Introduction to Basic Ultrasound

INTRODUCTION TO BASIC ULTRASOUND INTRODUCTION PAGE History and Definition 2-2 Sonographic Abbreviations & Terminology 2-3 Basic Formulae 2-4 PHYSICS AND INSTRUMENTATION 2-5 General Rules for Scanning 2-5 Scanning Modes 2-6 The Ultrasound Transducer 2-7 Image Resolution 2-9 The Ultrasound Beam 2-10

Wave Propagation Frequency Power Focusing

How Ultrasound works 2-12 Interaction with Matter Attenuation Compensation Image Processing Artifacts Image Quality

BASIC KNOBOLOGY 2-19 Gain Controls 2-19 Field of View 2-19 Power 2-19 Image Processing Controls 2-19 Measurement 2-22 Magnification 2-22 ID/Annotation 2-22 Image Display Information 2-22 Image Recording 2-23 BASIC SCANNING 2-24 Patient Preparation & Positioning 2-24 Preparing the Ultrasound Scanner 2-25

Transducer Selection Image Format/Polarity

Scanning Planes 2-26 Image Orientation 2-27 Assessing Image Quality When Scanning 2-28 BIOEFFECTS 2-29 PATIENT/USER SAFETY 2-30 ROUTINE MAINTENANCE 2-31 ULTRASOUND VS. X-RAY 2-32

RM: Physics and Instrumentation 2-2

THE HISTORY OF ULTRASOUND

The use of ultrasound as a diagnostic modality is a relatively new practice. The idea came after Langevin made use of a pulse-echo technique called SONAR (Sound Navigation and Ranging) in 1916. Later, in the late thirties, ultrasound was applied in industry by Firestone to detect metal flaws present in equipment and machinery. An Australian researcher named Dussik was the first to apply ultrasound to medicine. He used the metal flaw detector to evaluate the cerebral ventricles for midline displacements or defects. In 1949, Ludwig and Struthers improved the pulse-echo technique to detect foreign bodies in soft tissue. Soon this practice was applied to the gallbladder, in the search for gallstones. Howry and Bliss produced the first cross-sectional ultrasound image in 1950. The idea that this would be useful for imaging of the gravid uterus followed shortly thereafter. Ian Donald, in Scotland, was the first to obtain sonographic images of ovarian cysts and BPDs in 1958. In the 40 years since Dussik’s application of ultrasound to medicine, the quality of ultrasonic evaluation has improved dramatically to such a point that it is now considered an integral part of diagnostic imaging. The relatively low cost, ease of examination, and absence of significant bioeffects make ultrasound a favorable route to take among the many imaging modalities. As you will see, the ease with which one can master sonography makes this modality integral to modern Obstetric and Gynecologic practice. DEFINING ULTRASOUND - A brief look at how it works

The human ear can hear sound in the frequency range of 20-20,000 Hertz (cycles per second). Ultrasound is that sound which is at a frequency greater than 20,000 Hertz. Diagnostic Ultrasound operates in the range of 1 to 10 Megahertz, with 3.5 MHz being the standard operating frequency for obstetrical evaluation.

Ultrasound travels in the form of a longitudinal wave, in which particle motion is along the same direction as the wave is traveling. These waves are generated by the ultrasound transducer. Longitudinal waves transfer energy through the motion of regions of compression and rarefaction within the wave. Because it is in wave form, ultrasound physics is governed by many of the same principles as basic wave physics (see attached formulae).

The ability of the ultrasonic wave to travel through any medium is restricted by properties of that medium; these properties include the density and elasticity which make up the acoustical impedance specific to that medium. Transmission is also limited by the transducer frequency being used; higher frequencies have shorter wavelengths and penetrate less than lower frequencies.

As the ultrasound beam encounters tissues of different acoustic impedance, velocities are altered such that returning echoes are received by the transducer at different times and have different intensities. The differences in time and intensity are adjusted for by the user and the computer within the system. This information, along with the values for sound wave velocities in tissues is used by the ultrasound device to generate the diagnostic image on the monitor.


SONOGRAPHIC TERMINOLOGY 1. Acoustic Shadow - caused by absorption or reflection of sound and results in a lack of echoes

posterior to a structure (partial “dirty” or complete “clean”). 2. Anechoic - completely void of internal echoes not necessarily cystic unless through transmission is

noted. 3. Complex - displaying more than one sonographic characteristic; nonhomogeneous composition. 4. Cystic - describes a fluid-filled area, not necessarily a true cyst. 5. Echo free - completely void of internal echoes/interfaces (anechoic). 6. Echogenic - a relative term describing a structure that produces echoes dependent on the number of

internal interfaces it has. 7. Echogenicity - refers to the echo-producing ability that a structure has, dependent on internal

composition; a structure is associated with an echogenicity, which when altered is considered abnormal.

8. Echo poor - structure containing very few of low-level echoes; not cystic in nature thus will not

exhibit through transmission. 9. Homogeneous - structure that has uniform composition. 10. Hyper reflective - structure displaying more echoes than characteristic of it. 11. Hypo reflective - structure displaying fewer echoes than is characteristic of it. 12. Inhomogeneous - structure without uniform composition. 13. Interface - strong echo that is produced due to a large acoustic impedance mismatch; most

pronounced when the beam is perpendicular. 14. Mixed Echogenicity - complex; displaying more than one sonographic characteristic. 15. Sonogram - an ultrasound examination. 16. Sonographer - a professional trained in ultrasound technology (they are not “techs”). 17. Sonologist - a physician who specializes in ultrasound. 18. Sonolucent - completely void of internal echoes/interfaces. SONOGRAPHIC TERMINOLOGY, cont. 19. Texture - the echo pattern within a structure. 20. Through Transmission (Acoustic Enhancement) - caused by a lack of attenuating ability of a


superficial structure such that distal structures appear to have more echoes due to more sound passing through.

