Time of flight 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... 

Time-of-flight MRA is based on gradient echo sequences with very short repetition times (Laub and Kaiser 1988). The repeated HF excitations cause a relative spin saturation - i. e. signal reduction - in stationary tissue, while inflowing unsaturated blood is depicted with high signal. A saturation... 

Fig. 6.12 Internal carotid artery stenosis. Time of flight MRA (a) shows a segment of signal loss at the proximal ICA. The length of this focal signal loss is overestimated when compared with ceMRA (b). This could represent stenosis or occlusion, but was shown to be a high-grade stenosis on CT angiogram (c)...

Fig. 6.13 TOP MRA artifact. Blood (methemoglobin) in the prepontine cistern has high signal on the time of flight MRA (a) because signal from substances with short T1 is not adequately...

Fig. 6.20 Vasculitis, (a) MIP of 3D time of flight MRA shows scattered areas of narrowing in the right MCA (arrow), bilateral PCAs, and ACAs in this patient with primary CNS vasculitis.

Fig. 5.29a-f. MCA aneurysm with a small branch arising from the sac. More homogenous signal of the aneurysm was seen with 7 Tesla (b) b,c 3D Time of flight MRA demonstrating a paraophthalmic aneurysm on 1.5 T and 7 T, distal vessel depiction is nicely demonstrated on 7 T MRA e,f paraophthalmic aneurysm on DSA and well demonstrated on non-enhanced MPR age sequence imaged with 7 T... 

Johnston and Goldstein found that compared to DSA, there was a misclassifi-cation rate of 7.9% for combined US and time-of-flight (TOE) MRA compared to... 

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-...

Accuracy. CTA provides truly anatomic, nonflow-dependent data with regard to length of stenoses, residual lumen diameters and areas, and calcifications. How-dependent techniques such as MRA and US are not able to provide these data. Figure 4.3 illustrates the accuracy of CTA as compared to 3D time of flight (TOF) MRA. 

Fig. 4.3 (a) 3D time of flight (TOF) MRA there is apparent narrowing and irregularity of the distal right vertebral artery arrow head) and without definite flow in the right anterior inferior cerebellar artery arrow). 

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... 

Fig. 5.26. a Time-of-flight MR angiography of normal intracranial vessels, b Contrast-enhanced MRA technique with a large lield-of-view covering all vessels from the aortic arch to the circle of Willis... 

Time of flight (TOF) angiography TOF MRA relies on signal difference between stationary protons in the vessel wall and the surrounding soft tissues, compared to moving protons (flowing blood). 

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