## 3 Elastographic Methods
### Transient Elastography (TE)
#### 3.1.1 TE in Healthy Children  
Since 2008, following the release of a new probe with a smaller diameter (S-probe 5 mm) compared to the regular probe (M-probe 7 mm), LSM using TE could be obtained in small children and infants. The feasibility of LSM in children was assessed using the S-probe (thorax perimeter < 45 cm (S1) or 45-75 cm (S2)) and the M-probe (thorax perimeter > 75 cm) according to the manufacturer's recommendations [49]. TE was technically achievable in children of all age groups. TE is feasible also in infants, but confounding factors such as the probe choice, sedation, or food intake need to be taken  
Figure 1: Transient elastography (TE) (S1 probe) in a newborn with alpha-1 antitrypsin deficiency. The individual values of 10 measurements are shown as well the median of the 10 measurements and two quality parameters, the interquartile range and the interquartile range divided by the median.  
into account when interpreting the results. The success rate of TE decreased in children younger than 24 months even under ideal conditions. General anesthesia significantly increased LSMs in healthy children (5.4 vs. 4.2 kPa; \(p<0.01\)). Probe choice equally influenced results in paired comparisons (S1 5.5 vs. S2 4.8 kPa; \(p<0.01\)), as did food intake (5.9 vs. 5.4 kPa; \(p=0.015\)). Inter- and intra-observer agreements were good. Normal liver stiffness was 4.5 (2.5-8.9) kPa and did not vary significantly with age or sex [50]. However, another study found that LSMs were significantly age-dependent with values of 4.40, 4.73, and 5.1 kPa in children 0-5, 6-11, and 12-18 years (\(p=0.001\)) respectively (Table 1), while the interquartile range decreased with age (0.8, 0.7, and 0.6 kPa). The upper limit of normal (median plus 1.64 times standard deviation) was 5.96, 6.65, and 6.82 kPa, respectively. Girls between 11 and 18 years showed a significantly lower LSM than boys of the same age (4.7 vs. 5.6 kPa; \(p<0.005\)) [48]. In younger children, the number of invalid measurements increased significantly due to movement artifacts [48], however, the measurement was reliable from the age of 6 without sedation.  
\begin{table}
\begin{tabular}{c} **Study**  
As shown in Table 1, in a series of non-sedated control group of children LSM also increased with age; 0-2 years (3.5 \(\pm\) 0.5 kPa), 3-5 years (3.8 \(\pm\) 0.3 kPa) and 6-11 years (4.1 \(\pm\) 0.2 kPa), with healthy older children 12-18 years having values similar to adults (4.5 \(\pm\) 0.2 kPa). LSM did not vary significantly with gender (female, 4.5 \(\pm\) 0.2 vs. male, 4.8 \(\pm\) 0.2 kPa). Children with non-hepatic illnesses had higher LSM (5.2 \(\pm\) 0.2 kPa) compared to healthy children (4.1 \(\pm\) 0.1 kPa) [51].  
Another study has confirmed that LSM increased with age: it was 3.4 kPa (2.3-4.6 kPa) at ages 1-5 years; 3.8 (2.5-6.1) kPa at ages 6-11; and 4.1 (3.3-7.9) kPa at ages 12-18 (\(p\) = 0.001). The M-probe was suitable in a wide age range of children from age 1 year onwards. In children without evidence of liver disease, LSM showed an age-dependent increase [23].  
Still, when using the M probe in children with a thoracic perimeter below 45 cm, one should consider the \"underestimation\" phenomenon. It has been shown that LSM decreased with probe size (S1 \(<\) S2 \(<\) M) and caution is needed when interpreting the results [52].