21. Transducer - a device capable of changing one form of energy to another. 22. Transonicity - refers to a structure’s ability to allow sound to pass through it; usually qualified as

good or bad. 23. Ultrasound - sound with a frequency greater than 20,000 Hertz (cycles per second). 24. Unilocular - comprised of one cavity or compartment (i.e., simple or unilocular cyst). BASIC FORMULAE Sound is attenuated 1db/MHz/cm The velocity of sound is assumed to be 1540 m/sec, average - in human tissues Velocity: v=fλ Acoustic impedance: z= vc Attenuation Coefficient: dB=10 Log 10(I2/12) Snell’s Law: Sin i = V1

Sin r V2 Intensity (I): dB = 10 Log (Ar)2 = 20 Log Ar or Power (W)

(Ai)2 Ai Area(cm2) v - velocity (m/s) f - frequency (hertz) λ - wavelength (meters) z - acoustic impedance (g/cm2) c - density l - intensity i - incident beam r - refracted beam A - amplitude


PHYSICS AND INSTRUMENTATION This portion of the residency training program is designed to introduce the resident to the basic principles of Ultrasound Physics and Instrumentation. Other topics discussed include basic scanning, image format and manipulation, knobology and information specific to imaging of the uterus, ovaries and fetus. Using this information, in conjunction with basic scanning techniques, should enable the user to perform a basic ultrasound exam and/or localization. In this section you will become familiar with the numerous parameters of ultrasound image generation. Various types of display, transducers, and ultrasound beams will be discussed. An assessment of what makes ultrasound “work” as an imaging modality is also included. GENERAL RULES FOR SCANNING The ultrasound beam should be directed perpendicular to the object of interest for optimal visualization (this can be quite challenging with a moving target!) The user must choose a transducer which has the highest frequency allowable for the penetration required. A full bladder is required for optimal visualization of the uterus and ovaries (GYN) or lower uterine segment (OB). Scan all objects of interest in two planes 90 degrees to each other (i.e., scan each structure in its long and short axis).


SCANNING MODES Various scanning modes can be used, as is indicated by the particular examination to be performed. In OB/GYN ultrasound, the real time mode is the only one necessary. For informational purposes, however, the various other modes of scanning are reviewed. Bistable Scanning Bistable scanning is that which displays images in black and white only combinations. This type of display is obsolete today. Grey Scale Imaging Currently, Grey Scale Imaging is the standard among ultrasound users. An analog to digital scan converter transfers information from the transducer/receiver to the CRT. Manipulatable shades of grey enhance tissue characteristics and make the ultrasound image more aesthetic and realistic. Since only 16-32 shades of grey are discernible by the human eye, most systems employ no more than 32 grey shades (although many manufacturers claim to produce 128 or even 256 shades). A-mode Amplitude (A) mode displays present the amplitude of individual echoes as a function of distance of time of the CRT (cathode ray tube). This type of display is often present along side of the image, and may be helpful in determining cystic vs. solid characteristics. B-mode Brightness (B) mode displays present echoes as individual spots which are placed on the screen correspondent to their point of origin in the body. Differences in amplitudes of returning echoes manifest as different dot brightness. Using many picture elements (pixels) within the CRT, these numerous dots can be laid out and superimposed in such a way as to permit visualization of different shades of grey. Static B-scanning is a basically obsolete form of B-scanning, as it is currently expected that ultrasound images will be obtained in a real-time (dynamic) mode. M-mode Motion (M) mode is the result of the application of B-mode to a moving structure with an incorporated time function. In a strip recorder fashion, M-mode is often used as part of an echocardiographic examination, and to document viability in obstetrical scanning. Real-Time Dynamic or Real-Time imaging allows the user to document both the grey scale characteristics of tissue and the motion of interfaces. You “watch as it happens”. The brightness dots move among the CRT pixels as the actual interfaces move. State of the art sonography uniformly employs real-time imaging.


THE ULTRASOUND TRANSDUCER A transducer is a device capable of changing one form of energy into another. For ultrasound purposes, the transducer is the sender and a receiver of ultrasonic pulses and echoes. This transducer can change electrical impulses into mechanical waves and vice versa. As a transmitter, the transducer employs a piezoelectric crystal to create the waves that enter the body. As the receiver, the transducer has many functions including amplification, compensation, demodulation, compression and rejection.

Piezoelectric Effect

Man-made lead zirconate and lead titanate are used as crystals within most modern ultrasound transducers. These crystals are referred to as the piezoelectric (pressure-electric) crystals because they have the properties necessary for conversion of electrical stimulus to vibrations which produce pressure (sound) waves and vice versa. This unique quality enables transmission and receipt of the ultrasound signal by a single transducer.

Pulse Principles

Electricity is applied to the piezoelectric crystals at a specific pulse rate. This allows for transmission of the wave as well as listening for echoes. Generally, the transducer sends pulses of 1 microsecond duration with a repetition period of 999 micro seconds. Therefore, most of the time, the ultrasound scanhead is in “listening” mode. Pulse rate may also be controlled by the rate of oscillation of a mechanical scan head.

