Hennerici / Neuerburg-Heusler Vascular Diagnosis with Ultrasound

Clinical Reference with Case Studies Volume 1: Cerebral and Peripheral Vessels
2. Auflage 2005
ISBN: 978-3-13-257878-4
Verlag: Thieme
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 384 Seiten, ePub

Features more than 500 line drawings and images

The critically acclaimed Vascular Diagnosis of Ultrasound returns in a new two-volume second edition, offering the most comprehensive information available on the broad spectrum of vascular ultrasound applications. Volume 1: Cerebral and Peripheral Vessels retains the accessible design and structure of the first edition to discuss the available ultrasound technologies, including continuous and pulsed-wave Doppler mode, b-mode, and conventional and color-coded duplex analysis in frequency and amplitude power modes. This text covers anatomy, physiology, normal and abnormal findings, test accuracy and sensitivity, providing the reader with the information essential to managing common clinical situations.

Highlights:

- Provides comprehensive coverage of vascular ultrasonography in the arteries and veins of the cerebral circulation and the peripheral upper and lower limb circulation - Compares other diagnostic methods used in each region, such as conventional and noninvasive MR angiography - Assesses recent developments in ultrasound technology, including tissue perfusion studies, 3D and 4D imaging, contrast enhancement and microbubble applications, and their diagnostic, technological, and therapeutic implications - Challenging case studies for both the novice and the expert to review - With contributions from experts in the field and more than 500 line drawings and images, this text is an indispensable reference for radiologists, vascular surgeons, and residents and students in these specialties.

Zielgruppe

Ärzte

Autoren/Hrsg.

Hennerici, Michael G.

Neuerburg-Heusler, Doris

Fachgebiete

• Medizin | Veterinärmedizin Medizin | Public Health | Pharmazie | Zahnmedizin Klinische und Innere Medizin Kardiologie, Angiologie, Phlebologie
• Medizin | Veterinärmedizin Medizin | Public Health | Pharmazie | Zahnmedizin Medizinische Fachgebiete Bildgebende Verfahren, Nuklearmedizin, Strahlentherapie Sonographie, Ultraschall

Inhaltsverzeichnis

1 Physics and Technology of Ultrasound
2 Extracranial Cerebral Arteries
3 Intracranial Cerebral Arteries
4 Cerebral Veins
5 Peripheral Arteries
6 Peripheral Veins
7 Case Histories
8 Glossary

