the degree of arterial invasion is important in deciding whether tumor could be surgically resected . catheter digital subtraction angiography ( dsa ) is the diagnostic gold standard due to its high temporal and spatial resolution . preoperative embolization can be performed concurrently in some cases to prevent hemorrhage during the definitive surgery . however , the performance of dsa is limited in practice as it is an invasive and expensive test relying upon the use of ionizing radiation , , . computed tomography angiography ( cta ) and magnetic resonance angiography ( mra ) are non - invasive alternatives to dsa for the depiction of tumors , , , . with respect to mra , time resolved mra has been reported as superior to conventional contrast enhanced mra in several respects : lower contrast agent dose , decreased venous contamination , and added dynamic information , , , . twist ( time - resolved angiography with interleaved stochastic trajectories , twist ) imaging divided k - space into two regions  one central region responsible for overall image contrast and a peripheral , outer region responsible for image detail . to accomplish greater temporal resolution , the central regions are sampled more frequently than the peripheral regions based on a technique called view sharing . while twist has been reported to yield excellent image quality and to be accurate in the diagnosis of peripheral arterial stenosis , its value in the assessment of lower extremity tumors has not been studied to the authors knowledge . the aim of the current study is to compare twist with cta for the preoperative vascular evaluation of lower extremity tumors . inclusion criteria were : ( 1 ) a patient with a lower extremity mass ; ( 2 ) tumor protocol mri confirming presence of a mass ; ( 3 ) a surgical plan for open biopsy or definitive surgery ; ( 4 ) patient agreement to both cta and twist . exclusion criteria were : ( 1 ) patient gfr  <  30  ml / m/1.73  m ; ( 2 ) patient on dialysis ; ( 3 ) pregnancy ; ( 4 ) standard contraindications for mr or ct ( i.e. pacemaker , paramagnetic foreign bodies , claustrophobia , etc . ) . from january 2013 to march 2015 , 57 consecutive patients met the inclusion criteria and were enrolled in this trial . . however , 2 patients with positive results of iodine allergy test did not undergo cta . 5 cases were excluded because no tumor was ultimately found , thus 50 patients with lower extremity tumors were included in the final analysis . all twist examinations were performed on a 3.0 t whole - body mr scanner ( magnetom skyra , siemens medical solutions , germany ) . two eight - element body array coils were used to cover the area of interest , and were combined with the posterior integrated multi - channel spine array coil . twist was performed in the coronal plane with the following parameters : tr / te , 2.9/1.06  ms ; flip angle , 25 ; bandwidth , 700  hz ; fov , 448 mm    358.4  mm ; slice thickness , 1  mm ; matrix , 450    360 ; spatial resolution , 1 mm    1 mm    1  mm ; time resolution , 3.84s / frame ; a&b , 15%/20% ; generalized autocalibrating partially parallel acquisition ( grappa ) factor , 2 ; number of measurements , 25 . gadovist ( gadobutrol , bayer pharma ag , berlin , germany ) was utilized as the contrast agent in this study . injection of contrast ( 0.1  mmol / kg body weight ) was performed by an automatic injector ( accutron mr , medtron , germany ) at a rate of 2.5  ml / s , followed by a 20  ml saline flush at the same rate . for each patient , the cta examination was performed within 36  h after mra . a 128-row ct scanner ( discovery hd 750 , ge medical , america ) was used with the following parameters : tube voltage , 100  kv ; tube current , 150  ma ; pitch , 0.984:1 ; table speed , 55  mm / s ; slice thickness , 0.625  mm ; fov , 50  cm . iodinated contrast agent ( ultravist , bayer , germany , 1.2  ml / kg body weight ) was administered by an electronic power injector ( stellant , medrad , america ) through an 18 gauge antecubital injection at a rate of 3  ml / s . the bolus - tracking technique was used . a region of interest ( roi ) was positioned in the aortic bifurcation and image acquisition automatically started 5.5  s after the signal attenuation of roi reached the predefined threshold of 120 hounsfield units ( hu ) . imaging post - processing were performed on a dedicated siemens workstation ( syngo.via , siemens healthcare , germany ) . two readers with 10 years and 8 years experience viewed maximum intensity projections ( mip ) images of cta and arterial phase mip images of twist for image quality evaluation . cta mip images were reconstructed with a window setting of 600/300 ( window width / window level ) . the readers were blinded to diagnosis and patient