Purpose: The aim of this study was to use 7 T ex vivo magnetic resonance imaging (MRI) scans to determine the size of lymph nodes (LNs) in total mesorectal excision (TME) specimens and to increase the pathological yield of LNs with MR-guided pathology.
Materials and methods: Twenty-two fixated TME specimens containing adenocarcinoma were scanned on a 7 T preclinical MRI system with a T1-weighted 3-dimensional gradient echo sequence with frequency-selective lipid excitation (repetition time/echo time, 15/3 milliseconds; resolution, 0.293 mm) and a water-excited 3-dimensional multigradient echo (repetition time, 30 milliseconds; computed echo time, 6.2 milliseconds; resolution, 0.293 mm) pulse sequence.The first series of 11 TME specimens (S1) revealed the number and size of LNs on both ex vivo MRI and histopathology. The second series of 11 TME specimens (S2) was used to perform MR-guided pathology. The number, size, and percentages of yielded LNs of S1 and S2 were compared.
Results: In all specimens (22/22), a median number of 34 LNs (interquartile range, 26-34) was revealed on ex vivo MRI compared with 14 LNs (interquartile range, 7.5-21.5) on histopathology (P = 0.003). Mean size of all LNs did not differ between the 2 series (ex vivo MRI: 2.4 vs 2.5 mm, P = 0.267; pathology: 3.6 vs 3.5 mm, P = 0.653). The median percentages of harvested LNs compared with nodes visible on ex vivo MRI per specimen for both series were not significantly different (40% vs 43%, P = 0.718). By using a size threshold of greater than 2 mm, the percentage improved to 71% (S1) and to 78% (S2, P = 0.895). The median number of harvested LNs per specimen did not increase by performing MR-guided pathology (S1, 14 LNs; S2, 20 LNs; P = 0.532).
Conclusions: Ex vivo MRI visualizes more LNs than (MR-guided) pathology is able to harvest. Current pathological examination was not further improved by MR guidance. The majority of LNs or LN-like structures visible on ex vivo MRI below 2 mm in size remain unexplained, which warrants a 3-dimensional approach for pathological reconstruction of specimens.