We included nine neonates who had brain abnormalities visualized on US simultaneously with both a micro-convex probe and a very high-frequency probe. Basic population details are reported in Table S1 (Online Resource). As supplementary information, we included also eight figures from four infants depicting normal cranial US images obtained with the very high-frequency transducer (Online Resource, Fig. S1): note the accurate definition of brain structures. All infants underwent brain MRI at the corrected age (except for one due to death), which confirmed the normalcy of brain structures.

Intraventricular haemorrhage (IVH) in preterm infants usually originates from the immature germinal matrix, from where it can subsequently spread throughout the ventricular system. Its most common grading systems is based on sonographic findings, including the percentage of filling of the volume of the lateral ventricle and the parenchymal involvement [10]. When the haemorrhage remains confined to the subependymal germinal matrix (GMH or grade I IVH), the echodense collection is restricted to the caudothalamic groove. Detecting small GMH on cranial US can be challenging as they cannot be easily differentiated from the hyperechoic choroid plexus. However, the use of very high-frequency transducers enhances the resolution of images and may assist in characterizing cases that remain uncertain when using micro-convex transducers (Fig. 1; Figs. S2 and S3).

Intraventricular haemorrhage (IVH) grade I in a 29-week-gestation newborn, 10 days of life. a, b Right parasagittal scans. The definition of the subacute clot (inhomogeneous hyperechoic lesion on the ventricular floor, arrow) is lower using the micro-convex transducer (a) compared to the very high-frequency transducer (b). MRI performed at term age (c): the IVH is no longer visible, while a germinolytic cyst is detected (arrow)

While supratentorial IVH is usually easily visible by scanning through the anterior fontanelle, haemorrhages extending into the fourth ventricle, cisterna magna, and subdural spaces are more difficult to detect with this approach. The mastoid view enables better detection of haemorrhages in the posterior fossa (Fig. S4). In addition, scanning through the mastoid fontanelle gives a better overview of the third and fourth ventricle and aqueduct of Sylvius, facilitating accurate diagnosis of post-haemorrhagic hydrocephalus. When the fourth ventricle is dilated because of impairment in cerebrospinal fluid flow, the mastoid fontanelle can help identify the location of the obstruction and distinguishing between obstructive and communicating forms of hydrocephalus. In addition to post-haemorrhagic hydrocephalus, another complication of IVH is periventricular haemorrhagic infarction (PVHI). PVHI is a venous infarction due to the obstruction of the terminal veins consequent to the haemorrhage that impairs blood drainage from the medullary veins. On cranial US, PVHI is characterized by a unilateral or strikingly asymmetric echodensity in the periventricular white matter, ipsilateral to the haemorrhage. The echogenicity of the lesion gradually decreases and changes to echolucency, the ultrasonographic end-stage generally being a poroencephalic cyst or several smaller cystic lesions [11]. In the current study, the very high-frequency transducer allowed an earlier and reliable detection of PVHI compared to micro-convex transducers (Figs. 2 and 3).

Intraventricular haemorrhage (IVH) complicated by periventricular haemorrhagic infarction (PVHI) in a preterm infant (30-week gestation, 11 days of life). a, b Coronal scans. The PVHI is presented by an echogenic lesion (white arrow) in the left periventricular white matter, ipsilateral to the IVH (arrowhead). Note that the PVHI is an area of increased echogenicity that is asymmetric, wedge-shaped, and located above the anterior horn of the lateral ventricle. In preterm infants, these lesions should be distinguished from normal periventricular echodensities that are bilateral, symmetric, round-shaped, and located laterally to the anterior horns of the lateral ventricles (as shown in Fig. S1, a). c, d Left parasagittal scans. The micro-convex transducer (c) shows subtle hyperechoic lesions in the left periventricular white matter (arrows): however, this scan displays diffuse echogenicity with blurred margins, which can be confused with the typical periventricular hyperechogenic halo of prematurity. By contrast, with the very high-frequency probe (d), the PVHI appears inhomogeneous (arrows) with focal hyperechoic areas in contrast with physiological hyperechogenicities (Fig. S1, g). e MRI at term equivalent age. T2 gradient echo image (axial plane) at the level of the centrum semiovale shows small areas of venous infarctions in the left periventricular white matter, confirming the diagnosis of PVHI.

Bilateral intraventricular haemorrhage (IVH) grade 3 with post-haemorrhagic ventricular dilation and left periventricular haemorrhagic infarction (PVHI) in a preterm infant (30-week gestation, 9 days of life). a, b Coronal scans. Hyperechoic lesions involving the left periventricular white matter appear inhomogeneous (arrows) using the very high-frequency linear transducers (b) at a time when cranial ultrasound performed with the micro-convex transducer shows diffuse hyperechogenicities (a). c, d Parasagittal right scans. With the very high-frequency transducer (d), the increased echogenicity of the right periventricular white matter is better characterized as homogeneous compared to the micro-convex transducer (c). e, f Parasagittal left scans. The increased echogenicity of the left periventricular white matter (e) is similar to the right periventricular white matter (c) using the micro-convex probe. By contrast, using the very high-frequency linear transducer, increased echogenicity in the left periventricular white matter appears markedly inhomogeneous, findings compatible with PVHI (f). These hyperechoic lesions in the left periventricular white matter were confirmed as PVHI on MRI at term equivalent age (g, h). g Minute lesions with decreased intensity on T2-weighted image (axial plane) at the level of the centrum semiovale; h on T2 gradient echo image (axial plane) at the level of the centrum semiovale, localized areas of decreased signal intensity are visible in the left periventricular white matter

