Radiological imaging of neonates
Authors: Δουγένη Ευτυχία 
Published: January 2023

Neonates, and especially premature (with gestational age as low as 24 weeks and birth weight as low as 600 g) are predisposed to a variety of minor to life-threatening health complications, and often require special care during the first days of their life in the Special Baby Care Unit (SBCU). One of the primary methods of monitoring neonates in the neonatal unit is through radiography. During hospitalisation in the SCBU premature neonates may undergo a significant number of radiographic procedures to assist mainly in the diagnosis and management of lung diseases, such as respiratory distress syndrome and meconium aspiration syndrome, which represent one of the major clinical concerns in the neonatal care. Additionally, repeated radiographs are required to confirm correct positioning of the multiple tubes and lines (e.g. umbilical catheters, long-lines, endotracheal tubes) that are placed during the course of their management. Additionally, neonates may undergo a variety of other radiological investigations during the management of their condition. Although neonatal Computed tomography (CT) e.g. for assessment of congenital heart abnormalities or head trauma, is relatively infrequent compared to radiography, in view of the high organ doses involved with this modality, it is important for radiologists and operators to be able to easily estimate doses and cancer risks, and optimise protocols depending on patients size. Radiation damage is more extensive on dividing cells, thus exposure during childhood increases the risk of suffering the detrimental effects of ionizing radiation. Also, as a consequence of longer life expectancy, compared to adults, there is a greater period for the potential delayed effects of radiation to be expressed. Furthermore, the small sizes of the newborn bring critical organs frequently within or close to the primary beam resulting in a relatively higher overall exposure per examination than may be the case with adults. Therefore, it is emphasized that providing an image which is adequate for the clinical imaging task with the minimum radiation dose, whilst considering the best outcome of their treatment, is of particular importance to the neonate. Handling of the neonate inside the incubator should be minimized, since lifting or moving the infant to position the cassette, can lead to hypoxia, bradycardia, cerebral haemorrhage and could cause accidental dislodgment of tubes and lines. Modern incubators incorporate an imaging tray under the bed to facilitate placement of the radiographic cassette 10 with minimum disturbance to the infant and the conditions inside the incubator. The current study was divided in three parts. Part 1 The first objective of the study was to perform a survey on the doses encountered during chest radiography in the Special Care Baby Unit at the University Hospital of Patras in Greece. In our sample (378 chest and chest–abdomen radiographs) the majority of neonates were preterm and the mean number of radiographs was 3,6 per neonate (maximum 26). Entrance Surface Doses (ESDs) were estimated, Dose-Area Product (DAP) values were measured and the quality of radiographic image was evaluated for clinical film-screen neonatal images. The ESD values obtained are lower than, or comparable to, the values obtained in studies in other European countries and the majority showed good compliance with the recommended diagnostic reference levels proposed by the National radiological protection board and the European Commission (50 μGy and 80mGy respectively). A relatively wide variation in ESD (16-77 μGy, mean 38.2mGy) and DAP (1.2-15 mGy·cm2 , mean 7.2 mGy·cm 2 ) values is observed even for neonates of the same weight (3,5- fold in group 4). Image quality evaluation demonstrated that high tube voltage techniques, related to low ESD values, the image quality is adequate to answer the diagnostic question, thus adhering to the driving philosophy of the ALARA principle. An optimized standardized radiographic protocol is suggested depending on birth-weight for film screen radiography. Part 2 Since computed radiography is now the most used digital imaging modality in many centers, the study was further developed to compare techniques and exposure factors applied in SBCU of the Royal Victoria Infirmary hospital in Newcastle Upon Tyne, in the UK. The range of ESD values in the UK smaller that that found in the Greek hospital. All doses are below the Diagnostic Reference level. Comparing the two departments, a common overlap in the low range of doses is observed for high tube voltage techniques. That is in agreement with the optimised protocol proposed in Part 1 of this work. Additionally, the exposure index for most of the neonatal radiographs were in the range 1.8-2.1, as recommended from the Agfa manufacturer. With the use of an anthropomorphic neonatal phantom and thermoluminescent dosimeters (TLDs), our results demonstrated that the organs receiving the highest doses during neonatal chest AP radiography are the breast, liver lung and thyroid (when in the primary 11 beam) which are also amongst the most radiosensitive organs. Organ dose coefficients were deduced for lung and breast with respect to entrance kerma free-in air and entrance surface dose. Those are comparable to coefficients deduced in national reports and other studies. The phantom was irradiated with various techniques (58-64 kVp, 0.64-3.2 mAs) in order to compare image quality for the two radiographic methods, i.e. with the cassette ‘on the bed’ and ‘in the tray’. The Signal to noise ration (SNR) was measured for 10 regions of interest (ROIs) representing anatomical features such as lung, retrocardiac lung, rib on lung, retrocardiac rib on lung, tissue, rib on tissue, spine and was plotted in graphs for each kVp and for the corresponding mAs values used clinically. The contrast to noise ratio (CNR), was measured between the lung and the lung behind the heart, rib on the lung and lung, as well as between lung and tissue. In order to achieve the same dose to the detector when switching from “on the bed” to “in the tray” method a necessary increase in the mAs is required of approximately 40%, when using the same tube voltage. The reduction in the SNR values is more prominent for low xray beams, ranging from 45% on average for the low tube voltages to 27% for the higher tube voltages. An increase of 55% in mAs is required to maintain the SNR. The decrease in the contrast to noise ratio (CNR) between lung