Pat F. Fulgham1
(1)
Department of Urology, Texas Health Presbyterian Hospital of Dallas, 8210 Walnut Hill Suite 014, Dallas, TX 75231, USA
Pat F. Fulgham
Email: pfulgham@airmail.net
Ultrasound machine settings may be adjusted in order to obtain a good quality image. These settings include but are not limited to gain, time-gain compensation, frequency, focal zones, depth/size, field of view and cine function. A good quality ultrasound image will have sufficient and uniform brightness. It will be sharp and the focal zones will be set at the area of interest. The area of interest will be of adequate size and will be oriented properly and labeled for documentation purposes (Fig. 2.1).
Fig. 2.1
This image displays the characteristics of a good quality image by virtue of technical settings of user-controlled variables as well as proper labeling
Transducer Selection
The first step in performing ultrasound is to select the transducer with the optimal shape. A linear-array transducer produces a rectangular image and is generally used for scrotal ultrasound (Fig. 2.2). A curved-array transducer produces an image which is trapezoidal in shape and is generally used for abdominal and pelvic ultrasound (Fig. 2.2). The curved nature of the probe allows gentle pressure on the patient’s abdomen or flank resulting in contact of the entire transducer face with the skin. Endocavitary probes (transvaginal and transrectal probes) are curved array probes with trapezoidal image displays.
Fig. 2.2
(a) The linear array transducer produces a rectangular image field. (b) The curved array transducer produces a trapezoidal or pie-shaped image. The shape of the transducer affects the divergence of the wave as the wave propagates in the body
Transducers are usually multifrequency, meaning the frequency can be switched electronically over a range of frequencies (e.g. 3.5–5.0 mHz for transabdominal pelvic ultrasound). It is important to select the highest frequency which has adequate depth of penetration for the anatomic area of interest. The higher the frequency the greater the axial resolution and the better the anatomic representation of the image. However, the higher the frequency the lower the depth of penetration (Fig. 2.3).
Fig. 2.3
The selection of a transducer with the frequency of 7.5 mHz reflects the trade off between depth of penetration and good axial resolution. In this axial image of a large prostate, a lower scanning frequency may be needed to adequately visualize the anterior prostate
Scanning Environment
To produce the best quality image the sonographer should arrange the scanning environment so that access to the patient and equipment are optimized (Fig. 2.4). The table should be positioned at a height which allows the sonographer to stand or sit in a comfortable position so that the sonographer is able to stabilize the transducer against the patient’s body. This minimizes unwanted movement of the transducer and allows the sonographer to maintain the orientation and position of the transducer while adjusting machine features.
Fig. 2.4
The configuration of the equipment and proximity to the patient are critical in maximizing comfort, efficiency, and accuracy during scanning and documentation
The sonographer should be positioned so they are able to comfortably reach the physical console or touch screen in order to adjust the machine settings. Many ultrasound units provide the ability to freeze and unfreeze the transducer via a button on the transducer itself or by a footswitch. In either case the sonographer must be able to scan the patient with one hand while manipulating the console and documenting with the other hand.
The sonographer must have a clear direct view of the monitor. The angle of the monitor should be adjusted for viewing by the sonographer. The brightness settings on the monitor need to be adjusted for the conditions under which the scan is being performed. In general, dimming the room lights improves the ability to evaluate the image on the monitor.
Monitor Display
It is important when performing an ultrasound examination to understand the information that is being displayed on the monitor. Patient demographic information, type of exam and facility should be entered. The monitor will usually display information regarding which probe is active, the frequency of the probe, and the magnification of the image (Fig. 2.5). Information regarding overall gain and other settings is available on the monitor. Typically there will be a TGC (time-gain compensation) curve displayed on one side of the image as well as color bar which demonstrates the range of pixel brightness or hues available. In addition, there will be gradient markings on one side so that depth of field can be appreciated.
Fig. 2.5
Machine settings and icons displayed on the monitor help with adjusting machine settings to optimize image quality
Orientation
By convention, when scanning organs in the sagittal view, the upper pole of the organ (e.g. kidney or testis) is to the left of the screen and the lower pole is to the right of the screen (Fig. 2.6).
Fig. 2.6
In this sagittal image of the right testis, the superior pole of the testis (A) is to the left, the inferior pole of the testis (B) is to the right. The anterior aspect of the testis (C) is at the top of the image and the posterior aspect (D) at the bottom. Without the label (Sagittal testis right) there would be no way to distinguish the right from the left testis
In transverse scanning the right side of an anatomic structure is displayed on the left side of the image just as it would be when evaluating a conventional radiograph. These conventions should always be followed when documenting an ultrasound examination; however, it may be useful to also demonstrate the orientation of the probe using graphics or icons. When paired structures such as the kidneys or testes are imaged it is particularly important to designate the organ as right or left.
