Ultrasound-Guided Procedures and Investigations

A Manual for the Clinician

Armin Ernst, David J. Feller-Kopman, Armin Ernst, David J. Feller-Kopman

• Editore: CRC Press
• Anno: 2005
• ISBN:
  9781000611403
• Edizione: 1
• Argomento: Medicine
• Categoria: Medical Theory, Practice & Reference

Recognizing the increasing importance of ultrasonography in the evaluation and management of patients across a range of medical disciplines, this guide provides illustrative instruction on the performance and interpretation of ultrasound examinations in emergency, critical care, hospital, and outpatient settings.

1

ABCs of Ultrasound Imaging

James F. Greenleaf

Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, Minnesota, U.S.A.

Introduction

“Imaging” is a term applied to the graphic depiction of an attribute, usually physical, and usually in a two-dimensional format. “Medical imaging” usually refers to procedures that produce attribute images of tissues within the body.

Ultrasonography has had an increasing role in medical imaging over the past 20 years. Its use continues to expand because of the availability of real-time imaging and Doppler blood flow measurement, new transducer design, better signal processing, miniaturization and computerization of electronics, elimination of X-ray radiation exposure, and high information versus cost ratios of the images.

Cardiology, obstetrics, gynecology, and other areas of medicine have been greatly impacted by ultrasound (US) imaging technology.

The tissues of the body can be divided broadly into two types, soft and hard. In general, US is used to image the soft tissues (1); however, some imaging of hard tissues, such as bone, has been reported (2). There are two main reasons for medical imaging, detection and diagnosis of disease. Disease is detected and/or diagnosed by evaluating changes in US images, relative to images from normal individuals or relative to previous images in the same individual.

To make a US image, local variations in some acoustic property, usually backscatter are detected using ultrasonic energy and are mapped into a two-dimensional format (3). This requires localization of the energy in depth and azimuth, using appropriate spatial and temporal control of the energy. This chapter describes: (i) the elements of US medical imaging, (ii) an example of an imaging system, (iii) energy transduction, (iv) beam control, (v) signal processing, and (vi) display methods.

Elements of Ultrasound Imaging

Images are formed by sending a short pulse of high frequency sound (2–20 MHz) into the body and detecting weak reflections from scatterers within soft tissues. The method is similar to radar or sonar but on a different scale. Images of these echoes and their positions within two-dimensional planes are called B-scans and exhibit highly detailed representations of the internal structures. The speed of sound (c) in tissue (c_t) (approximately 1500 m/sec) allows approximately 100 to 200 echo lines to be obtained and displayed in the time of a television frame (1/30 sec); thus the method can produce real-time images of the internal organs and tissues of the body.

Ultrasonic waves are produced with a ceramic that changes dimensions when subjected to an electric field. Axial, lateral, and transverse localization of the ultrasonic beam must be accomplished to successfully image two- or three-dimensional distributions of scatterers within a volume of tissue.

Axial Localization

Axial localization of the energy is done with short pulses of ultrasonic energy propagating through the tissue, which reflect from scatterers distributed throughout the volume. The time of arrival of the reflected pulse encodes the spatial position of the pulse if the speed is known.

Ultrasonic Wave Speed

Speed in the soft tissues of the body is assumed to be approximately 1540 m/sec in modern imaging instruments. This assumption is violated in many cases; for instance, the speed of sound in fat can be as low as 1430 m/sec (4). Because the speed is not known exactly, errors can occur in the imaging system depending on how disparate the various tissue speeds are. For instance, fat layers within or overlying tissues of interest can cause aberrations in the image because the ultrasonic wave propagation speed in fat is very low.

Velocity/Timing

The depth (z) from which a pulse returns is determined from the speed of sound (c) and the travel time (t) to and from the scatterer; i.e., z = c t/2.

