Radiographic Imaging During Spinal Surgery

Imaging ‘minimally invasive spinal surgery’ presents particular challenges: the spine has vertebrae at widely varying angles of tilt and rotation and yet each vertebra must be imaged ‘straight on’ (the perpendicular view, also called Orthogonal). The image intensifiers highest dose and quality mode is often needed and image quality is only optimised when the smallest possible round collimation is used. The surgeon will require a good deal of room under the intensifier to strike the Jamshidi (Biopsy needle) to gain entry to the pedicles. This increases magnification and makes centring considerably more difficult than is usually the case. There is a need to achieve tightly coned views with precise caudal/cephalad and lateral angulation and re-centre between a true orthogonal lateral and orthogonal A.P. view, many times in one case. For the lateral view the heights of the vertebrae vary along the spine and left/right (pan) angulation may be required to give an orthogonal view. The upper Thoracic spine presents further difficulty with extreme angulation, small vertebrae and the shoulders obscuring  the lateral view. So in summary this is perhaps the most difficult imaging challenge for a Radiographer in theatre! Fortunately there are a range of techniques and tips which can (and do) make optimal images both achievable and repeatable.

Objectives:
  1. Orthogonal views

  2. Tight circular collimation

  3. Accurate Centring

  4. High Contrast AP views. Optimal Contrast Lateral Views.

 

C-arm Position Marking

Because collimation has to be so tight and magnification may be considerably increased it’s essential to mark the position of the cross arm before moving to lateral. I use a small ‘skittle’ magnet for this, sometimes moving the cross arm half a centimetre may be enough to make it ‘off centre’! It’s not necessary to mark the cross arm for the lateral position where the intensifier should simply be moved as close as possible to the patient.

 

Isocentre

One of the big problems with spinal imaging is the need to remain on centre with very tight collimation and large magnification. The answer is perhaps counterintuitive and that is to raise the intensifier higher than usual to twenty centimetres above the height from the lateral view. Raising the intensifier for the AP view by this precise amount from the lateral will give isocentric angulation from head to foot. This ‘constant’ is dependent on the geometry of your C-arm:

This works because the Isocentre is at the pivot height of the caudal/cephalad tilt of the C-arm. For many Intensifiers this will be twenty centimeters above the height for the lateral view. For example if the height of the centred lateral view is five centimeters, raise the C-arm to twenty five centimeters for the A.P. view. If the lateral height is nine centimeters raise the C-arm to twenty nine centimeters in height etc. This will mean that tilting the A.P. view caudal or cephalad will not move the vertebrae from the centre of the image. Before moving to lateral remove the cephalad/caudal angle and the vertebra will be in the centre of the field on the lateral. The surgeon will need most of this height for clearance in striking Jamshidi needles so it’s a real shame not to take advantage of this method. In some cases the caudal/cephalad angle may be too extreme to achieve the height for the Isocentre in which case the X-ray tube will touch the underneath of the table. Lower the height of the tube just enough to allow the angulation you need. The lateral will be close to but not be exactly in centre of field. To compensate for this roll the intensifier left or right rather than tracking the whole machine head to foot. This small adjustment will centre the vertebrae and the roll will be removed automatically when setting the caudal/cephalad angle again for the A.P.

Dose and Contrast

You will often have to use the high dose setting on your machine. If you are imaging the lumbar region of a small patient with good bone quality then you won’t need it so often. Many patients undergoing spinal surgery are larger and/or have poor bone quality so will absolutely need the highest dose and contrast you have. A.P. of both lumbar and Thoracic spine will often benefit greatly from the use of the highest contrast setting. For the lateral thoracic view you will need to lower the contrast setting sometimes going all the way to the lowest setting where the lung is over the vertebrae.

 

Collimation

The key to high quality images and low dose is the use of the tightest possible round collimation. For the A.P. view the optimal collimation will show less than three vertebrae in total, the top and bottom of the adjacent vertebrae should be collimated off. For the lateral view it is necessary to show the pedicle and the anterior vertebral body, the rest should be round coned. Opening the cones is necessary for counting to show vertebral level to be sure of which vertebra is being operated on. It is also used for final images to show the overall picture. In-between these all of the images should be as coned as possible. If using magnification the image should not fill the whole screen or dose will not be reduced. As a rule of thumb the proportion of the screen showing an image will approximate to the dose e.g. if the whole screen shows image the dose will be around the same as an un-magnified view. If half of the screen is collimated the dose will be approximately halved irrespective of the magnification level.

Orthogonal View A.P.

The A.P. view is taken with the image intensifier tilted to the same angle as the vertebra. Specifically the angle of the upper end plate of the vertebra. Rather than take a series of images at differing angles on the A.P. view trying to superimpose the anterior and posterior edge of the end plate the angle required can be measured from the lateral view. In order to do this you need to zero the rotation of the lateral image accurately e.g. with a digital zero button and the intensifier must be straight when the view is taken. Our old Siemens Siremobil Compact machines were not accurate in this respect and so a special marker board was fashioned to show horizontal markers.

 

From this image the tilt of the vertebrae can be measured with a digital inclinometer a really good one of these is around £20. Digipas Inclinometer

To Be Continued!...

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