User-Controlled Variables
One of the most commonly required adjustments during ultrasound scanning is an adjustment to the overall gain. The gain is a control which determines the degree to which the electrical signal produced by a returning sound wave when it strikes the transducer will be amplified for display. This needs to be differentiated from acoustic output which is defined as the power or amplitude of the afferent wave which is generated by the transducer.
Both gain and acoustic power can be controlled by the operator; however, acoustic output is limited by the manufacturer in compliance with industry safety standards [1]. In general, when the gain is increased the resulting image is brighter. When there is excessive overall gain the image often appears bright and washed out. When there is insufficient overall gain the image is often dark and it is difficult to distinguish between adjacent structures (Fig. 2.7).
Fig. 2.7
In this sagittal view of the bladder and prostate, (a) demonstrates a gain setting which produces bright echoes throughout the image, accentuating slice thickness artifact seen in the anterior bladder and obscuring the detail of the tissues posterior to the bladder because of increased through-transmission. (b) Demonstrates an overall gain setting which results in good contrast, less artifact and better overall tissue detail
It is generally more desirable to increase or decrease the gain rather than manipulate the acoustic output. However, there may be circumstances (e.g. a very thin or a very heavy patient) where increases or decreases in acoustic output would be appropriate. In every case, manipulations of gain or acoustic output are made to improve image quality. The principle of ALARA (as low as reasonably achievable) should always be honored when making these adjustments, imparting as little acoustic energy into the patient as will provide an adequate image.
Time-gain compensation(TGC) is another way to control the amplification of the signal from a returning sound wave. As opposed to overall gain, the amplification of these signals can be adjusted independently by region of the scanned field. That is, the electrical signal generated by sound waves returning from a specific region inside the patient can be individually amplified using time-gain compensation controls. Time-gain compensation controls usually involve a set of sliding switches which can increase or decrease the amplification of a signal at a particular depth in the scanned field. This is often displayed graphically on the monitor as a line or a curve which corresponds to the position of the physical sliders on the console (Fig. 2.8).
Fig. 2.8
TGC (time-gain compensation): the signal from a reflected (returning) sound wave can be amplified or diminished based on the depth of the reflector within the scanned field. In most machines the TGC is a set of sliders as seen in image (a). The TGC curve appears as a line on the screen (b)
Time-gain compensation is most commonly used to amplify the signal strength from regions of the image where there is high attenuation of the sound waves or to decrease the amplification of the signal strength when there are areas where sound waves are unattenuated. One frequent use of TGC in urologic scanning is to compensate for the relative hyperechogenicity of tissue distal to a fluid-filled structure such as the bladder or a large renal cyst. It is often necessary to decrease the time-gain compensation for that region of the image distal to the fluid filled structure so that structures in that location can be accurately represented (Fig. 2.9).
Fig. 2.9
(a) In this image of the bladder note how the shape of the TGC curve (white arrows) corresponds to the pixel brightness at given regions of the scanned field. When the TGC curve is deviated to the right, the signals produced by sound waves returning from that corresponding region of the ultrasound field are amplified and displayed as “brighter” pixels. (b) The TGC curve (open arrows) is adjusted so that uniform brightness is achieved throughout the image of the bladder and the surrounding tissues
Frequency adjustments allow a multifrequency probe to be switched between two or three main frequency ranges during scanning. For instance, a curved array probe for abdominal scanning will often have the ability to adjust from 2–4 mHz to 3.5–5 mHz to 4–6 mHz. These ranges are specifically designed to take advantage of greater axial resolution with the higher frequencies and greater depth of penetration with lower frequencies. It is useful during scanning to change between frequencies to determine which frequency range provides the best overall image quality (Fig. 2.10).
Fig. 2.10
The relative relationship between frequency and depth of penetration. Notice that to image a kidney 12 cm beneath the skin, a frequency of 2–4 mHz would be required to achieve an adequate depth of penetration
The frequency determines the axial resolution of the scan. Axial resolution is the ability to identify as separate, two objects in the direction of the traveling sound wave. The higher the frequency the better the axial resolution. The pulse that is sent from a transducer usually consists of two or three wavelengths and, as such, has a physical length. This pulse must fit completely between two objects in the axial plane in order to discriminate those objects as separate (Fig. 2.11).
Fig. 2.11
(a) The shorter pulse length associated with this higher frequency wave is able to fit between the two objects in the axial plane providing good axial resolution. (b) The longer pulse length is unable to fit between the objects, thus depicting the two distinct objects as a single “blurred” echogenic focus
Therefore, a pulse using a higher frequency wave has a shorter physical length than a pulse using a lower frequency wave. The shorter the pulse length, the better the axial resolution. A 5 mHz transducer produces a pulse length sufficient to produce an axial resolution of 1 mm (Fig. 2.12).
