Diagnostic Performance of Diffusion-Weighted Imaging for Multiple Hilar and Mediastinal Lymph Nodes with FDG Accumulation

Accurate staging of mediastinal and hilar lymph nodes is a critical factor in determining operability of patients with non-small cell lung cancer (NSCLC). Although several noninvasive procedures including computed tomography (CT) and positron emission tomography (PET) are widely used for diagnosing pulmonary nodules and nodal involvement, mediastinal node staging with 18-fluoro-2-deoxy-glucose (FDG)-PET/CT in coal workers is insufficient due to the high false-positive rates as a result of the presence of pneumoconiosis (Saydam et al., 2012). We have difficulty with assessing multiple hilar and mediastinal lymph nodes (MHMLN) with FDG accumulation in PET-CT. Recently diffusion-weighted magnetic resonance imaging (DWI) has been used to detect the restricted diffusion of water molecules. The principals of DWI exploit the random motion, or so-called Brownian


Introduction
Accurate staging of mediastinal and hilar lymph nodes is a critical factor in determining operability of patients with non-small cell lung cancer (NSCLC).Although several noninvasive procedures including computed tomography (CT) and positron emission tomography (PET) are widely used for diagnosing pulmonary nodules and nodal involvement, mediastinal node staging with 18-fluoro-2-deoxy-glucose (FDG)-PET/CT in coal workers is insufficient due to the high false-positive rates as a result of the presence of pneumoconiosis (Saydam et al., 2012).We have difficulty with assessing multiple hilar and mediastinal lymph nodes (MHMLN) with FDG accumulation in PET-CT.

Diagnostic Performance of Diffusion -Weighted Imaging for Multiple Hilar and Mediastinal Lymph Nodes with FDG Accumulation
Katsuo Usuda 1 *, Sumiko Maeda 1 , Nozomu Motono 1 , Masakatsu Ueno 1 , Makoto Tanaka 1 , Yuichiro Machida 1 , Munetaka Matoba 2 , Naoto Watanabe 2 , Hisao Tonami 2 , Yoshimichi Ueda 3 , Motoyasu Sagawa 1 movement, of water molecules in biologic tissue (Le Bihan et al., 1988).The primary application of DWI has been in brain imaging, mainly for the evaluation of acute ischemic stroke, intracranial tumors and demyelinating diseases (Tien et al., 1994;Sorensen et al., 1996;Schaefer et al., 2000).DWI makes it possible to detect malignant tumors based on the difference in the diffusion of water molecules among tissues.Diffusion of water molecules in malignant tumors is usually restricted compared to that in normal tissue, resulting in a decreased apparent diffusion coefficient (ADC) value (Szafer et al., 1995;Nasu et al., 2004;Takahara et al., 2004).In DWI, blood flow showing high diffusion and normal tissue with fat depression are likely to be undetectable, but cancer tissue with low Brownian motion of water molecules shows the restricted diffusion of water molecules, and it is likely to be detectable.In terms of the diagnosis of lymph nodes, MRI was reported to be helpful in distinguishing between progressive massive fibrosis of pneumoconiosis and lung cancer (Chong et al., 2006).DWI has advantages over PET-CT in diagnosing malignant from benign lymph nodes of lung cancers (Usuda et al., 2013).Although another report showed that the etiology of 100 PET-CT positive lymph node stations were metastasis in 14, anthracosis in 40, reactive in 39, granulomatous in 4, and silicosis in 3 patients (Koksal et al., 2013), it is uncertain whether DWI is useful in the assessment of patients who had MHMLN with FDG accumulation in PET-CT.If DWI is shown to have higher diagnostic efficacy than that by PET-CT for MHMLN with FDG accumulation, DWI may become a more useful examination tool in the assessment of MHMLN with FDG accumulation.The purpose of this prospective clinical study was to compare DWI and PET-CT in assessing MHMLN with FDG accumulation.

