How do they steer a catheter when they come to a fork in the road, so to speak? The vessels in the brain can get quite tiny and there must be many twists and turns.
The flexing control is in the tip. A tiny bend in the tip is enough to evoke a change in direction.
The old school ways of bending the tip involve a wire run up the catheter. (Pull the wire and the tip bends). Or hydraulically: change the pressure in a tube that ends near the tip.
Now it can be done electrically or even magnetically. The latter involves large magnets outside the body where the field is shifted to change the direction. This can sometimes be done with the surgeon located away from the patient to avoid recurring X-ray doses from working on several patients over time.
For a tricky case you can push past the fork a tiny bit, bend the tip, pull back a little and the tip pops into the side vessel. But the vessels have to be pretty robust. Popping one is a Bad Thing.
At the most basic level, catheters come in a variety of shapes, which can be used for various purposes (to access different vessels and their variations).
Here is a link to a site of a catheter manufacturer which includes a sampling of shapes.
Sometimes, a well formed catheter in a normal patient practically “falls” into the vessel with little effort. Sometimes you have to try harder. Torquing the catheter will change where it points and help you find the vessel.
Guidewires are used with catheters. They are actually a must for safe and practical use. Sometimes they have a very floppy tip and are not very steerable. These are safer, but do little to help you select a vessel. For more difficult vessels, one will often use an angled “torqueable” wire, that you can project from the tip of the catheter. Then one can torque the catheter while advancing and torquing the wire, allowing most vessels to be selected. But it’s not always easy.
Tip deflecting wires or magnetically steered wires may be the next level up, but these are not usually used for routine or even moderately difficult cases. *
Catheters and wires that go into the brain vessels are especially small and gentle. They get placed through guiding catheters that take them most of the way (up to the neck). They are also steerable, just like the big catheters are. Some are especially floppy and are carried to their destination by flow (flow directed catheters), but this only works if you want to get into a vessel with lots of flow, like supplying an AVM or a hyper vascular tumor.
A lot of this comes down to experience, careful work, and often patience.
*I should say that I’ve done this a bunch in the past, but it’s been years since I’ve done an angio now. Sometimes newer more expensive methods become “standard of care” for better or worse reasons. A good reason is that it leads to better outcomes. A worse reason is “it gets my job done faster, I’m not paying for it, and BTW, using it allows me to bill more!”