1 Imaging Techniques



10.1055/b-0036-138074

1 Imaging Techniques



1.1 Introduction


Appropriate clinical interpretation of pediatric neuroradiologic studies requires an understanding of the imaging techniques they use. Only through this understanding can appropriate tests be performed and subsequently interpreted. A multitude of resources exist to provide this information, but a brief review follows, including several pediatric-specific considerations.



1.2 Radiographs (Plain-Film)


Plain-film evaluation is the foundation of radiology; however, it is not commonly used for modern evaluation of the central nervous system (CNS). Plain-film radiography is still used with shunt-series to detect discontinuity or atypical placement of cerebrospinal fluid (CSF) diversion tubing, such as a ventriculoperitoneal shunt (Fig. 1.1). In children, radiographs may still be appropriate for evaluation of the spine after trauma. Radiographic evaluation for skull fractures, craniofacial abnormalities, and calvarial suture development is sometimes performed, but its diagnostic performance is inferior to that of computed tomography (CT).

Fig. 1.1 Example of a plain film image. (a) Anteroposterior and (b) lateral radiographs of the skull in a 9-month-old male show a ventriculostomy catheter, inserted through right parietal approach, with a discontinuity in its extracranial portion (between red arrows).


1.3 Ultrasonography


In the newborn period, sonographic evaluation of the intracranial contents can be performed through open fontanelles, most commonly the anterior fontanelle (Fig. 1.2). Evaluation through the posterior fontanelle can also be performed. Ultrasonography is also the primary screening tool for fetal imaging. These techniques, and the pathology evaluated, are further discussed in Chapter 5.

Fig. 1.2 Example of an ultrasound image. Coronal ultrasound image made through the anterior fontanelle in a 6-day-old female; a heterogeneous area of echogenicity in the expected location of the right caudate body, consistent with a focal hemorrhagic venous infarction (grade IV germinal matrix hemorrhage), is seen.

Transcranial Doppler ultrasonography is a technique of interrogation of flow velocities in branches of the circle of Willis and is used to identify abnormal blood-flow patterns in patients with sickle cell disease to determine the timing of transfusions. Ultrasonography is helpful in evaluating the soft tissues of the neck, including muscles, lymph nodes, cystic lesionsa, and infectious collections.


Prior to ossification of the posterior spinal elements (typically the first 3 months postnatally), the spinal cord can be evaluated with ultrasound to determine the position of the conus medullaris, thickness of the filum terminale, and motility of the cauda equina. Sacral dimples can also be evaluated with ultrasound, to identify the presence of dermal sinus tracts/pilonidal sinus tracts.



1.4 Computed Tomography


Computed tomography is a cross-sectional imaging technique that uses ionizing radiation, is widely available, and provides excellent osseous detail (Fig. 1.3). Soft tissue detail on CT is better visualized than with plain-film radiography but is limited in comparison to that provided by magnetic resonance imaging (MRI). Computed tomography is also the gold standard for detecting acute intracranial hemorrhage after trauma; however, modern MRI is more sensitive for detecting the chronic deposition of blood products/hemosiderin. Because CT is widely available and can be performed rapidly, usually without sedation, it is the mainstay imaging procedure for evaluating acute traumatic and infectious processes. Modern CT scanners can provide sagittal and coronal reformats, which significantly aid in the characterization of abnormalities in the pediatric head and are critical for imaging of the head and neck, as well as the spine. Three-dimensional (3D) reconstructions can be helpful in differentiating calvarial sutures from fractures, differentiating among craniofacial abnormalities, and characterizing complex dysraphisms of the spine.

Fig. 1.3 Examples of CT images. (a) Axial CT image of the head, displayed in a soft tissue window, shows an extracranial hematoma overlying the left frontal region, with an intracranial extra-axial (epidural) hematoma. (b) An axial CT bone algorithm image of the head shows a punctate focus of pneumocephalus and an overlying fracture. (c) 3-D reconstruction of the skull shows the left frontal fracture (red arrows) paralleling the coronal suture.

The density detected on CT is related to the underlying electron density of the evaluated tissue. This is typically evaluated in a visual manner, but it can be evaluated quantitatively, with density values reported in Hounsfield units (HU); see Table 1-1. For CT, pure water is defined as having a density of 0 HU, and air is defined as having a density of –1000 HU.



