Eligibility
The study protocol for examining DWI and PET-CT in patients with thoracic diseases was approved by the Institutional Review Board in Kanazawa Medical University (the approval number: No.189).Patients who had metal or pacemakers in their body or tattoos on their skin were excluded because of contraindication in MRI examinations.Written informed consent to participate in this study was obtained from all patients after discussing the risks and benefits of the study with their surgeons.

Patients
The clinical study started in January 2010.All patients who had three or more stations of MHMLN with SUVmax of 3 or more in PET-CT were included in the study.They underwent MRI including DWI within two weeks of PET-CT.Most MHMLN were pathologically diagnosed by resection, or by biopsy under CT or VATS, and the others were judged as benign or malignant by radiological follow-up study.

Positron emission tomography -computed tomography (PET-CT)
PET-CT scanning was performed with a dedicated PET camera (Biograph Sensation 16; Siemens Erlangen Germany) before surgery.All patients fasted for 6 hours before scanning.The dose of 18F-FDG administered was 3.7MBq/Kg of body weight.After a 60-min uptake period, an emission scan was acquired for 3 min per bed position and a whole-body scan ( from head to pelvis) was performed.After image reconstruction, a 2-dimensional (2D) round region of interest (ROI) was drawn on a slice after visual detection of the highest count on the fused CT image by the radiologist (N.W.) with 30 years of radioisotope scintigraphy and PET-CT experience who was unaware of the patients' clinical data.For the lesions with negative or faintly positive PET findings, the ROI was drawn on the fusion image with the corresponding CT.From those ROI, the maximum standardized uptake value (SUVmax) was calculated.The radiologist (N.W.) and one pulmonologist (K.U.) with 29 years of experience evaluated the FDG-PET data.A consensus was reached if there were any differences of opinion.The optimal cutoff value (OCV) of SUVmax for diagnosing malignancy in FDG-PET was determined to be 4.45 using receiver operating characteristics curve as previously reported (Usuda et al., 2013).Hilar and mediastinal lymph nodes with SUVmax of the same or more than the OCV was defined as positive.Hilar and mediastinal lymph nodes with SUVmax less than the OCV, or those that could not be detected on FDG-PET were defined as negative.

Magnetic resonance imaging (MRI)
All MR images were obtained with a 1.5 T superconducting magnetic scanner (Magnetom Avanto; Siemens, Erlangen, Germany) with two anterior sixchannel body phased-array coils and two posterior spinal clusters (six-channels each).The conventional MR images consisted of a coronal T1-weighted spin-echo sequence and coronal and axial T2-weighted fast spinecho sequences.DWIs using a single-shot echo-planar technique were performed with slice thickness of 6mm under SPAIR (spectral attenuated inversion recovery) with respiratory triggered scan with the following parameter: TR/TE/flip angle, 3000-4500/65/90; diffusion gradient encoding in three orthogonal directions; b value = 0 and 800 s/mm 2 ; field of view, 350 mm; matrix size, 128x128.After image reconstruction, a 2-dimensional (2D) round or elliptical region of interest (ROI) was drawn on the lesion which was detected visually on the ADC map with reference to T2-weighted or CT image by the radiologist (H.T.) with 39 years of MRI experience who was unaware of the patients' clinical data.Areas with necrosis were excluded from the ADC measurement.The procedure was repeated three times and the minimum ADC value was obtained.The radiologist (H.T.) and one pulmonologist (K.U.) with 29 years of experience evaluated the MRI data.A consensus was reached if there were any differences of opinion.The OCV of ADC for diagnosing malignancy in DWI was determined to be 1.70×10 -3 mm 2 /sec using receiver operating characteristics curve as previously reported (Usuda et al., 2013).Hilar and mediastinal lymph nodes with ADC value of the same or less than the OCV was defined as positive.Hilar and mediastinal lymph nodes with ADC value of more than the OCV or those that could not be detected on DWI were defined as negative.

