17-11-21 - Basics of Ultrasound Flashcards
Learning outcomes
- Understand the physics of ultrasound.
- Understand the basics of an ultrasound scanner including the different types of transducers.
- Understand the different modes of scanning used in clinical practice.
What are hertz?
What are they defined as?
What is diagnostic sonography?
What frequency do diagnostic sonography use?
- The hertz (Hz) is the unit of frequency in the international system of units (SI)
- Hertz is defined as the number of cycles per second
- Diagnostic sonography is the use of medical ultrasound for medical diagnosis
- Diagnostic sonography generally uses frequencies of 1-20MHz (x10^6)
How do diagnostic ultrasounds work?
What can attenuation limit?
- Ultrasound uses short high frequency sound pulses that are transmitted into the body
- These sound pulses can be reflected, scattered, refracted, or absorbed
- The combined effects of reflection, refraction and absorption result in attenuation (reduction of amplitude/force) in the intensity of the sound pulse as it travels through the mater
- Attenuation limits depth of imagine and is greater at higher transmit frequencies
What are 3 advantages of ultrasound?
What are 3 disadvantages of ultrasound?
• Advantages of ultrasound:
1) Ultrasound is safe
2) No ionizing radiation
3) The equipment required is portable, compact, and relatively inexpensive
• Disadvantages of ultrasound
1) Highly operator dependent
2) Structures surrounded by bone, such as the brain and spinal cord, do not give clinically useful images
3) The attenuation of the ultrasound signal at the air/tissue boundaries means that the technique is not suitable for imaging structures in the lung or abdominal organs obscured by gas in the overlying bowel
What is the role of ultrasound transducers/probes?
What are the 3 different types of transducers?
What frequencies do they function at?
- Ultrasound transducers produce and detect ultrasound
- Ultrasound transducers are capable of sending an ultrasound, detecting the sound, and converting it into an electrical signal to be diagnosed
• 3 different types of transducers/probes:
1) Linear array probe – 7 - 11MHz
2) Curved array probe – 2 - 5 MHz
3) Phased array probe – 1 – 5MHz
What are piezoelectric crystals?
What are they used for in diagnostic sonography?
How does this work?
How do higher amplitude echoes affect these crystals?
- Piezoelectric crystals are ceramic crystals that deform and vibrate when they are electronically stimulated
- They are used in diagnostic sonography to generate an electric pulse that is processed into an image
- Echoes that return to the transducer distort the crystal elements and generate an electric pulse that is processed into an image
- High-amplitude echoes produce greater crystal deformation and generate a larger electronic voltage
- These displayed on the image as brighter pixels than low-amplitude echoes
- Because of this, standard 2D grey-scale images are often referred to as B-mode (brightness mode) Images
What are images obtained with linear ray transducers like?
What is high frequency sound good and not good for?
- Images obtained with linear array transducers always have a flat superficial image and are designated with the letter L followed by the transmit frequency
- An example of this is HFL38/13-6 which indicates a high broadband frequency (13-6MHz) Linear transducer with a 38mm footprint
- High frequency sounds do not penetrate deep into tissues, so high-frequency probes are only useful for superficial structures
What are curved-array transducers?
What can curved array transducers with a short radius of curvature be used for?
What do images obtained with curved array transducers look like?
- Curved array transducers are made when the surface of a linear array is reformed into a curved convex shape
- Curved array transduces with a short radius of curvature can be used for endoluminal scanning
- Probes with a long radius of curvature can be used for general abdomen and obstetrical scanning
- Images obtained with curved array transducers always have a curved superficial surface and are designated with the letter C on the image, followed by the transmit frequency
How does the phased-array transducer work?
What type of image format is produced?
Where is it able to scan?
- With the phased array transducer, every element in the array participates in the formation of each transmitted pulse
- Because the sound beams are steered at varying angles from one side of the transducer to the other, a sector image format is produced
- The phased-array probe is smaller, and therefor is capable of scanning in areas where acoustic access is limited, such as between ribs
What are intraluminal probes?
Why can higher resolution images be obtained using these?
How are image-degrading properties minimized with this probe?
How does image quality of intraluminal probes compare with probes used in the standard transabdominal approach?
How have intraluminal probes been further modified?
- Intraluminal probes are small transducers that can be placed within various body lumens
- Because these transducers can be positioned close to the organ of interest, high frequencies can be used and higher resolution images obtained
- The ability to image organs without having to transmit the sound beam through the abdominal wall helps to minimize image-degrading properties of adipose tissue and the shadowing produced by bowel gas
- The images using an intraluminal probe are of much higher quality than images obtained with probes through the standard transabdominal approach
- Very small transducers have been added to flexible endoscopes and bronchoscopes to scan and guide biopsies in both the gastrointestinal tract and thorax.
