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TOF MRA is effective in diagnosing aneurysms > 3 mm at 15 T54 Voxel volume as low as 008 mm3 can be achieved with 3T systems55 Improved vessel contrast and diagnostic efficacy comparable to multidetector CT angiogram in aneurysm detection can also be achieved with 3T systems56 TOF- MRA is a useful technique for follow-up of intracranial aneurysms treated with endovascular detachable coils;57,58 however, it performance in follow-up of aneurysms less than 3 mm may not be as good59 Owing to susceptibility artefact associated with surgical clips, TOF-MRA is less effective in following up and accurate assessment of residual necks of aneurysms treated with titanium clips60 TOF-MRA may be suboptimal in evaluation of giant intracranial aneurysms because of slow and turbulent flow and CE-MRA may be more useful in evaluation of intracranial aneurysms >13 mm in size61 One important pitfall of TOF-MRA in the setting of intracranial hemorrhage is the potential of methaemoglobin in intracranial hemorrhage to mimic flow-related enhancement, owing to its inherent short T1 property |
Intracranial arteriovenous malformations and dural arteriovenous fistula are relatively less frequently encountered pathologies Assessment of these disorders require high spatial as well as temporal resolution that distinguishes between arterial feeders from draining veins Although digital subtraction angiography (DSA) remains the gold standard modality for evaluating these conditions; time resolved CE MRA can provide better temporal information than non contrast MRA TOF MRA has also been found to have lower sensitivity for evaluation of intracranial stenosis in primary CNS vasculitis62 FIGS 12A TO D Time-resolved CE-MRA in a case of cerebral AVM demonstrating the AVM nidus, its feeders (thick arrow) and venous drainage (thin arrows) in a dynamic manner comparable to catheter angiography |
FIGS 13A TO F Intracranial MRA of a 62-year-old female patient presenting with headache (A to C) MRA acquired with conventional technique (acquisition time 4 min 21 sec) and (D to F) MRA acquired with CS-SENSE with acceleration factor of 45 (acquisition time 2 min 34 sec) (A and B) and (D and E) coronal and axial maximum intensity projection images and (C and F) volume rendered images The images showed a saccular aneurysm arising from left internal carotid artery bifurcation and another small aneurysm arising from communicating segment of left internal carotid artery Both the observers considered the sets of images to have equivalent quality showing the aneurysmal morphology and bearing comparable diagnostic information |
Although, requirement for spatial resolution is less exacting in extracranial MRA, problems are likely to arise from vessel tortuosity and motion related artefact from patient鈥檚 swallowing Owing to the superior speed, coverage, and low sensitivity to flow-induced artifacts CE-MRA is generally the preferred method63 Magnetic resonance venography (MRV) is an important for evaluation of dural venous sinus thrombosis assessment To minimize the in-plane flow saturation, acquisition is done in an oblique sagittal plane 2D TOF MRV is able to depict flow as low as 2 cm/s;64 however, a combination of TOF and contrast-enhanced MRV has been advocated for greatest sensitivity65 Additionally, 3D PC imaging may be used for intracranial MRV, setting a low threshold velocity encoding of approximately 15 cm/s; however, it is a slower technique |
Susceptibility weighted imaging can also be used for evaluation of intracranial veins66 Abdominal Vascular Imaging Image quality of abdominal MRA is influenced by factors like respiratory motion, bowel peristalsis and artifacts from bowel contents, which also affects subtraction in both unenhanced and contrast enhanced MRA End expiratory breath hold techniques helps in minimizing artifacts due to cranial shift of kidneys and diaphragm seen during inspiratory breath hold Contrast enhanced MRA is the mainstay of abdominal vascular imaging which relies on test bolus method or bolus tracking technique for timing of acquisition Time resolved MRA allows multiphasic acquisition and useful in evaluation smaller vessels of hands and feet, and in pathologies like AVM in identifying nidus, multiple feeders and draining veins Parallel imaging techniques deliver ultra fast imaging thereby reducing breath hold duration In patients with impaired renal function non contrast MRA techniques like TOF and bSSFP are more commonly used |
Renovascular Hypertension Renal artery stenosis is the most common cause of renovascular hypertension and has higher prevalence among patients with end-stage renal disease and uncontrolled hypertension Etiological factors for RAS include atherosclerotic disease (90%) and fibromuscular dysplasia (10%) Renal parameters being frequently impaired in these patients non contrast techniques like TOF and bSSFP are more commonly used with good specificity and sensitivity (Fig 14) Braidy et al reported 85%, 96%, 94%, and 96% sensitivity, specificity, accuracy, and NPV respectively in detecting stenosis of renal arteries using non contrast balanced SSFP sequence as compared to CE-MRA67 FMD, vasculitis, dissection, atheroembolic disease and post-traumatic occlusion are the other rare causes of renovascular hypertension |
