flow data acquisition

Flow Imaging
Data Acquisition


Ecg-Triggering Strategies
Prospective Triggering
•

Measures less than entire cardiac cycle

•

Very sensitive to arrhythmias and variable heartrates

•

Acquisition Window manually adjusted

•

Cine frame-rate determined by # of measured segments

Retrospective Triggering
•

Measures through entire cardiac cycle

•

Arrhythmia rejection is available

•

Acquisition Window automatically adjusted

•

Interpolates to any desired cine frame-rate

Most Common


Ecg-Triggering Strategies

ECG

Trigger Pulses
Measured
Echoes

Retrospective
(most common)

Prospective

a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
1

2

3

4

5

N 1

6 .........

interpolated frames

1 2 3 4 5 6.......
measured frames

2

3

4

5

6 .........

N

interpolated frames

N

1 2 3 4 56.......

N

measured frames

Undersampled cardiac cycle = 10-20%


Temporal Resolution
•
•

Insufficient temporal resolution causes inaccurate velocity and flow
Always keep temporal resolution (TR) small as possible

Lotz, J. et al. Radiographics 2002;22:651-671
Copyright ©Radiological Society of North America, 2002


K-space Segmentation
• Multiple k-space lines measured per cardiac phase (segment)
• More lines/segment = faster scan, but larger TR (worse temporal resolution)

ECG

Trigger

Acquisition Window

Acquisition Window

TR

Echoes
Cardiac Phases

aaaaaaaaaa

. ..

aaaaa

a a a a a a a a aa

1234512345

. . .

12345

6 7 8 9 10 6 7 8 9 10

1

2
Echoes 1-5

N

1

. ..
..

2
Echoes 6-10

a a aa

a

6 7 8 9 10

N

repeat to
fill K-space


Multiple VENC’s
• Multiple velocity-encodings measured per cardiac phase (segment)
• More encodings/segment = more velocity info, but larger TR (worse temporal resolution)

ECG

Cardiac Phases

1

2

3

4

rx

rx

rx

rx

5
rx

1

2

3

rxyz

rxyz

rxyz

6
rx

7 8
rx

rx

4
rxyz

9 10 11 12 13 14 15 16 17
rx

5
rxyz

rx

6
rxyz

rx

rx

7
rxyz

rx

rx

8
rxyz

rx

9
rxyz

rx

1 Venc

rx

10
rxyz

3 VENC’s

TR
r = reference (zero) velocity encoding
x= velocity encoding in x direction

y= velocity encoding in y direction
z= velocity encoding in z direction

Single VENC
1 velocity in 1 direction
Rephased Images

Phase Images


VENC Scout
3 velocities in 1 direction
Rephased

VENC 150

VENC 90

VENC 200

aliasing


3-D VENC
1 velocity in 3 directions
Rephased

Right-Left

Ant-Post

Through-Plane

3-D VENC
1 velocity in 3 directions

Rephased

Head>Foot

H >> F

Through-Plane

Ant-Post


Typical Retrospective Triggering
1 VENC

3 VENC‘s

Calc Phases 25

Calc Phases 20

TR 39 ms

or

TR 59 ms

Segments 3

Segments 2

Time 20 sec

Time 30 sec

3x more velocity info
but.....
less temporal resolution
and.....
longer scan time