Ultrasound 2

Variable Focusing in Transmit Mode

Linear array transducers have a set focal spot distance, meaning variable focusing is not possible. The focal spot distance for linear array transducer depends on transducer diameter, frequency and presence of acoustic lenses on the element surface.

For phased array transducer, the focal spot is determined by time delays. Time delays applied to discrete elements results in phase shifts in the ultrasound pulses across the face of the transducer element. This results in the wave converging at a certain distance in the patient. Time delays can be changed according to the required focal distance.

• For shallow focal spot distances, the outer elements are fired before the inner elements.
• For greater focal spot distances, the outer elements are still fired before the inner elements but the time delay is reduced.
• For multi focal zones, we apply repeated transmissions but with difference time delays.

Rephasing In Receive Mode

For phased array transducers, the echoes received by all transducer elements are summed to create an ultrasound signal from a given depth. However, all of the echoes reach the transducer surface at different times; the echoes received by the outer elements travel a slightly longer distance than the echoes received at the centre, particularly for shallow depths.

Thus, the signals from individual transducer elements must be rephased to avoid a loss of resolution. Rephasing is done by applying electronic delays as a function of depth. At shallower depths, rephasing between adjacent transducer elements is greatest.

Near and Far Fields

Ultrasound waves exhibit two distinct beam patterns in a medium, the near and far fields. The near field is a slightly converging beam out to a certain distance, which is determined by the geometry and frequency of the transducer. It can be calculated by dividing the square of the transducer diameter by the wavelength times by 4.

The far field is a diverging beam beyond the near field. The beam divergence angle can be calculated (see notes for equation).

Spatial Resolution

There are three types of spatial resolution in ultrasound: axial, lateral and elevational. Axial is better than lateral resolution, and elevational resolution is the worst.

Axial resolution is the ability to discern two closely spaced objects in the direction of the beam. It requires returning echoes to be distinct and not overlapping. It depends on frequency, as higher frequency yields high resolution, and damping factor. Axial resolution is independent of depth.

Lateral resolution is the ability to discern two closely spaced objects perpendicular to the beam direction. Beam diameter determines lateral resolution in a linear array transducer, however transmit and focus electronics determine lateral resolution in a phased array transducer. Lateral resolution is dependent on depth.

Elevational resolution is perpendicular to the image plane. It is dependent on transducer element height and is typically the weakest measure of spatial resolution for array transducers. It is depth dependent.

Wave Interferences

If two waves are acting on the same body, the total displacement will be equal to the sum of the two separate displacements.

• Destructive interference is when the force of one wave on a particle is partially or completely counteracted by the force of another wave. This results in zero or less particle movement.
• Constructive interference is when the waves support each other, resulting in more particle movement.

Major Components of an Ultrasound System

• The beam former generates electronic delays for individual transducer elements to achieve transmit/receive focusing and beam steering.
• The pulser provides an electronic voltage for exciting PZT and controls the output transmit power. An increased transmit amplitude creates higher intensity sound, improving echo detection from weaker reflectors. Increasing the transmit amplitude improves the signal to noise ratio, but results in greater power deposition in patient.
• The transmit/receive switch is synchronised electronically with the pulser. In transmit mode, a high voltage is used for generating pulses. In receive move, we isolate sensitive amplifiers for the return echo-induced voltages in the piezoelectric element. The return echo is generally between 1V to 2 microV.

Time Gain Compensation

Throughout the patient, there will be some identical tissue interfaces. However, as depth increases, attenuation also increases and the return echoes for deeper interfaces will be of lower intensity than those closer to the patient surface. However, the amplitude of these echoes should technically be the same. Thus, this is corrected by amplifying the return echoes as a function of depth, i.e. amplification increases with increasing depth in the patient. This is known as time gain compensation.

Pulse Echo Operation

In pulse echo operation, the ultrasound is intermittently transmitted, with the majority of time reserved for listening for echoes. When all echoes are detected, the pulse is created again using the Pulser and a short voltage waveform. The transmitted pulse is generally 2 to 3 cycles long. The time delay between the transmit and receive is directly related to the depth of the interface.

In soft tissue, ultrasound assumes a speed of 0.1540cm/microseconds. The time delay between the transmission pulse and echo detection is directly related to the depth of the interface, D. See notes for equations.

Echo Display Mode

There are three modes of display for pulse echo operation; A (amplitude) mode, B (brightness) mode and M (motion) mode.

