The Normal Brain Scan

The Normal Brain Scan

Alfred Buck

Valerie Treyer

Bruno Weber

Specific Tracers


The great majority of the clinical brain scans are performed with fluorodeoxyglucose (FDG). It is important to bear in mind that FDG traces glucose metabolism and that all structures with high glucose demand will demonstrate high FDG uptake. In the head, not only the brain but also muscles, glands, and lymphatic tissue may display high FDG accumulation (see Fig. 33.21). An example of increased FDG uptake in the eye muscles is shown in Figure 21.1, and when searching for tumor metastases, the high uptake of FDG may be an imaging pitfall. In a healthy brain, it is actually possible to determine glucose metabolism quantitatively. The method was introduced by Sokoloff et al. (1). However, quantification of glucose metabolism is complex and usually not required for clinical purposes. Luckily, simple FDG uptake after a certain period is sufficient for clinical diagnosis. Typically, scanning is initiated 40 minutes following injection of FDG. In 3-D mode, enough counts are acquired during a 10-minute scanning period if the injected activity was on the order of 100 to 200 MBq. With lower activity, the scanning duration has to be extended accordingly.

Although the mass of the brain is only a small fraction of the mass of the whole human body, it utilizes about 25% of the total metabolic energy. Most of the energy demand is believed to be necessary for the maintenance of intra-extracellular ionic gradients by active sodium–potassium pumps. Moreover, the highest glucose consumption is believed to be localized around the pre- and postsynaptic zone. An excellent introduction on brain energy metabolism is given by Magistretti (2). Only in particular circumstances can substrates other than glucose be utilized by the brain.

A typical normal brain scan, together with a T1-weighted MR image, is shown in Figure 21.2. The resolution of the PET scanner used was around 6 mm (full width
at half maximum [FWHM]), while the cortex is approximately 2 to 4 mm thick. As a result, the actual concentration of FDG in the cortex is somewhat underestimated. Nevertheless, there is reasonable delineation of anatomic structures. It is easy to identify the cortex, the putamen, the head of the caudate, the thalamus, the cerebellum, and the brain stem. The high physiological FDG uptake in the brain is ideal for the study of brain metabolism.

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Jul 27, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on The Normal Brain Scan
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