Chapter 2 – Physics and Instrumentation




Abstract




The ultrasound machine console varies between companies in the layout of the knobs. However, the overall panels are very similar, labeled and become familiar with use.





Chapter 2 Physics and Instrumentation




Ultrasound Machine


The ultrasound machine console varies between companies in the layout of the knobs. However, the overall panels are very similar, labelled and become familiar with use.





Figure 2.1 Position of the bed, chair and machine console are adjusted for individual sonographer comfort.





Figure 2.2 Machine console.



Basic Scanning Protocol


Ultrasound machines vary in the actual layout of the console. The settings most often used for greyscale imaging are




  1. 1. New Patient: will remove all previous data acquired in a previous examination



  2. 2. Depth: adjusted as required to optimise the area being examined



  3. 3. Focus: positioned at or just posterior to the area of interest



  4. 4. Gain: adjusted to balance the overall greyscale of the image



  5. 5. Time gain compensation (TGC): adjusts the level of greyscale to be uniform from superficial to deep



  6. 6. Freeze: to capture the best image on the screen



  7. 7. Callipers: to measure the thickness of the endometrium and follicles



  8. 8. Print: to store the image


The machine is cleaned and prepared using the On/Off button to activate and New patient to enter data and ensure previous patient data are deleted. Select the Transducer and the Study (gynaecology). Factory settings for the study will be activated, which include depth, gain, TGC, focal depth, annotation, and measurement settings. These settings can be altered according to the appearance of the image on the screen. The Freeze button is activated so the image will appear in real time on the screen.


Transducers designed for transvaginal scanning provide good tissue definition. Allow sufficient depth in the field of view to locate the structures and then adjust the depth to optimise the required image. Figure 2.3





Figure 2.3 Transvaginal probe.



Physics of Ultrasound


The physics of ultrasound is important; however, a comprehensive discussion will not be covered in this text. For the practitioner, selecting the appropriate transducer and understanding the console to select the appropriate preset and controls to optimise image resolution are required. Developing the skill in spatial relationships, to manipulate the probe to provide the best acoustic window and to assess anatomy in greyscale imaging, comes with practice.


The ability to recognise artifacts and the limitations of ultrasound develops over time. Sonography is a new skill and requires supervision to start and independent competence with practice.


The following is a brief outline of how the image is produced, using high-frequency sound waves passing through tissue, with the image produced by the reflected echoes, from various interfaces, of the different tissues.


The ultrasound machine is composed of a computer hard drive, transducer, console, keyboard, monitor and printer or storage system. The software in modern machines is preprogrammed to suit each examination; however, adjustments can be made to maximise the image quality. The clarity of the image is essential for the accurate placement of callipers for measurements. New technology using silicon chips is advancing image quality.


Transducers are designed to optimise imaging of specific areas. Within the transducer is an array of piezoelectric crystal elements. The diagnostic ultrasound frequency range is 2–15 million Hertz or 2–15 MHz. The pulse length is frequency dependent. A higher frequency transducer will provide better resolution; however, it the beam will not penetrate as far as with lower frequency transducers. Abdominal transducers are 2–5 MHz, transvaginal transducers are 5–10 MHz, while transducers designed for superficial structures are in the 7–15 MHz range approximately.


The pulse-echo principle describes how the image is generated, as follows: (1) The electrical pulse strikes the crystal, which vibrates. (2) The produced sound beam propagates through tissue. (3) Echoes arising from tissue interfaces are reflected back to the crystal. (4) The crystal vibrates, generating an electrical impulse.


Each crystal element within the transducer sends out a pulse and receives echoes along the same line of sight, and the depth of each echo is related to the time from pulse transmitted to echo received. The image is composed of multiple lines from the array of crystals within the transducer.


As the pulse passes through the tissues of the body, echoes are generated by the many interfaces and scatterers. Echoes are reflected from interfaces of tissues with different acoustic impedance. Some energy is reflected back to the transducer, while the remainder is transmitted into the second medium. Interfaces with very large acoustic impedance difference, that is, soft tissue and air or soft tissue and bone, create a barrier and all of the energy is reflected and none is transmitted into the second tissue. The speed of sound in human tissues and liquids is determined by their density and stiffness, average 1540 m/s.


