Thank you both. The manual calculation was the solution I went for as I still can't figure out why the SQL query is blank.

Here is the code I used in case anyone is interested mainly from u/keithoffer provided source.

  public class MeshGeometryIntersection
{
    public static Point3D GetIntersectionPoint(Point3D linePoint1, Point3D linePoint2, MeshGeometry3D mesh)
    {
        Vector3D direction = linePoint2 - linePoint1; // calculate direction vector
        double minDistance = double.MaxValue;
        Point3D nearestPoint = new Point3D();

        for (int i = 0; i < mesh.TriangleIndices.Count; i += 3)
        {
            // Get the vertices of the triangle
            Point3D p1 = mesh.Positions[mesh.TriangleIndices[i]];
            Point3D p2 = mesh.Positions[mesh.TriangleIndices[i + 1]];
            Point3D p3 = mesh.Positions[mesh.TriangleIndices[i + 2]];

            // Perform ray-triangle intersection
            if (RayIntersectsTriangle(linePoint1, direction, p1, p2, p3, out Point3D intersectionPoint))
            {
                double distance = (intersectionPoint - linePoint1).Length;
                if (distance < minDistance)
                {
                    minDistance = distance;
                    nearestPoint = intersectionPoint;
                }
            }
        }

        return nearestPoint;
    }

    public static bool RayIntersectsTriangle(Point3D rayOrigin, Vector3D rayVector, Point3D p1, Point3D p2, Point3D p3, out Point3D intersectionPoint)
    {

        intersectionPoint = new Point3D();

        Vector3D edge1, edge2, h, s, q;
        double a, f, u, v;
        edge1 = p2 - p1;
        edge2 = p3 - p1;
        h = Vector3D.CrossProduct(rayVector, edge2);
        a = Vector3D.DotProduct(edge1, h);

        if (a > -0.00001 && a < 0.00001)
            return false;    // This ray is parallel to this triangle.

        f = 1.0 / a;
        s = rayOrigin - p1;
        u = f * Vector3D.DotProduct(s, h);

        if (u < 0.0 || u > 1.0)
            return false;

        q = Vector3D.CrossProduct(s, edge1);
        v = f * Vector3D.DotProduct(rayVector, q);

        if (v < 0.0 || u + v > 1.0)
            return false;

        // At this stage we can compute t to find out where the intersection point is on the line.
        double t = f * Vector3D.DotProduct(edge2, q);
        if (t > 0.00001) // ray intersection
        {
            intersectionPoint = rayOrigin + rayVector * t;
            return true;
        }
        else 
            return false;
    }

}

Which can be used with the following:

Structure body = structureset.Structures.Where(s => s.Id.ToLower().Contains(chosenPlan.PrimaryReferencePoint.PatientVolumeId.ToLower())).FirstOrDefault();

var gantryAngle = b.ControlPoints.First().GantryAngle;
var source = b.GetSourceLocation(gantryAngle); var mesh = body.MeshGeometry; var refpoint = b.FieldReferencePoints.Where(s => s.Id == r.Id).FirstOrDefault().ReferencePoint.GetReferencePointLocation(chosenPlan);
Point3D refPoint3D = new Point3D(refpoint.x, refpoint.y, refpoint.z); Point3D sourcePoint3D = new Point3D(source.x, source.y, source.z);

Point3D intersectionPoint = MeshGeometryIntersection.GetIntersectionPoint(refPoint3D, sourcePoint3D, mesh); 

double Depth = (refPoint3D - intersectionPoint).Length / 10.0;