Neurosonography is an ultrasound examination of the brain and spinal cord. It uses sound waves to create real-time images of neural structures, and it’s most commonly performed on newborns and infants, whose soft spots (fontanelles) provide a natural window into the skull. The technique is also used on fetuses during pregnancy, on adults through thinner areas of the skull bone, and even during brain surgery to help surgeons see tumors in real time.
How It Works
Like all ultrasound, neurosonography sends high-frequency sound waves into the body and captures the echoes that bounce back to build an image. What makes brain ultrasound unique is the challenge of getting sound through bone. In infants, this isn’t a problem: the anterior fontanelle, the diamond-shaped soft spot on top of a baby’s head, acts as an open acoustic window. The transducer is placed directly against it, giving a clear view of the brain’s internal structures.
In adults, sound waves must pass through the skull, which absorbs and scatters much of the signal. Clinicians use the temporal bone, a thinner region just above the ear that’s roughly 2 to 3 centimeters across and only 2 to 3 millimeters thick. Even so, between 8% and 29% of patients in a general population have “window failure,” meaning the bone is too thick or uneven for useful images. Lower-frequency transducers (around 1 MHz instead of the standard 2 to 2.5 MHz) can sometimes penetrate where higher frequencies cannot.
No special preparation is needed for the exam. It’s painless, uses no radiation, and can be performed right at the bedside, which makes it especially practical for fragile premature babies in the NICU who can’t be safely transported to an MRI scanner.
Neonatal Brain Screening
The most widespread use of neurosonography is screening premature infants for brain bleeds, known as intraventricular hemorrhage. These bleeds occur in the delicate blood vessels of the developing brain and are graded on a scale of severity. Grade 1 means blood is confined to the germinal matrix, a small area near the ventricles. Grade 2 means blood has partially filled a ventricle (less than 50%). Grade 3 means a ventricle is more than half filled with blood and has begun to swell.
Screening schedules depend on the baby’s birth weight. The smallest infants, those under 1,000 grams (about 2.2 pounds), follow the most intensive protocol: a first scan at 3 to 5 days of life, a second at 10 to 14 days, a third at 28 days, and a final scan before hospital discharge. Slightly larger babies (1,000 to 1,250 grams) get three scans, and those between 1,251 and 1,500 grams typically need only two, with additional scans if complications arise.
Beyond hemorrhage, neonatal neurosonography can detect hydrocephalus (fluid buildup in the brain), congenital brain malformations, signs of oxygen deprivation, congenital infections, and intracranial tumors. The American Institute of Ultrasound in Medicine published updated practice standards in 2024 that recommend using high-frequency linear transducers to get magnified views of the fluid spaces around the brain’s surface, measuring specific distances to track whether those spaces are normal or enlarging.
Fetal Neurosonography During Pregnancy
Fetal neurosonography is a targeted ultrasound of the baby’s brain and spine, typically performed during the midtrimester anatomy scan (around 18 to 22 weeks). Central nervous system malformations are among the most common birth defects, with neural tube defects alone occurring in roughly one to two per 1,000 births. Many intracranial abnormalities go undetected at birth and only become apparent later in life, which makes prenatal screening particularly valuable.
The exam captures the brain in multiple cross-sectional views at the level of the ventricles, the thalamus, and the cerebellum. The brain’s appearance changes dramatically throughout pregnancy. Certain structures, like the cavum septi pellucidi (a fluid-filled space between the brain’s hemispheres), only become visible from the early second trimester onward, so timing matters. Some abnormalities, particularly defects in early brain and spinal cord development, can be spotted as early as the end of the first trimester.
A targeted fetal neurosonography is recommended when a routine screening ultrasound raises suspicion of a brain or spinal malformation, when there’s a family history of inheritable central nervous system conditions, or when a previous pregnancy involved a brain or spinal defect. For open spina bifida, up to 97% of cases show a characteristic “banana sign” on ultrasound, caused by the hindbrain being pulled downward.
Transcranial Doppler in Adults
The adult version of neurosonography is called transcranial Doppler, or TCD. Rather than producing detailed structural images like an MRI, TCD measures how fast blood is flowing through the major arteries at the base of the brain. That flow speed reveals a lot: narrowed or spasming vessels push blood faster, much like water speeds up when you partially cover the end of a garden hose.
TCD has several established clinical roles. After a type of brain bleed called a subarachnoid hemorrhage, the surrounding arteries can go into spasm days later, dangerously restricting blood flow. TCD can track this process over time, helping clinicians decide when to intervene. It’s more sensitive at detecting spasm in the larger, more central arteries than in smaller branches deeper in the brain.
For children with sickle cell disease, TCD screening is now a routine part of preventive care. When the average blood flow velocity in certain brain arteries reaches or exceeds 200 cm/s, the child faces a significantly elevated risk of stroke. The landmark STOP trial showed that identifying these high-risk children with TCD and treating them with blood transfusions reduced the rate of first stroke fivefold. TCD is also used in evaluating acute ischemic stroke, traumatic brain injury, and detecting tiny blood clots (microemboli) traveling through brain arteries.
Use During Brain Surgery
Neurosurgeons use intraoperative ultrasound to guide tumor removal in real time. Once a section of skull has been removed to access the brain, there’s no bone barrier, and high-frequency ultrasound can produce detailed images of the tissue directly beneath the surface. Studies show that two-dimensional ultrasound correlates well with MRI for estimating tumor size and gauging how much tumor has been removed during surgery on both primary brain tumors and metastatic tumors.
Ultrasound offers something MRI taken before surgery cannot: it reflects what the brain looks like right now, not hours or days earlier. As the surgeon removes tissue, the brain shifts and deforms, making pre-surgical MRI images less accurate. Real-time ultrasound can identify residual tumor beyond what was visible on pre-surgical MRI and help distinguish actual tumor from surrounding swelling. Doppler imaging adds another layer, mapping blood vessels in real time so surgeons can avoid them during resection.
Strengths and Limitations
Neurosonography’s biggest advantages are practical. It’s portable, inexpensive compared to MRI or CT, involves no radiation, requires no sedation, and produces images instantly. For a premature infant who weighs less than a kilogram, or a critically ill patient on life support, these qualities make it irreplaceable.
Its limitations are equally clear. Image quality depends entirely on the acoustic window. In infants, the fontanelle closes over the first 12 to 18 months of life, gradually narrowing the window for scanning. In adults, skull thickness varies enough that a meaningful percentage of patients simply can’t be scanned through the temporal bone. The technique also can’t match MRI’s resolution for soft tissue detail, which is why it often serves as a first-line screening tool that flags problems for further investigation with more advanced imaging when needed.