Transducer Selection

The correct transducer must be chosen from a wide range of possibilities. Variables specific to each transducer must be carefully examined so as to choose the optimal transducer for the particular task at hand. Variables include frequency, image shape and inherent technology. Frequency

Most manufacturers offer a variety of transducers with frequencies ranging from 2-10 MHz. Some even possess the ability to vary the frequency within a single transducer. The operating frequency of a transducer is generally determined by the size of the piezoelectric crystals employed within.

As a rule, lower frequencies allow better penetration than those at upper frequencies. For this increase in penetration, there is a decrease in resolution, however. Lower frequency (2-3.5 MHz) should be used for third trimester examinations, and when you need additional penetration ability (as for obese patients). A 3.5 MHz transducer is standard equipment on most commercially available ultrasound systems.

Medium frequency transducers (5.0 MHz) are the best compromise between penetration and resolution. They can be used for all GYN and OB scans of patients with normal habitus. High frequency transducers have greater ability to resolve minute structures, but the user is limited by decreased depth of penetration. High frequency transducers (7.5-10 MHz) are employed for examination of small parts such as thyroid, breast and testicles. This may also be quite useful during the examination/localization of a superficial ovary, or for a very thin patient.

Application: ·Lower frequency probes are used for third trimester

pregnancy, and obese patients.


·Higher frequency probes are for superficial exams such as breast, thyroid, and testicles.·3.5 or 5.0 MHz probes are generally adequate for 1st and 2nd trimester OB studies.

General Rule: Use the highest frequency probe that will penetrate to the

back wall of the structure to be visualized. Transducer Mechanics Transducers may again be separated into categories based on their respective means of beam guidance and pulse generation. Mechanical Transducers/Rotating or Oscillating Many manufacturers offer mechanically controlled transducers. Since production of the ultrasound beam involves moving parts, this type of transducer is prone to break down. Mechanical transducers continually generate ultrasound waves, but only allow passage into tissue as a window or port rotates in front of the beam. This type of transducer may contain a single element, or numerous crystals, and generates a sector or pie shaped image. Wobblers are those mechanical sector scanners which operate with a back and forth rocking motion. The width of the section is determined by the distance that the wobbler is allowed to rock. Linear Array In a linear array transducer, multiple crystals are lined up along the face of the transducer in a linear fashion. Groups of crystals are stimulated together to form a beam which is displayed as a rectangle on the CRT. These provide a longer skin/transducer contact area and a larger near field of view.

Application: Use Linear transducers in third trimester OB scanning to measure femurs; the wide near field provides best visualization of entire length of femur.

General Rule: Do not use linear scanner to visualize lower uterine segment.


Phased Array In phased array transducers, the sequence in which the crystals are “fired” is electronically controlled in order to steer the beam and focus it. Phased array can be used to generate both sector and rectangular shaped images. Annular/Coaxial Phased array transducers may contain concentric crystals which are electronically fired and focused at a specific depth. Less frequently, the crystals may actually be rotated in a ring fashion. Images are sector shaped. Annular arrays are limited when compared to state of the art computed sonography in that they typically have only five (5) receiving channels to read information from numerous transmitters. The best systems incorporate characteristics of linear array and electronically focused phased array. This combination multiplies the user options and focusing possibilities for better image resolution. It is ideal to be able to read as many “mini beams” as you can generate. Thus, it is favorable to choose a transducer/system in which the number of transmitting crystals is equal to the number of receiving channels. IMAGE RESOLUTION Resolution is the ability of the transducer to separate and define small, closely spaced structures. The resolution capability of the transducer is of utmost importance when selecting the proper transducer for the task at hand. The scanner should choose the transducer that offers the best resolution at the needed penetration.

Lateral Resolution

Lateral resolution is the ability to separate and define small structures perpendicular to the beam axis. This resolution is measured in millimeters and is a function of beam width. Lateral resolution is optimized by focusing the beam at the area of interest, and using high frequencies.

If the beam width (controlled by crystal size) is greater than the separation between two objects, they are not resolved as separate objects. If the beam width is less than the separation distance, then we see the structures as two separate entities.

Axial Resolution

Axial, or longitudinal, resolution is the measure of the transducer’s ability to separate and define two structures along the axis of the beam. This is a function of pulse width, and is optimized by damping the crystal and increasing frequency.

The two investigated structures must be separated by a distance greater than or equal to one wavelength if they are to be resolved. Since the average ultrasound pulse contains two wavelengths, we want to use a higher frequency transducer to improve axial resolution because of its shorter wavelength.

THE ULTRASOUND BEAM The user should possess basic knowledge about the ultrasound beam in order to ensure safe and efficient application of diagnostic ultrasound. Information on wave propagation, frequency, power/intensity, focusing and pulse principles follows.


Wave Propagation

Propagation is the transfer of sound energy from one volume of space to another. As will be discussed later, propagation of the entire wave is highly unlikely, as obstacles to complete propagation are abundant within the human body. Thus, attenuation of the beam during propagation is inevitable.

Frequency

The frequency of the ultrasound beam is determined by the size of the crystals with the chosen transducer. Because the speed of sound in a particular tissue is relatively constant, we can determine the wavelength of the beam using the formula V = fλ. It is important to understand this because depth of penetration is directly proportional to the wavelength. Therefore, higher frequency transducers have the best resolution but the worst penetration.

Power/Intensity

Ultrasound power is expressed in a number of ways, the most useful being the decibel (dB). Intensity measures the amount of energy flowing through a unit area in a unit of time. See formulae.

Transmit intensity or power is manually adjustable on most modern devices. Keep in mind that the lowest possible power setting which allows adequate penetration should be employed for safety reasons.

