DIGITAL BEAM FORMING

The beam former is the system of electronics that determines the shape of the beam. Earlier beam formers used either analog electronics or a combination of analog and digital electronics. In modern transducers the beam former is totally digital which enables the ultrasound beam to be focused with greater precision.

The amount of focusing and the position of the focus in an ultrasound beam is a function of the beam former. Focusing is achieved by applying delays to the inner elements of the group of crystals that are used to produce a composite ultrasound pulse and to receive echoes from the subject. Accurate timing of these delays is critical in producing a narrow beam focusing at the correct depth. In addition, the more accurate the timing of the delays, the less noise is produced and the better the contrast resolution.

Using analog beam formers, it is not possible to produce delay timers with the accuracy required and this limits their performance. However, beam formers are now produced using digital electronics for the time delays. In addition to producing narrower beams, digital beam formers can operate at higher frequencies and can be used with broad-bandwidth transducers.

HIGH-FREQUENCY IMAGING

High-frequency ultrasound imaging using frequencies above 20 MHz is being developed to enable the imaging of superficial structures at a very high resolution. Using conventional medical ultrasound at 5 MHz, it is possible to penetrate up to 20 cm and achieve a spatial resolution of 0.5–1.0 mm. However, with high-frequency ultrasound it is possible to image with a resolution of 50 microns (1/20 mm). The disadvantage is that because the attenuation is increased at high frequencies, penetration is reduced to a few millimeters.

The initial clinical applications of high-frequency ultrasound include the anterior chamber of the eye, intravascular ultrasound of arterial walls, skin, and cartilage.

EXTENDED FIELD OF VIEW IMAGING

This is an imaging process which combines static B-mode techniques with real-time imaging so that a large subject area can be viewed on a single static image. Extended field of view (FOV) images are obtained by sliding the probe over the area of interest and as the images are acquired they are ‘stitched together’ electronically (Fig. 14.1). The result is a single slice image covering the whole area of interest, for example a full length view of the Achilles tendon (Fig. 14.2). Image feature recognition software is used to combine images.

Fig. 14.1 Diagram showing how images of the subject are acquired before being electronically ‘stitched together’

Fig. 14.2 Extended field image showing the full length of the Achilles tendon

This feature is now standard on most current ultrasound systems, and is particularly beneficial where large areas of the patient need to be visualized on one image, such as obstetrics or musculoskeletal imaging.

COMPOUND IMAGING

This technique combines electronic beam steering with conventional linear array technology to produce real-time images acquired from different view angles (see Fig. 14.3) Between 3 and 9 sector images are rapidly acquired and combined to produce a compound real-time image (see Fig. 14.4). Compound imaging improves image quality by reducing speckle, clutter, and other acoustic artifacts. It also gives better definition of the boundaries of structures. Because of the improved contrast resolution, compound imaging may be useful for the breast, peripheral blood vessels, and musculoskeletal applications.

Fig. 14.3 Diagram showing how electronic beam steering is used to acquire images from different angles

Fig. 14.4 Diagram showing how the images are combined to produce a compound real-time image

THREE-DIMENSIONAL IMAGING

In conventional two-dimensional imaging the operator integrates a large number of images representing slices of the subject to form a mental 3-D image of the subject’s anatomy; however this mental 3-D picture is only available to the operator and only during the scanning process. The challenge for equipment designers is to produce a 3-D image which can be reviewed after the examination by the operator and also by other staff and patients. With 3-D ultrasound, an image of the surface of a structure is produced; this can be rotated through different planes and the surface viewed from many angles. It is also possible for the operator to ‘peel away’ layers of a 3-D image and see inside the structure.

3-D Imaging Technology

The production of a 3-D image requires a volume of tissue to be scanned. The data from this volume are then used to construct the types of image required. There are three approaches to scanning a volume of tissue: free-hand, mechanical, and electronic scanning.

Free-hand 3-D imaging

In this approach the operator sweeps the probe across the volume of interest and a series of scanning planes are recorded according to their position on the patient (see Fig. 14.5). In order to register these planes, a method of determining the position of the transducer in space is required. This can be achieved by using a receiver in the probe which will detect a magnetic field generated by a transmitter situated next to the couch (see Fig 14.6). Each image slice will have image information and position information for use in 3-D construction. Another method of determining the position of the scan plane is to use a radio transmitter and radio detection coils attached to the probe.

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