Apparatus for use with an image of a wire within a blood vessel of a subject, the wire including a radiopaque portion, the apparatus comprising: an output device; and at least one computer processor configured: for each pixel within at least a portion of the image, to determine a wireness measure of the pixel and an intensity of the pixel, wherein the wireness measure of the pixel is a measure of an extent to which the pixel has a wire-like characteristic and is determined by measuring an extent to which the pixel, together with other pixels within the image, forms part of a long, thin set of pixels having a wireness-indicating characteristic; to determine that the tip of the radiopaque portion of the wire is located within a region within the portion of the image, by detecting that, within the region, there is at least one pixel at which there is a change, by more than respective threshold amounts, in both the wireness measure of the pixel relative to the wireness measure of one or more adjacent pixels with wireness-indicating characteristic and the intensity of the pixel relative to the intensity of at least some of the pixels with wireness-indicating characteristic; and to generate an output on the output device, in response to the determined location of the tip of the radiopaque portion of the wire within the image. an output device; and at least one computer processor configured: for each pixel within at least a portion of the image, to determine a wireness measure of the pixel and an intensity of the pixel, wherein the wireness measure of the pixel is a measure of an extent to which the pixel has a wire-like characteristic and is determined by measuring an extent to which the pixel, together with other pixels within the image, forms part of a long, thin set of pixels having a wireness-indicating characteristic; to determine that the tip of the radiopaque portion of the wire is located within a region within the portion of the image, by detecting that, within the region, there is at least one pixel at which there is a change, by more than respective threshold amounts, in both the wireness measure of the pixel relative to the wireness measure of one or more adjacent pixels with wireness-indicating characteristic and the intensity of the pixel relative to the intensity of at least some of the pixels with wireness-indicating characteristic; and for each pixel within at least a portion of the image, to determine a wireness measure of the pixel and an intensity of the pixel, wherein the wireness measure of the pixel is a measure of an extent to which the pixel has a wire-like characteristic and is determined by measuring an extent to which the pixel, together with other pixels within the image, forms part of a long, thin set of pixels having a wireness-indicating characteristic; to determine that the tip of the radiopaque portion of the wire is located within a region within the portion of the image, by detecting that, within the region, there is at least one pixel at which there is a change, by more than respective threshold amounts, in both the wireness measure of the pixel relative to the wireness measure of one or more adjacent pixels with wireness-indicating characteristic and the intensity of the pixel relative to the intensity of at least some of the pixels with wireness-indicating characteristic; and the wireness measure of the pixel relative to the wireness measure of one or more adjacent pixels with wireness-indicating characteristic and the intensity of the pixel relative to the intensity of at least some of the pixels with wireness-indicating characteristic; and