information and viewed cases in random order . the readers evaluated the mip images according to a 4 point scale : 0  =  non diagnostic image quality ; 1  =  poor image quality , observer not confident ; 2  =  fair quality , observer marginally confident ; 3  =  good quality , observer confident , 4  =  excellent quality , observer highly confident . if the readers gave different scores for the same image , the average of the two scores was utilized for the analysis . arterial stenosis caused by tumor was evaluated by two readers with 8 years and 6 years experience , using both source images and reconstructed images . the readers were blinded to diagnosis and patient information , and rated stenosis as follows : 0  artery free from tumor ; 1  artery displaced but not narrowed by tumor ; 2  tumor narrowing artery less than 50% ; 3  tumor narrowing artery 50% . the presence or absence of avf was determined by two readers with 10 years and 9 years experience in consensus . the two readers were asked to count feeding arteries in the mip images of each tumor . the viewing interval between cta and twsist for the same patient was at least 4 weeks in order to avoid recall bias . for each patient , a definitive operation or open biopsy was performed within 48  h following the cta examination . an amputation at the level of the right hip joint was performed in one patient . 1 ) . in the present study , who classification of tumors of soft tissue and bone 2013 ( 4th edition ) was used . 38 of 50 cases were primary soft tissue tumors and 12 of 50 cases were bone tumors with soft tissue involvement . the wilcoxon signed rank test was used to determine differences in image quality of cta and twist , and also used to compare number of feeding arteries identified by cta and twist . a chi - square test was used to determine the difference in avf identified by cta and twist . all data analysis was performed with spss ( version 21.0 , ibm , america ) . inclusion criteria were : ( 1 ) a patient with a lower extremity mass ; ( 2 ) tumor protocol mri confirming presence of a mass ; ( 3 ) a surgical plan for open biopsy or definitive surgery ; ( 4 ) patient agreement to both cta and twist . exclusion criteria were : ( 1 ) patient gfr  <  30  ml / m/1.73  m ; ( 2 ) patient on dialysis ; ( 3 ) pregnancy ; ( 4 ) standard contraindications for mr or ct ( i.e. pacemaker , paramagnetic foreign bodies , claustrophobia , etc . ) . from january 2013 to march 2015 , 57 consecutive patients met the inclusion criteria and were enrolled in this trial . . however , 2 patients with positive results of iodine allergy test did not undergo cta . 5 cases were excluded because no tumor was ultimately found , thus 50 patients with lower extremity tumors were included in the final analysis . all twist examinations were performed on a 3.0 t whole - body mr scanner ( magnetom skyra , siemens medical solutions , germany ) . two eight - element body array coils were used to cover the area of interest , and were combined with the posterior integrated multi - channel spine array coil . twist was performed in the coronal plane with the following parameters : tr / te , 2.9/1.06  ms ; flip angle , 25 ; bandwidth , 700  hz ; fov , 448 mm    358.4  mm ; slice thickness , 1  mm ; matrix , 450    360 ; spatial resolution , 1 mm    1 mm    1  mm ; time resolution , 3.84s / frame ; a&b , 15%/20% ; generalized autocalibrating partially parallel acquisition ( grappa ) factor , 2 ; number of measurements , 25 . gadovist ( gadobutrol , bayer pharma ag , berlin , germany ) was utilized as the contrast agent in this study . injection of contrast ( 0.1  mmol / kg body weight ) was performed by an automatic injector ( accutron mr , medtron , germany ) at a rate of 2.5  ml / s , followed by a 20  ml saline flush at the same rate . for each patient , the cta examination was performed within 36  h after mra . a 128-row ct scanner ( discovery hd 750 , ge medical , america ) was used with the following parameters : tube voltage , 100  kv ; tube current , 150  ma ; pitch , 0.984:1 ; table speed , 55  mm / s ; slice thickness , 0.625  mm ; fov , 50  cm . iodinated contrast agent ( ultravist , bayer , germany , 1.2  ml / kg body weight ) was administered by an electronic power injector ( stellant , medrad , america ) through an 18 gauge antecubital injection at a rate of 3  ml / s . a region of interest ( roi ) was positioned in the aortic bifurcation and image acquisition automatically started 5.5  s after the signal attenuation of roi reached the predefined threshold of 120 hounsfield units ( hu ) . imaging post - processing were performed on a dedicated siemens workstation ( syngo.via , siemens healthcare , germany ) . two readers with 10 years and 8 years experience viewed maximum