**1. Introduction**

The aim of this chapter is to briefly describe the topic of paediatric dosimetry in nuclear medicine. Paediatric administrated doses are considered, firstly.

Too high administrated activities increased radiation dose without adding diagnostic information but too low activities may not permit an adequate examination. The optimal activity amount gives the desired diagnostic information with the minimum patient radiation exposure.

The next session of this chapter refers to the rules and procedures that are established for the evaluation of quantities as absorbed dose, effective dose or Paediatric Dose Reference Levels (PDRL).

In general, patient-specific dosimetry for a child examination in nuclear medicine differs from that of an adult due to different biodistribution and kinetics and variability of body size. It requires the knowledge of many factors like age, weight, biological and molecular background of the pathology and of the developmental process of the child, shape, mass, volume, anatomy, physiological indices.

Different values of instrumental parameters will be used for personalized dosimetric measurements in planar, SPECT, PET, or hybrid (SPECT/CT, PET/CT) examinations in children's studies.

Single Photon Emission Computed Tomography (SPECT) is a nuclear medicine imaging modality that generates 3-dimensional pictures of the examined body.

Positron Emission Tomography (PET) investigates areas of abnormal activity by revealing relative glucose metabolic activity in tissues and organs.

SPECT and PET can be combined with Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) producing the so-called "hybrid" imaging.

Nuclear medicine imaging examinations are performed in paediatric patients for diagnosis of diseases or injuries. The necessity of these examinations by radiopharmaceuticals must be ensured and then should be performed securely.

The International Atomic Energy Agency (IAEA) helps medical professionals to improve quality and safety by providing standards and guidelines, training and information resources. Radiation Protection of Patients (RPOP) is also the leading resource for patients and the public on the safe and effective use of radiation in medicine (IAEA, Radiation Protection of Patients).

World Health Organization (WHO) has edited leaflets and posters titled: 'Nuclear medicine exams in children: what do we need to know?', for patients and families with many simple advices on what do they need to know about nuclear medicine examinations in children.

The performance of a nuclear examination should be based upon the 'justification' radioprotection rule as well as the 'optimization' rule. Both are part of responsible and ethical medical practice.
