Imaging of the Neonate: Term Infant

Collimation

Overcollimation is readily evident. Pertinent anatomy is excluded from the image. Conversely, undercollimation exposes extraneous anatomy to unnecessary ionizing radiation. In addition, the wider field of view increases scatter, decreasing image sharpness. In film/screen radiography, as well as in fluoroscopy, inclusion of very dense or very lucent regions (such as empty space outside the patient) increases the dynamic range, which falsely alters exposure parameters.

Projection/Angulation

It is simplest to image the newborn while supine, in anteroposterior (AP) projection. Fortunately, due to the geometry and small size of the infant thorax, there is no significant degradation of image quality or altered appearance of the mediastinum compared with the posteroanterior (PA) projection. As such, the AP projection is acceptable and widely used in the newborn and infant.

More problematic, however is unintended relative cranial angulation of the radiograph tube in relation to the chest, resulting in a lordotic projection ( Fig. 19.1 ). Due to the conical shape of the chest, a lordotic view often results when the baby lies flat. The effect is exaggerated when the arms are held back in extension and the midtorso is arched. To remedy this, a foam pad with a 15-degree incline (higher end toward the head) may be placed under the thorax. Alternatively, the tube may be angled slightly caudally. The appearance of lordotic positioning can be further exacerbated by a low centered beam, such as when acquiring a chest and abdomen on one cassette (“babygram”). In this instance the chest portion of the image is exposed by a more divergent beam. Ideally, the radiograph beam for a chest radiograph should be centered at the nipple line. The consequences of lordotic technique are outlined in Table 19.1 .

Positioning

A well-positioned patient is judged on a frontal radiograph by assessing the medial heads of the clavicle. The sternoclavicular junction should be an equal distance between left and right. In addition, the rib lengths should be symmetrical. When these findings are not met, the patient is rotated about the vertical axis. Spurious relative hyperlucency of one hemithorax is most commonly a result of simple patient rotation. Rotation-related changes in projection of mediastinal structures are particularly pronounced in children, largely because of the thymus. As a relatively bulky anterior chest structure, a small degree of patient rotation results in substantial differences in its projection ( Fig. 19.2 ). Indeed, this phenomenon can lead to a spurious impression of cardiomegaly or even upper lobe consolidation. Rotation uncovers the hilus of the side opposite the direction of rotation, resulting in accentuation of these structures (most typically on the left). With rotation, individual sternal ossification segments can appear as lung nodules or even as rib calluses ( Fig. 19.3 ).

Respiration

In contrast with adults or older children, neonates and young children are imaged while freely breathing. Thus exposure time must be kept to a minimum to avoid motion blur. Just as important, though, is the phase of respiration that the image is acquired in. This is, of course, most challenging in a neonate because the normal neonatal respiratory rate is 20 to 60 breaths/min. Ideally, the image is acquired near peak inspiration. Attempted imaging during peak inspiration requires carefully observing the rise and fall of the chest and abdomen to synchronize the exposure at or close to peak inspiration. As a rule of thumb, on the frontal projection, six anterior ribs should be seen above the diaphragm ( Fig. 19.4 ). In infants, counting anterior ribs is more reliable due to the more anterior dome of the diagram. Better yet is an overall assessment, including the degree of flattening of the diaphragm because other variables such as parallax as a result of positioning, will vary where the rib crosses the diaphragm.

Spurious findings commonly present because of inadvertent exposure during expiration are outlined in Table 19.2 . When carefully examined, the findings could be suggested to be due to technique, although differentiation from true pathology may be tricky. For example, the hazy increased density seen on an expiratory film may not seem to be made up from acinar infiltrates, nor does it appear as a result of reticular densities. Rather, there is a generalized haze and a crowded appearance to the hilar vessels. It may be erroneous, however, to confidently ascribe such findings to technique. This is even more challenging in neonates where there is much overlap already between different patterns of disease on the chest radiograph. Most importantly, true findings may be missed entirely on an expiratory film. For example, focal lung disease may be “hidden” by a high diaphragm or may be rendered inconspicuous by the already generally increased lung density that decreases contrast in the image.

