This study confirms the expected significant negative association between BGL and brain 18F-FDG uptake that has been identified in multiple prior studies.17–19 This observed effect is thought to be most likely due to competitive inhibition of 18F-FDG uptake by glucose at glucose transporters (GLUTs). Viglianti et al.19 previously suggested that variation in serum glucose levels affects 18F-FDG uptake in a non-linear/dual linear fashion due initially to saturation of GLUTs followed by saturation of intracellular hexokinase at higher serum glucose levels with a threshold of 125 mg/dl (i.e. 6.9 mmol/L). However, when we compared three possible models with and without transformation ((a) a linear model with log-transformed outcome variable, (b) a generalised linear model with log link function, and (c) a linear model with no transformation), we found that the linear model with no transformation had a comparable overall model fit to the other two more complex models, thus justifying the choice of the simpler model that we have used for our analyses. It is possible that this finding may be due to the majority of our patients’ BGLs falling within the euglycaemic range in which 18F-FDG uptake is still determined by the level of GLUT saturation.
We also found that increasing serum ketone levels have a suppressive effect on brain 18F-FDG uptake that is additional to, and independent of, the suppressive effect of BGL. Suppression of 18F-FDG brain uptake during ketosis has been observed previously in humans using PET scans under experimental conditions,20,21 but to the best of our knowledge this is the first time it has been found to have a measurable effect on 18F-FDG PET scans performed for clinical purposes. Although brain SUVbw varied with both serum glucose and serum ketone levels, it is interesting to note that in our patient group the effect of ketones was partially masked by BGL, being found to be more obvious once the relationship between BGL and 18F-FDG uptake has been accounted for. This complexity may explain why the suppression of brain 18F-FDG uptake due to ketosis has not previously been recognised in clinical scans.
The reduction we have found in 18F-FDG uptake associated with ketones most likely reflects a true reduction in glucose metabolism by the brain due to the preferential use of ketone bodies as an alternate energy substrate independent of glucose availability. Ketone bodies are a more efficient source of adenosine triphosphate (ATP) production per unit of oxygen than glucose.2,20−24 Our analysis indicates that the reduction in brain 18F-FDG uptake associated with increasing ketone levels can be modelled as a straight line so long as BGL is simultaneously considered. It has previously been shown that cerebral ketone uptake increases linearly with increasing serum ketone concentrations,21 and that an inverse relationship exists between brain glucose and ketone metabolism in normal adults during short-term moderate dietary ketosis,20 leading those authors to propose that overall cerebral metabolic rate (CMR) is a sum of CMRketones + CMRglucose.
These findings have direct clinical implications when SUV thresholds are used to help differentiate between pathologic and physiologic processes (e.g. comparing pathological uptake in glioma to that of normal brain tissue), suggesting SUVs may need to be corrected for serum ketone levels as well as BGL in such situations.
Our results indicate that BGL and serum ketones supress 18F-FDG uptake in all areas of the brain. However, we found that for both BGL and ketones, the degree of suppression is not uniform in all areas and appears to be more pronounced in the region of the precuneus. This might have implications in clinical scanning. A specific pattern of regional cerebral glucose hypometabolism is seen in AD characteristically involving the precuneus.25 Reiman et al.26 demonstrated similar patterns of regional glucose hypometabolism in cognitively normal patients at risk of late-onset AD (i.e. carriers of the apolipoprotein E e4 allele) several decades before the onset of symptoms and structural changes on anatomic imaging. However, assessing the uptake in the precuneus by normalising the uptake in the patient’s brain and comparing it to a standard normal database has been complicated by studies that suggest that hyperglycaemia suppresses the uptake of 18F-FDG in the precuneus to a greater degree than other regions of the brain.17 This would mean that when normalisation of 18F-FDG uptake using whole brain or cerebellum is used in the context of hyperglycaemia during comparison of brain uptake to normal databases, the precuneus may demonstrate an artefactual reduction in uptake potentially leading to a false positive diagnosis of AD. Our study strongly supports these previous findings that the measured reduction of brain 18F-FDG uptake related to increasing BGL is more marked in the precuneus than it is in the whole brain or the cerebellum. In addition, we have found that elevated serum ketone levels induce a similar difference in regional effect on 18F-FDG uptake, with the degree of suppression related to ketones appearing more marked in the precuneus than in the whole brain or cerebellum. This should also be taken into consideration when undertaking whole brain normalisation and comparison to a normal database, particularly when assessing for early Alzheimer’s disease.
There are several limitations of our study. Patients were provided with detailed information on the prescribed preparatory diet and the importance of strict adherence, however, there was inevitable variability in the quantities of glucose/carbohydrates consumed and degrees of dietary adherence. Interestingly, the patients who reported failure to adhere strictly to the diet were all in ketosis at the time of their PETs, while all the patients who were not ketotic reported strict dietary adherence. Of the twelve (23%) patients not in ketosis, five were diabetic including the single patient with T1DM, with two patients on regular insulin. Nonetheless, this has meant that we have a relatively large range of BGL and serum ketone levels to examine and does not impact our findings of a negative association between 18F-FDG uptake, and serum glucose and ketone concentrations.
We included both non-diabetic (n = 39, 75%) and diabetic (n = 13, 25%) patients in our study, with five (10%) of the diabetic patients being on regular insulin. Insulin is known to impact the actions of certain subsets of GLUTs as well as the normal ketone response.27 Our patients were instructed to withhold insulin during their fasting period. Due to fasting and insulin being withheld prior to PETs we consider that all our patients would be in a low insulin state, but we did not measure insulin levels to confirm this. Furthermore, intracellular uptake of 18F-FDG in the brain is primarily driven by GLUT1 and, to a lesser extent, GLUT3, the actions of both being independent of insulin.28,29 Therefore, we believe that insulin levels should have a negligible impact on the cerebral 18F-FDG uptake seen in our study.
Twenty-seven patients (52%) were administered unfractionated heparin intravenously 15 minutes prior to 18F-FDG injection to increase serum free fatty acid (FFA) levels. An increased availability of FFA has been shown to increase rates of ketogenesis and serum ketones levels in a relatively short period of time after administration (i.e. < one hour),30 more pronounced with unfractionated rather than low-molecular weight heparin, although transient for doses in the range used in our cohort.31 While the administration of heparin is a potential confounder, we did not find a significant relationship between cerebral 18F-FDG uptake and heparin administration.
Patients may occasionally self-administer benzodiazepines when undergoing imaging studies to ameliorate symptoms of claustrophobia. Benzodiazepines can result in a global reduction in cerebral 18F-FDG uptake.32 Given these studies were performed to assess for potential cardiac inflammation, we did not enquire about or record this information. It is possible that some of our cohort may have self-administered benzodiazepines, however, there is no reason to believe that this will be more or less likely to be related to BGL or ketone levels and is likely to represent a random effect.
It is standard at our institution to perform cardiac inflammation PET/CTs with patients’ arms above their heads to reduce beam-hardening artefacts through the thorax and heart. A few patients in our cohort were scanned with their arms down due to physical limitations. While this has the potential to affect the accuracy of the measured cerebral SUVbw, the impact is felt likely minimal given that our analysis was performed on the attenuation corrected PET images and with us evaluating whole brain SUVmean (corrected for bodyweight) and with the SUVmean extracted from relatively large brain regions thereby reducing regional inconsistencies in measured uptake.