MR Scanner Transmit Gain as an Indicator For Lead Heating

November, 2006

2 •

The Focus The Focus

MRI lead heating experiments using the same:

• Imaging parameters• Lead samples• Phantom• Setup & position• SAR level• MRI scanner

3 •

Experimental ResultsExperimental Results

4 •

MaterialsMaterials

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.

5 •

MaterialsMaterials

• 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

6 •

Imaging ParametersImaging Parameters

Slice Thickness 10 mm

Spacing 2.5 mm

Freq. 512

Phase 512

Phase FOV 1.0

NEX 5

Bandwidth 31.25 kHz

Body Mass 120 kg

Field Strength 1.5T

Coil Body

Sequence FSE-XL

Imaging Plane Axial

TE 60 ms

TR 4300ms

Echo T.L. 127

FOV 48 cm

Whole body average SAR = 2.0 W/kg

7 •

Experimental InvestigationExperimental Investigation

Three (3) samples positioned in identical configurations:

8 •

Lead Heating ResultsLead Heating Results

9 •

Experimental ResultsExperimental 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.

10 •

Lead Heating vs. Transmit GainLead Heating vs. Transmit Gain

11 •

Lead Heating vs. Transmit GainLead Heating vs. Transmit Gain

Note: TG = (constant) x (Log of Power).

12 •

Lead Heating vs. Transmit GainLead Heating vs. Transmit Gain

Note: TG = (constant) x (Log of Power).

13 •

Clinical SignificanceClinical Significance

For a single clinical MR scanner the reported transmit gain (TG) can vary.

* Data from University Medical Imaging, Rochester, NY

Transmit Gain: Min: 94; Max: 119; Avg: 104 %Δ: ~21%

Date: 7/03/06 7/10/06 7/17/06 7/24/06

TG: 113 96 96 94

Date: 7/31/06 8/14/06 8/21/06 8/28/06

TG: 97 117 119 97

14 •

DiscussionDiscussion

*See: Baker et al., “Evaluation of Specific Absorption Rate as a Dosimeter of MRI-Related Implant Heating”, Journal of Magnetic Resonance Imaging, 20:315-320, 2004

• 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.

15 •

ConclusionConclusion

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?

16 •

ConclusionConclusion

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.