current modern transducers. Their configuration on the probe varies and determines the appearance of the image. Three main types of transducer configuration are usually used for insonation of the brain-supplying arteries and veins (Fig.A1.14). For extracranial insonation, high-fre­quency transducers with a frequency range between 5 MHz and 15 MHz and a linear or sector ultrasound beam configuration are used. The linear transducer has the advantage of providing undistorted images with high spatial resolution, but it requires a large contact surface. For extracranial insonation however,this is generally not a problem, so that the linear probe is most frequently used. The high-frequency sector transducer has the advantage to provide images from a wider fan-shaped range but deliv­ers distorted images. For transcranial insonation, lower frequenciesin the range of 1–3MHzarerequiredtopermit transmission and reception of sound through the skull bone. However, this is limitedto regions of the skull where the bone is naturally thin, i. e. a “bone window” is present (for further details, see also Chapter 2, “General Structure of Arterial Ultrasound Examination,” p. 17, and Fig.A2.26). The use of a linear transducer for transcranial insonation would, if applied result in small, narrow images, whereas the application of the trapezoid-shaped ultrasound field allows insonation with maximum image information even through a small bone window (Fig. A1.14). The ultrasound fields of all transducer types are focused by either the spatial arrangement of the piezoelectrical elements on the surface of the transducer, by electronic control, or both. The ultrasound field can be divided into a near field and a far field. The transitional area between both is the focus zone with the best lateral spatial image resolution (Fig. A1.15). The localization of the focus zone can be ad­justed in all duplex ultrasound systems and should always be optimized according to theinsonated region of interest.

The brightness-mode (B-mode) permits two-dimensional visualization of tissue and is based on ultrasound signals being reflected from the insonated structures. The result­ing image is a compound of information from the parallel connected piezoelectric elements of the ultrasound trans­ducer. The reflected amplitudes along each single ultra­sound beam (A-mode data) are gray-scale coded, and within the image, arranged according to their reflection depth (Fig. A1.16).Thesumofallside-by-sideultrasound beams add up to the final B-mode image.

A particular problem of the above-mentioned conven­tional ultrasound modes is the lack of sensitivity for small bloodvesselsandthevascularperiphery.Withinthisre­gion the flow velocities are low. The resulting Doppler shift reaches the range of the spontaneous movements of the insonated tissue, which impedes the differentiation be­tweenbothsignals.Furthermore,thenumberofavailable reflectors is much lower within the microcirculation. The use of harmonic imaging in combination with ultrasound contrast agents overcomes this problem and permits the visualization of blood flow within the microcirculation. The underlying principle is that contrast agents not only purely reflect the emitted frequency but show resonance phenomena. Ultrasound contrast agents consist of micro­bubbles that vibrate or oscillate. Each bubble consists of a stabilizing shell and a gaseous core. If exposed to mechan­ical energy, they start to oscillate or if the insonation en­ergy becomes very high they may burst (Fig. A1.18). Oscil­lation phenomena occur marginally in resting tissue, and this facilitates the differentiation between tissue and the stimulated microbubbles, i. e., the perfused microcircula­tion. The oscillation phenomenon occurs in different fre­quencies (multiples of the original emission frequency). Therefore harmonic imaging requires transducers that can emit one frequency and receivce a different frequency at thesametime.Differenttechnicalsolutionshavebeen suggested to optimize the necessary differentiation be­tween the emitted fundamental and the reflected har­monic frequencies. Two commonly used examples, the second harmonic imaging mode and the phase inversion harmonic imaging mode, are briefly explained in Figures A1.19 and A1.20. Other modes that can be employed are the signal analysis via power modulation (use of two im­pulses differing in their intensity) or contrast pulse se­quencing (a combination of phase inversion and power modulation). However, the number of potentially useful techniques is rapidly increasing and an exhaustive discus­sion of these is beyond the scope of this book. For more information, we recommend that readers review current literature.

The analysis of the arterial vascular system of the brain and itspathologieshasbeenthemainfocusofextracranialand transcranial ultrasound examinations for many years. The increasing quality of the available ultrasound systems, especially for the intracranial examination approach, has led to more detailed visualization of even peripheral vessel segments. This subsequently requires constant adaptation of examination techniques which include excellent knowl­edge and a particular spatial sense of the extracranial and intracranial anatomy. In all applied diagnostic techniques including ultrasound, the anatomic descriptions used should be as precise as possible, e. g., for spatial orientation the approved notation should be used (Fig. A2.1). All ves­sels can be described and classified by their origin, course, caliber, and length, and are subdivided into different seg­ments within an angiographic nomenclature. The follow­ing glossary is a synopsis of several reference textbooks whose data are derived from postmortem examinations and angiographic studies (Huber 1982, Lang 2001, Osborn 1999, Yasargil 1984). Information from special reports is cited separately in the text.