Cardiac PET/CT

Cardiac imaging is a main stay within Nuclear Medicine in particular myocardial perfusion imaging which are often used to screen for acute coronary syndromes and the presence or absence of arteriosclerosis.  The more common isotopes used within Nuclear Cardiology are Thallium-201, Tc-99m and F-18.  Obviously other isotopes exist such as C-11 and N-13 for myocardial metabolism, but at our facility we generally stick to the "bread and butter" stuff.

The main focus of this section is on F-18 FDG cardiac imaging for viability.  Yes, Thallium-201 has been the champion of viability imaging for many years, but with the increase interest, the approved reimbursement (partial) of FDG cardiac imaging by the government and better imaging characteristics with PET, it is starting to catch on.

The imaging aspect of the myocardium is fairly straight forward with respect to PET/CT, since gating is not involved.  The tricky part is patient preparation to obtain good myocardial uptake of FDG.  We have had scans where the myocardium did not appear to have FDG uptake.  Needless to say, there was something amiss with the preparation.

Fig. 1  CT only, left ventricle.

Fig. 2  Fused PET/CT of the left ventricle.

Fig. 3  F-18 FDG AC of the left ventricle.  PET image only.

Why do we perform myocardial PET/CT scans in the first place?  

The rationale is to determine cardiac viability within patients with left ventricular dysfunction to figure out who will benefit from a re-vascularization procedure.  It is to find a better way to screen patients from those who will benefit from re-vascularization versus those who will not.  This really centres around the presence or absence of a "hibernating myocardium".  The PET/CT scan is compared to the patient's baseline Tc-99m sestamibi scan to see if there is an increased uptake of FDG in the region of the left ventricle where the defect is present (MIBI scan).  If there is an increase in FDG uptake, it suggests that a "hibernating myocardium" is likely, and if there is no FDG uptake the region in question is more likely to be a myocardial scar.  Thus the course of action is much simpler for the clinician with this type of information.

How do we prepare the patient to optimize the scan?

There are a variety of protocols, but the basic premise behind all of them is to drive the FDG into the myocardium.  The tricky part is usually working with diabetic patients and dealing with the blood glucose levels during the prep.  

Non Diabetics and Diabetics (oral glucose load)

-Patients fasting for 6-12 hours, minimum 6 hours

-Hold oral diabetic meds/insulin, if fasting overnight

-Diabetic patients are scheduled in the morning

Baseline blood glucose measurement (BGM) is taken, and depending on the level measured (mmol/L) there is a sliding scale in the amount of GLUCOLA (GLUTOL) - a very sugary drink, that is given.  If the initial BGM is lower than 5 mmol/L, then we give 50g of Glucola, if the BGM is higher than 5 mmol/L, then we give less of the drink, and if the BGM is really high we give the drink and a shot of insulin - yum!

1 hour post sugary drink, we get our second BGM, and again we have a sliding scale on how much insulin we should inject based on the measurement.  If the BGM is around 7.2 mmol/L, we give 1 unit of insulin, but if the BGM is higher in the 8 mmol/L - 11.0 mmol/L range, than higher units of insulin are given.

Injection of F-18 FDG is given during this time as well.  After the injection, there is a wait time anywhere between 45 - 60 minutes before the PET/CT scan is performed.

Diabetic Patients (insulin - glucose infusion)

-Patients fasting for 6-12 hours, minimum 6 hours

-Hold oral diabetic meds/insulin, if fasting overnight

-Diabetic patients are scheduled in the morning, but if they are scheduled for the afternoon, breakfast and medication as required

Baseline BGM.  If it is greater than 10 mmol/L, may require an insulin bolus before starting the insulin - glucose infusion.  Generally the insulin infusion is started first then the glucose afterwards, but if the patient had a low BGM initially the glucose would be infused first and then followed by the insulin.

Injection of F-18 FDG usually occurs when the patient's BGM stabilizes and or begins to decrease.  The ideal scan occurs around 5 mmol/L, but most times we are happy with the BGM dropping and stabilizes to an acceptable level.

PET/CT scan usually starts anywhere from 45 - 75 minutes, we generally try to keep it consistent between 45 - 60 minutes.  The insulin - glucose pumps do not go into the scan with the patient, they are disconnected and monitoring of the patient continues post imaging.