White matter injury (WMI) is the most frequent form of preterm brain injury, affecting up to 50% of very low birth weight infants. Due to improvements in neonatal care, cystic WMI, also referred to as cystic periventricular leukomalacia (PVL), has decreased in incidence, whereas the non-cystic, diffuse form of WMI has become prevalent [7]. The sensitivity of cranial US in the detection of diffuse, non-cystic WMI is poor [10]. However, very high-frequency linear transducers may allow the detection of a wider range of white matter lesions and provide a better evaluation of the nature and extent of white matter abnormalities. Indeed, they can also assist in distinguishing normal white matter echodensities in preterm infants from more inhomogeneous and/or echogenic areas that probably represent WMI. In fact, bilateral symmetrical homogeneous periventricular hyperechogenicities are normal before term equivalent age and are typically seen in early scans [12]. They include blushing around the anterior frontal horn and in the parieto-occipital junction of the lateral ventricles (Fig. S1f), which dissipate without cyst formation or ventricular dilatation. These findings likely reflect maturational processes that typically occur during the final trimester of pregnancy and are attributed to the anisotropic effect of layers of migrating cells along radial glia fibers [13]. The assessment of potential injury in periventricular white matter can be greatly facilitated by imaging through the posterior fontanelle, as it takes advantage of a different insonation angle and can separate anisotropic effect from true increased echogenicity due to pathology [13]. The very high-frequency transducers, providing higher resolution, can also help in distinguishing these maturational changes from WMI, where areas of increased echogenicity are mostly inhomogeneous, present with patchy appearance and bilateral but asymmetric distribution [7]. These lesions may evolve into cystic lesions. Cysts resulting from WMI should not be confused with frontal pseudocysts due to germinolysis which are below the line passing through the upper portions of the anterior horns of the lateral ventricle [14].

The duration of periventricular hyperechogenicities correlates with the severity of the injury and with the long-term outcome, even when they do not evolve into cystic lesions. Prolonged hyperechogenicities have been found to predict white matter abnormalities on MRI [7]. Examples of diffuse WMI are shown in Fig. S5. In diffuse, non-cystic WMI, ex vacuo ventriculomegaly may be seen instead of cysts in later scans close to term age (Fig. S5c, d). Figure 4 presents an example of cystic PVL.

Microcystic periventricular leukomalacia (PVL) and ex vacuo dilatation of lateral ventricles in a preterm infant (29 weeks gestation) with in utero twin-to-twin transfusion. a, b Posterior coronal scans, 15 days of life. Hyperechoic changes affecting bilateral periventricular white matter that appear diffuse and homogeneous when cranial ultrasound is performed with the micro-convex transducer (a). By contrast, using the very high-frequency linear transducer hyperechoic lesions appears more inhomogeneous, and multiple small cysts are visible in the left periventricular white matter (b). MRI T2-weighted sequence (c, d) detected focal punctiform hypointensities in the white matter adjacent to the roof of both middle cells (asterisks). Cavitated lesions in the white matter adjacent to the posterior profile of the left lateral ventricle (arrows). Squared appearance of the posterior aspect of both ventricles and loss of the tapetum, indicative of white matter injury

Through the anterior fontanelle, visualization of infratentorial structures is usually suboptimal due to the distance from the transducer to the posterior fossa and to the interposition of the echogenic tentorium. Data on cerebellar infarcts are limited because, without the routine use of mastoid window views, the diagnosis of cerebellar infarctions and sequelae of cerebellar haemorrhages is challenging. However, the distance between the probe and the posterior fossa is smaller in the preterm infant: the use of a very high-frequency transducer allows a sufficient depth penetration to visualize infratentorial structures, in order to exclude haemorrhages and dilation of the aqueduct and the cisterna magna. Furthermore, by utilizing the mastoid fontanelle as an additional acoustic window, as recommended in European guidelines on practices in pediatric neuroradiology [15], the transducer is closer to the posterior fossa structures and approaches them at a different angle, avoiding the tentorium. Also with this approach, very high-frequency transducers can help in the detection of cerebellar lesions (Fig. 5).

Cerebellar cystic lesion in a moderately preterm infant (gestational age 30 weeks), smaller twin born after dichorionic diamniotic pregnancy complicated by birth weight discordance. a, b Day 1 of life: coronal ultrasound scan using the left mastoid fontanel as acoustic window. Loss of parenchymal tissue of cerebellar vermis (arrow) is visible with the micro-convex transducer (a). Using the very high-frequency transducer (b), the echogenicity of the cerebellar parenchyma surrounding the cyst appears abnormally increased, raising suspicion of a prenatal cerebellar lesion with cystic evolution. MRI at term equivalent age (T1-weighted sequence, axial plane at the level of the brainstem) confirmed the cystic lesion (arrow), although its etiology remained undefined

Table 1 presents a summary of included brain lesions, comparing US findings using a standard medium-frequency micro-convex transducer, very high-frequency transducers, and magnetic resonance imaging at term corrected age.