SHORT COMMUNICATION 

Annals of Nuclear Medicine Vol. l5, No. 1, 69-73, 200l 

Evaluation of resting brain conditions measured by two different methods (i.v. and oral administration) with 18-F-FDG-PET 

Mehedi MASUD,* Keiichiro YAMAGUCHI,* Hisashi RIKIMARU,* Manabu TASHIRO,* Kaoru OZAKI,* Shoichi WATANUKI,* Masayasu MIYAKE,* Tatsuo IDO** and Masatoshi ITOH* 

*Division of Nuclea Medicine and **Division of Nuclear Pharmacology, Cyclotron Radioisotope Center, Tohoku University 

Our aim was to evaluate regional differences between brain activity in two resting control conditions measured by 3D PET after administration of FDG through either the intravenous (i.v.) or the oral route. Ten healthy male volunteers engaged in the study as the i.v. group (mean age, 26 +- 9.3 years, +- S.D.) who received FDG intravenously and another 10 volunteers as the oral group (mean age, 27.9 +- 11.3 years, +- S.D.) who received FDG per os. A set of 3D-PET scans (emission and transmission scans) were performed in both groups. To explore possible functional differences between the brains of the two groups, the SPM-96 software was used for statistical analysis. The results revealed that glucose metabolism was significantly higher in the superior frontal gyrus, superior parietal lobule, lingual gyrus and left cerebellar hemisphere in the i.v. group than in the oral group. Metabolically active areas were found in the superior, middle and inferior temporal gyrus, parahippocampal gyrus, amygdaloid nucleus, pons and cerebellum in the oral group when compared with the i.v. group. These differences were presumably induced by differences between FDG kinetics and/or time-weighted behavioral effects in the two studies. This study suggests the need for extreme caution when selecting a pooled control population for designated activation studies. 

Key words: 3D-PET, FDG, oral intake, resting condition, pooled control 

INTRODUCTION 

POSITRON EMISSION TOMOGRAPHY (PET) is an imaging tool to visualize brain function by using energy metabolism 1-5 or cerebral blood flow distribution 6-9 as a marker. The FDG-PET method has been widely used to explore regional cerebral glucose metabolism to assess neuronal function. By applying appropriate statistical analysis (SPM in particular) over the brain image data, the task-induced regional activations were detected (Friston et al. 1995). 10 For the time being, FDG was administered by the intravenous route for human studies.  

Received July 24, 2000, revision accepted December 4, 2000. 
For reprint contact: Mohammad Mehedi Masud, M.D., Division of Nuclear Medicine, Cyclotron Radioisotope Center, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980-8578, JAPAN. 
E-mail : mehedi@cyric.tohoku.ac.jp 

An attempt to introduce FDG by the oral route to evaluate glucose absorption by the alimentary system was carried out. This method is also suitable for assessing the regional metabolic rate for glucose (rCMRGlc) in organs other than the digestive tract. Oral-FDG introduction is less intrusive and more suitable in pediatric patients or adults who are nervous about having intravenous injections. And the procedure is more demanding concerning the quality control process for the radiopharmaceuticals than the intravenous procedure. In brain science, fMRI has been replaced by PET due to its short time resolution. To trace human cognitive function, quick time resolution of less than I second is required which is out of reach for PET. Nevertheless, the longer time resolution of PET may have an advantage over fMRI in the study of emotion and behavior. The aim of this study is to determine whether the resting brain images obtained by i.v. injection and oral intake of FDG are identical or not.

SUBJECTS AND METHODS 

Twenty healthy male volunteers participated in this study as the i.v. and oral groups. The i.v. group (mean age, 26.9 +- 9.3 y, +- S.D.) were studied by intravenous FDG administration while resting, whereas the oral group (27.9 +- 11.3 y) received FDG orally. Each group comprised 10 male subjects. They served as resting control subjects for the other studies. Written informed consent was obtained from all the subjects after detailed explanation of the study protocol. This study was approved by the Clinical Committee for Radioisotope Use of Tohoku University School of Medicine. All the subjects abstained from eating and drinking for at least 3 hours before the start of the experiment. In this experiment, the resting condition was defined as remaining in the sitting posture after the intake of FDG in a light quiet room, with eyes open and without ear-plugs. An intravenous blood sample was obtained from all the subjects to measure the plasma glucose level just before the FDG injection. 

