Modes
A-Mode (A for Amplitude)
One beam of ultrasound is passed through the material and the returning echoes are recorded giving a 1D representation of the structures the beam passes through. A-mode was the first use of medical ultrasound and was used to show midline shift in the brain. An ultrasound beam was passed through the skull and the bones and falx would return the echoes showing their position. Now, it is mainly used in ophthalmology to investigate retinal detachment etc.
B-Mode (B for Brightness)
This is now the main mode of ultrasound used. The echoes returned are shown on screen in a grey-scale corresponding to their intensity. The structures are shown as a 2D image on screen.
M-Mode (M for Motion)
Ultrasound waves are released in quick succession in A or B-mode and recorded. This creates an image analogous to a video recording.
As organ boundaries reflecting the sound waves move, the velocity can be calculated e.g. heart valves.
Doppler
Uses the Doppler effect to measure flow e.g. blood flow. As a sound wave hits a moving object the returning sound wave changes in frequency. If the object is moving towards the transducer the frequency increases, if the object is moving away from the transducer the frequency decreases. Doppler can be pulsed or continuous.
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
B-Mode transducer
Basics
- 100 or more A-lines are fired sequentially
- These are reflected from the tissue interfaces
- The amplitude of the returning waves are received and converted into brightness
- An image is built up line-by-line forming a cross-sectional image
Transducer
The transducer converts mechanical energy into electrical energy and vice versa. It acts as both a transmitter and a receiver of sound.
Acoustic Insulator: stops the transducer vibrating in the hand
Backing Material: stops vibrations reverberating back into the piezoelectric material. It determines the length of the ultrasound pulse by determining how much it is dampened (Q value, see “Producing an ultrasound beam” chapter)
Piezoelectric Material: material that forms ultrasounds and receives echoes. It is ½ wavelength thick and is usually composed of 256 crystals.
Matching Layer: always ¼ wavelength thick to reduce wavelength impedence difference.
Safety
The advantages of ultrasound imaging are:
- No radiation exposure
- Non-invasive
- Fast
- Inexpensive
- Real-time imaging
- Can measure velocity e.g. of blood
- Cross-sectional imaging
There are also disadvantages to ultrasound:
- Can’t image through bone or gas
- Very dependent on operator skill
Although ultrasound is generally safe and there has been no confirmed evidence of damage from diagnostic ultrasound exposure, there are some theoretical risks. There are a few values that must be monitored and kept within certain limits to reduce the likelihood of these risks.
- Time-averaged intensity < 100 mWcm-2
- Total sound energy < 50 Jcm-2
- Thermal index
- Mechanical index
Thermal Index (TI)
The TI measures the ability of the ultrasound to heat up the local tissue.
TI = power emitted / that required to increase temperature by 1°c
An idex of <0.5 is below the threshold level for any effect and considered safe. As the TI increases the scanning time should be decreased in proportion e.g. with a TI of 3 a patient may be scanned for 10 min. If the patient has a fever, the threshold for complications is lower and the scanning time should be shortened e.g. with TI of 3 and a patient with a temperature of 39°c scanning for even less than 1 min would not be safe.
Sensitive tissues that are more susceptible to thermal damage are:
- An embryo less than eight weeks after conception
- The head, brain or spine of any fetus or neonate
- The eye (in a subject of any age).
Mechanical Index (MI)
The MI is the measure of the maximum amplitude of the pressure pulse and indicates the risk of cavitation.
MI = peak negative pressure / √(ultrasound frequency)
The British Medical Ultrasound Society (BMUS) states that general diagnostic ultrasound and obstetric ultrasound must have an MI < 0.7, especially with the use of contrast agents which theoretically increase the cavitation risk with higher MIs. The MI in general neonatal imaging should be <0.5.
Complications of ultrasound
- Local heating
- Cavitation: The pressure changes cause microbubbles in a liquid to expand then collapse. There is an increased risk of cavitation in:
- Gas-containing structures (e.g. bowel, lung)
- Low frequency pulses (i.e. longer wavelengths)
- Higher power or intensity of pulses
- Use of ultrasound contrast agent.
- Mechanical damage to cell membranes
Σ Summary
Modes
- A-Mode – (amplitude) 1D representation of structures
- B-Mode – (brightness) most commonly used form of diagnostic ultrasound
- M-Mode – (motion) used in cardiac imaging
- Doppler – measures flow and velocity
Transducer
- Backing material: determines pulse length via Q-value
- Piezoelectric crystals: produce and receive ultrasound beam. ½ wavelength thick
- Matching layer: reduces impedence difference. ¼ wavelength thick
Safety
- Time-averaged intensity < 100 mWcm-2
- Total sound energy < 50 Jcm-2
- Thermal index
- Indicates risk of local heating
- TI 0 – 1.0 safe
- Decreased threshold in: febrile patients, fetal scanning, eye
- Should never use TI > 3 in fetal scanning
- Mechanical index
- Indicates risk of cavitation
- MI < 0.7 for general use
- MI < 0.5 for fetal scanning
- MI > 0.7 should never be used with ultrasound contrast agents
- Complications
- Local heating, cavitation, mechanical damage