Focusing The beam emitted by the ultrasound transducer in an unfocused state has two separate “fields”: The near field (Fresnel Zone) and the far field (Fraunhoffer Zone). The near field is the same width as the transducer face and extends to the focal point (focal points are variable with electronic images). Beyond this point, the beam diverges and is called the far field. Because of this divergence, resolution is worse in the far field than in the uniform near field.

Ideally, a transducer should be focused to the specific area of interest. This optimizes the resolution of the structures in that area. Focusing may be accomplished with the use of an acoustic lens, which converges the beam to one focal point, a mirror, which reflects the beam to one focal point, or a curved crystal surface, which directs the beam to one point from its origination. Most commonly employed today, however, is the variable electronic focus of the phased array type of transducer.


Many transducers have a pre-selected focus which is only minimally changeable. Most commercially available transducers have the following focal lengths:

Short 3-5 cm Medium 5-7 cm Long 9-13 cm

In this case, the focal point becomes a major factor in proper transducer selection.

State of the art ultrasound transducers read the returning echoes in such a way that they are better focused throughout, permitting less operator error. These devices allow the user to change both the focal zone and the number of focal zones.

Application: Since most modern equipment is focus variable, change the

focus as you scan each area of the image in order to maximize resolution.

General Rule: You should be changing the focus for each different anatomical

region examined. HOW ULTRASOUND WORKS The diagnostic image that we see on the monitor is a compilation of the previously discussed variables and certain known values. Understanding the way in which the beam interacts with tissues is important in understanding image formation.

Interaction with Matter

The useful information in an ultrasound image is the result of echoes received as the beam interacts with the tissues it traverses. These interactions contribute to image formation and are as variable as the tissues themselves. Because of these variables, it is important to have some known values for use in computation and image formation. Such known values include acoustic impedance and speed of sound in certain tissues.

Acoustic Impedance

Acoustic impedance (z) is a function of the elasticity and density of a particular tissue. Materials with high z transmit sound faster than others, but do not allow for continued compression by the impending wave. Some common acoustic impedance values are:

Blood 1.6 x 105 gm/cm2

Bone 7.8 x 105 gm/cm2

Fat 1.4 x 105 gm/cm2

*Soft Tissue Average 1.6 x 105 gm/cm2

Air .004 x 105 gm/cm2

Water 1.5 x 105 gm/cm2

Application: ·Bone appears to be white on the ultrasound image

because it is hyper reflective.


·Blood or fluids appear to be black on the image because they are anechoic.·Soft tissue appears as grey (salt and pepper) on the image because it is of medium echogenicity.

General Rule: Use plenty of gel to remove the air interface between the

transducer and the skin. The physical properties of air do not allow passage of the ultrasound beam even though the Acoustic impedance has a low value.

Speed Variables

The following list contains known values for the speed of sound in specific tissues:

Blood 1570 M/sec Bone 4080 Fat 1450 *Soft Tissue Avg. 1540 Air 330 Water 1480

While it is evident that the ultrasonic wave traverses different tissues at different speeds, the scanning devices must be able to calculate some fixed speed in order to place brightness dots in the correct location on the monitor. For this reason, ultrasound devices assume that all tissues traversed have a velocity of 1540 meters per second (soft tissue average).

Attenuation

The sound beam is attenuated while traveling through the various tissues within the body. The beam may be scattered as it encounters varying tissues (with different acoustic impedances) or it may be reflected back toward the transducer. Refraction (beam bending) and absorption may also attenuate the beam. The amount of energy removed from the beam per unit depth is expressed as the attenuation coefficient: dB = 10 Log10 (I2/I1). This is simply a logarithmic expression of the ratio of intensity of the returning echoes to the intensity of the original sound beam. As the frequency increases, the attenuation coefficient also increases. As a general rule, the beam is attenuated one dB per each MHz for each cm of tissue traversed.

Reflection

When the beam reaches a tissue interface within the body at a perpendicular incidence, the energy is reflected back toward the sound source. The amount of energy reflected is proportional to the difference in acoustic impedance between the structures forming the tissue interface. These reflected echoes are used to create the diagnostic image.

Refraction


If the beam is not exactly perpendicular to the object of interest, refraction of the sound beam may occur. In this instance, the beam is bent much like light is bent at an air water interface. The exact angle of refraction can be calculated using Snell’s Law:

Sin I = V1Sin R = V2

Not all non-perpendicular beams are refracted. Some are scattered haphazardly about the tissue interface. These scattered echoes are also used to create images of the tissue boundaries which are not perpendicular to the beam, as well as fill in tissue parenchyma.

Absorption

Another cause of beam energy loss is absorption by the traversed tissues. Depth of the object of interest is the major absorption determinant. This lost energy is imparted to the tissue in the form of heat. (This is the basis of therapeutic ultrasound used in sports medicine).

Compensation

Because of the many attenuating processes involved with ultrasound transmission, some form of compensation must be employed in order to visualize internal structures in an aesthetic and diagnostic manner. The differences in the intensity and amplitude of returning echoes are compensated for using mechanisms referred to as Time Gain Compensation and Depth Gain Compensation. These compensators give equal amplitudes to displayed echoes of equally impedant structures on the final image regardless of the depth traversed or time elapsed between pulse and echo.

Time Gain Compensation

Since echoes in the near field take less time to return to the transducer than those in the far field, Time Gain Compensation (TGC) is used to correct the varying intensities (and dot matrix placement) of these echoes. To do this, the user would increase the TGC gains in the far field and decrease the gains in the near field. One must keep in mind that over or under compensation can produce artifacts and limit the diagnostic quality of the final image.

