Venous pathology in the lower extremities may be suspected in patients with engorged calf veins, unhealed shadowing wounds, and asymmetrically swollen legs. [1, 3, 13] US is operator-dependent, time-consuming, and inadequate at providing information about the pelvic and abdominal areas. Conventional venography has been considered the gold standard for detection of DVT in patients with VV; however, it is invasive, time-consuming, and requires the use of radiation plus contrast media.[5, 14] . Although CT venography is useful for exclusion of pulmonary embolism in patients with signs of thrombosis in the legs, it still cannot replace US as the first line image for detecting DVT.[15-18] MRI with contrast media, or time-of-flight (TOF) MRV, has been revealed as highly sensitive for detecting pathology in a variety of blood vessels when compared with conventional angiography. In TOF-MRV, blood flow is used as the intrinsic contrast agent and signal is based on an in-flow effect. The signal in the vessel depends on the flow up to a threshold speed, which is calculated by the slice thickness, in mm, divided by repetition time, in ms. However, the vessels can be observed most clearly when they are orthogonal to the two-dimensional plane, because in-plane vessels sometimes experience loss of signal.[19-21] In summary, TOF-MRV is less invasive than conventional venography and CT venography, avoids the side effects of iodinated contrast material such as renal damage, and is less operator-dependent than US. The main disadvantage of TOF-MRV is that the FOV is small for each image acquisition and need much time to obtain a whole image of the lower extremity. MRI with gadolinium-based contrast media is an alternative and relatively rapid method for imaging lower extremity. Although MRI does not involve radiation exposure, the non-iodinated contrast agents involved still have undesirable effects. NSF is a common complication of gadolinium-based contrast agents in patients with pre-existing impairments of kidney function.[7, 8] The TRANCE technique in MRI, which exploits differences in vascular signal intensity during the cardiac cycle with subsequent image subtraction, was first described by Wedeen in 1985.[22] TRANCE-MRI has been applied in cranial neurologic diseases and arterial diseases; however, few applications of this technique in venous pathology in the lower extremities can be found. [19, 23-26]
The principle of TRANCE-MRI technique is that different blood flow velocities will have different signal intensities on TSE sequence. High signal intensity, as well as bright color, reflects slow velocity, such as venous blood flow and diastolic arterial blood flow. The high velocity of systolic arterial blood flow will result in a flow void effect and is dark, as well as low signal intensity. TRANCE MRI can present high resolution and isolated vascular structures, such as arteries or veins. Presenting only the venous structure without accompanying the arterial structure is difficult to be achieved on MRI or CT with use of contrast medium, because the proper acquisition time is short and variable. Therefore, TRANCE MRI is a useful tool for venous pathology of the lower extremities because it provides additional pelvic information, no contrast agents, and no radiation toxicity. This study highlight that TRANCE MRI may be a safe and useful tool for lower extremity imaging, especially venous pathology. TRANCE-MRI may be preferred in patients with chronic renal insufficiency, history of abdominal/pelvic/orthopedic surgery and allergy to contrast media.
In this study, we do not specifically describe how to distinguish between acute and chronic thrombosis. Distinguishing acute from chronic DVT is a potential advantage of MRI, with irregular wall thickening in the presence of collaterals and diminutive lumen suggestive of chronic DVT. Our MRI protocol provide coronal and axial images, as well as 3D MRA and MRV images. We will use the original unremoved background image to examine possible tumors 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 is located between the right common iliac artery and the spine, which is an anatomically 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 middle femoral veins that might be difficult to detect by US. 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.
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. TRANCE-MRI is a non-invasive examination without use of contrast medium, which provide not only images of the arteries and veins in the lower extremities but also information about the pelvis and abdomen. If a patient has a pelvic vein problem or complicated varicose veins before surgery, we recommend an MRI.
We did learn of some drawbacks to TRANCE-MRI from this study. First, TRANCE-MRI of the venous system may cause a false positive in the left iliac vessels, which could be attributed to the complex anatomy and overlapping of the vessels with different blood flow directions. Other observations, such as increasing diameter and number of collateral veins, constant filling defect, and application of intravascular ultrasound, may decrease the risk of incorrect diagnosis. Second, the 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. Lastly, TRANCE-MRI is expensive and not widely used at our institution yet.
The major limitation of this investigation was that it was a nonrandomized study with few patients. This study also limited with lacking in comparison of inter-observer variability and adequate validation with other image 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 pelvic/abdominal assessment and accuracy for TRANCE MRI is needed before versatile clinical applications. The TRANCE-MRI may provide more useful information regarding optimal therapeutic protocols in treating complicated vascular diseases.