Ultrasound Flashcards
What is ultrasound?
Ultrasound is high frequency waves (above 20kHz) generated by piezoelectric crystals within the ultrasound transducer. The piezoelectric effect occurs whereby the piezoelectric crystals are placed in between 2 electrodes – this applies a voltage to the crystals, causing them to vibrate which is sound energy (converting electrical energy to mechanical energy). These mechanical waves go into tissues of the patient and bounce back which in turn vibrate the crystals which produces a voltage that is processed by a computer and is converted to an image.
- Ultrasounds are sound waves which are longitudinal waves whereby the particles move parallel to the direction of the wave. In some areas, the particles are closer together (compression) whilst in other areas, they are further apart (rarefaction). They require a medium.
- Medical ultrasound uses frequencies of 2-18MHz and they can be used for diagnostic purposes, therapeutic and small animal lab imaging.
- The transducer acts as both emitter and receiver of ultrasound waves which allows us to determine which structures are present. The sound waves are reflected off the body structures and detected by the probe/transducer –> pulse echo principle. The ultrasound waves are reflected by solid, so they appear bright (as ultrasound waves go back to probe); transmitted by fluids, so they appear dark. Scattering of ultrasound waves occurs when the waves hit a small dense structure (when an object is smaller than the wavelength) causing the ultrasound beam to spread out, producing a blurry image.
- So ultrasound waves can interact with tissues in 4 ways: absorption (where the ultrasound energy is converted to heat); reflection (energy is reflected from a boundary of 2 dissimilar materials); refraction (some or all the energy is diverted from it’s original path; scattered (energy is dispersed in all directions).
What is accoustic impedence?
density of tissue x speed of sound in tissue.
We assume the speed of sound to be about 1540 m/s in every tissue so acoustic impedance is essentially a measure of how easily a material allows sound waves to pass through. The greater the acoustic impedance between tissues, the greater the reflection of the ultrasound waves and the brighter the image.
What is accoustic shadowing?
Distal to a bright object, we see dark acoustic shadowing e.g., kidney stone is so dense that it reflects all of the ultrasound waves back to the probe so that none of it is transmitted, hence everything behind the stone appears black.
What is accoustic enhancement?
When ultrasound waves go through fluid filled structure; the ultrasound waves are not reflected, they get transmitted, and the next dense structure will appear brighter than it would actually. This is the case for urinary bladder and cysts.
It’s a type of artefact.
What is time-gain compensation?
Reflection of ultrasound waves decrease with distance e.g., someone who is fatter will have more tissues that the ultrasound waves have to go through hence more waves will be absorption, compared to a thinner person. So, we have a method called time-gain compensation which makes use of mathematics to counteract this difference, so it measures the time where the beam is emitted to when it’s received – this allows us to take into consideration the absorption of the ultrasound waves.
Why do we use gel on patients?
We use gel on the patient’s skin to allow the ultrasound waves to pass into the body without reflecting off intervening air. This is because in air a lot of energy of the ultrasound waves is lost between the particles.
What is the use of different probes?
Different ultrasound wave frequencies can be used for different diagnoses. Higher frequencies of ultrasound have a shorter wavelength so they cannot travel far distances (less penetrating), so they’re used to detect more superficial structures such as muscles or thyroid gland; higher frequency ultrasound is absorbed more easily so are less penetrating. However, high frequency ultrasound waves give a better resolution image.
Lower frequencies of ultrasound have longer wavelengths so they can travel further distances (hence more penetrating) and can detect deeper structures such as deep abdomen (e.g pancreas) and gynaecological structures.
To try and get a high resolution of structures, we can use a high frequency ultrasound emitting probe inside the patient so that less ultrasound is lost through attenuation.
What are the different modes?
We have different modes of ultrasound such as brightness mode, amplitude mode and motion mode.
The brightness mode ultrasound is the main mode used and produces a 2D image where tissues/organs are of different brightness and relies on the pulse echo principle.
The amplitude mode is when one beam of ultrasound is passed through the material and the returning echoes are recorded to give a 1D representation (graph) of the structures the beam passes through – it can be used to show midline shift of the brain.
The motion mode is when ultrasound waves are released in quick succession in A or B mode then recorded which creates an image analogous to a video recording and is used in cardiology to image heart valves and vessels.
What is Doppler ultrasound?
Doppler ultrasound is used to estimate blood flow through blood vessels using ultrasound. It occurs when the detector and source are moving relative to each other.
