pneumothorax or pleural effusion from normal tissues and it identifies the level of the diaphragm as it moves with respiration Technique Any probe can be used to interrogate the lung If identifying a pneumothorax, the linear probe is appropriate because it provides the best visualization of the pleural line However, for the actual thoracentesis procedure a phased array or curvilinear probe is preferred because it will provide a wider and deeper view of the chest cavity (Fig 131.24 ) For identifying fluid in the chest, the probe should be placed in the midaxillary line or posteriorly where the intended catheter will be placed If the static technique is used, the level of the diaphragm should be marked along with the deepest pocket of intrathoracic fluid Pitfalls If using the static technique, moving the patient after the area of fluid is marked could lead to fluid shifts and inappropriate placement of the catheter or needle It is also important to identify the level of the diaphragm when diagnosing a pleural effusion to ensure that the visualized fluid is not confused with intra-abdominal free fluid FIGURE 131.24 Pleural effusion identification using phased array probe Bladder Catheterization Bladder catheterization is commonly performed in the pediatric ED, especially in infants While the catheterization is not technically challenging, it may be uncomfortable for the patient; hence, the importance of avoiding multiple attempts Using ultrasound to identify urine in the bladder may be helpful, especially if the child has recently voided or had a dry bladder on prior catheterization For children who require suprapubic aspiration, it is important to use ultrasound to not only identify that there is urine in the bladder, but also to identify the epigastric vessels prior to the procedure Technique A phased array or curvilinear probe is an appropriate transducer for identifying urine in the bladder prior to urethral catheterizations (Fig 131.25 ) For neonates and smaller infants, a linear probe is also acceptable For suprapubic aspiration, the linear probe is also recommended as it is easier to visualize the needle entering the bladder FIGURE 131.25 Bladder measurements FIGURE 131.26 Bladder and intra-abdominal free fluid Pitfalls It is important that clinicians are comfortable identifying the bladder and are able to differentiate it from free fluid or fluid within bowel (Fig 131.26 and Video 131.27 ) Lumbar Puncture Guidance POCUS is a useful adjunct when performing lumbar punctures Ultrasound can readily identify landmarks such as the spinous process and interspinous space These landmarks can be challenging to palpate in infants, in whom the anatomy is smaller, and in obese children, in whom the landmarks may be obscured by body habitus In infants, the spine is mostly cartilage; therefore, ultrasound can identify deeper anatomy, including the spinal canal, conus medullaris and cauda equina fibers In all age groups, the ability to visualize the necessary landmarks may improve success rates and lead to fewer traumatic lumbar punctures Technique A linear transducer is recommended for imaging the spine In obese children, however, a curvilinear or phased array probe may be required to provide adequate depth The transducer should be placed on the spine in both the longitudinal and transverse planes In the longitudinal axis, the transducer should be placed along the spine with the interspinous space in the center and a spinous process on each side In older children, the spine will appear hyperechoic with clean, anechoic shadows posteriorly, and the interspinous space will appear darker (Fig 131.27 ) In the transverse axis, clinicians should move the probe up and down to ensure the spinous process is midline and then identify the interspinous space before the next spinous process appears ( Video 131.28 ) Clinicians can then mark the interspinous space with a marker or object such as a paperclip After marking the area, sterile technique should be used In infants, both the longitudinal and transverse images will display more anatomical structures than in older children The spinous processes will still appear hyperechoic, but because the bones are not completely calcified, sound waves are transmitted through the spine to the spinal canal (Fig 131.28 and Video 131.29 ) Clinicians can visualize the dura mater, which appears hyperechoic, and surrounds the anechoic cerebrospinal fluid Within the spinal canal, the conus medullaris and cauda equina fibers are also often visualized (Fig 131.29 ) ... phased array probe Bladder Catheterization Bladder catheterization is commonly performed in the pediatric ED, especially in infants While the catheterization is not technically challenging, it