Statistical analysis
Statistical analysis was performed using StatView for Windows (Version 5.0; SAS Institute Inc. Cary, NC, USA).The data are expressed as the mean ± standard deviation.A two-tailed Student t test was used for comparison of ADC values or SUVmax in several pathological factors.The sensitivity, specificity, and accuracy of DWI versus PET-CT for mediastinal tumor were compared by using McNemar test.A P value of <0.05 was considered statistically significant.
Diagnostic ability by PET-CT for MHMLN with SUVmax of 3 or more are presented in Table 2.The sensitivity by PET-CT was 86% (6/7), the specificity by PET-CT was 31 % (5/16) and the accuracy by PET-CT was 48% (11/23).Diagnostic ability by DWI for MHMLN with SUVmax of 3 or more are presented in Table 3.The sensitivity by DWI was 71% (5/7), the specificity by DWI was 100 % (16/16) and the accuracy by DWI was 91% (21/23).
Concerning sensitivities between DWI and PET-CT for 7 patients with malignant MHMLN with SUVmax of 3 or more, 4 (57%) were true-positive (TP) with DWI and PET-CT, 2 (29%) were TP with PET-CT but falsenegative (FN) with DWI and 1 (14%) were TP with DWI but FN with PET-CT.The sensitivity (86%) by PET-CT was not significantly higher than that (71%) by DWI for 7 patients with MHMLN with SUVmax of 3 or more in the McNemar test (P=1.0)(Table 4).
Concerning specificities between DWI and PET-CT for 16 patients with benign MHMLN with SUVmax of 3 or more, 5 (31%) were TN with DWI and PET-CT, 11 (69%) were TN with DWI but FP with PET-CT (Table 5).The specificity (100%) by DWI was significantly higher than that (31%) for 16 patients with benign MHMLN with ×10 -3 mm 2 /sec) of malignant lymph nodes was significantly lower than that (1.99±0.24×10 -3 mm 2 /sec) of benign lymph nodes (P=0.0437).(B) The SUVmax (10.0±7.34) of malignant lymph nodes was not significantly higher than that (6.38±4.31) of benign lymph nodes (P=0.15)Concerning accuracies between DWI and PET-CT for all 23 patients with MHMLN with SUVmax of 3 or more, 9 (39%) were correct with DWI and PET-CT, 12 (52%) were correct with DWI but incorrect with PET-CT, 2 (9%) were correct with PET-CT but incorrect with DWI (Table 6).The accuracy (91%) by DWI was significantly higher than that (48%) by PET-CT for all 23 patients with MHMLN with SUVmax of 3 or more in the McNemar test (P=0.0129).

