Neuroimaging technologies continue to evolve, but there remain three basic methods utilized in the neonatal population – Cranial Ultrasound, Magnetic Resonance Imaging (MRI), and Computed Tomography (CT). These technologies are complex, but a basic understanding of how these technologies work, their benefits, risks, and indications is attainable for every bedside clinician.
So let’s dive in!
Cranial ultrasound remains the most widely used neuroimaging technique for premature babies. An ultrasound technician uses the large anterior fontanel, and to a lesser degree the smaller posterior fontanel as an “acoustic window” into the brain. The tech applies conductive gel to the skin overlying the open fontanels and uses a probe that transmits sounds through the conductive gel and into the brain. These high-frequency sounds bounce back to the probe and a computer uses the sound waves to create an image.
- No radiation
- Images are captured in real-time
- Portable technology
Risks & Limitations
- Only one level of contrast and intensity which limits resolution
- Difficulties seeing small bleeds or infarcts
- Difficulties seeing cerebellum bleeding or infarcts
- Potentially noxious to sick and premature neonates
- May result in hypothermia due to cold gels, open portholes and raised incubator lids
- Intracranial bleeds
- Germinal matrix hemorrhages and sequelae (IVH, PVL, PHH)
- Detecting masses such as tumors or cysts
- Evaluation of a sacral dimple
Computed Tomography (CT)
CT uses ionizing radiation to capture a series of x-ray images at various angles to create images via specialized computer programming. The computer uses the x-ray images captured to create a 3-D model of the part of the body being studied, in our case, the brain.
- Useful for detecting bleeding and calcifications
Risks & Limitations
- Exposure to radiation
- Requires travel off unit
- Potential for hypothermia
- Trauma such as non-accidental trauma cases
Magnetic Resonance Imaging (MRI)
MRI scanners, typically large tube-shaped magnets, use a magnetic field and radio waves. The magnetic field generated by the scanner temporarily realigns hydrogen atoms in the body of the patient. Radio waves are used to generate faint signals from these aligned atoms. The signals create cross-sectional images, just like slices in a loaf of bread. There are many options for using these MRI properties, and most Neuro-Radiologists have a standard set of parameters for different populations. These parameters are collectively known as “sequences” or “protocols”.
Different “sequences” can give different clinical information. The most common neonatal sequences are, T1 Weighted and T2 Weighted images for anatomy, and Diffusion Weighted Imaging (DWI) with Apparent Diffusion Coefficient (ADC Maps) for edema, and a newer sequence that has emerging value with small bleeds (micro-bleeds) is Susceptibility Weighted Imaging (SWI). It is also possible to construct 3-D images from the many slices taken which gives you a view of the brain from different angles.
Magnetic Resonance Spectroscopy (MRS)
MRS may be used as an adjunct to MRI to measure biochemical changes in the brain. Metabolites in the basal ganglia and thalamus are typically measured as these areas are particularly sensitive to acute anoxic events such as those occurring with HIE. One such metabolite, N-acetyl-aspartate (NAA) may be measured in the thalamus — high levels indicate health tissue, whereas low levels indicate damage.
Functional MRI (fMRI)
FMRI scans are similar to MRI. fMRI allows us to create images of metabolic function in different areas of the brain or body. Images generated by standard MRI create 3D pictures of anatomical structure, whereas fMRI generates 3D images of metabolic activity within structures. Newer technology combines MRS with functional MRI to create Functional MRS (fMRS).
Benefits of MRI
- No ionizing radiation
- Improved sensitivity for evaluating infarction and white matter changes compared to CT
Risks & Limitations
- Sedation requirements
- Length of study (usually 40 to 60 minutes)
- Off-Unit Transport to MRI Scanner
- Need for MRI compatible equipment (ventilators, IV pumps)
- Priming 10+ feet of tubing if infant on IV fluids or drips
- Focal cerebral injury (such as stroke)
- Neonatal encephalopathy (such as Hypoxic Ischemic Encephalopathy)
- Congenital Malformations
- Etiology of Neonatal Seizures
- Anatomical detail of spine malformations
New Bedside Technology!
Recently, Aspect Imaging has released the Embrace Neonatal MRI, the first FDA approved neonatal MRI scanner. This new technology allows for safe and efficient MRI scanning right in the NICU! Check out this exciting new technology at https://www.embracemri.com/
The first Embrace Neonatal MRI scanner in the United States was recently installed at Brigham and Women’s Hospital in Boston. Check out the article HERE.