As a sound wave hits a moving object, the returning wave changes frequency – if an object is moving towards a transducer, the frequency increases; if an object is moving away from the transducer, the frequency decreases. It detects movement relative to the probe by looking at whether the reflected waves are compressed. Each wavelength is associated with a colour, so the faster the blood moves, the more red the colour. It detects the direction of blood flow in the heart and vessels; it can be used to find the presence of vessels in an organ or tumour or confirm lack of flow in a deep vein thrombosis. It can also be used to measure the diameter of a vessel lumen in carotid atherosclerosis.
We can have duplex ultrasound where both B mode ultrasound and doppler are used (overlay of doppler colours over the basic grey ultrasound image).
How can ultrasound be used for biopsy?
Ultrasound-guided biopsy can be performed on superficial lesions such as on your thyroid gland. It is used to get a diagnosis to confirm whether you have a disease or not. It is less invasive than surgical biopsy
What is thermal index?
Power emitted/power required to increase temperature 1 degrees. This measures the ability of ultrasound to heat up local tissue. An index of less than 0.5 is below the threshold for any effect and is considered safe. Certain tissues are more susceptible to thermal damage than others, hence thermal index has to be considered in embryos with less than 6-week gestation, head/brain/spine of neonate, bone as it absorbs a lot of ultrasound. The maximum thermal index allowed by the scanner is 6.
How can ultrasound be used for gallstones?
The bile will appear black because it transmits the ultrasound waves; fat and calcium of the gallstones appear white as there is greater reflection of the ultrasound waves. We can differentiate between the gallstone and fat because the calcium is much more dense than fat so it reflects all of the ultrasound so that the waves are not further transmitted – so we have acoustic shadowing.
How can ultrasound be used to diagnose Down’s syndrome?
Ultrasound can also be used to measure the nuchal fold at the back of foetus neck. Increased thickness can be a sign of chromosomal abnormality such as Down’s Syndrome. It is measured in the 2nd trimester (20-week scan).
Nuchal fold of more than 6mm is abnormal. It is fluid so it will appear black (more transmission).
What are the advantages and disadvantages of ultrasound?
The advantages of using ultrasound are that there is no radiation exposure, it’s non-invasive, it’s inexpensive, it’s fast, it can measure velocity (e.g., of blood). However, the disadvantages is that it can’t image bone or gas (e.g., not good for most cases involving bowel); it’s very dependant on the skill of the operator.
How can ultrasound be used for pregnancy?
Pregnancy – can see the yolk sac and fetal pole in the uterus. The yolk sac appears black as it transmits the ultrasound; the fetal pole appears brighter as it reflects the ultrasound waves. You can have a transabdominal ultrasound which will have to be of lower frequency as more ultrasound waves have to penetrate through to reach the different layers to reach the fetus. There is also transvaginal ultrasound whereby a probe of higher frequency can be used because the waves don’t have to penetrate as far; the image produced will be of higher resolution. -Ultrasound can also be used to measure the nuchal fold at the back of foetus neck. Increased thickness can be a sign of chromosomal abnormality such as Down’s Syndrome. It is measured in the 2nd trimester (20-week scan). Nuchal fold of more than 6mm is abnormal. It is fluid so it will appear black (more transmission).
There is no ionising radiation so no radiation risk to the baby (so reduced damage to DNA by direct and indirect effects of X-rays), no pain, non-invasive and real time image. However, the image produced in ultrasound is more blurry than X-rays, CTs and MRIs (so ultrasound has a lower resolution) which may lead to misdiagnoses – but the benefit of reduced radiation dose overrides the fact that the image is in low resolution. The image produced is operator and patient dependent, so we need to make sure we have an experienced professional doing this. Provided the thermal index of the ultrasound is less than 0.5, the effect of the ultrasound is thought to be safe (however, it’s important to consider that some fetal tissues such as bone, head, and spine of fetus may absorb more ultrasound, heating these tissues up more). We use gel on the patient’s skin to allow the ultrasound waves to pass into the body without reflecting off intervening air. This is because in air a lot of energy of the ultrasound waves is lost between the particles. Doppler ultrasound can also be used to measure blood flow, such as in the umbilical artery (in normal cases, the blood should flow forwards) and uterine artery. MRI is not recommended for pregnancy as we are not sure about the effects of the magnetic fields on the growing fetus.
