MRI-guided interventional procedures are gaining increasing clinical acceptance^1,2. Positioning of treatment devices under real-time MRI is challenging and time consuming due to tight space inside MRI scanner bore. Compact Ultrasound (US) imaging system provides an option for real-time guidance because of its small foot print and low amount of magnetic components. The purpose of this study was to define and test site-specific conditions for safe use of a compact Samsung UGEO model HM70A US system in an interventional magnetic resonance imaging (iMRI) suite hosting a 1.5T Siemens Espree MRI scanner. The investigation also included evaluation of the effects of the presence of US system in the iMRI suite on both MR and US image quality.

The proposed configuration for clinical use included placement of the US scanner with transducer connector at the foot of the MRI table and the transducer to be operated at the opening of the MRI scanner bore (Figure 1). During MRI scanning, the US scanner and transducer were repositioned onto an MR safe stool at the foot of the table. A series of tests were performed to compare magnetic displacement (MDF) and static friction (SFF) forces exerted on each of the US system components at the proposed locations to assess the likelihood of displacement towards the bore.

Components that did not exhibit any magnetic attraction under a hand-held magnet test were positioned on a smooth Plexiglas surface, and moved to proposed location to directly assess if field-induced displacement occurs.

For US system components displaying magnetic attraction, a separate measurement protocol, listed below, was followed, to obtain a conservative estimate of the ratio, R, of the SFF to MDF at the proposed location,z_loc.

1. MDF exerted by the hand-held magnet on US system components and a steel wool test pad were measured using a scale outside iMRI suite.

2. Magnetic field strength along the scanner z-axis inside the iMRI suite was mapped using a Gauss meter and the field gradient at each location was derived, gradient_B(z).

3. Steel wool test pad was used inside iMRI suite, to determine the location z_disp where the pad starts to get displaced by B0 of MRI scanner. (Figure 2)

4. Using data from the above experiments, R was calculated as follows,

$$R=\frac{F_{test\ pad}}{F_{US}}\times\frac{m_{US}}{m_{test\ pad}}\times\frac{gradient_{B}(z_{disp})}{gradient_{B}(z_{loc})}$$

where F_US and F_{test pad} are MDF exerted by the hand-held magnet on the US system component and the test pad, respectively, and m_us ,m_{test pad} are their masses. A minimum R value of 10 is set as the criteria for the tethered US system components to be safely advanced towards the proposed location.

Image quality (IQ) tests of MRI system in the proposed configuration were evaluated with the US system in different operating modes, using an ACR MRI accreditation phantom to assess MRI IQ degradation caused by the presence of the US scanner. Similar US IQ assessments were conducted to detect any field-induced degradation of US image quality.

Using a hand-held magnet, US scanner, transducer connector and A/C adapter demonstrated attraction, while transducer scan head did not. Displacement tests for the transducer scan head demonstrated no displacement at scanner bore opening. Magnetic field strength measurements are shown in Figure 3. Estimated R ratios for US scanner, transducer connector, and A/C adapter were found to be 1063, 66, and 3850, demonstrating the SFF to be at least an order of magnitude greater than MDF at the proposed locations, which met our criteria. The US scanner, transducer connector and A/C adpater were subsequently tethered and advanced towards the proposed locations inside iMRI suite, at which no tendency towards magnetic displacement was found.

MR image quality met the American College of Radiology (ACR) standards³ for clinical scanning. Increases in noise levels and RF interference of varied degrees were observed, depending on operating mode of the US scanner. The “sleep mode” was identified as having been associated with absence of RF interference and minimal noise level increases (Figure 4). This mode allows the physician to begin and end US scanning quickly, without significant delay. At the proposed locations, no clinically-significant impact in US image quality was observed.

Our results suggest that the Samsung UGEO model HM70A US system is suitable for use in our well-defined site-specific configuration inside the iMRI suite. Following the tests presented above, the US system has been successfully used in our clinical practice for image guidance of treatment device positioning (Figure 5) during MRI-guided intervention procedures.