1. Comparison of Surface Coil and Automatically-tuned, Flexible Interventional Coil Imaging in a Porcine Knee R. Venook 1, B. Hargreaves 1, S. Conolly 1, G. Gold 2, G. Scott 1 Department of Electrical Engineering 1 Department of Radiology 2 Stanford University Stanford CA, USA

2. 2 Introduction Prior work has demonstrated the advantage of using a surface coil versus a body coil to achieve high local SNR [1,2,3]. To do better one can place a smaller, invasive probe closer to the region of interest. Under controlled probe shape and loading conditions, such as with endorectal and intravascular imaging, this has worked well [4,5]. However, more general interventional techniques require remote operation and a variable shape, which both introduce practical problems with tuning and matching. We have developed circuitry for automatic tuning of a flexible interventional probe (Figure 1) [6]. Here we present results from an in vitro porcine knee study comparing high resolution images from a 3-inch surface coil and a minimally invasive, automatically tuned, 1-inch flexible interventional coil (Figure 2). Figure 2: Flexible coil with 12 cm lead, Q-spoiling, and varactor-diode voltage-controlled tuning. Probe designed for catheter-based introduction to interarticular space. 2.5 cm DC Tuning Voltage Signal Figure 1: Automatic tuning electronics diagram

3. 3 Here we derive a rough estimate for the SNR improvement of our 1” interventional coil versus a 3” surface coil. There are three key observations to make: 1) We compare the coils at both of their centers, and on the same object, so the signal for each will be the same, and the sensitivity scales as 1/D*. 2) ‘Body noise dominance’ conditions, wherein the noise from the coil is mostly coupled noise from the subject, would allow the noise analysis to basically rely upon the noise volume difference between the coils. Unloaded vs. loaded Q measurements define the ratio between intrinsic coil noise and coupled noise from the sample. 3) The surface coil has noise volume below it, on only one side of the coil. The interventional coil has noise volume on both sides because it is surrounded by tissue. This introduces a factor of √2 to the analysis. Theory We assume that the 3” surface coil is body- noise dominated, and measure our coil to have a loaded/unloaded Q ratio of 0.8 (coil- noise dominant). Finally, we use the ratio of the radii (3:1) and estimate the SNR advantage of our coil to be 4.9. * on-axis sensitivity of a simple RF coil where D is the coil diameter: QuantityScaling Signal Noise total Noise sample (N S ) SNR

4. 4 Methods To evaluate the performance of our flexible, 1-inch interventional coil and automatic tuning device, we imaged the patella-femoral cartilage of an in vitro pig knee in a GE Signa 1.5T scanner (Figure 3). We surgically implanted the flexible coil within the interarticular space of the patella- femoral joint, and performed electronic autotuning. We made 256 2, 10cm 2 FOV axial images using three types of sequences with cartilage-specific parameters from the literature [7,8,9]: 3D FIESTA (SSFP, 28° flip, 32x0.7mm ‘slice’, 1:40); 3D SPGR (8/50 TE/TR, 30° flip, 32x0.7mm ‘slice’, 6:53); and FSE (30/4000 TE/TR, 8 etl, 3mm slices, 4:32). After disconnecting the interventional coil, we placed a 3-inch surface coil over the patella and imaged using identical scan parameters. We measured the SNR of the complex image data using MATLAB (Mathworks, Inc.). By manually defining a background noise and a signal box in each image (Figure 4), we calculated SNR for our ROI in different slices. This method was repeatable within 4%. Figure 3: Experimental setup. Flexible RF coil implanted within the interarticular space of in vitro pig knee. Figure 4: Example 3” surface coil image with manually defined ‘noise’ and ‘signal’ regions for SNR calculation. 3” Surface Coil Noise Signal

5. 5 Results The SNR of images with the flexible coil was substantially higher than the SNR of the surface coil images. Figure 5 displays similar slices of the same pig knee—with identical scan parameters—from the 3-inch surface coil and from the surgically- implanted coil. In these images, the flexible coil exhibits SNR gain of a factor of 4.2 within the center of the patella-femoral cartilage (53 vs. 12.5 +/- 2%). Figure 6 summarizes the results for the same pig knee imaged with different sequences. As expected, the SNR gains are sequence-independent, and scale with SNR. 3” Surface Coil SNR = 12.5 1” Flexible Coil SNR = 53 Figure 5: 3D SSFP images, 3” surface coil vs. 1” flexible coil. 10cm 2, 256 2 (390µm 2 ), 32x0.7mm ‘slice’, 28° flip, 1:40 scan time Figure 6: Measured SNR for 3” surface coil and 1” flexible coil images of patella-femoral cartilage, different sequences

6. 6 Conclusions These experiments show that, as expected, a 1” coil can produce images with superior local SNR to those of a 3” coil. Comparing it within the best ROI of the 3” coil (patella- femoral cartilage) we measure a factor of 4.2-4.7 increase, which is consistent with our theoretical prediction of 4.9. This improvement directly enables resolution increases by a factor of 2 without sacrificing either SNR or scan time. The real promise in using a small, flexible interventional coil for clinical work is that it retains high local SNR for arbitrary depths and different shapes of target anatomy—if well-matched to the preamplifier. A surface coil’s SNR, in contrast, falls off predictably with depth. With automatic tuning of a flexible, interventional design, we have shown that we can maintain SNR while allowing the coil to conform to anatomy. Clinically, these gains could make the difference between a minimally-invasive image-based diagnosis, and arthroscopy in cases such as cartilage flaps or transplants, where high resolution is critical. [1] Hoult, D. et al, JMR 1976, 24: 71-85 [2] Hayes, C. et al, Med Phys 1985, 12(5): 604-607 [3] Edelstein, W. et al, MRM 1986, 3: 604-618 [4] Martin, J. et al, Radiology 1988, 167(1):268-270 [5] Ocali, O. et al, MRM 1997, 37(1): 112-118 [6] Venook, R. et al, Proc. ISMRM 2002: 893 [7] Hargreaves, B. et al, MRM 2003, 49(4):700-9 [8] Recht, M. et al, Clin Orthop. 2001 (391):S379-96 [9] Potter, H. et al, J Bone Joint Surg Am. 1998, 80(9):1276-84 References Acknowledgements We gratefully acknowledge the following support of this work: GE Medical Systems NIH - R33 EB 00777 - R01 EB 00346 - 5R24CA092862-03 Whitaker Foundation