CARDIAC CT

Most machines have one X-ray tube that shoots out X-rays but multiple detector plates that pick up the X-rays that have come through the patient.
So, it doesn't take long for a region of the patient to be caught on film because that region is quickly moved past the X-ray tube and detection plates by the moving table.

As the region moves past, the X-ray tube varies its output so that maximum tube current and therefore radiation is only given during diastole. This significantly reduces the total radiation dose to the patient. Why diastole? - because the heart is not moving as much and so images are crisper. Why not give no dose during systole then? Because paradoxically sometimes the systolic phase - at 35% of the RR interval - gives the crispest image (so you don't want to lose that window of opportunity)!

Before the patient even gets to the machine a few thing happen:
- they abstain from food for 3 hours but keep on drinking because dehydration leads to tachycardia
- they abstain from coffee for 24 hours so that the heart rate is not fast
- they abstain from viagra, etc., for 24 hours because GTN will be given once they are lying on the scanner to maximize the size of the coronaries. You use a full size GTN for everybody unless they weigh 55 kilos or less (a nice number to remember!), also because the dose of the metroprolol is 5mg :)
- they get beta-blocked with IV metroprolol to a HR of <65bpm. The dose used is 2.5-5mg and one doesn't go beyond a dose of 40mg.
- if they have a pacemaker then it is set to a HR of 60bpm and beta-blockade is then given until they reach 60bpm.

There are actually several shots taken by the machine, not just the coronary shot.
The first is a "scout" shot - it looks just like a plain film CXR.
On this scout one then draws a box to tell the machine to only detect X-rays in this box. The box typically begins a little above the heart, and ends a little below the diaphragm, with the side margins of it covering the lung fields all the way out to the chest wall.

A very small blob of Iodine fluid is then injected into an peripheral IV and one measures how long the iodine takes to reach the aorta. This will tell you how long to tell the machine to wait after the injection before it starts to shoot out X-rays (else will get unnecesary radiation).

Then, the real deal.

Things to know:
1. Because the dose of iodine is so small, it is very concentrated (360mg/ml) and shot into the veins very fast - 5ml/sec (that number 5 again!). Typical doses are 75ml or 100ml if CAGS.
2. If you want to evaluate the right heart too then you give a second bolus of contrast - 25ml at 2.5ml/sec.
3. The machine is typically scanning for 10-15 seconds.
4. Slices are no thinner than 2/3rds of a MILLImetre. That means that 1/3 of each MILLImetre is not captured. The reconstruction that is first tried is at 70-80% of the RR interval. However, for the low current part of the scan, the reconstruction lumps 2mm worth of data together and then ignores the next 2mm of data.
5. If the heart rate cannot be brought down then no dose modulation is given because you may need the systolic frames. Thus, the dose to the patient is twice as high.