Axial Resolution

The axial resolution of the US imaging system depends on the spatial length of the pulse, which is inversely proportional to the frequency bandwidth (Δf) of the pulse (Fig. 1). The spatial length of the propagating pulse determines the distance between consecutive scatterers that can be delineated in the image, and is thus called the axial resolution. A term used to define the resolution is full width half maximum amplitude (FWHMA), which refers to the width at half the peak amplitude of the envelope in the axial direction of the pulse of pressure traveling in the tissue. The relationship is:

Figure 1
The resolution cell is the smallest volume from which the signal is received at one point in time. It depends on the aperture (D), the band width (Δf), the speed of sound (c), and the distance to the transducer (z).

${FWHM}_A=\frac{1.37mmMHz}{\Delta F}(1)$

Imaging can be improved by ensuring that the pulse has low side-lobes in the time dimension. This can be done by providing pulses that have smooth Fourier transforms, ensuring a smooth time domain response (5).

Lateral Localization

Lateral localization of the focusing ultrasonic beam is accomplished by electronically controlling the phase and amplitude of the motion of the transducer surface in transmit mode and by dynamic focusing in receive mode. Most systems localize the transmit energy and the receive mode sensitivity to a line collinear with the transmit direction. Each line of sensitivity, or imaging beam, is transmitted, the echoes are received, and then a new line is incrementally scanned in the imaging plane. This procedure is repeated, scanning quickly within a plane to produce data for real-time backscatter imaging in the plane. Sometimes several adjacent receive lines are computed by the scanner in the beam forming section for each transmit. This increases the line rate and, thus, the frame rate.

Lateral resolution depends on the size of the aperture (D), the distance (z) from the surface of the transducer to the imaged point, and the effective wavelength (λ = c/F) of the pressure wave. The relationship is (6):

${FWHM}_L=\frac{\lambda z}{D}(2)$

This relationship gives the best-case resolution, stating that the larger the aperture (diameter or length of the transducer probe) in wavelengths, the better the resolution. Most medical transducer probes are approximately 2.5 cm across and typically run at center frequencies of 3.5 to 7.0 MHz, giving a resolution at a depth of 10 cm of 1.7 to 0.85 mm at best. Because of variations in the speed of sound in the tissue, some defocusing occurs, limiting the resolution to values somewhat worse than those estimated with Equation 2 (7).

Aperture Shading for Enhanced Contrast

An important aspect of medical imaging that is not important in nondestructive evaluation or other acoustic imaging disciplines is the contrast of the image. Contrast refers to the ability to image holes or regions of low scatter buried within regions of high scatter. Imaging with high contrast is desired because it produces images with very dark cysts and vessel lumens. This requires that the lateral and axial responses of the system have low side lobes.

High side lobes smear energy into the dark regions of the image, decreasing contrast. It is well known that the lateral field response of a square aperture is a sine function, which has high side lobes. Shading of apertures refers to changing the amplitude of the pressure over the surface of the imaging probe to decrease the side lobes. The lowest side lobes can be produced with Gaussian-like shading across the face of the probe. Many commercial medical imaging instruments shade the amplitude of the US signal over the aperture of the probe, in addition to tailoring the amplitude of the pulse in time to obtain the lowest side lobes possible in both the lateral and the axial directions.

Transverse Localization

Transverse localization refers to the control of the direction of the beam in the direction transverse to the scanning plane. Beam control is usually accomplished in this direction with physical acoustics. Lenses or ot...

Indice dei contenuti

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Contributors
• 1 ABCs of Ultrasound Imaging
• 2 Ultrasound Guidance for Central Venous Catheterization
• 3 Ultrasound-Guided Thoracentesis
• 4 Ultrasound-Guided Percutaneous Drainage
• 5 Ultrasound-Guided Transthoracic Needle Biopsy
• 6 The Use of Ultrasound in Trauma
• 7 Emergent Pelvic Ultrasound
• 8 Basic Cardiac Echo
• 9 Documentation
• 10 Ultrasound: Future Directions
• 11 Ultrasound Competency
• Index