intensity projections ( mip ) images of cta and arterial phase mip images of twist for image quality evaluation . cta mip images were reconstructed with a window setting of 600/300 ( window width / window level ) . the readers were blinded to diagnosis and patient information and viewed cases in random order . the readers evaluated the mip images according to a 4 point scale : 0  =  non diagnostic image quality ; 1  =  poor image quality , observer not confident ; 2  =  fair quality , observer marginally confident ; 3  =  good quality , observer confident , 4  =  excellent quality , observer highly confident . if the readers gave different scores for the same image , the average of the two scores was utilized for the analysis . arterial stenosis caused by tumor was evaluated by two readers with 8 years and 6 years experience , using both source images and reconstructed images . the readers were blinded to diagnosis and patient information , and rated stenosis as follows : 0  artery free from tumor ; 1  artery displaced but not narrowed by tumor ; 2  tumor narrowing artery less than 50% ; 3  tumor narrowing artery 50% . the presence or absence of avf the two readers were asked to count feeding arteries in the mip images of each tumor . the viewing interval between cta and twsist for the same patient was at least 4 weeks in order to avoid recall bias . for each patient , a definitive operation or open biopsy was performed within 48  h following the cta examination . an amputation at the level of the right hip joint was performed in one patient . 1 ) . in the present study , who classification of tumors of soft tissue and bone 2013 ( 4th edition ) was used . 38 of 50 cases were primary soft tissue tumors and 12 of 50 cases were bone tumors with soft tissue involvement . the wilcoxon signed rank test was used to determine differences in image quality of cta and twist , and also used to compare number of feeding arteries identified by cta and twist . a chi - square test was used to determine the difference in avf identified by cta and twist . all data analysis was performed with spss ( version 21.0 , ibm , america ) . from january 2013 to march 2015 , 50 consecutive patients ( 31 men , 19 women , age range 1880 years , average age 42.7 years ) with lower extremity tumors underwent twist and cta , without any adverse events . 44 twist cases and 48 cta cases were rated as excellent by both readers ( fig . image quality of twist was lower than that of cta in 6 of 50 cases . total image quality of twist was slightly lower than that of cta , not reaching statistical significance ( 3.88    0.37 vs 3.97    0.16 , p  =  0.135 ) . intra - modality agreement between twist and cta ( see table 2 ) was excellent for both readers ( kappa  =  0.806    0.073 and 0.805    0.073 ) . readers identified avf at 27 cases using twist , while at 14 cases using cta ( 54.0% vs 28.0% , p the number of tumor feeding arteries identified by twist was equivalent to that by cta ( fig . 3 ) . for 18 cases , twist identified one or more feeding arteries than cta ( fig . the mean number of tumor feeding arteries identified by twist was greater than that by cta ( 2.08    1.72 vs 1.62    1.52 , p  =  0.02 ) . the sensitivity and specificity of mra and cta have been reported as comparable to that of dsa for the diagnosis of vasculopathy , , however , data regarding comparing these non - invasive techniques for preoperative assessment of lower extremity tumors is lacking . in the present study , twist using gadobutrol at 3  t was evaluated for this purpose . with advances in mr hardware and software , time - resolved mra can now be obtained with concurrently high spatial and temporal resolution , , . the spatial resolutions utilized in this work were 1  mm isotropic for twist and 1 mm    1 mm    0.625  mm for cta . time - resolved cta was not performed due to the current prohibitively high level of ionizing radiation exposure required , . the main founding of the present study is that twist is comparable to cta in identifying tumor feeding arteries and avf , as well as assessing arterial stenosis caused by tumors . selection of contrast agent is important for contrast - enhanced mra , including twist , because the concentration and relaxivities of the contrast agent affect image quality . gadobutrol has a high relaxivity and is the only gadolinium ( gd)-based contrast agent approved for clinical use at 1  m concentration . found that gadoburol contrast - enhanced mra resulted in improved accuracy of renal artery stenosis assessments relative to gadoterate meglumine . thus , gadobutrol was chosen as the contrast agent for twist in the present study . in the present study , image quality of mra was slightly lower than that of cta , with the difference not