Exposure

Arguably, in the digital era, exposure factors have become less crucial to set with the same degree of precision as in the past. This is due to the much wider latitude digital radiography affords because it can record a far wider dynamic range than can film/screen. Still, setting proper exposure parameters is important for several reasons. First, the dynamic range is impressive but not infinite. At the extremes of density (such as dense, overlapping osseous structures and aerated lung), there still can be loss of detail that is not recoverable by adjusting the window and level settings on the display monitor ( Fig. 19.5 ). In addition, regardless of overall exposure, the inherent contrast is still affected by exposure parameters such as kilovoltage peak (kVp). Importantly, an underexposed film can usually be brought back to the “correct” contrast on the display panel; however, the resulting increased noise in the film from quantum mottle renders a suboptimal image. This underexposure then imparts a “grainy” or hazy appearance to the image. When interpreting such a film, examining the soft tissues for these effects will help the reader differentiate this phenomenon from true lung pathology.

Within a large range of exposure values, overexposure in digital radiography may not compromise overall image quality. Because it is not visually readily evident, this actually creates a problem as there is no longer a feedback to the technologist to keep the dose in check. Whenever there is a quality issue, the dose can easily be raised to compensate for another deficiency. It is said that the exposure and final image product have been “decoupled.” This results in ever-increasing radiation doses, known as “dose creep,” and is something that both the technologist and radiologist should be aware of to adhere to the as low as reasonably achievable (ALARA) principle.

In addition to underexposure, the final image is affected by postprocessing techniques, such as use of an unsharp mask filter, or edge enhancement or changes in the gradient shift (see Fig. 19.2C ). This can adjust the conspicuity of fine linear structures, including catheters, fissures, and the vascular markings. A detailed discussion of this, as well as other postprocessing techniques, is beyond the scope of this book.

Visualization of the lower vertebral endplates and pedicles through the mediastinal structures on a frontal radiograph suggests proper exposure. In addition, vascular markings should be easily seen in the central two-thirds of the lungs.

Humeral Ossification Centers

Presence of the proximal humeral ossification centers strongly suggests that the newborn is greater than 38 weeks of gestation ( Fig. 19.7 ). The converse, however, is not true. That is, the humeral ossification centers are not always present yet in the full-term infant (see Fig. 19.4 ). Moreover, the appearance of the proximal humeral ossification centers, as well as the coracoid ossification center, is variably delayed in different populations.

Broken Clavicle

At delivery, occasionally a clavicle fracture may occur. Radiographs are often obtained to document and confirm the fracture ( Fig. 19.8A ). Not all broken-appearing clavicles are fractured. When the ends appear rounded and corticated, the possibility of a congenital pseudoarthrosis of the clavicle should be raised (see Fig. 19.8B ). Similar to clavicular birth fractures, these may present with soft tissue swelling and, occasionally, reduced movement of the arm. It is most often seen on the right side.

The appearance of pseudoarthrosis is distinct from the appearance of the clavicle in patients with cleidocranial dysostosis. Cleidocranial dysostosis is a disorder that affects intramembranous ossification and, to a lesser extent, endochondral ossification. As such, patients with cleidocranial dysostosis will present with hypoplastic or aplastic clavicles (see Fig. 19.8C ). Common coexisting radiographic abnormalities include wormian bones and absence of ossification of the symphysis pubis (see Fig. 19.8D ).

Tracheal Buckling

Not only is lateral and anterior tracheal buckling normal in the neonate, its presence actually assists in determining the side of the aortic arch. The typical convex rightward buckle of the trachea is consistent with a normal left-sided aortic arch. A leftward buckle is expected with a right-sided aortic arch. The buckling is more pronounced during expiration or with the head turned. It can give the false impression of a mass on the frontal view or prevertebral soft tissue thickening on the lateral view. If there is any uncertainty, repeating the radiograph with attention to head positioning and respiratory phase may confirm the buckling to have been only transient.