18 Chapter 1 use strong magnetic fields and radio waves to generate images, which in this thesis will be used to investigate images of the brain in children with craniosynostosis. T1 weighted image A T1-weighted (T1W) image is a basic MRI pulse sequence and depicts differences in signal based upon intrinsic T1 relaxation time of various tissues (Figure 4). Clinically, T1-weighted images are used for depicting the anatomy of the brain, where tissue fluid characteristics are presented in grey scale differences. In the upcoming studies we will use T1w images to measure anatomical structures i.e. the size of ventricles, the herniation of the cerebellar tonsils and the thickness of the cortex. Arterial spin labeling Two studies will use Arterial Spin Labeling (ASL). ASL is a MRI technique that provides injection-free measurements of the absolute brain perfusion by magnetically labelling water protons in the blood vessels of the patient.11 ASL is used for several pediatric imaging applications, for example in vascular diseases, tumors, epilepsy and seizures, to detect cortical hyper perfusion or hypoperfusion.86 The main advantage of the ASL technique is that it measures absolute brain perfusion and that it is does not require administration of an exogenous contrast agent.87 ASL magnetically labels arterial blood water when it flows through the neck perpendicular to the cervical arteries. By a radiofrequency pulse the water protons in the blood will get inverted; the blood will get a negative magnetization.88, 89 After a post-labeling delay (PLD) in which the labelled blood flows into brain tissue, an image of the brain will be acquired (Figure 5).90 As depicted in Figure 6, by making two images a third image can be provided with the weighted perfusion.91 The control image is the image without labelling of the blood. The second image contains both the normal stationary blood as well as the negative magnetization from the inflowing labeled blood.92 Therefore, an ASL image is weighted for the cerebral blood flow (CBF), i.e. the volume of blood that flows through 100 grams of brain tissue each minute (mL/100g/min). Figure 4. T1-weighted image.
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