Depth Gain Compensation

Echoes returning from varying depths also have varying intensities and amplitudes. Because echoes traveling deeper into the body are attenuated more than those returning from superficial structures, one should employ Depth Gain Compensation (DGC) to correct for these energy losses.

Since Depth related energy loss and Time related energy loss are directly related, most modern scanners unite these two functions in the form of area gain pods to make corrective manipulation easier for the sonographer. Simply move the gain pod which corresponds to the portion of the image you are trying to improve. Pods move to the right to increase gain and to the left to decrease the gain.

Overall (System) Gain


Once the image appears to be of homogeneous echo quality throughout both the near and far fields, we can increase or decrease the overall brightness of the entire image. This is accomplished by turning the Overall Gain Control (pod or dial) up or down. This control affects all areas of the image equally by controlling the amount of amplification performed by the receiver.

Image Processing

Digital manipulation of the ultrasound image can improve both image quality and the diagnostic capability. The user can alter the appearance of the image to enhance sonographic findings.

Pre-processing

Pre-processing refers to compression of the various amplitudes into number values. These numbers are used in the digital creation of the ultrasonic image. This allows preferential enhancement of certain tissue types and better characterization of the internal structure of organs. Pre-processing is performed before or while scanning (in real-time) and the changeable options are persistence, edge enhancement, and compression.

Persistence subtle tissue differences Edge Enhancement vessel and organ boundaries Compression grey scale available for image formation

Post-processing

Post-processing takes the numbers assigned by pre-processing and in turn assigns them to grey shades available on the video monitor. Post-processing is performed after the image is frozen. It can be used to give more or less display brightness to weaker echoes.

Artifacts Artifacts are those lines or dots (or even absence of such) which appear on the display in a position that is inaccurate or non-existent. Artifacts are relatively non-diagnostic in nature and often impede or prevent diagnosis. In some instances, however, artifacts are an important aid to sonographic diagnosis. You will recall that certain assumptions are made by the scanner concerning gain controls. First, the computer assumes that the speed traveled by the beam is 1540 M/sec (regardless of the varying tissues traversed). Second, the beam attenuation is supposed to be 1dB/MHz/cm. If the actual tissue traversed possesses properties much different that these assumptions, artifacts may occur. Refraction This artifact, you will recall, is beam bending which takes place when the incident beam is not exactly perpendicular to the object of interest. Refraction commonly occurs at bone/soft tissue interfaces. Beam deviance is misread by the transducer (receiver) and may produce images of structures placed in inappropriate positions on the monitor. This may hamper accurate measurement and depth assessment in some instances. Keeping the incident beam perpendicular to the target object helps to prevent this type of artifact. Reverberation Another common artifact is reverberation. In this case, echoes appear on the screen that do not represent any structure in the body. This happens when the beam encounters an interface of acoustic impedance difference (such as the skin/bladder interface). Reverberations are easily identified in that they have the appearance of equally spaced, bright rings which decrease in brightness as the distance from the transducer face increases.


Reverberations appear in the near field as a bright “bang” with rings descending down the image and are often seen within the full bladder. Acoustic Shadowing Shadowing occurs when the entire beam is directed back toward its source by a bright reflector. This leaves an area of the image with complete absence of echoes posterior to that reflector. This area is generally crisply delineated. Shadowing is used to diagnostically differentiate between bone and soft tissue, and may also indicate the presence of air within an abscess (though shadows from air are not quite as sharp as those produced by bone or calcium containing stones). Enhancement/Through Transmission Often, the ultrasound beam enters a tissue or field such as a fluid-filled cyst which has a speed value greater than the assumed value of 1540 M/sec. Enhancement, or through transmission, becomes apparent in these regions. Here, the echoes posterior to the “fast” region appear closer to the source (since the echoes return more quickly) and are displayed much brighter than the adjacent structures. Enhancement is a key element in the diagnosis of a true cyst and may be used to evaluate pathologic characteristics of other masses. Image Quality Basically, there are three main parameters to consider when speaking about the quality of an ultrasonic image. These are detail resolution, contrast resolution and image uniformity. Detail Resolution Detail resolution refers to our ability to visualize and distinguish small structures in a clear and precise manner. Lateral resolution is an aspect of detail resolution. It is important, however, not only to be able to separate close objects, but one wants to be able to identify them as well. For example, the sonographer should be able to distinguish the medullary pyramids of the fetal kidney from any present hydronephrosis. Contrast Resolution In order to differentiate several similar types of tissues, and visualize subtler structural differences, a scanner must possess good contrast resolution. The device should be capable of picking out subtle differences in the presence of bright reflectors as well. This will be important in differentiating soft tissue structures in the presence of bones. Image Uniformity If the detail and contrast resolution are to be of any diagnostic use to the interpreter, we must have complete image uniformity. This parameter measures the devices ability to maintain contrast and detail resolution throughout the entire field of view. We want to see the anterior side of the fetus as well as we see the posterior. The aforementioned parameters can be measured using various phantoms and beam plotting devices which are employed by the various manufacturing companies to assure performance quality. While these particular parameters are inherent in the device (and not changeable), it is important to note that the ratio of the patient’s weight to height is the single most important image quality determinate. This is why it is difficult to evaluate fetal anatomy in obese patients.


