Phase contrast MRA

The MRA techniques discussed in the following base on gradient echo sequences and can generally be divided into flow-dependent and flow-independent ones. The main flow dependent techniques are time-of-flight MRA (TOF-MRA) and phase contrast MRA. CE-MRA, in contrast, is blood flow-independent, although flow phenomena can still influence... 

Fig. 6.14 Normal and abnormal phase contrast MRA of the circle of Willis, (a) Normal 2D phase contrast MRA with direction encoding in the rightdeft direction (vessel is bright if flowing right to left and dark if flowing left to right). There is normally...

Clinically, PC-MRA is often used for laminar flow with few pulsations as for example in the cerebral venous sinuses. Many users apply it as a thick slab 2D technique with short acquisition times and primary projective vessel depiction. 3D PC-MRA demands relatively long measurement times for data acquisition and is somewhat sensitive to patient movements. Principally, phase contrast methods additionally enable a quantification of blood flow velocity and the assessment of flow directions. 

Benjamin MS, Gillams AR, Carter AP (1997) Carotid MRA - what advantages do the turbo field-echo and 3D phase-contrast sequences offer Neuroradiology 39 469-473 Bogousslavsky J, Pierre P (1992) Ischemic stroke in patients under age 45. Neurol Clin 10 113-124 Bouthillier A, van Loveren HR, Keller JT (1996) Segments of the internal carotid artery a new classification. Neurosurgery 38 425-432... 

Table 18.2 summarizes the sequences and MR techniques which in our experience can be proposed (as mandatory or optional) in the diagnostic management of veno-occlusive disorders of the brain. Venous MRA can either be performed with the time-of-flight (TOF) or with the phase-contrast (PC) technique. In addition to the tomographic images, a flow sensitive gradient-echo sequence should be obtained if CVST is in question. As a fast screening examination we prefer a TOF 2D FLASH sequence (Table 18.2 Fig. 18.4, see 18.6c), oriented 90 degree to the flow direc-...

MRA is broadly divided into noncontrast and contrast-enhanced techniques. Noncontrast MRA can be acquired with phase contrast (PC) or time of flight (TOP) techniques, and both can be acquired as 2D slabs or 3D volumes. 

In the neck, time of flight techniques are preferred over phase contrast techniques due to the longer scan times of phase contrast techniques needed to provide the same coverage and spatial resolution. 2D TOF MRA provides superior flow-related enhancement and allows coverage of the entire neck. Compared to 2D TOF MRA, 3D TOF MRA provides superior spatial resolution and is less susceptible to phase dispersion artifacts, but is more susceptible to saturation effects and cannot cover a large area. 3D TOF flight techniqnes are, therefore, used to delineate the carotid bifurcation only. 2D PC techniques are used to evaluate flow direction in the vertebral arteries when subclavian steal is... 

D Phase contrast (2D PC MRA) Can show direction and magnitude of flow No artifact from T1 hypeiintense lesions Can show very slowly moving blood. Can be obtained after contrast Can be repeated if suboptimal Low spatial resolution More susceptible to turbulent dephasing than TOF MRA Can have abasing artifact 2D PC of neck is longer than 2D TOF of neck Can determine collateral flow around COW Can detect subclavian steal and abnormal flow direction in neck Can detect slow flow if near-occlusion is suspected... 

D Phase contrast (3D PC MRA) High spatial resolution Does not show high signal artifact from T1 hyperintense lesions Time-consuming 3D TOF MRA is faster, with similar resolution Rarely used Used if intravascular clot confuses 3D TOF MRA interpretation... 

Evaluation of the hepatic vasculature is essential before tumor resection. MRI or CT can show vascular involvement even without contrast enhancement. Other modalities such as 3D MRA have been used for this purpose, with the advantage that the images can be studied in both the arterial and venous phases (Haliloglu et al. 2000). 

A pseudoaneurysm at the anastomotic site can be seen at CTA and MRA as a round mass that enhances avidly during the arterial phase. Portions of the pseudoaneurysm may contain thrombus and therefore do not enhance after administration of contrast. Furthermore, flow within the pseudoaneurysms may be slow and hence enhancement is only appreciated during the portal or delayed venous phases. 

Wang Y, Yu Y, Li D, et al. (2000) Artery and vein separation using susceptibility-dependent phase in contrast-enhanced MRA. Journal of Magnetic Resonance Imaging. 12 661-670. 

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