and lung behind the heart, rid and rib behind the heart, and lung and tissue caused by the use of the tray was 40-46% for low and 20-25% for higher tube voltages. The figure of merit, CNR2 /ESD was calculated and it was concluded that the higher tube voltages are preferable for imaging neonates with the cassette positioned in the incubator imaging x-ray tray. Reducing neonatal handling may ultimately improve outcome and speed up therapy and recovery, therefore structure and composition of materials of incubators should be improved e.g. thinner transparent bed, made from low attenuation materials or a grid of carbon fibre material can be tested. Part 3 Part three is presenting a dosimetric methodology for neonatal CT, updated based on the new tissue radiosensitivity factors, published in the new report from the International Commission on Radiological Protection 2007. That impact of the new factors on the effective dose and the dose conversion coefficients in neonatal and paediatric CT . In order to obtain size dependant coefficients, it was extended to paediatric CT scans, including the ages of 1, 5 and 10 years old. Despite the relatively low frequency of CT examinations in neonates compared to radiography, since CT is related to high organ doses, optimized protocols adjusted to clinical 12 task and patient size should be implemented for neonates. Neonates represent the most radiosensitive group of patients. Additionally, in view of their longer life expectancy, and possible repeated radiological examinations, knowledge of organ and effective dose is vital for the estimation of cancer risks. The dosimetric methodology presented in this study, utilizing anthropomorphic neonatal phantom, was updated based on the new set of tissue weighting factors suggested by the ICRP 2007. The tissue weighting factor for breast increased from 0.05 to 0.12 and the factor for gonads has decreased from 0.20 to 0.08. Also liver, bladder, esophagus and thyroid have changed from 0.05 to 0.04. The effect of these changes in the calculation of E through the EDLP coefficients is investigated in this work. Effective dose is a parameter related to the relative biologic risk and is not a physical parameter that can be measured. Four non-overlapping anatomical areas were scanned, namely, head, chest, abdomen and pelvis. Organ doses were measured utilising TLDs and effective dose was calculated for each scan. This work was primarily focused on neonatal CT, however, in order to obtain size dependant conversion coefficients it was extended to paediatric CT scans, including the ages of 1, 5 and 10 years old. A standard protocol was used for all patient sizes and anatomical areas. Effective dose conversion coefficients, normalised to dose-length product, were derived for neonates and children as a function of patient size for four anatomical areas (head, chest, abdomen, pelvis). The highest coefficient was found to be for the abdomen. The coefficients for chest, abdomen and pelvis were approximately 2-3,5 times higher than those for head, similarly to those published previously. The changes in the dose coefficients range from an increase of up to 25 % for chest CT scans to a decrease of 30–40 % for scans of the pelvis. No significant changes were observed for scans of the head and abdomen. The results of this study were validated by irradiating the neonatal phantom on a clinical ‘chest neonatal protocol’ (base of the neck to end of the liver). The predicted effective doses, using the deduced coefficients, were within 20% of the measured values. The results suggest that even for scanners of different manufacturers and maximum slice widths, the coefficients can be used to predict E however, the current results are only indicative of the influence of the new ICRP103 weighting factors on the paediatric EDLP coefficients, and further investigation is necessary, including the effect of varying tube voltages. 13 The neonatal phantom was additionally irradiated on two other different scanners (Toshiba and the Siemens) on the default clinical chest neonatal protocol in a paediatric hospital. It was observed that the main difference between the two protocols on the scanner was the tube voltage. For the neonatal protocol in the Toshiba scanner, the default tube voltage was set to 80kVp, whereas on the Siemens it was 120kVp, as used for adults. The scan parameters were used to calculate the E from the IMPACT software program. The values produced through the IMPACT dose calculator, for those clinical protocols, compared good with those estimated by application of the deduced dose coefficients in this work, and verified our results. Also, actual patient data were collected during the survey in various hospitals. The predicted values using the coefficients produced dose values which were within the range of the dose values given by the IMPACT dose calculator. That signifies that the use of the deduced size dependant EDLP conversion coefficients for paediatric patients is a quick and straightforward method for estimating E. Accurate dosimetry is crucial for children and neonates due to the greater susceptibility of radiation effects in younger ages. That can assist in the optimisation of protocols for children. Patient doses in paediatric CT vary considerably as a function of patient age and weight and are highly dependent on the optimisation procedures and the type of the CT scanner. It was observed that data for neonatal scans are scarce due to the low frequency of the examinations. Only half of the 15 hospitals included in the survey have responded to the survey as the majority has not set protocols particularly for neonates. Only those departments dedicated to paediatric imaging applied low tube voltage techniques (80kVp or 90kVp) versus 120kVp used for adults, and less than half used tube current modulation. Finally, organ doses for radiosensitive organs (breast, lung, stomach and liver) were normalized to the CT dose index. The organ doses can be estimated to within 20%. The dose coefficients presented in this study provide a practical and effective way for radiologists/radiographers to estimate effective dose in routine clinical practice. This can contribute in the optimisation of scan parameters and assist in the calculation of radiation cancer risks for infants and paediatric patients of any size. The method and the results of this study can assist radiologists and CT users understand the radiation dose estimates and the risks associated with this modality. Awareness of the risks can reinforce the justification criteria