BASIC KNOBOLOGY Knobology is a term used to describe the use of the various controls (knobs) on a scanner in an attempt to perfect diagnostic quality of the ultrasound image. A complete user’s manual for each scanning device in the department is available to residents rotating through the lab. These manuals contain definitions and usage advice for certain controls and provide an excellent, quick way to orient yourself to the specific machine you will be using.

Gain controls

TGC/DGC controls and Overall System gains controls are generally found together on the ultrasound control panel. Remember that these controls help the user to compensate for attenuation and can enhance less reflective structures.

Depth or Field of View

The depth and field of view controls allow the user to incorporate a larger or smaller area of the patient’s anatomy into a given image size. As a basic rule, you should scan only to a depth just below the most posterior object of interest, and use a field of view just large enough to display all important aspects of your target. (Altering the field of view changes your transmitter to receiver channel ratio and may affect resolution).

Power Settings

The power coming out of the transducer should be at the lowest possible setting which allows adequate penetration of the patient with visualization of structures of interest. Check the display information on the screen to see the power setting. The scanning devices all boot up with acceptable power levels and should not need to be changed (except with extremely obese patients). Always compensate with gains before changing the power setting.

Image Processing

Once the echo information is stored within the computer, it may be altered to suit the user’s needs through manipulation of the image processing controls.

Pre- and Post -processing, which were described earlier, affect the assignment of binary numbers and grey shades to the returning echoes. For gynecological and obstetrical sonograms, we want to maintain a smooth image, having a medium contrast level, with pre and post-processing curves which allow even distribution of numbers and grey shades among all tissues displayed. During special procedures such as fetal echocardiography, however, it is to our advantage to be able to alter the processing curves in such a manner as to selectively display the bright echoes from the cardiac walls as clearly different from the blood-filled cavities. Pre-processing is generally a real-time function, while post-processing is performed after the image is frozen.

Grey Scale

Ultrasound intensity is related to brightness displayed on the screen. This can be changed to suit the


user. It is the amount of “color” in the image. The image will be either very grey and subtle, or very black and white.

Persistence

Persistence is the amount of smoothing done to the image to make it aesthetically pleasing. Lower levels of persistence make it possible to visualize motion of very minute structures and may be employed during real-time monitoring of an active fetus, or for cardiac examination. Persistence enhances tissue differentiation to allow visualization of subtle tissue movement and changes.

Edge Enhancement

Edge enhancement provides enhancement of tissue and vessel boundaries. You would use this to clearly mark or target an object for needle guidance or mass measurement.

Compression

Another type of contrast, image enhancer. Range of grey shades assigned to different levels of echo intensity.


Many scanners have a special ‘memory’ for technique settings. This will allow the user to program a combination of settings most desirable to him/herself. By entering all of these settings directly into the scanning device the user can easily change all settings at once to her preference. Check the image display area for current settings and manipulate them individually to determine your preferences. Remember your personalized settings for future scans.

Measurement

Calipers for measurement are standard equipment on modern ultrasound scanners. Joy sticks or roller balls are used to move the calipers about a structure which is to be measured. The caliper cross hairs should be placed exactly on the edge of the structure for greatest accuracy. Calipers can also be used to measure the distance from the transducer to a target during amniocentesis or PUBS. This ensures accurate needle placement.

Magnification

Modern ultrasound devices possess the ability to “blow up” the entire image, or a preselected portion of it for closer examination. Magnification may allow you to visualize a small structure, but it generally affects resolution negatively. A magnification control will blow up the entire field of view.

Zoom controls will allow you to blow up a specific area of the image. You will be able to select the area you wish to enlarge, and may scan in this enlarged format. You should not use zoom when doing amniocentesis, because you will lose the outer borders of the image and will not be able to see the angle of the shaft of the needle until it is well into the uterus.

Expand options allow the user to magnify the image from the top down (therefore, you maintain visualization of the entire transducer/skin interface). This is the only enlargement option suitable for needle guidance.

Patient ID and Annotation

Every ultrasound scanner allows the user to input and display some form of patient and hospital identification information. Entering the patient information is important for legal documentation and identification of films and records. By entering the NEW PATIENT mode, you can simply type all important information at the specified line using the alphanumeric keyboard. Annotation and labeling of the image is helpful as a teaching tool and to describe scanning planes used. Comments and notations can also be entered using the alphanumeric keyboard when in the text/comment/annotate mode.

Image Display Information

All settings defaulted in the system and those chosen and/or changed by you will be represented in the image display information area on the monitor. You should look at this area to see what settings and presets are being employed and to verify power and transducer selections.

Recording the Image

The user will need to produce some form of study documentation for later review and patient records. There are several ways to do this:


Most scanners can now convert an image to a digital format (DICOM). Once in DICOM format, the image can be stored or sent electronically. A PAC (photo archive/capture) system is then used to display the images on to a monitor. A PACs system can permanently store the images to digital linear tape (DLT), optical disks, CDs or DVDs.

A simple device which can produce hard copies is a thermographic paper printer (Sony, Mitsubishi, etc.).

Real-time imaging may be recorded by integrating a video recorder into the monitor. Using this, an examination can be viewed by interested parties without actually having to be in the room. This is useful as a teaching tool and for outside physician consultation.


BASIC SCANNING It is much easier to perform an OB/GYN ultrasound examination and/or localization if basic preparatory measures are taken to assure accuracy and safety. Be sure that both the patient and the ultrasound scanner are ready for examination before you start to manipulate the scanner.

