**12. Conclusion**

Pituitary adenomas can appear as hypermetabolic lesions on FDG PET [68, 69], with higher uptake seen in macroadenomas than in microadenomas [69]. Increased DOTATATE uptake has been reported in intracranial metastases of pituitary carcinoma and may be useful in the decision to treat withpeptide receptorradionuclide therapy [70, 71]. METsuccessfullydetected

PET/MRI is a relatively novel hybrid diagnostic imaging device that can simultaneously acquire PET and MR images of the brain and other body regions. PET images show the distribution of an intravenously injected radiotracer, whilst MRI depicts the local responses of atomic nuclei to high-frequency radio waves when placed in a strong magnetic field. PET/MRI represents an advance on hybrid PET/CT imaging systems that are currently used in routine

**a.** *Reduced radiation exposure for patients.* The use of MRI to correct PET images for the attenuation of emitted radiation by overlying tissue avoids the ionising radiation of CT. Because the acquisition time for MRI often exceeds that for PET, there are also opportu‐ nities to reduce the amount of radiotracer administered, by increasing PET acquisition

**b.** *More accurate anatomical localisation of areas of radiotracer uptake.* The simultane‐ ous acquisition of MR and PET images reduces the likelihood of patient movement

**c.** *Compensation for some limitations of PET.* The ability of PET to identify tumour sites is constrained by background physiological tracer uptake in some organs. For the most commonly used clinical PET radiotracer, FDG, these organs include the brain (as well as the liver and bone marrow) which are frequent sites of tumour recurrence after initial treatment. PET interpretation is also complicated by processes other than tumour infiltration that can cause radiotracer uptake, the most notably inflammation which can be particularly problematic when assessing cancer status after treatment, especially

Many of these advantages are particularly relevant to brain tumour imaging, and indeed, the first exploration offeasibility of hybrid PET/MRIin clinical oncology was in brain tumours [73].

PET/MRI could theoretically harness the advantages of PET imaging with various radiotrac‐ ers to accurately distinguish tumours from surrounding normal brain tissues—and the ability of advanced MRI techniques such as fMRI and diffusion tensor imaging to map the spatial relationship between tumours and adjacent functional brain tissues and white matter tracts—

clinical practice for the assessment of patients with cancer and other diseases.

The integration of PET with MRI rather than CT has several advantages:

causing misregistration of the two image sets.

surgery or radiotherapy.

at the same time.

all cases of craniopharyngioma in 10 patients [72].

**11. PET/MRI**

196 Neurooncology - Newer Developments

time.

Whilst MRI remains the gold standard for imaging of brain tumours, future applications in‐ tegrating PET—with its enlarging gamut of radiotracers—and MRI are likely forthcoming. This chapter briefly summarised the current status of the most commonly used radiotracers for the molecular imaging of brain tumours (**Table 2**). It will hopefully also serve as a useful guide that the reader can refer back to and build upon with future reading.



**Table 2.** Summary of radiotracers most commonly used in PET imaging of brain tumours.
