A quantitative meta-analysis was performed on prospective studies, comparing MRI with and without USPIO, against histological diagnosis after surgery or biopsy (Will et al., 2006). The primary endpoints were sensitivity, specificity, area under the receiver operating characteristic (ROC) curve, and diagnostic odds ratio of the imaging technique used. Sensitivity analysis was performed to evaluate the diagnostic precision of USPIO-enhanced MRI for different body regions imaged (head and neck, chest, abdomen and pelvis), studies using a 1.5 T magnet and studies using 1.7 or 2.6 mg/kg doses of ferumoxtran-10.
A total of 19 prospective studies were published between 1994 and 2005 and were used for data analysis. These comprised 3,004 lymph nodes in 631 patients who underwent comparable MR imaging with histological verification as shown in Table 3.1. The overall sensitivity and specificity for MRI with USPIO (17 studies) were 88.1% and 96.2% respectively, with an area of 84.2% under the ROC curve and a diagnostic odds ratio of 123.1. When unenhanced MRI was evaluated, there was a significant reduction in the overall sensitivity and specificity (63% and 92.7% respectively), with an area of 84.2% under the ROC curve and a diagnostic odds ratio of 26.7 (Table 3.2). When analyzed specifically by body region, USPIO-enhanced MRI had a high sensitivity and specificity for lymph node status in the abdomen and pelvis compared with the chest or head and neck. The highest sensitivity and specificity was noted for the two studies, reporting results only for prostate cancer. This may be explained by the fact that the lymph flow in the legs, pelvis and abdomen is more pronounced compared to thorax and head and neck. Similarly, the diagnostic precision of USPIO-enhanced MRI appeared to be better when a 3 T MRI scanner was used at 2.6 mg/kg contrast dose, than when using a 1.5 T field strength (Heeaskkers, 2006).
Analysis by nodal size (without USPIO injection) was reported by two studies, with just one specifically examining lymph nodes of less than 5 mm in diameter (Table 3.2). For lymph nodes between 5 and 10 mm, the sensitivity and specificity were 96.4% and 99.3%, respectively, and for lymph nodes under 5 mm the sensitivity and specificity was reduced to 41.1% and 98.1%, respectively. These results, although less than the sensitivity reached for nodes of a larger diameter, were superior to that obtained without USPIO. Even at 3T MRI has a limited resolution with 1 mm being at the limit of the highest definition. As sagittal cut section thickness is commonly around 3 mm, it is clear that micro-metastases with partial node destruction can be missed.
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