**5. Paediatric diagnostic reference levels (PDRLs)**

The International Commission on Radiological Protection (ICRP) Publication 73 was first to introduce the term 'Diagnostic Reference Level' (DRL) in 1996, a concept that was further developed further. The DRL has been proven to be an effective tool towards the optimisation of protection in the medical exposure of patients in diagnostic procedures.

#### **5.1 Planar-SPECT imaging procedures**

For planar nuclear medicine imaging, DRLs have been set either by administered activity (MBq) or by administered activity per body weight (MBq/kg).

For SPECT imaging procedures, DRL values should be used in the same way as for planar nuclear medicine procedures. DRL values for SPECT studies are usually slightly higher than for the same radiopharmaceuticals used for planar imaging.

#### **5.2 Positron emission tomography (PET)**

Specific radiopharmaceuticals are used for PET imaging, depending on the scope of the study. F18-fluorodeoxyglucose (F18-FDG) is used for diagnosing, staging and assessing therapeutical schemes in cancer, inflammation, viable myocardium and brain diseases by revealing relative glucose metabolic activity in tissues and organs. N13-ammonia or Rb82-chloride assesses myocardial perfusion. Ga68-DOTATATE and DOTATOC in neuroendocrine tumours reflecting the status of somatostatin receptors. As the physical half-lives of radionuclides and biological half-times of radiopharmaceuticals are different, DRL values have to be set for each one.

European guidelines provide a calculation system according to body weight, image acquisition method (two-, or three-dimensional), scan speed (minutes per table position) and table overlap during the following PET acquisitions.

### **5.3 Hybrid imaging (PET-CT, SPECT-CT and PET-MRI, SPECT-MRI)**

PET and SPECT have been combined with the modality of CT generating the so-called hybrid systems of PET-CT and SPECT-CT accordingly. Nowadays, they have been combined with the Magnetic Resonance Imaging (MRI) modality too, as these combinations increase diagnostic accuracy by providing both functional and anatomical images of the body.

The acquisition of accurately co-registered anatomical and functional images is a major strength of combined modality (hybrid imaging) devices. The patient dose from a PET-CT or SPECT-CT examination is the combination of the radiation exposures caused by the radiopharmaceutical and by the CT study via the exposure to ionising radiation.

The MRI component of PET/MRI or SPECT/MRI does not increase the patient dose considering that it uses non-ionising radiation, so from a radiation protection point of view, this hybrid imaging is preferable in paediatric examinations.

In the framework of a PET/CT or SPECT/CT, the CT portion of the examination consists of a localiser radiograph and the helical CT scan. If a CT is solely performed for attenuation correction and co-localisation, the acquisition parameters (tube current, voltage, slice thickness, rotation time, and pitch) should be selected in order to minimise the patient's radiation exposure. A low-dose CT used in hybrid imaging is sufficient for attenuation correction and anatomic localisation and proper for paediatric examinations.
