Lisette van Dam

Chapter 2 26 with the upper limit of the 95%CI well below the predefined 6.5% safety threshold, as was the failure rate and upper limit of the confidence interval in all patients with a MRDTI negative for DVT. MRDTI is a non-invasive technique that can visualize the metabolism of a fresh thrombus. When red bloods cells are trapped within a thrombus, hemoglobin within the red blood cells undergo oxidative denaturation to methemoglobin, which will cause shortening of T1-signal and this results in a high signal on a T1- weighted sequence. 11 Before the DTI signal will become positive, methemoglobin must be formed reliably within an acute clot. Profuse acquired or congenital methemoglobinemia will therefore not result in a positive DTI signal. 11 MRDTI was first described to diagnose a first episode of DVT, an observation that was confirmed inseveral cohorts. 11-13,15 Histological proof of theabilityofMRDTI todetect acute thrombosis has been provided in the setting of chronic thromboembolic pulmonary hypertension: the location of a positive MRDTI signal in the pulmonary artery correlated 1:1 with fresh clots found in the surgical specimens of pulmonary artery endarterectomy performed one day after the MRDTI. 22 The main advantage of the MRDTI technique in the setting of suspected recurrent ipsilateral DVT is the clear distinction between acute and chronic thrombosis, leading to a large reduction of inconclusive diagnoses from 30% in a previous cohort (mainly due to the poor interobserver agreement of the thrombus diameter measurement by CUS and the unavailability of reference CUS examinations) to less than 1% (2/305) in the present study. 3 The interobserver agreement of the MRDTI in our study was excellent (kappa statistic 0.91). This finding is consistent with the interrater agreement observed in a prospective study that evaluated the diagnostic accuracy of MRDTI for distinguishing acute recurrent ipsilateral DVT from chronic thrombi in leg veins (kappa statistic 0.98). 13 Moreover, MRDTI proved to be a feasible and reproducible diagnostic test across international academic and non-academic study sites. An important methodological aspect of our study requires comment. From August 2015 onwards, patients with suspected acute recurrent ipsilateral DVT while on therapeutic anticoagulant treatment were allowed in the study as they were found to represent a high proportion of the screened study population. Canadian researchers have recently reported that 15%of VTE patients in a largemanagement study were subjected to testing for suspected recurrence within the first year of treatment, underlining our experience. 23 In the setting of our study, many of the clinical presentations of recurrent DVT during anticoagulant treatment could likely