Rationale and Objectives
To investigate the feasibility and performance of 7T magnetic resonance imaging compared to 1.5T imaging to discriminate benign (normal and inflammatory changed) from tumor-bearing lymph nodes in rabbits using ultrasmall particles of iron oxide (USPIO)-based contrast agents.
Materials and Methods
Six New Zealand White rabbits were inoculated with either complete Freund’s adjuvant cell suspension (n = 3) to induce reactively enlarged lymph nodes or with VX2 tumor cells to produce metastatic lymph nodes (n = 3). Image acquisition was performed before and 24 hours after bolus injection of an USPIO contrast agent at 1.5T and afterward at 7T using T1-weighted and T2*-weighted sequences. Sensitivities, specificities, and negative and positive predictive values for the detection of lymph node metastases were calculated for both field strengths with histopathology serving as reference standard. Sizes of lymph nodes with no, inflammatory, and malignant changes were compared using a Mann-Whitney U -test.
Results
All 24 lymph nodes were detected at 1.5T as well as at 7T. At 1.5T, sensitivity amounted to 0.67, while specificity reached a value of 1. At the higher field strength (7T), imaging was able to reach sensitivity and specificity values of 1. No statistical differences were detected concerning lymph node sizes.
Conclusions
Magnetic resonance lymphography with USPIO contrast agents allows for differentiation of normal and reactively enlarged lymph nodes compared to metastatic nodes. First experiments at 7T show promising results compared to 1.5T, which have to be evaluated in further trials.
The detection of lymph node metastases is part of nearly any cancer staging as lymph node involvement has a poor prognosis for many tumor entities and can directly impact a patient’s management .
Computed tomography (CT) and magnetic resonance imaging (MRI) are established tools for staging malignant tumors. A short-axis diameter of >10 mm has been the major malignancy marker for lymph node evaluation of many cancer types (e.g., uterine cervical and other pelvic cancer). On the basis of this technique, false-positive results may occur for reactively enlarged lymph nodes without metastatic infiltration as well as false-negative results if a lymph node shows (micro)metastasis (focal small tumor infiltration <2 mm) in a nonenlarged node . Therefore, new MRI techniques including diffusion-weighted MRI, high-field MRI, and contrast-enhanced lymphography have been evaluated in the past years.
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Materials and methods
Animal Model
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MRI
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Table 1
Imaging Parameters for T1-Weighted and T2*-Weighted Sequences at 1.5T and 7T
Repetition Time (ms) Echo Time (ms) Flip Angle (°) Field of View (mm) Voxel Size (mm 3 ) Bandwidth (Hz/px) Time of acquisition (min:s) T2*-weighted 7T 1000 13 30 149 × 180 0.47 × 0.47 × 1.5
interpolated
0.23 × 0.23 × 1.5 435 2:30 T2*-weighted 1.5T 28 15 30 192 × 256 1.12 × 1 × 1.2
interpolated
0.56 × 0.5 × 1.2 500 2:20 T1-weighted 7T 6.12 2.63 10 157 × 180 0.4 × 0.35 × 0.5
interpolated
0.4 × 0.35 × 0.4 315 2:56 T1-weighted 1.5T 553 12 90 250 × 250 0.98 × 0.98 × 1.2
interpolated
0.49 × 0.49 × 1.2 130 2:21
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Histopathological Analysis
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MRI Evaluation
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Results
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Histopathological Analysis
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MRL Analysis
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![Figure 1, A healthy lymph node ( arrow ) in the right popliteal fossa shown at 1.5T T2*-weighted imaging (multiple echo data image combination [MEDIC]; a) and T1-weighted imaging (b) 24 hours after contrast as well as on 7T T2*-weighted imaging (MEDIC, c) and T1-weighted imaging (d) . Note the different image aspects of the sequences due to different field strength and the high iron uptake of the nonaffected lymph node, which even turns it dark on T1-weighted imaging.](https://storage.googleapis.com/dl.dentistrykey.com/clinical/DiscriminationofBenignandMalignantLymphNodesat70TComparedto15TMagneticResonanceImagingUsingUltrasmallParticlesofIronOxide/0_1s20S1076633213004121.jpg)
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Table 2
Sensitivity, Specificity, and Positive and Negative Predictive Values for 1.5T and 7T
1.5T 7T Sensitivity 0.67 1 Specificity 1 1 Positive predictive value 1 1 Negative predictive value 0.9 1
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Discussion
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