Additional to the standard SE sequences, an increasing number of various sequences have become available. However, the imaging characteristics on these sequences and thus the size of tumors might vary artificially. Key to a correct evaluation of study patients in a comparable way is to keep standardized imaging sequences during follow-up. A standard imaging protocol should contain T2/FLAIR (or proton density PD) sequences. FLAIR alone is not sufficient, because the cell density of a tumor cannot be sufficiently evaluated without a T2-sequence. The only performance of a T2 or FLAIR bears the risk to miss pathology (Fig. 4.1a–c). They should be combined with T1-weighted sequences before and after intravenous (i.v.) administration of contrast material. For children, the slice length should not exceed 4 mm. For small structures even much smaller slice lengths might be necessary. Although many surgical or radiotherapy treatment planning systems require three-dimensional sequences, these should only be additional to the core of standard imaging, because the contrast behavior of tumors on SE series and on 3-D-MPR series can differ considerably [117] making a comparison impossible (Fig. 4.1f–i). An automatized 3-D volume calculation of brain tumors can only be used in single or limited center studies because the acquisition parameters have to be uniform.

Diffusion weighted MRI (DWI) with the additional calculation of the ADC allows not only the depiction of infarcted brain but also an estimation of cellular density in the absence of hemorrhage [118]. Together with the signal intensity on T2-weighted MRI the ADC is a very useful tool for differential diagnosis [119]. Susceptibility weighted sequences (SWI) are useful for the identification of calcifications or blood degradation products. However, in pediatric brain tumors with the exception of craniopharyngiomas this feature is of little importance (see also Fig. 3.​9f) compared to its value in the differentiation of adult high-grade gliomas [115, 116]. As SWI sequences are prone to disturbances by susceptibility artifacts the skull base is not a useful region.

A residual tumor after resection can only be identified within the first 3 days after resection because after this time period a nonspecific reaction of the brain to the surgical trauma can create contrast enhancement which is virtually indistinguishable from residual tumor [120] (Fig. 4.2a–c). Unfortunately also during and very early after resection surgery induced enhancement may cause problems in the identification of a possible residual tumor especially in case of the use of electrocoagulation [121] (Fig. 4.2d–g). Therefore and because of frequent increased artifacts induced by air in the intracranial cavity (Fig. 4.2h, i), we do not advice to enter the MRI directly from the operating theatre to perform the early postoperative MRI, which seems very attractive in terms of logistics. Ideal for the early postoperative MRI are day 1 and 2 after surgery. However, if in case of a contrast enhancing tumor this time period is missed, then the correct identification of a residual tumor might not be possible for a long time or even forever (Fig. 4.2a–c). Nonenhancing tumors can only be identified on the basis of their T2/FLAIR or PD characteristics. Therefore, the comparability of MRI to the preoperative time point is of utmost importance. Also a change of magnetic field strength is problematic and should be avoided for the pre- and postoperative comparison and also for further follow-up.