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.

Burnt Out

Top:  Anterior feet (detector 1); Bottom:  Posterior feet  (detector 2)

What's wrong with this picture?  

First off, this is an In111-WBC scan to localize infection in the ankle.  The top image is the anterior view (detector 1) and the bottom image is the posterior view (detector 2).  What is interesting is that there is a "bite" taken out of detector 2.  As you know, this is probably not something that you would want in your image, but rather a technical artifact in which a photomultiplier tube has burnt out during the image acquisition.

There are 2 things to discuss here:

1.  White Blood Cell (WBC) scans:  

The decision to use Tc-99m WBC versus In-111 WBC is based on the a decision tree approved by our physician.  It really revolves around whether the patient is diabetic and or has a prosthetic or implant post surgery.  With WBC scans, it's primarily used to determine infection such as osteomyelitis, which for the most part represents 99% of our cases.

a.  Not diabetic or no implant - Tc-99m WBC

b.  Diabetic with (?) foot infection and/or has prosthetic implant - In111-WBC 

All WBC cases have a bone scan first followed by either the Tc-99m or In-111 WBC scan afterwards.  The one main thing to note is that if the In-111 route is taken, they are given Tc-99m sulphur colloid first before the In-111 WBC injection on the same day to determine if there is any bone marrow involvement with the site in question.

Having said this, I have yet to see a positive scan to date.  The current case above was to determine post surgical infection of the right ankle.  The scans for both the sulphur colloid and the In-111 WBC were unremarkable, but there were post surgical changes on the right distal fibula and ankle (image not shown) on the bone scan.

What about gallium?  At our site, gallium is not the isotope of choice unless discitis is indicated.  In-111 WBC's are not as effective in relation to Ga-67 in conjunction with a bone scan, to determine discitis/vertebral osteomyelitis.  Accuracy for WBC's are generally low for this type of imaging (Henkin et al. Part IV p. 1129)

2.  Photomultiplier tubes (PMT)

Basic premise with PMT's are to convert light into an electrical current.  There are a multiple of reasons why PMT's burn out.  Daily quality control and monthly preventative maintenance programs are important to detect these irregularities.  PMT's have been around for many years, but with new solid state cameras like the CZT (Cadmium Zinc Telleride) equipped systems, PMT's may be a thing of the past.  Who knows?

Whole Body Gallium - Lymphoma

Contrary to most standards of practise, gallium-67 whole body imaging is still being performed for neoplastic staging of lymphomas.  Many centres have now moved away from this and use F18-FDG as part of their protocol.  However having said that, a whole body scan along with a four bed SPECT/CT was performed to aid in the staging process.

Indication:  Staging of neoplasm.  Lymphoma of the left tibia.

Findings:  Intense gallium avid lesion in the left proximal tibia and fibula, corresponding to a know bony lesion at this site.  Further gallium avid activity are localized in 3 sites on the left parietal region of the skull, along with another foci of activity in the left parietal skull.  In addition, abnormal gallium activity is found midline to the abdomen below, but separate to the transverse colon.  No other soft tissue abnormality is observed.

Hx:  Imaging with gallium-67 has been around for almost 40 years, first described by Edward and Hayes in 1969 (J Nucl Med. 1969;10:103–105).

It has been used in a variety of indications such as inflammation, infection and tumour localization (osteosarcomas, lymphomas, testicular tumours, melanomas), but for the most part, gallium has been used in oncology to stage and monitor the response of tumours to treatment.

With regards to imaging, most patients are scanned 72 hours post injection. Encouraging them to drink lot's of fluids within the first 24 hours is done to eliminate gallium from the genitourinary system, afterwards the patient may take a laxative to help in eliminating the gallium through the bowels.

Upon 72 hours, a whole body gallium image is taken.  Depending on the clinical site, the arms may be raised to allow better visualization of the axillary regions or they may be left down by their side.  SPECT/CT is also used afterwards to improve the specificity and sensitivity of the study.

F18-FLT PET/CT Imaging

FLT = [F-18] Fluorothymidine

FLT is a unique PET tracer in which it targets cellular proliferation, the rate of DNA synthesis, in patients to evaluate the effectiveness of anti-neoplastic drugs affecting DNA replication.  The thymidine component of the FLT does not actually get incorporated into the DNA replication process because of a phosphorylation event creating an ionic FLT-MP (monophosphate).  It is FLT-MP that gets trapped within the proliferating cells.

Majority of the patients that we see are already on their cycles of 

anti-neoplastic treatment when imaging with F18-FLT.  Normal 

bio-distribution of FLT includes the bone marrow, lymphoid tissue, kidneys and bladder.

Imaging with FLT is no different than FDG, except for the fact that the injected dose is based on 3MBq/kg rather than 5MB/kg with respect to FDG.  At least at our institution. The post injection imaging time is still relatively the same, approximately 60 minutes +/- 10 minutes however it is not necessary to obtain blood glucose measurements since the mechanism of action is different than that of FDG.