Transcript HotTopicTGPresentationV8

MR Scanner Transmit Gain as an
Indicator For Lead Heating
November, 2006
The Focus
MRI lead heating experiments using the same:
• Imaging parameters
• Lead samples
• Phantom
• Setup & position
• SAR level
• MRI scanner
2 •
Experimental Results
3 •
Materials
Test Samples Used:
• An industry available standard active
fixation bipolar pacing lead.
• Prototype bipolar active fixation pacing
lead with resonant circuit in distal tip.
(One of Biophan’s solutions to the MR
lead heating problem.)
• Prototype bipolar active fixation pacing
lead without resonant circuit in distal tip.
4 •
Materials
• Luxtron® model 3100 fluoroptic thermometry
system with SSM model optical probes
• Gelled-saline solution: 5.8 g PAA, 0.8g NaCl
per liter of de-ionized water in head/torso
phantom (ASTM F21820-02a)
• GE 1.5 T Signa, Excite version 12x with M5
software patch
5 •
Imaging Parameters
Whole body average SAR = 2.0 W/kg
Field Strength
1.5T
Slice Thickness
10 mm
Coil
Body
Spacing
2.5 mm
Sequence
FSE-XL
Freq.
512
Imaging Plane
Axial
Phase
512
TE
60 ms
Phase FOV
1.0
TR
4300ms
NEX
5
Echo T.L.
127
Bandwidth
31.25 kHz
FOV
48 cm
Body Mass
120 kg
6 •
Experimental Investigation
Three (3) samples positioned in identical
configurations:
7 •
Lead Heating Results
8 •
Experimental Results
• Initially, it was believed that MR scanner
reported SAR was the primary parameter to
hold constant in our studies.
• It was determined that the Transmit Gain
had significant impact on the amount of
energy transferred to the lead even when
MR scanner reported SAR (and all other
parameters) were held constant.
9 •
Lead Heating vs. Transmit Gain
10 •
Lead Heating vs. Transmit Gain
Note: TG = (constant) x (Log of Power).
11 •
Lead Heating vs. Transmit Gain
Note: TG = (constant) x (Log of Power).
12 •
Clinical Significance
For a single clinical MR scanner the reported
transmit gain (TG) can vary.
Date:
TG:
Date:
TG:
7/03/06
7/10/06
7/17/06
7/24/06
113
96
96
94
7/31/06
8/14/06
8/21/06
8/28/06
97
117
119
97
Transmit Gain: Min: 94; Max: 119; Avg: 104
%Δ: ~21%
* Data from University Medical Imaging, Rochester, NY
13 •
Discussion
• It is known that the scanner calculated and
reported SAR level can be different between
different MRI scanner types.*
• For the same experimental setup on the
same MRI scanner using the same imaging
sequence and reported SAR level, lead
heating can significantly vary due to varying
“transmit gain” values.
*See: Baker et al., “Evaluation of Specific Absorption Rate as a Dosimeter of MRIRelated Implant Heating”, Journal of Magnetic Resonance Imaging, 20:315-320, 2004
14 •
Conclusion
Experimental Relevancy Concerns:
• Determination of SAR from calorimetry (heat
generated in tissue) studies may not be the
SAR level used during lead testing if TG is
not fixed.
• How should one compare lead heating
experiments with MR scanners that do not
have a “transmit gain” parameter?
15 •
Conclusion
Lead Construction for Patient Safety:
• The lead with Biophan’s resonant circuit
solution to the lead heating problem
performed well below the safety limits for all
values of TG tested.
16 •