Transcranial Sonography—Anatomy

Heiko Huber    Department of Neurodegeneration, Hertie Institute of Clinical Brain Research and German Center of Neurodegnerative Diseases (DZNE), Tübingen, Germany

I Three Standardized Planes

For the investigation of patients with movement disorders, it is important to know three standardized planes of B-Mode sonography.

The investigation is started by placing the ultrasound probe at the middle or the posterior temporal acoustic bone window (Fig. 1) in parallel to the imagined orbito-meatal line (Fig. 1, OM). By doing so, one usually visualizes the plane of the mesencephalic brainstem (Fig. 1, A1). By tilting the probe slightly upward for about 10–20˚ the ventricular plane is reached (Fig. 1, A2). These are the two standard planes for routine TCS in a movement disorder patient. In previous papers, the cella media plane has been mentioned as well. However, this does not contribute to the evaluation of movement disorders––therefore, this plane will not be commented upon in this chapter. However, in addition to the two axial planes, the plane of the cerebellum (Fig. 1, A3) can be assessed in a standardized fashion by rotating the ultrasound probe for about 30–45˚ to visualize the posterior fossa.

A The Plane of the Mesencephalic Brainstem

The mesencephalic brainstem plane constitutes a nearly axial section through the midbrain. As shown in Fig. 2, the key structure within this plane is the butterfly-shaped midbrain, which is hypoechogenic contrasting the strongly echogenic basal cisterns. In comparison to the standard MRI images, TCS allows to differentiate a number of different structures within the midbrain, i.e., substantia nigra (SN), red nucleus (RN), brainstem midline raphe (BR), and aqueduct. Abnormalities in these midbrain structures have shown to be valuable markers for a variety of disorders, including idiopathic Parkinson’s disease, restless legs syndrome, and major depression.

In addition to the midbrain structures, also other structures, including vessels in the basal cisterns, the hippocampus, the lower frontal lobe, parts of the temporal and occipital lobes as well as the upper and medial parts of the cerebellum, can be clearly distinguished within this plane.

B The Plane of the Third Ventricle

The plane of the third ventricle is reached by tilting the ultrasound probe slightly 10–20˚ upward starting from the midbrain plane. As shown in Fig. 3, this mild angulation of the probe results in a cross section through the brain, which is not compatible to the axial MRI slices and does not allow direct comparison of the left and right sides. Because of the wavelengths used to penetrate the skull, resolution close to the probe is only limited. Therefore, only structures close to the midline (e.g., the third ventricle) are evaluated at the side ispilateral to the insonating probe while structures more distant from the midline (like the basal ganglia) are generally evaluated from the contralateral side (see below). Landmark structure for orientation in this plane is the pineal gland, which normally appears strongly hyperechogenic due to calcification and is located adjacent to the posterior surface of the third ventricle. This plane allows assessment of the ventricular system including not only the third ventricle, but also the anterior and posterior horns of the lateral ventricles. Measurement of the third ventricle has been shown to be valid and reliable and correlates well with the findings of other structural neuroimaging methods (Berg et al., 2000). Normal values for the width of the ventricular system have been published (Seidel et al., 1995). From this and other studies, age-related normal values have been derived, which should be referred to when the diagnosis of an enlargement of the ventricular system due to brain atrophy, hydrocephalus, etc. is made (see Table I). Moreover, the thalamus and the basal ganglia including lentiform and caudate nuclei can be assessed. Normally, these structures appear isoechogenic compared to the surrounding tissue. In some cases, however, hyperechogenic areas may occur at the anatomical site of these structures, which is abnormal and related either to a pathological condition or to an abnormality without clear clinical implications, such as slight calcification of the basal ganglia.

C Plane of the Posterior Fossa

The posterior fossa can be visualized by rotating the anterior pole of the probe upward for about 45˚ starting from the imagined orbitomeatal line (compare Fig. 1). This plane also does not correspond to the common axial or coronal MRI slices (Fig. 4). It allows assessment of the fourth ventricle as well as of the cerebellar hemispheres, the vermis, and especially the dentate nucleus, which can sometimes be found to be slightly hyperechogenic compared to the surrounding tissue. Other structures that can be pictured in this plane, such as the brainstem, the hippocampus, or the cerebral lobes, are better assessed in one of the other planes and have not been evaluated so far in this plane. The plane of the posterior fossa does not belong to the routine workup so far, and proper depiction of structures in this plane is more dependent on the quality of the temporal acoustic bone windows. Therefore, data on the pathophysiological findings in this plane are limited and standardization of assessments in this plane is still in progress.

II Measurements and typical pathological findings

Well examined and of special interest to the evaluation of patients with movement disorders are the ventricles, the lentiform nucleus, the substantia nigra, and the mesencephalic midline raphe. The methods for quantitative assessment and examples of typical pathological findings for these specific structures are shown below.

A Ventricular System

The width of the third ventricle and the anterior horn of the contralateral lateral ventricle of the ventricular system must be measured in a regular exam. Especially, the width of the third ventricle has been shown to correlate well with the measurements on CT scans. Both structures are best visualized in the third ventricular plane (compare IB). For assessment of the third ventricle, the largest distance between the inner surfaces of both ependymal linings is measured perpendicularly in the frozen and about a twofold magnified image. In the same plane, the contralateral anterior horn is evaluated by using the midline as orientation. The distance between the medial (nearest to the midline) to the lateral ependymal lining (farthest from the midline)—again perpendicularly from the midline—is used for measurement of the anterior horn. Figure 5B shows a case of a pathologically enlarged ventricular system.

The posterior horn is in general not measured (there exist no standard values so far); however, the impression of an enlarged posterior horn should be documented.

B Lentiform Nucleus

The lentiform nucleus (LN) is also visualized in the third ventricular plane. Regularly, it is isoechogenic to the surrounding tissue and therefore cannot be quantitatively assessed. Hence, usually a semi-quantitative rating applying a three-item scale is used, rating “normal LN echogenicity,” “moderate LN hyperechogenicity,” and “marked LN hyperechogenicity.” Moderate LN hyperechogenicity...