Study Protocol 

I.V. group: The i.v. group subjects rested for 5 minutes on a chair and then were administered FDG manually through an antecubital vein. The FDG infusion time was about 20 seconds and the radioisotope dose averaged 39.7 +- 6.1 (+- s.d.) MBq. The subjects remained in the same position for another 45 minutes in the same room before PET measurements were taken. After voiding they lay on a PET examination table and emission scanning started at 60 min after FDG injection. A 3D whole brain emission scan was performed with a PET Scanner (SET-2400W, Shimadzu Co., Japan) with an intrinsic in-plane spatial resolution of 3.9 mm at full width at half maximum (FWHM) and a 200 mm axial field of view 11 The emission scan lasted for 5 to 7 minutes depending on the subjects' physique. Transmission scan (post-emission transmission) which continued 5 to 7 minutes, was performed with a 68Ge/68Ga external rotating line source (370 MBqs at purchase) to correct the tissue attenuation of emission photons. 

Oral group: The oral group subjects rested for 5 minutes on a chair before the administration of FDG. They were given FDG (40.4 +- 3.9 MBq) orally and kept in the same sitting posture for another 15 minutes prior to the PET scan. After voiding, the subjects lay on the PET examination table and dynamic whole body emission scan (3 min/scan) was performed in 3 minute segments from the pelvis to the vertex with the PET scanner. The brain images, scanned at 60 min after the oral intake of FDG, were used for the analysis. The post-emission transmission scan, a 3 min/scan, was performed to correct the attenuation of emission photons. 

PET image data were reconstructed with a filtered 3D back projection algorithm in a Tohoku University super-computer (SX-4/128H4).11,12
 
Statistical Analysis 

The statistical parametric mapping (SPM-96) software was used for the analysis. First, the brain images were spatially normalized to minimize anatomical variations between subjects. An SPM blood flow template was used for this normalization. Smoothing was performed with a 14-14-14 mm isotropic Gaussian filter kernel. Voxel based statistical analysis was perfonued on these smoothed images by choosing "compare-groups: one scan per subject analysrs" (Friston et al 1995 and 1991). 10,13 The locations of relative hypermetabolic brain regions for each group were identified in the x, y and z standard co-ordinates (Talairach and Tournoux, 1988) 14 with the statistically significant threshold level at p < 0.001 without correction for multiple comparisons, but a significantly lower level of p < 0.005 was used to threshold result images to minimize possible type 2 errors in the statistical inference. 

RESULTS 

Figure 1 and Figure 2 depict the relative hypermetabolic regions in the i,v. and the oral group, respectively. Applying the statistical analysis, the i.v. group showed hypermetabolic brain regions in the right superior frontal gyrus, left superior parietal lobule, left lingual gyrus (Brodmann' s area (BA), BA6, BA7, and BA18) and left cerebellar hemisphere (Table 1 and Fig. 1 ). The oral group showed signs of relative regional hypermetabolism in the left superior temporal gyrus, bilateral middle temporal gyrus, right inferior temporal gyrus and left parahippocampal gyrus (BA38, BA21 , BA20 and BA35 respectively). The left amygdaloid nucleus, right pontine nucleus and bilateral cerebellar areas also showed signs of higher metabolism (Table 2 and Fig. 2). 

