The scan converter is a memory device that converts the RF signal processed by the receiver into a video display made up by picture elements (pixels).

The image plane is typically divided into 262,144 (512 x 512) equally sized squared pixels. A number is digitally stored in each pixel. The number assigns a shade of gray (brightness) to the pixel. The brightness is proportional to the amplitude of the returning echo.

The positional (X,Y) and echo amplitude (Z) information is stored in a solid-state semiconductor device. A single binary digit (= a bit) can be 0 or 1. If the memory device were a single layered grid only two shades of gray could be displayed. A modern eight layered grid allows 256 shades of gray (2^8) for each pixel.

The voltage developed on the transducer elements is sensed by the beamformer, that amplifies and dynamically focuses the very weak electrical impulses from the transducer elements.

Adjustment of the amplitude and delays of the received signal on each element makes it possible to receive from a chosen angular direction.

The receiver processes the radio-frequency (RF) signal from the beamformer by amplification, compensation, rejection, compression, and demodulation.

Amplification strengthens the electrical signals. Compensation amplifies the signals in proportion to the depth from which the echoes return. Rejection eliminates signals below a minimum amplitude value in order to eliminate noise. Compression reduces the range of signals by logarithmic amplification that strengthen weaker signals more than stronger signals. Demodulation smoothens the signals into simple forms that are easier to process.

It is possible for the user to control the amplification of the signals by adjusting the gain settings. Some systems only allow a few steps of user gain control. Others have multiple gain controls (Time Gain Compensation TGC) allowing adjustment of gain in narrow bands at all depths of the 2D image displayed on the monitor.

The phased array principle: instead of one single wave emitter which has to be oriented mechanically, the phased array system uses many emitters with independent adjustment of the signals in order to create the desired orientation of the beam.

A phased array probe applies voltage pulses to all crystals simultaneously but with tiny time differences (phasing).

All the waves are added up to a single beam with a single wave front travelling at an angle steered electronically by preset programming of the pulse timing. Controlled by the electronic steering (beam steering), the beam can make a sweep and send the beam in one direction at a time. In this way the beam can sweep a sector despite a rather small transducer footprint.

Phase steering can also be used to make a concave wavefront. The result is that the beam is focused at the narrowest part of the beam.

Combining beam steering and beam focusing, a focused beam can be swept across a sector.

The transmit beamformer controls and coordinates the timing of the transmit high voltage pulses in order to produce a continuous thin acoustic beam with focal point uniformity and high sensitivity and resolution at all depths.

Modern transmit beamformers use dedicated processing channels for each individual transducer element. The system calculates the optimum delay pattern real-time for each transducer element.

In a linear transducer the crystals are embedded in a straight array. The generated ultrasound beams are parallel to each other producing a rectangular image.

The linear transducer generates parallel beams in sequence – one at a time. The created field is as wide as the length of the footprint (see figure) of the transducer.

The linear array transducer is ideal for guiding vascular access in veins and arteries and for superficial peripheral nerve blocks.

In a curvilinear array transducer the piezoelectric crystals are embedded in a curved array, creating a far field that is wider than the footprint of the probe. This allows a relatively small footprint. The trade-off is reduced lateral resolution in the far field as the scan lines diverge.

In modern ultrasound systems the piezoelectric crystals are arranged in an array which is structured to allow the sound waves generated by one crystal to interact with those from other crystals.

A 2D image is constructed by a sequence: (1) an ultrasound beam is emitted from one crystal; (2) the echo signal is retained; (3) an ultrasound beam is emitted from the next crystal in the array etc.

Strictly speaking, a transducer is one piezoelectric crystal. Transducers are the crystals within a probe. The probe contains hundreds of transducers and a lot of hardware to form and steer the beam.

Beam width is the major determinant of lateral resolution. Lateral resolution is practically identical to beam width.

In modern electronically focused transducers the lateral resolution is defined at the focal plane.

In a modern linear array probe, the ultrasound system controls the number of crystals used to create a line of ultrasound data. It is possible for the user to control the depth of the beam focus.

The lateral resolution is best at the focal plane of the ultrasound beam.