Patient Preparation and Positioning

Patients having any type of pelvic or obstetrical ultrasound should be instructed to drink 32 oz. of clear liquids (water is preferable) at least one half hour prior to their appointment. A full bladder is needed to maximize visualization of pelvic structures. Patients should be instructed not to use the bathroom after drinking the fluids.

Why a Full Bladder?

1) pushes the uterus up and extends it for better visualization of lower uterine

segment (placental position in the gravid uterus).

2) pushes bowel out of the way (remember, ultrasound cannot penetrate air).

3) provides an acoustic “window” by placing fluid (rapid sound transmission) in front of the structures we want to see.

4) provides a sonographic reference (comparison) for other fluid-filled

structures (vessels, cysts).

The patient must remove clothing from the area to be examined. This can generally be accomplished by having the patient pull her pants/skirt and undergarments to her thighs. Be sure that clothing is well below symphysis or it will impede scanning. (This is also more uncomfortable for a patient with a full bladder). A towel should be used to cover the patient from the symphysis down.

The patient should be positioned supine (or slightly toward her left side in advanced stages of pregnancy).

An acoustic couplant such as scanning gel must be placed over the area of interest. Gel couplant assures maximum transmission of the ultrasonic beam from the transducer into the patient by eliminating air interfaces. Be sure to use an adequate amount of warmed gel.

Application: Be sure to inform patient about the importance of a full bladder

when you schedule her ultrasound. Diagnostic mistakes are made the patients are not properly filled, and much time is wasted in the lab every day waiting for patients to fill up.

General Rule: Inform the patient NOT to drink soda as her liquid. This fills the pelvis with gas and makes the ultrasound more difficult.

Schedule pelvic ultrasound studies BEFORE barium contrast studies. Ultrasound cannot penetrate an abdomen filled with barium (and air used in contrast studies).


Preparing the ultrasound scanner The ultrasound scanner should be turned on and allowed to warm up for at least 5 minutes before scanning commences. Any image recording devices which will be used should also be turned on at this time. Transducers should be cleansed and placed within easy reach before starting the exam. Power settings and techniques should be checked to insure adequate imaging. Transducer selection Depending upon the patient’s body size, weight and habitus, the proper transducer must be selected. A 3.5 MHz or 5.0 MHz transducer with a variable focus is adequate in most cases. After selecting a transducer, perform a quick initial scan to evaluate image orientation, depth and penetration capabilities, and processing needs. Image polarity The image should be presented as a series of white dots placed on a black background. Polarity can be reversed (black image on white background), but the standard is white on black. Black areas on the screen are hypoechoic regions such as fluid; white regions are hyperechoic structures such as bone. Beam direction While the beam only enters the body from the skin surface and travels toward the inside, the display may be controlled to present the apex (point of entrance) at the top or bottom of the screen (often employed in endocavitary sonography). When performing obstetrical and gynecologic ultrasound examinations, the apex should be left in the standard “up” position. Scanning Planes There are three major scanning planes referred to in sonographic terminology: Longitudinal (or sagittal), transverse, and coronal. In actual practice, however, ultrasound is unique in that any desired plane of examination is attainable through manipulation of the transducer. Therefore, the scanning places described below are used as constant references.

Longitudinal plane

The longitudinal or sagittal scanning plane is that which runs from the head to the foot and would divide the patient into right and left halves. The uterus and cervix, aortic and IVC are examples of structures you will see in a truly longitudinal plane when scanning at midline.

Transverse plane

Transverse scanning is useful to image body structures which lie transversely within the body such as the pancreas or a fetus in a transverse lie. The transverse plane divides the body into upper and lower portions and extends from side to side.


Coronal plane

The coronal plane divides the patient into anterior and posterior halves. This plane is employed during evaluation of the adult kidneys or other laterally placed structures within the body.

The above-mentioned terms are important to understand, but when you are actually scanning it is most important that the scanning plane you choose, or variation of it, allows the beam to reach the target at a perpendicular incidence. This means that your employed plane will continually change throughout the exam, depending upon the position of the target organ. (This is further complicated by the fact that your target may be a moving one!) Image Orientation The sonographic image must be properly oriented in order to obtain correct anatomic relationship information. You must determine if you are correctly oriented before starting to scan. When scanning the maternal abdomen in a longitudinal plane: ·the apex of the image is the patient’s anterior i.e. skin surface·the bottom of the image is the patient’s posterior maternal spine·the left of the image is superior uterine fundus ·the right of the image is inferior maternal bladder In other words, if you are holding the transducer in the longitudinal plane and the bladder appears on the left side of the screen YOU ARE BACKWARDS! When scanning in the transverse plane (similar to a CT scan): ·the apex of the image is the patient’s anterior i.e. skin surface·the bottom of the image is the patient’s posterior maternal spine·the left of the image is the patient’s right liver, right ovary·the right of the image is the patient’s left spleen, left ovaryIf you are scanning in the transverse plane, and you angle the transducer toward the patient’s right hand side, information pertaining to the right should appear on the left side of the screen. A simple test to determine your scanning orientation (to figure out if you are backwards before you start the exam!!):

1. hold the transducer so that the long axis is parallel to the patient’s long axis (longitudinal scan plane), your thumb towards patient’s head. IT DOES NOT MATTER WHERE THE DOT IS!!!

2. place the transducer on the patient’s midline, just above the symphysis pubis.

3. angle the transducer down towards the patient’s bladder while watching the image.

4. the bladder (a caudal structure) should appear on the right side of the screen.

If the bladder appears on the left, you are backwards. Rotate the transducer 180 degrees or hit the


“right/left” switch on the scanner before beginning to scan again.