Leseproben

 1     Physics and Technology of Ultrasound Basic Ultrasound Physics Sound is mechanical energy that is transmitted through a medium such as air. Periodic changes in air pressure are created by forces acting on air molecules, causing them to oscillate. A pressure wave is transmitted from one location to another when vibrating molecules interact with neighboring molecules. This molecular motion is necessary for the transmission of sound and explains why sound cannot be transmitted in a vacuum. Sound waves above a frequency of 20 kHz are termed “ultrasound.” Like all sound waves, ultrasound propagates through various media in the form of a pulsating pressure wave. Waves are basically of two types—longitudinal and transverse. Longitudinal waves are those in which particle motion is along the direction of propagation of the wave energy. Sound waves are longitudinal. Transverse waves are perpendicular to the direction of propagation of the wave energy. Wave motion resulting from a stone being thrown into water is an example of a transverse wave. Bone is the only biological tissue that can cause the production of transverse waves, which are also referred to as “shear waves” or “stress waves.” Properties of Waves When particle displacement is plotted against distance, the wavelength (?) of a wave is the distance from crest to crest, or from trough to trough. A wave cycle is a sequence of changes in the amplitude that recur at regular intervals. The frequency (f) of a wave is the number of cycles passing a given point in one unit of time (usually one second). The unit of frequency is the hertz (Hz; one cycle per second) (Fig. 1.1). The speed of wave propagation through a medium is known as the acoustic velocity, c. This speed depends on the density and compressibility of a medium. For sound to propagate, it is essential for a medium to be present. In addition, the medium also has to be compressible—that is, it must be able to deform temporarily and then return back to its original shape. The velocity of sound in a medium is inversely proportional to the square root of the medium's density, . Therefore, the denser the medium, the slower the velocity of sound. The relationship between sound velocity and compressibility is also inversely proportional to the square root of the medium's compressibility, . Since dense materials such as bone have low compressibility, sound propagates through them at a high velocity. In contrast, since gas molecules in air are far apart and easily compressible, the velocity of sound in air is slower. Fig. 1.1 Schematic drawing of ultrasound wave When the effects of compressibility and density are combined into a single equation, acoustic velocity can be defined as: Compressibility and density are interdependent—changes in density are accompanied by opposing changes in compressibility. However, since compressibility varies more rapidly than density does, it becomes the dominant factor in determining the relative acoustic velocity through a medium. Reflection Ultrasound reflections occur when there is an interface between two media with differing levels of resistance to ultrasound. Such interfaces are known as “specular reflectors.” Resistance to ultrasound is termed acoustic impedance. Acoustic impedance depends on the speed of ultrasound propagation in tissue and on the density of the tissue concerned; the greater the difference in impedance, the stronger the reflection. Mathematically, acoustic impedance (Z) is the product of the density of the tissue () and the velocity of sound in the medium (c): Acoustic impedance is thus is a measure of the resistance to sound passing through a medium. High-density materials are associated with high sound velocities and therefore high acoustic impedances. Similarly, low-density media such as gases have low acoustic impedances. It is not important which impedance is larger or smaller for two media forming an interface; the same reflection occurs whether sound is propagating from high-impedance tissue to low-impedance tissue or vice versa. The angle of reflection of a sound beam is equal to the angle of incidence of the sound beam. To obtain maximum detection of the reflected ultrasound signal, the transducer, which both sends and receives signals, has to be oriented in such a way that the ultrasound beam generated strikes the interface perpendicularly (normal incidence). Refraction If a sound beam strikes an interface at an angle other than 90°, the part of the beam that is transmitted further is refracted or bent away from the straight path that would have been expected. This refraction takes place in accordance with Snell's law of optics, which relates the angle of transmission to the relative velocities of sound in the two media. The bending occurs because the portion of the wavefront in the second medium travels at a different velocity compared with that in the first medium. As may be expected, the amount of deviation from the expected path changes with the angle of incidence and with the velocities in the associated media. If the velocity of sound is the same in both media, then there will be no refraction, even though there may be different acoustic impedances. In diagnostic vascular ultrasound, problems of refraction are encountered in transcranial applications, in which refraction occurs at both bone–tissue and tissue–bone interfaces when insonating through the transtemporal bone window. This may result in a significant reduction of the image quality. Scattering and Diffraction Tissue particles that are relatively small in relation to the wavelength (e.g., blood cells), and particles with differing impedance that lie very close to one another, cause scattering or speckling. Reflecting media that lie at an angle to the ultrasound propagation axis can only be recognized due to these scattering phenomena, which are accompanied by an attenuation in the echo intensity. Speckling effects result from the extinction of reflexes from structures lying adjacent to each other or close behind each another at a distance of ?/4. Table 1.1 Attenuation of various human tissues at 1 MHz Tissue Np/cm Blood 0.021 Fat 0.069 Brain 0.098 Liver 0.103 Kidney 0.115 Skull bone 2.3 Lung 4.6 Np: neper. Attenuation The stronger the reflections are at the interfaces, the less ultrasound energy is available to reach deeper tissue. If a total reflection occurs at an interface to air, bone, or calcium-containing tissue, then an ultrasound shadow results. Most of the ultrasound energy is converted into kinetic energy within the tissue. The term attenuation characterizes the reduction in the amplitude of an ultrasound wave as it propagates through a medium, due to both scattering and absorption. Attenuation is described by an exponential function. The attenuation coefficient is given by the sum of the scattering coefficient as and the absorption coefficient a: a= as + a The coefficients quantitate the respective fractional loss in amplitude per unit length from absorption, scattering, and both processes together. The special unit used for these coefficients is the neper (Np) per centimeter. The attenuation coefficients at 1 MHz for various tissue types are shown in Table 1.1. Intensity and Power The intensity of the ultrasonic beam is a physical parameter that describes the amount of energy flowing through a unit of cross-sectional area each second. Acoustic intensity is expressed in mixed units of watts per centimeter squared. One watt is equal to one joule per second. The intensity of an ultrasonic beam is proportional to the square of the pressure amplitude. The instantaneous intensity (i) is given by where p is the acoustic pressure, c the velocity of sound and the density. The intensity of the beam decreases exponentially with distance, the magnitude of which depends on the amplitude attenuation coefficient. The Doppler Effect The flow velocity of corpuscular elements in the blood can be detected with ultrasound by using a principle named after Christian Andreas Doppler (1803–1852). The physical context of the Doppler effect may well be familiar from everyday experience: when a truck blowing its horn approaches a stationary observer on a highway, the observer will hear the sound of the horn rising in pitch until the truck is directly in front of him. When the truck has passed and drives away from the observer with its horn still blowing, the pitch of the horn starts to fall. When we use this principle to investigate blood flow, we refer correspondingly to a change in frequency called the Doppler shift, ?f(Hz), which is proportional to the flow velocity of blood, v (cm/s), and to the transmission frequency of ultrasound, f (MHz). Exact measurement of the Doppler shift requires that the angle a between the ultrasound beam and the longitudinal axis of the blood vessel is known: ?f=2f...

Über Autor(innen)

Michael G. Hennerici, Doris Neuerburg-Heusler