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FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on B 1 + Field and Specific Absorption Rate (SAR) Narayanan Krishnamurthy 1 and Tamer S. Ibrahim 1,2 Departments of 1 Bioengineering & 2 Radiology University of Pittsburgh, PA, USA

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Page 1: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

FDTD Simulation of Receiver-Only Array Inserts for 7TeslaTransmit-Only Head Coil :

Influence on B1+ Field and Specific Absorption Rate (SAR)

Narayanan Krishnamurthy1 and Tamer S. Ibrahim1,2

Departments of 1Bioengineering & 2RadiologyUniversity of Pittsburgh, PA, USA

Page 2: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Overview:

• Evaluate the Influence of Different ReceiverArray Geometry on Transmit Field viz. scatterparameters, B1

+ field and RF absorption or SAR

• Issues with current receiver design & receiverbuilt by our RF group

• Simulation Results

Page 3: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Motivation: Issues with Current Receiver Array

• Smaller loop arrays produce higher SNR images• Massively parallel receive systems have been built for

high field MRI

• Loops of array constructed with wireIssues:

• Although wires are an improvement over 5mm coppertape (reduced coverage)

• Close conforming loops of receiver array could causean increase in RF absorption ?

• Current receiver helmets do not account for differenthead shapes and sizes

Page 4: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Four Piece Helmet Design

The assembly was designed to fit 90 percentile of heads*L 7.5 + 0.6” W 5.9 + 0.6”

HeadsUSA

Length (inches)(min,mean,max)

Width (inches)(min,mean,max)

MaleFemale

7.1, 7.7, 8.46.4, 7.1, 7.8

5.6, 6.1, 6.75.2, 5.7, 6.3

*The measure of man and woman, human factors in design A. R. Tilley, John Wiley & Sons2002 p27

Grid localizes array geometry

Page 5: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

PEC Wire Model of 32 Channel Overlapped Array

• Created fromCAD modelusing Matlab

• 3d Splinediscretized to1.6mm resolution

Page 6: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Overview:

• Evaluate the Influence of Different ReceiverArray Geometry on Transmit Field viz. scatterparameters, B1

+ field and RF absorption or SAR

• Issues with current receiver design & receiverbuilt by our RF group

Page 7: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Different Receiver only designs in TEM coil

Receive Array Geometry:

• Cylindrical overlap, staggered overlap and close fittinghelmet array of varying copper trace widths modeled

• Detuning of loops modeled as open circuit

• Two anatomically detailed head models of 1.6mmresolution (Head2 18 tissues & 3.6Kg, Head3 22 tissues& 4.7Kg) were used

Page 8: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Transverse electromagnetic (TEM) Resonator

TEM coil Schematic: The coil was tuned and matched to50Ω at 300 MHz

• Four ports of coil were connected in quadratureexcitation.

• Voltage per port was scaled by √(1 – reflection_port^2)to ensure equal transmission

Page 9: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Results: Effect on Scatter Parameters

Bottom: Change in coupling with differentreceiver inserts is < 6%. Right: Match betweenscatter parameters using a network analyzerand FDTD simulation.

• The coupling between ports of TEM coil donot change with different receiver inserts

Table1 Scatter Parameters Head2 dB Scatter Parameters Head3 dB8Loop 16Loop 32Loop 8Loop 16Loop

Honly H+.125” H+.25” H+.125” H+.25” H+.063” Honly H+.125” H+.25” H+.125” H+.25”S12 -15.88 -16.36 -16.22 -15.71 -15.66 -15.72 -16.74 -17.24 -17.09 -16.57 -16.57S13 -11.36 -11.89 -11.77 -11.37 -11.4 -11.70 -12.65 -12.63 -12.51 -12.18 -12.18

Percent change in coupling Head2 Percent change in coupling Head3S12 -5.4 -3.8 1.9 2.6 1.9 -5.6 -3.9 2.0 2.0S13 -5.9 -4.6 -0.1 -0.5 -3.8 0.2 1.6 5.6 5.6

Page 10: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Results: Effect on RF Absorption & Coil Radiation

Could copper of receiver arrayscatter RF and increase TEMcoil radiation?

Total absorption = 0.5 σ E^2

Global absorption in the twoheads increased 9-18%

Input Power = PowerAbsorbed + Radiated

Radiated power from TEM coildecreased

Absorbed Power in head for equal transmissionof four ports in quadrature excitation

Table 2 Power (Watt) Percent Change

Head2 Head3 Head2 Head3Head only 2.57 2.65H+8 loop array0.125” copper

3.02 3.09 17.77 16.59

H+8 loop array0.25” copper

2.97 3.05 15.87 14.78

H+16 loop array0.125” copper 2.95 2.98 14.94 12.34

H+16 loop array0.25” copper 3.02 2.98 17.79 12.34

H+32 array.063” copper

2.80 9.01

Page 11: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Results: Effect on B1+ Field

