So today we will stop talking about registration and what we will be talking about is some

topic that also uses ideas of registration but also different ideas to come up with essentially

an interventional 4D reconstruction and the topic that we'll be talking about will be

cardiovascular reconstruction. So reconstructions of the heart and the vasculature of the heart.

Very well. So reconstruction was quite heavily introduced in last semester in diagnostic

medical image processing but in case you haven't attended it we will do a short recap of how

you do reconstruction but we will stay at a very superficial level here so we won't

discuss details of filtering and so on. But let's do a first refresher of reconstructions.

So in angiography, 3D angiography, we already have such reconstructions since the 90s and

when we are talking about 3D angiography what we are essentially referring to is a reconstruction

that is performed, so first developed in the 90s and since 1990 really in clinical use

and this is to be meant to be used directly in the interventional room. So you typically

have a special system like this one, so this is a C-arm system and there you do the acquisition

in the interventional room and you do the reconstruction and in the end you get a 3D

image of the vasculature. And you can do that pretty well if you have flat panel detectors.

What you may want to do is a digital subtraction angiography so you actually perform two scans.

You do one scan without contrast agent and one scan with contrast agent and then you

subtract the two and only the contrast agent remains. And this allows you to get quite

nice images of the vasculature and since you have these flat panel detectors they have

like 30 by 40 centimeters in height and width, you can reconstruct objects that are pretty

much fits an entire skull, an entire head. And this is pretty nice so if you do neuro

interventions and you want to go with a catheter through the vasculature you can do such a

3D image once so you only inject contrast agent once and then you can use this 3D image

as overlay to navigate through the vessels even if you change the angulation. So even

if you change the 3D projection or the projection of your Siam system so you rotate or move

a bit but you can still overlay this 3D image to guide you through the vessels. So this

is very nice but as you might remember one inherent assumption that you have in 3D reconstruction

is that all views are taken of a static object. And now if you have the skull and the vessels

in the skull even if you go in with a catheter there is only slight deformations of these

vessels. And usually you do the contrast agent injection so during the reconstruction itself

you will have a more or less static image. You also have to keep in mind that the rotation

of the Siam system, this is in an interventional room so there is all kinds of stuff around

and the typical CT gantry rotates four times a second. Now if you have a heavy x-ray tube

and this pretty large flat panel detector and you rotate at this speed it's open so

if there is somebody close to it or if there is some kind of machinery close to it it will

get hit. And if you move really quickly it will accelerate items so there is a high risk

of moving too fast. So if you move too fast you could hurt somebody. So this is the reason

why those systems rotate at a much slower speed. So a fast rotation is between 2 to

4 seconds for 200 degrees and it can take up to 20 seconds for one acquisition. So you

have pretty slow rotation speeds here. And in neuro this is still okay. So in neuro if

you have a patient and he is lying on the table and he is more or less attached to the

table as long as he is not suffering from an acute stroke event where he might move

or things like that then you can still get decent reconstructions. But if you have something

that inherently moves like the heart then you run into trouble. So your heart beats

60 times a minute and if you are in an interventional situation there may also be acute stress events

where the heart can beat considerably faster. So sometimes you have 120 beats per minute

that the heart is beating actually. And if you need 4 seconds for the rotation you have

several heartbeats in this single rotation. So you need to do something about it. Generally

for the reconstruction how it works is we are using filtered back projection. We are

using filtered back projection of course because it is fast. So we need the image quickly so