TRANCE is a non-contrast-enhanced MR technique that was first described by Wedeen in 1985[13]. TRANCE-MRI has been applied in cranial neurologic diseases and arterial diseases; however, few applications of this technique in venous pathology have been found [14-18]. The principle of the TRANCE-MRI technique is that different blood flow velocities will have different signal intensities on the TSE sequence. It can display ultra-clear and background-removed blood vessel images, including arteriography along, venography along, or background-removed arteries and veins. According to our initial experience performing TRANCE-MRI, varicose veins and their territories can be clearly displayed, and venous thrombosis and compression can be found and distinguished, which may be helpful for pre-intervention assessment and planning. Presentation of only the venous structure without the accompanying arterial structure is difficult to achieve on contrast-enhanced MRI or CT because the proper acquisition time is short and variable. This study highlights that TRANCE-MRI can be used as an alternative and objective tool for assessing lower extremity diseases, especially suspected venous pathology.
Doppler ultrasonography is considered the first-line tool for evaluating lower limb swelling. In this present study, inter-rater agreement for DVT of the thigh between the ultrasonography (gold standard) and TRANCE-MRI results was substantial agreement. The sensitivity, specificity and accuracy of TRANCE-MRI were 85.7%, 88.9% and 88%, respectively. Therefore, we believe that TRANCE-MRI can be used as an alternative and objective tool for assessing lower extremity diseases, especially suspected venous pathology. Furthermore, in ultrasonography-negative cases, TRANCE-MRI could detect further cases of DVT, venous compression, vena cava anomaly, occult PAD and occluded bypass grafts. Compared with TRANCE-MRI, ultrasonography played a relatively small role in assessing varicose veins of the lower extremities and deep veins of the pelvis and abdomen. We still consider that ultrasound should be used preferentially when assessing venous lesions in the lower extremities because it is non-invasive and cost-effective. If a patient has an abdominal pelvic venous problem or complicated varicose veins, non-contrast-enhanced MRI techniques, such as TRANCE, may be helpful for pre-intervention assessment and planning.
In the present study, we designed a complete MR protocol (total acquisition time, 60 minutes) for imaging acquisition of all (infra-diaphragmatic) lower extremity arteries (MRA; acquisition time, 35 minutes) and veins (MRV; acquisition time, 25 minutes) to fully explore its clinical utility and potential diagnostic value. Thus, this protocol is not suitable for critical and irritable patients and should be modified to reduce the imaging acquisition time in selective patients. In this study, the majority of cases (92%, 23/25) were attributed to venous disease only, and the MRA results were negative. Therefore, we recommend performing a TRANCE-MRV protocol (acquisition time, 25 minutes) instead of the full protocol (MRV+MRA) in the clinical setting in patients with venous scenarios.
In this study, we did not specifically describe how to distinguish between acute and chronic thromboses. Distinguishing acute from chronic DVT is a potential advantage of MRI, with irregular wall thickening in the presence of collaterals and a diminutive lumen suggestive of chronic DVT. Our MRI protocol provides coronal and axial images, as well as 3D MRA and MRV images. We used the original unremoved background image to examine possible tumours or other causes of compression for all vascular lesions (Figure 4). TRANCE-MRV showed that many subjects had equivocal interruption of the left common iliac vein but no venous thrombosis, collateral vessels or related symptoms. This may be because the left common iliac vein is located between the right common iliac artery and the spine, which is an anatomically and relatively narrow location (Figure 5).
Several advantages of TRANCE-MRI application in venous pathology in the lower extremities exist. First, TRANCE-MRI provides not only images of the arteries and veins in the lower extremities but also information on the pelvis and abdomen, which is valuable in patients with a venous scenario of DVT. DVT may be mistaken as external compression of the pelvic vessels. Moreover, it is notorious as a sign of occult malignancies. Among the 11 patients with a venous scenario of DVT, four of them (36.4%) had no DVT and the symptoms were attributed to malignancy, external compression by degenerated hip prosthesis, external compression by knee effusion, and congenital anomaly. Second, the thrombi and collateral veins can be clearly outlined, including deep femoral vein that might be difficult to detect by ultrasonography. This may be helpful in catheter-based thrombolytic therapy and rescue therapy in recurrent VV after truncal ablations of GSV. Finally, because TRANCE-MRI has no radiation and does not use contrast media, it is safe for patients with impaired renal function.
We did learn of some drawbacks to TRANCE-MRI according to this study. First, TRANCE-MRI of the venous system may cause false-positive results in the left iliac vessels, which could be attributed to the complex anatomy and overlapping of the vessels with different directions of blood flow. Other observations, such as increasing the diameter and number of collateral veins, constant filling defects, and the application of intravascular ultrasound, may decrease the risk of incorrect diagnosis. Second, this TRANCE-MRI protocol requires 60 minutes for imaging acquisition, 25 minutes for MRV, and 35 minutes for MRA. Thus, it is not suitable for critical and irritable patients. We suggest that the MRI protocol should be determined according to the patient's condition, and it is not necessary to perform the whole TRANCE-MRI protocol. Finally, TRANCE-MRI is expensive and not widely used at our institution.
The major limitation of this investigation was that it was a non-randomized study with few patients. This study was also limited by a lack of comparison of inter-observer variability and adequate validation with other imaging studies. However, we attempted to identify the values and pitfalls of TRANCE-MRI in venous pathology. This was the first prospective study to apply TRANCE-MRI for assessing venous pathology in the lower extremities. Further evaluation of the pelvic/abdominal assessment and accuracy of TRANCE MRI is needed before implementing versatile clinical applications. TRANCE-MRI may provide more useful information regarding optimal therapeutic protocols for the treatment of complicated vascular diseases.