it is very important and significant for doctors to determine the status of vascular - related diseases such as thrombosis and arteriosclerosis and to precisely visualize the three - dimensional ( 3d ) structure of the vasculature before surgery [ 13 ] . in dental fields , understanding the 3d structure of the vasculature is necessary and important for surgical operations on tumors and cysts . the most precise evaluation technique is conventional angiography ( cr ) using contrast medium . in cr , if the treatment should be applied for the status of vessels , the technique was continuously followed by the interventional radiology such as the embolization of arteries with arteriosclerosis . therefore , the cr is a firm position for elucidating the status of vascular - related disease such as thrombosis and arteriosclerosis , and precisely visualization of 3d vasculatures at the preoperations . however , the risk of a new focal neurological deficit caused by cr is about 0.14% ~ 0.5% , and this risk rises to 1.3% ~ 2.6% in patients investigated for stroke and transient ischemic attacks . therefore , it is important to identify the 3d structure of the vasculature of the arterial and venous systems as noninvasively as possible . to determine the 3d structure of the vasculature as noninvasively as possible , ultrasonography ( us ) is easy to use clinically . furthermore , the 3d structure of the vasculature can not be visualized using computed tomography ( ct ) without contrast medium . of course , ct angiography ( cta ) using contrast medium was a very useful tool for the 3d structure of the vasculature . however , the risk of complications in cta should be higher because of iodine contrast medium . recent advances in magnetic resonance imaging ( mri ) , 3d data acquisition , and postprocessing technologies are expanding the potential applications of 3d displays [ 516 ] . as a noninvasive technique , magnetic resonance angiography ( mra ) has been developed to identify and characterize peripheral blood vessels located in the thoracic and abdominal regions and the lower extremity [ 1722 ] . of course , mra is very useful to visualize the 3d structure of the vasculature in the oral and maxillofacial regions , and it is clinically significant for evaluating the relationship between lesions and vessels in the oral region [ 14 , 16 , 2326 ] . general dentists should know and easily understand the clinical applications of mra in the oral and maxillofacial regions . therefore , the present paper provides a clear and easily understandable explanation of mra , the method , and its characteristics , as well as its clinical applications and characteristics , for general dentists . mra commonly uses a bright - blood method , in which the signal from the moving protons is accentuated relative to the stationary protons through pulse sequences and measurement parameters . in fact , mra is a technique using vessel flow or contrast agents such as gadolinium to detect vessel systems such as arteries and veins . therefore , mra has the least complication and is the easiest technique for 3d visualization of vessels , unlike cr using contrast medium such as iodine - related medium . at the same time , this technique is very useful for identifying vascular diseases and abnormalities , as well as the relationships between diseases and surrounding vessels . the various techniques of mra include mr sequences such as time of flight ( tof ) , phase contrast ( pc ) , fresh blood imaging ( fbi ) , and contrast - enhanced mra [ 511 , 13 , 15 , 16 , 2737 ] . mra using tof ( tof - mra ) is the most time - efficient method for obtaining mra images . a single measurement is performed , with the stationary tissue signal suppressed relative to the flowing tissue signal . mra using pc ( pc - mra ) is a technique in which the background tissue signal is subtracted from the flow - enhanced image to produce flow - only images , analogous to digital subtraction x - ray angiography . mra as a new technique using fbi that has been applied to the 3d evaluation of vasculature [ 811 , 13 , 15 , 16 , 3437 ] . fbi , which uses 3d half - fourier fast advanced spin echo ( fase)-triggered mr with electrocardiographic ( ecg ) gating , is one of the newest non - enhanced - mra techniques [ 811 , 13 , 15 , 16 , 3437 ] . contrast - enhanced mra makes good use of the contrast agent , which affects the relaxation times of the water protons in the nearby tissue . in mra with contrast medium , as disadvantages , intravenous injection