a follow-up MDCT scan is sometimes suggested to ensure full resolution. No appropriate time interval has been recommended for this follow-up MDCT scan. Some have suggested obtaining a follow-up scan several weeks after the initial examination in patients in whom infection or inflammation is believed to be the cause of the finding. This approach will not allay fears of underlying malignancy should these nodes persist on the subsequent scan. Others have suggested longer follow-up intervals, but no consensus is available. Although we did not obtain any formal follow-up data to determine whether the MDCT scans of our study population were truly normal, we did search the hospital database records in all cases. In no patient was there evidence, either by hospital visit or on radiologic imaging, to suggest the interval development of disease at 1-year follow-up. The possibility always exists that patients went to other institutions for further medical care. We acknowledge that this is a limitation of our study; however, we believe that it is reasonable to take this trauma population as representative of the healthy population.
We have found that lymph nodes up to 4.6 mm in the maximum short-axis diameter and 3.6 mm in the mean size are frequently found in the mesentery of the healthy population. These nodes are a normal finding and require no follow-up imaging. This approach results in fewer unnecessary MDCT scans and allows patients to avoid unnecessary radiation exposure.
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