Discussion
Although PET-CT is widely accepted as the imaging modality of choice in tumor staging, false positive results of hilar and mediastinal lymph nodes by PET-CT were reported to be due to pneumoconiosis, silicosis, pulmonary tuberculosis, and sarcoidosis (Jain et al., 2011;Lin et al., 2012;Usuda et al., 2013;Maturu et al., 2014).In PET-CT, care should be taken in lymph node staging for patients who have other pulmonary complications, including interstitial pneumonitis, previous pulmonary tuberculosis and (Konishi et al., 2003).For early stage lung cancer, the false positive rates by PET-CT scan in N1 and N2 nodes were reported to be 70% and 78%, respectively, primarily due to inflammatory process (anthracosis as the leading cause) (Lin et al., 2012).Silicotic lesions of hilar and mediastinal lymph nodes were reported to have moderate accumulation of FDG in PET-CT, and were likely to be judged as false-positive (Usuda et al., 2013).PET had some limitations for evaluating sarcoidosis (Jain et al., 2011).The SUVmax of sarcoidosis were usually high: 12.4 (early), and 16.2 (delayed) (Maturu et al., 2014).PET gives false-negative results for well-differentiated pulmonary adenocarcinoma (Higashi et al., 1998;Cheran et al., 2004;Usuda et al., 2014), and false-positive results for inflammatory nodules (Goo et al., 2000;Nomori et al., 2004).
Recently, there have been advancements in MR gradient technology.In this study, specificity and accuracy by DWI for MHMLN with SUVmax of 3 or more were shown to be significantly higher than that by PET-CT.In cervical lymph nodes, the mean ADC value of malignant nodes was significantly lower than that of benign nodes (Abdel et al., 2006;Perrone et al., 2011).DWI was reported to be a new promising technique for differentiating inflammatory from metastatic lymph nodes on animal model (Xue et al., 2008).Some reports indicate the superiority of DWI in comparison with PET-CT.First, DWI was reported to be superior to PET-CT in detection of primary lesions and nodal assessment of non-small cell lung cancers (Usuda et al., 2011).DWI with ADC value and signal intensity can be useful in the differentiation of malignant and benign mediastinal lymph nodes (Kosucu et al., 2009).Nomori et al., 2008 reported that the accuracy of N staging in the 88 patients was 0.89 with DWI, which was significantly higher than the value of 0.78 obtained with PET-CT, because of less overstaging in the former.The superiority of DWI can be explained not only by DWI giving fewer false-positive results for N staging of non-small cell lung cancer compared with PET-CT (Nomori et al., 2008), but DWI also gave fewer falsenegative results for N staging of non-small cell lung cancer compared with PET-CT (Usuda et al., 2011).PET-CT is likely to show false-positive results when lymph nodes contain inflammation, and is likely to show false-negative results when the lymph nodes contain a small amount of cancer cells.Second, DWI has easier accessibility, and is relatively cheaper compared with PET-CT.The number of hospitals equipped with PET-CT is limited because of the difficulty in handling the radioisotope of 18F-FDG, but MRI with DWI is usually available in hospitals nowadays.In addition, an MRI examination carries no risk of radiation exposure, whereas a PET-CT examination carries some risk of radiation exposure.In a DWI examination patients do not have to fast before the examination, do not need exogenous contrast medium, and less time is required for the examination.
There are limitations of DWI.The evaluation of several areas such as brain, spinal cord, spleen, kidney, and bone marrow may not be useful using DWI because an impeded diffusion can also be seen in these normal structures (Kwee et al., 2010).Furthermore, in interpretation of DWI, it should be kept in mind that a number of benign lesions can exhibit restricted diffusion on images, thus mimicking malignant lesions (Humphries et al., 2007;Feuerlein et al., 2009).Abscesses and thrombi are believed to impede the diffusivity of water molecules because of their hyperviscous nature (Desprechins et al., 1999;Kwee et al., 2010).
DWI is shown to have higher diagnostic efficacy than that by PET-CT for MHMLN with FDG accumulation, and DWI may become a more useful examination tool in the assessment of MHMLN with FDG accumulation.DWI is a new imaging modality, adding diagnostic performance to PET-CT.

Figure 2 .
Figure 2. 66 y.o.Male with Pulmonary Squamous Cell Carcinoma, but Without Lymph Node Metastasis.The patient was diagnosed as n2 disease by PET-CT, but n0 disease by DWI.DWI showed lymph node station #4R as negative (d), and lymph node station #7 as negative (e).The lymph nodes were diagnosed as pneumoconioisis pathologically

Figure 3 .
Figure 3. 74 y.o.Male with Pulmonary Adenocarcinoma, but Without Lymph node Metastasis.The patient was diagnosed correctly by DWI, not by PET-CT.The patient was diagnosed as n2 disease by PET-CT, but n0 disease by DWI.Lymph node station #4L was diagnosed positive by PET-CT (b), but negative by DWI (e).Lymph node station #11 was diagnosed positive by PET-CT (c), but negative by DWI (f).The lymph node stations were diagnosed as sarcoidosis pathologically