should be applied such as cr , and gadolinium could induce many kinds of complications including nephrogenic fibrosing dermopathy . mra without contrast medium has no complications and no need of intravenous injections , and its technique can easily and additionally apply for many patients in whom vascular information should be evaluated . in the present paper , the focus is on three techniques ( tof , pc , and particularly fbi ) as the main mra techniques without contrast medium that are useful in the oral and maxillofacial regions . on tof- or pc - mra , arteries and veins which are to some degree slow appear as bright homogeneous linear structures and stationary tissues except vessels with flow which appear for the most part blurred(figure 1 ) . therefore , vessels with flow can be primarily visualized on mra with these two techniques ( figure 1 ) . on the other hand , vessels without flow can not be visualized at all using mra with tof and pc techniques . of the two techniques in use for patients , pc - mra is better for demonstrating slow - flowing blood vessels ( < 10  cm / s ) , owing primarily to an optimized velocity - encoding gradient ( venc ) and perhaps also to smaller saturation effects and better background suppression . this sequence offers a better examination of the external carotid system than tof - mra . though the main carotid artery and its branches can be visualized on pc - mra , tof - mra can visualize the main carotid artery , but not its branches ( figure 1 ) . in addition , 3d - pc - mra has a better signal - to - noise ratio and a better spatial resolution than 2d - pc - mra ( figure 2 ) . therefore , in the oral and maxillofacial regions , 3d - pc - mra routinely tends to be selected for additional and preoperative vascular evaluations in mr examinations . however , in the oral and maxillofacial regions , the vasculature is complex , with blood flow rates varying from high in the carotid arteries to low in the lingual arteries . thus , 3d visualization of the vasculature in these regions is very difficult to obtain even using 3d - pc - mra . therefore , in very recent reports , mra using fbi ( fbi - mra ) was used for the 3d evaluation of the vasculature in the oral and maxillofacial regions [ 10 , 16 , 23 ] . most prior studies using fbi primarily examined the main vessels of the chest , abdomen , and lower extremities [ 9 , 11 , 13 , 15 , 3437 ] . in previous reports , 3d - fbi - mra could provide better 3d visualization of the vasculature of thin blood vessels , including the lingual and facial arteries , than 3d - pc - mra ( figure 3 ) . the axes of the lingual and facial arteries are under 2 - 3  mm . 3d - fbi - mra permits the depiction of slow - flowing blood vessels by acquiring the images with ecg triggering during the slow - flow cardiac phase . 3d - fbi - mra can depict slow - flowing vessels due to the t2 effects , so that even relatively static blood vessels can be depicted on images , unlike imaging with 3d - pc - mra . therefore , 3d - fbi - mra can even visualize vessels with very slow flow that can not be visualized on 3d - pc - mra [ 7 , 9 , 13 , 16 ] . in addition , the depiction level of thick blood vessels such as external and internal carotid arteries using 3d - fbi is the same as that using 3d - pc - mra . in other word , peripheral small arteries look well visualized on 3d - fbi - mra compared to 3d - pc - mra , but not for large arteries . carotid , maxillary , facial , and lingual arteries can be more precisely visualized on 3d - fbi - mra without contrast medium than on 3d - pc - mra ( figure 3 ) . at the same time , the internal jugular , maxillary , facial , and lingual veins can also be better visualized on 3d - fbi - mra without contrast medium than on 3d - pc - mra ( figure 3 ) . 3d - tof- or pc - mra depicts only blood vessels , but not mass lesions such as hemangiomas ( figure 4 ) [ 16 , 23 ] . therefore , the 3d relationship between the mass lesions such as hemangiomas and surrounding blood vessels should be visualized by fusing fat - saturation t2-weighted images with 3d - pc - mra . by doing so , hemangiomas and surrounding vessels can be visualized on the fusion images ( figure 4 ) . the software to allow superimposition of both 3d - pc - mra and t2-weighted images is integral to the mr system . however , image resolution degradation is a disadvantage of fusion imaging ( figure 4 ) . deterioration of the contrast - to - noise and signal - to - noise ratios on superimposed views occurs because noise on the