Tmri

The dramatic reduction in imaging times obtained with the fast gradients of high-field MR is especially useful in patients with cerebral ischaemia, who often exhibit a severe clinical condition and maybe unable to cooperate.

The parameters used for the standard sequences are reported in Table 14.1.

In the hyperacute phase, the total imaging time for axial T1, coronal or axial FLAIR, DWI and 3D PC MRA is approximately 8 min. A PWI sequence lasting only around 0.44 s will also be performed in thrombolysis candidates.

T2 FSE sequences in different planes of acquisition, a T2* GRE sequence to detect possible blood components, 3D TOF MRA and spectroscopy are performed in the later phases, when longer examination times are better tolerated.

An important advantage of high-field DWI in stroke patients is that it enables a greater anatomical coverage than 1.5 T MR. Another significant difference is its high diagnostic sensitivity in hyperacute ischaemia also at low b values (500, rather than 1,000 as in 1.5 T systems), thus reducing gradient stress (Fig. 14.7) [14].

The high-field PWI technique most frequently used in ischaemic stroke is dynamic susceptibility contrast (DSC) with a contrast agent as an exogenous tracer. DSC is faster and is the most widely used technique in clinical practice. It yields a much greater signal change with high-field compared with 1.5 T magnets, resulting in more reliable images [24].

The high signal/noise ratio of 3.0 T MR systems enables data collection using smaller voxels (greater spatial resolution) at shorter intervals (greater temporal resolution). The gradients are so fast that, even using short TE, temporal resolution is in the order of 1 s, the time required to follow the passage of the contrast bolus through the brain and its vessels. In addition, the greater sensitivity of 3.0 T machines to magnetic susceptibility allows a much smaller dose of paramagnetic contrast agent to be used than in lower-field PWI to obtain the same result. Use of higher concentrations of the agent (1 mol) allows to reduce the dose to 1/4.

Table 14.1. Parameters used for standard sequences

Sequence

TE

TR

Other parameters (IT, FA, ETL)

FOV

Th/Sp

NEX

No. of slices

Matrix

Time (min:s)

T1 FSPGR

Min

275

FA 75

22

4/1

1

25

512X256

1:50

FLAIR

130

11,000

IT 2,250

22

4/1

1

24

288X192

2:56

T2 FSE

81

4,840

ETL 15

22

4/1.5

2

24

448X320

2:39

Axial GRE T2*

Min

525.0

FA 20

22

4/1

4

25

512X224

5:56

DWI

Min

11,000

b values 1,000

22

4/1

1

26

128X128

0:44

GE-EPI DWI

48

1,700

FA 90

32

5/1

1

12

164X164

0:44

3D PC MRA

30

FA 20

22

35

6

256X224

2:41

3D TOF MRA

Min

26

FA20

16

1.4

1

60

288X224

5:53

ZIP 512

ZIP 2

H-MRS Press

30

2,000

24

Voxel = 20

8

1

4:45

30-144

144

Fig. 14.7. Acute-phase ischaemic stroke studied with DWI with different b values (500 in a, 1,000 in b, 2,000 in c). Optimum depiction of the ischaemic area is also obtained with low b values (a)

Fig. 14.7. Acute-phase ischaemic stroke studied with DWI with different b values (500 in a, 1,000 in b, 2,000 in c). Optimum depiction of the ischaemic area is also obtained with low b values (a)

Unraveling Alzheimers Disease

Unraveling Alzheimers Disease

I leave absolutely nothing out! Everything that I learned about Alzheimer’s I share with you. This is the most comprehensive report on Alzheimer’s you will ever read. No stone is left unturned in this comprehensive report.

Get My Free Ebook


Post a comment