Bottom: Axial, coronal & sagittal sliceof B1

+ in brain of heads2&3 for 1 Wattof absorption

Right: % change in B1+ with different

receiver array is <4%

The existing receiver array designdo not alter mean B1

+ of TEM coil

Table 3 B1p per 1 Wattabsorbed power

(Micro Tesla)Mean

Percent change

B1p Mean

Head2 Head3 Head2 Head3Head only 0.77 0.60H+8 loop array0.125” copper

0.76 0.59 < -1.0 < -1.0

H+8 loop array0.25” copper

0.76 0.59 < -1.0 < -1.0

H+16 loop array0.125” copper

0.79 0.61 2.93 2.32

H+16 loop array0.25” copper

0.79 0.61 3.25 2.32

H+32 array.063” copper

0.77 0.98

1.4e-6

1

0.5

0

Page 12: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Results: Effect on local SAR W/kg per 10gm

Bottom: Axial, coronal & sagittalslice of SAR in heads 2&3 averagedover 10gm of tissue, for mean B1

+ inbrain of 1 micro Tesla

Right: % change in peak SAR

Peak Local SAR increased 2-15%with different receiver array

Table 4 Peak SAR(W/Kg Per

10gm) for meanB1p (1Micro

Tesla in Brain )

Percent changeIn Peak SAR

Head2 Head3 Head2 Head3Head only 6.72 8.99H+8 loop array0.125” copper 7.20 9.68 7.11 7.70

H+8 loop array0.25” copper 7.17 9.65 6.65 7.37

H+16 loop array0.125” copper 6.83 8.95 1.59 <-1.0

H+16 loop array0.25” copper 6.85 8.95 1.89 <-1.0

H+32 array.063” copper 7.71 14.69

W/Kg

8

4

0

Page 13: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Results: Effect of Copper Trace Width on RF absorption

Copper width of 8 & 16 loop array varied & absorption over 9 tissues analyzed

Absorption increased with trace width for 16 Loop array while it decreased forthe 8 Loop array

Overall effect of trace width on RF absorption was inconclusive

Head2 Skin CSF WM BoneEyes Muscle GM CBE Fat

Head3 Skin CSF WM BoneEyes Muscle GM CBE Fat

Percent change in absorption (W)25

20

15

10

5

0

-5

8 loop 0.125 Copper8 loop 0.25 Copper16 loop 0.125 Copper16 loop 0.25 Copper32 loop 0.625 Copper

Page 14: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Results: Percent Change in SAR with Receiver Inserts

SAR increased atregions with verydifferent RFproperties; e.g. atboundaries ofSinuses, Bone,CSF, GM & Eyes

Conductivity @300MHz:

Percent change SAR

8

6

4

0

-4

-62.2 CSF, 1.54 Eye,

0.97 CBE, 0.77 Muscle,

0.69 GM, 0.64 Skin,

0.41 WM, 0.08 Bone,

0.0 Sinus, 0.03 FATSAR (W/Kg) per 10gm

Head 2

Head 3

7

6

5

4

3

2

1

0

8

6

4

2

0

Page 15: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Results: Changes in Peak Local SAR

• Peak SAR (hot-spot) intensity increased with different receiverinserts

• Hot-spot location in some tissues changed when receiver arrayinserts are used for head2

• Close conforming helmet array caused greatest change in peak SARdistribution when compared to cylindrical arrays

Head2 Skin CSF WM BoneEyes Muscle GM CBE Fat

31

Head3 Skin CSF WM BoneEyes Muscle GM CBE Fat

Percent change in peak SAR (W/Kg)15

10

5

0

-5

-10

Page 16: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

Conclusion : Influence of Different ReceiveArray Inserts on Transmit Field

• Receiver inserts do not change coupling of the TEM coil (<6 %)

• Mean B1+ in brain per Watt of absorption did not change (<3%).

• For same input power : SAR increased with # of receive channels withcorresponding decrease in radiation from the coil.

Tissue specific analysis• A global increase in absorption (9-18%)• Vulnerable structures like Eyes and tissue adjoining sinuses

showed a notable increase in absorption• Peak SAR location and intensity in tissues changed when different

receiver inserts were used• Close conforming helmet array affected peak SAR distribution

more than the cylindrical arrays• Overall effect of copper trace width on RF absorption was

inconclusive

Page 17: FDTD Simulation of Receiver-Only Array Inserts for …nak54/ACES_2012_NK_PPT.pdf · FDTD Simulation of Receiver-Only Array Inserts for 7Tesla Transmit-Only Head Coil : Influence on

RF GroupTamer S Ibrahim

Former Members: Current Members:Anthony Defranco (RF Tech)Yujuan Zhao (PhD Student)Daniel Stough (MS Student)Shailesh Raval (PhD Student)

Yik-Kiong Hue (Post-Doc) Jung-Hwan Kim (PhD Student)Lin Tang (PhD Student) Sossena Wood (PhD Student)

Acknowledgements

AcknowledgementNIH 1R01EB00984

Siemens