Sizing rings: a simple technique for measuring intracranial lesions.

There is now a need to measure accurately the sizes of intracranial lesions, particularly arteriovenous malformations (AVMs) and aneurysms. Until recently, this was not necessary, because the treatment options available for these lesions were limited to open surgery or nothing. Aneurysm size, for example, was relatively unimportant; what was important was only the issue of its location, whether it had a clippable neck, and whether there was vasospasm. Recent dramatic improvements in endovascular and stereotactic therapies have placed increasingly severe demands on accurate measurements from external references. The first use of this sort of measurement was for pelvimetry. To be useful, the sizes of bony and fetal structures must be corrected for their relationships to the x-ray beam geometry. For cranial work, these measurements previously have been made by placing objects of known size on the surface of the head and taking radiographs. Typically, only one marker is used. When this is done, it is impossible to be accurate when correcting for magnification, because magnification is not constant within the head. By knowing where in the head a lesion to be measured is and by using structures of known size on opposite sides of the head, it is possible to correct for the changing geometric magnification that is intrinsic to radiographs. For the past several years, we have used a simple system consisting of two pairs of disks. We place one disk on each side of the head, one in front, and one in back (Fig 1). Each disk (Fig 2) has a 1and 2-cm concentric circular groove inscribed; each groove contains a radioopaque material. Although it is not necessary for there to be two rings, the pair is easier to see than a single ring. We initially used (cleaned) platinum ends scavenged from used Target guide wires. Since then, these sizing rings (Vascutech, Andover, Mass) have been made by painting radioopaque ink onto medical grade tape. The tape then is mounted onto clear Lucite squares to make the structure rigid. This rigidity is necessary to prevent the rings from buckling; as Fig 1. Positions of rings on head. Black arrows signify directions of central beams.