- Intravascular probes that fit on the end of catheters are also widely used in vascular applications
What is A-mode imaging (amplitude mode)?
How is it represented?
- A-mode imagine (Amplitude mode) is a method of supplying echoes acquired in 1 dimension
- Depth is represented along 1 axis, and an echo amplitude is displayed along a perpendicular axis
What is B-mode (Brightness/ 2D mode)?
How does it work?
- Brightness, or B-mode, is the most commonly used ultrasound mode
- In this mode, the spike is converted to a dot, and the brightness of the dot represents the amplitude of the returning signal
- The position of the dot on the display represents the depth from which the signal is returning and depends on the round-trip time of the US signal
- Multiple scan lines across a plane are combined to produce a single two-dimensional (2D) image.
- A series of frames are then displayed in rapid succession to give the impression of constant motion, the quality of which depends on the number of images displayed per second, (the frame rate)
What is echogenicity?
What are the 4 different types?
What area do various structures fall into?
- Echogenicity is the ability to bounce an echo e.g return the signal in ultrasound examinations
- Anechoic – free from echo
- Isoechoic – producing ultrasound echoes equal to those of neighbouring or of normal tissues
- Fluid is anechoic (black – free from echo). This includes blood, urine, cysts.
- Pus in an abscess is usually anechoic (black) or hypoechoic (dark grey).
- Sometimes abscesses may be the same shade of grey as the surrounding skin tissue (isoechoic) or brighter than surrounding tissue (hyperechoic).
- Bone, gall stones and metallic objects are white (hyperechoic) since they bounce the sound back very well, leaving a dark shadow behind them.
- Solid organs such as the liver, kidney and spleen will appear grey with black inside where the vessels/urine/bile tracks
What are the 3 different types of Doppler Ultrasound?
What do they each do?
• Different types of Doppler Ultrasound:
1) Colour doppler
• Measures and colour codes the direction and magnitude of the mean Doppler frequency shifts that occur in moving red blood cells and superimposes a colour depiction of these data on the grey-scale image
2) Power colour doppler
• Depicts the amplitude, or power, of the Doppler signals.
• This allows better sensitivity for visualization of small vessels, but at the expense of directional information.
3) Pulsed Doppler
• Allows a sampling volume (or gate) to be positioned in a vessel visualized on the gray-scale image and displays a spectrum of the full range of blood velocities within the gate plotted as a function of time
What does BART ultrasound show?
• BART ultrasound shows the movement of blood in colours, with blue being away from the probe and red being towards the probe
What is m-mode (motion mode) designed for?
How does it work?
What is this mode important for the study and documentation of?
- Motion mode is designed to document and analyse tissue motion
- Using the S2 image is a guide, a particular scan line is selected to correspond to the moving structure of interest
- This mode is particularly important in studying cardiac valve and wall motion, and in documenting mental foetal heart rate activity
How does 3D ultrasound imaging work?
What is the biggest clinical application of 3D ultrasound imaging?
- Data for 3D sonography are acquired as a stack of parallel cross sections with the use of a 2D scanner
- The biggest clinical application of 3D ultrasound is in the evaluation of gynaecologic and foetal anatomy
What does the gain control adjust on Ultrasound machines?
What effect does too little and too much gain have on the image?
- The gain control adjusts the amplification of the returning acoustic signals and is used to optimise the US image
- Reduce gain produces a dark image and detail is masked
- Too much gain produces a white image and detail is saturated?
What is the direction of the orientation marker on the probe when performing a longitudinal and transverse scan?
- The orientation marker on the transduce is directed cephalad (towards the head) when performing a longitudinal scan
- It is directed towards the right side of the patient when performing a transverse scan
How does acoustic enhancement work?
What are examples of where it occurs?
- Some structures allow sound to pass through them more easily than others.
- The most dramatic example is watery fluid, such as in an effusion or in a cyst.
- Because only a minimal amount of energy is absorbed by the fluid, the region that lies behind will receive more sound than the processor expects for that depth.
- This area will therefore appear uniformly brighter.
- This effect is called Enhancement.
- A good example is the effect of the bladder, acting as a window to deeper tissues.
What are the 4 manipulation manoeuvres of the US probe?
How should the probe be orientated to the structure of interest?
• The probe should usually be at 90 degrees to the structure of interest, as this provides the maximum return echo
What is anisotropy?
Why may this be problematic?
What is an example of this?
What is the anisotropic effect dependent upon?
- Anisotropy is an artefact encountered in ultrasound, notable in muscles and tendons during a musculoskeletal ultrasound
- In MS applications, the artefact may prompt an incorrect diagnosis
- E.g if the artefact causes a normal tendon to appear hypoechoic (more solid than usual), it may falsely lead to a diagnosis of tendinosis or tear
- The anisotropic effect is dependent on the angle of the ultrasound beam
