Duplex ultrasonography (duplex scan) consists of two ultrasound modalities to study blood flow and the perivascular tissue. This includes B-mode / gray scale imaging used in combination with spectral Doppler / pulsed-wave Doppler.
The real-time visualization of the vessels and tissue by the B-mode component improves the PW Doppler positioning and the direction of blood flow can be inferred. The angle between the direction of the PW Doppler signal and the estimated direction of blood flow can be measured.
Duplex techniques are available on phased array, linear array, and mechanical scanners. A phased array probe is able to create nearly simultaneous images and flow information. A linear array transducer can also do this if the Doppler probe is attached separately to one end of the scanhead. A mechanical transducer freeze the image; the crystals must be static to produce a Doppler image. The first two transducers are therefore the best choice for Duplex.

See also Compound B-Mode, and Duplex Scanner.

Sonography of the gallbladder is a reliable technique for diagnosing e.g., gallstones, cholecystitis, tumors, polyps, or ductal obstruction. Patient should be examined with empty stomach and on a low fat diet the night before. Barium studies, endoscopy, ERCP, colonoscopy, and abdominal CT should be performed after this examination.
Gallbladder ultrasound is best performed with a 5 MHz curved array or a linear array transducer in cases of a very superficial gallbladder. In obese patients or in patients with difficult sonographic access, a 3.5 MHz sector or curved linear transducer is advantageous.
Gallbladder and biliary tree are usually imaged in supine and posterior oblique (LPO) positions. Sometimes very small gallstones are better visible in upright and prone position.

A linear probe contains one or more acoustic linear array transducer elements arranged in a line to send pulses of sound into a material. The linear array gives a large probe surface (footprint) and near field. In Doppler mode, a linear probe operates a subset of its elements as a linear phased array and can thus steer the Doppler beam at a selected angle to the imaging beam. This is a popular configuration for peripheral vascular and perivascular scanning.

In the field of medical ultrasound imaging, the term 'probe' specifically refers to the ultrasound transducer and represent the handheld device that emits and receives ultrasound waves during an examination.
The probe encompasses various components such as the elements, backing material, electrodes, matching layer, and protective face that are responsible for both emitting and receiving the sound waves. Aperture, known also as the footprint, is the part of the probe that is in contact with the body. When the emitted sound waves encounter body tissues, they generate reflections that are received by the probe, which then generates a corresponding signal. In most cases, the probe emits ultrasound waves for only about 10% of the time and receives them for the remaining 90%.
Probes are available in different shapes and sizes to accommodate various scanning situations. The footprint is linked to the arrangement of the piezoelectric crystals and comes in different shapes and sizes e.g. linear array transducer//convex transducer. The transducer plays a huge role in image quality and is one of the most expensive parts of the ultrasound machine. Mechanical probes steer the ultrasound beam driven by a motor and are capable of producing high-quality images, but they are prone to wear and tear. Mechanical probes have been mostly replaced by electronic multi-element transducers, but mechanical 3D probes still remain for abdominal and Ob-Gyn applications.
In summary, the terms 'ultrasound transducer,' 'probe,' and 'scanhead' are often used interchangeably to refer to the same component of the ultrasound machine. Probes consist of multiple components and are available in different shapes and sizes depending on the sonographer's needs.

See also Handheld Ultrasound, Ultrasound System Performance, Omnidirectional, Probe Cleaning, and Multi-frequency Probe,

Real-time mode has been developed to present motion like a movie of the body's inner workings, showing this information at a high rate. The special real-time transducer uses a larger sound beam than for A, B or M-modes. A linear array transducer with multiple crystal elements displays real-time compound B-mode images with up to 100 images per second.
At each scan line, one sound pulse is transmitted and all echoes from the surface to the deepest range are received. Then the ultrasound beam moves on to the next scan line position where pulse transmission and echo recording are repeated.

See also Compound B-Mode, Pulse Inversion Doppler, and Frame Averaging.