Fig.2.12
An example of calculating the axial resolution for a 5 mHz probe
Focal zone adjustments are made in an attempt to bring the narrowest portion of the ultrasound beam into the location where maximal lateral resolution is desired. Lateral resolution is defined as the ability to discriminate as separate, two points which are equidistance from the transducer (Fig. 2.13).
Fig.2.13
Lateral resolution is optimized when beam width is narrow enough to fit between two objects equidistant from the transducer. In (a) the objects would be correctly displayed as separate objects. In (b) the beam width is too thick to fit between the objects and they would be displayed as a single “blurred” focus
Lateral resolution is a function of the width of the sound wave beam. The more focused the beam the better the lateral resolution; that is, even closely spaced objects can be differentiated. Most transducers have a focal point producing the best lateral resolution and a focal range producing adequate lateral resolution (Fig. 2.14).
Fig. 2.14
The shape of the ultrasound beam determines its lateral resolution. The narrowest portions of the beam is its focal point or focal zone. The location of the narrowest point of the beam can be adjusted by manually setting foci
The location of the narrowest portion of the ultrasound beam can be set by adjusting the focal zone. However, the thickness of the beam (known as the elevation or Azimuth) is determined by the characteristics of the transducer crystals and design. In general, the focal zone should be placed at or just distal to the area that is of maximum clinical interest (Fig. 2.15).
Fig. 2.15
The shape of the ultrasound beam is simulated in this drawing (purple). The focal zone (A) is located to produce the best lateral resolution of the medial renal cortex (white arrows). The location of the focal zone is designated by the arrowhead (B). The location of the focal zone can be adjusted by the operator
It is possible to set multiple focal zones; however, this requires the software to sequentially interpret returning sound waves from specific locations of the scanning field (Fig. 2.16).
Fig. 2.16
In this sagittal view of the bladder the focal zone is set at the level of the bladder stone
Multiple focal zones result in a slower frame refresh rate and may result in a display motion that is discontinuous. In most urologic scanning applications a slower refresh rate is not a significant liability. Multiple focal zones are most useful in urologic scanning when fine anatomic detail throughout a solid structure is desirable (notably, in testicular scanning). When it is desirable to produce and interpret a twinkle artifact during Doppler scanning it is useful to place the focus just at or distal to the object producing the twinkle artifact.
Depth/size function allows the user to select that portion of the scanned field which will be displayed on the monitor. By adjusting the depth of field it is possible to allow the structure of interest to occupy the appropriate proportion of the visual field. By limiting the area of the scanned field from which returning echo signals will need to be interpreted and displayed the amount of work performed interpreting that returning information will be diminished and frame refresh rates will be improved. The depth/size function has no effect on the axial resolution of the image. Appropriate depth of field adjustments can improve the ability to visually discriminate certain structures during urologic scanning and improves the overall performance of the equipment (Fig. 2.17).
Fig. 2.17
(a) Depth of field has been set so that the testis fills the available display space but produces a grainy image. (b) Depth of field has been increased so that the testis occupies a very small portion of the available display. Tissue posterior to the testis which is not relevant, occupies a large percentage of the display
Field of view is an adjustment to limit the width of an image so that only a portion of the available ultrasound information is interpreted. As with changes in depth of field narrowing the field of view will reduce the amount of work necessary to interpret the returning echo data and improve frame refresh rate. It also limits the visual distraction of tissues which are irrelevant to a specific exam (Fig. 2.18).
Fig. 2.18
The full ultrasound field is displayed in (a). Limiting the field of view to the kidney (b) decreases the time necessary to interpret returning echo information and improves the frame refresh rate
The cine function of most machines provides an opportunity to save a sequence of frames from the most recent scanning session and allows these frames to be played back one by one. This is a very useful feature when scanning organs such as the kidney which may be affected by respiratory motion. When a subtle finding is identified the machine can be placed in the freeze mode and then the sequential images captured in the cine memory can be scanned backwards until the most appropriate image for measurement and documentation is identified. The cine function is invaluable in clinical office urology because it significantly decreases the time necessary to perform and document a complete examination.
Conclusion
Ultrasound is ultimately an exercise in image recognition. Clinicians tend to see what they know and that with which they are familiar. Great care must be taken to ensure optimal image quality so that the unexpected and unfamiliar may also be recognized and correctly diagnosed. While ultrasound equipment has preset applications which allow scanning of most patients without the need to make individual adjustments, there are many clinical circumstances in which the ability to make individual adjustments is invaluable for making a clinical diagnosis or clarifying an artifact. Knowledge of the physical properties of ultrasound and the judicious use of basic instrumental controls such as TGC, Depth, Gain and Focus will allow the clinician to maximize the diagnostic capability of this modality of imaging in evaluating pelvic floor disorders.
Reference
1.
Guidance for industry and FDA staff information for manufacturers seeking marketing clearance of diagnostic ultrasound systems and transducers. Document issued on: 9 Sept 2008. http://www.fda.gov/downloads/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/UCM070911.pdf.