Image Overlay Template Alignmemt
IOTA is a aystem of transparent navigation and guidance overlays which are mounted magnetically or held by hand on screen. They are magnification corrected so that each centimetre of the overlay represents one of the patients anatomy in the chosen plane.
EXACT CENTRING FOR PERFECT RADIOGRAPHY
In order to centre exactly on a structure you need to know how far to move the in each direction. This is achieved by working at a fixed distance from the anatiomy. At a given distance magnification is a constant, this fixed distance can be ascertained with a laser or by reference to a lateral view. Because a C-arm's rotation has fixed geometry once you have corrected your height on the lateral view this relates directly to the distance on the AP view.
To understand this better think of an AP and lateral hip during surgery. The Radiographer will centre the femur on the lateral view by altering the height of the intensifier/tube until the femur appears central on the screen. Rotating the C-arm back to the AP view the height above the hip is now fixed. On the Siemens Siremobil Compact this is 20cms. On the Philips Endura it is 19.5cms. The magnification factor this represents is 1.74 for the Siemens and 1.69 for the Philips machine.
So now it is a relatively simple matter to produce a clear overlay at this magnification showing centimetres in a grid, which allow the Radiographer to see how far from centre a chosen point lies.
SURGICAL GUIDANCE OVERLAYS
Planning software has been available for some time now allowing surgeons to represent the positioning of fixation implants onto pre-operative images. Unfortunately the fracture is often not in the position that it will be once the patient is anaesthetised and the fracture reduced on traction in theatre.
The solution is to enact the software on the Image Intensifier in theatre however it can be time consuming and also expensive to do so. Another way to approach this was needed so I developed transparent overlays which are both magnification and geometry adjusted to represent the apparent positioning of implants during surgery.
As with the magnification correct navigation overlays it is also possible to print outline plans of implants such as the IMHS (Intra Medullary Hip Screw). As long as the C-arm is perpendicular to the implant and the height is unchanged from centre on the lateral view, the overlay will correctly represent the implant. In the AP image below you can see an IMHS with its’ IOTA overlay in place. With this overlay it is possible to accurately preview the final position of the lag screw guide wire as the nail is inserted. This helps ensure that the nail is inserted to the correct depth for the chosen screw position in the femoral head
Magnetically mounted navigation overlay
Measure your C-arm movements as you Pan left or right
This is a printable overlay scale for Siemens Siremobil C-arms which shows one degree increments. It's in Powerpoint format so that the majority of users will be able to print it. There is no need to print it in colour if that is not readily available. Cover with Helicopter tape and stick over existing scale with sellotape double sided tape. Some care is required!