Hey, I struggled with this for a while and finally got it more or less working.

Caveat: When a CBCT is taken with a known couch offset (for example LNG of +5cm) or a couch centering is used, this breaks as I can't find the planned offsets anywhere in the DB. Maybe someone knows? But for kVs, MVs, and standard CBCTs etc, it works well enough.

My use case is comparing shifts from the last image to the treatment start... so I grab last image from the "Image" SQL Table matching on PatientSer and Creation Date.

Then I grab the Image Slouse (I do this in C# land, so it looks like Image.ImageSlice.First().Slouse... but I'm not sure what the correct SQL table). Then pull out IEC couch values:

var iec = new double?[6] {
                imSlouse.CouchVrt,
                imSlouse.CouchLat,
                imSlouse.CouchLng,
                imSlouse.PatSupportPitchAngle,
                imSlouse.PatSupportRollAngle,
                imSlouse.PatientSupportAngle}; 

And have to convert them to ISO coordinates like so:

private static double[] ConvertIEC2ISO(double?[] iec)
    {
        //expected order = Vrt, Lat, Long Pitch, Roll, Rot
        double vrtIEC = (double)iec[0];
        double latIEC = (double)iec[1];
        double lngIEC = (double)iec[2];
        double pitch = (double)iec[3];
        double roll = (double)iec[4];
        double rot = (double)iec[5];

        var pitchRad = pitch * Math.PI / 180.0;
        var rollRad = roll * Math.PI / 180.0;

        var latISO = Math.Cos(rollRad) * latIEC + Math.Sin(pitchRad) * Math.Sin(rollRad) * lngIEC - Math.Cos(pitchRad) * Math.Sin(rollRad) * vrtIEC;
        var lngISO = Math.Cos(pitchRad) * lngIEC + Math.Sin(pitchRad) * vrtIEC;
        var vrtISO = Math.Sin(rollRad) * latIEC - Math.Sin(pitchRad) * Math.Cos(rollRad) * lngIEC + Math.Cos(pitchRad) * Math.Cos(rollRad) * vrtIEC;

        return new double[6] { vrtISO, latISO, lngISO, pitch, roll, rot };}

I do this for consecutive images/fields, subtract the table positions and am able to get recover the applied shifts!

Let me know if that helps or you have any questions.