DISCUSSION AND CONCLUSION 

We attempted to compare resting brain images obtained through the use of oral or intravenous administration of FDG to the normal resting subjects, although the i.v. and oral subjects were engaged in different study protocols, while endeavoring to keep similar time schedules with regard to FDG administration and brain imaging. Intravenous administration of FDG is one of the most common methods used in clinical PET practice. 15-17 Since glucose is the principal energy source for brain tissue, regional cerebral FDG uptake is a clear indicator of the functional level of the brain in physiological l8,19 and pathological 20,21 conditions. The most remarkable differences between studies with i.v, and oral FDG administration were found in the limbic structures (parahippocampal gyrus and amygdaloid nucleus) in the oral group and visual association cortex (lingual gyrus) in the i.v. group (Figs. 2 and 1). A previous report which compared i.v. and oral-FDG administration as a case report, suggested that there was no difference in regional glucose metabolism in the brain.22 But in the case report a simple subtraction instead of the statistical image comparison was used. We searched regional brain areas with statistical differences by applying the SPM analysis. 

We used post-injection transmission for tissue attenuation correction of emission data. The contamination of emission components in these transmission data is known to reduce the accuracy of quantification 23 Although we did not correct emission contamination, these effects would be less in the 3D emission scans because a smaller dose of radioactivity was used. Selection of the SPM template is another problem in our study. As FDG templates are not available, the blood flow template was used for spatial normalization. The difference between radioactivity distribution for the two tracers, 15O-H2O and 18F-FDG is substantial in the periphery of the brain as radioactivity exists in the vascular compartment in the 15O-H2O study only. As these confusing effects do exist in both studies, they do not have to be taken into consideration and can be canceled out in the SPM group comparisons, but the use of an unmatched template for spatial normalization could introduce significant errors in the anatomical locations. Normalized FDG images may differ spatially from the Talairach space, but our study was not concerned with the precise identification of anatomical locations. 

The significant differences found in our study between via i.v. and oral FDG administration might emerge from inter-subject variations because there were different subjects in each group. This is a limitation of this study. When FDG is given orally, blood pool radioactivity may not be negligible due to slow FDG absorption, but no trace of large brain vessels was seen on the oral FDG images, and significant areas revealed by SPM analysis were not close to large vessels. We therefore considered that the effects of blood radioactivity in the oral FDG did not affect our results. 

Figure 3 shows average input functions of FDG-tracer in normal volunteers for oral and i.v. routes respectively obtained in our other studies. Regional FDG uptake is a function of regional transfer or rate constants. If the regional distributions of these rate constants differ substantially. FDG accumulation is possibly changed regionally. The determinant parameters in FDG tissue uptake are Kl and k3, since k4 is very small and k2 is related to Kl by the distribution volume (Vd). Kl and k2 are responsible for the influx and efflux of glucose between plasma and tissue, and k3 is the rate constant responsible for phosphorylation by hexokinase. Brain image taken 60 minutes after i.v. injection are considered to reflect radioactivity of the metabolized compartment, whereas brain radioactivity at a similar time after oral introduction includes both unmetabolized and metabolized compartments because FDG input remains in the blood. Therefore any regional rate constant changes may affect SPM results in i.v. and oral input comparisons. Nevertheless, we have no report that confirmed the systemic regional rate constant changes in the limbic structures, especially in the parahippocampal gyrus and amygdaloid nucleus which were identified in our study. 

Although we tried to have similar resting conditions during PET scans, the scanning set-ups for the two groups were not completely matched due to different study purposes. Sequential blood samplings were taken in the oral group only. Habituation and discomfort should also be taken into account due to slow FDG kinetics in the tissue. Since arterial input reaches a peak at around 60 minutes after oral introduction, FDG brain images by the oral route reflect brain condition in the later phase. Therefore the effect of behavioral distress caused by longer scanning time should have a greater affect on the oral FDG brain images than by the i.v. route. The identification of the limbic structures in the oral study also indicates this possibility. Although the precise mechanism involved in the difference in brain activity for oral and i.v. FDG introduction is not known, this study indicates the need for substantial caution in FDG PET activation studies especially when normally resting subjects are to be used for a pooled control resting experiment. 

ACKNOWLEDGMENTS 

The authors would like to convey special thanks to all the staffs of Cyclotron Radioisotope center for their courageous collaborations and technical supports. This study was conducted in collaboration with the multi-center PET activation projects in Japan (Senda M. et al). 

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