A Mode

A mode involves a single pulse echo. The echoes return from tissue boundaries and scatterers, and a digital signal proportional to echo amplitude is produced as a function of time. There is one A line of data produced per pulse repetition frequency.

NB: A mode is used in ophthalmology for precise distance measurements of the eye.

B Mode

B mode converts the A line information into brightness modulated dots. The brightness of the dots is proportional to the echo signal amplitude. B mode generates a 2D image and converts a plane of interest, rather than just one single line of transmit/receive information. B mode display is used for M mode and 2D grey scale imaging.

M Mode

M motion uses B mode information to display the echoes from a moving organ from a fixed transducer position and beam direction. Only one anatomical dimension is represented using the M mode technique. However, 2D echocardio, Doppler and colour flow have since replaced the need for M mode.

Pulse Repetition Frequency

Pulse repetition frequency is the number of times the transducer is pulsed per second. It typically ranges from 2000 to 4000 pulses per second. The maximum PRF is determined by the time required for echoes from the most distant structures to reach the transducer. High frequency transducer elements produce high spatial resolution and have low penetration. Thus, PRF is high because there is not as greater distance to travel. It can be expressed in Hz.

Pulse Repetition Period

Pulse repetition period is the time between pulses and is equal to the inverse of PRF.

Benefit and Drawback from High Transmit Amplitude

• Benefit: improved echo detection from weaker reflectors, increased signal to noise ratio
• Drawback: more power deposition in patients

Contrast Agents

Contrast agents are mainly used for vascular and perfusion imaging. The contrast agents are micro bubbles of 3-5 micrometer in diameter, filled with air, nitrogen or other insoluble gaseous compounds encapsulated by a material that will maintain stability during the scan, e.g. human albumin.

Gases have unique acoustic impedance values and are highly compressible. This means there is large difference in acoustic impedance between the contrast and fluid/tissues, resulting in high reflection or scattering. The bubbles are small compared to the wavelength of the ultrasound and produce scatter in every direction.

3D/4D Ultrasound

3D ultrasound uses a large 2D matrix with up to 10,000 transducer elements which fire ultrasound in multiple directions. The returning echoes are process a 3D volume image of a foetus surface or internal organ. 4D ultrasound simply provides a moving 3D image.

Elastography

Elastography is mainly used for breast lesion characterisation. The characteristic properties of ultrasound change depending on the elastic properties of the propagation medium. Based on these changes, a 'stiffness' map can be created. Benign tumours are easily deformed and tumours are stiffer than surrounding tissues.

Ultrasound Artefacts

There are five types of artefact observed in ultrasound imaging.

1. Rarefaction artefacts
2. Shadowing and enhancement
3. Reverberation or multiple artefacts (comet-tail artefacts)
4. Speed displacement artefacts
5. Side lobes or grating lobes

Rarefaction artefacts are when an anatomical structure is is missed or displaced from its true position due to rarefaction of the beam.

Shadowing is when there is reduced echo intensity behind a highly attenuating or reflecting object, creating a shadow.

Enhancement is when there is a high echo intensity behind a minimally attenuating or reflecting object.

Reverberation is when there is multiple representation of the same interface on the display. It common occurs between two strong reflectors, such as an air pocket and the transducer array. The echoes bound back and forth between the boundaries and produce equally spaced signals of diminishing amplitude.

Speed displacement artefact is caused by the variability of speed of sound in different tissue. In particular, the lower speed of sound in fat (1450m/s) results in displacement of the returning echoes from distal anatomy by 6% of the distance travelled through fat. Speed displacement artefacts result in range and distance uncertainty.

Side lobes or grating lobes are normally of much lower amplitude. However, if they strike a strong reflector, this may give rise to visible echoes. Anatomy outside of the beam may then be mapped into the main beam.

Biological Effects of Ultrasound

There are two main biological effects of ultrasound and these are mechanical and thermal effects.

Mechanical Effects

Mechanical effects are the mechanical movement of the particles of the medium due to radiation pressure, as well as small scale fluid motion termed micro streaming. Cavitation is the creation and collapse of microscopic bubbles and production of cavitation is measured using the Mechanical Index (MI).

Thermal Effects

Biological tissues absorb ultrasound energy, which is converted into heat. Thermal effects depend on the rate of heat deposition and how fast the heat is removed through blood flow. The Thermal Index (TI) is the ratio of the acoustic power produced by the transducer to the power required to raise the tissue in the beam area by 1 degree celsius.

NB: MI and TI are displayed on the ultrasound equipment. The sonographers must minimise power deposition within the patient of foetus, as per the ALARA principle.