T

3.0 T MRA offers significant advantages (Figs. 5.3-5.6) [8-11].

First of all, it allows to perform all the angiographic sequences applied routinely in clinical practice with lower field systems, such as 2D and 3D TOF and PC, as well as ultrafast dynamic sequences after administration of a bolus of contrast agent (CE-MRA).

Similarly to standard brain MRI, the technical parameters of MRA sequences also need to be optimized at higher magnetic fields (Table 5.1).

The higher SNR improves scan quality largely through wider acquisition matrices (up to 1,024) without giving rise to significantly grainy images [12-15]. In addition, the change in T1 relaxation time, i.e. the increased T1, yields greater background stationary tissue suppression due to decreased R1 (the rate of longitudinal or spin-lattice relaxation), and greater flow enhancement since blood R1 is roughly constant, thus improving vessel-tissue contrast [12,16,17].

This advantage makes the application of magnetization transfer less effective than it is with 1.5 T systems [18, 19] and is mainly evident when using 3D TOF sequences, since the short TR usually saturates stationary tissue but not circulating blood [20], and the tag per-

Fig. 5.3. Normal arterial cerebral circulation imaged using the same 3D TOF sequence at 3.0 T (a) and 1.5 T (b). In a,vessel conspi-cuity is greater than in b, yielding better and more detailed depiction of superficial, smaller calibre vessels

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I ft f A

I ft b

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Fig. 5.4. Coiling of a branch of the right middle cerebral artery studied with a 3D TOF sequence at 3.0 T (a) and 1.5T (b). The coil is better depicted in a than in b, where the image mimics a small aneurysm

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b

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Fig. 5.5. Sacciform aneurysm of middle cerebral artery imaged using the same 3D TOF sequence performed at 3.0 T (a) and 1.5T (b). The sacciform vascular dilatation is depicted in greater detail and exhibits a more intense signal in a than in b

Fig. 5.6. Large right arteriovenous malformation: unenhanced 3D TOF images acquired at 3.0 T (a) and 1.5 T (b). Greater background suppression and flow enhancement afford better spatial depiction of the malformation at 3.0 T
Table 5.1. Optimization of technical parameters of MRA sequences at higher magnetic fields

Sequence

TR (ms)

TE (ms)

Other parameters (TI, FA, ZIP)

Slice thickness (mm)

No. of slices

FOV

Matrix

NEX

Examination time (min:s)

3D TOF 1.5 T

30

6.9

FA 30

1.2

32

24

352X224

1

3:08

3D TOF 3.0 T

26

Min

FA 20 ZIP 512 ZIP 2

1.4

60

16

288X224

1

5:53

HD 3D TOF 3.0 T

30

Min

FA 20 ZIP 1,024 ZIP 2

1.4

48

19

384X320

1

6:18

2D TOF 1.5 T

Min

Min

70

1.5

23

256X224

1

Variable in relation to number of slices

2D TOF 3.0 T

Min

Min

50

1.4

22

256X192

1

Variable in relation to number of slices

3D PC 1.5 T

33

20

50

20

256X192

8

3:23

3D PC 3 0 T

30

20

35

22

256X224

6

2:41

sists over a longer time. These effects afford greater vessel detail and conspicuity, especially with regard to small calibre structures, including surface vessels not clearly depicted on 1.5 T images (Figs. 5.7-5.12) [21, 22].

For the same reasons, 3.0 T MRA appears to be a promising technique to enhance vessel conspicuity in neonatal intracranial vessels or to further reduce scanning time [23]. Neonatal brain vessels are small, they exhibit lower blood flow velocity than adult vessels, and frequently have a turbulent flow. MRA protocols therefore need to be adapted to the specific needs and features of these patients, e.g. by reducing acquisition time to prevent motion artefacts, by using low flip angles and out-of-phase imaging better to saturate subcutaneous fat (which sometimes masks vessels on 3D MIP sequences), and by implementing ramped RF pulse and multiple thin volume strategies to visualize the intra-vascular signal at distal cortical branches.

Since the relaxivity of paramagnetic contrast media remains largely unchanged at higher magnetic field strengths, contrast administration further improves 3.0 T imaging given that its high SNR can be transformed into spatial resolution: resolution of 300 |im on the plane of section and of 400 |im on the slice thick-

ness, i.e. 300x300x400 interpolated voxels (ZIP 4 and ZIP 1,024) (Fig. 5.13).

Dynamic techniques, which afford shorter acquisition times, further contribute to improving contrast-enhanced MRA.

Additional gains are obtained with parallel imaging, which significantly reduces examination times and increases anatomical coverage providing the same image quality. In this case, a TOF sequence is usually applied with a wide matrix and hence greater spatial resolution [24].

(Continued on p. 41)

Fig. 5.7. Vestigial artery: unenhanced 3D TOF study at 3.0 T. MIP image (a); single partitions (b)

Finally, although the deflection and torsion movements of biomedical devices (such as the aneurysm clips commonly used in interventional and therapeutic neuroradiological procedures) and resulting susceptibility artefacts increase at higher magnetic field strength, the newer devices appear to entail no particular safety or compatibility risk [25, 26]. Patients with tested biomedical implants can therefore safely undergo 3.0TMRA [27, 28].

Fig. 5.9. Small berry aneurysm of left middle cerebral artery: 3D TOF study at 3.0 T. MIP image (a); single partitions (b)

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