Once correct longitudinal orientation is achieved, turn the transducer 90 degrees so that your thumb is toward you for correct transverse orientation. These planes are used in GYN scanning when talking about maternal anatomy with the pregnant patient. When scanning the fetus, the orientations apply to the long axis of the fetus (i.e. long axis of fetus may be transverse axis of maternal uterus).

Images are read with the same assumed orientation as that used in scanning. Scan orientation is therefore extremely important. If you are scanning backwards a right ovarian mass may be misread as a left ovarian mass, or you could create a fetus with situs inversus!!

TEST to Assess Image Quality

You want your examination to offer the most diagnostic information possible. Therefore, you should adjust and set all image parameters before starting scan. After choosing a transducer and any preset technique settings, and after your have confirmed that you are scanning in the right direction (correct image orientation), you are ready to perform a quick test to assess the image quality.

·take a quick image of something in your field of view. FREEZE this image.

·look carefully at the displayed image.Are there:

too many echoes in the near field (top of image)? too many echoes in the far field (bottom of image)? too few echoes in near field? too few echoes in far field?

·using TGC (time gain compensation) gain pods or dials, correct areas noted above as inappropriate.

·once entire image, top to bottom, is uniform then assess the overall image quality:

too few echoes overall (image very dark)? too many echoes overall (image very white)?

·using the OVERALL gain control (pod or dial), correct image gain

BIOEFFECTS Any person using an ultrasound device for diagnostic purposes must investigate the hazards of using such a device. Risk information comes from experimentation, epidemiology and instrument output data. Benefit information is learned through the experience of prudent use of ultrasound. Together these risks and benefits must be weighed to create a policy for the use of ultrasound in medicine. While knowledge of ultrasound bioeffects is incomplete, most data support the idea that ultrasound used at diagnostic power levels is completely harmless. The American Institute of Ultrasound in Medicine (AIUM) reports as of 1985, “no confirmed adverse biologic effects on patients resulting from [diagnostic ultrasound] usage have ever been reported.” At much higher power levels, heat formation and cavitation have been demonstrated.


Prudent application of diagnostic ultrasound should follow the “ALARA” (As Low As Reasonably Achievable) principle for power settings. This is similar to the principles of safety used by radiographers. Statement on Mammalian in Vivo Ultrasonic Bioeffects Reaffirmed October 1992.

In the low megahertz frequency range there have been no independently confirmed significant biological effects in mammalian tissues exposed to intensities* below 100 MW/cm2. Furthermore, for ultrasonic exposure times** less than 500 seconds and greater than 1 second, such effects have not been demonstrated even at higher intensities, when the product of intensity* and exposure time** is less than 50 joules/cm2.

*Spatial Peak Temporal Average as measured in a free field of water. **Total time; this includes off-time as well as on-time for a repeated pulse regime.

Statement on Clinical Safety: No confirmed adverse effects on patients or instrument operators caused by exposure at intensities and exposure conditions typical of present diagnostic instrument and examination practices have ever been reported. Experience from normal diagnostic practice may or may not be relevant to extended exposure times and altered exposure conditions. At this time no hazard has been identified that would preclude the use of diagnostic ultrasound in education and research. Diagnostic ultrasound has been in use for over 25 years. Given its known benefits and recognized efficacy for medical diagnosis, including its use during human pregnancy, the American Institute of Ultrasound in Medicine herein addresses the clinical safety of such use: No confirmed biologic effects on patients or instrument operators caused by exposures at intensities typical of present diagnostic instruments have ever been reported. Although the possibility exists that such biological effects may be identified in the future, current data indicate that the benefits to patients of the prudent use of diagnostic ultrasound outweigh the risks, if any, to the patient. PATIENT/USER SAFETY DO NOT drop or mishandle the transducers. Transducers are very expensive to replace. Cracked housings are an electrical threat to the user. Report any damage to a transducer immediately. Freeze the image when you are not actively scanning. Use power settings which are “as low as reasonably achievable” (ALARA) yet still provide diagnostic information. Clean transducers with alcohol before and after each examination. Perform quick reference scan before beginning the examination to assure proper image orientation and transducer selection. ROUTINE MAINTENANCE OF ULTRASOUND EQUIPMENT Proper care and maintenance of ultrasound equipment ensures safety and prolongs equipment life. It is simple to perform a few easy steps in order to keep scanning quality at a maximum.

·Clean and store ultrasound transducers when not in use.

·Clean probe with alcohol or Cidex after each patient.


·Clean the unit with a damp cloth at the end of the day.

·Do not hang or drape transducers around the device. Place transducers upright in specially designed transducer holders.

·Do not put straight bleach directly on transducer head.

·Do not allow transducer to soak in Cidex for more than one hour (Cidex or bleach will destroy the acoustic membrane on the transducer face).

·Never gas or autoclave a transducer, these processes will destroy the transducer crystals.

·Use acoustic coupling gel specified by the manufacturer of the scanner. Do not use mineral oil.

·Clean or vacuum air filters on back of scanner once a month.

ULTRASOUND VS. X-RAY Ultrasound examination and localization is favorable as compared with x-ray in that:

1. Ultrasound is non-invasive and non-ionizing and can be safely used in pregnancy

2. There is no need for oral or injected contrast media

3. Ultrasound is less expensive.

4. Almost any anatomic structure can be visualized from a variety of different reference points.

5. Boundaries of organs and masses are clearly displayed permitting accurate measurement.

6. Tissue characteristics can be assessed.

Introduction to Basic Ultrasound

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