respective images is doubled without increasing the signal intensity of the tumor and vessels . in addition , production of the superimposed view might be unsuccessful if the patient should move in the time gap between capture of the respective images . on the other hand , both hemangiomas and thin blood vessels around tumors can be clearly visualized on 3d - fbi - mra without fusion of mra with tumor images ( figure 5 ) . the carotid arteries , internal jugular veins , lingual arteries , lingual veins , facial arteries , facial veins , maxillary arteries , and maxillary veins can be seen as bright homogeneous linear structures ( figure 5 ) . at the same time , hemangiomas are clearly visualized as masses with high signal intensity ( figure 5 ) . except for the carotid arteries , 3d - fbi - mra is better than 3d - pc - mra based on visual scores despite the short acquisition time . artifacts from patient movement and shortened acquisition times reduce image quality more with 3d - pc - mra than with 3d - fbi . another advantage of 3d - fbi - mra is that one can distinguish between arteries and veins without the use of contrast medium [ 811 , 13 , 15 , 16 , 23 , 3437 ] . the blood vessels depicted on 3d - fbi - mra are most of the arteries , including the carotid , maxillary , facial , lingual , and other arteries ( figure 6 ) . flow - spoiled gradient pulses do not affect the signal intensity of stationary background tissues . veins are similarly less affected by the flow - spoiled pulse during diastole and systole as a result of their relatively constant slow flow throughout the cardiac cycle . by applying the flow - spoiled pulses , thus , diastolic and systolic subtraction provides better delineation of the arteries . however , except for 3d - fbi , the distinction between arteries and veins has not been possible with mra sequences such as 3d - tof and 3d - pc without contrast medium so far . it is hoped that 3d - fbi - mra will be a significant , epoch - making technique . however , there are some potential disadvantages with 3d - fbi - mra for clinical applications in the oral and maxillofacial regions [ 16 , 3437 ] . the first is that the selection of the appropriate flow - spoiled gradient is important for determining the image quality of hemangiomas and surrounding vessels . even a slight mistake affects the visualization of branch arteries , such as the maxillary and lingual arteries [ 16 , 23 ] . to select the appropriate flow - spoiled gradient pulses is very difficult for operators , because blood vessels in the oral and maxillofacial regions have a wide range of flow rates , from high to static . therefore , the images tend to be degraded in patients with movements of the mandible and tongue . moreover , the third disadvantage is that patients with arrhythmias or tachycardias are not suitable candidates for fbi examinations , because ecg triggering is used . mra can not perfectly replace cr for examinations of vascular - related diseases and suspected hemangiomas in the oral and maxillofacial regions . that is why mra can not have more contrast between vessels and tissues except vessels and can not be more precisely visualized than cr . however , performing mra for diseases in the oral and maxillofacial regions should be recommended in addition to common mr examinations before cr . in particular , 3d - fbi - mra is relatively easy to perform and can precisely visualize thin vessels except a little difficulty of parameters setting . therefore , examinations using 3d - fbi - mra are very useful and important for preoperative evaluations for various kinds of diseases , in particular the 3d relationship between the vasculature and diseases . higher spatial resolution gadolinium - mra can be achieved at 3 tesla with a sustained or greater signal - to - noise ratio of enhanced vasculature , relative to 1.5 tesla . therefore , mra with higher resolution in the oral and maxillofacial regions can be achieved with a 3-tesla mr system . very recently , it has been found that existing mra is suboptimal for assessing the hemodynamics of arteriovenous malformations [ 4250 ] . as a completely noninvasive method , 4d dynamic mra offers hemodynamic information with a temporal resolution of 50100  ms for the evaluation of arteriovenous malformations and can complement existing methods , such as dynamic subtraction analysis and tof mra [ 4246 ] . therefore , the evaluation of hemodynamics in hemangiomas will be performed by mra in the oral and maxillofacial regions in the future .