Table 1.1 Density of substances

Substance


Density (HU)


Air


–1000


Fat


–200 to –30


Water


0


Proteinaceous fluid


10 to 30


Acute blood


60 to 80


Muscle


~80


Bone


~600 to 1000


Metal


>1000



1.5 Magnetic Resonance Imaging


Magnetic resonance imaging provides the best noninvasive characterization of the central nervous system (CNS) and soft tissues of the head and neck (Fig. 1.4). Studies done with MRI must be tailored to the patient’s specific clinical symptoms, with the selection of specific imaging sequences and acquisition planes to highlight the suspected pathology. Because MRI is probably the most important tool in evaluation of the pediatric CNS, the basics of different MRI sequences are discussed below. Note that although CT measures electron density, MRI evaluation has more to do with the molecular constituents and the percent of protons that are in free water, proteins, and lipids. A “bright” signal in a given sequence is most accurately described as having a “hyperintense” and not a “hyperdense” appearance. Likewise, a “dark” signal is “hypointense.” Other terms related to a given sequence can be used; for instance, a hyperintense signal on T1 weighted (T1 W) imaging can be described as T1 shortening, and a hyperintense signal on T2 weighted (T2 W) imaging as T2 prolongation. A hypointense signal on T1W imaging is T1 prolongation, and a hypointense signal on T2W imaging is T2 shortening. Familiarity with these terms is helpful in understanding imaging reports produced by other persons, and in understanding journal articles and teaching resources, even if the reader does not choose to use “shortening” and “prolongation” in his or her reports (s. Tab.).

Fig. 1.4 Normal MRI image. (a) Axial T1W image of the brain in a patient with a mature myelination pattern shows intermediate intensity of the peripheral cortex (gray matter) and relatively hyperintense signal of the white-matter structures. (b) Axial T2 W image of the patient shows a relatively hypointense appearance of the myelinated white matter, intermediate hyperintense appearance of the gray matter, and hyperintense appearance of CSF. (c) Axial FLAIR image shows suppression of the hyperintense signal of CSF as compared with that in the T2W image. (d) Axial diffusion-weighted image and (e) ADC map. (f) Axial directionally encoded fractional anisotropy map shows the normal white matter anatomy of the brain of the patient, with transversely oriented fibers within the corpus callosum (red), anteroposteriorly directed fibers within the optic radiations (green), and cranio–caudally directed fibers in the posterior limb of the internal capsule, representing fibers of the corticospinal tract.
































































Table 1.2 Appearance on magnetic resonance imaging of various substances

Substance


T1 W


T2 W


FLAIR


Fat


Bright


Bright


Bright


Water/CSF


Dark


Bright


Dark


Proteinaceous fluid


Intermediate


Intermediate to bright


Intermediate


Methemoglobin


Bright


Dark (extracellular) Bright (intracellular)


Variable


Deoxyhemoglobin


Dark


Bright


Variable


Hemosiderin


Dark


Dark


Dark


Gray matter


Intermediate dark


Intermediate bright


Intermediate bright


White matter (myelinated)


Intermediate bright


Dark


Dark


White matter (unmyelinated)


Intermediate dark


Intermediate bright


Intermediate dark


White matter (incompletely myelinated)


Intermediate bright


Intermediate


Variable/bright


Magnetic resonance imaging uses powerful magnetic fields to manipulate hydrogen protons for obtaining diagnostic information. The stronger the magnet, the better the information obtained. Modern clinical imaging is performed with scanners having a high field strength, with a static magnetic field of either 1.5 or 3.0 teslas. Scanners with higher field strengths than this exist, most commonly for research purposes. Older scanners with lower field strengths still exist, but should only be used if a high-field-strength alternative is unavailable. There also exist “open” MRI scanners, which use a lower field strength and do not circumferentially encompass the area being scanned. Although the concept of an open scanner is appealing from a marketing standpoint, such scanners provide limited diagnostic information in many circumstances and should be avoided unless there is an absolute contraindication to standard MRI.



1.5.1 T1 Weighted Imaging


T1 weighted (T1 W) imaging is one of the two primary MRI sequences, and is related to the longitudinal relaxation of hydrogen protons. Fat, protein, melanin, and gadolinium are among substances that demonstrate T1 shortening (or “bright” signaling).

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May 28, 2020 | Posted by in NEUROLOGICAL IMAGING | Comments Off on 1 Imaging Techniques
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