FIG 14 Noncontrast-MRA of renal arteries showing adequate visualization of bilateral renal artery origin, course and segmental branches |
Hepatic Vasculature Recent advances in newer image reconstruction algorithms and dedicated contrast agents have enabled MR angiography to achieve higher spatial resolution and better image quality for imaging hepatic vasculature The noncontrast technique, most effective in evaluating the hepatic arteries, is the respiratory triggered, 3-dimensional, steady-state free precession (SSFP) sequence68 Contrast enhanced 3D MR portography shows efficacy similar 64-slice MDCT for portal vein thrombus detection, and is able to identify portal vein anatomic variants with sensitivities and specificities of 100%69 CE- MRA is found to be superior to color Doppler ultrasound for the evaluation of portal hypertension as it shows evidence of thrombosis and occlusion of the main portal vein and portosystemic collaterals70 In cases of portal hypertension MRI can also help in assessment of liver parenchyma, focal hepatic lesions, biliary tree and vessels for shunt surgery or transplant Phase contrast MRA can be used for quantification of increase in portal flow after transhepatic shunting or the noninvasive calculation of total hepatic blood flow rates71 In evaluating Budd Chiari syndrome MR venogram can be used to assess hepatic vein occlusion or stenosis, and IVC stenosis or thrombosis Intrahepatic collaterals appear as comma-shaped enhancing vessels and bypass occlusions The identification of level of obstruction helps to establish the cause and planning the intervention |
Peripheral Vascular Disorders 3D contrast-enhanced MRA facilitates accurate and detailed assessment of peripheral arteries without catheterization, ionizing radiation, or potentially nephrotoxic iodinated contrast agents Unlike DSA, CE-MRA provides 3D data set that can be reformatted to produce multiplane DSA-like displays of the vessels that highlight information most relevant to prognosis and treatment planning (eg, arterial wall inflammation, plaque composition, and mural and intramural thrombus formation)72 MRA has advantage over CT angiography in assessment of heavily calcified arteries whereas CTA scores over MRA in assessment of in stent restenosis Newer hybrid MRA protocols use two stage acquisition, first stage is high-spatial-resolution MRA of the calf and foot and second stage is aortoiliac and femoral bolus-chase MRA after the second injection, and are more accurate for evaluating the trifurcation and foot vessels than single-injection multistation MRA Time resolved 3D MRA techniques eliminate the need for timing acquisitions or triggering methods, and also minimizes venous contamination |
Non contrast MRA techniques are particularly useful in patients with impaired renal functions and newer techniques such as ECG-gated 3-D FSE, 3D FSE-SPACE, balanced SSFP with arterial spin labeling, and quiescent-interval single shot MRA using 2-D SSFP have good sensitivity and sensitivity in detecting stenosis, comparable to CE-MRA (Figs 15A and B) |
For evaluation of vascular malformations MRI with MRA can be helpful for better characterization of lesions, assessment of high-flow versus low-flow malformations and supplying, and draining vessels |
Time-resolved CE-MRA are especially useful for vessel-specific information with clear separation of the arterial and venous phases, for detection of venous shunting, and assessment of flow direction73 FUTURE PERSPECTIVES Presently MRA techniques are well established tool for vascular imaging however still there is need of decrease acquisition times, increase temporal and spatial resolution with more dynamic information |
With advent of 3T MR scanner these problems has been addressed in clinical imaging Use of high fields such as 7T are in developing and research phase and the recent studies reveals that CE-MRA at 7T demonstrates the feasibility and current constraints of ultrahigh filed contrast enhanced MRA relative to NC MRA74 Also there are evidences of work under research reveals that TOF MRA at 7T have increase SNR and spatial restoration, especially for the assessment of brain AVM and high grade gliomas Other applications like arterial spin labeling are mentioned in other chapter on perfusion in this book Newer techniques like SWI-MRA are also in developmental phase, it makes possible to image both artery as well as vein simultaneously and can be evaluated separately75 Veins are dark due to T2* effect with SWI processing, same time arteries are seen bright due to TOF inflow effect (Figs 16A to D) In addition, it can also provide the information about hemorrhage and calcification whether parenchymal or mural in case of atherosclerotic disease |
FIGS 15A AND B (A) Multistation CE-MRA of the abdomen, pelvis, and lower extremities reveals focal high-grade stenosis of right distal SFA and collateral formation (B) Noncontrast-MRA of lower extremities showing adequate visualization of bilateral below knee arteries |
Lastly, there is lot more in future for MRA application due to advent of ultra-high field scanners and development of advances imaging sequences |