Let's say you have a referral for a CT Chest/Abdomen/Pelvis, and the clinical indications say: 1) ?Primary Lesion
2) ?Pulmonary Emboli
Or perhaps there is a CT IVP referral, and the radiologist wants to look at both the ureters and the renal parenchyma, but wants you to minimise radiation dose.
These scenarios don't arise very frequently, but when they do it's a great opportunity to show off some of your technician skills, while maintaining ALARA principles. This article shows you how a split bolus CT scan is achieved.
Let's look at the first example: CT CAP + CTPA
Let's say that our scan delay times for each scan on their own are:
CT CAP = 70 sec post injection
CTPA = 10 sec post injection
Since CT CAP has the longest scan delay, we'll set the scan delay to match this. Therefore, our first injection bolus will start at time = 0 seconds of the delay countdown and will be used for the Portal Venous phase of the CT CAP. Let's say we inject 70 ml at 3.5 ml/sec = 20 second injection time.
The CTPA bolus will need to begin 10 seconds before the scan, therefore there needs to be a 40 second pause in between the two injection boluses (70-20 = 50 - 10 = 40). We know that for a good CTPA, the contrast injection has to finish 3 seconds before the scan begins, in order to flush the SVC free of contrast to limit beam hardening artefact. So that means the CTPA contrast bolus will last for 7 seconds at 5 ml/sec = 35 ml.
If we put it all together, the protocol looks like this:
Scan delay = 70 seconds
Injection A:
Time = 0 seconds
Volume/Rate= 70 ml at 3.5 ml/sec
Duration = 20 seconds
Pause = 40 seconds (20 to 60 seconds post injection)
Injection B:
Time = 60 seconds post injection
Volume/Rate= 35 ml at 5 ml/sec
Duration = 7 seconds
Saline flush = 5 seconds immediately after injection B.
The second example is much more straight forward: CT IVP + Portal Venous Abdomen
Because contrast remains in the ureters for an extended period of time, we don't need to be as precise with our bolus timing. Simply inject a smallish amount of contrast (40-50 ml is usually enough) and wait anywhere between 5 and 30 minutes, or even longer! Note that the time it takes for contrast to enter the renal pelvis and ureters will depend on the patient's eGFR - the lower the number, the longer it takes.
When you have reached the desired time after the first injection, set the scan delay to 70 seconds (however some radiologists prefer a dedicated "nephrographic phase" for these cases, which is around the 80-100 second mark) and begin the second injection at time 0 of delay countdown, using a similar injection bolus to a PV abdomen (70-100 ml at 3 ml/sec).
Pro Tip: Because every patient will have a different delay time for an optimal IVP phase, perform a low dose single slice scan through the urinary bladder before beginning the second injection to determine if enough contrast has passed through the ureters.
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