statistically significant ( p  =  0.135 ) . one possible explanation is that twist images relied upon mask subtraction , which was susceptible to patient motion . fortunately , the lower extremity is not affected by respiration or bowel peristalsis like the chest and abdomen . venous contamination was not observed in the arterial phase of twist in this study , which also contributed to the image quality of twist . bolus - chase contrast - enhanced mra is an alternative technique to twist in lower extremity imaging , but was not utilized in this study due to the potential for venous contamination , . the spatial resolution of twist ( 1 mm    1 mm    1  mm ) utilized in this study was proved sufficient for detection of small feeding arteries of tumor . the isotropic resolution also enables rotation of the 3d - mip images of twist out of the coronal plane without significant vessel blurring , which is important for accurate identification of the tumor feeding arteries . although the spatial resolution of cta ( 1 mm    1 mm    0.625  mm ) was even higher , cta did not identify more feeding arteries that twist , one possible explanation was that non - subtracted cta was used in the present study , whose vessel - to - background contrast was not high ( fig . excellent intra - modality agreement was observed between twist and mra in the current study . twist seems an ideal substitute for cta in the assessment of arterial stenosis caused by tumor . however , it is worth noting that twist is performed in the coronal plane , while cta in the transverse plane . the detection of mild anterior - posterior stenosis with twist was slightly limited , thus underestimating stenosis degree relative to cta . feeding arteries and avf are usually of very small size , limiting their angiographic demonstration in some cases , . with the development of more technically sophisticated mr hardware and software , spatial resolution and snr furthermore , the conventional cta had only one frame of imaging , while the twist contained multiple arterial and venous phases , enabling readers to view the dataset dynamically , similar to dsa , twist enabled readers to select the best arterial phase for evaluation of feeding arteries and avf . the dynamic nature of the twist dataset also resulted in improved identification of avf , whereas single phase cta may fail to detect avf duo to time - point missing . one limitation of the current study is that only one mr contrast agent was utilized for twist . subtracted cta was superior to non - subtracted cta in vessel - to - background contrast , so it may identify more feeding arteries . third , a relatively heterogeneous tumor population was also studied , including both primary soft tissue tumors and primary bone tumors with soft tissue components . fourth , this is a single - center study with a small sample size , decreasing the statistic power . twist is a reliable imaging modality for the assessment of lower extremity tumors and is comparable to cta for the identification of arterio - venous fistulae and feeding arteries .
purposeto evaluate the diagnostic value of time resolved magnetic resonance angiography with interleaved stochastic trajectory ( twist ) using gadobutrol for preoperative evaluation of lower extremity tumors.materials and methodsthis prospective study was approved by the local institutional review board . 
 50 consecutive patients ( 31 men , 19 women , age range 1880 years , average age 42.7 years ) with lower extremity tumors underwent twist and computed tomography angiography ( cta ) . 
 image quality of twist and cta were evaluated by two radiologists according to a 4-point scale . 
 the degree of arterial stenosis caused by tumor was assessed using twsit and cta separately , and the intra - modality agreement was determined using a kappa test . 
 the number of feeding arteries identified by twist was compared with that by cta using wilcoxon signed rank test . the ability to identify arterio - venous fistulae ( avf ) were compared using a chi - square test.resultsimage quality of twist and cta were rated as 3.88    0.37 and 3.97    0.16 , without statistically significant difference ( p  =  0.135 ) . 
 intra - modality agreement was excellent for the assessment of arterial stenosis ( kappa  =  0.806    0.073 for reader 1 , kappa  =  0.805    0.073 for reader 2 ) . 
 readers identified avf with twist in 27 of 50 cases , and identified avf with cta in 14 of 50 ( p  <  0.001 ) . 
 mean feeding arteries identified with twist was significantly more than that with cta ( 2.08    1.72 vs 1.62    1.52 , p  =  0.02).conclusiontwist is a reliable imaging modality for the assessment of lower extremity tumors . 
 twist is comparable to cta for the identification of avf and feeding arteries .