Resolution is the ability to distinguish echoes in terms of space, time and strength, and good resolution is critical to the production of high-quality images. Resolution is determined by the frequency of the transducer in the axial plane and the beam width in the lateral plane. Beam width artifact is caused by divergence of the beam in the far field deep to the focal zone.


Acoustic windows are areas of the patient through which the ultrasound beam can pass to visualise deeper structures. When the organ is located the probe is slowly moved to study the structures and produce the best image. Examine organs in longitudinal, transverse and oblique planes to assess size, shape, margins and echogenicity.


Greyscale imaging in ultrasound is a cross-sectional image of a slice of the body, with a limited depth. The range is from white through shades of grey to black. Strong reflectors are white, echogenic. Fluid, that is, urine, bile, blood or fluid within a cyst, is black, anechoic. Soft tissue is demonstrated in shades of grey. The sound wave will be totally reflected at a tissue/air and tissue/bone interface.


The Gain button on the console can be adjusted to make the image darker or brighter. The TGC sliders will adjust the greyscale of the image in the near field to the far field. Even shades of grey should be seen superficially and at depth.


Echogenicity is a term used in ultrasound to describe the shades of grey. Lighter shades are more echogenic (hyperechoic) and darker ones less echogenic (hypoechoic), while black is anechoic. Complex mass refers to a structure having both solid and cystic components and is described as predominantly solid or cystic.





Figure 2.4 Left ovary located between the uterus and the internal iliac vein. Greyscale imaging defines the outline of the ovary and the cluster of arcuate vessels of the uterus.





Figure 2.5 Greyscale imaging demonstrating the white outline (hyperechoic) of the uterus and the endometrium.





Figure 2.6 The full bladder is pushing the uterus away from the optimal plane of 90 degrees, making the endometrium less well defined.





Figure 2.7 Greyscale imaging shows the ovary, with anechoic follicles, located in the pouch of Douglas posterior to the cervix. The margin between the cervix and ovary is the hyperechoic line.





Figure 2.8 Gain setting needed to be increased to visualise the membrane between the two small follicles.





Figure 2.9 Overall gain setting is too high, and the image brighter white than required, diminishing the contrast between the ovary and the bowel.





Figure 2.10 Unstimulated ovary is hypoechoic compared to the surrounding structures. A small follicle is measured.





Figure 2.11 Normal bowel pattern seen deep to the uterus. Bowel patterns vary due to content; however, peristalsis will be seen with real-time imaging.





Figure 2.12 Bowel wall muscle is hypoechoic. Mucosal layer is more echogenic. Content of the bowel will create a variety of appearances. Peristalsis can be observed in real-time imaging.





Figure 2.13 Haemorrhagic cyst with hypoechoic low-level echoes, and two anechoic follicles, within the right ovary.



Acoustic Artifacts


There are multiple ways that artifacts are produced in images using ultrasound. This brief discussion will cover enhancement, shadowing, comet tail, ringdown, beam width and side lobe artifacts, commonly seen with transvaginal scanning.



Enhancement


Enhancement is seen behind a non-attenuating structure. There is no attenuation through a fluid-filled cyst and the area directly deep to the cyst will appear whiter (enhanced) than the adjacent tissue (Figure 2.14).


Characteristics of a simple cyst are




  • Rounded thin wall structure



  • Fluid filled (black)



  • No internal echoes



  • Acoustic enhancement posterior (Gill, 2012, pp. 53–69)





Figure 2.14 Acoustic enhancement deep to a fluid-filled cyst.


With the increase in the fluid content of the blood vessels and glands, in the endometrial lining, posterior enhancement may be seen in the myometrium.





Figure 2.15 Transverse view of the uterus showing acoustic enhancement in the myometrium, deep to the less attenuating endometrium.





Figure 2.16 The acoustic enhancement is blended with the hyperechoic echoes behind or deep to the ovary.



Shadowing


Multiple artifacts are produced by the sound beam passing through tissue. An acoustic shadow is produced behind a strong reflecting interface or strong attenuating tissue. The interface between soft tissue and air, bone or calcified structures will produce shadowing, posteriorly.





Figure 2.17 Acoustic shadowing deep to calcification of a uterine fibroid.


Feb 23, 2021 | Posted by in GYNECOLOGY | Comments Off on Chapter 2 – Physics and Instrumentation
Premium Wordpress Themes by UFO Themes