The brain seen by ultrasound: the tumour is not where the navigation screen puts it

What the navigation screen shows

Before the operation the patient has an MRI. In theatre the head is fixed in a frame and a camera tracks the instruments, whose tip appears on the screen inside the scan. To make screen and head agree, the image is aligned to markers on the head; for the Trondheim patients, five markers were glued on before the MRI 1.

On the left of the monitor is what navigation sees in the plane under the probe: the FLAIR MRI, in which the glioma shows white. The gold outline, traced with an intensity threshold, follows that white area. Grade 2 gliomas like this one grow slowly and spread into the brain without a clean edge.

What the ultrasound sees in the same place

On the right is the 3D ultrasound taken through the craniotomy, usually before the dura was opened 1. The image is a slice through the volume recorded at the time, on exactly the same plane as the MRI. The glioma shows brighter than the brain around it, and the white lines are mostly the grooves between folds.

The gold outline is placed on the ultrasound at the same coordinates. The bright tumour sits beside it. The database’s authors marked the same anatomical points on the MRI and on the ultrasound: in this patient the two points of each of the 16 pairs are 19.7 mm apart on average. Gold rings are the points on the MRI, blue dots the same landmarks on the ultrasound; those within 4 mm of the plane are shown.

Move the probe

Drag the probe across the brain to move the plane; the ⟲ ⟳ buttons or the Q and E keys rotate it. Hold the right button and drag to turn the head, or use the corner buttons to turn it step by step. On a phone, drag a finger across the monitor images to move the probe. The ultrasound only covers what the recorded sweep covered; outside it the image is black.

The landmarks do not all move the same way: the brain deforms. On the navigation screen the gap combines an imperfect alignment at the start of surgery with the brain’s movement since the MRI was taken; brain shift is what makes navigation unreliable as the operation goes on 5.

In the typical patient, 3.3 mm

This patient is close to the middle of the group: the two points of each of the 16 landmark pairs are 3.3 mm apart on average. The median across the 22 patients is 3.5 mm. In 8 of them the average gap is more than 5 mm; in 3, more than a centimetre 1.

On the ultrasound, the glioma next to the gold outline is the same one, only moved. When a tumour lies close to areas that control movement or speech, a few millimetres separate what can be removed from what must be left.

During the resection

Ultrasound is repeated during the resection and at the end. The buttons below step through the three volumes recorded for this patient. Where the tumour was, a cavity opens, and the tissue around it settles differently.

Compared with the first ultrasound, in 17 patients the landmarks had moved by 3.46 mm on average during the resection and by 3.55 mm at the end; in this patient by 2.4 and 2.0 mm 1. The two averages are almost equal, though patients differ: in one of them the distance grew from 3.6 mm during the resection to 7.6 mm at the end.

Where would you stop?

Another hospital: Brigham and Women’s in Boston, which published 114 brain operations with their 3D ultrasound and an MRI taken in theatre during surgery 3. This is a glioblastoma in the left temporal lobe, in an area classed as eloquent, meaning important for functions such as speech or movement. The gold outline is the tumour as the authors drew it on the pre-operative MRI: 28.5 mL. On the right is the ultrasound taken after the dura was opened.

Click and drag on the ultrasound to colour what you would remove. You can move and rotate the probe to work in other planes; each stroke is a 5 mm sphere. The ↶ button on the image or the Backspace key undoes the last stroke.

The MRI in theatre

When the surgeon judged the resection finished, an MRI was taken in theatre. The button below puts it on the left, on the same plane: the cavity is the dark area and the red outline is the tumour left behind, as the authors drew it, 2.8 mL. On the right is the last ultrasound, taken just before that MRI.

According to the clinical data, the resection did not continue after this MRI, and 1 to 5 mL of tumour remained after surgery 3. Near eloquent areas, part of a tumour is sometimes left on purpose; the data do not say why it was left here. The ultrasound was brought into the MRI’s coordinates only roughly, and the brain can also move between the ultrasound and the MRI in theatre, so the overlap with your strokes is approximate.

• tumour on the pre-operative MRI
• tumour left, on the MRI in theatre
• what you coloured