Visualization of active devices and automatic slice repositioning ("SnapTo") for MRI-guided interventions

Abstract

The accurate visualization of interventional devices is crucial for the safety and effectiveness of MRI-guided interventional procedures. In this paper, we introduce an improvement to the visualization of active devices. The key component is a fast, robust method ("CurveFind") that reconstructs the three-dimensional trajectory of the device from projection images in a fraction of a second. CurveFind is an iterative prediction-correction algorithm that acts on a product of orthogonal projection images. By varying step size and search direction, it is robust to signal inhomogeneities. At the touch of a key, the imaged slice is repositioned to contain the relevant section of the device ("SnapTo"), the curve of the device is plotted in a three-dimensional display, and the point on a target slice, which the device will intersect, is displayed. These features have been incorporated into a real-time MRI system. Experiments in vitro and in vivo (in a pig) have produced successful results using a variety of single- and multichannel devices designed to produce both spatially continuous and discrete signals. CurveFind is typically able to reconstruct the device curve, with an average error of approximately 2 mm, even in the case of complex geometries.

Figures

Figure 1

a: The three perpendicular projection images of the active device shown as the sides of a cube (shown in reversed grayscale for clarity). b: The iteration is initialized by picking the voxel of highest magnitude in the product image (large dot). The initial direction (two-headed arrow) is the principal eigenvector of the moment of inertia matrix of the local sub-image. c: The iterative steps in which a slice of the product image is interpolated perpendicular to the estimated curve. These steps are described in detail in d–g. d: The next curve point is initially predicted as described in step (i). The large dot denotes the current point p→i The arrow extending from it represents the current curve direction d→i e: A local 2D slice h(u1,u2) (black-bordered square) is computed as described in step (ii). The vectors v→1 and v→2 are represented by the arrows in the plane of the square. f: The centroid (denoted by the center of the large `x') of the local 2D slice is computed as described in step (iii). g: A correction is made to the predicted point by displacing it by the offset of the centroid. The new curve point p→i+1 is the point at the end of the arrow. h: One half of the found curve. i: The whole curve and the SnapTo slice that contains it.

Figure 2

a: 3D display of a slice of interest with the curve of the active device plotted in red. The point on the target slice at which the active device is expected to intersect the slice of interest is denoted by the yellow dot. b: A photograph of the Aortic phantom with the guide-wire inside (whose tip is marked by the arrow).

Figure 3

Active guidewire in aortic phantom. Top row shows projection images: Sagittal (a), Coronal (b), and Transverse (c). Bottom row shows the slice before (d) and after (e) SnapTo has been executed. In d the white arrows point to the device (faintly seen in red).

Figure 4

Details of projection images from experiments of guidewire with more complex turns in water bath. Sagittal (a) and Coronal (b) projections of Experiment #1 from Table 3. Dashed arrows point to areas of diminished signal located along the loop of the device and solid arrows point to areas of strong signal within the loop of the device. Sagittal (c) and Coronal (d) projections of Experiment #4 from Table 3. The error in Experiment #4 is lower than that in Experiment #1.

Figure 5

Transfemoral active guidewire in aorta of a pig. Sagittal, Coronal and Transverse (from left to right) projection images with the found curve of the device overlaid in red. The white arrows point to an area of strong signal within the loop of the device.

Figure 6

Active guidewire in the aorta of a pig. Imaged slice before (a) and after (b) SnapTo is activated. The device channel image is shown in green in both images. The image in b is the 3D display with the identified curve plotted in red. The breaks in the red curve occur when the identified curve is on the far side of the central plane of the SnapTo slice. The white arrow points to a region in which the device identification is incorrect. The imaged slice is 6mm thick even though it is depicted as infinitesimally thin in the 3D display. Any apparent discrepancy in the position of the red line and the green signal in the proximal guidewire is merely due to the parallax error caused by the non-infinitesimal thickness of the slice combined with the rising of the guidewire out of the plane of the image.

Figure 7

Active needle, with a discontinuous signal consisting of a series of point-sources, entering the chest of a pig. a: A closeup of the Sagittal projection image of the device showing the five point-sources. b: 3D display of the imaged transverse slice through the thorax overlaid with the curve trajectory in red.

Figure 8

Coupling artifacts. a: Sagittal projection of loopless shaft channel. b: Sagittal projection of solenoidal tip channel. c: Transverse projection of solenoidal tip channel. In b and c the artifact due to electromagnetic coupling is denoted by the dashed arrow, and the true point sources are denoted by whole arrows. In c the brightest pixel is the artifact and not either of the true point sources.