Ultrasound is widely used in the clinical diagnosis of deep venous thrombosis of the lower extremities. However, this imaging technique is more dependent on the personal experience and skills of the operating physician. The imaging field is small and easily leads to misdiagnosis of DVT. Ultrasound can only determine the distribution of the thrombus; there is great difficulty in judging the age or the source of the thrombosis. Compared with ultrasonography, CE-MRV has obvious advantages for the diagnosis and analysis of lower extremity deep venous thrombosis [4, 5]. However, magnetic resonance contrast-enhanced scanning requires the use of gadolinium-containing contrast agent. As early as 2006, the U.S. Food and Drug Administration (FDA) released to the public a health monitoring report demonstrating that nephrogenic systemic fibrosis (NSF) and nephrogenic fibrosing dermopathy (NFD) are associated with gadolinium-containing contrast agents used by patients during an enhanced MRI scan or CE-MRA. RJ McDonald et al.  found that gadolinium deposition in neural tissues after GBCA administration occurs in the absence of intracranial abnormalities that might affect the permeability of the blood–brain barrier. These findings challenge the current understanding of the biodistribution of these contrast agents and their safety. Therefore, the use of non-enhanced MRI diagnosis of DVT will be the basis of future development of this method.
Karla Maria Treitl et al.  conducted a comparative study of 13 patients with DVT (11 cases were confirmed by ultrasound and 2 were suspected due to the presence of pulmonary embolism) on a 3.0T magnetic resonance scanner. As a result, VISTA-MRI and CE-MRI image quality and diagnostic confidence were as follows: 3.54 vs. 3.55 and 3.80 vs. 3.77, respectively (P < 0.001). There was high consistency between VISTA and CE-MRI in the diagnosis of DVT based on CE-MRI. VISTA (some companies call this method SPACE) has the advantages of a high signal-to-noise ratio, fast imaging speed, and natural flow signal suppression. However, its suppression of blood flow signals, which are generally used for arterial angiography with faster blood flow, is quite limited . Due to very slow venous flow, VISTA cannot completely suppress the blood flow signal in the venous cavity, making imaging ineffective. It is difficult to distinguish the vessel wall, blood flow, and thrombus, and it is therefore difficult to assess the age or source of the thrombus.
The DANTE-SPACE sequence was prepared with a blood flow-suppressing pulsed DANTE, and the data were read using SPACE. DANTE is sensitive to motion signal, and SPACE also has a blood flow signal suppression effect. Therefore, DANTE -SPACE can well suppress slow-flowing blood flow signals, clearly showing the non-flowing components in the lumen and the state of the venous blood vessels, which will facilitate the detailed observation of venous thrombosis . Figures 1 and 2 show DANTE-SPACE images in which normal venous blood flow shows a significant low signal (black blood technique), venous thrombosis shows varying degrees of hyperintense, and the diseased segment vein wall is rough.
CE-MRV needs enhanced contrast agents for its operation. Under the contrast agent, the normal blood flow in the endovascular cavity is bright and has a high signal (white). In general, it is difficult for the contrast agent to enter the thrombus, which is obviously low signal (black). Therefore, “intracavity filling defect” and “blood flow interruption” are important signs of conventional CE-MRI for the diagnosis of deep venous thrombosis in the lower extremities. DANTE-SPACE uses black-blood technology, it is contrast-free, and it shows markedly low signal (black) in blood flowing normally in the vessel bottom, whereas hemoglobin and collagen fibers in different oxidized states inside the thrombus show different levels of high signal (different levels of white). Thus, “abnormally high intracavity signal” and “interrupted flow void” phenomena are important signs for the DANTE-SPACE diagnosis of deep venous thrombosis.
In this experiment, the thrombus/cavity signal intensity ratios in the DANTE-SPACE and CE-MRV images were as follows: 20.5 ± 12.96 vs. 0.51 ± 0.46, n = 51 (P < 0.05); and the ratios of thrombus/muscle signal intensity were as follows: 1.74 ± 0.70 vs. 0.99 ± 0.53, n = 51 (P < 0.05). In DANTE-SPACE, thrombus and cavity, and thrombus and surrounding muscle tissue have a stronger signal contrast. Relatively high-signal thrombus lesions are more likely to draw the attention of the diagnostician, who is more apt to then accurately diagnose the presence and distribution of intravenous thrombosis given evidence of a bright lesion on a dark background.
In the past, there were three stages of deep venous thrombosis in the lower extremities, based on the different times of onset of the disease. In the acute period, the onset time is less than two weeks; in the subacute period, onset time is between two weeks and six months; and in the chronic period, the onset time is more than six months. This staged method is primarily based on clinical symptoms and ignores the actual process of thrombosis; therefore, it is more likely that staging and clinical efficacy cannot be used to verify one another. Phinikaridou et al.  believed that magnetic resonance motion imaging and diffusion-weighted imaging can identify the protein components of the thrombus and delineate the different stages of thrombosis.
As normal flowing blood coagulates, hemoglobin during coagulation undergoes a series of characteristic oxidative changes over time and exhibits different MRI signal characteristics. In the acute phase, deoxygenated hemoglobin is primarily localized to the cells, and T1WI has an equal signal. In subacute coagulation, erythrocyte disintegration occurs, methemoglobin containing Fe3 + is paramagnetic, and T1WI has a high signal. In the chronic phase, hemoglobin disintegration and hemosiderin deposition occur, and T1WI has a slightly lower signal. The different states of hemoglobin render it like an endogenous contrast agent, suggesting different “ages” of the thrombus [11,12,13]. In addition, Prakash Saha et al.  established a mouse DVT model and found that there was a corresponding relationship between the T1 relaxation time and thrombus age in the venous thrombus, with the longest T1 time in the super acute phase and the shortest T1 time in the subacute phase. The T1 time of the chronic stage gradually became longer and closer to the T1 value of normal blood.
When acute thrombosis of the lower extremities occurs, it often leads to thrombophlebitis. An acute inflammatory reaction can cause capillary leakage in the corresponding area. In the conventional CE-MRV sequence, capillary leakage will cause an increase in static contrast agent extravasation, manifested as tissue around the vein thrombus strengthening, showing a high signal, with the thrombus showing a relatively low signal, a pattern comprising the “bull’s eye” sign . This sign can be enhanced or decreased with a reduction of the inflammatory response and can also be used as a basis for the assessment of acute venous thrombosis by CE-MRV. In this respect, because DANTE-SPACE does not use contrast agent, the display of pre-thrombotic phlebitis is less obvious than that seen with CE-MRV. However, compared with conventional CE-MRV, DANTE-SPACE has the same ability to detect venous thrombosis in the lower extremity.
In my experiment, DANTE-SPACE had some limitations. (1) DANTE-SPACE takes 4 min to complete a segment scan of the lower limb, and it takes about 12 min to cover the whole lower limb. Compared with CE-MRI, DANTE-SPACE had a longer scanning time and a higher specific absorption rate. (2) DANTE-SPACE technology requires high uniformity of magnetic field (B0). In the center area of the image, the magnetic field uniformity is good and the image quality is good; in the edge area of the image, the magnetic field uniformity is poor and the image quality is easy to deteriorate. (3) Poor display of small diameter veins in the calf.
This study had several limitations. First, the sample size was small, and more patients are needed to further prove the experimental conclusions drawn. Second, there was no comparison for the clinical effect, and the value of guiding the clinical treatment needs further study.